Abstract
Plasma serine disposal rate (DR), decarboxylation, and conversion to fetal plasma glycine by the placenta were measured in six fetal lambs at 72 +/- 1 days gestation. L-[1-13C]serine, L-[U-14C]serine, and 3H2O were infused over 3 h into the fetal circulation, the latter for measurement of uterine and umbilical blood flow. The fetal plasma serine DR was 8.7 +/- 1.0 mumol/min or 61.8 +/- 4 mumol.min-1.kg fetus-1. Approximately 90% of the DR represented placental uptake of fetal serine. There was no detectable release of fetal serine into the maternal circulation. The fetal arterial plasma glycine-to-serine enrichment ratio was approximately 0.30. The conversion rate of fetal serine to 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 mumol/min, respectively. Thus, at midgestation, there is a rapid fetoplacental serine exchange that constitutes most of the fetal plasma serine turnover. Placental conversion of serine to fetal glycine and serine oxidation together account for only 10% of the placental uptake of fetal serine.
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