Intrarenal Aminopeptidase N Inhibition Restores Defective Angiotensin II Type 2-Mediated Natriuresis in Spontaneously Hypertensive Rats

Abstract

Abstract-The preferred ligand of angiotensin (Ang) II type 2 (AT 2 R)-mediated natriuresis is Ang III. The major enzyme responsible for the metabolism of Ang III is aminopeptidase N, which is selectively inhibited by compound PC-18. In this study, urine sodium excretion rates (U Na V), fractional excretion of sodium, fractional excretion of lithium, glomerular filtration rate, and mean arterial pressures were studied in prehypertensive and hypertensive spontaneously hypertensive rats (SHRs) and compared with age-matched Wistar-Kyoto rats (WKYs Key Words: natriuresis Ⅲ angiotensin receptors Ⅲ hypertension Ⅲ angiotensin III Ⅲ aminopeptidase N Ⅲ SHR S pontaneously hypertensive rats (SHRs) develop hypertension at Ϸ6 weeks of age and are widely used as a model to study the development and maintenance of human genetic hypertension. 1 One of the proposed mechanisms of the initiation of hypertension in SHRs involves a primary defect in renal sodium (Na ϩ ) excretion. 2-8 Over time, this defect necessitates an increase in renal perfusion pressure, an adaptation that becomes central to the development and maintenance of hypertension. 9,10 In normal rodents, both the intrarenal renin-angiotensin (Ang) system and the renal dopaminergic system play important roles in renal proximal tubule Na ϩ handling. Basal Na ϩ excretion rates are generally determined by the activity of the intrarenal renin-Ang system, whereas the dopaminergic system regulates Na ϩ excretion in response to high-salt intake. Although an acute sodium load or rise in blood pressure increases sodium excretion in normal rodents, the following 3 significant pharmacological manipulations also induce natriuresis: (1) blockade of intrarenal Ang II type 1 receptors (AT 1 Rs); (2) stimulation of renal dopamine D 1 -like receptors; and (3) activation of intrarenal Ang II type 2 receptors (AT 2 Rs). Recent studies have shown that natriuresis, resulting from both AT 1 R blockade and D 1 -like receptor stimulation, are mediated, at least in part, by AT 2 R activation, because concomitant blockade of renal AT 2 Rs in these situations abolishes the natriuresis. 11,12 Regarding direct renal AT 2 R-induced natriuresis, renal interstitial (RI) Ang III, but not Ang II, results in increased Na ϩ excretion when AT 1 Rs are blocked systemically. 11 This effect is also abolished by concomitant infusion of a selective AT 2 R antagonist, highlighting the important direct role of renal AT 2 R activation by Ang III in natriuresis. 11 In the kidney, aminopeptidase A (APA), an enzyme normally expressed on the brush border of renal proximal tubule

AT 2 Receptor Activation Prevents Sodium Retention and Reduces Blood Pressure in Angiotensin II–Dependent Hypertension

“…Fenoldopam translocates AT 2 Rs to the apical plasma membranes of RPTCs via a cAMP- and protein phosphatase 2A-dependent signaling pathway (19). SHR fail to develop either AT 2 R recruitment or natriuretic responses to Ang III and both can be restored by increasing Ang III levels via blockade of aminopeptidase N. Together with the present observations, the evidence indicates that AT 2 R recruitment to the apical plasma membranes of RPTCs is likely a common mechanism initiating and sustaining long term natriuretic responses to endogenous AT 2 R agonists dopamine and Ang III, as well as exogenous agonist C-21 (19–21). Because AT 2 Rs do not internalize in renal epithelial cells (22,23), their recruitment and continuous apical plasma membrane expression may stabilize and reinforce the natriuretic response.…”

AT 2 Receptor Activation Induces Natriuresis and Lowers Blood Pressure

“…In addition, dopamine D 1 receptor (D 1 R) activation with fenoldopam induces natriuresis in an AT 2 R-dependent fashion and translocates AT 2 Rs to the apical plasma membranes of RPTCs via cyclic AMP and protein phosphatase 2A signaling mechanisms (32). Interestingly, hypertensive 12-wk-old spontaneously hypertensive rats (SHR) fail to recruit AT 2 Rs or mount natriuretic responses to Ang III, but both receptor translocation and natriuresis can be restored by inhibition of aminopeptidase N, increasing Ang III formation (20, 31). Combined with our previous findings, the present results suggest that AT 2 R recruitment to the apical plasma membranes of RPTCs may be a critical common mechanism initiating and/or supporting sustained natriuretic responses to dopamine, Ang III, and C-21.…”

“…An open bore micro-infusion catheter (PE-10) was inserted under the renal capsule into the cortex of the left kidney to ensure renal interstitial (RI) infusion of vehicle D 5 W or pharmacological agent at 2.5 μL/min with a syringe pump (Harvard; model 55-222) as reported previously (14-17, 20). When two agents were infused, separate catheters were employed.…”

Nonclassical Renin‐Angiotensin System and Renal Function

“…In contrast, aminopeptidase N (AN) has preference for basic residues such as the N-terminal arginine and converts Ang III to Ang-(3-8) or Ang IV (78; 102). Although Ang III was shown to stimulate an AT 1 R-dependent increase of Ca ++ in various tubular elements of the rat kidney (69), Carey and colleagues have presented additional findings that Ang III may be the endogenous ligand for the AT 2 receptor within the rat kidney (99-101). Further N-terminal processing of Ang III by AN or direct conversion of Ang II by dipeptidyl aminopeptidase IV (DAP IV) yields another vasoactive peptide Ang-(3-8) (Ang IV) that convey biological actions not through a classical G-protein coupled receptor but by interacting with the insulin-regulated aminopeptidase (IRAP) to stimulate NO (6; 76).…”