Effect of ANP on sustained aldosterone secretion stimulated by angiotensin II

The sustained aldosterone secretory response to angiotensin II (ANG II) depends on receptor-mediated increases in membrane diglyceride (DG) and an increase in calcium influx rate. These signals serve to activate membrane-associated protein kinase C (PKC) and result in enhanced phosphorylation of a unique set of proteins. These events can be mimicked by the addition of a phorbol ester, 12-O-tetra decanoyl phorbol 13-acetate (TPA), and a calcium ionophore, A23187, that bypass the initial receptor-associated events. We studied the inhibitory action of atrial natriuretic peptide (4-28 hANP) on the sustained secretory response to ANG II in isolated bovine adrenal glomerulosa cells. Although 10 nM ANP inhibited aldosterone secretion, it did not significantly alter the ANG II-elicited rise in ⁴⁵Ca²⁺ influx rate [control (CON): 0.44 ± 0.06; ANG II: 1.11 ± 0.12 (P<0.001); ANG II + ANP: 1.18 ± 0.14], the steady-state level of aequorin luminescence [intracellular [Ca²⁺] ([Ca²⁺](i))], or the rise in cellular DG content [CON: 0.132 ± 0.01; ANG II: 0.194 ± 0.01 (P<0.005); ANG II + ANP: 0.202 ± 0.01 nmol/10⁶ cells]. In addition, ANP was able to inhibit aldosterone secretion stimulated by the combined addition of A23187 + TPA. When protein phosphorylation in the ANP-inhibited cells was evaluated, ANG II-induced protein phosphorylation events were preserved. In contrast to the effect of ANP, the calcium channel blocker nitrendipine abolished the ANG II-induced rise in ⁴⁵Ca²⁺ influx rate, reduced the steady-state level of [Ca²⁺](i), and returned the phosphoproteins to their control states. Taken together, these data suggest that the mechanism of ANP inhibition is not via an effect on ANG II-elicited second messenger generation and probably lies beyond the activation of PKC.