Intrafollicular action of estrogen in regulating pituitary-induced ovarian progesterone synthesis and oocyte maturation in Rana pipiens: temporal relationship and locus of action.

Previous studies demonstrated that estrogen inhibited frog pituitary homogenate (FPH) and progesterone-induced oocyte maturation. In order to determine whether estrogen interfered with intrafollicular progesterone synthesis, experiments were designed to study the effect of exogenous estrogen (estradiol-17 beta) on FPH-induced progesterone production in amphibian (Rana pipiens) ovarian follicles cultured in vitro. Intrafollicular progesterone concentrations were monitored directly using radioimmunoassay and the occurrence of germinal vesicle breakdown (GVBD) was used as a biological indicator of the action of steroids on oocyte maturation. FPH elicited a rapid and dramatic increase in follicular progesterone concentration (1000-3000 pg per follicle) which preceded germinal vesicle breakdown. Addition of estrogen to the culture medium inhibited FPH-induced progesterone production and the accompanying GVBD in a dose-dependent fashion. The presence of estrogen did not enhance the degradation of preloaded progesterone, suggesting estrogen impeded progesterone synthesis rather than enhanced progesterone metabolism. The temporal relationship between estrogen and FPH interaction was assessed by varying the relative time of hormone addition, after which intrafollicular progesterone concentration and GVBD were monitored. Progesterone production and GVBD were drastically inhibited when estrogen was added before or simultaneously with FPH. However, when addition of estrogen was delayed until after FPH simulation, a progressive loss of the inhibitory effect of the steroid on progesterone accumulation and GVBD was observed. Thus, the estrogen-sensitive phase was confined to the early portion of FPH stimulation. Continuous presence of estrogen in the culture system was not required to inhibit FPH-induced events. A short exposure (15 min) of follicles to estrogen was sufficient to inhibit oocyte maturation, whereas progesterone synthesis was not significantly affected. With longer exposure, however, FPH-induced progesterone production was impeded. Washing estrogen-treated follicles did not reverse the inhibitory effect of estrogen, however the follicles remained responsive to exogenous progesterone stimulation and exhibited GVBD. Results suggest that the inhibitory effects of estrogen on FPH action and progesterone production were not reversible under the in vitro culture conditions. To determine whether specific follicular components were involved in estrogen inhibition, progesterone production was assessed following selective removal of different follicle components by microdissection prior to being treated with FPH and estrogen.(ABSTRACT TRUNCATED AT 400 WORDS)