Inhibitory action of the gonadopeptide inhibin on amphibian (Rana pipiens) steroidogenesis and oocyte maturation.

In view of recent reports on the production of inhibin- and activin-like proteins in lower vertebrates and their important role during development, we have examined the effects of the gonadopeptide inhibin in the process of oocyte maturation using amphibian (Rana pipiens) fully grown preovulatory ovarian follicles cultured in vitro. In the presence of frog pituitary homogenate (FPH), which stimulates progesterone (P4) levels and the subsequent germinal vesicle breakdown (GVBD), purified porcine inhibin (35-50 IU) inhibited both of these responses in a dose-dependent manner. Inhibin also blocked GVBD initiated by exogenously added P4 in intact as well as denuded oocytes. Thus, inhibin seems to act at the follicle (granulosa) cells because it blocked steroidogenesis and at the oocyte because it altered the steroid-induced oocyte maturation. The P4-treated follicles were susceptible to the inhibin action during the first 3 hr of steroid stimulation, which indicates that inhibin affects some early events during the process of GVBD. Maximum inhibitory effect was observed when P4 and inhibin were added simultaneously at the beginning of the incubations. Moreover, the inhibitory effect on GVBD caused by the gonadopeptide was dependent on the length of exposure of the follicles to inhibin. The continuous presence of inhibin in the culture was required to block GVBD efficiently. Data also indicate that the inhibitory effect of inhibin was reversible. Taken together, results from this study present evidence that inhibin may be a relevant paracrine/autocrine regulator of ovarian functions.