AJP - Cell Physiology, Vol 262, Issue 5 C1189-C1196, Copyright © 1992 by American Physiological Society
Active transport of dopamine in human placental brush-border membrane vesicles
S. Ramamoorthy, F. H. Leibach, V. B. Mahesh and V. Ganapathy
Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta 30912-2100.
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.
