Plasma Membrane Mediated Action of Progesterone in Amphibian (Rana dybowskii) Oocyte Maturation

The mechanism of progesterone action within the ovarian follicle was investigated inRana dybowskii,by using immobilized progesterone. Fluorescein isothiocyanate-labeled progesterone 3-O-carboxymethyloxime–BSA (P-BSA) was localized on the outside surface of the denuded oocyte, which indicated that P-BSA did not cross the barrier of cell surface. Progesterone–BSA induced germinal vesicle breakdown (GVBD) of denuded oocytes in a dose-dependent manner but failed to induce GVBD of follicle wall-enclosed oocytes. The time course of P-BSA-induced GVBD in denuded oocytes was similar to that observed with progesterone. Furthermore, both P-BSA and progesterone induced oocyte maturation in the presence of RU486, a well-known nuclear progesterone receptor antagonist. Treatment of denuded oocytes with P-BSA resulted in a threefold increase in inositol triphosphate (IP3) and a fourfold increase in diacylglycerol levels within 10 min. Additionally protein kinase C (PKC) activity was markedly increased by 30 min of incubation following exposure to P-BSA. Such changes were not observed in denuded oocytes exposed to β-estradiol-6-O-carboxymethyloxime–BSA, which failed to induce GVBD. These results suggest that progesterone acts initially at the oocyte surface where it triggers generation of membrane-mediated second messengers during oocyte maturation in amphibians.