Demonstration of system y+L activity on the basal plasma membrane surface of rat placenta and developmentally regulated expression of 4F2HC mRNA.

Na(+)-independent cationic amino acid transport in the rat placenta occurs by leucine-sensitive and leucine-insensitive pathways. The ontogeny of these transport mechanisms within the rat placenta has been described recently. To assign the leucine-inhibitable portion of uptake definitively the uptake of [3H]arginine was studied in the presence of both BCH (to inhibit system Bo,+) and varied concentrations of leucine. Uptake of arginine into basal-enriched membrane vesicles derived from rat placenta was, in the presence of sodium, inhibited by micromolar concentrations of leucine, consistent with assignment of this activity to system y+L. In contrast, the majority of arginine uptake into apical-enriched membrane vesicles was leucine insensitive. Messenger RNA derived from rat placenta at days 14, 16, 18 and 20 of gestation was hybridized with full-length rat cDNA probes against NBAT and 4F2HC (thought to encode proteins associated with system bo,+ and y+L activities, respectively). No NBAT mRNA was detected, whereas 4F2HC mRNA was present at all gestational stages, increasing 12-fold over the last third of gestation. It is concluded that system y+L is present in the basal plasma membrane of the rat placenta syncytium and is subject to developmental regulation by a mechanism that alters the steady content of 4F2HC mRNA.