Modulation of cultured mouse Leydig cells adenylate cyclase by forskolin and hCG

The diterpene, forskolin, stimulated cAMP accumulation about 15-fold over basal levels in purified mouse Leydig cells; however, it remained far less potent than hCG. Simultaneous addition of forskolin and hCG resulted in a striking synergistic stimulation of cAMP production. In contrast, forskolin-enhanced testosterone accumulation was never synergistic with that produced by maximal concentrations of hCG. hCG (3 X 10(-9) M) lowered about 6-fold the ED50 for forskolin-elicited cAMP accumulation and increased the maximal response to forskolin about 16-fold. Conversely, forskolin 10(-6) M) reduced the ED50 for hCG 2-fold but had a much smaller effect (2-3-fold) on maximal response. Moreover, pretreatment with hCG induced only a homologous desensitization of adenylate cyclase, whereas the enzyme became partially resistant to both hCG and forskolin in cells pretreated with forskolin. The homologous hCG-induced desensitization and the partial heterologous one induced by forskolin suggest that more than the catalytic unit of the cyclase is required for the diterpene activation.