Alkaline phosphatase activity in the membrane of Xenopus laevis oocytes: effects of steroids, insulin, and inhibitors during meiosis reinitiation.

The mechanism of steroid hormone-induced reinitiation of meiosis in Xenopus laevis oocytes in vitro involves interaction of the hormone with an ooplasma membrane receptor and early changes of enzymatic activities (adenylate cyclase, p48 protein kinase). In full-grown (stage 6) oocytes, we have observed cytochemically, at the ultrastructural level, alkaline phosphatase activity in the ooplasma membrane of microvilli, its decrease by 2 hr of progesterone action, and its complete disappearance at the time of germinal vesicle breakdown (GVBD). Insulin (30 micrograms/ml) also provoked a decrease of phosphatase activity, although it did not promote GVBD under these circumstances. When oocytes were exposed simultaneously to progesterone (1 microM) and insulin (30 micrograms/ml), the enzymatic activity disappeared earlier than with any one of them, correlating with the faster occurrence of GVBD. Inhibitors of alkaline phosphatase activity and competitive substrates potentiated progesterone action on GVBD. Insulin and beta-glycerophosphate potentiating activities were additive. These results suggest that the ooplasma membrane alkaline phosphatase may be implicated in the course of reinitiation of meiosis in X. laevis oocytes.