Interaction between dopamine and its transporter: role of intracellular sodium ions and membrane potential

The present study addresses the effect of intracellular Na+ and membrane potential on the binding of dopamine (DA) to the dopamine transporter (DAT). Perforation of plasma membranes of DAT-expressing cells with gramicidin diminished DA uptake and decreased the potency (increases K-i) of DA in inhibiting the binding of cocaine analog [H-3]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT). It also compromised the ability of external Na+ to reduce DA K-i. No substantial effect on DA K-i was observed upon gramicidin treatment in Na+-free buffer, membrane depolarization with high [K+](o), or elevation of [Na+](i) with monensin under non-depolarizing conditions. Elevation of DA K-i was greater at more positive potentials when [Na+](i) was raised to a similar level, or at higher [Na+](i) when the membrane was depolarized to a similar level. In cells expressing D313N DAT, DA K-i was significantly higher but less sensitive to gramicidin than that in wild-type (WT) cells. In contrast, DA K-i in cell-free membranes was insensitive to Na+, gramicidin, and D313N mutation. The data suggest that (i) intracellular Na+ plays a role in affecting the external access to DA binding sites at DAT on depolarized plasma membranes of cells, and (ii) access to DA binding sites in cell-free membranes may occur from the intracellular side of the membrane. Unlike DA binding, CFT binding to both cells and membranes was sensitive to Na+ and D313N mutation but insensitive to gramicidin, consistent with exclusively external access to sites that are different from but conformationally linked to those for DA.