Subclasses of angiotensin II binding sites and their functional significance

Specific binding sites for angiotensin II were identified in the rabbit adrenal gland and uterus and in the rat liver and were divided into two subclasses based on inhibition by nonpeptide ligands. Peptide ligands affected binding equally in all three tissues. However, the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II binding to adrenal and liver homogenates at nanomolar concentrations but exerted only a minimal effect on binding to uterine homogenates. The nonpeptide PD 123319 potently blocked angiotensin II binding to uterine homogenates but had no effect on adrenal or liver homogenates at concentrations up to 10 microM. Further analysis of angiotensin II binding in uterus showed that both sites are present, with the PD 123319-sensitive site predominating. Additionally, the nonhydrolyzable GTP analogue 5'-guanylyl-imidodiphosphate was able to modulate binding to liver and to the Dup 753-sensitive site in uterus but not that to the PD 123319-sensitive site. Saralasin and the nonpeptide antagonists Dup 753 and Exp 6803 blocked angiotensin II-stimulated accumulation of inositol phosphates in cultured Clone 9 cells and also relaxed aortic rings previously contracted with angiotensin II. In contrast, PD 123319 had no effect on either angiotensin II-stimulated inositol phosphate accumulation or vasoconstriction. Saralasin and Exp 6803, but not PD 123319, lowered blood pressure in renal hypertensive rats following intravenous administration. These results suggest the existence of two subclasses of angiotensin II binding sites, which differ in their tissue distribution and affinity for the nonpeptide ligands Dup 763, Exp 6803, and PD 123319. Although no functional role for the PD 123319-sensitive subclass has yet been identified, the Dup 753/Exp 6803-sensitive subclass plays an important role in mediating inositol phosphate metabolism, vascular contractile activity, and blood pressure regulation.