GLUTAMATE METABOLISM IN FETUS AND PLACENTA OF LATE-GESTATION SHEEP

Glutamate is produced by the fetal liver and taken up by the placenta, To explore the functional meaning of this exchange, the disposal rate (DR), clearance, conversion to glutamine, and decarboxylation rate of fetal plasma glutamate were studied at 129 +/- 2 days of gestation in seven fetal lambs infused via a systemic vein with L-[2,3,3,4,4-H-2(5 )]glutamate and L-[1-C-14]glutamate. In two experiments, L-[1-C-13]glu tamate was also infused. The mean glutamate DR and clearance were 11.9 +/- 1.3 mu mol . min(-1) . kg(-1) and 200 +/- 8 ml. min(-1) . kg(-1), respectively. The placenta extracted 88.5 +/- 0.8% of the tracer glut amate carried by the umbilical circulation and contributed to 61.3 +/- 3.2% of the glutamate DR, Most of the C-14 infused as L-[1-C-14]gluta mate was converted to (CO2)-C-14: 37 +/- 4% by the fetus and 41 +/- 6% by the placenta. Of the labeled glutamate taken up by the placenta, 6 .2 +/- 1.5% was returned to the fetus as glutamine. The glutamine-to-g lutamate enrichment ratio in fetal arterial plasma was 0.066 +/- 0.008 . We conclude that fetal plasma glutamate has an exceptionally high cl earance because the flux of glutamate into the placenta is virtually e qual to umbilical glutamate delivery rate. The main pathway of fetal p lasma glutamate disposal is oxidation by placental and fetal tissues. Placental conversion of glutamate to fetal glutamine is a relatively s mall component of the placental metabolism of fetal glutamate.