Reducing PIP2 hydrolysis, Ins(1,4,5)P3 receptor availability, or calcium gradients inhibits progesterone-stimulated Xenopus oocyte maturation.

In this report, we investigated the possibility that a product of phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), and its downstream effector, Ca2+, is involved in oocyte maturation. Microinjection of monoclonal antibody specifically directed to PIP2 into oocytes prior to progesterone addition inhibited meiotic maturation. In addition, preventing or suppressing the progesterone-induced increase in [Ca2+]i and maintaining cytosolic free Ca2+ of oocyte at 0.1 or 0.3 microM by Br2-BAPTA buffered Ca2+ solutions, completely blocked maturation. However, raising cytosolic Ca2+ concentration of oocyte to 1.0 or 3.0 microM, the inhibitory Br2-BAPTA effects on oocyte maturation was greatly diminished. Finally, microinjection of heparin, a potent antagonist of Ins(1,4,5)P3 receptor, inhibited progesterone-induced oocyte maturation in a manner that is linearly proportional to its cytoplasmic concentration. These results strongly support the hypothesis that PIP2 turnover as well as Ins(1,4,5)P3-induced Ca2+ release play crucial roles in regulating normal meiotic cell division in Xenopus oocyte.