THE ROLE OF CA2-INDUCED GERMINAL VESICLE BREAKDOWN OF XENOPUS-LAEVIS OOCYTES - THE SYNERGIC EFFECTS OF MICROTUBULE DEPOLYMERIZATION AND CA2+( IN PROGESTERONE)

By monitoring Ca-45(2+) influx and efflux from oocytes a transient inc rease followed by a transient decrease in the Ca2+-content of progeste rone-treated oocytes was observed. Chelation of intracellular Ca2+ wit h EGTA or BAPTA-type buffers inhibited progesterone-induced GVBD. Buff ers with a mid-range K-d (similar to 1.5 mu M) were most effective in inhibiting GVBD whereas buffers with a K-d above or below this value w ere less effective. These observations indicate that intracellular C2, probably in the form of a localized release, is required for progest erone-induced oocyte maturation. However, Ca2+ alone was insufficient to induce GVBD. When the effects of nocodazole and taxol upon this Ca2 +-requirement were tested, we observed that taxol-induced microtubule polymerization not only delayed progesterone-induced GVBD but also com pletely inhibited it in combination with BAPTA-AM. Conversely, nocodaz ole-induced microtubule depolymerization in combination with ionophore A23187 not only accelerated progesterone-induced GVBD, but also induc ed GVBD in the absence of progesterone. The combined treatment of oocy tes with nocodazole and InsP(3), or with cold microtubule depolymeriza tion synergistically promote GVBD. In both nocodazole-and cold-treated oocytes, the GV was displayed to the periphery of the oocytes and und erwent GVBD when treated with A231187. However, when the GV was displa ced to the cortex by a centrifugel force under conditions that would n ot cause microtubule depolymerization and the oocyte was treated with A231187, oocytes did not undergo GVBD.