The Adrenal Capsule Alters the Response of Zona Glomerulosa Cells to Atrial Natriuretic Peptide*

Atrial natriuretic peptide (ANP) is a potent inhibitor of potassium-stimulated aldosterone secretion. In the present study, we observed rat alpha ANP to inhibit aldosterone secretion stimulated by 10 mM potassium with an IC50 of 0.15 +/- 0.02 nM (mean +/- SE) in dispersed rat adrenal glomerulosa cells. However, when rat adrenal capsules, which contain the zona glomerulosa, were superfused in vitro, ANP had no effect on aldosterone secretion. Superfusion with 10 mM potassium increased aldosterone secretion 3- to 4-fold above baseline. Addition of 10 nM ANP to the superfusate did not lower potassium-stimulated aldosterone secretion. When this same ANP-containing superfusate was incubated with dispersed adrenal glomerulosa cells, potassium-stimulated aldosterone secretion was inhibited by 90%, proving sustained biological potency of the superfused ANP. Incubation of [125I]iodo-ANP with adrenal capsules for 60 min resulted in 83% degradation of [125I]iodo-ANP, whereas no detectable degradation was observed with dispersed adrenal glomerulosa cells. Removal of blood from the adrenal capsules or culturing the capsules for 48 h did not render them responsive to superfused ANP. In contrast, superfusion of 0.1 mM cycloheximide inhibited potassium-stimulated aldosterone secretion by 90%. These results suggest that the adrenal capsule contains an ANP-degrading enzyme(s). This enzyme may be produced by adrenal glomerulosa cells. The local existence of a degrading enzyme for ANP may allow the zone glomerulosa to regulate its response to ANP.