XENOPUS OOCYTE MATURATION - CYTOPLASM ALKALIZATION IS INVOLVED IN GERMINAL VESICLE MIGRATION

In Xenopus laevis oocytes a transient increase in intracellular pH has been reported to occur during progesterone-induced maturation. Using a cytological approach, we have systematically analyzed germinal vesic le breakdown and meiotic spindle formation in various experimental con ditions either preventing or promoting pH(i) changes. Injection of a n eutral buffer (MOPS pH 6.9) induced a cytosolic acidification of 0.3 p H unit and inhibited by 30% the formation of the maturation white spot after progesterone exposure; in oocytes displaying a white spot, only half showed a spindle, often located far from the plasma membrane. Si milar results were observed with a Na-free medium which prevents oocyt e alkalization. Injection of an alkaline buffer (Tris pH 9) was able t o induce migration of the germinal vesicle in 25% of the oocytes in th e absence of progesterone, but failed to induce GVBD. Taken together, these results suggest that the increase in pH, observed during maturat ion may be involved in the migration of the germinal vesicle towards t he plasma membrane. We also incubated oocytes in the presence of proca ine, a weak base often used to artificially alkalize the oocyte cytopl asm. The changes induced by exposure to procaine were different from t hose resulting from alkaline buffer injection. Indeed procaine promote d GVBD, as well as spindle formation and chromosome condensation. Howe ver these events appeared without migration of the germinal vesicle, s uggesting that the expected alkalization did not occur.