Fetal serine fluxes across fetal liver, hindlimb, and placenta in late gestation
Cetin, I.; Fennessey, P.V.; Sparks, J.W.; Meschia, G.; Battaglia, F.C.
American Journal of Physiology 263(4 Pt 1): E786-E793
1992
ISSN/ISBN: 0002-9513 PMID: 1415701 Document Number: 393141
Eleven studies of fetal serine fluxes were performed in chronically catheterized fetal lambs by continuous infusion of (1-13C)- and (U-14C)serine into a fetal branchial vein. At tracer serine steady state, samples were collected from the fetal abdominal aorta, umbilical vein, fetal hepatic vein, and fetal femoral vein and from the maternal femoral artery and uterine vein. Analyses were perrformed for plasma serine and glycine concentration, for serine and glycine 13C mole percent enrichment, and for whole blood 14CO-2 and O-2 concentrations. Uterine and umbilical blood flows were also measured. The placenta had a significant net uptake of fetal serine (2.1 +- 0.5 mu-mol cntdot min-1 cntdot kg-1, P lt 0.01). Fetal plasma serine disposal rate (DR) was 42.5 +- 3.9 mu-mol cntdot min-1 cntdot kg-1. CO-2 production from decarboxylation of fetal plasma serine represented 7.9 +- 0.5% of DR, or 10.1 +- 1.2 mu-mol CO-2 cntdot min-1 cntdot kg-1. Fetal plasma glycine enrichment was 59.7 +- 4.9% of fetal plasma serine enrichment. There was a significant loss of tracer serine from the fetal circulation into the placenta accounting for apprx 45% of infused tracer. Fifteen percent of this was converted to glycine and released into the umbilical circulation. There was a significant uptake of tracer serine by both fetal liver and fetal hindlimb with a significant CO-2 production by both sites with serine oxidation predominantly in the carcass. These results indicate a high fetal serine disposal rate in the limb, with rapid fetoplacental serine exchange, resulting in a net uptake of fetal serine by the placenta. Significant placental glycine production from fetal plasma serine was demonstrated, supporting the concept of interorgan cycling of serine and glycine between the fetal liver and placenta.