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 vesicle breakdown and meiotic spindle formation in various experimental conditions either preventing or promoting pHi changes. Injection of a neutral buffer (MOPS pH 6.9) induced a cytosolic acidification of 0.3 pH 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. Similar results were observed with a Na-free medium which prevents oocyte alkalization. Injection of an alkaline buffer (Tris pH 9) was able to induce migration of the germinal vesicle in 25% of the oocytes in the absence of progesterone, but failed to induce GVBD. Taken together, these results suggest that the increase in pHi observed during maturation may be involved in the migration of the germinal vesicle towards the plasma membrane. We also incubated oocytes in the presence of procaine, a weak base often used to artificially alkalize the oocyte cytoplasm. The changes induced by exposure to procaine were different from those resulting from alkaline buffer injection. Indeed procaine promoted GVBD, as well as spindle formation and chromosome condensation. However these events appeared without migration of the germinal vesicle, suggesting that the expected alkalization did not occur.