Metabolism of pyruvate by the early human embryo

Pyruvate is added to all media used for human in vitro fer-tilization and embryo culture, but its function(s) in the early embryo is unknown. We tested the possibility that pyruvate can act as an oxidizable energy source by measuring the consump-tion of pyruvate and oxygen by Day 2 and Day 3 human em-bryos, using microfluorometric techniques. Oxygen consump-tion (19.6 pmol/embryo per hour) could account for the oxi-dation of only 56 % of the pyruvate consumed (13.9 pmol/em-bryo per hour). Oxygen was also consumed in the absence of exogenous substrates. Lactate appeared in the incubation me-dium with pyruvate (0.47 mM) as sole exogenous substrate at a rate of 12.1 pmol/embryo per hour, at a similar rate (10.85 pmol/embryo per hour) in the presence of 1 mM glucose and 0.47 mM pyruvate, and at 2.25 pmol/embryo per hour in the absence of exogenous substrates, suggesting that a high propor-tion of the pyruvate taken up by early human embryos is con-verted to lactate. Pyruvate uptake in the presence of UK5099, a pyruvate transport inhibitor, was reduced to 10 % of control values, consistent with the presence of the mornocarboxylate carrier in the human embryo plasma membrane