INCORPORATION OF SUBSTRATE CARBON FROM [U-C-14]ACETATE BY THE SHEEP CONCEPTUS RECOVERED FROM THE UTERUS ON DAYS 13 TO 19 OF PREGNANCY

Of the substrate carbon incorporated from 1.12 mM [U-C-14]acetate as s ole energy substrate during a 2.5-h incubation, a large proportion was isolated in the acid-soluble fraction. Although there was no signific ant change over time in the rate of entry into this pool for embryos, the rate of accumulation by the trophoblast and yolk sac increased as development progressed. At Days 13 and 15 of pregnancy, incorporation of acetate into the acid-insoluble fraction of embryos accounted for a lmost half the total label accumulated. The rate of this incorporation fell rapidly over time and by Day 19 less than 30% of carbon accumula ted was in this fraction. By contrast, the rate of incorporation into this fraction by trophoblastic tissue was low at Day 13 but rose drama tically as development progressed. Incorporation by the yolk sac into acid-insoluble components also rose with time. At the early stages of pregnancy studied, lipid synthesis accounted for the majority of aceta te carbon accumulated by the conceptus in the acid-insoluble pool. At later stages of development, incorporation into lipids constituted a m inor pool of acetate carbon. Some acetate carbon was found in the glyc ogen fraction of the conceptus. The rate of incorporation into the aci d-soluble glycogen fraction by embryos was constant throughout the per iod studied. By contrast, trophoblast increased its rate of incorporat ion markedly into both acid-soluble and acid-insoluble glycogen pools as did the yolk sac from Day 17. The addition of 0.28 mm glucose to th e medium for extraembryonic membranes had no significant effect on inc orporation into either of the acid-soluble pools or into lipid. Howeve r, it did reduce incorporation into the non-lipid macromolecular pool of the allantois and yolk sac. Comparisons of these results for carbon incorporation from acetate with published results for carbon incorpor ation from glucose indicate less effective utilization of acetate than glucose by all tissues other than the trophoblast. For the trophoblas t, the ability to incorporate acetate carbon increased with time, and by Day 19 the amount of carbon incorporated from acetate was twice tha t from glucose.