PLACENTAL TRANSPORT OF THREONINE AND ITS UTILIZATION IN THE NORMAL AND GROWTH-RESTRICTED FETUS

Placental transport and fetoplacental utilization of threonine (Thr) w ere compared at 130 +/- 1 days gestational age between seven control e wes (C) and six ewes in which intrauterine growth restriction (IUGR) h ad been induced by exposure to high ambient temperature from 33 +/- 1 to 112 + 2 days of gestation. Thr fluxes were measured using simultane ous intravenous infusions of L-[1-C-13]Thr into the mother and L-[U-C- 14]Thr into the fetus. The IUGR group had less fetal weight (1.27 +/- 0.14 vs. 3.10 +/- 0.10 kg, P < 0.01) and placental weight (120 +/- 17 vs. 295 +/- 14 g, P < 0.01) than the C group. The direct flux of mater nal Thr into the fetal systemic circulation was less in the IUGR fetus es, both relative to fetal weight (1.40 +/- 0.19 vs. 2.19 +/- 0.18 mu mol.min(-1).kg fetus(-1), P = 0.0107) and placental weight (1.5 +/- 0. 2 vs. 2.3 +/- 0.2 mu mol.min(-1).100 g placenta(-1), P = 0.0187). In b oth groups, there was excretion of CO2 produced from fetal Thr. The ra te of CO2 production from fetal plasma Thr carbon by fetus plus placen ta was reduced in the IUGR group (1.50 +/- 0.23 vs. 2.86 +/- 0.32 mu m ol.min(-1).kg fetus(-1), P = 0.0065). We conclude that the flux of mat ernal Thr into the IUGR fetus is markedly reduced because of a reducti on in placental mass and because of a weight-specific reduction in Thr placental transport. The reduced flux is routed into fetal Thr accret ion via a decrease in fetal Thr oxidation.