CATIONIC AND ANIONIC REQUIREMENTS FOR THE BINDING OF TA-CARBOMETHOXY-3-BETA-(4-FLUOROPHENYL)[3H]TROPANE TO THE DOPAMINE UPTAKE CARRIER

The present study reports the ion dependency of ta-carbomethoxy-3beta- (4-fluorophenyl)[H-3]tropane ([H-3]CFT) binding to the dopamine transp orter in the rat striatum. The results indicate that [H-3]CFT binding to synaptosomal P2 membranes requires low concentrations of Na+ (peak binding between 20 and 50 mM Na+), is stimulated by phosphate anion or I-, but is unaffected or only slightly affected by F-, Cl-, Br-, NO3- , or SO42-. Concentrations of Na+ of >50 mM become inhibitory except i n the presence of I-, which shifts peak binding levels toward higher N a+ concentrations and also elevates the peak binding level. K+ strongl y decreased [H-3]CFT binding with a shallow inhibition curve, and Nacould not overcome this effect. Saturation analySiS of [H-3]CFT bindin g revealed a single binding site changing its affinity for CFT dependi ng on the concentration of sodium phosphate buffer (6, 10, 30, 50,130, or 200 mM; 1 mM plus 49 mM NaCl versus 1 0 mM plus 40 mM NaCl; or 1 m M plus 129 mM NaI versus 1 0 mM plus 120 mM Nal). No differences were observed in the density of CFT binding sites between any of the condit ions examined.