Uptake of choline into syncytial microvillus membrane vesicles of human term placenta.

The uptake of the quaternary ammonium compound choline was studied in syncytial microvillus membrane vesicles of human term placenta. Uptake was stimulated by an inside negative membrane potential and by loading the vesicles with unlabeled choline. Imposition of an inwardly directed Na+ or outwardly directed H+ gradient did not stimulate choline uptake. Several organic cations were able to inhibit choline transport in the following order: hemicholinium-3 > or = choline > or = mepiperphenidol > cimetidine > or = famotidine. The kinetics of uptake involved a saturable process for choline with high affinity (Km = 550 microM). Our results confirm the presence of a carrier mediated transport system in human placental syncytial microvillus membranes. The system appears to be electrogenic, and able to transport choline efficiently from the maternal circulation into the placenta.