The renin-angiotensin system (RAS) is undisputedly one of the most prominent endocrine (tissue-to-tissue), paracrine (cell-to-cell) and intracrine (intracellular/nuclear) vasoactive systems in the physiological regulation of neural, cardiovascular, blood pressure, and kidney function. The importance of the RAS in the development and pathogenesis of cardiovascular, hypertensive and kidney diseases has now been firmly established in clinical trials and practice using renin inhibitors, angiotensin-converting enzyme (ACE) inhibitors, type 1 (AT1) angiotensin II (ANG II) receptor blockers (ARBs), or aldosterone receptor antagonists as major therapeutic drugs. The major mechanisms of actions for these RAS inhibitors or receptor blockers are mediated primarily by blocking the detrimental effects of the classic angiotensinogen/renin/ACE/ANG II/AT1/aldosterone axis. However, the RAS has expanded from this classic axis to include several other complex biochemical and physiological axes, which are derived from the metabolism of this classic axis. Currently, at least five axes of the RAS have been described, with each having its key substrate, enzyme, effector peptide, receptor, and/or downstream signaling pathways. These include the classic angiotensinogen/renin/ACE/ANG II/AT1 receptor, the ANG II/APA/ANG III/AT2/NO/cGMP, the ANG I/ANG II/ACE2/ANG (1–7)/Mas receptor, the prorenin/renin/prorenin receptor (PRR or Atp6ap2)/MAP kinases ERK1/2/V-ATPase, and the ANG III/APN/ANG IV/IRAP/AT4 receptor axes. Since the roles and therapeutic implications of the classic angiotensinogen/renin/ACE/ANG II/AT1 receptor axis have been extensively reviewed, this article will focus primarily on reviewing the roles and therapeutic implications of the vasoprotective axes of the RAS in cardiovascular, hypertensive and kidney diseases.

Abstract-Hypertension induced by long-term infusion of angiotensin II (Ang II) is associated with augmented intrarenalAng II levels to a greater extent than can be explained on the basis of the circulating Ang II levels. Although part of this augmentation is due to AT 1 receptor-dependent internalization, the intracellular compartments involved in this Ang II accumulation remain unknown. In the present study, we sought to determine whether Ang II trafficking into renal cortical endosomes is increased during Ang II hypertension, and if so, whether the AT 1 receptor antagonist, candesartan, prevents this accumulation. Compared with controls (nϭ12; 114Ϯ2 mm Hg), Ang II-infused rats (nϭ12; 80 ng/kg/min, SC, for 13 days) developed hypertension with systolic blood pressure rising to 185Ϯ4 mm Hg by Day 12. In Ang II hypertensive rats, plasma renin activity was suppressed, whereas plasma and kidney Ang II levels were increased by 3-fold (348Ϯ58 versus 119Ϯ16 fmol/mL) and 2-fold (399Ϯ39 versus 186Ϯ26 fmol/g). Intracellular endosomal Ang II levels were increased by more than 10-fold (1100Ϯ283 versus 71Ϯ12 fmol/mg protein), whereas intermicrovillar cleft Ang II levels were increased by more than 2-fold (88Ϯ22 versus 37Ϯ7 fmol/mg protein T he importance of angiotensin II (Ang II) in the development and maintenance of hypertension is well documented in several animal models of experimental hypertension, such as two-kidney, one-clip renal hypertension (2K1C), the Ren-2 gene transgenic rats, and the Ang IIinfused model. [1][2][3][4] The Ang II-dependent increases in arterial blood pressure are commonly associated with higher levels of circulating and intrarenal Ang II levels, structural abnormalities, and functional derangements in the kidney. [1][2][3][4][5][6] One important feature in the renin-angiotensin system profile observed in these rats is that renal Ang II levels are greater than can be explained on the basis of circulating Ang II and suppressed renin expression. 2,4,[7][8][9] This suggests that angiotensin peptides continue to be generated intrarenally via a renin-independent pathway, or that circulating Ang II accumulates in one or more compartments within the kidney. 4,9 Previous studies have shown that intrarenal Ang II levels are increased in the contralateral nonclip kidney of 2K1C hypertensive rats and in kidneys of Ang II-infused rats and Ren-2 transgenic rats. 2,4,[7][8][9] Because blockade of the AT 1 receptor with losartan normalizes blood pressure and prevents augmentation of intrarenal Ang II levels, this enhanced uptake of Ang II within the kidney appears to be mediated by the AT 1 receptor. 9,10 These results suggest that Ang II levels are augmented in intracellular compartments by an AT 1 receptormediated internalization mechanism and are protected from degradation to some extent.Recent studies have localized angiotensin peptides in renal endosomes and intermicrovillar clefts, thus implicating them as potential sites for intracellular accumulation in the kid-