UPTAKE AND TRANSPORT BY THE OVINE PLACENTA OF NEUTRAL NONMETABOLIZABLE AMINO-ACIDS WITH DIFFERENT TRANSPORT-SYSTEM AFFINITIES

Placental uptake and transport of three nonmetabolizable amino acids w ith different reactivities for transport systems were studied in sheep under normal physiologic conditions. Methylaminoisobutyric acid (MeAI B), which has specific affinity for the sodium-dependent A system tran sporters, demonstrated placental concentrative uptake from the uterine and the umbilical circulations, but virtually no transport from mothe r to fetus. By contrast, aminoisobutyric acid (AIB) and aminocyclopent ane-1-carboxylic acid (ACP), which have affinity for both sodium-depen dent and sodium-independent transporters, demonstrated both concentrat ive uptake and transport from mother to fetus. ACP transport rate to t he fetus was approximately twice the AIB transport rate. It is conclud ed that a neutral amino acid which interacts almost exclusively with t he weakly reversible system A transporters may be transported rapidly into the placenta and may attain high concentrations within this organ but cannot escape from placenta to fetus down its own concentration g radient because the exit route is controlled by reversible amino acid transporters at the fetal surface of the placenta. Conversely, high af finity for reversible Na-independent transporters may be a necessary c ondition for the rapid transport of an amino acid from placenta to fet us. (C) 1998 W. B. Saunders Company Ltd.