METABOLISM AND TRANSPORT OF MATERNAL SERINE BY THE OVINE PLACENTA - GLYCINE PRODUCTION AND ABSENCE OF SERINE TRANSPORT INTO THE FETUS

The present study compares the transplacental transport Of L-[l-C-13]s erine and L-[l-C-13]leucine in sheep. An in vivo preparation using twi n gestations was set up such that the arterial circulation to one uter ine horn, including its placenta and fetus, was infused with tracer se rine and leucine while the umbilical circulations of both fetuses were sampled. Uterine and umbilical blood flows were measured in each horn . Plasma serine enrichments were 14.6 +/- 2.7% and 4.3 +/- 1.6% in the uterine veins draining the experimental and control horns, respective ly. Fetal plasma leucine enrichments in the umbilical veins were 50 an d 55% of the uterine venous enrichments in the control and experimenta l fetuses, respectively. By contrast, during 280 min of infusion, ther e was no detectable serine enrichment in either fetal circulation. How ever, significant plasma glycine enrichment was present in the fetal c irculation of the experimental horn and venous glycine enrichments in the experimental horn were significantly greater than arterial glycine enrichments for both the umbilical (p < 0.02) and uterine (p < 0.001) circulations. We conclude that under conditions in which leucine tran sport is easily demonstrable there is no significant transplacental tr ansport of maternal serine and that maternal plasma serine is used wit hin the uteroplacental tissues for producing glycine, some of which is delivered into the fetal circulation.