Pyruvate is added to all media used for human in vitro fertilization a
nd embryo culture, but its function(s) in the early embryo is unknown.
We tested the possibility that pyruvate can act as an oxidizable ener
gy source by measuring the consumption of pyruvate and oxygen by Day 2
and Day 3 human embryos, using microfluorometric techniques. Oxygen c
onsumption (19.6 pmol/embryo per hour) could account for the oxidation
of only 56% of the pyruvate consumed (13.9 pmol/embryo per hour). Oxy
gen was also consumed in the absence of exogenous substrates. Lactate
appeared in the incubation medium with pyruvate (0.47 mM) as sole exog
enous substrate at a rate of 12.1 pmol/embryo per hour, at a similar r
ate (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 ex
ogenous substrates, suggesting that a high proportion of the pyruvate
taken up by early human embryos is converted to lactate. Pyruvate upta
ke in the presence of UK5099, a pyruvate transport inhibitor, was redu
ced to 10% of control values, consistent with the presence of the mono
carboxylate carrier in the human embryo plasma membrane.