PLACENTAL TRANSPORT AND FETAL UTILIZATION OF LEUCINE IN A MODEL OF FETAL GROWTH-RETARDATION

Placental transport and fetal utilization of leucine were studied at 1 30 days of gestation in six control ewes and in seven ewes in which in trauterine growth retardation (IUGR) had been induced by exposure to h eat stress. Leucine fluxes were measured during simultaneous intraveno us infusion of L-[1-C-13]leucine into the mother and L-[1-C-14]leucine into the fetus. In the IUGR group, the following leucine fluxes, expr essed as mu mol . min(-1). kg fetus(-1) were reduced compared with con trol: net uterine uptake (3.44 vs. 8.56, P < 0.01), uteroplacental uti lization (0.0 vs. 4.7, P < 0.01), fetal disposal rate (6.4 vs. 8.9, P < 0.001), flux from placenta to fetus (5.0 vs. 7.1, P < 0.01), direct transport from mother to fetus (1.6 vs. 3.4, P < 0.01), flux from fetu s to placenta (1.5 vs. 3.2, P < 0.001), and oxidation of fetal leucine by fetus plus placenta (2.1 vs. 3.2, P < 0.02). Uterine uptake, utero placental utilization, and direct transport were also significantly re duced per gram placenta. We conclude that maternal leucine flux into t he IUGR placenta is markedly reduced. Most of the reduced flux is rout ed into fetal metabolism via a decrease in placental leucine utilizati on and a decrease in the leucine flux from fetus to placenta.