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We cloned the Xenopus laevis form of Gq alpha subunit to study its effects on oocyte maturation. Injection of Xenopus Gq alpha mRNA into stage 6 oocytes activated the phospholipase C/phosphatidylinositol pathway. The oocyte membrane became permeable to calcium ions and was able to generate transient inward currents (T(in)), due to the opening of Ca(2+)-dependent Cl- channels. The T(in) amplitude developed over several hours and disappeared by 24 hr. Diacylglycerol levels were found to parallel the appearance and disappearance of the T(in). The concurrent decline of T(in) values and diacylglycerol was not due to a failure in the synthesis of Gq alpha protein, which was produced continuously for > 24 hr. After Xenopus Gq alpha mRNA injection, germinal vesicle breakdown (GVBD) was variable (0-100%) in stage 6 oocytes, whereas none of the stage 4 oocytes underwent GVBD. In contrast, stage 6 oocytes injected with mRNA encoding the Go alpha G protein consistently underwent GVBD but did not acquire T(in). Our results show that activation of phospholipase C is not an absolute requisite for the induction of maturation, although in oocytes of some frogs phospholipase C activation can trigger a pathway to GVBD.
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