Ca+2 Is an Inhibitor of Adenylate Cyclase in MA-10 Leydig Tumor Cells*

Using a clonal strain of cultured Leydig tumor cells (designated MA-10), we have examined the effects of Ca+2 on the activation of cAMP accumulation and steroid biosynthesis by hCG. Our results show that addition of Ca+2 ionophores (A23187 or ionomycin) leads to inhibition of the activation of cAMP accumulation by hCG. The magnitude of this effect is dependent on the concentrations of ionophore and hCG used, becoming more pronounced as the concentration of hCG increases. A detailed examination of the effects of A23187 and removal of extracellular Ca+2 on the rates of cAMP synthesis and degradation in intact cells revealed that A23187 inhibits the rate of cAMP accumulation activated by hCG, but does not affect the rate of degradation of cAMP. On the other hand, removal of extracellular Ca+2 had no effect on the rate of cAMP accumulation activated by hCG or on the rate of degradation of cAMP. Removal of extracellular Ca+2, however, completely prevented the inhibitory effect of A23187 on the rate of cAMP accumulation. Additional experiments show that the effects of A23187 or removal of extracellular Ca+2 on hCG-activated steroidogenesis closely parallel those described for cAMP accumulation. We conclude that Ca+2 is an inhibitor of the hCG-activated adenylate cyclase in Leydig tumor cells, and that this inhibition imposes a limitation on the ability of hCG to activate steroid biosynthesis.