ONTOGENY OF CATIONIC AMINO-ACID-TRANSPORT SYSTEMS IN RAT PLACENTA

Gestational regulation of the placental transfer of amino acids from maternal to fetal circulations is essential for the proper development of the fetus. The cationic amino acid transport systems of the microvillous (maternal facing) and basal (fetal facing) membranes of the rat placental syncytiotrophoblast were examined. Inhibition analysis documented the presence of three kinetically distinct cationic amino acid transport mechanisms: a single Na+-dependent mechanism in the microvillous membrane, which increased in activity from 14 to 20 days gestation but was absent from the basal membrane throughout the entire gestational period (system B-0,B-+), and two Na+-independent transport systems in both membrane domains, one that is completely inhibited by leucine, which increased in activity in both the microvillous and basal membrane domains, and the other that is leucine insensitive, which remained fairly constant in the basal membrane and increased throughout gestation in the microvillous membrane (system y(1)(+)). Northern analysis with the system y(1)(+) cDNA revealed a specific band of similar to 7.4-7.9 kb, which increased with increasing gestational age.