Microcirculación y circulación linfática

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1 Microcirculación y circulación linfática
Capítulo 17 Función final sist cv mantenr ambiente tejidos sitio de intercambio gases, agua nutr. Prod de deshecho función nutri pero too otras func por ej caps glomerulaeres formación de un filtrado, piel reg temp. Señalaizacipon distr hormonas o defensa plauqetas Dra. Aileen Fernández Ramírez M.Sc. Profesora catedrática Departamento de Fisiología Escuela de Medicina, UCR

2 Microcirculación Capilares
5 a 10 um una capa de cel endoteliales rodeda por una membr basal y longitud 0.5-1mm v media F: 1 mm/s Membrana basal gruesa o delgada dependiendo de Esfínteres precapilares: musc liso no está inervado responde a cambios locales controlan el flujo a una parte de la red capilar modula Cierre o apertura de caps produce peq cambios de P que puede revertir el flujo en algunos sectores Caps 2 a 5 um una capa de cel endoteliales rodeda por unamembr basal Alta densidad capilar Corazón, musc y gándulas Baja densidad cap. Tejido subcutáneo cartílago Cel endoteliales tienen actina y miosina y pueden modifcar su forma como resp a det estimulos químicos Por su pequeño radio pueden soportar grandes presiones internas sin estallar T=aumentado P x r disminuido aun cuando la P cap suba a 100 (de pie) la T no se eleva mucho 3000 veces menor que en la aorta a pesar de que alcanzó la misma P En la aorta en condic promedio caps 25 mmHg y aorta 100 mm Hg la T es veces menor en los caps. Metarteriola atajo por el capilar, tiene musc liso pero no está inervado 10-20um, Ganong dice que no se ha comprobado que sea reg por SS (musc esq no tiene) y esfinteres no están presentes en todos los tejidos Sangre fluye por las metarteriolas cuando los capilares están colapsados y esfínteres precapilares cerrados cuando aumenta la Pc y se supera la P crítica de cierre los esfínteres precap se dilatan y relajación del musc liso metarteriolas ye esfínteres por metabolitos liberados localmente Silverthorn La sangre que ingresa por las metarteriolas puede ingresar directamente a los caps y si los esfínteres están cerrados pasa de arteriolas a vénulas Anastomosis arteriovenosas son vasos musculares 20 a 130 um de ancho que unen art a vénulas sin pasar por caps. En la piel de dedos nariz labios y orejas, reg temp. La densidad cap está adaptada Vasomoción: contracción y relac. De musc liso de arteriolas, cambia el flujo cap. Es una caract intrínseca del musc indep de influencias externas Aumento de la P transmural por aumento de P venosa o dilatación produce contracción de las arteriolas y la dismiución P t produce relajacións Vasomoción se puede abolir por influencias externas por ej por factores metab. Arteriolas de gran calibre y metaarteriolas too subren vasomoción. Contr de las arteriolas terminales puede deterner el F pro completo El flujo por los capilares puede modificarse por la contraccion y relajaciónd e las areterias pequeñas, arteriolas y metarteriolas. Cortocircuito : F no nutricional puntas de los dedos derivaciones AV Too F no nutri cuando hay baja act metab y los esfínteres precaps están cerrados Cuando hay baja act metab y caps caps: F nutricional Intercambio de nutrientes Filtración Regulación de la temperatura Distribución de sustancias Las ramas de las art más peq se ramfican en arteriolas de primer orden que se siguen dividiendo hasta formar arteriolas terminales que son las últimas que tienen musc liso Cada arteriola terminal origina un módulo o grupo de capilares de 500 a 1000 um de largo y 4 a 8 de ancho. Metarteriolas semejantes a las arteriolas pero más cortas musc liso disc no inervadadas Boron: El lado venoso forma vénulas pericíticas de 15 um de ancho que no tienen musc liso y son muy permeables al agua y juegan un rol imp. En inflamación Vénulas de 30 a 50 um reaparece el musc liso. Arteriolas y vénulas con musc liso

3 Capilares Membrana basal Células endoteliales Fibras de colágeno
Pericitos: Células endoteliales Fenestraciones Vesículas Uniones o hendiduras inter-endoteliales Caps 5 a 10 um una capa de cel endoteliales rodeda por una membr basal Membrana basal: fibras de colágeno y en algunos tejido pericitos. Pericitos: contráctiles y liberan agentes vasoactivos. Sintetizan constituyentes de la membrana basal Mas gruesa en tejidos que soportan altas presiones transcapilares y otras fuerzas Cel contráctiles muy ramificadas rodean a los caps y forman una capa reticular ext. Entre endotelio cap y liq int. Contribuyen a det la permeabilidad de los caps. Entre más pericitos menos permeable caps cerebrales están reodeados por pericitos y cel gliales y uniones estrechas barrera hemato. Influyen en el crec. Pueen idferenciarse y convertirse en nuevas cel endot o cel musc lisas. Predominan en capilares continuos y en la barrera hematoencefálica Poros y vesículas: endocitosis, transcitosis de agua y compuestos Uniones interendoteliales : unen cel endoteliales espacio entre células puede ser estrecho 4 nm o muy amplio. Uniones estrechas membranas celulas se fusionan Otros tienen conductos llamados fenestraciones que atraviesan toda la cel.(glandulas, vellosidades intestinos, partes de lso riñones brechas 20 a 100 nm y dejan pasar moléculas grandes vuelven porosos a los caps.

4 Tipos de capilares: clasificación estructural
4 a 15 nm Es el más común Músculo, piel, pulmones Berne: la malyor parte del agua pasa por poros, hendidurasLevick: Clasificacion de acuerdo a permeab al agua. Sinosoidales: grandes espacios entre las células endoteliales son discontinuos La barrera hematoencefálica Continuous (found in muscle, skin, lung, central nervous system) – basement membrane is continuous and intercellular clefts are tight (i.e., have tight junctions); these capillaries have the lowest permeability.  Levck 2 a 3 cel edote permite paso de agua y solutos peq no solubles en lípidos el glicocalix (barrera de macromoléculas cargada neg en la superficie interna o luminal del endotelio excluye a las prot. , sistema de caveolas -vesículas Fenestrated (found in exocrine glands, renal glomeruli, intestinal mucosa) – perforations (fenestrae) in endothelium result in relatively high permeability.  se encuentran en tejidos especializados en intercambio de liq. Espcial para filtración, Discontinuous (found in liver, spleen, bone marrow) – large intercellular gaps and gaps in basement membrane result in extremely high permeability.   Permite migración de células como glóbulos rojos, leucos, Large surface area and relatively high permeability (especially at intercellular clefts) to fluid and macromolecules make capillaries the primary site of exchange for fluid, electrolytes, gases, and macromolecules. In some organs, precapillary sphincters (a circular band of smooth muscle at entrance to capillary) can regulate the number of perfused capillaries. 50 a 100 nm epitelio intestinal, glomérulos renales y glándulas 100 a 1000 nm Hígado, médula ósea, bazo En la barrera hematoencefálica estos espacios no existen (uniones apretadas)

5 Mecanismos de intercambio capilar
Intercambio transcapilar: a través de la pared endotelial difusión filtración y pinocitosis ACUAPORINAS: canales para el agua mec transcelular más impo para el move de agua. Los poros, fenestraciones y hendiduras son la via paracelular más impor. El mec más impo para la transferencia de líquidos a través del cap es la convección Berne: papel pasivo del endotelio Intercambio por difusión, filtración y pinocitosis. Dif mas imp y pinocitosis menos imp. Filtración y absorción: 0.06ml H2O / min /100 g tejido Difusión: 300 mlH2O/min / 100g Lo describe la ley de Fick Difusion: se dezplaza mayor vol de líquidos El intercambio transcapilar de solutos se produce principalmetne por difusión que depdende de el coef de difusión, el area de sección transv, y gradiente de conc. En el caso de la pared capilar el flujo se puede expresar como: J= PS(Ci-Cec) Permeabilidad al soluto y S superficie del capilar PS no se modifica mucho en condici fisiol. Pero si tras la picadura de una abeja. La difusión de sust insol en los lípidos se prod por poros o canales llenos de agua. Molec peq como NaCl urea y gluc psan facilmente por los poros poca resist coef de reflesión bajo. Gradiente bajo porque intercambio es muy rápido Peso molec mayor de la difusión se hace mínima. En el caso de molec peq la ´punica limitac es la veloc del flujo que transporta las moelc haci el cap limita por el flujo La cant de molec que han entrado por extr arte y salen por extr venoso depende del tamño: peq canti desprecialbe.- Solo la limita la dist entre caps y cel y edema muy grandes sale la misma cant que entro poruqe no ingresó por los poros. Difusión es el factor limitante limitada por la difusión Poros existe relac con las cargas de los solutos, la configuración de los poros 300 ml/minx 100 g tejido mientras que por filtración y absorción 0.06 ml/minx100 g Dif de 5000 veces malyor Intervambios transcapilar de solutos reg por difusiónj O2 liposol sat se ha reducido en un 80% a salida de los caps por la difusión por arteriolas y peq arterias. Int de CO2 se prod en los vasos precap O se además de int capialr se prod intercambio entre arteriolas y vénulas contracorriente Copyright © The Author(s) 2008 who observed that capillary endothelium contained large numbers of small (∼70-nm diameter) vesicles [31–33]. He named these plasmalemmal vesicles and they are now more commonly referred to as caveolae (Fig. 2a, b). The majority of caveolae are found connected to the luminal and abluminal plasma membranes by means of stomata that are generally closed by thin diaphragms. Little is known about the composition of these diaphragms other than that they contain a unique protein, PV–1, and likely sulfated proteoglycans [34]. Palade postulated that caveolae shuttled across capillary endothelium carrying cargoes of plasma fluid and proteins and this was subsequently demonstrated experimentally with tracers (reviewed in [29]). Thus it seemed that the large pores postulated by physiologists were not pores at all but shuttling caveolae and that transport of large molecules across capillaries was anything but passive. Endothelial permeability is regulated in part by the dynamic opening and closure of cell-cell adherens junctions (AJs). In endothelial cells, AJs are largely composed of vascular endothelial cadherin (VE-cadherin), an endothelium-specific member of the cadherin family of adhesion proteins that binds, via its cytoplasmic domain, to several protein partners, including p120, β-catenin and plakoglobin. Endogenous pathways that increase vascular permeability affect the function and organization of VE-cadherin and other proteins at AJs in diverse ways. For instance, several factors, including vascular endothelial growth factor (VEGF), induce the tyrosine phosphorylation of VE-cadherin, which accompanies an increase in vascular permeability and leukocyte diapedesis; in addition, the internalization and cleavage of VE-cadherin can cause AJs to be dismantled. From the knowledge of how AJ organization can be modulated, it is possible to formulate several pharmacological strategies to control the barrier function of the endothelium Difusion O2, CO2, ácidos grasos, hormonas y vitaminas liposolubles. Agua, etanol. Movimiento neto: según gradiente de concentración (compartimiento más concentrado → menos concentrado). Bulk Flow (Convection) Poros y perforaciones (fenestraciones) Bulk flow of fluid and electrolytes occurs through "pores" and intercellular clefts (d,e,f in preceding figure) This mechanism of exchange is particularly important in renal glomerular capillaries; however, it occurs to variable extent in nearly all tissues. Bulk flow follows Poiseuille’s equation for hydrodynamic flow. Therefore, changes in pressure driving forces (either hydrostatic or osmotic) and in the size of "pores" or intercellular clefts will alter exchange. Contraction of capillary endothelial cells by substances such as histamine and bradykinin increases intercellular pore size and greatly augments fluid and electrolyte movement by increasing the capillary filtration constant. There is some evidence that vesicles can fuse together creating pores across endothelial cells (c in preceding figure). Vesicular Transport Vesicular transport is involved in the translocation of macromolecules across capillary endothelium (b in preceding figure). Active Transport Some molecules (e.g., ions, glucose, amino acids) can also be taken up by vascular endothelial cells by transport mechanisms; however, this is not normally thought of as a mechanism for exchange between plasma and interstitium, but rather between an individual cell and its surrounding extracellular environment.

6 Mecanismos de intercambio
Solutos lipofílicos: O2, CO2, anestésicos área total del cap disponible para el intercambio, la permeab para O2 es tan alta que se escapa de las arteriolas y pasa a las vénulas directamente, no toda la desoxigenación se debe a transf a tejidos Solutos lipofóbicos hidrofílicos : electrolitos, glucosa, lactato, AA vitaminas, hormonas asdrenalina e insluina, drogas por uniones intercelulares y fenestraciones. Las hendiduras ocupan 0.2 a .4 % de la sup capilar Cada fenestra es un grupo de poros Dif en permeabilidad no dependen tanto del tamaño del poro sino de su número Glicocalix responsable de la selectividad del tamaño Difusión: forma dominante de intercambio transcapilar de gases, sustratos y productos de deshecho Se analiza la difusión de gases porque la difusion de solutos pequeños y solubles en agua depdne de la permeabilidad y de la gradiente de difusión. Arrastre por solvente: Acarrea solutos. Transferencia neta de líquidos Proteínas plasmáticas pueden atravesar la membrana por fenestraciones o por transporte vesicular Ley de Fick: la velocidad de difusión es proporcional a la superficie a través de la cual se produce la difusión y al gradiente de concentración Gases ruta transcel con la misma facilidad que difunden a través de los tejidos vecinos Fick’s First Law of diffusion: Where dn/dt is flux in moles/sec, D is diffusion constant, A is surface area, DC is concentration difference, and DX is thickness of barrier to diffusion. The movement (or flux) of a molecule is directly related to its diffusion constant across the barrier, the surface area available for diffusion, and the concentration gradient across the barrier. Aunque O2 y CO2 libre permeabilidad ofrece barrera de intercambo conotras sustancias insolubles en lípoidos los slutos hidorfilicos más pequeños qeu la albúmina pudeen atravesar por difusión por la vía paracelular: poros o uniones interendoteliales y fenestraciones, si están presentes La cantidad de soluto que atreviesa por unida de tiempo es el flux o flujo. (Jx: moles/(cm2/s) Jx= Px x (conc c-conc liq intesticial Px (es el coeficiente de permeabildiad) es una combinación del grosor de la pared cm, (dificil de medir) con el coficiente de difusión cm2/seg Indica la facilidad con que una sustancia puede atravesar la membrana cap. Como es dificl medir el área superrficial de los caps no se calcula el flujo de un soluto que se experesa por unidad de área, sion que es más comun calcular el floujo en masa Q. que es la cantidad de soluto transferido por unidad de tiempo moles /seg. Como es dificil estimar los coeficientes de permeabilidad para u soluto en diferentes capilares o diferntes solutos en un capilar, se usa una medida indirecta que es la fraccón de extracción de todo el órgano. dif arteriovenosa del soluto normalizad. O sea describe el grado en queún órganos remueve un soluto de la circ. Pero en el caso de solutos hidorfílicos además del F depdne de las prpiedades de intercambio de los caps o sea de la permedabilidad y el área cap. In the case of O2 diffusing from the exchange vessels into the tissue, increasing the partial pressure of oxygen (pO2) in the plasma, or increasing the surface area for exchange (i.e., increasing the number of open capillaries), increases the total amount of O2 per unit time moving out of the blood and into the tissue. Movimiento convectivo de agua (arrastre por solvente): Macromoléculas con radio de más de 1 nm como prot. plasmaticas pueden atravesar la membra cap a una tasa baja por hendiduras y fenestraciones cuando están presentes. Otro mec es por transcitosis. Pero en el caso de prot peq too inflluye la carga. Las prot cargadas neg el flujo es menor que las neutras. Y las cargadas positivamente tienen Px más alto. Permeab selectiva basada en la carga eléctrica Transcitosis (ganong) Vesículas en el citoplasma de cel edoteliales prot transportadas por endocitosis fuera de los caps. A través de las cel endoteliales en el lado cap. seguidoa de exocitosis en el lado intersticial. Vesiculaes transporatadoras 50 a 100 nm . Berne: Debe haver equilibrio entre las lmacromoelc disulestas en el lumen cap y la fase liq denro de la vesicula, rotura de la vesicula, fusion con otras vesiculare en el citoplasma mezcla de coantenidos vesiculares, fusión devesiculas con la membrana plasmatica y equilibrio con el liqextracel opuesto. Aunque el mov transciitótico no es en si un flujo no cumple con las leyes de la difusión si se han calculado la permeabilidad aparente de caps típicos a macromoléculas. La permeab total refleja el mov total de las macromolec sin importar la vía de paso, esta se reduce mucho con el radio molecular (tamizaje). Pueden formarse cadenas de vesículas fusionadas a lo ancho de la cel endot. Algunas de las macromolec transportadas pueden ser procesadas por la cel. No es un ferry que carga y descarga. En los caps cerebrales no ocurre transporte por cadenas de vesículas fusionadas y transp too se da por transictosis Filtraciòn y absorciòn: 0.06 ml de agua /min/100 g tejido por la pared capilar Difusiòn: 300 ml de agua /min/100 g tejido 5000x más Intercambio transcapilar de solutos tambièn está regulado por la difusiòn. LA DIFUSION ES EL FACTOR CLAVE EN EL INTERCAMBIO DE GASES, SUSTRATOS Y P`R0ODUCTOS DE DESHECHO ENTRE CAPS Y CEEL. La transferencia neta de lìqu a trecvés del endotelio puded atribuirse sobre todo a la filtraciòn y absorciòn Transporte limitado por el Flujo: molec pequeñas la ùnica limitac que tienen para atravesar es la velocidad de tranpsorte por el F sang. Limitado por difusiòn: tamaño: molec grandes La perameabilidad puede modificarse se reduce por estim de receptores b adrenérgicos asociados a AMPc que aumental la unión entre espacios de la hendidura. Y la permeab es auemntada por el aumento de flujo y PNA a traves GMPc. En inlfamación se forman grandes hendiduras entre las cel endot venulares . Transferencia rápida de port plasmática y agua lleva a transudación. Acute vascular hyperpermeability (AVH) A rapid increase in vascular permeability occurs when the microvasculature is exposed acutely to any of a number of vascular permeabilizing factors, e.g., VEGF-A, histamine, serotonin, PAF, etc. Some of these agents (e.g., histamine, serotonin, VEGF-A) are normally stored in tissue mast cells [37–39] and so may be released by agents that cause mast cell degranulation, e.g., allergy, insect bites, etc. Single exposure to any of these permeability factors results in a rapid but self-limited (complete by 20–30 min) influx of plasma into the tissues. Not only is the quantity of extravasated fluid greatly increased above that found in BVP but its composition is greatly changed. As already noted, the fluid passing from the circulation into normal tissues under basal conditions is a plasma filtrate, i.e., a fluid consisting largely of water and small solutes but containing very little plasma protein. However, the fluid that extravasates in AVH is rich in plasma proteins, approaching the levels found in plasma, and is referred to as an exudate. Among the plasma proteins that extravasate are fibrinogen and various members of the blood clotting cascadMore recently, a structure was discovered in venular endothelium, the vesiculo-vacuolar organelle (VVO), that offers an alternative, trans-endothelial cell route for plasma extravasation in response to permeability factors [44–48]. VVOs are grape-like clusters comprised of hundreds of uncoated, cytoplasmic vesicles and vacuoles that together form an organelle that traverses venular endothelial cytoplasm from lumen to albumen (Figs. 3(a, b), 4a)

7 Flujo de solutos: varía entre tejidos y condiciones fisiológicas
Número de capilares abiertos (perfundidos) Estado contráctil de las arteriolas terminales y esfínteres precapilares Densidad capilar (capilares/mm2) Área superficial para intercambio Distancia intercapilar: tiempo de difusión Alta actividad metabólica Tiempo de tránsito Permeabildlid se define como la tasa de difusión de soluto (Jx) a través de una unidad de área de la membrana por unidad de dif de concentración a través de la membranas Ecuación de permeabilidad: P= Jx/ S grad C unidades: cm/s S: area superficial del capilar Tiempo disponible por cada unidad de sangre para descargar O2, glucosa etc. Y recargar CO2 urea etc. En caps bien perfundidos el tiempo tr´nasico es de s velocidad de flujo de 300 a 1000 um/s En ejerccio puede caer a 0.25 s Macromoléculas con radio de más de 1 nm como prot plasmaticas pueden atravesar la membra cap a una tasa baja por hendiduras y fenestraciones cuando están presentes. Otra posibilidada en transcitosis. Vesiculaes transporatadoras 50 a 100 nm . No de caps que se abren reclutamietno caps} No solo aumenta el numero de cpas perfundidos sino que aumenta el area sup de intercambio y reduce la distancia intercapilar

8 Flujo de solutos a través de la pared de un capilar
Permeabilidad endotelial Ultraestructura del capilar (geometría de los poros o hendiduras) Propiedades del soluto (tamaño, liposolubilidad) Gradiente de concentración Superficie de intercambio Principio de Fick La ecuación de difusión de soluto es una modificación de la ley de Fick de difusión La definición de permeabilidad incorpora el coef de difusión el área y el paso Permeabilidad endotelial Tamaño del poro en relación con el tamaño del soluto Cantidad de poros/unidad de área Longitud de las hendiduras El área de poro por unidad de área endotelial es la extensión de las hendiduras Coef reflexión para prot en caps continuos 0.8 a .95 Levick Difusión depende de la Ley de Fick: Jx = -DAx grad C/grad x D coef de difusión: facilidad con que el soluto se desliza por el solvente. Relac inversamente con el tamaño de la molec por la fricción. Glucosa es peq. Albumina es grande Grad x distancia El signo neg indica queel transporte es hacia el grad de conc El coeficiente de reflexión

9 Tasa de extracción de O2 : fracción del soluto removido del plasma durante su tránsito por el lecho capilar [O2] a - [O2] v VO2 = F EO2 = [O2] a - [O2] v [O2] a [20 ml/dl] - [15 ml/dl] [20 ml/dl] 5 = 0.25 20 VO2 = [O2] a - [O2] v F Levick Extracción: fracción del soluto removido del plasma durante su tránsito por el lecho capilar Regulac fisiol de la transf de solutos, reclutamiento Grad de conc más pronunciado, cuando aumenta tasa metab se aumenta la grad porque se reduce C intracel. Flujo sanguineo: aumemento de NO Otra forma de establecer lo que los tejidos remueven es con la tasa de extración esta es una dif AVO2 normalizada al conatenido arterial de la sustancia En este ejemplo el músculo remueve y quema el 25% del O2 que le es presentado por la sangre arterial Factores determinantes son los mismos del modelo de Krogh Un amyor F suple más O2 por lo que el tejido extrae una fracción más peq cuando la demanda es la misma. Otra forma de ver el principio de Fick Músculo esquelético solamente el 20% de los caps están perfundidos en reposo, pero durante el ejercicio la resistencia de los vasos y los esfínteres precap se dilatan lo que se equipara con la demanda Algo semejante a reducir el radio del cilindro tisular del modelo de Krogh La veloc del F too aumenta durante el ejercicio lo que hace que la PO2 se reduzca menos a lo largo del lumen del capilar pero en el caso del ejercicio too aumento VO2 entonces PO2 si cae bruscamente. Factores determinantes: Flujo capilar Demanda metabólica (VO2) 9

10 Intercambio de líquidos y solutos: flujo masivo de agua (fuerzas de Starling)
Direcci´pon y la intensidad deldesplazamiento del agua a través de la pared es la suma algegraica de la P hidorstática sy osmóticas a Gradiente de P hidrostática transcapilar Gradiente de P osmótica efectiva (P coloidosmótica)

11 Gradiente de P hidrostática transcapilar
P c πc Pi πi Gradiente de P hidrostática transcapilar P intravascular o P hidrostática capilar (Pc) P extravascular o P hidrostática tisular (Pi) Gradiente de P osmótica efectiva P coloidosmótica capilar: proteínas del plasma (πc) P coloidosmótica tisular: proteínas intersticiales y proteoglicanos (πif) Proporcional a la diferencia en la [proteínas] P hidrostática : F principal filtrac Pi se aproxima a O

12 Jx = Lp [(Pc - Pif) – σ( πc- πif)]
=+35 π c = +25 = +15 if = 0.1 = 3 Pif = -2 Reabsorción Filtración Pif = -2 PNF = (Pc - Pif) - (πc - πif) PNF= ( ) – ( ) PNF= +12 (a favor de la filtración) PNF = (Pc - Pif) - (πc - πif) PNF= ( ) – ( ) PNF= -5 (a favor de la reabsorción) Berne: extremos venosos más permeales que extr. Arterial mayor número de poros en las vénulas entonces elevada permeablidad. Qf unidades de flujo por unidad de gradiente de P a través del capilar por unidad de superficioe cap. Pero como la viscosidad y el espesor de la pared son constantes (k) entonces se puede expresar como: Qf= k delta P=ml/minx 100 g tejidox mmHg Quemaduras o toxinas puede modificar la permeab y porlo tanto el coef de filtrac. Pero no otras var fisiolo como Rpre o post o pH o PO2, como en condic fisiol permeab es cte entonces el coef de filtración puede usars para det el número de caps abiertos o sea la sup capilar disponible para la filtrac. (ej. Reclut capilar)’ El coef de filtración para todo el org. Es de 0.006ml/minx100 g de tejidoxmmHg BoronF : se multiplica por una constante de conductividad hidráulica (coeficiente de permeabildiad) es la proporcionalidad que ralciona la fuerza neta al flujo y expresa la permeabilidad total provista por las acuaporinas y vía paracelular. La Lp depende de el area superficial y la conductancia. Lp es la conductancia hidráulica, The filtration coefficient is the constant of proportionality. A high value indicates a highly water permeable capillary. A low value indicates a low capillary permeability. The filtration coefficient is the product of these two components: Kf = Area x Hydraulic conductivity capillary surface area capillary hydraulic conductance A ‘leaky’ capillary (eg due to histamine) would have a high filtration coefficient. The glomerular capillaries are naturally very leaky as this is necessary for their function; they have a high filtration coefficient. Abstract. Capillary filtration coefficient is a critical determinant of fluid flux across the microvascular wall. Changes in capillary filtration coefficient have been described in a number of disease processes. Measurement is typically made by venous occlusion plethysmography using either the upper or lower limb, but a variety of measurement protocols have been used and the importance of the site of measurement remains unclear. In this study, forearm and calf capillary filtration coefficient were measured in healthy volunteers, either simultaneously (group A; n = 11) or sequentially in random order (group B; n = 11) using venous occlusion plethysmography, with the subject supine and the limb at heart level. In both studies capillary filtration coefficient was significantly higher when measured at the forearm than at the calf (group A: versus , p < 0.01; group B: versus , p < 0.01). Isovolumetric venous pressure (the maximum pressure at which there is neither net filtration nor absorption at the microvascular wall) was similar in upper and lower limbs in both groups of subjects. We conclude that limb capillary filtration coefficient is dependent on the site of measurement. Caution is required when comparing data recorded at different sites even if corrected for the volume of soft tissue under study. Grupo A Forearm 6.1 calf 3.7 x 10-3 ml/min x mmHg x 100 ml An isogravimetric preparation was used and in paired experiments the value for Kf obtained in glands perfused with albumin-Krebs solution, ml min-1 mmHg g-1, was not significantly different from that in blood-perfused glands, / Capillary Filtration Coefficients in Forearm Segment in Environmental Temperature of 22° Mean = (ml/100 ml per mln per mm Hg) f de reflexión describe la membr semipermeab imperfecta .8 a .95 o sea 80 a 95 % de la P osmtótia coloida es ejercidea por prot. ley van t Hoff indica que la dif P osmótica téorica es proporcional a la dif de conc de prot. Pero como el cap excluye de forma imperfecta las prot la dif P coloidosmótica observada es menor que la ideal la razón P coloid obs/teórica es el coeficiente de reflexión que expresa como una barrera excluye o refleja solutos conforme el agua se mueve a través de la barrera movida por gradientes de P hidrostática o osmóitca The oncotic pressure difference (pC - pT) should be multiplied by the reflection coefficient that represents the permeability of the capillary barrier to the proteins responsible for generating the oncotic pressure. Because both hydrostatic and oncotic forces are normally expressed in units of mmHg. The net driving force (NDF) for fluid movement is the net pressure gradient determined by the sum of the individual hydrostatic and oncotic pressures.   Sigma varía entre 0 y 1 Si = 0 el agua mueve el soluto perfectamente y no ejerce P osm y si es = 1 la barrera excluye totalmente el soluto cuando el agua se mueve y ejerce su fuerza osmótica ideal. Las prot plasmáticas tienen un coef de reflexión muy cercano a 1. Na y Cl cruzan facilmente el endotelio entonces sigma = 0 y no se incluyen en la ecuación de Starling Las membranas plasmáticas cel tienen sigma = 1 el Na y Cl si ejercen fuerza osmótica su conc si cambia el agua enre los comportamientos intracel e intersticial. Si sigma =1 el liq que deja el cap es libre de prot. (ultrafiltración) Normalmente en extremo art filtración y extremo venoso hay reabs pero hay excepciones a esta regla general For a given NDF, the amount of fluid filtered or reabsorbed per unit time (fluid flux, or JV) will be determined by the permeability of the capillary barrier and by the surface area available for exchange. The permeability is usually referred to as the filtration constant (KF), and is determined by the physical properties of the barrier (i.e., size and number of "pores" and the thickness of the barrier). For example, fenestrated capillaries have a higher KF than continuous capillaries. Furthermore, substances such as histamine, which are released in response to tissue injury or inflammation, increase KF. The surface area (A) is related to the length, diameter, and number of capillaries available for exchange. The surface area is dynamic in vascular beds such as skeletal muscle where the number of perfused capillaries increase several-fold during exercise. To summarize:   JV = KF A [(PC – PT) – s(pC - pT)] The expression in brackets represents the NDF. If this is positive, filtration occurs, and if negative, reabsorption occurs. For a given NDF, the JV is determined by the product of KF and A. The Starling equation reads as follows: where: ([Pc − Pi] − σ[πc − πi]) is the net driving force, Kf is the proportionality constant, and Jv is the net fluid movement between compartments. By convention, outward force is defined as positive, and inward force is defined as negative. The solution to the equation is known as the net filtration or net fluid movement (Jv). If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when pathologic processes grossly alter one or more of the variables. [edit] The variables According to Starling's equation, the movement of fluid depends on six variables: Filtration coefficient ( Kf ) Reflection coefficient ( σ ) Pressures are often measured in millimeters of mercury (mmHg), and the filtration coefficient in milliliters per minute per millimeter of mercury (ml·min-1·mmHg-1). In essence the equation says that the net filtration (Jv) is proportional to the net driving force. The first four variables in the list above are the forces that contribute to the net driving force. Reflection coefficient The reflection coefficient is often thought of as a correction factor. The idea is that the difference in oncotic pressures contributes to the net driving force because most capillaries in the body are fairly impermeable to the large molecular weight proteins. (The term ultrafiltration is usually used to refer to this situation where the large molecules are retained by a semipermeable membrane but water and low molecular weight solutes can pass through the membrane). Many body capillaries do have a small permeability to proteins (such as albumins). This small protein leakage has two important effects: the interstitial fluid oncotic pressure is higher than it would otherwise be in that tissue not all of the protein present is effective in retaining water so the effective capillary oncotic pressure is lower than the measured capillary oncotic pressure. Both these effects decrease the contribution of the oncotic pressure gradient to the net driving force. The reflection coefficient (σ) is used to correct the magnitude of the measured gradient to 'correct for' for the ineffectiveness of some of the oncotic pressure gradient. It can have a value from 0 up to 1. Glomerular capillaries have a reflection coefficient close to 1 as normally no protein crosses into the glomerular filtrate. In contrast, hepatic sinusoids have a low reflection coefficient as they are quite permeable to protein. This is advantageous because albumin is produced in hepatocytes and can relatively freely pass from these cells into the blood in the sinusoids. The predominant pathway for albumin and other proteins to enter the circulation is via the lymph. En el riñon la PHc es suficiente para que se produzca filtracion a lo largo de todo el cap en los intestinos hay absorciòn a todo lo largo del cap Tradicionalmene se consideraba que filtr del lado art y absorción del lado venoso, en cap idealizado pero la vasoc de art reduce Pc de modo que se prod abs transitoria. Si persiste vasoc predomina la abs a lo largo del tiempo porque va aumentando Ponc int se va equilibrando Quemaduras o toxinas puede modificar la permeab y porlo tanto el coef de filtrac. Pero no otras var fisiolo como Rpre o post o pH o PO2, como en condic fisiol permeab es cte entonces el coef de filtración puede usars para det el número de caps abiertos o sea la sup capilar disponible para la filtrac. (ej. Reclut capilar)’ Lp: constante de filtración, σ: coeficiente de reflexión 12

13 Presión capilar Pc media +25 Localización de los capilares Tiempo
Extremo arterial Extremo venoso Pc=+35 Pa 60 Pc media +25 Pc= +15 Pv 15 Presión capilar Localización de los capilares Pc riñones: +50 mm Hg Pc pulmones: +5 a +15 mm Hg Tiempo Cambios en radio arteriolar y el tono de los esfínteres precapilares (solo filtración o absorción) Gravedad Por debajo corazón Pc >que por encima del corazón Berne: Pc principal factor estado contráctil de los vasos precap mayor efecto de los cambios de P venosa que de P arterial Pc aumenta más cuando aumenta Pv En condiciones normales la tensiòn arterial Pv R poscap rila re Phi y Poi y la PO plasma permanecen relativamente constantes por lo que los cambios en resistencia precap son lo que det el moovimiento de liq}Sólo un 2% del plasma se filtra y el 85% de lo que se filtró se reabsorrbe, el resto vulve con linfa en conj con prot Pulmones PHc 8 mmhg favorece absorciòn Cuando P arteriolar es de 60 y P venosa de 15 la Pc media es de 25 debido a que Rpos/Rpre 0.3 Dado que Rpre es mayor que Rpos entonces Pc sigue a Pv si aumenta Pa incremento leve de Pc pero si aumenta Pv incremento mayor de Pc PC riñones favorece la filtración y Pc pulmones reduce la filtración y previene el edema. Klabunde Capillary Hydrostatic Pressure (PC ) This pressure drives fluid out of the capillary (i.e., filtration), . Depending upon the organ, the pressure may drop along the length of the capillary (axial pressure gradient) by mmHg. The axial gradient favors filtration at the arteriolar end (where PC is greatest) and reabsorption at the venular end of the capillary (where PC is the lowest). The average capillary hydrostatic pressure is determined by arterial and venous pressures (PA and PV), and by the ratio of post-to-precapillary resistances (RV/RA). An increase in either arterial or venous pressure will increase capillary pressure; however, a given change in PA is only about one-fifth as effective in changing PC as the same absolute change in PV. Because venous resistance is relatively low, changes in PV are readily transmitted back to the capillary, and conversely, because arterial resistance is relatively high, changes in PA are poorly transmitted downstream to the capillary. Therefore, PC is much more influenced by changes in PV than by changes in PA. Furthermore, PC is increased by precapillary vasodilation (particularly by arteriolar dilation); precapillary vasoconstriction decreases PC.  Venous constriction increases PC, whereas venous dilation decreases PC. Solo se filtra el 2% del plasma y de este el 85% se absorbe en los caps y vènulas. El resto vuelve al sist vascular con la linfa junto con la albùmina

14 P capilar Berne: Aumento de Pa o Pv aumenta Pc
El 80% del aumento de P venosa se transmite a los caps. P cap depende principalmente del estado cont´ractil de los vasos precap Boron Se dijo que delta P depende de la R entre dos puntos (corriente arriba y corriente abajo) La P intravascular local o absoluta depende de la distribuciòn de la resistencia vascular Determinantes de la P absoluta en un solo punto 1. P corriente arriba 2. P corriente abajo 3. Distribución de la R entre los puntos. Ej con la microvasculatura Pc = (Rpos/Rpre) x Pa + Pv P arteriolar P venular 1 + R pos/Rpre Pc no es un promedio artimetico de la Pa y Pv porque R afecta. Esto ocurre solamente si R es igual, pero no es igual. O sea cuando Rpos/ Rpre= 1 Pero como la Pc no es 37.5 media sino 25 esto significa que 1. Rpre es mayor que R pos Rpos/Rpre menor 1 normal: 0.2 a 0.4 2. Si la suma de Rpos y Rpre es cte, la Pc no cambia, los cambios recíprocos no producen cambios totales de R del circuito y por lo tanto deja Pa y Pv cte. Si la relac Rpos/ R pre aumenta, aumenta too Pc Rppos/Rpre mayor 1 aumenta Pc aumenta y se acerca a Pa Rpos/Rpre menor 1 disminuye Pc disminuye y se acerca a Pv Si Rpre = 0 pero aumenta Rpos no se produce una disminuciòn de P a lo largo del cap. Pc=Pa Si Rpos=0 pero aumenta Pre Pc = Pv Dependiendo Rpos/Rpre la Pc puede ser mas sensible a cambios de Pa o de Pv Si relac es baja por ej Rpre mayor que Rpos: Pc tiende a seguir P corriente abajo Pv. Como en condiciones normales la relaciòn es baja entonces Pc tiende a seguir Pv, si Pv aumenta entonces Pc tambièn aumenta Explica : edema tobillos cuando persona està mucho de pie, se eleva P en las grandes venas y se traslada a un aumento de Pc que lleva a la transudación de liq. De los caps a liq intesrt. Si se elevan los pies se reduce P en las venas y se revierte el edema. La R vascular varia con el tiempo y depende en forma crìtica de la acciòn del musc liso vascular que modifica el radio R depene de r lo cual depende de musc liso en los vasos. El lugar de mayor control de R vasc en la circ sistémica es en la arterias pequeñas terminales o arterias feed y en las arteriolas. Rpre está dada por las arteriolas pequeñas precap de resist Klabunde

15 Presión hidrostática tisular:
Pi= -8 Pi= +6 Fase sólida y líquida H2O libre es escasa en el intersticio Fase sólida: colágeno y proteoglicanos P en los tejidos encapsulados o no encapsulados Riñones: +1 a +3 mmHg, cerebro: +6 mmHg Piel:- 2 mmHg Pulmones: -8 mmHg Phtisular: intersticio consiste en una fase sólida (geles agua se mueve por difusión)y líquida agua se mueve de acuerdo con fuerzas convectivas. La fase solida es fibras de colágeno y proteoglicanos. La fase líquida consiste en una pequeña fracción que es agua totalmente libre y capaz de moverse de acuerdo con las fuerzas convectivas. La mayoría del agua está atrapada en geles (proteoglicanos) en el cual el agua y los solutos pequeños se mueven por difusión. Pif es subatmosférica (neg) Proteoglycans are a major component of the animal extracellular matrix, the "filler" substance existing between cells in an organism. Here they form large complexes, both to other proteoglycans, to hyaluronan and to fibrous matrix proteins (such as collagen). They are also involved in binding cations (such as sodium, potassium and calcium) and water, and also regulating the movement of molecules through the matrix. Evidence also shows they can affect the activity and stability of proteins and signalling molecules within the matrix. Individual functions of proteoglycans can be attributed to either the protein core or the attached GAG chain. En tejidos sueltos o flojos como el pulmón o la piel Pif ) -1 a -2 por la remoción de agua por los vasos linfáticos Adv physiol 07 The negative interstitial pressures in subcutaneous tissue and other tissues are usually attributed to the action of lymphatics. Unfortunately, the existence of this has not yet been proved, but mechanisms by which the terminal lymphatic system could act as a suction pump have been proposed.” Muscle contractions compress and evacuate lymph vessels. Pero en compartimientos rígidos es positiva En órganos encapsulados como el riñon es +1 a + 3 Pif positiva: cerebro (cráneo), hígado (capsula), riñores (capsula), músculo por fascias Pif negativa: pulmones espacio no restringido debido a que a nivel tisular existe libertada para el flujo de aire), tejido subcutáneo y submucoso( no es compresible por aire externo) Negative values are normally found only in lungs and subcutaneous tissue. These are the only two body parts freely compressed by outside air pressure. In both tissues under normal perfusion, outflow capacities (venous blood and lymph) are greater than needed for the inflow volume and capillary permeability. Large outflow capacities drain most of the interstitial water and reduce its hydrostatic pressure to 0. The draining process is augmented by the physical work of neighboring muscles in subcutaneous tissue or by breathing La expansión de vasos de alta P en el riñon empuja el líquido intesticial conatra una capsula fibrosa que no cede , aumentando Pif. En le musc esq que es rodeado por capas de fascia too paso esto. La P sobre los caps puede ser ejercida por otro compartimiento, por ej la capsula de Bowman en el riñon (10 mm Hg) o los alvelos en los pulmones Intersticio es un sistema de baja complianza: aumenta mucho Pif con peq cant liqu agregadopero si se rompe la fase sólida de fibras colágenos y proteoglicanos y aumenta la distensibilidad y en órganos no encapsulados se puede acumular mucho líquido, edema. Edema: del griego inflamación. Exceso de sal y agua en el espacio estracelular, intestcio. Asociado con enf asoc a retención de sal y expanción del vol EC, por ej enf renales cardiacas o hepaticas. Peor el edema too pouede ocurrir sin retención de sal or altgeraciones microcircu. Que afectan las fuerzas de Starling. Fuerzas hidrostáticas: Persona de pie por unt tiempo prolongado Pv aumenta y Pc aumenta por la gravedad, el resultado es el mov de liq en el espacio tisular, en la mayoría de los casos el sistema linfático puede extraer el el liq extra y regresarlo al espacio vascular lo que mantiene el eq de líq. El retorno de liq rquiere la contracción de los musc de las piernas que comprimen las venas y los linfáticos y rpopulsan el liq hacia arriva a través de las valv, si la persona de pie no contrae estos musc. Ocure tansduación de liqu que puee esceder el retorno linfatico y causar edema. Un org muy sensible al eq deliq es el pulmon. Solamente con peq cambios de Pc por ej hipertensión pulmonar puede causar edema pulmonar, se reduce la complianza pulmonar loque la inflación del pulmonon más dificil y mpuede comprometer el intercambio de gases. La insfu carddiaca izq. Cuasa quela sngre se reresa hacia los vasos del pulmnón y auemnta la P pulmnoar vascular y causa edema pulmonar. En el corzón derecho, insuf prduce el flujo retrógrado hacia las venas sitémicas lo que aumenta la P venosa central (P en la grandes venas sitémicasO loque cuasa un aumento de Pc en las extrem inf y en las visceras abdomnales.. El liq transudado de capilares hepáticos e intestinales puede dejarn el espacio intersticial y entran en la canvidad peritoneal lo que produce una condición conocida como ascitis. Klabunde Tissue (Interstitial) Hydrostatic Pressure (PT) This pressure is determined by the interstitial fluid volume and by the compliance of the tissue, which is related to the ability of the tissue volume to increase.  Normally, PT is near zero. In some tissues it is slightly subatmospheric, whereas in others it is slightly positive. Tissue compliance is generally low; that is, small increases in tissue volume as occurs during states of enhanced filtration or lymphatic blockage result in dramatic increases in PT. The rise in PT that occurs with increased interstitial fluid volume decreases the hydrostatic gradient across the capillary thereby limiting filtration. Advances in physiol 07 Although the reported values differ, pressures probably range from - 8 mmHg in the lungs to -3 or -2 mmHg in subcutaneous tissue to 0, 1, and 2 mmHg in the liver and kidneys or 6 mmHg in the brain (4, 6). Interstitial pressure can reach very high positive values in tumors (from 20 to 40 mmHg) (2, 3). Reported findings in freshly burned tissue describe even more negative interstitial pressures that can reach -20 to 30

16 Presión oncótica capilar
Extremo arterial Extremo venoso πc=25 mm Hg: [prot. plasmáticas ]=7 g/dl Albúmina ejerce mayor fuerza osmótica σ varía entre órganos σ= Músculo< cerebro πc varía con la composición de las proteínas plasmáticas (peso molecular) Berne La albúmina es la prot plasmática más imp. Para det. P onc. Mide la mitad molec globulina, pero su conc es el doble, además ejerce mayor fuerza osmótica porque se une al Cl le aumenta la carga neg. Y la capac de retener más Na dentro de los caps. P osmótica total del plasma 6000 mm Hg Ponc cap 25 mmhg. Albúmina 4.5 g/dl Electrolitos conc idéntica a los dos lados del endotelio Berne: Coeficiente de reflección dificultad relativa para el paso de una sust. A traves de la membr cap Una peq parte de prot salen y ejercen una P 0.1 a 5 mmhg a bajas conc no puede potenciar fuerza osmótica Boron P de prot del plasma: álbumina, globulinas y fibrinógeno. Conc prot totales plasmáticas = 7 g/dl, quecorresonde a 1.5 mM de prot. Según la ley de vant Holf las porteíans ejercen una P osm de arox 28 mm Hg si son reflejandas perfectamente por la pard capilar (sigma= 1). La P osm total del plasma es de 6000 mmHg y de las prot sòlo de 25 sin embargo existen la misma concentraciòn de electrolitos (electrolitos principals responsables de la P osmòtica del plasma) en el espacio intravascular que en el lìquido intersticial por lo tanto Albùmina det presiòn contòtica fuerza osmòtica superior a la explicable uicamente por el nùmero de molèculas disueltas. En el plasma. Fuerza osmòtica complementaris se debe a: carga negativa de al albùmina atracciòn y retenciòn de cationes (Na), tambièn se une al CL lo que aumenta su carga negativa y su capac para reterner Na, el aumento de la cocentraciòn electrolìtica potencia la fuerza osmòtica σ= 1 si membr refleja perfectamente prot Posm ? 28 mm Hg pero es de 25 para las prot principales albúmina 3.5 a 5.5 g /dl y glubulinas 2 a 3.5 g/dl. El valor es le mismo si los solutos osmoticamnete ativos se presntean en con 1.3 mm. Solutos peq ocn osmolalidad de 290 mOsm no se toman en cuenta. Pi no varía a lo largo del cap. Enla malyoría de los caps se filra menos de 1% de porte lo que no concentra el plasma a lo largo del cap y no aumenta pi. Los lab clinicos reportan conc prot plasmatica en g /dl y no todas las port tienen el miso peso mole. Una conc de prot plasm de 7 g /dl prued porducir dif valores de pi c dependiendo de la composición de las port del plasma, la albumina tine un peso molec mucho menor que la gamma globulinas, entonces 1 g de gama glbulinas mas pesado que 1 g de albumina elevaría pi. Entondes la P colioidosmotica aumenta en forma más pronuciada aunque la relacion alb/glob se mantenga cte. Las prot plasmáticas too acarrean cargas negativas y el efecto de Donnan produce un aumento de la conc de cationes y de P coloidosmótica en el lumen capilar. Klabunde Capillary Plasma Oncotic Pressure (PC ) Because the capillary barrier is readily permeable to ions, the osmotic pressure within the capillary is principally determined by plasma proteins that are relatively impermeable. Therefore, instead of speaking of "osmotic" pressure, this pressure is referred to as the "oncotic" pressure or "colloid osmotic" pressure because it is generated by colloids. Albumin generates about 70% of the oncotic pressure. This pressure is typically mmHg. The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration (e.g., in renal glomerular capillaries), because the filtering fluid leaves behind proteins leading to an increase in protein concentration. Normally, when oncotic pressure is measured, it is measured across a semipermeable membrane – i.e., a membrane that is permeable to fluid and electrolytes but not to large protein molecules. In most capillaries, however, the wall (primarily endothelium) does have a finite permeability to proteins. The actual permeability to protein depends upon the type of capillary as well as the nature of the protein (size, shape, charge).  Because of this finite permeability, the actual oncotic pressure generated across the capillary membrane is less than that calculated from the protein concentration. The effects of finite protein permeability on the physiological oncotic pressure can be determined knowing the reflection coefficient (s ) of the capillary wall.  If the capillary is impermeable to protein then s = 1.  If the capillary is freely permeable to protein, then s = 0. Continuous capillaries have a high s (>0.9), whereas discontinuous capillaries which are very "leaky" to proteins have a very low s .  In the latter case, plasma and tissue oncotic pressures may have a negligible influence on the net driving force

17 Fuerzas de Starling Jx = Lp [(Pc - Pif) – σ( πc- πif)]
20 L/día a 18 L/día Fuerzas de Starling PNF muy variable: Mucosa intestino: Pc<πc Glomérulo: Pc> πc Filtración neta: 2 a 4 L/día Berne Este es el ejemplo de un capilar ideal, alguos solo filtran y otros solo absorven, Además si ocurre Vasoconstricción: disminuye Pc, disminuye filtración y predomina la absorción entonces se aumenta P onc tisular Solo 2% del plasma que fluye por los caps se filtra} 85% se reabsobe Ej pulmones: Pc 8 mm Hg, Ponc 25 mm Hg, Pi 15 mm Hg PNf favorece abs Desequilibrios: Hemorragia aumenta la abs se reduce P capilar De pie aumenta la P venosa y P cap y filtración pero la aumentar la P transmural se estim un mec miógeno, los vasos se cierran se reduce el coef de filtrac área disponible para el inte y no sale gran cant del liq hacia el espacio intersticial. Una persona de pie por 10 min prod filtrac de 342ml pero no se produce edema por sist linf Boron Intestino Pc menor que pi c entonces hay absorción a lo largo del cap. Se filtran 20 L/día del lado arteriolar y se reabsorben de 16 a 18 L/día del lado venoso Equilibrio de Starling: equilibro de fuerzas que regulan la distribución de líquidos en el espacio intravascular e intersticial Filtración neta sin contar laf filtración glomerular Filtración neta varía entre órganos y depende de la PNF, el coeficiente de permeabilidad de los capilares y el área superficial Jx = Lp [(Pc - Pif) – σ( πc- πif)] reabsorción ultrafiltración

18 Circulación linfática
Acoplada al exceso de filtración capilar (2 a 4 l/día) Linfa desemboca en las venas subclavias Ausentes en huesos, cartílago, miocardio y cerebro Abundante en la piel, TGI, pulmones. Berne. Comprende vasos linf, ganglios y tejido linfoide. Semejantes a los caps sanguineos pero no tienen uniones estrechasfilamentos finhos anclan a los vaso al teji conjuntivo, Estos filamentos finos tiran de los vasos linf durante la contr musc

19 Circulación linfática
Unico mecanismo que permite a las proteínas regresar a la sangre Filtración de proteínas: crea una gradiente a favor de su movimiento hacia los vasos linfáticos Linfáticos regresan de 100 a 200 g/día de proteínas El sistema linfàtico devuelve liq y solutos que escapan de los caps Linf devuelven de una cuarta parta a la mitad de lals prot circulantes por la sangre en un día Filtra el linfa en los ganglios linfáticos y elimina las partículas extrañas como bacterias Plinf: -1 a +1 mm Hg

20 Flujo linfático ↑Pi (> Plinf) Bombeo pasivo Bombeo activo
Movimiento líquido hacia vasos linfáticos iniciales Bombeo pasivo Compresión extravascular: contracciones musculares, intestinales y movimientos respiratorios→ ↑ Plinf Bombeo activo Linfáticos colectores Mecanismo miogénico intrínseco Linfaticos inciales antes terminales son semejantes a los caps uniones interendot se comportan como microválvulas de una vía, válvulas linfáticas primarias Bombeo activo Mecanismo miogénico intrínseco: contracción del músculo liso al aumentar P linf Terminales linfáticos: semejante a capilares pero con múltiples uniones interendoteliales, hendiduras y fenestraciones. Algunos pueden estár colapsados, la contracción musc los deforma Pared semejante a las venas con cel endotelialesm musc liso y válvulas restringe el flujo retrógrado Oclusión corriente abajo aumenta P linf y aumenta la contracción musc liso, pero una oclusión corriente arriba reduce el F linf P en vasos linfáticos grandes va de +1 a +10 aumenta después de cada válvula Flujo linf responsable de Pif negativa Filtración de macromoléculas no regresan a la circulación y crea una gradiente a favor su movimiento hacia los vasos linfáticos Contracciones intermitentes del musc esq., contracciones de los vasos linfàtico s y sist de vàlvulas unidireccionales. Los huesos, sistema nervioso central, los cartìlagos no tienen vasos linfàticos Sino se eliminan las prot del espacio intersticial se acumulan en el liq interts y actuarian como una fuerza oncòtica para extraer lip desde los caps Too filtra la linfa en el sist linfàtico y retira partìculas extrañas como bacterias. Si vol supera capac de drenaje o vasos se bloquean como en elefantiasis (filarosis infestaciòn de helmintos) el lie intersticial se acumula edema Terminal Lymphatics Composed of endothelium with intercellular gaps surrounded by highly permeable basement membrane and are similar in size to venules – terminal lymphatics end as blind sacs. Larger lymphatics also have smooth muscle cells. Spontaneous and stretch-activated vasomotion is present which serves to "pump" lymph. Sympathetic nerves can modulate vasomotion and cause contraction. One-way valves direct lymph away from the tissue and eventually back into the systemic circulation via the thoracic duct and subclavian veins (2-4 liters/day returned).

21 El flujo linfático depende de la presión intersticial
Zona de alta complianza: (intersticio expandido) Filtración ↑ poco Pif →no ↑ F linf No compensa la filtración excesiva de líquido Edema genera más edema Rango de alta complianza Rango de baja complianza Doble fase de distensibilidad intersticial: primero es baja y luego es alta. Adición de líquidos al compartimiento intersticial Poca cantidad: sistema de baja complianza Pif aumenta mucho con pequeñas cantidades de líquido acumulado Mucha cantidad: sistema de alta complianza fragmenta la fase sólida de fibras colágenas y proteoglicanos (se acumula mucho líquido con poco cambio de P, edema intersticial) Edema produce más edema Además del mov convectivo de agua, está el mov difusional que es mucho mayor L salen y los caps y lo mismo vuelve a entrar Solutos pequeños: glucosa disuelta en plasma 100 mg/dl x 2.75 L/min deplasma el corazón bombea 4000 g de glucosa esta entra en el intersticio por dos mec: Filtración: 20 L x 100 mg/dl = 20 g glucosa por día Difusión g de glucosa La mayor parte de la gluc que difunde hacia intersticio regresa a la sangre. Proteínas: 7g/dl x 2.75 l/min de plasama bombeo 277,000 g prot por circ cada día Convección: 100 a 200 g/día disuleta en agua filtrada arrastre por solvlente. Solo un peq cant regresa 5 g/día, casi toda la port filtrada depnde del flujo linfático para ser recuperada. Prot totales en el plasma para una persona de 70kg es de solamnte 200 a 250 g y el sist linfático recuepra de 195 g/día Rango de baja complianza Filtración ↑ mucho Pif →↑ F linf Poco líquido acumulado Pif >Plinf 21

22 Edema πif= 0.1 Aumento de la filtración Disminución de la reabsorción
Pc=+35 πc= +25 Pc= +15 πif= 0.1 Pif= -2 πif= 3 Aumento de la filtración Disminución de la reabsorción Fuerzas hidrostáticas: Persona de pie por unt tiempo prolongado Pv aumenta y Pc aumenta por la gravedad, el resultado es el mov de liq en el espacio tisular, en la mayoría de los casos el sistema linfático puede extraer el el liq extra y regresarlo al espacio vascular lo que mantiene el eq de líq. El retorno de liq rquiere la contracción de los musc de las piernas que comprimen las venas y los linfáticos y rpopulsan el liq hacia arriva a través de las valv, si la persona de pie no contrae estos musc. Ocure tansduación de liqu que puee esceder el retorno linfatico y causar edema. Un org muy sensible al eq deliq es el pulmon. Solamente con peq cambios de Pc por ej hipertensión pulmonar puede causar edema pulmonar, se reduce la complianza pulmonar lo que la inflación del pulmonon más dificil y mpuede comprometer el intercambio de gases. La insfu carddiaca izq. Cuasa quela sngre se reresa hacia los vasos del pulmnón y auemnta la P pulmnoar vascular y causa edema pulmonar. En el corzón derecho, insuf prduce el flujo retrógrado hacia las venas sitémicas lo que aumenta la P venosa central (P en la grandes venas sitémicasO loque cuasa un aumento de Pc en las extrem inf y en las visceras abdomnales.. El liq transudado de capilares hepáticos e intestinales puede dejarn el espacio intersticial y entran en la canvidad peritoneal lo que produce una condición conocida como ascitis. Presión coloidosmótica reducción de la P coloidosmótica por pérdida de prot. Por ej en orina con síndrome nefrótico. Se reduce la habilidad de los caps para retener liq. Edema generalizado. Durante el embarazo la síntesis de prot por la madre no es proporcional con la expansión del vol. Rducción relativa de los niveles de port. Too en malnutrición protéinca. Edema en las extremidades. Propiedades de la pared capilar: Inflamacipon causa la liberación de sustancias vasodilatadoras como histamina y citoquinas. Y aumenta el número de caps abiertos lo que aumenta el área superficial. Las citoquicnas too aumentan las hendiduras interendoteliales y producen una disminución del coef de reflexión. Inflamación se acompaña de edema. Edema cerebral por rompimiento de las uniones estrechas lesión en cabeza. Puede producir oclusión de la micorcirculación cerebral porque el cráneo no permite la expansión Durante la isquemia el flujo sang a un tejidos se reduce o se detiene totalmente se deterioran los vasos y causan un aumento de la conductividad hidráulica y el coef de reflexión disminiye. Cuando el flujo se restablece estos cambios producen edema local. Se es imortante se filgran proteínas al espacio intersticial lo que disipa la gradiente de P coloidosmótica y agrava el edema. Drenaje linfático: Remoción de nódulos linfáticos por eje por cirugia cancer o cuando los nódulos linf están obstruidos por ej por degeneración maligna de los nódulos linfáticos, Hopkin produce edema corriente arriba de los nódulos afectados Modificación de las propiedades de la pared capilar: aumento de la permeabilidad Reducción del drenaje linfático

23 Regulación de la microcirculación)
F = ΔP R F = Pa - Pv Rpre + Rcap + Rpos Resistencia total de la microcirculación Rpre + Rcap + Rpos R cap muy pequeña y Rpre > Rpos Mecanismo más importante para modulación F: cambios en el tono muscular de arteriolas Modulación de la [Ca++] o cambios en la actividad de la cinasa de la cadena ligera de miosina (MLCK) Agentes nerviosos o humorales que actúan a través de diversas proteínas de membrana (canales, receptores y transportadores) y vías de transducción de señales que modifican la actividad de la MLCK

24 Musc esq, tracto gastrointestinal y riñones reciben 2/3 partes
Musc esq, tracto gastrointestinal y riñones reciben 2/3 partes. M esq, pasa de un 20% del GC a un 85% en el ejerc. 24

25 Regulación de la resistencia vascular
F sanguineo = PAM R Control local Autorregulación Mecanismos miogénicos Metabolitos vasoactivos: Mecanismos metabólicos Mecanismos endoteliales Control extrínseco Nervioso Endocrino Berne: Tono basal indep del sist nervioso (miogénico, alta PO2 o Ca). Boron Contracciòn y relajaciòn del mùsculo liso vascular regula el flujo sang. Màs musc. Liso en las arteriolas Control local o intrìnseco:Autorregulaciòn y mec. Miógeno Condciones de los tejidos que rodean a los vasos Tono: contracción parcial del musc liso vascular Contracción y relajac musc liso vascular: control RPT, tono venoso y arterial. Si aumenta P intravasc se estiramiento pasivo reduce R y si disminuye P R aumenta por retracciòn del mùsc Pharmacological reports Gap junctions are formed in the cardiovascular system by connexin40 (Cx40), Cx37, Cx43, and Cx45. These low resistance channels allow the transfer of ions and small molecules between cells. The longitudinal coupling of endothelial and smooth muscle cells via gap junctions allows the spread of changes in membrane potential along the vascular wall and hence provides conduction pathways within the vessel itself. Functionally, this tight coupling is reflected by the spread of locally initiated vasomotor responses along the arteriole which are termed conducted responses. Conducted dilations are initiated by the application of endothelium-dependent stimuli which result in local hyperpolarization. This signal spreads along the wall, most likely along the endothelial cell layer, to elicit a coordinated dilation of the arteriole over a considerable distance. Likewise, the opposite signal (depolarization) spreads along the vessel giving rise to a conducted constriction. The latter response is however most likely transmitted along the smooth muscle cell layer. Thus, conducted responses reflect the synchronized behavior of the cells of the vascular wall. It is assumed that conducted responses are critical for the matching of oxygen delivery and tissue needs because they contribute to an ascending dilation which lowers resistance along the length of the arterioles and upstream vessels in a well-tuned Fashion Vasomoción: algunos vasos como los del musc esq. Contracción rítmica de arteriolas indep de estímulos y ocurren varias veces por min. Oscilación coordinada de Ca citosolico participan uniones de hendidura, Too las oscilaciones en vasos grandes pueden deberse a oscilación del Vm

26 Control de la actividad del músculos liso
La relajaciòn del musc liso requiere de la defosforilaciòn de la cadena ligera de miosina Grado de fosfolrilación de la cadena ligera de la miosina det grado de contracción depdende MLCK act o inhyibición MP Inac MP por RHO

27 Regulación del tono del músculo liso
Variación gradual de Vm (neutrotransmisores y hormonas) Regulación de la [Ca2+] en un rango muy amplio Complejo Ca-CaM Variación gradual de Vm (neutrotransmisores y hormonas) Liberación de Ca2+ intracelular inducida por segundos mensajeros (IP3) Sensibilidad al Ca2+ de las proteínas que regulan la contracción Inhibición de la MLCP permite una mayor contracción a una menor [Ca2+] Depende del equilibrio entre la fosfolrilaciòn y defosf de la MLC la tasa de fosf es reg por el complejo Ca CaM que a su vez depende de los niveles de Ca Variación gradual de Vm (neutrotransmisores y hormonas Ca. No requiere PA Liberación de Ca2+ intracelular inducida por segundos mensajeros (IP3 Algunos NT inhiben la fosfatasa MLC Otro de reg de la sensibilidad al Ca es modificando fosforilaciòn de MLCK por medio de kinasas que incluyen la PKA, PKC y kinasas dep de Ca CaM si se fosforila MLCK disminuye la sensibilidad de la MLCK a la activaciòn por el complejo Ca CaM Webb, R. C. Advan. Physiol. Edu. 27: ; doi: /advan Copyright ©2003 American Physiological Society

28 Regulación local: metabolitos tisulares
Sobrepasa el control SNA Regulación local: metabolitos tisulares

29 Metabolitos vasodilatadores
29

30 Mecanismo miogénico Copyright ©2008 American Physiological Society
Berne: en sujetos sanos la PAM se mantiene cte. Poca particip miogénico Berne dice que el mec se ha relacionado con que un aumento de P aumenta PLC, aumenta diaclglicerol, activación del canal TRP, despol y apetura de canales de Ca tipo L Canal iónico sensible al ácido (ASIC) y canal epitelial de sodio (ENaC) FIGURE 1. Proposed signaling pathway underlying conversion of pressure-induced vessel stretch into vasoconstriction Stretch-induced changes in vascular wall tension are believed to activate a mechano-dependent event that results in VSMC depolarization and subsequent activation of voltage-gated Ca2+ channels and vasoconstriction to reduce wall tension A new trick for an old dogma: ENaC proteins as mechanotransducers in vascular smooth muscle. Drummond HA, Grifoni SC, Jernigan NL. Physiology (Bethesda) Feb;23: Review. FIGURE 1. Proposed signaling pathway underlying conversion of pressure-induced vessel stretch into vasoconstriction Stretch-induced changes in vascular wall tension are believed to activate a mechano-dependent event that results in VSMC depolarization and subsequent activation of voltage-gated Ca2+ channels and vasoconstriction to reduce wall tension. However, the identity of the molecule(s) responsible for this event and how tension may regulate the molecule(s) is unknown. FIGURE 3. Proposed model of a mechanosensor in VSMCs This model is based on the mechanotransducer model established in the nematode. The mechanosensor may be a heteromultimeric complex. ENaC and/or ASIC proteins form the ion-transducing heart of the mechanotransducer. These proteins are anchored to the extracellular matrix and cytoskeleton by associated linking proteins and are yet to be identified. The application of a mechanical stimulus, such as strain, gates channel activity and allows influx of Na+/Ca2+. Copyright ©2008 American Physiological Society Drummond, H. A. et al. Physiology 23:

31 Autorregulación del flujo sanguíneo: F estable aunque cambie PAM
Elimina aumentos de F cuando el órgano está bien perfundido. Mantiene F y Pc cuando disminuye P perfusión (corazón, cerebro y riñones) Autorregulación: cambios en la P perfusiòn se compensan con cambios en la resistencia vascular. Si se eleva la P de perfusiòn : aumenta F sang y los vasos se contraen. Mec. Miògeno:El mecanismo responsable del F sang cte en preencia de modificaciones en la P perfusiòn El musc liso vascular se contrae debido a un estiramiento de las fibras y se relaja al disminuir el estiramietno. Dado que la PA se mantiene constante el mec miògeno quedarìa reducido al minimo Cuando una persona cambia de posiciòn decùbito a bipedestaciòn aumenta la P transparietal mayor estiramiento y contricciòn arteriolar, reduce la filtración, esto ocurre hasta que se equilibra la P oncotica e intersticial con la P hidrostàtica elevada. Normalmente se aumenta la P produce dilatación arteriolar que reduce la resistneica y produce un mayor F. Pero en algunos lechos vasculares los lechos vascualres se compartan diferente . Grandes cambios de PAM mantienen el F wn un rango muy estrecho. Mas bien un aumento de P produce puede produce aumentos de R que mantienen F en un rango muy pequeño El comportameinto autorregulatorio toma tiempo en desarrolarse y se debe a un porceso9 activo. Si la P prerfusi´pon aumenta abruptamente, se observa que el aumento de P se observa como se comporta un tubo rígido pero lentalmente el tono vascular se ajusta lentamente a si misomo para producir el diagrama P F la contracción del MLV que aoya la ayotoregulación es aytomonomo , o sea muy locan e indepeindiente de facotres neurales o endocrinos. Los mecanismos míogénicos y metab´loicas juegan un papel imp en los ajustes del musc liso vacular durante la autorreg. Por ej, el estiramiento MLV que acompaña un aumento de P Perf desncadena la contracción miogénica La autorreg es útil porque: Con una aumento de P perf la autorreg reduce el gasto de perf de orga en que el F es suficiente. Una dismincu9ión de perfusión la ayorrreg mantiene el F cap y P cap. Autorreg es muy impo en el corazón, cerebro y riñones que son muy sensitivos a la isquemia o hipoxia o para órganos (riñones ) en que su trabajo es fitlrar la sangre.

32 Autorregulación del flujo sanguíneo
↑P ↓ Flujo sanguíneo ↑Estiramiento: ↑ Flujo P ↓PCO2 vasoconstricción metabólica contracción miogénica Autorregulación: cambios en la P perfusiòn se compensan con cambios en la resistencia vascular. Si se eleva la P de perfusiòn : aumenta F sang y los vasos se contraen. Mec. Miògeno:El mecanismo responsable del F sang cte en preencia de modificaciones en la P perfusiòn El musc liso vascular se contrae debido a un estiramiento de las fibras y se relaja al disminuir el estiramietno. Dado que la PA se mantiene constante el mec miògeno quedarìa reducido al minimo Cuando una persona cambia de posiciòn decùbito a bipedestaciòn aumenta la P transparietal mayor estiramiento y contricciòn arteriolar, reduce la filtración, esto ocurre hasta que se equilibra la P oncotica e intersticial con la P hidrostàtica elevada. Normalmente se aumenta la P produce dilatación arteriolar que reduce la resistneica y produce un mayor F. Pero en algunos lechos vasculares los lechos vascualres se compartan diferente . Grandes cambios de PAM mantienen el F wn un rango muy estrecho. Mas bien un aumento de P produce puede produce aumentos de R que mantienen F en un rango muy pequeño El comportameinto autorregulatorio toma tiempo en desarrolarse y se debe a un porceso9 activo. Si la P prerfusi´pon aumenta abruptamente, se observa que el aumento de P se observa como se comporta un tubo rígido pero lentalmente el tono vascular se ajusta lentamente a si misomo para producir el diagrama P F la contracción del MLV que aoya la ayotoregulación es aytomonomo , o sea muy locan e indepeindiente de facotres neurales o endocrinos. Los mecanismos míogénicos y metab´loicas juegan un papel imp en los ajustes del musc liso vacular durante la autorreg. Por ej, el estiramiento MLV que acompaña un aumento de P Perf desncadena la contracción miogénica La autorreg es útil porque: Con una aumento de P perf la autorreg reduce el gasto de perf de orga en que el F es suficiente. Una dismincu9ión de perfusión la ayorrreg mantiene el F cap y P cap. Autorreg es muy impo en el corazón, cerebro y riñones que son muy sensitivos a la isquemia o hipoxia o para órganos (riñones ) en que su trabajo es fitlrar la sangre. F recupera su nivel 20 a 60 s ↑P perfusión →contracción ↓P perfusión→dilatación

33 Papel vasoactivo del endotelio y de los tejidos
Fuente de sustancias que producen contracción o relajación del MLV Berne: Liberación de NO. Estim x Ach, ATP, bradicinina, serotonina, sutP, hist. Fuerzas de cizalla X act guanilato ciclasa aumento GMP c reduce la sens x filamentos ac. araquidónico Aumento de la tensión de cizallamiento por aumento v F aumenta la síntensis de PGI2 a partir de ácido araquidónico produce relajación MLV PGI2 (estim F cizalla) too inhibe la agregación plaquetaria y adherencia al endot. Previene formación de coágulos PGI2 estim AMPc fosforila MLCK (inact) Tejidos parenquimatossos producen : adeosina, H, CO2, K NP fármaco nitroprusiato Endotlina

34 William E. Sonntag, Delrae M. Eckman, Jeremy Ingraham, and David R
William E. Sonntag, Delrae M. Eckman, Jeremy Ingraham, and David R. Riddle Regulation of Cerebrovascular Aging ch12 * * I. INTRODUCTION Normal function of virtually all tissues depends on adequate blood flow. As one would expect, deficits in blood flow under basal or stimulated conditions result in diminished metabolic capacity and impairments in function. Importantly, functional deficits in organs and tissues are one of the hallmarks of biological aging but the etiology of these deficits and the potential relationship between alterations in blood flow and the deterioration of tissue function with age remain enigmatic. Empirical and rigorous scientific evidence demonstrates that functional deterioration of many tissues begins in early adulthood and progresses throughout life. Concurrently, there is an increase in tissue pathology, including deposition of insoluble collagen and tissue fibrosis. Despite the structural changes with age, which are generally considered permanent or irreversible, tissue function can be improved even in late ages by several disparate types of interventions, supporting the conclusion that age-related impairments in cellular and tissue function, and perhaps some aspects of aging itself, remain “plastic.” Whether such “plastic” changes depend on increased basal blood flow or the capacity to increase blood flow in response to metabolic challenge remains unknown. The strong relationship that exists between cellular metabolic capacity and regional blood flow leads to the conclusion that a clear understanding of age-related changes in the regulation of blood flow (including microvascular architecture, plasticity, and vessel reactivity) is essential for understanding the progressive decline in cellular metabolic activity and eventually tissue function with age. Nevertheless, there are a limited number of studies that have considered the potential interrelationships between these variables. The guiding principle of this chapter, and for which there remain insufficient data, is that alterations in the vasculature have the capacity to impact biological aging. These alterations may include, but are not limited to, changes in microvascular density (density of arterioles, arteriolar to arteriolar anastomoses, capillaries, and venules); ultrastructure (cellular components that comprise the vasculature); plasticity (e.g., elaboration, regression, or replacement of microvessels that may occur over days, weeks, or months); and the dynamic regulation of blood flow through the vasculature (e.g., vessel reactivity). Despite the importance of each of these components, historical and technical aspects of research in these areas isolate the scientific disciplines and they are rarely considered a functional unit. This chapter reviews the current state of information in each of these areas as it relates to functional changes within the central nervous system with age. III. FUNCTIONAL CHANGES WITHIN THE CENTRAL NERVOUS SYSTEM Impairments in tissue function are common phenomena in the aging population; however, compared to other tissues, loss of function within the central nervous system (CNS) has the potential to have more profound social and psychological consequences and can be an important factor in loss of independence. Although marked variability exists between individuals, there are numerous reports demonstrating a decline in cognitive function with age unrelated to a specific disease process. In otherwise healthy individuals, perceptual-motor performance and information processing speed, visual and auditory attention, as well as fluid intelligence are generally compromised with age [2, 3]. In addition, impairments in synaptic efficacy, neurogenesis, glucose metabolism, neurotransmitter levels, and long-term potentiation (an electrophysiological correlate of memory) are evident. Also, risk for degenerative diseases (e.g., Alzheimer’s disease, Parkinson’s disease, among others) increases and recovery from stoke damage is impaired. Although a number of cellular and subcellular correlates of this decline have been identified to date, there is no single unifying hypothesis for the decline in function of the CNS and increased risk of disease with age. A. Cerebral Blood Flow Whether the decline in CNS function with age is the result of a decrease in cerebral blood flow (CBF) remains a topic of considerable debate in which no consensus has emerged [4]. In part, this lack of consensus is related to some of the issues discussed previously. However, equally important is the technical difficulty in associating functional defects that occur in highly specific brain regions with an accurate measurement of flow to these same regions. Generally, the vast majority of analyses provide a “snapshot” of alterations in CBF in a large region of brain obtained under static conditions. However, the regulation of CBF is not static, and localized brain regions have the ability to regulate blood flow in response to minute alterations in metabolic requirements of the surrounding tissues. As glial and neuronal metabolism increase, blood flow increases concomitantly to support the increased metabolic requirements. Thus, local cellular metabolism and blood flow are tightly coupled in the CNS. The extremely limited data assessing the dynamic processes of changes in cerebral blood flow represent an important missing link in our analyses of age-related changes in CBF. Despite the limitations described above, investigators using a variety of imaging methods have reported that CBF is significantly reduced in aged humans compared to young adults (see, for example, [5–7]). Similar conclusions were reached when comparing average measures of blood flow in groups of young and old subjects [8] or correlating CBF and age in individuals [9, 10]. Significantly, age-related changes in CBF appear to be regionally distinct [5, 6, 11–13] and may begin as early as middle age [10]. In addition to studies of humans, regionally specific, aging-related declines in CBF are found in both rodents and nonhuman primates [14–18]. Of course, several mechanisms regulate flow into and through each capillary bed, including the density of precapillary arterioles and capillaries [19], the structure of the vessels, and the reactivity of the arterioles (discussed in [7, 20, 21]). All or some of these mechanisms may be compromised with age. B. Microvascular Density FIGURE 12.1 (SEE COLOR INSERT FOLLOWING PAGE 204) Representative (more...) . (SEE COLOR INSERT FOLLOWING PAGE 204) Representative photographs of the cortical surface microvasculature in 13- and 29-month-old Brown-Norway rats, as seen through a cranial window. The entire parietal cranium has been removed. The cortex visible in this window extends from the frontal to the occipital cortex. Vascular measurements were made over the sensorimotor cortex. (Source: From [27]. With permission.) FIGURE 12.2 Summary of arteriolar (left), arteriole-to-arteriole (more...) Summary of arteriolar (left), arteriole-to-arteriole anastomotic (center), and venular endpoint (right) density in male Brown-Norway rats. Data represent mean ± SEM for 18 young, 14 middle-age, and 13 old animals. (Source: From [27]. With permission.) The absence of an appropriate vascular network (including arterioles, arteriolar-arteriolar anastomoses, pre-capillary arterioles, and capillaries) supplying a tissue has the potential to result in inadequate blood flow either under basal conditions or conditions that require an increase in blood flow to meet metabolic demand. There are, in fact, numerous reports of age-related rarefaction or loss of arterioles in many tissues throughout the body (including cardiac and skeletal muscle [22–24]); however, only a few investigators have studied the effects of aging on the density of cerebral arterioles. Knox and Oliveira [25] reported that the number of arterioles in a strip of cortex extending from the pia to the white matter was similar in rats at 3 and 24 months of age, and Bell and Ball [26] reported an increase in arteriolar density in the subiculum of the aging human hippocampus. More recently, a substantial age-related rarefaction of the surface arterioles that supply the parenchymal vessels of the cerebral cortex was reported [24, 27]. In otherwise healthy aging rats, the density of arterioles on the cortical surface was almost 40% lower in senescent animals than in young adults (29 vs. 13 months of age, Figure 12.1). Similar decreases were evident in arteriole-arteriole anastomoses and venules (Figure 12.2), suggesting that surface vessels and possibly vasculature in deeper layers of the cortex are affected by aging. Although it is difficult to reconcile the differences in these studies due to the specific ages, species, and brain regions compared, the substantial changes observed on the cortical surface and corresponding decreases in regional blood flow provide the first evidence that rarefaction of arterioles may be an important contributing factor to decreases in blood flow with resulting impairments in cortical function. TABLE 12.1 Capillary Changes in the CNS with Age Species Region(s) Change Ages Examined Ref. Report Aging-Related Decrease in Capillary Density Rat (Wistar; F) Fr, Occ ↓a 8 vs. 27 mos [30] Human (F, M) Precentral gyrus ↓ (17%) 74–94 years [35]a Human (F, M) Hipp ↓ (16%) Avg 38 vs. avg 74 years [26] Rat (Wag/RIj x BN, F) Fr ↑ followed by ↓ 3 vs. 19 vs. 29 mos [36] Rat (Crl: CD(SD)BR, M) Olfactory bulb ↓ (15%) 3–36 mos (27–36) [37] Rat (SD, M) MTB ↓ (30%) 3–33 mos (3/6–27/33) [38] Human (F, M) Calcarine cortex ↓ (16%) Avg 38 vs. avg 74 years [39] Human (F, M) Fr, Temp ↓ (Fr only) 26–96 years (correlation analysis) [34] Rat (Long Evans, M) Multiple ↓ (10–30%) 8–10 vs. 28–33 mos [40] Rat (F344, M) CA1, Par ↓ 18 vs. 28 mos [41, 42] Human (F, M) V1 ↓ (16%) Avg 31 vs. avg 79 years [43] Human (F, M) Hypothalamus ↓ (16%) 30–76 years (correlation analysis) [44] Rat (Wistar, M) Hipp, Fr, Occ ↓ 12 vs. 27 mos [31] Rat (Wistar, M) Hipp, Fr, Occ, Cb ↓ 12 vs. 24 mos [32] Report No Aging-Related Decrease in Capillary Density Rat (SD, ns) Occ ↑ (19%) 6 vs. 30 mos [45] Human (F, M) PCG ↑ (17%) 19–74 years [35]b Human (ns) Fr, Occ, PCG, PsCG, Temp No change 19–94 years [33] Rat (Wistar, F) Occ No change 3, 12, and 24 mos [25] Human Cerebral Cortex ↑ [46] Rat (SD, F) Par No Change 12 vs. 36 mos [47] Rat (Wistar, M) Fr ↑c 3 vs. 20 mos [29] a Approximately 20% decrease, no statistics reported. b Authors emphasize age-related increase in capillary density but data also reveal later decrease in density. c Apparent increase restricted to superficial region of cortex. Abbreviations: Cb: cerebellum; F: female; Fr: frontal cortex; Hipp: hippocampus; M: male; MTB: medial nucleus of the trapezoid body; NS: sex not specified; Occ: occipital cortex; Par: parietal cortex; PCG: precentral gyrus; PsCG: postcentral gyrus; Temp: temporal cortex; V1: primary visual cortex. The putative decrease in afferent vessels raises the question of whether there is a corresponding decrease in cerebral capillaries supplied by the afferent arterioles. As in other tissues, the length of capillary per volume of tissue arguably represents the fundamental measure of microvascular status because it determines, at the simplest level, the surface area for exchange between tissue and blood and how far a given cell is from the source of oxygen and nutrients. Surprisingly, there is as yet no striking consensus for the effects of aging on capillary density within the brain. One recent review concluded that there is little, if any, decrease in capillary density [20], whereas others report compelling evidence for an aging-related decline in capillary density [7]. How such disparate conclusions can be drawn becomes clear when one reviews the literature from the past three decades (summarized in Table 12.1, from [28]), which includes reports of aging-related decreases in capillary density, stability of capillary density from adulthood through senescence, and of increased capillary density (presumably as a result of neural atrophy). Several possibilities exist for the different conclusions. First, although significant differences exist among species, these differences alone do not account for the disparate findings reported to date. All possible age-related changes in capillary density (decrease, increase, or no change) have been reported for both humans and rats, the subjects of virtually all available studies. Second, differential responses in localized regions of the brain most likely contribute to the disparity in the literature, but regional differences alone cannot explain the varied results. In some cases, the same neural region in the same species has been examined in different laboratories and conflicting conclusions have been drawn (e.g., rat frontal cortex: [29] vs. [30–32]; rat occipital cortex: [25] vs. [30]; human frontal cortex: [33] vs. [34]). Third, capillary changes with aging may be multiphasic. At least two studies suggest that capillary density increases during late adulthood and then declines during later senescence [35, 36]. If supported, the comparison of young and old animals without the appropriate middle-aged group may potentially bias interpretation of experimental results. Finally, previous studies indicate that the magnitude of age-related changes in capillary density, where they occur, do not exceed approximately 10 to 20%. Taken together, the available literature suggests there is a substantial rarefaction of surface arterioles and limited but significant changes in capillary density in some regions of the brain, including the hippocampus and cerebral cortex. However, the evidence for capillary loss is tenuous and more extensive studies are required. In part, the disparate findings for capillary density changes with aging result from methodological differences among laboratories and improved stereological methods may resolve many of these issues [48–50]. To reliably reveal the regional and temporal patterns of microvascular changes, however, it will be necessary to analyze many neural areas in a single species at multiple ages using a consistent methodology. Such studies not only are required to establish the structural bases of aging-related changes in blood flow, but also are necessary to provide the critical baseline data for studies of microvascular plasticity. C. Microvascular Plasticity Capillary density is highest in regions rich in synapses, somewhat lower in regions containing primarily cells bodies, and lowest in fiber tracts [7]. Even within the gray matter of the cerebral cortex, sensory and association regions have higher densities of microvessels than motor regions; within a cortical region, layers and modules with greater cytochrome oxidase activity (indicative of greater synaptic activity) have higher capillary densities [72–74]. Thus, there must be developmentally regulated mechanisms by which the elaboration of the microvasculature is matched to the local level of neuronal signaling, presumably contributing to the differential growth of more active regions of the brain [73, 75, 76]. In principle, microvascular density in each region of the brain could be genetically programmed. The developmental mechanisms must be dynamic, however, because changes in neural activity during development result in predictable changes in microvascular density. Raising animals from the time of weaning in complex environments significantly increases synaptogenesis and the growth of neuropil in the cerebral cortex [77, 78]. Greenough and colleagues also demonstrated that the associated increase in metabolic demand from this paradigm produces significant growth of new capillaries [79, 80], even after the end of the normal period of developmental angiogenesis [81]. Conversely, decreased activity reduces vascular growth. Raising animals in complete darkness is commonly used to investigate the effects of activity deprivation on cortical development. Argandona and Lafuente [82, 83] demonstrated that dark-rearing rats from the time of birth significantly decreased the elaboration of the microvascular bed in the primary visual cortex. When examined as adults, the capillary density was 22% lower in dark-reared animals than in age-matched controls. Thus, the microvasculature within the CNS is actively modified during development to maintain a match between the local level of neural activity and the level of metabolic and vascular support necessary for that activity. For many aspects of neural development, there are critical periods during which specific aspects of neural structure or function change in response to alterations in activity [84, 85]. Critical periods may be absolute, after which no significant change is possible, or they may be relative, such that change remains possible but only in response to greater perturbations than are required to elicit plasticity during earlier development. A variety of studies indicate that, for the cerebral microvasculature, plasticity is not limited to the developmental period; rather, the microvasculature in adult animals can be altered to maintain or improve function in response to changes in activity, damage, or other perturbations. The extent to which such plasticity is maintained during aging has not been clearly defined but undoubtedly depends on the specific factors eliciting the microvascular response. Greenough and co-workers [86] have used the enriched environment paradigm to test whether microvascular plasticity in response to increased neural activity is limited to developing animals or is maintained in adults. These investigators demonstrated that housing adult rats (2 months of age) in complex environments resulted in microvascular growth within 10 days, just as in developing rats. Additional studies revealed, however, that the extent of the effect was reduced with age. The response in middle-aged rats was less than that in young adults, and no statistically significant change in microvascular density was elicited in old rats [87]. This age-related decrement in microvascular plasticity is consistent with reports that synaptic and dendritic plasticity also are reduced in old animals [79]. Thus, it is difficult to establish whether vascular plasticity decreases with age because neuronal plasticity declines, or whether neuronal plasticity is lost because there is insufficient vascular plasticity to support the generation and maintenance of new synapses [87]. Clearly, however, this type of microvascular plasticity is maintained into adulthood but sustained only poorly, if at all, during senescence [88]. Microvascular plasticity in the adult cerebellum was also investigated to assess the influence of learning vs. increased neural activity associated with simple motor activity. Microvascular growth was seen after motor learning, which also elicited synaptogenesis and growth of neuropil, and also after simple exercise, which had no effect on synapses and neuropil [89]. Voluntary exercise increased neural activity, as evidenced by increased glucose utilization [90], suggesting that microvascular growth occurs to meet the greater metabolic demands of increased neuronal signaling, even in adult animals. Consistent with this hypothesis, recent studies find that chronic exercise in adult animals, with no motor learning, promotes microvascular growth in the cerebral cortex [91, 92]. Unfortunately, to our knowledge there exists little or no data on the effect of exercise or motor learning on vascular plasticity in aged animals. In addition to activity-induced angiogenesis, adult microvascular plasticity is important clinically. The success of strategies for treating neurodegenerative diseases by implanting stem cells or neurons critically depends on microvascular plasticity to establish metabolic support for the foreign cells [93, 94]. Thus, several laboratories have investigated the ability of the microvasculature to invade and support new tissue or cells after implantation in the adult CNS. Following transplantation, solid tissue allografts are infiltrated and supported primarily by host blood vessels [95, 96], although some donor vessels also are maintained within the graft [97]. Host blood vessels also elaborate to support grafts of dissociated cells [98–100], in which microvascular growth is more effective than in solid grafts [101]. Given that grafts can survive in aged brains [102–105], the necessary microvascular plasticity must be maintained during senescence. Significant microvascular plasticity also occurs in response to chronic hypoperfusion or ischemia [7, 106, 107]. The restoration of blood flow after arterial occlusion relies in part on increased blood flow through collateral vessels supplying the ischemic region [108, 109], but significant new microvascular growth also occurs [110]. This includes both sprouting from preexisting capillaries and de novo vasculogenesis involving bone marrow-derived endothelial progenitor cells [107]. Similar to the microvascular changes that accompany changes in neural activity, postischemic microvascular plasticity appears to decrease in aged animals and humans [111, 112]. Several pharmacological agents promote microvascular growth and plasticity in the adult brain. Treatment of aging animals with a calcium channel blocker from 21 to 27 months of age, for example, significantly increased capillary density in the cerebral cortex and hippocampus [31, 32]. Similarly, 4 to 6 weeks of treatment with the plant alkaloid vincamine or its derivative increased capillary density in the cortex and hippocampus of adult rats [41, 47]. The extent of microvascular response appeared to be similar in 1-year-old and 3-year-old rats. Thus, at least for the responses to these pharmacological agents, there is no significant decline in plasticity during aging. Clarifying the mechanisms that regulate microvascular growth and function in the developing and adult brain, and establishing how those mechanisms are affected by age, is critical for understanding the nature and functional consequences of age-related changes in the brain and for assessing prospects for preventing or reversing cognitive deficits. Recent studies by Sonntag et al. [27] demonstrate that, although microvascular plasticity may be reduced, growth of precapillary arterioles can be elicited even in aged animals. The studies also suggest a potential mechanism for age-related changes in the microvasculature and related plasticity. Microvascular density on the surface of the cerebral cortex was reported to decline with age [27]. Notably, the density of surface arterioles correlated with plasma insulin-like growth factor 1 (IGF-1) levels at the time of vascular mapping. That correlation suggested that microvascular rarefaction may be the consequence of the age-related decline in circulating growth hormone and IGF-1, particularly given evidence that growth hormone and IGF-1 are important anabolic growth factors that may influence many aspects of blood vessel growth and repair [113]. Consistent with that hypothesis, twice-daily injections of growth hormone to 30-month-old animals, sufficient to increase plasma IGF-1, dramatically increased the number of cortical arterioles. Thus, growth hormone and IGF-1 appear to mediate, at least in part, age-related changes in the microvasculature and potentially cerebral blood flow [113]. Moreover, although microvascular plasticity may decline in aged animals, it can be restored if appropriate trophic conditions are present. In addition to growth hormone and IGF-1, other trophic factors profoundly influence the microvasculature. Although their potential roles in the etiology of age-related vascular changes have not been established, both basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) significantly influence angiogenesis and microvascular plasticity. Both factors exert a mitogenic effect on human microvascular endothelial cells in culture [114, 115]. VEGF prevents cultured microvascular endothelial cells from entering replicative senescence [116]. Additionally, VEGF is upregulated in spatial and temporal coincidence with angiogenesis associated with various CNS pathologies [117–119] and is involved in exercise-induced neovascularization [120]. Expression of bFGF is upregulated endogenously in response to focal cerebral infarction in rats, and exogenous administration of bFGF improves functional recovery, potentially via induction of angiogenesis in the damaged brain [121]. These factors do not appear always to benefit the microvasculature, however, because VEGF has been implicated in the breakdown of the blood-brain barrier associated with various CNS insults [122, 123]. Thus, the roles of these factors in modulating angiogenesis are complex and require further clarification. In addition to trophic factors such as VEGF and BDNF, endocrine growth factors influence neuronal turnover in the adult brain. Plasma-derived IGF-1 reverses the decline in hippocampal neurogenesis that is produced by hypophysectomy [124] and also ameliorates the age-related decline in neuronal turnover [125]. These findings suggest that IGF-1 is an important regulator of neurogenesis, and that the decline in neuronal turnover during senescence is the result of decreased IGF-1 levels. The effects of aging on the multiple sources of IGF-1 within the CNS remain unclear, but the decline in plasma levels of IGF-1 may be exacerbated in many regions by decreased blood flow and by microvascular rarefaction, which also would reduce local production of IGF-1 and potentially other important growth factors by endothelial and smooth muscle cells. Understanding the relationship between microvascular plasticity and neural activity, and how aging-related changes in the microvasculature affect metabolic support for neuronal signaling, is essential for clarifying the basis of cognitive changes during senescence. Accumulating evidence suggests that age-related changes in the microvasculature also may influence other critical aspects of neural function and plasticity. As noted previously, new neurons are continually produced in some regions of the adult brain [126–128] (see also Chapter 6), and the microvasculature is critically involved in regulating adult neurogenesis, both as a local source of factors that create an appropriate milieu for neurogenesis and as the source of blood-borne factors that influence proliferation. The production of new granule neurons in the subgranular zone of the adult dentate gyrus occurs within “neuroangiogenic foci” where neuronal, glial, and endothelial precursors divide in tight clusters [129, 130]. Proliferative precursors in other regions of the hippocampus are not found within such a vascular niche and do not give rise to neurons, only glia. Thus, the association between endothelial and neuronal proliferation in the subgranular zone suggests that either signals originating from somatic tissues or from the CNS act simultaneously to stimulate neurogenesis and angiogenesis, or that the initiating signal activates proliferation of one cell type, which then stimulates proliferation of the other. A recent demonstration that intracerebroventricular infusion of VEGF into the adult brain increases the genesis of both endothelial cells and granule neurons is consistent with a mechanistic link between angiogenesis and neurogenesis [131]. Also supporting this hypothesis is evidence that the reduction in neurogenesis that follows whole brain irradiation is due, in part, to alterations in the microenvironment, including disruption of microvascular angiogenesis [132]. More direct evidence that endothelial-produced factors regulate neurogenesis comes from the demonstration that culturing precursor cells from the adult rodent forebrain subependymal zone (SZ) on monolayers of endothelial cells, rather than on astrocytes or fibroblasts, increases neurogenesis and neuronal survival [133]. Thus, much remains to be determined concerning the complex interactions between vasculature and neurons. D. Microvascular Ultrastructure Alterations in microvessel ultrastructure potentially contribute to alterations in both blood flow and the transport of materials across the capillary wall, even where capillary density is unchanged. The effects of aging on arteriolar and capillary ultrastructure have been recently reviewed [7, 20]. In arterioles, aging appears to decrease the distensible components of the vessel wall (smooth muscle and elastin) and increase less distensible components (collagen and basement membrane; see [51]). Along with thickening of the endothelial basement membrane, cerebral arterioles in aged animals often contain flocculent material and intracellular inclusions that are not evident in the vessels of young animals [52, 53]. These alterations in arteriolar structure presumably contribute to age-related changes in arteriolar reactivity (see below). Age-related alterations have been described in capillary endothelial cells, their basement membrane, in pericytes, and in the astrocytic endfeet that are opposed to the abluminal vascular surface (reviewed in [7]; see also [54]). The increase in thickness of the basement membrane and the abnormal inclusions reported for aging arterioles, including enlarged perivascular space, also are evident in capillaries [37, 38, 55–61]. The complete source and mechanisms of basement membrane changes remain to be established [7], but the flocculent deposits observed in aging capillaries have been attributed to the degeneration of pericytes [62]. The various age-related ultrastructural changes in the capillary wall must be regulated by several mechanisms, because chronic treatment with calcium channel blockers decreases the deposition of extracellular collagenous fibers but has no effect on the degeneration of pericytes [58, 59]. The functional impact of these age-related alterations in the capillary wall are numerous, and are likely to include both increased leakage of materials normally excluded by the blood brain barrier and reduced transport of substances (e.g., glucose, amino acids, growth factors) that are actively transported into the brain parenchyma. Recent studies suggest that the age-related increase in leakage may be less than previously suggested [63], but that the decrease in carrier function appears to be pronounced [7]. Reports of age-related changes in the shape of arterioles and capillaries were reported in the early 20th century [64]. Although the extent of such abnormalities during normal aging is still debated [21], there are sufficient descriptions of microvessel looping, tortuosity, and twisting to conclude that such changes occur in many neural regions [20, 52, 65–69]. Whether such alterations reflect primary changes in blood vessels or secondary effects of atrophy of surrounding neural tissue is not clear, but regardless of the specific mechanism, one would expect modification of vessel shape to produce profound hemodynamic and rheological changes within the microvascular bed [52, 68–71], contributing to decreased blood flow and reduced delivery of oxygen and nutrients to the brain parenchyma. E. Summary In many regions of the aging brain, metabolic support for neuronal signaling may be compromised by decreased blood flow. Rarefaction of arterioles and changes in vessel shape and structure most likely contribute to reduced flow although the possibility of capillary rarefaction and the regulation of capillary density with age remains an area for additional research. The metabolic impact of reduced blood flow may be exacerbated by altered transport across the capillary wall. In addition to influencing blood vessel structure and function, aging reduces microvascular plasticity such that capillaries respond less to increases in neural activity although responses to other factors that promote angiogenesis may be maintained. The age-related loss of plasticity certainly influences neural plasticity as well, because neuronal turnover in the adult brain is linked mechanistically to capillaries and their growth. Trophic factors produced by endothelial cells are additional important regulators of ongoing neurogenesis within the adult hippocampus, and impairments in secretion of these factors may have independent actions on the aging brain. Thus, aging-related changes in microvascular structure and plasticity potentially contribute in multiple ways to the decline in cognitive function that accompanies brain aging. IV. VASCULAR REACTIVITY A. Age-Related Changes in Vascular Reactivity Although decreases in vascular density and alterations in vessel ultrastructure have the ability to contribute to decreased blood flow and tissue dysfunction with age, alterations in vascular tone (basal vessel diameter) and vascular reactivity (dynamic changes in vessel diameter) are equally important because matching blood flow to tissue metabolic demand is critical for normal cellular function. Despite recent advances in the field, there are conflicting reports of age-related alterations in cerebrovascular tone and reactivity in the literature. For example, Thorin-Trescases [134] observed no effect of age on arterial tone in isolated human pial vessels with diameters ranging between 300–1200 microns, whereas Geary and Buchholz [135] reported increased middle cerebral artery tone in aged Fisher-344 rats. This inconsistency is potentially the result of a disparity in the size and location of the vessels examined, as well as species-based differences in vascular responses. However, recent studies in our laboratory indicate that there are age-related changes in vascular tone and reactivity, and that these alterations are secondary to changes in the balance between endothelium-derived relaxing and contracting factors released from either the micro- and/or macro-circulation (D.M. Eckman et al., unpublished data). B. Role of the Endothelium in Aging FIGURE 12.3 (SEE COLOR INSERT FOLLOWING PAGE 204) The degree of (more...) (SEE COLOR INSERT FOLLOWING PAGE 204) The degree of vascular constriction (vascular tone) regulates blood flow and depends on a close communication between endothelial cells (ENDO) and smooth muscle cells (SMC). Endothelium-dependent relaxing factors (EDRFs) include endothelium-dependent hyperpolarizing factor (EDHF), NO, and PGI2. These compounds result in hyperpolarization of smooth muscle cells and dilation. Other factors such as endothelium-derived constriction factors (EDCFs), including PGH2 and TXA2 result in smooth muscle cell depolarization and vessel constriction. Many of the reported endothelium-dependent dilations depend on the actions of PKA/PKG and PKC. These kinases act on various intracellular targets that regulate/modulate [Ca2+]i and/or directly influence ion channel activity. With aging, the contribution of endothelium-derived relaxing factors (EDRFs) decreases, whereas the contribution of endothelium-derived contracting factors (EDCFs) increases in many vascular beds. A decrease in endothelium-derived NO and PGI2 leads to a relative decrease in the production of PKA/PKG, reducing membrane hyperpolarization, and increasing vascular tone. Furthermore, aging-associated elevation in PGH2 and/or TxA2 could increase PKC, thus decreasing K+-channel activity and leading to greater increases in vascular tone. The additive effects of decreased PKA/PKG activity along with elevated PKC activity result in membrane depolarization, increasing [Ca2+]i entry via voltage-dependent Ca2+ channels (VDCCs), leading to increased vascular tone/decreased blood flow in the cerebral circulation. (Source: Modified from [177]. With permission.) Ultimately, blood flow through a vessel depends on the close interactions of several cell types that compose the vessel wall. Endothelial cells and smooth muscle cells form the basis of this interaction; the complex interactions between relaxing and constricting factors derived from endothelial cells result in a dynamic regulation of vascular smooth muscle cell activity, vessel dilation or constriction, and hence regulation of blood flow to tissue. Mechanistically, endothelium-dependent relaxing factors (EDRFs) result in hyperpolarization of smooth muscle cells and dilation, whereas endothelium-derived constriction factors (EDCFs) result in smooth muscle depolarization and vessel constriction. The roles of each of these factors are discussed below. Despite the recognized importance of these factors in the regulation of cerebral blood flow, there is generally little information available on age-related changes in vessel tone and reactivity in the cerebral circulation. Therefore, data from other vascular beds are presented such that they might provide some insight into potential age-related changes in the cerebrovascular system. 1. Endothelium-Derived Relaxing Factors (EDRFs) a. Nitric Oxide (NO) The contribution of nitric oxide (NO) to the modulation of vascular tone has been the focus of multiple human and animal studies. The primary source of NO in the cerebral circulation is endothelial nitric oxide synthase (eNOS). As the name suggests, eNOS is located in the endothelial cells that line the lumen of the blood vessel. NO diffuses to vascular smooth muscle cells where it increases cGMP formation through the activation of cyclic guanylate cyclase (cGC). Activation of this pathway results in the dilation of vascular smooth muscle via multiple mechanisms. While the role of NO signaling in healthy aging-associated vascular dysfunction is not well understood, there are both human and animal studies that provide compelling data suggesting that altered vascular function associated with advancing age may be attributed, at least in part, to perturbations in NO signaling [136–144]. For example, in carotid arteries from aging mice [143], as well as forearm vessels of healthy elderly humans [142], vessel dilation following acetylcholine (ACh) administration (which stimulates endothelial NO production) is significantly reduced, suggesting a blunted production of NO occurs in endothelial cells with age. However, both of these studies also showed reduced vessel dilation in response to sodium nitroprusside (SNP), which directly donates NO to smooth muscle cells. Taken together, these data suggest both endothelial-dependent NO production and endothelial-independent NO sensitivity are altered with aging. In contrast, at least one study reported no age-related differences in coronary artery responses to ACh or nitroprusside have been observed in 24-month-old male F344 rats [145]. These inconsistent findings may reflect methodological differences but more importantly suggest that the mechanisms of age-related vascular dysfunction may be unique to each vascular bed and species. The mechanisms underlying the disrupted NO signaling with aging identified thus far appear to be complex. In the systemic circulation, both an increase and a decrease in nitric oxide synthase (NOS) activity have been reported [146, 147], as well as an age-associated decrease in agonist-induced vascular cGMP levels [141, 148–150]. In middle cerebral arteries from Fisher-344 rats, Geary and Buchholz [135] demonstrated an increase in arterial tone with advancing age, a finding that appears to be, at least in part, secondary to the dysfunction of eNOS-sensitive mechanisms. Others report declines in the levels of the β subunit of soluble guanylate cyclase (sGC) and reduced sGC activity [141], reduced protein kinase G-1 (PKG-1) activation [151], and decreased eNOS mRNA expression [144, 147, 152] and phosphorylation [153]. Finally, either unchanged [145, 154] or increased eNOS protein expression has been reported in aged vasculature [138, 154–159]. b. Endothelium-Derived Hyperpolarizing Factors (EDHFs) It is well accepted that endothelium-derived hyperpolarizing factors (EDHFs) are important regulators of vascular tone in resistance-size arteries. However, the role of EDHFs in modulating vascular tone in the aging systemic circulation, especially in cerebral circulation, has received little attention. Interestingly, aging appears to significantly impair EDHF-mediated relaxations in isolated human gastroepiploic arteries, distal microvessels [160], and gracilis arterial segments [161]. In contrast, the magnitude of EDHF-mediated relaxation of renal arteries from WKY rats appears unaffected by increasing age [162]. 2. Endothelium-Derived Contracting Factors (EDCFs) a. Endothelin There are limited data regarding the role of the potent vasoconstrictor, endothelin, in the modulation of vascular tone with age. Barton and colleagues [152] reported a rise in plasma endothelin-1 (ET-1) levels with increasing age in old female Ro-Ro Wistar rats (32 to 33 months). Furthermore, they have shown attenuated ET-1-induced constriction in the aorta but not the femoral artery of old female rats [152]. In contrast, the contractile response to ET-1 has been shown to increase in coronary arteries from aged Wistar-Kyoto [163] and Fisher-344 rats [145], as well as in basilar arteries from aged female rats [164]. Selective inhibition of endothelin-A receptors in young animals abolishes ET-1-induced coronary artery constriction, whereas inhibition of these receptors in aged animals has no effect on ET-1-induced constriction [145]. Thus, the role of endothelin in the modulation of vascular tone and reactivity in the aging vasculature appears complex and requires further investigation. b. Prostanoids The role of prostanoids, which include prostaglandins, prostacyclin, and thromboxane, in vascular regulation is well known. These molecules are created from arachidonic acid by cyclooxygenase (COX-1 and COX-2) and prostaglandin H synthase (PGHS-1 and PGHS-2) in most mammalian cells. In vasculature, prostanoids, especially thromboxane, stimulate vasoconstriction. Recent data suggest that aging results in a significant increase of thromboxane A2 and the prostaglandins PGE2, PGF2alpha, and PGI2 [165, 166]. Other studies have shown that the blunted vasodilatory response of aged vessels to administration of ACh or SNP, while partially attributable to dysregulation of NO, could be partially or completely reversed by inhibition of COX [138, 167] or PGHS2 [168]. Additionally, Davidge and colleagues [167] found that COX inhibition reversed an age-related hypersensitivity to administration of the vasoconstrictor phenylephrine [167]. Furthermore, age-related impairments in vasodilatory responses can be ameliorated by thromboxane A2/PGH2 receptor blockade [138, 167, 168]. Finally, age-associated endothelial dysfunction appears to occur as early as 12 months of age via inhibition of the synthesis of COX-2-derived constrictors as well as superoxide anions [169], suggesting an important role for prostanoids in the vascular dysfunction seen with aging. c. Reactive Oxygen Species (ROS) There are extensive data suggesting that oxidative stress plays an important role in the mechanisms of aging in multiple tissues [170] (see also Chapter 15). This concept is supported by data revealing a direct correlation between the activity of superoxide dismutase (SOD), an endogenous antioxidant, and lifespan in several species [171]. Although it is well recognized that changes in the antioxidant profile occur with advancing age, relatively little is known regarding the role of oxidative stress in aging-associated vascular dysfunction. In cultured aortic vascular smooth muscle cells from 16-month-old mice, reduced SOD activity has been shown to result in increased levels of reactive oxygen species (ROS) as well as increased lipid peroxidation and damage to mitochondrial DNA[172]. In aging rats and mice, increased ROS production has been linked to decreased NO bioavailability [143, 173], with concomitant quenching of NO by the formation of peroxynitrite followed by nitration and inhibition of mitochondrial MnSOD [174]. These alterations have the potential to interfere with endothelial NO signaling, resulting in increased vasoconstrictive tone in aged vessels. Indeed, coronary arterioles from aging rats show significantly diminished flow-induced, NO-mediated dilation as a result of increased O2− anion and peroxynitrite production [144]. In contrast to the above data, at least one study reported that, despite increasing oxidative stress in both male and female Fisher-344 × Brown Norway rats with advancing age, vascular function appears to be preserved in mesenteric arteries [175]. Thus, specific vascular beds may be sensitive to increased oxidative stress associated with age, whereas others may be relatively insensitive to such effects. C. Summary Perturbations in multiple signaling pathways (NO, EDHF, ROS, prostanoids) have been described in different vascular beds in the healthy aging animal. The majority of the research to date suggests that endothelium-dependent responses are attenuated, primarily due to decreased NO production and/or impaired downstream signaling in the NO pathway. The apparent decrease in endothelium-dependent vasodilators coincides with an increase in endothelium-dependent vasoconstrictors in many vascular beds. Additionally, there is an evolving literature suggesting that both ROS and prostanoids may be responsible for elevated vasoconstrictor activity. As most of the aforementioned information is derived from peripheral vascular studies, the mechanism(s) underlying cerebral vascular dysfunction in the aging animal remain unknown. It is well accepted that the cerebral circulation is highly autoregulated. Small changes in arterial tone result in rapid adjustment of regional cerebral blood flow to meet neuronal and metabolic demands. Although it can be argued that the effects of aging on cerebral circulation are predictable based on other vascular beds, the diversity of changes observed in other vascular beds with age argue against this position [176]. In addition, the basic control mechanisms in the cerebral circulation are unique compared to other vascular beds and include, but are not limited to, features such as the blood-brain barrier, perivascular innervation, intracellular communication between neurons, perivascular glial cells, and smooth muscle cells, a high tissue metabolic rate, lack of anoxic tolerance, and the presence of collateral arteries in some species. Therefore, extrapolation of findings from other vascular beds to the cerebral circulation is difficult, and further studies of the altered regulation of the cerebrovasculature in aged animals are necessary. V. CONCLUSION Cerebrovascular aging can be viewed from several perspectives, including alterations in vascular density (the number of capillaries and arterioles), vascular plasticity (the dynamic regulation of vessel density or structure), and vascular reactivity (the adjustment of vessels to acute metabolic changes that occur in tissues). There is evidence that in otherwise healthy humans and animals, age-related changes occur in each of these variables. Data on vascular changes in the brain with aging are limited by the absence of highly controlled studies, as well as by the complexities common to gerontological investigations. Nevertheless, there are substantial data that the density of some cerebral vessels, especially precapillary arterioles, decreases with age. However, the analysis of capillary density can be more challenging, and the introduction of stereological analyses may aid in the development of more precise analyses and resolve some of the current controversies. Certainly the growth of new vessels appears to be compromised with age, which has important implications for management of disease processes including stroke. Finally, matching acute metabolic changes to alterations in blood flow is critical for normal tissue function. Unfortunately, the majority of data on microvascular reactivity has been gathered from peripheral vascular beds. Whether changes in the cerebrovasculature are similar to those in the periphery remain to be established. However, each vascular bed appears to have unique regulatory properties and therefore additional direct studies will be required. The underlying basic question to be addressed is whether age-related alterations in blood flow or transport of nutrients from blood to brain limit tissue function in highly localized areas of the brain and directly or indirectly lead to impaired function. Obviously this is a complex question that will require non-invasive imaging techniques that are still in development. 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Woodman CR, Price EM, Laughlin MH. Aging induces muscle-specific impairment of endothelium-dependent dilation in skeletal muscle feed arteries. J Appl Physiol. 2002; 93: [PubMed] 177. Eckman DM. et al. Calcium sparks and membrane potential. In: Vanhoutte PM., editor. EDHF Taylor and Francis Inc; New York: pp. 35–46. Copyright © 2007 Taylor & Francis Group, LLC Bookshelf ǀ NCBI ǀ NLM ǀ NIH Help ǀ Contact Help Desk ǀ Copyright and Disclaimer 12 Table of Contents Browse on In this page Brain Aging INTRODUCTION EXPERIMENTAL CAVEATS AND POTENTIAL CONFOUNDS IN AGING RESEARCH FUNCTIONAL CHANGES WITHIN THE CENTRAL NERVOUS SYSTBrain Aging2007 Role of the Endothelium in Aging F (SEE COLOR INSERT FOLLOWING PAGE 204) The degree of vascular constriction (vascular tone) regulates blood flow and depends on a close communication between endothelial cells (ENDO) and smooth muscle cells (SMC). Endothelium-dependent relaxing factors (EDRFs) include endothelium-dependent hyperpolarizing factor (EDHF), NO, and PGI2. These compounds result in hyperpolarization of smooth muscle cells and dilation. Other factors such as endothelium-derived constriction factors (EDCFs), including PGH2 and TXA2 result in smooth muscle cell depolarization and vessel constriction. Many of the reported endothelium-dependent dilations depend on the actions of PKA/PKG and PKC. These kinases act on various intracellular targets that regulate/modulate [Ca2+]i and/or directly influence ion channel activity. With aging, the contribution of endothelium-derived relaxing factors (EDRFs) decreases, whereas the contribution of endothelium-derived contracting factors (EDCFs) increases in many vascular beds. A decrease in endothelium-derived NO and PGI2 leads to a relative decrease in the production of PKA/PKG, reducing membrane hyperpolarization, and increasing vascular tone. Furthermore, aging-associated elevation in PGH2 and/or TxA2 could increase PKC, thus decreasing K+-channel activity and leading to greater increases in vascular tone. The additive effects of decreased PKA/PKG activity along with elevated PKC activity result in membrane depolarization, increasing [Ca2+]i entry via voltage-dependent Ca2+ channels (VDCCs), leading to increased vascular tone/decreased blood flow in the cerebral circulation. (Source: Modified from [177]. With permission.)

35 Sustancias vasodilatadoras producidas por el endotelio
Oxido nítrico: fuerzas de cizalla, Ach, bradicinina, ATP, VIP ↑CaCaM→↑NOS III ↑GMPc→↑PKG→↓ MLCK→ ↓ fosforilación de MLC Factor hiperpolarizante derivado del endotelio: bradicinina, Ach Abre canales K+ activados por Ca++ Prostaciclina (PGI2): fuerzas de cizalla ↑AMPc→↑PKA→↓ MLCK→ ↓ fosforilación de MLC Agonistas o estímulos NO Bradicinina y Ach estimulans NOS III aumento del las fuerzas de cizalla estimula NO[SIII que depende de Ca y calmodulina cataliza la formaciónh de NO a partir de arginina. Fosf MLCK inhibe MLCK y reducen la fosf de MLC disminuye la interacción entre miosina y actina. Relajación Levick Shear stress se transuce o sensa por integrinas que anclan el endotelio co la lámina basal. Se activa fosfatidil inosito 3 kinasa que fosforila PKB que aumenta la act de eNOS (fosforilación). Aunque la lámina más cercana a la pared está inmovil ejerce una especie de jalón o tirón sobre el endotelio porque la lámina que sigue está jalando sobre esta. Agonistas que activan CaCaM en cel endoteliales (aumentan Ca)i activan eNOS y NO Agosnistas bradicinina, trombina, sust P, ATP, ADP, acetilcolina por M3, VIP, insulina e histamina en algunos tejidos algunos d estos agonistas son liberados por tejidos inflmado por lo que NO contribuye con el enrojecimiento de la inflamación (vasodilatación). NO predomina en arterias grandes Contribuye al tono basal Uso nitroglicerina El EDHF too se libera como respuesta Ach EDHF en arterias peq y arteriolas Ach y bradicinina inician hiperpol y relajac musc liso. Se bloquea NO por lo tanto es otro factor indep de NO o protaciclina. Posiblemente contribuye más que NO a vasodil en vasos peq. Endothelium plays a crucial role in the regulation of cardiovascular homeostasis through the release of vasoactive factors. 1: Fundam Clin Pharmacol Aug;22(4): Besides nitric oxide (NO) and prostacyclin, increasing evidences show that endothelium-derived hyperpolarizing factors (EDHF) participate in the control of vasomotor tone through the activation of calcium-activated potassium channels. In humans, the role of EDHF has been demonstrated in various vascular beds including coronary, peripheral, skin and venous vessels. The mechanisms of EDHF-type relaxations identified in humans involved the release by the endothelium of hydrogen peroxide, epoxyeicosatrienoic acids (EETs), acido epoxieicosatrienicoides potassium ions and electronical communication through the gap junctions. The role of EETs could be particularly important because, in addition contributing to the maintenance of the basal tone and endothelium-dependent dilation of conduit arteries, these factors share many vascular protective properties of NO. Protacilcina derivados del ac araquidónico aumenta AMPc y promueve fosforilación MLCK reduce fosf cadena ligera de miosina. PGI2 es ijmpo en dilatar los vasos pulmnoares al nacimiento. Levick: se genera por acción de la enzima ciclooxigenasa sobre ac. Araqudiónico (AG no sat). Agonistas: trombina fuer

36 Sustancias vasoconstrictoras producidas por el endotelio
Endotelina: hipoxia, angiotensina II ↑PLC→↑IP3→↑Ca++→ CaCaM→↑act. MLCK→ fosforilación de MLC Tromboxano A2 ↑ act. Canales Ca++ tipoL→ ↑Ca++→ Endotelinas hpoxia promueve la liberac endot. Receptor Eta median vasoc predominan en las partes de alta P aumenta conc Ca fosfolipasa C para generar inositol trifosfato IP3, liberac de Ca de reservas IC Levick Vasoc potenta que tarda 2 a 3 horas, estim proliferación miocitos cardiacos y vasc. Bajo nivel basal de síntesisi, poca contribución a tono basal Estim hipoxia Agonistas angiotnesina II vasopresina y tormbina Tromboxano: Pq metabolizan ácido arquidónico por medio de ciclooxigenasa para prod TXA2 activa receptores TP que abren canales CatipoL Factor constrictor derivado del endotelio en arteriolas de perros la anoxia produce constr.

37 Tono basal es indep del sist nervioso, algún factor metab respe miogénica inducida por la P arterial, elevado PO2 de la sngre art o la presenca de Ca Las venas responden más a la constricción por SS a la misma frec. Pero no tienen receptores beta adr. Tono basal bajo en las venas Tono basal

38

39 Concentración alta Concentración baja
B2 = corazón, hígado y M esquelético = vasodilatador. Concentración baja 39

40 Estímulos adrenérgicos
vasoconstricci ón (α1 ) vasodilatación (β2) Agonista: norepinefrina activa Gq → ↑PLC →↑IP3→ ↑Ca++→ CaCaM→↑act. MLCK→ ↑fosforilación de MLC Agonista: epinefrina → activa Gs→↑AMPc→↑PKA→↓act. MLCK → ↓ fosforilación de MLC Músculo esquelético, miocardio e hígado Vasoconstrictores: VASOPRESINA, Nucleo supraóptico y paraventricular del hipotálamo, transpo por asones hasta hipofisisi posterior donde se libera, estim por aumento de la osmolaridad del plasma (deshidratación) o una disminución de PAM o volemia (hemorragia) Reg. Excreción de agua. Renina angiotensina aldolterona ANGIOTENSINA, Estim por disminución de la P arteria renal, aumento del tono simp renal y adrenalina, disminución de la carga de NaCl que pasa por la mácula densa de los túbulos renales renina. Tiene acción tónica vasoc., además aumenta tono SS por acción sobre el área postrema y neuromodulación de la liberac de NT en la teminal simp. VIP too aumenta AMPc vasodil

41 Mecanismos de control extrínseco Control endocrino
Vasoconstrictores Catecolaminas: Noradrenalina R ∝ adrenérgicos Angiotensina II Vasopresina Serotonina Sustancia P Vasodilatadores Catecolaminas: Adrenalina R. 2 adrenérgicos Péptidos natriuréticos Péptido intestinal vasoactivo

42 Anexos

43 Microcirculación Metarteriola atajo por el capilar, tiene musc liso pero no está inervado Esfínteres precapilares: musc liso no está inervado responde a cambios locales controlan el flujo a una parte de la red capilar modula el Flujo sang tisular por 1 orden de magnitud Cierre o apertura de caps produce peq cambios de P que puede revertir el flujo en algunos sectores Metarteriolas (musc esq no tiene) y esfinteres no están presentes en todos los tejidos Caps 2 a 5 um una capa de cel endoteliales rodeda por unamembr basal Sangre fluye por las metarteriolas cuando los capilares están colapsados y esfínteres precapilares cerrados cuando aumenta la Pc y se supera la P crítica de cierre los esfínteres precap se dilatan y relajación del musc liso metarteriolas ye esfínteres por metabolitos liberados localmente Silverthorn La sangre que ingresa por las metarteriolas puede ingresar directamente a los caps y si los esfínteres están cerrados pasa de arteriolas a vénulas Anastomosis arteriovenosas son vasos musculares 20 a 130 um de ancho que unen art a vénulas sin pasar por caps. En la piel de dedos nariz labios y orejas, reg temp. La densidad cap está adaptada Intercambio de nutrientes Filtración Regulación de la temperatura Distribución de sustancias Las ramas de las art más peq se ramfican en arteriolas de primer orden que se siguen dividiendo hasta formar arteriolas terminales que son las últimas que tienen musc liso Cada arteriola terminal origina un módulo o grupo de capilares de 500 a 1000 um de largo y 4 a 8 de ancho. El lado venoso forma vénulas pericíticas de 15 um de ancho que no tienen musc liso y son muy permeables al agua y juegan un rol imp. En inflamación Vénulas de 30 a 50 um reaparece el musc liso. 43

44 J. Smooth Muscle Res. (2008) 44 (2): 65–81
Physiological roles of K+ channels in vascular smooth muscle cells. Ko EA, Han J, Jung ID, Park WS. J Smooth Muscle Res Apr;44(2): Review K+ channels contribute to the regulation of the membrane potential in electrically excitable cells, including those found in smooth muscle. Apertura de canales de K Membrane hyperpolarization due to an efflux of K+ results from the opening of K+ channels in vascular smooth muscle. This effect is followed by the closure of voltage-dependent Ca2+ channels, leading to a reduction in Ca2+ entry, and vasodilation (Nelson and Quayle, 1995). Cierre de canales de K In contrast, inhibition of K+ channels function leads to membrane depolarization and vasoconstriction. To date, four distinct types of K+ channel have been identified in vascular smooth muscle: voltage-dependent K+ (Kv) channels, Ca2+-activated K+ (BKCa) channels, ATP-sensitive K+ (KATP) channels, and inward rectifier K+ (Kir) Voltage-dependent K+ channels (Kv channels) Broad voltage-dependent K+ (Kv) channels expression has been detected in vascular smooth muscle cells. Kv channels open to allow an efflux of K+ in response to depolarization of the membrane potential (Fig. 1A), resulting in repolarization and a return to the resting membrane potential. Small-scale depolarization in vascular smooth muscle cells leads to an influx of Ca2+ through L-type Ca2+ channels and activation of the contractile machinery. Taken together, this indicates that Kv channels function to limit membrane depolarization and maintain resting vascular tone . The Kv channel inactivation results from sustained depolarization. Compared to the process of activation, Kv channel inactivation is relatively slow and involves an initial peak in the Kv current due to voltage-dependent activation followed by a drop in the current due to voltagedependent inactivationr) channels Ca2+-activated K+ channels (BKCa channels) Large-conductance (200 ~ 250 pS) Ca2+-activated K+ (BKCa) channels are a persistent feature of vascular smooth muscle cells (Fig. 1B). BKCa channels, which are activated by changes aumentos in the intracellular Ca2+ concentration and membrane depolarization, are believed to contribute to the maintenance of the membrane potential in small myogenic vessels . The efflux of K+ that results from BKCa channel activation can be used to counteract pressure- or chemical-induced depolarization and vasoconstriction Estabilizan el potencial de membrana, parcialmente activos en estaddo basal La hiperpol cierra canales de Ca, y contrarresta los cambios que estimularon su apertura. ATP-sensitive K+ channels (KATP channels) ATP-sensitive K+ (KATP) channels have been first identified by in cardiac muscle and then, they have been found in various cells including vascular smooth muscle (Fig. 1C) th in vitro and in vivo that a block in KATP channels leads to vasoconstriction and membrane depolarization in various types of vascular smooth muscle KATP channel activation is closely associated with several pathophysiological responses, including systemic arterial dilation during hypoxia, reactive hyperemia in coronary and cerebral circulation and acidosis- and endotoxic shock-induced vasodilation . moreover, the inhibition of KATP channels leads to impaired coronary and cerebral autoregulation Parcialmente activos en estado basal por la fosf de PKA Inward rectifier K+ channels (Kir channels) Inward rectifier K+ (Kir) channels are abundant in the smooth muscle of small-diameter resistance vessels (Fig. 1D) Though the exact function of Kir channels in vascular smooth muscle is still incomplete, there are two basic possibilities. First, Kir channels contribute to the resting membrane potential and resting tone in small-diameter vascular smooth muscle. Second, Kir channel activation in response to moderate increases in the extracellular K+ concentration (to 10–15 mM) may cause vasodilation. Effect of vasoconstrictors on K+ channels: role of PKC A number of vasoconstrictors inhibit K+ channel activity, which contributes to membrane depolarization. Generally, vasoconstriction is initiated at membrane receptors that are coupled through a GTP-binding protein (Gq) to phospholipases, which generate the second messengers diacylglycerol and inositol 1,4,5-triphosphate (IP3), which activate protein kinase C (PKC) . Several PKC isoforms, including α, β, ε and ζ have been identified in vascular smooth muscle according to their Ca2+-dependence. Classic PKCs (α and β) activation requires Ca2+, diacylglycerol, and phosphatidylserine; in comparison, the novel PKC (ε) requires diacylglycerol and phosphatidylserine, but not Ca2+, and the atypical PKC (ζ) is activated by phosphatidylserine Endothelin and angiotensin have shown to inhibit Kv currents via Ca2+-independent PKCε Activation More recently, Kv channel inhibition by thromboxane A2 reportedly involves PKCζ In vascular smooth muscle, several vasoconstrictors such as angiotensin, endothelin, vasopressin, noradrenaline, histamine, serotonin, and neuropeptide Y inhibit KATP channels function via PKC activation (primarily Ca2+-independent PKCε) ( Effect of vasodilators on K+ channels: role of PKA and PKG A number of vasodilators including calcitonin gene-related peptide, β-adrenergic agonists, vasoactive intestinal peptide, and adenosine, activate adenylyl cyclase, thereby increasing the intracellular concentration of cAMP, which activates cAMP-dependent protein kinase (PKA). Several types of vascular K+ channels are activated in this mechanism (Fig. 2). Though the regulation of Kv channels by vasodilators has been largely ignored, β-adrenoceptor stimulation has been shown to activate Kv currents through PKA in rabbit vascular smooth muscle cells (Aiello et al., 1995; Standen and Quayle, 1998). More recently, it was suggested that the potent vasodilator, adenosine also activates Kir currents through PKA (Park et al., 2005e; Son et al., 2005). Kir channels, like other K+ channels, may also be modulated by other important vasodilators; additional studies are required to solve this issue. Alteration of K+ channels in pathological conditions Impaired K+ channel function in vascular smooth muscle cells has been detected in various pathological conditions including hypertension, diabetes, ischemia/reperfusion, and brain injury Levick un miocito tiene alrededor de canales de K una gran fracción están abiertos Hipoxia abre canales de K hiperpol cierra canales de Ca relajación Indian J Med Res 129, March 2009, pp KATP channels in vascular smooth muscle play an important role in mediating systemic vasodilatation during hypoxia thus increasing blood flow to the heart, brain and muscle. This vasodilatation is attributed to the release of NO or adenosine as a result of hypoxia as well as due to the direct effect of hypoxia33. During hypoxia, KATP channels may be opened by fall in ATP, pH, oxygen tension or by a rise in intracellular lactate and ADP33. These factors may activate the channel either directly or by facilitating NO activation. In sepsis, KATP channel is activated by NO, peroxynitrite, reactive oxygen species and cGMP resulting in vasodilatation and hypotension34. Vasodilators elevating cAMP levels like CGRP, adenosine and prostacyclin also activate K+ channel and produce smooth muscle relaxation Haemodynamic studies have shown that KCOs decrease blood pressure in a dose-dependent manner in both normotensives and hypertensives. Potassium channel openers (KCOs) are potent arterial vasodilators in systemic, pulmonary and coronary circulation. Regional blood flow in cerebral, renal, mesenteric and skeletal muscle vascular bed is less affected compared to coronary circulation. Hence, despite their potent coronary vasodilator effect, KCOs do not cause coronary steal phenomenon39. KCOs like minoxidil, diazoxide, nicorandil, pinacidil, cromakalim and levcromakalim act by enhancing the ATPase activity of SUR1 subunit and the resultant channel opening causes hyperpolarization J. Smooth Muscle Res. (2008) 44 (2): 65–81

45

46 Mecanismos asociados con la respuesta miogénica
Estiramiento MLV activa canales sensibles al estiramiento que producen despolarización y contracción Mantiene flujo sanguíneo constante en presencia de cambios en la P perfusión (autoregulación) Músculo liso vascular se relaja cuando se reduce el estiramiento y se contrae cuando se aumenta el estiramiento Mec. Miògeno:El mecanismo responsable del F sang cte en preencia de modificaciones en la P perfusiòn Dado que la PA se mantiene constante el mec miògeno quedarìa reducido al minimo Cuando una persona cambia de posiciòn decùbito a bipedestaciòn aumenta la P transparietal mayor estiramiento y contricciòn arteriolar, reduce la filtración, esto ocurre hasta que se equilibra la P oncotica e intersticial con la P hidrostàtica elevada Hiperemia activa proceso or medio del cual el aumento de F se asocia con un aumento de la act metab. In fact, there is growing evidence that the myogenic response possesses a high degree of redundancy at the mechanosensor and intracellular signalling pathway levels. For details, please, refer to the main text. AbbTRP – transient receptor protein cation channel, ENaC – degenerin/epithelial sodium cation channel; Cav1.2 – voltage-operated Ca2+ channel; CCE – capacitative calcium entry; (Ca2+)4CAM – Ca2+-calmodulin complex; MLCK – myosin light-chain kinase; MLC20 – myosin light-chain regulatory domain (20 kDa); Sk1 – sphingosine kinase 1; PLC – phospholipase C; GPCR – G-protein-coupled receptor; IP3 – inositol triphosphate; DAG – diacylglycerol; PKC – protein kinase C; IP3R – IP3 receptor; RyR – ryanodine receptor; sER – smooth endoplasmic reticulum; CYP4A – cytochrome P450 4A; MLCP – myosin light-chain phosphatase. CGrowing evidence suggests that mechanisms which regulate the Ca2+ sensitivity of the contractile apparatus in vascular smooth muscle cells form the backbone of pressure-induced myogenic vasoconstriction. The modulation of Ca2+ sensitivity is suited to partially uncouple intracellular Ca2+ from constriction, thereby allowing the maintenance of tone with fully conserved function of other Ca2+-dependent processes. Following a brief review of ‘classical’ Ca2+-dependent signalling pathways involved in the myogenic response, the present review describes the emerging mechanisms that promote myogenic vasoconstriction via modulation of Ca2+ sensitivity. For the purpose of this review, Ca2+ sensitivity reflects the dynamic equilibrium between myosin light-chain kinase and myosin light-chain phosphatase activities in terms of its impact on vascular tone. Several signalling pathways (PKC, RhoA/Rho kinase, ROS) which have been identified as prominent regulators of Ca2+ sensitivity will be discussed. Although Ca2+ sensitivity modulation is clearly an important component of the myogenic response, attempts to integrate it into existing mechanistic models resulted in a two-phase model, with a predominant Ca2+-dependent ‘initiation/trigger’ phase followed by a Ca2+-independent ‘maintenance’ phase. We propose that the two-phase model is rather simplistic, because the literature reviewed here demonstrates that Ca2+-dependent and -independent mechanisms do not operate in isolation and are important at all stages of the response. The regulation of Ca2+ sensitivity, as an equal and complimentary partner of Ca2+-dependent processes, significantly enhances our understanding of the complex array of signalling pathways, which ultimately mediate the myogenic response.anal de catión del receptor potencial (TRP) transitorio cationes proteico de receptor transitorio t should be noted that myogenic behaviour in small arteries includes two phenomena: myogenic tone (i.e., tone at a constant pressure level) and the myogenic response (i.e., the alteration of tone in response to a change in pressure).2 The myogenic response is, of course, capable of regulating blood flow within the microcirculation since (i) the vessel wall of flow-regulating arteries (resistance arteries or arterioles) is predominantly composed of smooth muscle cells and (ii) in the distal sections of the vascular tree, changes in mechanical load are a frequent phenomenon. Accordingly, myogenic responses have been described in several vascular beds (e.g. the mesenteric, skeletal muscle, cerebral, renal, and coronary circulation2–5). The description by Bayliss, which we have quoted to introduce this subject to the reader, focuses primarily on the local effect of the myogenic response in resistance arteries: the regulation of tissue perfusion. However, to lay the groundwork for a more complete understanding of this phenomenon, two other important functions of the myogenic response merit mention: (i) it contributes to the protection of adjacent capillary beds against fluctuations of systemic pressure and (ii) it feeds back to systemic blood pressure (which is a product of total peripheral resistance and cardiac output). The systemic effects of the myogenic response are prominent, and they significantly amplify (through a positive feedback mechanism) the immediate vasoconstrictor effects of pressor agents on systemic pressure.6 n summary, the ‘myogenic response’ mechanism is involved in virtually every aspect of resistance artery function. It contributes to the maintenance of arteriolar blood flow following an alteration of pressure load: an increase in transmural pressure will result in constriction of the artery; while a loss in pressure will be followed by dilation (Figure 1). Consequently, the relationship between arteriolar resistance and transmural pressure is proportional, a phenomenon that maintains capillary pressure within relatively constant limits.2 Recently, antisense oligonucleotide techniques have provided the first functional evidence that stretch-activated channels, specifically TRPC6, TRPM4 and possibly ENaC, are indeed involved in the myogenic response.24,25,28 It should be noted that the activities of voltage-dependent calcium channels29 and calcium-activated potassium channels30,31 may also be modulated by transmural pressure, and therefore these channels are additional candidates. Calcium-activated potassium channels (K+Ca) carry a hyperpolarizing current proportional to the intracellular calcium concentration. An increase in K+Ca activity would result in inhibition of the myogenic response, a negative-feedback mechanism which may serve to limit the magnitude of the response There are endogenous mediators, for example the cytochrome P-450 metabolite 20-HETE, that inhibit K+Ca38,39 and could, therefore, modulate the myogenic response.40 Indeed, the inhibition of cytochrome P-450 (and hence 20-HETE generation) has been shown to attenuate myogenic responses in several artery preparations Recently, direct measurement of chloride fluxes has overcome these limitations, and alterations in transmural pressure indeed results in increased chloride fluxes,51 at least in some vessel preparations. However, the general inconsistency of the reported data makes it difficult to assign a specific role for these channels in the myogenic response. Finally, it should be noted that voltage-gated potassium (Kv) channels have been investigated in the context of the myogenic response. These channels do not promote myogenic vasoconstriction per se, rather, they appear to provide a negative-feedback mechanism that limits depolarization in response to elevation of pressure and therefore limits myogenic vasoconstriction.52–54 We believe that the delay between the immediate increase in calcium and full vasoconstriction reflects a continuous increase in calcium sensitivity. The modulation of Ca2+ sensitivity is an emerging concept, which has only recently been extensively reviewed by the Somlyos.72 Their definition of the phenomenon states that ‘Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes’. In this regard, the ‘lag phase’ observed in Figure 2 may result from the ongoing process of establishing a new equilibrium between the activities of MLCK and MLCP. These more recently described mechanisms act primarily through regulation of MLCP, allowing for changes in MLC20 phosphorylation at a constant level of MLCK activity. Such alterations in Ca2+ sensitivity provide, for the first time, a mechanistic solution to the enigmatic observation that vasoconstriction continues despite unchanged calcium levels in the post-initiation phase of the myogenic response (Figure 2). Relying on the definition provided by Somlyo and Somlyo,72 the appropriate measure of Ca2+ sensitivity would be the degree of MLC20 phosphorylation at a constant level of intracellular Ca2+ (and in principle, MLCK activity), through an alteration in MLCP activity PKC RhoA estimulas MLCP y aumentan resp miogénica Especies reactivas de oxígeno ROS However, recent evidence involving the role of sphingosine kinase 1 (Sk1) in the modulation of the myogenic response suggests otherwise. In isolated hamster resistance arteries, Sk1 signalling is known to enhance Ca2+ sensitivity by a RhoA/Rho kinase-dependent mechanism.89 However, Sk1 signalling also possesses clear immediate effects on pressure-induced Ca2+ elevation.89 Thus, it was concluded that Sk1 signalling (presumably via its metabolite sphingosine-1-phosphate) orchestrates both components of the myogenic response as an upstream modulator. Copyright restrictions may apply. Schubert, R. et al. Cardiovasc Res :8-18;

47 Diferencia consumo de O2 y a-vO2 en diferentes órganos
Diferencia a-vO2(ml/100ml) VO2(ml/100g) Corazón 10-12 8 (70) Músculo esquelético 2-5 1 (50) Riñón 2-3 5 Piel 1-2 0.2 Berne: Alta densidad capilar :con eleada tasa metab por ej corazón, glándulas , musc esq. Baja densidad cap. Baja tasa metab tejido subcut. Too varía el diámetro caps. OrganAO2-VO2 (vol %) heart10-12 skeletal muscle (resting)2-5 kidney 2-3 intestine4-6 skin1-2 OrganO2 Consumption (ml O2/min per 100g) Brain3 Kidney5 Skin0.2 Resting muscle1 Contracting muscle 50 Arrested heart2Resting heart rate8Heavy exercise70 En ejercicio la vasodil prod aumento F y la dnesidad de capilares perfundidos, equivalente la disminuir el radio del cilindoro de Krogh si las otros factores se mantienen iguoa la disminución de la s ditancias de difusioón aumentan la PO2 tisular La veloc de F too aumenta, lo que hace que la PO2 no disminuyea tan ráido a lo largo del cap, pero hay que tomar en cuenta que VO2 aumenta entonces PO2 si disminuye Si se calclua Px que es la facilidad con que pasan sustancias hidrosolubles el coazón es miuy alta seguida de los pulmones e intestinos y es muy baja en el músculo esquelético, lo que indica las difernecias de dnesidad de poros y perforaciones en los fi tejidos. Además una mayor fracción de los caps est{an abiertos en el corazón entonces el area superficial es mucho mayor . Las uniones estrechas del cerebro no permiten el flujo paracelular de solutos hidorfi{ilicos y exheiben muy baja permeabildiad, mientras que su permeabilidad al agua es similar a otros org. El area no es cote por que lel número de caps abiertos varía en resp a moleculas de señalización como citokinas y las cel endot reorganizan su citoesqueleto y cambian de forma. Queamplia los poros y aumenta Px. Wj secre histamina por mastocitos y basófilos. Recordas quela permabildiad se reduce de acuerdo cn el radio molecular. Las que solo atraviesan por poros o hendidudas llenas de agua tienen un baja permabildilda (molec polares peq) que es una fracción pequeña del área capilar total. En el lado venso del cap hay más poros y fenestraciones y son mas anchos por eso la pemeabilidad Px aumenta a lo largo del cap. El flujo seria mayor del lado venoso. With an arterial and venous blood sample, and a simple equation, we have for the first time demonstrated that absolute and serial peripheral oxygen extraction are powerful predictors of organ failure and mortality following major injury. Usaron la ecuación de fick para calcular la extracción de O2 Table 1 Survived Died Total Extract < 150 m Extract > 150 ml Total Robertson et al. Critical Care (Suppl 2):P351   doi: /cc5511 (músculo en contracción)

48 Fuerzas hidrostáticas:
Persona de pie por unt tiempo prolongado Pv aumenta y Pc aumenta por la gravedad, el resultado es el mov de liq en el espacio tisular, en la mayoría de los casos el sistema linfático puede extraer el el liq extra y regresarlo al espacio vascular lo que mantiene el eq de líq. El retorno de liq rquiere la contracción de los musc de las piernas que comprimen las venas y los linfáticos y rpopulsan el liq hacia arriva a través de las valv, si la persona de pie no contrae estos musc. Ocure tansduación de liqu que puee esceder el retorno linfatico y causar edema. Un org muy sensible al eq deliq es el pulmon. Solamente con peq cambios de Pc por ej hipertensión pulmonar puede causar edema pulmonar, se reduce la complianza pulmonar loque la inflación del pulmonon más dificil y mpuede comprometer el intercambio de gases. La insfu carddiaca izq. Cuasa quela sngre se reresa hacia los vasos del pulmnón y auemnta la P pulmnoar vascular y causa edema pulmonar. En el corzón derecho, insuf prduce el flujo retrógrado hacia las venas sitémicas lo que aumenta la P venosa central (P en la grandes venas sitémicasO loque cuasa un aumento de Pc en las extrem inf y en las visceras abdomnales.. El liq transudado de capilares hepáticos e intestinales puede dejarn el espacio intersticial y entran en la canvidad peritoneal lo que produce una condición conocida como ascitis. Presión coloidosmótica reducción de la P coloidosmótica por pérdida de prot. Por ej en orina con síndrome nefrótico. Se reduce la habilidad de los caps para retener liq. Edema generalizado. Durante el embarazo la síntesis de prot por la madre no es proporcional con la expansión del vol. Reducción relativa de los niveles de prot. Too en malnutrición protéinca. Edema en las extremidades. Pies se hinchan o inflaman 30 ml/h De pie de 15 a 40 min aumenta ultrafiltración de plasma en tejidos dependientes y reducen el vol plasmatico de 6 al 20% P onc estudiantes que se sientan 8 hrs durante las lecciones o al leer pasa de 25 a 29 mm hg Compensación : vasocontr postural mec miogénico Hemoconc local por disminución de F con aumento de Pc aumenta la fracción de filtración aumenta Ponc e los capilares venosos y se atenúa el aumento de la tasa de filtración Disminuye capaci de filtración por cierre de caps por vasocontr? Bomba musc. Transudado: edema con bajo cont prot por aumento Pc o disminución Ponc cap Exudado: edema con alto cont prot por aumento de la permeab cap. Inflamación por traumas o infección, mediadores liberados por cel.