Descubrimiento de la renina El artculo clsico sobre el descubrimiento de la renina por el fisilogo finlands Robert Tigerstedt y su estudiante sueco Per Bergman en 1898 [1] se basa en experimentos realizados desde 1896 hasta 1897 en el Instituto Karolinska. Extractos salinos de rin de conejo, se mostr a aumentar lentamente la presin arterial (PA) cuando se inyecta en conejos. El principio causando esto estaba presente en la corteza del rin, pero no en la mdula y fue destruida por calentamiento. Los autores llegaron a la conclusin de que la sustancia era una protena y que la llam renina. Ellos especularon que 'renina podra de alguna forma directa o indirecta se asoci con la hipertrofia del corazn se encuentra en la enfermedad renal y presin arterial alta. Sin embargo, estos primeros resultados no se pueden repetir en otros laboratorios, y no fue hasta finales de 1930 cuando la renina fue 'redescubierto'. Circulacin 'RAS clsica "Una inmensa cantidad de las primeras investigaciones sobre el sistema renina-angiotensina (RAS) allan el camino para una mejor comprensin de su fisiologa y fisiopatologa. A principios de la dcada de 1970, se identificaron los principales componentes del "clsico" que circulan RAS (Fig. 1) y no hubo evidencia convincente para indicar un papel importante para RAS en la regulacin del equilibrio de lquidos y BP. En ese momento, sin embargo, hubo un escepticismo generalizado sobre el papel de RAS en la enfermedad cardiovascular. Lo No fue sino hasta el descubrimiento de la angiotensina eficaz por va oral de la enzima convertidora (IECA), el primero de los cuales fue el captopril, que estaba siendo apreciado la importancia primordial de RAS en la homeostasis cardiovascular y la enfermedad. La introduccin de losartn, el primer tipo de receptor de angiotensina 1 bloqueador oral activo y eficaz fortalece an ms este concepto. Nueva vista ampliada de RAS El concepto relativamente simple "clsico" de la "RAS circulante ', (Fig. 1) con el angiotensingeno (AGT) generada por el hgado, la renina por los riones y el principal pptido efector, la angiotensina II (Ang II) generada por ACE en la vasculatura se completa con la clonacin de los receptores AT1 y AT2. Sin embargo, las implicaciones fisiolgicas de los RAS han seguido creciendo (Fig. 2) y no hemos visto el cuadro completo todava. Se ha convertido gradualmente evidente que, adems de la 'RAS circulante,' hay un local de RAS tejido "en la mayora de los rganos y tejidos estudiados. De hecho, se ha reportado incluso la generacin intracelular de Ang II. Esto hace RAS no slo un endocrinas, sino tambin un paracrinos y un sistema de intracrina. Por otra parte, tanto la angiotensina heptaptide 2-8 (Ang III) y el hexapptido 3-8 (Ang IV) han demostrado ser biolgicamente activa. La angiotensina 1-7 heptapptido (Ang 1-7) parece jugar un papel importante en contrarrestar muchas de las acciones de la Ang II. La angiotensina II y Ang III acciones estn mediadas por los receptores AT1 y AT2 solamente. Descubrimientos recientes han puesto de manifiesto los receptores funcionales especficos para Ang IV, Ang 1-7, y quizs lo ms sorprendente, incluso para la renina / prorrenina. Por lo tanto, nuestro presente vista ampliada de RAS (fig. 2) es bastante complejo y de mltiples capas. Los roles establecidos de Ang II La angiotensina II ejerce sus acciones a travs de los receptores AT1 y AT2 que, en principio, aunque no siempre, median funciones opuestas. Receptores AT1 mediar acciones con consecuencias potencialmente perjudiciales, si no compensado adecuadamente. Receptores AT2 se cree que median las acciones de proteccin, la relevancia clnica de los cuales an no se ha establecido claramente. La angiotensina II es un importante regulador de equilibrio de lquidos y de sodio y la hemodinmica, sino tambin del crecimiento celular y el remodelado cardiovascular. As, los receptores AT1 median la vasoconstriccin, la sed y la liberacin de vasopresina y aldosterona, fibrosis, el crecimiento celular y la migracin. Ms recientemente, la Ang II se ha demostrado que causa la generacin de radicales oxidantes a travs de los receptores AT1 y para participar en los procesos inflamatorios, incluyendo la aterosclerosis y envejecimiento vascular. Receptores AT2 median la vasodilatacin, la liberacin de xido ntrico (NO) y por lo general la inhibicin del crecimiento. Nuevas funciones mediadas por los receptores AT1 infusin de angiotensina II causados disminucin de adiponectina en plasma, un sensibilizador de insulina, aparentemente a travs de los receptores AT1 en la rata. Supresin de la adiponectina puede representar un mecanismo mediante el cual la Ang II hace que la tolerancia alterada a la glucosa. Otras acciones metablicas de la Ang II incluyen la modulacin pro-inflamatoria, aumento de la secrecin de insulina, la apoptosis de clulas B, reduccin de la gluconeognesis y la produccin de glucosa heptica y el aumento de los triglicridos en plasma. Decidimos no entrar en el complejo campo de la AT1 intracelular y la sealizacin del receptor AT2 y por lo tanto consulte lector a crticas recientes. Nuevas funciones mediadas por receptores AT2 angiotensina receptor de tipo 2 se informa generalmente para mediar efectos opuestos y que contrarrestan los mediada por los receptores AT1 (fig. 1), por ejemplo, vasodilatacin, la liberacin de NO y la inhibicin de la proliferacin y el crecimiento. Sin embargo, los receptores AT2 tambin pueden mediar efectos neurotrficos en el sistema nervioso central. Por otra parte, los receptores AT2 upregulated en el cerebro-peri isqumica pueden ejercer la proteccin contra el dao isqumico. Los autores especulan que un efecto protector mediado por receptores AT2 podra explicar en parte la proteccin superior contra los accidentes cerebrovasculares en pacientes tratados con losartn versus tratamiento con atenolol en el estudio LIFE. Otra explicacin puede ser que el losartn reduce el centro de BP de manera ms eficaz que atenolol. En rin de rata, Ang III pero no Ang II se inform recientemente para inducir natriuresis mediada por receptores AT2. Esta natriuresis fue aumentada por el bloqueo de aminopeptidasa N, una enzima que metabolizan Ang III de la Ang IV (fig. 2). Los autores especularon que los bloqueadores de aminopeptidasa N pueden ser desarrollados para tratar enfermedades caracterizadas por la retencin de sodio y fluido, tal como la hipertensin y la insuficiencia cardaca. En teora, tales inhibidores tambin pueden ejercer acciones beneficiosas a travs de la reduccin de los niveles tisulares de Ang IV (vase ms adelante). Curiosamente, lquido intersticial renal se ha demostrado que contienen ms o menos 1.000 veces mayores concentraciones de Ang II y Ang III que el encontrado en el plasma. Efectos de la estimulacin de los receptores AT2 sin oposicin son un poco controversial. Por lo tanto, los efectos beneficiosos incluyen efectos vasodilatadores bradicinina-NO, natriurticos y efectos antifibrticos. Los efectos potencialmente perjudiciales son la apoptosis, el factor nuclear kappa B (NF-JB) la transduccin de seales y la induccin de quimiocinas. A pesar de una gran cantidad de resultados experimentales prometedores que sugieren enrgicamente las acciones beneficiosas de estimulacin AT2 la prueba clnica final es insuficiente. Aunque el tratamiento con bloqueadores de los receptores AT1 (ARA II) aumentar sustancialmente los niveles de Ang II en plasma y presumiblemente causan aumento de la estimulacin de los receptores AT2, no hay pruebas concluyentes para demostrar la relevancia clnica del aumento de la actividad del receptor AT2. Vas alternativas de generacin de la Ang II de la angiotensina II pueden ser generados enzimticamente a partir de Ang I por quimasa (Fig. 2) en ciertas condiciones patolgicas. La quimasa se almacena en complejo macromolecular con proteoglicanos heparina en los grnulos secretores de los mastocitos. Para llegar a ser enzimticamente activa, quimasa acomplejado debe ser liberado de los grnulos de los mastocitos, por ejemplo, por dao vascular causado por abombamiento u otros daos. Por lo tanto, la quimasa es enzimticamente inactiva en el tejido vascular normal y puede producir la Ang II slo en las paredes arteriales daados o aterosclerticas. Es de sealar que los inhibidores de la proteasa serina endgenos presentes en el fluido intersticial son potentes inhibidores de la quimasa. Los inhibidores de la quimasa informa, prevenir lesiones neontima tras el injerto de vena o distensin arterial en perros, mientras que los inhibidores de la ECA son ineficaces. Sin embargo, los efectos de los inhibidores de la quimasa pueden depender de otros efectos de estos compuestos tales como disminucin en factor de crecimiento transformante b (TGF-b) la generacin y la estabilizacin de grnulos de los mastocitos y no disminuido en la formacin de Ang II. Adems, los ARA II que bloquean las acciones Ang II, independientemente de la generacin de la enzima (s) no han demostrado ser superiores a los inhibidores de la ECA en grandes ensayos clnicos. A pesar de los resultados experimentales en animales con inhibidores de quimasa son prometedores, la posible importancia de la generacin de Ang II por quimasa est clara y los inhibidores de la quimasa que son seguros y tiles para ensayos en humanos an no se han desarrollado. La angiotensina 2-8 heptapptido La angiotensina III ha sido conocido desde la dcada de 1970 para causar vasoconstriccin y liberacin de aldosterona. Se genera a partir de la Ang II por aminopeptidasa A (Fig. 2). Ang III ejerce sus acciones, en principio, similares a los de la Ang II, a travs de los receptores AT1 y AT2. Mientras que la Ang II es considerado el principal efector de RAS, Ang III puede ser tanto o ms importante en algunas de las acciones mediadas por los receptores AT1, por ejemplo, liberacin de vasopresina. La infusin sistmica de la Ang II o III de la Ang perros conscientes ha sido recientemente demostrado resultar en efectos equipotentes a la misma concentracin de plasma en BP, la secrecin de aldosterona, la excrecin de sodio y la actividad de la renina plasmtica; todos los efectos inhibidas por candesartn. Sin embargo, la tasa de aclaramiento metablico de Ang III fue cinco veces mayor que la de Ang II. Este estudio indic que la Ang II juega un papel dominante como un efector de la 'clsica RAS circulantes. La angiotensina 3-8 hexapeptide La angiotensina IV puede ser generada a partir de Ang III por aminopeptidasa M (Fig. 2). Este pptido biolgicamente activo ha alcanzado creciente inters tras el descubrimiento y clonacin de los receptores de amino peptidasa de insulina-regulada (IRAP), un sitio de unin y un receptor probable (AT4) de la Ang IV. Acciones de Ang IV mediadas por IRAP (fig. 3) incluyen vasodilatacin renal, la hipertrofia y la activacin de NF-JB que conduce a una mayor expresin de activador de plaquetas inhibidor-I (PAI-1), protena quimiotctica de monocitos (MCP-1) interleuquina-6 y factor de necrosis tumoral-a. Varios estudios sugieren que la Ang IV tiene importantes funciones reguladoras en la cognicin, el metabolismo renal y dao cardiovascular. Ang IV regula el crecimiento celular en los fibroblastos cardacos, clulas endoteliales y clulas musculares lisas vasculares. Parece que la Ang IV est implicado en la respuesta inflamatoria vascular y, por tanto, podra desempear un papel en la fisiopatologa cardiovascular. La angiotensina 1-7 heptapptido La angiotensina 1-7 heptapptido se pens durante mucho tiempo que se carece de acciones biolgicas, a pesar de los primeros informes sobre los efectos biolgicos. La importancia de Ang 1-7 fue enfatizado por la relativamente reciente descubrimiento de un "nuevo" ACE2. Esta enzima genera Ang 1-7 de la Ang II. Ang 1-7 tambin puede ser generada a partir de Ang I o la Ang II por otras peptidasas. Ya en 1988, Ang (1-7) se demostr que la liberacin de vasopresina tan eficazmente como Ang II a partir de explantes neurohipofisarias. ANG (1-7) se encontr que tienen acciones opuestas las de la Ang II (Fig. 3), a saber, la vasodilatacin y efectos antitrophic y la amplificacin de la vasodilatacin causada por la bradiquinina. Numerosos experimentos sugieren una importante interaccin entre Ang (1-7) y los sistemas de prostaglandina-bradicinina-NO. Ang (1-7) parece compensar varias acciones de Ang II. ANG (1-7) se une al receptor Mas (figuras 2 y 3) que media vasodilatadora y acciones antiproliferativas de la heptapptido. Enzima convertidora de la angiotensina 2 Enzima convertidora de angiotensina 2 fue descubierto y clonado en lugar recientemente. Este descubrimiento trajo tanto ACE2 y su producto principal, Ang 1-7 en el foco de una intensa investigacin. ACE2 es una carboxipeptidasa, que escinde un residuo de la Ang I para generar angiotensina 1-9 y un nico residuo de la Ang II para generar Ang 1-7 (fig. 2). ACE2 es ms abundante en el endotelio vascular del rin, corazn, hipotlamo y la pared artica. ACE2 tambin se encuentra en los testculos [40]. Regulacin de la expresin de ACE2 an no se ha aclarado completamente. Ni los inhibidores de la ECA ni las ARB inhibe la actividad de ACE2, pero ambos parecen regular al alza la expresin de ACE2 en el miocardio de rata y la corteza renal. La expresin de ACE2 en el corazn se incrementa por infarto de miocardio. Se inform de Interrupcin del gen de ACE2 en ratones para dar lugar a un defecto de la contractilidad cardiaca grave, el aumento de los niveles plasmticos de Ang II y la regulacin positiva de genes inducidos por hipoxia-cardacos. Los autores concluyeron que ACE2 es un "regulador esencial de la funcin del corazn '. La ablacin gentica de la ECA en ACE2 ratones nulos normaliz completamente el fenotipo cardaco, que se ajusta a los papeles mutuamente de contrapeso propuestos para las ACE / Ang II y ACE2/Ang 1-7 brazos de RAS. Los autores interpretaron estos resultados como evidencia de ACE2 es un regulador esencial de la funcin cardaca. Esta interpretacin ha sido recientemente cuestionada por un estudio tambin en ACE2 nula ratones, mostrando "ningn efecto detectable en las dimensiones cardacas o fraccin de eyeccin en ratones conscientes, en condiciones basales. Estos investigadores reportaron una mayor sensibilidad presora a la infusin de Ang II y los niveles plasmticos ms altos y concentraciones renales de Ang II durante la infusin en ACE2 ratones nulos. Este estudio sugiere un papel importante para ACE2 en la degradacin de la Ang II y regulacin de las respuestas vasculares a la Ang II. La abundancia de ACE2 en los riones, en particular en el tbulo proximal es de particular inters. ACE2 puede ser crtico en la regulacin del equilibrio entre los efectos renales de la Ang II y Ang (1-7) y, por tanto, puede convertirse en un objetivo para futuras estrategias teraputicas. ACE2 parece tener un papel protector en el rin. Enzima convertidora de angiotensina 2 y Ang 1 a 7 mayo desempear un papel importante en la fisiologa cardiovascular y la fisiopatologa, por ejemplo, modulando o contrarrestar el exceso de actividad de la RAS "clsico". La expresin y actividad de ACE2 en el corazn y el rin se incrementa de manera diferente por el tratamiento con inhibidores de la ECA. Esto conduce a la organi-sabio regulada aumento de la produccin local de Ang 1-7, como se ha demostrado en ratas. Esto ofrecera un efecto beneficioso adicional de los inhibidores de la ECA, que puede explicar en parte por qu los inhibidores de la ECA y los BRA siguen siendo eficaces a pesar de la mayor actividad de la renina plasmtica y las concentraciones de pptido angiotensina. Receptores renina / prorrenina Receptores de renina se identificaron y clonaron en lugar recientemente y se muestran para ser funcional. Receptores de renina se unen tanto prorrenina y renina. Dos receptores se han caracterizado; el / receptor de M6P-factor de crecimiento insulnico II, un receptor de aclaramiento y el receptor de la renina especfica, que activa la sealizacin intracelular y mejora la actividad cataltica de la renina unido al receptor en la superficie celular (Fig. 4). Receptores de la renina son abundantes en el corazn, el cerebro y la placenta, los niveles ms bajos se encuentran en los riones y el hgado. El tejido adiposo visceral tambin expresa el receptor de renina, mientras que el tejido adiposo subcutneo expresa receptores de menos de renina. Parece que los receptores de renina participan en la produccin local de la Ang II y pueden contribuir a los niveles de Ang II sistmicos tambin. La unin de prorrenina al receptor de renina (fig. 4) conduce a la activacin de prorrenina de renina activa, la activacin de protenas quinasas activadas por mitgeno p44 / 42 y TGF-b, en ltima instancia, el aumento de la contractilidad, la hipertrofia y la fibrosis. Se ha sugerido que el bloqueo de los receptores de renina puede ser un nuevo objetivo para la proteccin de los tejidos. La "manija y la puerta de 'hiptesis de la activacin del sistema renina unida al receptor ofrece perspectivas interesantes. Un pentapptido reproduccin de la regin de 'mango' del prosegmento de prorrenina (fig. 4) que cubre el sitio activo ha proporcionado resultados prometedores en el tratamiento de la nefropata diabtica y en la prevencin de la glomerulopata hipertensiva en ratones. Si traducido en patologa humana, estas observaciones pueden cambiar profundamente nuestra visin del pro-renina y la (pro) receptor de renina como actor dentro de los RAS tejido y un objetivo potencial del tratamiento. Tejido RAS (locales) Sistemas RAS locales se han identificado en la mayora de los rganos y tejidos investigados como revisado recientemente. Ellos contienen todos los componentes necesarios para la produccin de Ang II y otros pptidos de angiotensina y sus respectivos receptores, y adems, los receptores de renina / prorrenina. La mayora de los estudios indican que la mayora si no todos de la renina que se encuentra en los sistemas locales de RAS se deriva de la renina renal. Sistemas RAS tisulares ejercen diversas acciones en muchos rganos. En algunos rganos, funcionan independientemente de la "RAS circulante ', por ejemplo, las glndulas suprarrenales y el cerebro. Otros sistemas de RAS locales, por ejemplo, corazn y los riones funcionan en estrecha interaccin con el RAS 'circulando'. Por lo tanto, los componentes circulantes de RAS como la renina y AGT pueden ser absorbidos por los tejidos. Circulacin de RAS y RAS tisulares locales se cree que funcionan de manera complementaria [56], no opuestos entre s. Un equilibrio adecuado entre la regulacin y la contra-regulacin de factores de RAS tejido parece importante en el mantenimiento de las funciones fisiolgicas normales de muchos rganos. La RAS circulante est visto como un regulador de volumen sistmico y el equilibrio de electrolitos y de BP homeostasis, mientras que los sistemas RAS 'locales' tienen efectos tisulares locales que implican funciones de proliferacin, crecimiento, la sntesis de protenas y de rganos, por ejemplo, en los riones, el corazn, el cerebro, los rganos reproductores y el pncreas. Algunos descubrimientos recientes sobre los sistemas locales de RAS pueden merecer especial inters, es decir, las del corazn, el cerebro y el tejido adiposo. Por lo tanto, la expresin de ACE2 se incrementa en el corazn despus de un infarto de miocardio, la insuficiencia cardaca y durante el tratamiento con inhibidores de la ECA o los ARA II. ACE2 es el principal generador de Ang 1-7 de la Ang II en el corazn, y la cantidad de Ang 1-7 se incrementa en el rea peri-isqumica despus de un infarto de miocardio. Componentes de RAS cerebro median en una gran variedad de actividades neurobiolgicos que estn siendo entendidos progresivamente. Por ejemplo, los receptores AT1 neuronales median Ang II efectos sobre la PA, la sal y el agua de admisin y de la secrecin de vasopresina mientras que los receptores AT2 median, por ejemplo, la apoptosis y la regeneracin neuronal posiblemente despus de la lesin neuronal. Adems de la Ang II y Ang III, IV y Ang Ang 1-7 parecen estar implicados en la modulacin de las funciones del cerebro, incluyendo las respuestas de aprendizaje y memoria. El tejido adiposo tambin contiene todos los componentes de RAS, incluyendo receptores de renina funcionales co-localizar con renina y puede estar implicado en la regulacin de la acumulacin de tejido adiposo visceral. Por lo tanto, visceral RAS puede desempear un papel en la fisiopatologa del sndrome metablico. El tejido adiposo ha demostrado ser una importante fuente de tanto AGT local y circulante y por lo tanto podra participar en la regulacin sistmica de la PA. Sin embargo, esto no se ha demostrado en los seres humanos. ACE de testculo (aproximadamente 100 kDa), una isoforma ms pequea de la ECA (150-180 kDa) ha demostrado recientemente que desempear un papel crucial en la fertilizacin mediante la liberacin de un glicosilfosfatidilinositol (GPI) anclado protenas de las clulas de esperma. Knock-out clulas de esperma de la ECA mostraron deficiencia de unin de vulos. El impacto de esta observacin espera una mayor clarificacin. Sin embargo, no se reporta el tratamiento con inhibidores de la ECA para interferir con la fertilidad masculina. RAS intracelulares Evidencia que sugiere la existencia de un RAS intracelular completa y funcional dentro de las clulas se ha proporcionado recientemente. RAS intracelular se inform para mediar en los cambios en los flujos de Ca2 + y la activacin de los genes. Intracelular de la Ang II informes, caus hipertrofia cardiaca in vivo en ratones. En estos experimentos, un plsmido construir bajo el control de un promotor de un-miosina causado un aumento intracelular de Ang II y aumento del 68% en peso relativo del corazn. Los mecanismos por los cuales intracelular Ang II ejerce sus acciones no se entienden completamente. Por lo tanto, intracelularmente aplican los BRA pueden bloquear slo en parte intracelular de Ang II. El papel de la RAS intracelular actualidad no est claro. Participacin de ACE2 en la gripe aviar Sorprendentemente, ACE2 se ha demostrado que funcionan como un receptor del sndrome respiratorio agudo severo (SARS) coronavirus. ACE se cree que contribuyen al dao de tejido pulmonar y edema mediante la generacin de Ang II. ACE2 se cree para contrarrestar normalmente estos efectos nocivos, pero despus de la fijacin del virus del SARS a ACE2 y replicacin, expresin ACE2 se ve disminuida, menos Ang 1-7 se forma a partir de Ang II y la activacin del receptor AT1 se intensifica. En apoyo de esta afirmacin, la inyeccin de ACE2 recombinante en ratones protege estos ratones de la lesin pulmonar aguda causada por sepsis o aspiracin cida. Por lo tanto, el funcionamiento ACE2 puede proteger contra la lesin pulmonar potencialmente mortal asociado con el SARS. RAS y arterial Envejecimiento envejecimiento se asocia con alteraciones de varias propiedades estructurales y funcionales de las grandes arterias. El aumento de espesor de la pared y la rigidez, velocidad de onda de pulso y pulso de onda de aumento y el deterioro de la funcin endotelial son signos caractersticos del envejecimiento arterial. Estas alteraciones forman suelo frtil para la enfermedad cardiovascular asociada a la edad. Por el contrario, la enfermedad cardiovascular provoca la aceleracin de estos cambios perjudiciales. Varias lneas de evidencia apoyan un papel importante de RAS en arterial envejecimiento, as como en la enfermedad cardiovascular. Componentes arteriales de la cascada de sealizacin de Ang II aumentan con el envejecimiento. Ang II sealizacin a travs de receptores AT1 aumenta la produccin de colgeno dentro de la pared arterial, promueve formas reducidas de nicotinamideadenine actividad oxidasa dinucletido fosfato y mejora la migracin de clulas musculares lisas vasculares. Aumento de la formacin de especies reactivas de oxgeno (ROS) conduce a la activacin de la metaloproteasa, menos la biodisponibilidad de NO y la disfuncin endotelial. Formacin de ROS inducida por la Ang II puede contribuir a envejecimiento de los tejidos y enfermedad cardiovascular relacionada con la edad. Ang II tambin causa la activacin de NF-JB va proinflamatorias y citoquinas. Por lo tanto, a juzgar por los criterios mecanicistas, la Ang II parece jugar un papel central en muchos estmulos que rigen envejecimiento arterial y sus respuestas funcionales. Un estudio reciente mostr que el tratamiento de ratas Wistar machos con un inhibidor de la ECA (enalapril 10 mg kg da) o un ARB (losartn 30 mg kg da) durante 18 meses o largo de la vida como resultado la prolongacin de la esperanza de vida en un 21 % (enalapril) o 19% (losartn) en comparacin con las ratas control no tratados. La diferencia en la duracin de la vida no podra ser explicada por la proteccin cardiovascular. Los autores especulan que la prolongacin de la esperanza de vida podra ser explicado por la reduccin por RAS inhibidores de la formacin de ROS y la reduccin de la carga oxidativa. Gentica de la RAS De acuerdo con la importancia generalmente reconocido de RAS en la fisiopatologa de la enfermedad cardiovascular, muchas mutaciones en los genes de los componentes de RAS estn asociadas con la hipertensin y las enfermedades cardiovasculares. El gen AGT se ha asociado con la hipertensin, pero los intentos de predecir respuestas a frmacos antihipertensivos basados en el polimorfismo AGT han producido resultados inconsistentes. Las variantes de la AT1 y los genes de los receptores AT2 se informa, asociados con la hipertensin, sino que tambin estn asociados de manera incompatible con la respuesta a la terapia antihipertensiva. En una cohorte de pacientes hipertensos china, la asociacin con AGT combinada y de los receptores AT1 de polimorfismos de nucletido nico (SNP) haplotipos fue modesta (13% para la sistlica, el 9% para la reduccin de la PA diastlica con inhibidor de la ECA. En un estudio prospectivo que comprende 2.579 hombres del Reino Unido, el receptor AT1 genotipo 1166CC se asoci con un mayor riesgo cardiovascular, independientemente de la presin arterial, mientras que la AT alelo 1675A receptor II se asoci con un mayor riesgo slo en la PAS elevada (> 165 mm Hg). En general, la magnitud de la capacidad de prediccin del gen RAS, SNPs ha sido ms bien modesta. Un posible papel para el receptor AT2 en el sistema nervioso central se sugiri por primera vez por el comportamiento exploratorio atenuada en (knockout) los ratones deficientes en el receptor AT2. En los seres humanos, la ausencia de o mutaciones en el gen del receptor AT2 se muestran para ser asociado con el cromosoma X de retraso mental ligado severa, que muestra un enlace entre un componente de RAS y el desarrollo de las funciones cognitivas. Curiosamente, una mutacin nica del gen del receptor de renina ms tarde se demostr estar presentes en pacientes con retraso mental ligado a X y epilepsia. Anlisis funcional revel que el receptor de renina mutado podra obligar renina y aumentar la actividad cataltica de la renina. "Este hallazgo confirma la importancia de la RAS en los procesos cognitivos y seal un nuevo papel especfico para el receptor de renina en las funciones cognitivas y el desarrollo del cerebro". Considerable ampliacin y profundizacin de nuestra comprensin de la fisiologa y pathopysiology RAS 'se ha logrado mediante la manipulacin gentica de animales de experimentacin, por ejemplo, ratas [86] y en ratones. Por ejemplo, 'ACE 1/3 de los ratones', heterocigotos compuestos para los genes de la ECA no tienen ACE endotelial, pero sin embargo son capaces de mantener la fisiologa normal. La explicacin para esto pareca ser un aumento compensatorio en la produccin de renina renal seguido por un aumento de la generacin de la Ang II por ACE nonendothelial, que muestra la plasticidad de RAS. Este es slo un ejemplo de la manipulacin gentica fascinante que revela secretos de la fisiologa RAS que de otra manera habran permanecido enigmtico. Inhibicin RAS, logros y expectativas inhibidores y ARA II (Fig. 5) de la enzima convertidora de angiotensina estn bien establecidas las piedras angulares en la prevencin y tratamiento de la hipertensin y la enfermedad cardiovascular, como se ha demostrado por numerosos ensayos clnicos y prctica clnica en todo el mundo. De acuerdo con un reciente meta-anlisis [89], los inhibidores de la ECA y los ARA tienen efectos BP-dependientes similares para los riesgos de accidente cerebrovascular, enfermedad coronaria e insuficiencia cardiaca. Para los inhibidores de la ECA, pero no para los ARA II, no hay evidencia de efectos BP-independientes en el riesgo de eventos de enfermedad coronaria. Los beneficios de los inhibidores de la ECA o los ARA II sobre los resultados renales probablemente el resultado de efectos BPlowering mientras que los beneficios renoprotectores en los pacientes diabticos pueden depender en parte de factores ms all de disminucin de la PA. De hecho, varios estudios sugieren que tanto los inhibidores de la ECA y los BRA ofrecen beneficios adems de los mediada por disminucin de la PA slo. Varios ensayos han demostrado una reduccin del 15-30% de diabetes de nueva aparicin durante el tratamiento con inhibidores de la ECA o los ARA II. El mecanismo detrs de este efecto protector de la inhibicin del SRA no est claro, pero ofrece una ventaja significativa para los inhibidores del SRA que estamos viviendo una epidemia global de la creciente incidencia de la diabetes. Bloqueo combinado de la RAS por los inhibidores de la ECA y los ARA-II se ha demostrado que proporcionan beneficios adicionales en comparacin con cualquiera de las clases de drogas. Sin embargo, estas expectativas no fueron confirmados por el estudio ONTARGET recientemente publicado que compar el tratamiento con telmisartn, ramipril o ambos frmacos combinados en una megatrial comprende 25.620 pacientes con alto riesgo cardiovascular. De particular inters es el uso en ONTARGET de telmisartn, de lejos, el activador ms prominente de proliferador de peroxisoma activados por los receptores-C, un mediador de una serie de acciones metablicas favorables. En el estudio ONTARGET, PA media fue menor en tanto el grupo de telmisartn (0,9 / 0,6 mm de Hg mayor reduccin) y el grupo de terapia de combinacin (2,4 / 1,4 mm de Hg mayor reduccin) que en el grupo de ramipril. Telmisartan fue equivalente a ramipril en trminos de medidas de resultado primarias en pacientes con enfermedad vascular o diabetes. La combinacin de telmisartn y ramipril se asoci con ms efectos adversos (hipotensin, disfuncin renal) sin un aumento de los beneficios. La reciente introduccin de la primera inhibidor eficaz por va oral de la renina, aliskiren, ha aumentado el inters adicional en nuevas posibilidades de bloqueo casi completo de RAS como una herramienta (Fig. 5), tal vez, ms eficaz que antes, en la prevencin y tratamiento de la enfermedad cardiovascular . Los primeros informes sobre la utilizacin de aliskiren son prometedores, mostrando como mnimo, un efecto antihipertensivo de aliskiren potentes como los de otros frmacos antihipertensivos [93]. En particular, la combinacin de inhibidores de la renina con inhibidores de la ECA y los ARA II puede ofrecer una solucin al fenmeno 'escape renina', lo que implica que los inhibidores de la ECA o los ARA II pueden perder parte de su efecto durante el tratamiento a largo tiempo. Muchas de las preguntas tambin se plantean, por ejemplo, lo que seran las consecuencias si el pptido angiotensina 'beneficioso', Ang 1-7 no se gener en absoluto o efectos mediados por el receptor AT2 'benficos' desaparecieron por completo? Experimentos y ensayos bien realizados nicamente pueden responder a estas preguntas. Podemos esperar aos interesantes por delante a la espera de los resultados y respuestas.

The critical role of the circulating RAS1 in the regulationof arterial pressure and sodium homeostasis hasbeen recognized for many years. Ang II is the mostpowerful biologically active product of the RAS, althoughthere are other bioactive Ang peptides, includingAng III, Ang IV, and Ang 1-7. Ang II directly constrictsvascular smooth muscle cells, enhances myocardial contractility,stimulates aldosterone production, stimulatesrelease of catecholamines from the adrenal medulla andsympathetic nerve endings, increases sympathetic nervoussystem activity, and stimulates thirst and salt appetite.Ang II also regulates sodium transport by epithelialcells in intestine and kidney. There has also been agrowing appreciation of the organ-specific roles exertedby Ang II acting as a paracrine factor (Navar et al., 1996;Paul et al., 2006). In addition to its physiological roles,locally produced Ang II induces inflammation, cellgrowth, mitogenesis, apoptosis, migration, and differentiation,regulates the gene expression of bioactive substances,and activates multiple intracellular signalingpathways, all of which might contribute to tissue injury.Clinical and preclinical studies on the effects of pharmacologicalinvestigations with ACEIs and ARBs supportthe notion that Ang II exerts a cardinal role in thepathogenesis of hypertension and renal injury via activationof AT1 receptors when inappropriately activated(Timmermans et al., 1993; Navar et al., 2000). Importantly,because the kidney plays a crucial role in thedevelopment of hypertension, hypertension is both acause and consequence of renal disease (Navar, 1997, 2005; Paul et al., 2006). Accordingly, the Seventh Reportof the Joint National Committee (JNC7), the EuropeanSociety of Hypertension/European Society of Cardiology(2003 ESH-ESC), and the Japanese Society of Hypertension(JSH2004) recommended that ACEIs and ARBs beused in concert with diuretics as first-line therapy toreduce blood pressure in patients with hypertension andrenal disease (Chobanian et al., 2003; Cifkova et al.,2003; Ikeda et al., 2006).Recent attention has been focused on findings thatlocal Ang II levels are differentially regulated in thekidney. Because there often is not clear evidence formarkedly elevated circulating renin or Ang II concentrations,identification of local RAS activity is essential forunderstanding the mechanisms mediating pathophysiologicalfunctions. In particular, the Ang II contents inrenal tissues are much higher than can be explained onthe basis of equilibration with the circulating concentrations(Navar et al., 1997, 1999a,b; Navar and Nishiyama,2004). Furthermore, the demonstration of muchhigher concentrations of Ang II in specific regions andcompartments within the kidney indicates selective localregulation of intrarenal Ang II (Navar and Nishiyama,2001, 2004; Ichihara et al., 2004b; Pendergrasset al., 2006). Thus, it is now apparent that intrarenalAng II levels are regulated in a manner distinct fromcirculating Ang II concentrations. It has also been revealedthat Ang II produced locally in the kidney exertsan important regulatory influence on renal hemodynamicsand functions as a paracrine factor (Navar et al.,2000; Paul et al., 2006). Further studies demonstratethat reduced renal function and its structural changesare associated with inappropriate activation of the intrarenalAng II, leading to the development of hypertensionand renal injury (Navar et al., 2003; Navar, 2005).In this review, we will briefly summarize the paracrineroles of intrarenal Ang II and review recent findingsrelated to its independent regulation with specialemphasis on roles in the pathogenesis of hypertensionand renal injury. We will also discuss evidence regardingthe effects of pharmacological intervention with antihypertensiveagents on intrarenal Ang II. The molecularmechanisms responsible for Ang II-induced cellinjury have been reviewed by Kim and Iwao (2000) andTouyz and Schiffrin (2000) and will not be discussed indetail in this review.II. Physiological Actions of Angiotensin II in theKidneyA. Role of Angiotensin II in the Regulation of RenalHemodynamicsExogenous administration of Ang II elicits dose-dependentdecreases in renal blood flow and glomerularfiltration rate (Yamamoto et al., 2001; Paul et al., 2006).Although there is agreement that Ang II exerts substantialdirect effects on the renal microvasculature andglomerular mesangium, there remains controversy regardingthe intensity of actions at various sites and therelative contribution of systemically and intrarenallyformed Ang II to the overall regulation of renal hemodynamics.The observation that Ang II increases thefiltration fraction has frequently been used to supportthe notion that Ang II predominantly constricts the postglomerulararterioles (Schor et al., 1980; Heller andHoracek, 1986; Alberola et al., 1994). It should be emphasized,however, that this misconception is based onthe failure to recognize that an increase in filtrationfraction can occur as a consequence of parallel increasesin both pre- and postglomerular arteriolar resistances(Navar and Rosivall, 1984; Rosivall et al., 1984; Carmineset al., 1987). Indeed, in vivo micropuncture studiesin rats have clearly demonstrated that Ang II elicitsreductions in single nephron glomerular filtration rateand glomerular plasma flow and increases in both afferentand efferent arteriolar resistance (Blantz et al.,1976; Baylis and Brenner, 1978; Schor et al., 1980; Rosivalland Navar, 1983). The decreases in glomerularfiltration rate are also attributed to the effects of Ang IIto reduce the glomerular filtration coefficient, which isthought to be due to changes in contractility of mesangialcells (Blantz et al., 1976; Baylis and Brenner, 1978;Schor et al., 1980; Paul et al., 2006). Because both AT1and AT2 receptors are expressed in mesangial cells(Sharma et al., 1998), these may influence the glomerularfiltration coefficient. However, the exact mechanismby which Ang II regulates the glomerular filtration coefficientremains to be clarified.Although it was originally reported that Ang II did notconstrict isolated rabbit afferent arterioles, there aremany reports demonstrating that Ang II constricts bothafferent and efferent arterioles (Carmines et al., 1986;Mitchell and Navar, 1988; Loutzenhiser et al., 1991;Ichihara et al., 1997; Yamamoto et al., 2001). Ito et al.(1991, 1993), and Yoshida et al. (1994) showed thatinhibition of nitric oxide synthesis markedly augmentedthe afferent arteriolar responses to Ang II, indicatingthat high levels of nitric oxide may be present in thedissected afferent arterioles perfused with cell-free solutions.Studies using the in vitro blood-perfused juxtamedullarynephron preparation (Carmines et al.,1986; Ichihara et al., 1997), renal tissue transplantationinto hamster cheek pouch (Click et al., 1979), and hydronephroticrat kidneys (Steinhausen et al., 1987; Dietrichet al., 1991; Loutzenhiser et al., 1991; Inman etal., 1995) also showed similar results. Yamamoto et al.(2001) used an intravital tapered-tip lens-probe videomicroscopysystem and demonstrated that intrarenalinfusion of Ang II constricts both afferent and efferentarterioles in anesthetized dogs. These collective observationsindicate that, rather than predominantly constrictingefferent arterioles, Ang II elicits vasoconstrictoractions on both pre- and postglomerular resistancevessels; however, the experimental circumstances may influence the reactivity of the afferent more than of theefferent arterioles.It should be recognized that Ang II elicits the glomerularhemodynamic changes described above withoutcausing significant proteinuria. In both animals andhumans, acute Ang II infusion sufficient to change renalhemodynamics does not elicit proteinuria (Loon et al.,1989; Pagtalunan et al., 1995). These observations are inagreement with the prediction based on the mathematicalmodeling that alterations in glomerular pressurecan cause less change in macromolecule filtration if thecapillary wall structure is not altered (Bohrer et al.,1977). However, sustained elevation of intrarenal Ang IIinduces proteinuria accompanied by progressive injuryof the glomerular filtration barrier, which is composed ofthe glomerular endothelium, glomerular basementmembrane, and podocytes (glomerular visceral epithelialcell) (Miller et al., 1991; Hoffmann et al., 2004;Whaley-Connell et al., 2006). Locally produced Ang IIdirectly induces podocyte injury via activation of AT1receptors, independent of hemodynamic changes(Durvasula et al., 2004; Liang et al., 2006; Liebau et al.,2006). Therefore, pharmacological interventions of theseeffects of Ang II are useful for reducing proteinuria inpatients with renal injury.The overall renal hemodynamic responses to Ang IIblockade with ACEIs and ARBs have been quite variablebecause of the counteracting influences of the associateddecreases in systemic arterial pressure. If arterial pressureremains within the renal autoregulatory range,renal blood flow is generally increased by Ang II blockade(Navar et al., 1996; Paul et al., 2006); however, theglomerular filtration rate responses have been muchmore variable, either increased (Kimbrough et al., 1977;Rosivall et al., 1986; Tamaki et al., 1993), unchanged(Omoro et al., 2000), or decreased (Hall et al., 1979b). Invivo micropuncture studies showed that Ang II blockadeincreases single nephron filtration rate as well as singlenephron plasma flow when arterial pressure is notmarkedly reduced (Kon et al., 1993; Cervenka et al.,1998; Cervenka and Navar, 1999; Paul et al., 2006).Similarly, intrarenal infusion of subpressor doses ofARBs significantly increased both whole kidney renalblood flow and glomerular filtration rate (Nishiyama etal., 1992; Tamaki et al., 1993), suggesting that Ang IIblockade increases the glomerular filtration coefficient.Most clinical studies also show that the glomerular filtrationrate remains stable when Ang II blockade isinstituted (Andersen et al., 2000; Fridman et al., 2000;Agodoa et al., 2001). The most direct way to explainincreases in renal blood flow without changes in glomerularfiltration rate is by combined decreases in both preandpostglomerular arteriolar resistance. In some studies,glomerular filtration rate has been shown to beincreased slightly in response to treatment with ACEIsand ARBs (Fridman et al., 1998; Peche`re-Bertschi et al.,1998). However, a significant reduction in the glomerularfiltration rate has often been seen in patients withrenal disease (Hansen et al., 1995; Apperloo et al., 1997).Decreases in arterial pressure in response to Ang IIblockade are pronounced during sodium-depleted states(Navar et al., 1996; Paul et al., 2006). Usually, in hypertensivepatients with renal disease, ACEIs and ARBsare often added to other drugs, including diuretics, underthe conditions where intake of sodium is restricted.Thus, it seems likely that Ang II blockade with ACEIsand ARBs causes a marked reduction in blood pressure,leading to decreases in glomerular filtration rate whenextracellular fluid volume is low. In addition, in patientswith established glomerular disease, it may be difficultto maintain the glomerular filtration rate by sufficientincreases in glomerular filtration coefficient when glomerularpressure is reduced by treatment with ACEIsand ARBs. In patients with more severe renal disease,the afferent arterioles may also become less responsiveto ACEIs and ARBs.In addition to its direct constrictor effects on glomerulararterioles and mesangium, Ang II also regulatesrenal hemodynamics by exerting a modulatory influenceon the sensitivity of the tubuloglomerular feedbackmechanism (Navar et al., 1996; Paul et al., 2006). Thismechanism provides a balance between the reabsorptioncapabilities of the tubules and the filtered load by regulatingthe glomerular filtration rate (Nishiyama et al.,2004a). When flow-dependent changes in the tubularfluid solute concentration at the level of the maculadensa in the terminal part of the loop of Henle aresensed, signals are transmitted to the afferent arteriolesand glomerular mesangium to constrict or dilate tomaintain stability of the filtered load (Navar et al., 1996;Paul et al., 2006). The tubuloglomerular feedback mechanismalso participates in autoregulatory responses ofrenal vascular resistance and glomerular filtration rate(Nishiyama et al., 2004a; Paul et al., 2006). Although itwas demonstrated that Ang II does not directly mediatethe tubuloglomerular feedback response, its level of activityexerts an important modulatory influence on thesensitivity of the vascular and mesangial elements thatrespond to signals from the macula densa cells (Ploth,1983; Schnermann and Briggs, 1986; Mitchell et al.,1992; Braam et al., 1995; Schnermann et al., 1997;Traynor et al., 1999). The tubuloglomerular feedbackresponsiveness is enhanced during either systemic orperitubular capillary infusion of exogenous Ang II(Schnermann and Briggs, 1986; Mitchell et al., 1992).Furthermore, Ang II blockade with ACEIs and ARBsmarkedly attenuates the tubuloglomerular feedback responsivenessas assessed by stop-flow pressure feedbackresponses to increases in distal nephron perfusion rate(Ploth, 1983; Braam et al., 1995). Similarly, both AT1receptor knockout and ACE-deficient mice have markedlyattenuated tubuloglomerular feedback responses toincreases in distal nephron perfusion rate (Schnermannet al., 1997; Traynor et al., 1999). Collectively, these findings indicate that Ang II enhances the sensitivity ofthe vascular and mesangial elements that mediate tubuloglomerularfeedback-induced alterations in singlenephron function. These effects probably are mediatedby direct actions on the vascular smooth muscle cellsand mesangial cells as well as by modulating theNa/H exchange activity of the macula densa cells(Peti-Peterdi and Bell, 1998; Kovacs et al., 2002). Amodulatory influence of Ang II on tubuloglomerularfeedback responsiveness shifts the operating point of thesystem and allows the nephron filtration rate to bemaintained at a lower distal nephron volume delivery(Navar et al., 1996; Paul et al., 2006). During conditionsof elevated intrarenal Ang II levels, the modulatory influenceof Ang II on tubuloglomerular feedback responsivenessis of pivotal importance in maintaining the AngII-mediated stimulation of proximal tubular reabsorptionand the consequent decrease in distal nephron volumedelivery. In this manner, the interactive effects ofincreased Ang II levels to enhance both proximal tubularreabsorption rate and sensitivity of the tubuloglomerularfeedback mechanism elicit sustained decreasesin distal nephron volume delivery and, thus, urinarysodium excretion.Enhanced preglomerular vascular tone and bluntedmicrovascular autoregulatory responsiveness to changesin perfusion pressure are observed in Ang II-dependenthypertensive models (Ichihara et al., 1997; Inscho et al.,1999). The blunted autoregulatory responsiveness of theafferent arteriole in Ang II-dependent hypertension apparentlyresults from chronic elevation of Ang II levelsbecause acute exposure to 10-fold greater concentrationsof Ang II does not affect autoregulatory behavior (Inschoet al., 1996). Chronic treatment with ARBs prevents thedeterioration of renal autoregulatory responsiveness inAng II-infused rats (Inscho et al., 1999). However, Ang IIblockade does not affect renal autoregulatory behaviorin normal animals (Navar et al., 1986; Persson et al.,1988).B. Role of Angiotensin II in the Regulation of TubularFunctionAng II is one of the most powerful sodium-retaininghormones in the body. The direct intrarenal actions ofAng II that contribute to increased tubular reabsorptionare complex, including constriction of glomerular arterioles,which alter peritubular capillary dynamics andrenal medullary blood flow, and direct actions on tubularepithelial cell transport. Although the quantitative contributionof each of these hemodynamic and tubularactions may vary in different physiological circumstances,high intrarenal Ang II levels contribute to saltand water retention through direct actions on renal tubulartransport function when inappropriately stimulated(Navar and Nishiyama, 2004).Ang II is also one of the bodys most important regulatorsof aldosterone, which stimulates sodium reabsorption,primarily through the mineralocorticoid receptorsin the connecting and cortical segments of the collectingtubule. Furthermore, Ang II directly enhances urinaryconcentration in the collecting tubule and collectingducts.Because all of the components of the RAS are found inthe kidney and significant amounts of Ang II can beformed locally, considerable interest has focused on thepossibility that intrarenally formed Ang II may be moreimportant than circulating Ang II in controlling renalfunction. Several studies demonstrated the fact that intrarenalinfusion of ARBs or ACEIs, at rates that producedno changes in plasma aldosterone concentrationand minimal effects on systemic hemodynamics, increasedsodium excretion (Kimbrough et al., 1977; Hallet al., 1979a; Klag et al., 1996; Cervenka et al., 1998).Intrarenal infusion of Ang I, to stimulate local formationof Ang II, also reduced sodium excretion (Rosivall andNavar, 1983). These results emphasize the contributionof intrarenally formed Ang II in regulating sodium excretion.In addition to maintaining fluid and electrolyte homeostasis,Ang II participates in a variety of tubularfunctions, including induction of cellular hypertrophyand oxidative stress. Details of biological function specificfor each tubular segment are described below.1. Proximal Tubules. Normally, the potent antinatriureticeffects of Ang II are due primarily to increasedtubular reabsorption rather than to reductions in glomerularfiltration rate (Hall et al., 1986; Mitchell et al.,1992). In vivo perfusion of rat proximal tubules with anultrafiltrate-like solution containing either ACEIs orARBs decreased the volume reabsorption, suggestingmodification of proximal tubule transport by locally producedAng II independent from the systemic RAS (Quanand Baum, 1996).Microperfusion studies of isolated proximal tubuleshave shown that the Ang II effect on proximal tubulesodium transport is bimodal; Ang II at physiologicalconcentrations (picomoles per liter) significantly stimulatesproximal tubule sodium reabsorption, whereaspharmacological micromole per liter concentrations inhibittransport (Harris and Young, 1977; Schuster et al.,1984). Reabsorption of sodium by Ang II in proximaltubules is coupled with bicarbonate reabsorption, whichis mediated by inhibition of adenylate cyclase (Liu andCogan, 1989). Using in vivo microperfusion in theMunich-Wistar rat, Liu and Cogan (1987) showed thatadministration of luminal Ang II increased proximaltubule bicarbonate reabsorption. These findings wereconfirmed using electrophysiological methods in isolatedperfused rabbit renal proximal tubules (Coppola andFromter, 1994a,b). Perfusion of rabbit proximal tubuleswith luminal Ang II after treatment with ACEIs increasedvolume and bicarbonate reabsorption (Baum etal., 1997), supporting the role of intrarenally produced Ang II to stimulate proximal tubule volume and bicarbonatetransport.Molecular mechanisms of direct stimulation of fluidreabsorption by Ang II within the proximal tubule involveincreased transcellular sodium and bicarbonatereabsorption via activation of apical Na/H exchange,basolateral Na-HCO3 cotransport, and basolateralNa/K-ATPase and via insertion of H-ATPase intothe apical membrane (Liu and Cogan, 1988; Garvin,1991; Mitchell et al., 1992; Eiam-Ong et al., 1993; Wangand Giebisch, 1996). Stimulation of Na-HCO3 cotransportby Ang II is mediated by diverse signalingpathways, including activation of the Src family of tyrosinekinase and the classic mitogen-activated proteinkinase pathway (Espiritu et al., 2002; Robey et al.,2002).In view of these observations that the intratubularactivation of the RAS stimulated proximal fluid reabsorption,recent analysis of tissue-specific ACE knockoutmice using micropuncture techniques gave unexpectedresults (Hashimoto et al., 2005). In this unique model,tissue ACE is deleted, but ACE is selectively expressedin the liver (Cole et al., 2003). Whereas disruption ofACE often causes low blood pressure, which complicatesrenal functional studies, this model is able to maintainsufficient plasma levels of ACE and subsequently normalblood pressure. Proximal tubular fluid reabsorptionof these genetically altered mice was comparable withthat observed in wild-type mice despite the essentiallycomplete absence of tissue ACE. These findings are incontrast with the previous findings that an acute reductionin local Ang II formation exerted a profound inhibitoryeffect on fluid reabsorption. The discrepancy maylie in the chronicity of Ang II blockade, and chronic ACEdeficiency is apparently associated with compensatoryevents that normalize fluid reabsorption along the proximaltubule. It is also possible that proximal tubular AngII is formed through alternative pathways not requiringACE.In addition to regulation of fluid and electrolyte balance,Ang II plays an important role in hypertrophy ofproximal tubular cells. In rat proximal tubular epithelialcells, Ang II induces cellular hypertrophy and activatesrelevant downstream signal transduction pathways(Wolf et al., 1993; Hannken et al., 1998, 2000; Guoet al., 2004). The Ang II-induced tubular cell hypertrophyis inhibited by ARBs, suggesting that the AT1 receptorcontributes to the tubular cell hypertrophy (Chatterjeeet al., 1997). Cells undergoing hypertrophy arearrested in the G1 phase of the cell cycle, and p27Kip1,an inhibitor of cyclin-dependent kinases, is required forAng II-induced hypertrophy of proximal tubular cells(Wolf and Stahl, 1996; Terada et al., 1999; Wolf et al.,2001, 2003).Transfection of AT1 receptors into a renal proximaltubular cell line LLCPKcl4, which does not express endogenousAng II receptors, increased protein synthesiswithout DNA synthesis in response to Ang II, as indicatedby increased [3H]leucine incorporation without increasesin [3H]thymidine incorporation (Burns and Harris,1995). The stimulation of protein synthesis and cellhypertrophy without increasing cell number was mediatedby activation of the epidermal growth factor receptor(Chen et al., 2006). Recent studies also demonstratedinvolvement of connective tissue growth factor in mediatingAng II-induced tubular cell hypertrophy (Liu et al.,2006). In cultured proximal tubular cells, Ang II stimulatedthe expression of connective tissue growth factorand increased the total protein content as well as cellsize, which were markedly inhibited by cotreatmentwith an antisense oligonucelotide for connective tissuegrowth factor.With blockade of transforming growth factor- receptor,Ang II-mediated hypertrophy can be converted intocell proliferation. Rats that received Ang II infusion hadan increased number of proliferating cell nuclear antigen-and transferase dUTP nick-end labeling-positivecells in proximal tubules with a possible involvement ofAT2 receptors (Cao et al., 2000), suggesting that Ang IIalso triggers both proliferation and apoptosis in tubularepithelial cells under certain circumstances.A role for Ang II in induction of oxidative stress in thekidney has been extensively studied. Treatment ofWistar-Kyoto rats with subcutaneous Ang II infusionsfrom osmotic minipumps induced oxidative stress in associationwith increased expression of the p22phox componentof NADPH oxidase and decreased expression ofextracellular superoxide dismutase in the renal cortex(Welch et al., 2005). These effects were mediated viaAT1 receptors and were offset by protective effects ofAT2 receptors (Chabrashvili et al., 2003). Measurementof PO2 in the lumen of proximal tubules and distaltubules gave low values, which can be ascribed to inefficientutilization of O2 due to oxidative stress. Therefore,it is likely that Ang II induced oxidative stress inboth proximal and distal tubules.Another function of renal proximal tubule cells regulatedby Ang II is endocytosis of urinary protein components.Ang II at physiological concentrations as low as 1nM increased albumin endocytosis through AT2 receptorslocated on the luminal side and triggered the activationof protein kinase B in a porcine proximal tubularcell line (Caruso-Neves et al., 2005). This report clearlyindicates that Ang II is also involved in the regulation ofendocytosis of urinary protein in renal proximal tubulecells under physiological conditions.2. Distal Tubules. Ang II infusion increased distalfractional sodium reabsorption (Olsen et al., 1985). Intravenousinfusion of Ang II stimulated distal bicarbonatereabsorption during microperfusion experiments(Levine et al., 1994). Ang II also regulated distal bicarbonatereabsorption during modifications of food intakein the rat (Levine et al., 1996). Studies of separate perfusionsof early and late segments of cortical distal tubule showed that Ang II stimulated early distal bicarbonatereabsorption, whereas the late distal effect wasmostly on amiloride-sensitive sodium reabsorption, i.e.,on sodium channels (Wang and Giebisch, 1996). Ang IIacts to stimulate Na/H exchange in both early andlate distal segments via activation of AT1 receptorsand the vacuolar H-ATPase in late distal segments(Barreto-Chaves and Mello-Aires, 1996). Experimentsperformed in distal tubules of nephrectomized rats indicatedthat AT1 receptor blockade caused marked reductionof synthesis and insertion of apical H-ATPase inA-type intercalated cells (Levine et al., 2000). The effectsof Ang II on sodium reabsorption in distal tubular segmentsfurther enhance and amplify the effects in proximaltubules, leading to much greater overall efficiencyof sodium conservation.3. Collecting Ducts. In the proximal and distal tubules,Na serves as a counterion for H secretion. Thus,Ang II augments H secretion and Na absorption inthese tubular segments. Despite a dramatic up-regulationof H secretion in the proximal and distal tubules by AngII, infusion of Ang II does not produce a metabolic alkalosis,suggesting a compensatory regulation of acid secretionin other segments of the nephron. To support this notion,Ang II decreased H secretion in the perfused rat outermedullary collecting ducts (Weiner et al., 1995; Wall et al.,2003). This can be explained by a reduction in H-ATPaseactivity (Tojo et al., 1994; Valles and Manucha, 2000).However, different results regarding the effect of Ang II onH secretion have been reported. Whereas selective aldosteronedeficiency created by adrenalectomy with glucocorticoidreplacement resulted in down-regulation in the expressionof the H-ATPase B1 subunit in medullarycollecting ducts, Ang II increased the expression of the B1subunit of H-ATPase in the medullary collecting ductsand thus may up-regulate H secretion in this tubularsegment of these animals (Valles et al., 2005).Ang II also plays an important role in regulation of thesodium channel in the collecting ducts via a mechanismthat is not dependent on circulating aldosterone. In isolatedperfused rabbit cortical collecting ducts, Ang IIdirectly stimulated apical membrane epithelial sodiumchannel activity (Peti-Peterdi et al., 2002). With low-saltdiets, associated with activation of the RAS, the expressionof the -epithelial sodium channel was markedlydecreased in AT1a receptor knockout mice (Brooks et al.,2002).The inner medullary collecting ducts are responsiblefor the final concentration of the urine. Mice with genedeletion of the AT1a receptor exhibit defects in urinaryconcentrating ability (Oliverio et al., 2000). The effects ofRAS activation in the inner medullary collecting ductsmay be mediated by stimulation of urea transport,which maintains the medullary interstitial osmotic gradient.In rat terminal inner medullary collecting ducts,low concentrations of basolateral Ang II increases vasopressin-stimulated urea permeability and induces phosphorylationof the urea transporter (Kato et al., 2000).These data suggest that Ang II stimulates the urinaryconcentrating mechanism, leading to increased waterreabsorption.In addition to direct effects, Ang II regulates functionof the collecting ducts via aldosterone. Ang IIstimulates the zona glomerulosa of the adrenal cortexto produce the sodium-retaining hormone, aldosterone.Aldosterone stimulates ionic transport in theprincipal cells by increasing the number of open sodiumand potassium channels in the luminal membraneand the activity of Na/K-ATPase pump in thebasolateral membrane. Thus, aldosterone promotessodium chloride reabsorption and potassium secretionin the principal cells of the cortical collecting tubularsegment of the nephron. It further stimulates H secretionin the intercalated cells of the cortex and tubularcells in the outer medulla (Navar et al., 1996;Paul et al., 2006).III. Regulation of Circulating Renin-AngiotensinSystemClassic Renin-Angiotensin SystemPathwaysAng II is produced systemically via the classic RAS.An aspartyl protease, renin, in the plasma is releasedprimarily from the juxtaglomerular cells on the afferentarterioles of the kidney (Hackenthal et al., 1990; Schnermannet al., 1997). Although circulating active renin andprorenin are released mainly from the kidney, othertissues also secrete prorenin into the circulation, andprorenin can be converted to renin by limited proteolysissuch as that with trypsin activation in the circulation(Sealey et al., 1986). Angiotensinogen is primarilyformed and constitutively secreted by hepaticcells into the circulation, thus allowing systemic formationof Ang II throughout the circulation (Brasierand Li, 1996). On release into the circulation, renincleaves angiotensinogen at the N terminus to form thedecapeptide, Ang I (Navar et al., 1997). The circulatingconcentrations of angiotensinogen are abundant,being more than 1000 times greater than the plasmaAng I and Ang II concentrations (Navar and Nishiyama,2001). Although some species variation exists,changes in renin activity thus determine the rate ofAng I formation in the plasma from the huge stores ofcirculating angiotensinogen (Ichihara et al., 2004b;Paul et al., 2006). Figure 1 shows the representativeplasma angiotensinogen concentrations measured inanesthetized rats and expressed as nanomoles perliter; the Ang I and Ang II concentrations are expressedas picomoles per liter, indicating that theactive Ang II concentration in the plasma is a smallfraction of the available Ang II in the form of angiotensinogen.Therefore, even small relative changes inthe rates of Ang I and Ang II generation may makelarge absolute differences in the circulating concentrations. As is well known, renin is synthesized andstored in substantial quantities in the granules ofjuxtaglomerular cells and is released in response tovarious stimuli (Schweda and Kurtz, 2004; Paul et al.,2006). Thus, large changes in plasma renin levels canoccur rapidly, leading to changes in Ang I generation.The concentrations of angiotensinogen in the plasmaare close to the Michaelis-Menten constant of the proteolyticactivity of renin such that changes in substrateconcentrations can also influence the Ang Igeneration rate; however, changes in angiotensinogensynthesis occur slowly and thus are less responsiblefor the dynamic regulation of plasma Ang I and Ang IIthan renin (Deschepper, 1994; Brasier and Li, 1996).Ang I is easily converted to Ang II, due not only to thecirculating dipeptidyl carboxypeptidase, ACE, butalso to the widespread presence of ACE on endothelialcells of many vascular beds including the lung (Navaret al., 1997; Ichihara et al., 2004b; Paul et al., 2006).Although other pathways for Ang II formation havebeen identified in certain tissues (Fig. 1), the circulatinglevels of Ang II reflect primarily the consequencesof the renin and ACE enzymatic cascade on angiotensinogen(Erdos, 1990; Johnston, 1994). The resultantincreases in plasma Ang II exert powerful actionsthroughout the body through activation of AT1 receptors(Timmermans et al., 1993; Paul et al., 2006).Several angiotensinases and peptidases are then ableto metabolize Ang II further (Reudelhuber, 2005). It isrecognized that several of the smaller peptides, includingAng III, Ang IV, and Ang 1-7, have biologicalactivity, but their plasma levels (except for Ang1-7) are much lower than those of Ang II (Haulica et al.,2005; Pendergrass et al., 2006).IV. Mechanisms Responsible for IndependentRegulation of Intrarenal Renin-AngiotensinSystemThe RAS has been acknowledged as an endocrine,paracrine, autocrine, and intracrine system (Navar etal., 2002; Re, 2003; Kobori et al., 2006; Re and Cook,2006; Suzaki et al., 2006b), and, thus, it has been difficultto delineate the quantitative contributions of systemicallydelivered versus locally formed Ang peptidesto the levels existing in any given tissue. Emergingevidence suggests that local formation is of major significancein the regulation of the Ang levels in many organsand tissues. For example, there is substantial evidencethat the Ang peptide levels in the brain are regulated inan autonomous manner (Baltatu et al., 2000). Althoughevery organ system in the body has elements of the RAS,the kidney is unique in having every component of theRAS with compartmentalization in the tubular and interstitialnetworks as well as intracellular accumulation.Recent attention has been focused on the existenceof unique RASs in various organ systems. Various studieshave demonstrated the importance of the tissue RASin the brain, heart, adrenal glands, and vasculature aswell as in the kidney (Mitchell and Navar, 1995; Navaret al., 2006). In this regard, the kidneys, as well as theadrenal glands, are unique in terms of the tissue concentrationsof Ang II, which are much greater than canbe explained by the concentrations delivered by the arterialblood flow (Ingert et al., 2002a). There is substantialevidence that the major fraction of Ang II present inrenal tissues is generated locally from angiotensinogendelivered to the kidney as well as from angiotensinogenlocally produced by proximal tubule cells. Ang I deliveredto the kidney can also be converted to Ang II (Rosivalland Navar, 1983; Komlosi et al., 2003). Reninsecreted by the juxtaglomerular apparatus cells and deliveredto the renal interstitium and vascular compartmentalso provides a pathway for the local generation ofAng I (Hackenthal et al., 1990; Schnermann et al.,1997). ACE is abundant in the rat kidney and has beenlocated in the proximal and distal tubules, the collectingducts, and renal endothelial cells (Casarini et al., 1997).Therefore, all of the components necessary to generateintrarenal Ang II are present along the nephron.A. AngiotensinogenAlthough most of the circulating angiotensinogen isproduced and secreted by the liver, the kidneys alsoproduce angiotensinogen (Kobori et al., 2006). Intrarenalangiotensinogen mRNA and protein have been localizedto proximal tubule cells, indicating that the intratubularAng II could be derived from locally formed andsecreted angiotensinogen (Darby and Sernia, 1995). Theangiotensinogen produced in proximal tubule cellsseems to be secreted directly into the tubular lumen inaddition to producing its metabolites intracellularly andsecreting them into the tubule lumen (Lantelme et al.,2002). Proximal tubule angiotensinogen concentrationsin anesthetized rats have been reported in the range of300 to 600 nM, which greatly exceed the free Ang I andAng II tubular fluid concentrations (Navar et al., 2001).Because of its molecular size, it seems unlikely thatmuch of the plasma angiotensinogen filters across theglomerular membrane, further supporting the conceptthat proximal tubule cells secrete angiotensinogen directlyinto the tubule (Rohrwasser et al., 1999). To determinewhether circulating angiotensinogen is a sourceof urinary angiotensinogen, Kobori et al. (2003b) infusedhuman angiotensinogen into normotensive rats; however,circulating angiotensinogen was not detectable inthe urine. The failure to detect human angiotensinogenin the urine indicates limited glomerular permeabilityand/or tubular degradation. These findings support thehypothesis that urinary angiotensinogen originatesfrom the angiotensinogen that is formed and secreted bythe proximal tubules and not from plasma in rats (Koboriet al., 2003b). Formation of Ang I and Ang II in thetubular lumen subsequent to angiotensinogen secretionmay be possible because some renin is filtered and/orsecreted from juxtaglomerular apparatus cells. Theidentification of renin in distal nephron segments mayalso provide a possible pathway for Ang I generationfrom proximally delivered angiotensinogen. Intact angiotensinogenin urine indicates its presence throughoutthe nephron and, to the extent that renin and ACE areavailable along the nephron, substrate availability supportscontinued Ang I generation and Ang II conversionin distal segments (Ding et al., 1997; Davisson et al.,1999). Once Ang I is formed, conversion readily occursbecause there are abundant amounts of ACE associatedwith the proximal tubule brush border. Casarini et al.(1997) found that ACE activity is present in tubularfluid throughout the nephron except in the late distaltubule. They demonstrated that the ACE activity ishigher at the initial portion of the proximal tubule butthen decreases to the distal nephron and increases againin the urine. This evidence suggests proximal ACE secretion,degradation, and/or reabsorption associatedwith secretion in the collecting ducts. Therefore, intratubularAng II formation may occur not only in theproximal tubule but also beyond the connecting tubule(Fig. 2). Thus, renal tissue ACE activity is critical tomaintain the steady-state Ang II levels in the kidney.Indeed, Modrall et al. (2004) demonstrated that knockoutmice that do not exhibit bound tissue ACE in thekidney have 80% lower intrarenal Ang II levels comparedwith wild-type mice. In addition to the markedreduction of intrarenal Ang II levels, this tissue ACEknockout mouse showed significant depletion of its immediateprecursor Ang I in renal tissue, which supports the concept that Ang II exerts a positive feedback loop onproximal angiotensinogen (Ingelfinger et al., 1999; Koboriet al., 2001a,b, 2002).The proximally formed angiotensinogen that is secretedinto the tubular fluid flows into the distalnephron, allowing intraluminal Ang II formation to continuethroughout the length of the nephron with theresidual angiotensinogen appearing in the urine (Dinget al., 1997; Rohrwasser et al., 1999). Ding et al. (1997)demonstrated in mice harboring the gene for humanangiotensinogen fused to the kidney-specific androgenregulatedprotein promoter that human angiotensinogenwas localized primarily to proximal tubule cells (seesection V.A.1.c.). They found abundant human angiotensinogenin the urine but only slight traces in thesystemic circulation. This finding suggests that most ofthe angiotensinogen formed in proximal tubule cells isdestined for secretion into the lumen. Rohrwasser et al.(1999) demonstrated luminal localization of angiotensinogenin proximal tubular cells in vivo and showed,in monolayer proximal tubule cell cultures, that most ofthe angiotensinogen was detected near the apical membrane.They also reported that angiotensinogen was detectedat low nanomoles per liter concentrations in urinefrom mice and human volunteers. Kobori et al. (2002)evaluated the changes in urinary angiotensinogen excretionrates in Ang II-infused rats maintained on high-saltdiets to suppress basal levels and observed an approximately4-fold increase with Ang II infusion (80 ng/min)in urinary angiotensinogen excretion rates. Angiotensinogenwas measured using both Western blot analysisand radioimmunoassay determination of generatedAng I after incubation with excess renin, thus demonstratingthe fact that urinary angiotensinogen containedintact active angiotensinogen. They extended these resultsfurther to show that chronic Ang II infusions tonormal rats significantly increased the urinary excretionrate of angiotensinogen in a time- and dose-dependentmanner that was associated with elevations in systolicblood pressure and kidney Ang II levels but notwith plasma Ang II concentrations (Kobori et al., 2003b).To determine whether the increase in urinary angiotensinogenexcretion was simply a nonspecific consequenceof the proteinuria and hypertension, furtherstudies were done in rats made hypertensive with DOCAsalt plus a high-salt diet. Although urinary protein excretionin DOCA salt-induced volume-dependent hypertensiverats was increased to the same or to a greaterextent, urinary angiotensinogen was significantly lowerin volume-dependent hypertensive rats than in Ang IIdependenthypertensive rats and was not greater thanin control rats. This study also demonstrated that therewas a significant relationship between urinary angiotensinogenand kidney Ang II content in rats given differentdoses of Ang II to achieve different levels of hypertension.These results provide further evidence thaturinary angiotensinogen may be a useful index of intrarenalAng II activity (Kobori et al., 2002, 2003b, 2004)(Fig. 2). Recently, two independent groups have developedan enzyme-linked immunosorbent assay system tomeasure angiotensinogen directly (Lantelme et al.,2005; Suzaki et al., 2006a). Outcomes of clinical studiesare expected in the near future.B. Renin and ProreninStrictly speaking, renin is not a hormone; however, itcan be considered as such because of its role in determiningAng I generation and because it is subject totight control. Hence, the plasma renin concentration oractivity is often used as a measure of the overall activityof the RAS. In most species, renin synthesized by thejuxtaglomerular apparatus cells is the primary source ofboth circulating and intrarenal renin levels. However,some strains of mice also produce substantial amountsof renin in the submandibular and submaxillary glandsas an expression of the duplicated renin gene, Ren2(Catanzaro et al., 1983).The secreted active form of renin contains 339 to 343amino acid residues after proteolytic removal of the 43-amino acid residue at the N terminus of prorenin. Circulatingactive renin and prorenin are released mainlyfrom the kidney, but other tissues also secrete prorenininto the circulation (Sealey et al., 1986). Besides servingas the precursor for active renin, it has been suggestedthat circulating prorenin is taken up by some tissueswhere it may contribute to the local synthesis of Angpeptides (Prescott et al., 2002). In the heart under normalconditions, renin is not produced and its transcriptis undetectable or extremely low (Ekker et al., 1989).Nevertheless, transgenic rats expressing the Ren2 reningene exhibit high circulating prorenin levels in the absenceof the cardiac transgene, prorenin internalizationinto cardiomyocytes with generation of Ang, and cardiacdamage (Peters et al., 2002). These effects suggest thatuptake of circulating prorenin but not active renin mayplay an important role in cardiac hypertrophy.