PLACENTAL AND FETAL SERINE FLUXES AT MIDGESTATION IN THE FETAL LAMB

Plasma serine disposal rate (DR), decarboxylation, and conversion to f etal plasma glycine by the placenta were measured in six fetal lambs a t 72 +/- 1 days gestation. L-[1-C-13]serine, L-[U-C-14]serine, and (H2 O)-H-3 were infused over 3 h into the fetal circulation, the latter fo r measurement of uterine and umbilical blood flow. The fetal plasma se rine DR was 8.7 +/- 1.0 mu mol/min or 61.8 +/- 4 mu mol.min(-1).kg fet us(-1). Approximately 90% of the DR represented placental uptake of fe tal serine. There was no detectable release of fetal serine into the m aternal circulation. The fetal arterial plasma glycine-to-serine enric hment ratio was similar to 0.30. The conversion rate of fetal serine t o fetal plasma glycine by the placenta was 5.8 +/- 0.7% of the serine DR. Fetal and placental CO2 production from fetal plasma serine carbon was 1.9 +/- 0.4 and 1.2 +/- 0.4 mu mol/min, respectively. Thus, at mi dgestation, there is a rapid fetoplacental serine exchange that consti tutes most of the fetal plasma serine turnover. Placental conversion o f serine to fetal glycine and serine oxidation together account for on ly 10% of the placental uptake of fetal serine.