Endothelin Stimulates Steroid Secretion by Frog Adrenal Glandin Vitro: Evidence for the Involvement of Prostaglandins and Extracellular Calcium in the Mechanism of Action of Endothelin*

Endothelin (ET-1) is a pleiotropic regulatory peptide which exerts multiple endocrine actions on the cardiovascular system. In the present study, we have investigated the possible role of ET-1 in the regulation of adrenocortical cells using perifused frog interrenal (adrenal) slices. Graded doses of ET-1 from 10(-11)-10(-9) M stimulated both corticosterone and aldosterone production in a dose-dependent manner. Repeated 20-min pulses of ET-1 (10(-9) M), given at the frequency of one pulse per 90 min, resulted in a marked reduction of the secretory response after the second pulse. Prolonged administration (3 h) of ET-1 induced a rapid increase in corticosterone and aldosterone output, followed by a gradual decline of corticosteroid secretion. Perifusion of frog adrenal tissue with ET-1 (10(-9) M) caused a significant increase in the release of prostaglandin E2 (PGE2) and 6-keto-PGF1 alpha, the stable metabolite of the prostacyclin PGI2. The enhancement of PG biosynthesis preceded by 10 min the peak of corticosteroids. When repeated pulses of ET-1 were administered, a significant diminution of the production of PGE2 and 6-keto-PGF1 alpha was observed after the second pulse. The cyclooxygenase inhibitor indomethacin (5 microM) totally suppressed the stimulatory effect of ET-1 on corticosterone and aldosterone secretion; in contrast, indomethacin did not affect ACTH-evoked corticosteroid secretion. Perifusion of adrenal slices with a calcium-free solution or addition of cobalt (4 mM) induced total inhibition of the stimulatory effect of ET-1. These results demonstrate that ET-1 is a potent stimulator of corticosterone and aldosterone secretion from frog adrenal gland in vitro. Our data show that repeated or prolonged administration of ET-1 induces a rapid desensitization phenomenon. The data also indicate that ET-1-evoked corticosteroid secretion is mediated by an increase in PG biosynthesis and requires the presence of extracellular calcium.