Umbilical uptake of amino acids in the unstressed fetal lamb

Abstract

The observation that several amino acids, both essential and nonessential, can cross the placenta rapidly and against a concentration gradient has led to the suggestion that the placenta may transport to the fetus all of the amino acids which are needed for protein synthesis. This study was designed to measure the whole blood concentrations of 22 amino acids in a chronic, unstressed fetal lamb preparation. Samples were taken daily from the umbilical artery, umbilical vein, and maternal artery over the latter quarter of gestation; 73 sets of samples (from the umbilical artery and vein and the maternal artery) from 13 animals were analyzed for amino acid levels. Oxygen contents were determined simultaneously in 48 sets (umbilical artery and vein) to relate fetal oxygen consumption to amino acid uptake via the umbilical circulation. The results indicate that there is no umbilical uptake of the acidic amino acids, glutamate and aspartate; there is, in fact, a net flux of glutamate out of the fetus into the placenta. As both of these amino acids are major constituents of body proteins, the data indicate that they are formed within the fetus. The umbilical uptake of some neutral and basic amino acids (e.g., valine, leucine, isoleucine, arginine, phenylalanine, and tyrosine) is in considerable excess of estimated growth requirements, suggesting that some amino acids undergo extensive transamination and oxidative degradation in the fetus. Finally, the net uptake of nitrogen, carbon, and calories by the growing ovine fetus in the form of amino acids, glucose, and lactate is compared to estimated requirements as determined in previous studies.