The role of Ca(2+) in progesterone-induced germinal vesicle breakdown of Xenopus laevis oocytes: the synergic effects of microtubule depolymerization and Ca(2+).

By monitoring (45)Ca(2+) influx and efflux from oocytes a transient increase followed by a transient decrease in the Ca(2+)-content of progesterone-treated oocytes was observed. Chelation of intracellular Ca(2+) with EGTA or BAPTA-type buffers inhibited progesterone-induced GVBD. Buffers with a mid-range Kd (∼1.5 μM) were most effective in inhibiting GVBD whereas buffers with a Kd above or below this value were less effective. These observations indicate that intracellular Ca(2+), probably in the form of a localized release, is required for progesterone-induced oocyte maturation. However, Ca(2+) alone was insufficient to induce GVBD. When the effects of nocodazole and taxol upon this Ca(2+)-requirement were tested, we observed that taxol-induced microtubule polymerization not only delayed progesterone-induced GVBD but also completely inhibited it in combination with BAPTA-AM. Conversely, nocodazole-induced microtubule depolymerization in combination with ionophore A23187 not only accelerated progesterone-induced GVBD, but also induced GVBD in the absence of progesterone. The combined treatment of oocytes with nocodazole and InsP3, or with cold treatment and ionophore A23187 also induced GVBD in the absence of progesterone. Thus, Ca(2+) and microtubule depolymerization synergistically promote GVBD. In both nocodazole- and cold-treated oocytes, the GV was displaced to the periphery of the oocyte and underwent GVBD when treated with A23187. However, when the GV was displaced to the cortex by a centrifugal force under conditions that would not cause microtubule depolymerization and the oocyte was treated with A23187, oocytes did not undergo GVBD.