Glycolysis and glucose oxidation by the sheep conceptus at different oxygen concentrations.

The effect of changes in oxygen concentration on the catabolic utilization of glucose by the sheep conceptus at selected periods between Days 3 and 19 of preimplantation development was examined by estimating the production of CO2 and lactate from [U-14C]glucose during a 2.5-h culture in vitro in the presence of 20%, 5% and 1% O2. In general, lowering O2 significantly altered the catabolism of glucose with a changing pattern of response depending on the stage at which the conceptus was explanted. For embryos at Days 3 and 6 post insemination, reduced O2 caused no significant change in oxidative utilization of glucose and a small decrease in conversion of the substrate to lactate. By contrast, lowering O2 concentration during incubation of the structures of the advanced conceptus from Day 13 through to Day 19 of pregnancy significantly restrained oxidative utilization of glucose but stimulated its conversion to lactate. The effects of these changing levels of O2 on the generation of energy from glucose in the form of ATP was estimated. Except for the Day-13 conceptus, reduction in O2 concentration had little influence on the calculated amount of ATP produced from glucose, with glycolysis making up the deficit in energy production when reduced O2 inhibits oxidation of the substrate at the later stages of development. At Day 13, the switch in the metabolism of glucose to glycolysis is not fully effective and energy production fell as O2 concentration was reduced. The results indicate a major shift towards dependence by the preimplantation sheep conceptus on the glycolytic pathway for energy generation from glucose as development progresses. This move to glycolysis is increased by low O2 concentration. As a low concentration of O2 most probably exists in the lumen of the sheep uterus, the results indicate that, in utero, the energy required for the rapid growth of the conceptus depends progressively more on glycolysis than oxidative metabolism of glucose. The finding that the Day-13 conceptus has not fully adapted to this method of ATP generation at low O2 concentrations may make it especially vulnerable during development in utero.