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Research was conducted to test whether the exchange of amino acids between plasma and fetal red blood cells is slow enough to permit the use of plasma concentrations and plasma flows for estimating umbilical amino acid uptakes. The umbilical uptake of each amino acid was compared with its normal fetal accretion rate while hemoglobin and O2 saturation were measured spectrophotometrically. Results showed that uterine and umbilical uptakes of amino acids can be determined on either metabolic pathways or transport systems.

Effect of dexamethasone on fetal hepatic glutamine-glutamate exchange. Am J Physiol Endocrinol Metab 278: E839-E845, 2000.--Intravenous infusion of dexamethasone (Dex) in the fetal lamb causes a two- to threefold increase in plasma glutamine and other glucogenic amino acids and a decrease of plasma glutamate to approximately one-third of normal. To explore the underlying mechanisms, hepatic amino acid uptake and conversion of L-[1-[sup.13]C] glutamine to L-[1-[sup.13]C] glutamate and [sup.13][CO.sub.2] were measured in six sheep fetuses before and in the last 2 h of a 26-h Dex infusion. Dex decreased hepatic glutamine and alanine uptakes (P [is less than] 0.01) and hepatic glutamate output (P [is less than] 0.001). Hepatic outputs of the glutamate (R[sub.Glu,Gln]) and [CO.sub.2] formed from plasma glutamine decreased to 21 (P [is less than] 0.001) and 53% (P = 0.009) of control, respectively. R[sub.Glu,Gln], expressed as a fraction of both outputs, decreased (P [is less than] 0.001) from 0.36 [+ or -] 0.02 to 0.18 [+ or -] 0.04. Hepatic glucose output remained virtually zero throughout the experiment. We conclude that Dex decreases fetal hepatic glutamate output by increasing the routing of glutamate carbon into the citric acid cycle and by decreasing the hepatic uptake of glucogenic amino acids. fetus; amino acids; fetal liver; placenta