Studies on the Mechanism of Action of Estradiol in Regulating Follicular Progesterone Levels: Effects on cAMP Mediated Events and 3β‐Hydroxysteroid Dehydrogenase

Previous studies demonstrated that estradiol interferes with pituitary‐induced progesterone production and oocyte maturation in cultured amphibian (Rana pipiens) ovarian follicles. To elucidate the mode of action of estradiol in modulating follicular progesterone accumulation we have examined its effects on cAMP‐induced progesterone production and enzymatic conversion of pregnenolone to progesterone by 3β‐hydroxysteroid dehydrogenase (3β‐HSD). Follicular cAMP levels were manipulated with forskolin (an adenylate cyclase activator), isobutyl methyl xanthine (IBMX‐phosphodiesterase inhibitor) and exogenously added cAMP. Progesterone production induced by forskolin alone or forskolin in combination with frog pituitary homogenate (FPH) was inhibited by estrogen. Addition of estradiol to culture medium markedly inhibited follicular progesterone accumulation following treatment of follicles with cAMP and IBMX. In the presence of exogenous pregnenolone, non‐FPH stimulated ovarian follicles effectively converted the 3β‐HSD substrate to progesterone. Treatment of follicles with estradiol inhibited conversion of pregnenolone to progesterone. The results indicate that estradiol acts, following FPH stimulation, at one or more steps subsequent to elevation of cAMP levels to regulate intrafollicular progesterone accumulation and oocyte maturation. Estrogen appears to directly influence the enzymatic (3β‐HSD) conversion of pregnenolone to progesterone.