Intrarenal Angiotensin III Is the Predominant Agonist for Proximal Tubule Angiotensin Type 2 Receptors

In angiotensin type 1 receptor–blocked rats, renal interstitial (RI) administration of des-aspartyl 1 -angiotensin II (Ang III) but not angiotensin II induces natriuresis via activation of angiotensin type 2 receptors. In the present study, renal function was documented during systemic angiotensin type 1 receptor blockade with candesartan in Sprague-Dawley rats receiving unilateral RI infusion of Ang III. Ang III increased urine sodium excretion, fractional sodium, and lithium excretion. RI coinfusion of specific angiotensin type 2 receptor antagonist PD-123319 abolished Ang III–induced natriuresis. The natriuretic response observed with RI Ang III was not reproducible with RI angiotensin (1-7) alone or together with angiotensin-converting enzyme inhibition. Similarly, neither RI angiotensin II alone or in the presence of aminopeptidase A inhibitor increased urine sodium excretion. In the absence of systemic angiotensin type 1 receptor blockade, Ang III alone did not increase urine sodium excretion, but natriuresis was enabled by the coinfusion of aminopeptidase N inhibitor and subsequently blocked by PD-123319. In angiotensin type 1 receptor–blocked rats, RI administration of aminopeptidase N inhibitor alone also induced natriuresis that was abolished by PD-123319. Ang III–induced natriuresis was accompanied by increased RI cGMP levels and was abolished by inhibition of soluble guanylyl cyclase. RI and renal tissue Ang III levels increased in response to Ang III infusion and were augmented by aminopeptidase N inhibition. These data demonstrate that endogenous intrarenal Ang III but not angiotensin II or angiotensin (1-7) induces natriuresis via activation of angiotensin type 2 receptors in the proximal tubule via a cGMP–dependent mechanism and suggest aminopeptidase N inhibition as a potential therapeutic target in hypertension.