Sodium independence of the binding of [³H]GBR 12783 and other dopamine uptake inhibitors to the dopamine uptake complex

When Na⁺ was the only cation present in the incubation medium used for the determination of the specific binding of [³H]GBR 12783 in rat striatal membranes, the Na⁺-dependence between 10 and 210 mM Na⁺ was not observed. In media with low (10 mM) or high (130 mM) Na⁺ concentration, mazindol and nomifensine competed with [³H]GBR 12783 for its specific binding site with the same affinities. With the exception of amineptine, all the tested catecholamine uptake inhibitors were equally potent to compete with [³H]GBR 12783 when Na⁺ concentration was decreased from 130 to 10 mM. These data suggest that the media previously used for the binding studies of tritiated inhibitors of dopamine uptake (Tris-ions buffer and Krebs-Ringer medium) contain ions which could exert inhibitory effects on the specific binding at low Na⁺ concentration.