Effect of calcium on corticosteroid secretion by isolated frog interrenal gland.

The direct effect of extracellular calcium concentrations on corticosteroidogenesis has been examined in the frog, using a perifusion system technique. The release of corticosterone and aldosterone in the effluent medium was monitored by specific radioimmunoassays. Increasing concentrations of Ca2+ (from 2 to 15 mM) gave rise to a dose-related stimulation of corticosteroid release, whereas the increment of either Na+ or K+ concentrations did not modify steroid production. Iterative administration of a moderate concentration of calcium (6 mM) led to a reproducible stimulation of steroid secretion whereas the same dose infused during 6 h induced a transient rise in corticosteroid secretion followed by a plateau. The direct effect of Ca2+ on steroidogenesis was confirmed by the dose-dependent stimulation of steroid secretion induced by the calcium ionophore A 23187. Perifusion with a calcium-free medium or blockade of Ca2+ channels by 4 mM Co2+ both resulted in a significant decrease in steroid production. Conversely, the administration of verapamil (up to 10(-4) M) did not affect steroidogenesis. These results provide evidence that extracellular calcium ions are required for basal production of corticosteroids in amphibians and that Ca2+ influx does not occur through voltage-dependent channels. Since, in the frog, blood Ca2+ concentrations vary in a rather large range, these results suggest that circulating Ca2+ levels may regulate corticosteroid production in these animals.