A comparison of amino acid arteriovenous differences across the liver and placenta of the fetal lamb.

Amino acid and ammonia concentrations as well as oxygen content were measured in either the right or left hepatic vein, the umbilical vein, and the umbilical artery in 13 fetal lambs in late gestation. There was an uptake of all of the essential and most of the nonessential amino acids by both lobes of the fetal liver. The umbilical venous-hepatic venous amino acid concentration differences were similar in the two hepatic lobes. While glutamine and glycine were taken up by both hepatic lobes, their metabolically related amino acids, glutamate and serine, were released by the fetal liver into the systemic circulation. There was a reciprocal net placental uptake from the umbilical circulation of glutamate and serine and a net fetal of glutamine and glycine, suggestive of interorgan cycling of these amino acids between the placenta and fetal liver. Total fetal umbilical nitrogen uptake was 0.91 g N.kg-1.day-1. The umbilical venous-hepatic venous differences of ammonia were positive and not significantly different in the two lobes. There was a significant umbilical uptake of ammonia (12.8 +/- 1.8 microM; 0.0078 microM NH3/microM O2). However, 0 the ratios of NH3 to O2 were much higher in each lobe (right, 0.060; left, 0.079; each P less than 0.01) than in the umbilical circulation.