Complex ionic control of [H-3]GBR 12783 binding to the dopamine neuronal carrier

At 20 degrees C, [H-3]GBR 12783, {1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-([1-H-3]propenyl)-piperazine} dissociated from the dopamine neuronal carrier present in rat striatal membranes with a t(1/2) value of 27 min. At this temperature, KCl, CaCl2, and MgCl2 increased the binding dissociation, revealing that they recognize a binding site which is not mutually exclusive with that of [H-3]GBR 12783. The comparison of the ability of KCl to increase the binding dissociation (by 160% at 30 mM KCI) with its potency as a binding inhibitor (K-i = 2.6 +/- 0.3 mM) suggests an involvement of two recognition sites for K+ in binding inhibition, a not mutually exclusive site and another, mutually exclusive, site. Divalent cations mainly inhibited the binding via a mutually exclusive site since 3 mM Ca2+ and 10 mM M(2+) increased the binding dissociation by 90% at 20 degrees C whereas their K-i values were 0.049 +/- 0.006 and 0.141 +/- 0.035 mM, respectively. Involvement of this mutually exclusive site was also supported by the persistence of the binding inhibition elicited by Ca2+ and Mg2+ at 0 degrees C, a temperature at which they reduced the binding dissociation. At 20 degrees C, 100 mM NaCl did not modify [H-3]GBR 12783 binding but it antagonized the binding dissociation elicited by inhibitory cations. Ca2+ reduced the off-rate of [H-3]GBR 12783 binding at 0 degrees C and in the presence of 100 mM Na+. Finally, [H-3]GBR 12783-binding dissociation was increased by high 'cytosolic' K+ while 'synaptic' concentrations of Na+ K+, Ca2+, Mg2+ and Cl- were ineffective. A reduction of H2PO4-/HCO3- from 10 to 5 mM and a substitution of 5 mM H2PO4-/HCO3- by 5 mM Cl- increased the binding dissociation, suggesting that an anion-binding site could also regulate the binding.