CONTRIBUTION OF THE PENTOSE-PHOSPHATE PATHWAY TO GLUCOSE-UTILIZATION BY PREIMPLANTATION SHEEP EMBRYOS

The activity of the pentose phosphate pathway of glucose metabolism in early sheep embryos and in the structures of the advanced conceptus f rom Day 13 to Day 19 of pregnancy was measured quantitatively during a 2.5-h incubation with glucose as sole energy source. For embryos duri ng cleavage, activity of this pathway accounted for 6-9% of total gluc ose utilized. The proportion of glucose metabolized through the pentos e pathway fell progressively with development and by Day 19 represente d 1-2% of glucose turnover. However, total turnover of glucose increas ed eight fold between the 2-cell and blastocyst stage and the amount o f glucose processed through the pentose pathway increased over this ti me despite the fall in the proportion utilized in this way. ID contras t, glucose turnover by the advanced embryo and its extra embryonic mem branes progressively decreased as the structures developed. As a resul t, estimates of the amount of glucose utilized through the pathway per mug dried weight per hour declined to low values at Day 19 following the peak in activity at about the time of blastulation. Trophoblast an d yolk sac processed less glucose through the pentose pathway per mug dried weight than embryonic tissue but the allantois was similar to th e embryo. Overall, the pentose pathway accounted for a relatively cons tant proportion of the CO2 produced from glucose under these experimen tal conditions with values generally between 15 and 20% of total CO2 p roduced. When activities in the components of the advanced conceptus w ere expressed as the total amount of glucose processed through the pat hway per hour, turnover in the embryo, allantois and yolk sac increase d progressively with time. By contrast, there' was a substantial troug h in the activity of the trophoblast on Day 17 of pregnancy.