The Participation of Energy Substrates in the Control of Meiotic Maturation in Murine Oocytes

Abstract

In this study we have examined the effects of various energy substrate combinations on both the maintenance of meiotic arrest and follicle-stimulating hormone (FSH)-induced germinal vesicle breakdown (GVB) in isolated mouse oocytes treated with 4 mM hypoxanthine (HX). When cumulus cell-enclosed oocytes were cultured in HX minus glucose, the addition of pyruvate dose-dependently sustained oocyte viability and induced GVB (from 25% GVB at 0 mM pyruvate to 98% GVB at 1 mM pyruvate). FSH had a limited stimulatory effect at all pyruvate concentrations. Oxaloacetate, but no other intermediates of the tricarboxylic acid (TCA) cycle, mimicked the action of pyruvate on viability and GVB. Lactate alone induced maturation in the absence of pyruvate but required NAD for this effect when the medium included pyruvate. Of two TCA cycle inhibitors tested, malonate had no effect on pyruvate-induced GVB, and fluoroacetate was inhibitory only at the highest concentration (10 mM). Pyruvate, oxaloacetate, and lactate also stimulated maturation in denuded oocytes. Adding glucose back to the medium produced an inhibitory effect on meiosis in cumulus cell-enclosed oocytes up to a concentration of 23 mM and was associated with increases in ATP. Exposure to the glycolytic inhibitor, iodoacetate, eliminated both the meiotic arrest and ATP production mediated by glucose. High concentrations of glucose (111 mM) proved stimulatory to oocyte maturation. Glucose had no effect on denuded oocytes. FSH-induced maturation was dependent upon the presence of glucose but was not causally related to increases in lactate production. In addition, raising the pyruvate concentration 10-fold did not overcome the meiotic block. Coculture of denuded oocytes with dissociated cumulus cells, but not intact oocyte-cumulus cell complexes, stimulated GVB in HX-containing medium when glucose was added. However, both the cumulus cells and the complexes produced an increase in pyruvate production, indicating that the effect of the dissociated cumulus cells was not due principally to secretion of this carbohydrate into the culture medium. This study has demonstrated the importance of energy substrates in the meiotic response of oocytes in culture. Evidence failed to support a role for increased TCA cycle activity in the stimulatory action of pyruvate. The inhibitory effect of glucose is the result of glycolytic activity that produces increased levels of ATP, but production of ATP and glycolytic intermediates does not appear to be the mechanism whereby glucose mediates a stimulatory action on meiosis, either in response to high glucose levels or FSH treatment. Other avenues of glucose metabolism or interaction may be responsible for this effect.