Fully grown Xenopus oocytes are physiologically arrested at the G2/prophase border of the first meiotic division. Addition in vitro of progesterone or insulin causes release of the G2/prophase block and stimulates meiotic cell division of the oocyte, leading to maturation of the oocyte into an unfertilized egg. The possibility that the products of polyphosphoinositide breakdown, diacylglycerol and inositol-1,4,5-trisphosphate (IP3-, are involved in oocyte maturation was investigated. Microinjection of IP3 into oocytes just prior to addition of progesterone or insulin accelerated the rate of germinal vesicle breakdown (GVBD) by up to 25%. Half-maximal acceleration occurred at an intracellular IP3 concentration of 1 microM. Treatment of oocytes with the diacylglycerol analog and tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA) induced GVBD in the absence of hormone. Half-maximal induction of GVBD occurred with 150 nM TPA and was blocked by pretreatment of oocytes with 10 nM cholera toxin. Microinjection of highly purified protein kinase C from rat brain into oocytes did not induce maturation but markedly accelerated the rate of insulin-induced oocyte maturation. However, injection of the enzyme had no effect on progesterone action. In oocytes with a basal intracellular pH below 7.6, TPA increased intracellular pH, but GVBD occurred with TPA in Na-substituted medium. Neomycin, a putative inhibitor of polyphosphoinositide breakdown, reversibly inhibited insulin- but not progesterone-induced maturation. Half-maximal inhibition occurred at 1.6 mM neomycin. These results indicate that protein kinase C is capable of regulating oocyte maturation in Xenopus.
Animals Female