NITRIC-OXIDE INHIBITS ALDOSTERONE SYNTHESIS BY A GUANYLYL CYCLASE-INDEPENDENT EFFECT

To investigate the mechanism of nitric oxide (NO) inhibition of aldost erone release, this study compared the effects of type A natriuretic p eptide and heat-stable enterotoxin to a nitric oxide donor, deta nonoa te, on cGMP production and angiotensin II-stimulated aldosterone synth esis in primary cultures of bovine adrenal zona glomerulosa cells. Typ e A natriuretic peptide (10(-10)-10(-6) M) and deta nonoate (10(-6)-10 (-3) M) stimulated concentration-related increases in cGMP production. Heat-stable enterotoxin (10(-6) hi) failed to stimulate cGMP synthesi s in zona glomerulosa cells. Type A natriuretic peptide and deta nonoa te attenuated angiotensin II-stimulated aldosterone production over th e same concentration range that stimulated cGMP production. Heat-stabl e enterotoxin (10(-6) M) was without effect on aldosterone release. To further test the hypothesis that cGMP mediated the inhibition of aldo sterone synthesis, the selective inhibitor of soluble guanylyl cyclase , 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) was used. ODQ pret reatment (10(-5) M) completely prevented deta nonoate-stimulated cGMP production without altering the inhibitory effect of deta nonoate on a ngiotensin II-stimulated steroidogenesis. Consistent with its selectiv ity for inhibiting soluble guanylyl cyclase, ODQ did not block type A natriuretic peptide-stimulated cGMP synthesis or type A natriuretic pe ptide inhibition of steroidogenesis. Deta nonoate completely blocked 2 5-hydroxycholesterol- and progesterone-stimulated aldosterone synthesi s in zona glomerulosa cells and inhibited the conversion of 25-hydroxy cholesterol to pregnenolone in mitochondrial fractions from bovine adr enal cortex. Deta nonoate-derived NO gave an absorbance maximum of the mitochondrial cytochrome P450 of 453 nm and inhibited the absorbance at 450 nm caused by carbon monoxide binding to the enzyme. These resul ts suggest that deta nonoate reduces steroidogenesis independent of gu anylyl cyclase activation and that NO. has a direct effect to inhibit the activity of cytochrome P450, probably by binding to the heme group s of the cytochrome.