Autoradiographic Identification of Kidney Angiotensin IV Binding Sites and Angiotensin IV-induced Renal Cortical Blood Flow Changes in Rats

The present investigation initially determined that specific binding sites for the hexapeptide angiotensin IV (AngIV) are present in the rat kidney cortex and outer medulla but not in the inner medulla, using in vitro autoradiographic techniques. This binding site has been termed AT4, is distinct from the previously characterized AT1 and AT2 sites, and does not bind the specific AT1 receptor antagonist DuP753 or the AT2 receptor antagonist PD123177. Renal artery infusions of AngIV produced a dose-dependent increase in cortical blood flow without altering systemic blood pressure. In contrast, the infusion of angiotensin II (AngII) induced a dramatic decrease in cortical blood flow, accompanied by a significant elevation in systemic blood pressure. The infusion of [D-Val(1)]AngIV, an analog that does not bind at the AT4 receptor site, and the C-terminal truncated analogs AngIV (1-4) and AngIV (1-5) that possess lower affinity for this site, produced no change in cortical blood flow. The infusion of [Nle1]AngIV and [Lys1]AngIV, analogs that bind with high affinity at the AT4 receptor site, produced increases in cortical blood flow with no influence on blood pressure. Pretreatment with a specific AT4 receptor antagonist, Divalinal-AngIV, completely blocked AngIV-induced elevations in blood flow, but failed to influence AngII-induced decreases in blood flow, suggesting that these ligands are acting at different receptor sites. Pretreatment with the nitric oxide synthase inhibitor, NG-Monomethyl-L-Arginine, also blocked subsequent AngIV-induced increases in cortical blood flow. These data support the notion that AngIV exerts a unique influence upon renal hemodynamics via the AT4 receptor subtype, and suggest that AngIV-induced elevations in blood flow may be mediated by nitric oxide.