DEMONSTRATION OF SYSTEM Y(-MEMBRANE SURFACE OF RAT PLACENTA AND DEVELOPMENTALLY-REGULATED EXPRESSION OF 4F2HC MESSENGER-RNA()L ACTIVITY ON THE BASAL PLASMA)

Na+-independent cationic amino acid transport in the rat placenta occu rs by leucine-sensitive and leucine-insensitive pathways. The ontogeny of these transport mechanisms within the rat placenta has been descri bed recently. To assign the leucine-inhibitable portion of uptake defi nitively the uptake of [H-3]arginine was studied in the presence of bo th BCH (to inhibit system B-o,B-+) and varied concentrations of leucin e. Uptake of arginine into basal-enriched membrane vesicles derived fr om rat placenta was, in the presence of sodium, inhibited by micromola r concentrations of leucine, consistent with assignment of this activi ty to system y(+)L. In contrast, the majority of arginine uptake into apical-enriched membrane vesicles was leucine insensitive. Messenger R NA derived from rat placenta at days 14, 16, 18 and 20 of gestation wa s hybridized with full-length rat cDNA probes against NBAT and 4F2HC ( thought to encode proteins associated with system b(o,+) and y(+)L act ivities, respectively). No NBAT mRNA was detected, whereas 4F2HC mRNA was present at all gestational stages, increasing 12-fold over the las t 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 con tent of 4F2HC mRNA. (C) 1997 W. B. Saunders Company Ltd.