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

Abstract

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.