Comparison of characteristics of dopamine uptake and mazindol binding in mouse striatum

Summary Biochemical and pharmacological studies suggest that the binding of ³H]mazindol is functionally related to the dopamine uptake carrier complex in rodent striatum. In order to study further the relationship between the substrate recognition site for dopamine uptake and the high-affinity binding site for mazindol the uptake of ³H]dopamine and the binding of ³H]mazindol was studied in BALB/cBy mouse striatum in various buffers (Tris, HEPES, bicarbonate-phosphate). Kinetic analysis showed that theK _d, of the binding of ³H]mazindol and theK _m of the uptake of ³Mdopamine was changed by different sodium concentrations and/or by the presence of Tris, while theB _max, and theV _max remained essentially the same. However, the shape of the Na⁺ dependency curves was not the same for mazindol binding and dopamine uptake in the various buffers. The inhibitory effect of other cations such as K⁺ and Tris was also different on binding and uptake under similar experimental circumstances. Dopamine did not slow down the dissociation of mazindol from its site and this effect was not sodium-sensitive. These complexities can be accomodated by a model that involves overlapping sites for mazindol and dopamine on the dopamine uptake carrier complex, and translocation -reorientation steps.Abbreviations K _d dissociation rate constant - HEPES N-2hydroxyethyl-piperazine-N-2-ethanesulfonic acid - Km half-saturating concentration for uptake - B _max maximal binding capacity - V _max maximal initial uptake rate - k _–1 dissociation rate constant - IC₅₀ concentration of inhibitor causing 50% inhibition - DA 3,4dihydroxyphenylethylamine (dopamine) On leave from the Institute of Experimental Medicine, H-1450, Budapest, P.O.B. 67, Hungary. Send offprint request to I. Zimányi at her present address