XENOPUS G(Q)ALPHA SUBUNIT ACTIVATES THE PHOSPHATIDYLINOSITOL PATHWAY IN XENOPUS OOCYTES BUT DOES NOT CONSISTENTLY INDUCE OOCYTE MATURATION

We cloned the Xenopus laevis form of G(q) alpha subunit to study its e ffects on oocyte maturation. Injection of Xenopus G(q) alpha mRNA into stage 6 oocytes activated the phospholipase C/phosphatidylinositol pa thway. The oocyte membrane became permeable to calcium ions and was ab le to generate transient inward currents (T-in), due to the opening of Ca2+-dependent Cl- channels. The T-in amplitude developed over severa l 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 G(q) alpha protein, which was produced continuously f or >24 hr, After Xenopus G(q) alpha mRNA injection, germinal vesicle b reakdown (GVBD) was variable (0-100%) in stage 6 oocytes, whereas none of the stage 4 oocytes underwent GVBD, In contrast, stage 6 oocytes i njected with mRNA encoding the G(o) alpha G protein consistently under went GVBD but did not acquire T-in. Our results show that activation o f phospholipase C is not an absolute requisite for the induction of ma turation, although in oocytes of some frogs phospholipase C activation fan trigger a pathway to GVBD.