Angiotensin II receptor subtypes and phosphoinositide hydrolysis in rat adrenal medulla

Abstract

Angiotensin 11 (ANG) receptor subtypes were characterized by quantitative autoradiography after incubation with the ANG agonist [ 125I]Sar 1-ANG in rat adrenal medulla. ANG receptors are highly localized in adrenal medulla. Specific binding was displaced by 4% and by 95% with the AT 1 receptor blocker losartan and the AT 2 receptor competitor CGP 42112A, respectively. Analysis of competition curves indicated relative binding potencies for the AT2 population of CGP 42112A>PD 123319> PD 123177. ANG stimulated inositol phosphate formation in a dose-dependent manner in rat adrenal medulla. Losartan at concentrations of 10 −9 to 10 −5 M antagonized the effect of ANG, whereas PD 123177 or PD 123319 had no antagonistic action. However, at a higher concentration (10 −5 M) PD 123177 or PD 123319 potentiated the effect of ANG on InsP 1-accumulation. In the presence of PD 123319 (10 −5 M) ANG dose-response curve was shifted to the left with no change in the maximal effect. This affect was blocked by the addition of losartan (10 −5 M). On the contrary, the addition of CGP 42112A (10 −5 M) inhibited ANG-induced increase in InsP i-accumulation. On the other hand, ANG and CGP 42112A reduced basal cyclic GMP formation, this effect was partially reverted by sodium orthovanadate, a phosphotyrosine phosphatase inhibitor. Our results further demonstrate the presence of two ANG receptor subtypes in adrenal medulla: ANG binding to AT 1 receptor stimulates inositol phospholipid metabolism, whereas ANG binding to AT 2 receptors decreases both inositol phosphate production and cGMP formation.