Active transport of dopamine in human placental brush-border membrane vesicles.

Brush-border membrane vesicles isolated from normal human term placentas were found to accumulate dopamine against a concentration gradient when an inwardly directed NaCl gradient was imposed across the membrane. The activity of the transport system was obligatorily dependent on Na+ as well as Cl-. Intravesicular H+ and intravesicular K+ stimulated the transport activity. The system possessed high affinity for dopamine and norepinephrine, with considerably lower affinity for serotonin. The stoichiometry of Na(+)-Cl(-)- dopamine was 1:1:1. The system was electrogenic because the NaCl-dependent dopamine uptake was stimulated by an inside-negative membrane potential, and this characteristic was observed in the presence and in the absence of intravesicular K+. Kinetic analysis revealed that the uptake was due to a carrier-mediated component plus a diffusion/binding component. The apparent Michaelis-Menten constant for dopamine for the carrier-mediated component was 19 +/- 7 nM. The transport system was clearly distinct from the serotonin transporter. Analysis of the inhibition of dopamine uptake by various monoamine uptake inhibitors showed that the uptake occurred via a transport system that is similar to the neuronal norepinephrine transporter.