COMPARISON OF LEUCINE, SERINE AND GLYCINE TRANSPORT ACROSS THE OVINE PLACENTA

To estimate the transport rate of maternal glycine across the placenta [1-C-13]glycine and L-[1-(13)]serine were infused intravenously in pr egnant sheep using both continuous and bolus infusions. Each tracer wa s infused together with L-[1-C-13]leucine, to enable a comparison with the placental transport of an essential amino acid. At steady state, fetal plasma leucine enrichment was 40 per cent of maternal enrichment . indicating that approximately 60 per cent of the entry rate of leuci ne into fetal plasma is derived from protein breakdown in the placenta and fetus. Fetal plasma glycine enrichment was 11 per cent of materna l and there was no detectable fetal serine enrichment. The direct flux of maternal leucine into the fetal circulation was approximately 3.0 (bolus experiments) to 3.6 (continuous infusion experiments) mu mol/mi n (kg fetus) and greater than the estimated 1.4 mu mol/min (kg fetus) direct flux of maternal glycine, despite the fact that the net umbilic al uptake of glycine exceeds that of leucine. This supports the conclu sion that placental glycine production is a quantitatively important c ontribution to fetal glycine uptake via the umbilical circulation. The fetal glycine supply from the placenta is provided by a relatively sm all direct maternal glycine transplacental flux and a larger contribut ion derived from serine utilization within the placenta for glycine pr oduction. (C) 1996 W. B. Saunders Company Ltd