Precursors to glycogen in ovine fetuses

Postprandial hepatic glycogenesis in the adult animal is now felt to proceed largely through gluconeogenic pathways rather than directly from glucose. The ovine fetus, like the mature sheep, lacks specific hepatic glucokinase. Therefore, we examined the role of lactate as a fetal glycogenic precursor in seven chronically catheterized 125-day sheep fetuses. Fetuses were infused with L-[U-14C]lactate and D-[3-3H]glucose (50 microCi load, 50 microCi/h for 5 h), while maternal glucose was maintained at 50 mg/dl. Mean fetal hepatic glycogen specific activity (microCi/mg x 10(3] was 0.82 +/- 0.08 for 14C and 2.6 +/- 0.4 for 3H, whereas fetal renal glycogen specific activity was 0.46 +/- 0.22 for 14C and 0.78 +/- 0.16 for 3H. In contrast, [14C]glucose specific activity was undetectable in blood (limit of detectability 1 microCi/mg x 10(3] and mean [3H]glucose specific activity was 8.9 +/- 1.3 microCi/mg x 10(3]. The least detectable specific activity of [14C]glucose did not differ significantly from the [14C]glycogen enrichment in liver, whereas [3H]glucose specific activity was significantly (P less than 0.02) greater than [3H]glycogen enrichment. We conclude that glycogenesis from glucose is partly through the indirect gluconeogenic route and that lactate may be a glycogenic precursor in the ovine fetus.