The renin-angiotensin system (RAS) is well recognized as one of the oldest and most important regulators of arterial blood pressure, cardiovascular and renal function. New frontiers have recently emerged in the RAS research well beyond its classic paradigm as a potent vasoconstrictor, an aldosterone release stimulator, or a sodium-retaining hormone. First, two new members of the RAS have been uncovered, which include the renin/(Pro)renin receptor (PRR) and angiotensin-converting enzyme 2 (ACE2). Recent studies suggest that prorenin may act on the PRR independent of the classical ACE/ANG II/AT1 receptor axis, whereas ACE2 may degrade ANG II to generate ANG (1-7), which activates the Mas receptor. Second, there is increasing evidence that ANG II may function as an intracellular peptide to activate intracellular and/or nuclear receptors. Third, currently there is a debate on the relative contribution of systemic versus intrarenal RAS to the physiological regulation of blood pressure and the development of hypertension. The objectives of this article are to review and discuss the new insights and perspectives derived from recent studies using novel transgenic mice that either overexpress or are deficient of one key enzyme, ANG peptide, or receptor of the RAS. This information may help us better understand how ANG II acts, both independently or through interactions with other members of the system, to regulate the kidney function and blood pressure in health and disease.

ANG II is the most potent and important member of the classical renin-angiotensin system (RAS). ANG II, once considered to be an endocrine hormone, is now increasingly recognized to also play novel and important paracrine (cell-to-cell) and intracrine (intracellular) roles in cardiovascular and renal physiology and blood pressure regulation. Although an intracrine role of ANG II remains an issue of continuous debates and requires further confirmation, a great deal of research has recently been devoted to uncover the novel actions and elucidate underlying signaling mechanisms of the so-called intracellular ANG II in cardiovascular, neural, and renal systems. The purpose of this article is to provide a comprehensive review of the intracellular actions of ANG II, either administered directly into the cells or expressed as an intracellularly functional fusion protein, and its effects throughout a variety of target tissues susceptible to the impacts of an overactive ANG II, with a particular focus on the proximal tubules of the kidney. While continuously reaffirming the roles of extracellular or circulating ANG II in the proximal tubules, our review will focus on recent evidence obtained for the novel biological roles of intracellular ANG II in cultured proximal tubule cells in vitro and the potential physiological roles of intracellular ANG II in the regulation of proximal tubular reabsorption and blood pressure in rats and mice. It is our hope that the new knowledge on the roles of intracellular ANG II in proximal tubules will serve as a catalyst to stimulate further studies and debates in the field and to help us better understand how extracellular and intracellular ANG II acts independently or interacts with each other, to regulate proximal tubular transport and blood pressure in both physiological and diseased states.

The present study directly tested the hypothesis that the NHE3 (Na ⁺ /H ⁺ exchanger 3) in the proximal tubules of the kidney is required for the development of Ang II (angiotensin II)-induced hypertension using PT- Nhe3 ^−/− (proximal tubule-specific NHE3 knockout) mice. Specifically, PT- Nhe3 ^−/− mice were generated using the SGLT2-Cre / Nhe3 ^loxlox approach, whereas Ang II-induced hypertension was studied in 12 groups (n=5–12 per group) of adult male and female wild-type (WT) and PT- Nhe3 ^−/− mice. Under basal conditions, systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure were significantly lower in male and female PT- Nhe3 ^−/− than WT mice ( P <0.01). A high pressor, 1.5 mg/kg per day, intraperitoneal or a slow pressor dose of Ang II, 0.5 mg/kg per day, intraperitoneal for 2 weeks significantly increased systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure in male and female WT mice ( P <0.01), but the hypertensive response to Ang II was markedly attenuated in male and female PT- Nhe3 ^−/− mice ( P <0.01). Ang II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in Ang II-infused PT- Nhe3 ^−/− mice ( P <0.01). AT ₁ receptor blocker losartan completely blocked Ang II-induced hypertension in both WT and PT- Nhe3 ^−/− mice ( P <0.01). However, inhibition of nitric oxide synthase with L-N ^G -Nitroarginine methyl ester had no effect on Ang II-induced hypertension in WT or PT- Nhe3 ^−/− mice (not significant). Furthermore, Ang II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney may be a therapeutical target in hypertension induced by Ang II or with increased NHE3 expression in the proximal tubules.