MAIN ROUTES OF PLASMA LACTATE CARBON DISPOSAL IN THE MIDGESTATION FETAL LAMB

The turnover rates of plasma lactate, normalized for O-2 consumption r ate, are higher in the fetus than in the adult. This occurs despite ve ry low rates of fetal gluconeogenesis which preclude the recycling of lactate carbon into glucose. In an effort to establish the main routes of disposal of fetal plasma lactate, 12 midgestation ovine fetuses (a ge 74 +/- 1 days) were infused intravenously at constant rate with L-[ U-C-14]lactate for a 4-hour period. At the end of the infusion, the am ounts of C-14 retained by the fetus and by the placenta, and the distr ibution of the retained C-14 in free and protein-bound amino acids and in lipids were measured. Of the total C-14 infused, 17.0 +/- 1.4% was recovered in the placenta, 4.0 +/- 0.3% in the fetal liver, and 15.0 +/- 0.8% in the extrahepatic fetal tissues. Of the retained radioactiv e carbon, 45-57% was recovered in the free and protein-bound amino aci d fractions and 11-17% in the lipid fractions. Approximately 90% of th e C-14 in the free amino acid fractions was present as glutamate/gluta mine, serine, glycine, and alanine carbon. In conjunction with data on fetal CO2 production from lactate carbon, these results demonstrate t hat the main routes of fetal lactate disposal are oxidation and synthe sis of nonessential amino acids and lipids.