The Influence of Glucose, Cumulus Cells, and Metabolic Coupling on ATP Levels and Meiotic Control in the Isolated Mouse Oocyte

Abstract

The effects of glucose and cumulus cells on oocyte ATP levels and germinal vesicle breakdown (GVB) in isolated mouse oocytes have been examined. Oocyte-cumulus cell complexes or denuded oocytes (DO) from pregnant mare serum gonadotropin-primed immature mice were cultured in minimum essential medium containing 4 mM hypoxanthine and 1 mM pyruvate, in the absence or presence of 0.55 mM glucose. After 17-18 hr in the presence of glucose, ATP in both the oocyte-cumulus cell complexes and oocytes derived from such complexes (cumulus cell-enclosed oocytes; CEO) was elevated, and less than one-half of the oocytes had resumed maturation (48% GVB). Removal of glucose caused a decrease in ATP levels in complexes and CEO and reversed the meiotic arrest in CEO (98% GVB), and these effects were mimicked by iodoacetate treatment. Higher frequencies of GVB were observed in glucose-free medium after more than 6 hr of culture, while ATP levels were reduced within 3 hr. Glucose had no effect on ATP levels or the meiotic state of denuded oocytes. Interestingly, iodoacetate had a stimulatory effect on GVB in DO (86% GVB compared to 57% in controls), but did not effect ATP levels. Glycerrhetinic acid, a gap junction uncoupler, completely suppressed oocyte-cumulus cell coupling in cultured complexes (0.15% coupling compared to 16.6% in controls) and reversed the inhibitory effect of glucose on oocyte maturation (91 and 95% GVB at 10 and 25 μM compared to 42% GVB in controls). This agent also prevented follicle-stimulating hormone-induced meiotic maturation in dibutyryl cAMP-arrested CEO. These results thus implicate mediation by gap junctions of both inhibitory and stimulatory signals from the cumulus cells. Comparison of ATP levels in spontaneously maturing CEO in vitro with those from oocytes maturing in vive in response to human chorionic gonadotropin revealed that a decrease in oocyte ATP preceded or accompanied GVB in spontaneously maturing oocytes but not in those maturing in vivo. The results of this study indicate that glucose-derived elevation of oocyte ATP contributes to meiotic arrest in cumulus cell-enclosed oocytes that is dependent on patent gap junctions. Furthermore, a model for reinitiation of oocyte maturation is supported in which spontaneous GVB results from cessation or interruption of inhibitory influences, while ligand-provoked GVB is brought about by the generation of stimulatory signals that override inhibitory input.