Evidence for modulation by ATP of amino acid transport in human placenta

The effect of ATP on placental amino acid transport was studied by measuring the uptake of alpha-(methylamino)-isobutyrate in brush border microvillous plasma membrane vesicles prepared from human full-term placental syncytiotrophoblasts which were incubated with or without ATP. The presence of a Na+ gradient from the outside to the inside of the vesicles prepared after incubation with ATP resulted in a higher initial rate and an increased transport of alpha-(methylamino)-isobutyrate, while Na+ gradient-independent alpha-(methylamino)-isobutyrate uptake was not different in either type of membrane vesicle. The increase in transport activity was not inhibited by cycloheximide. Kinetic analysis showed that ATP enhanced transport activity by increasing the maximal velocity (Vmax) of transport, without significant changes in the affinity (Km) of the carrier for the substrate, suggesting an increase in carrier number in placental syncytiotrophoblasts incubated with ATP.