Influence of oocyte-secreted factors and culture duration on the metabolic activity of bovine cumulus cell complexes.

Intracellular communication between the cumulus cell complex and the oocyte is essential for numerous processes during oocyte maturation. The aim of this study was to determine the interaction between oocyte-secreted factors and the metabolic activity of bovine cumulus cell complexes during in vitro maturation (IVM). Cumulus-oocyte complexes (COCs) were aspirated from ovaries derived from an abattoir and divided into four treatment groups: (i) intact COCs, (ii) oocytectomized complexes (OOX), in which the ooplasm was microsurgically removed, (iii) OOX co-cultured with denuded oocytes (OOX+DO) and (iv) DO. The complexes were cultured individually in IVM media. After 0-4, 10-14 and 20-24 h of culture, the utilization of oxygen, glucose, pyruvate and L-lactate by the complexes was measured. The metabolic activity of the DO was undetectable. There were no significant differences in metabolic measurement among any of the treatment groups, indicating that the metabolism of the cumulus complex is not affected by the presence of the oocyte. When metabolic activity for the complexes was analysed relative to time in culture, there was an approximate twofold increase in the consumption of oxygen, glucose and pyruvate over the 24 h period (P<0.05), although production of L-lactate remained constant. The relationship between total glucose uptake and L-lactate production indicated that the majority of glucose consumed at the start of culture was being utilized via glycolysis, but by the cessation of the maturation period, there was significant utilization of glucose elsewhere, possibly for the formation of cumulus extracellular matrix. These results indicate that metabolism of COC does not reflect biochemical activity of the oocyte. Nevertheless, the metabolic requirements of the COC increase throughout maturation.