EFFECTS OF SEVERAL CATIONS ON THE NEURONAL UPTAKE OF DOPAMINE AND THE SPECIFIC BINDING OF [H-3] GBR 12783 - ATTEMPTS TO CHARACTERIZE THE NA+ DEPENDENCE OF THE NEURONAL TRANSPORT OF DOPAMINE

We have studied the effects of several cations on (1) the neuronal uptake of [H-3]dopamine ([H-3]DA) and (2) the specific binding of 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-H-3]propenyl)piperazine ([H-3]GBR 12783) to a site associated with the neuronal carrier of DA, in preparations obtained from rat striatum. When studied under the same experimental conditions, both the uptake of [H-3]DA and the binding of [H-3]GBR 12783 were similarly impaired by the gradual replacement of NaCl by sucrose. In both processes, no convenient substitute for Na+ was found. Furthermore, potential substitutes of Na+ acted as inhibitors of the uptake with a rank order of potency as follows: K+ = Li+ greater-than-or-equal-to Cs+ greater-than-or-equal-to Rb+ > choline+ > Tris+ > sucrose, which was somewhat different from that observed in binding studies, i.e., Cs+ > Rb+ > choline' greater-than-or-equal-to K+ > Li+ > Tris+ > sucrose. In the presence of either 36 mM or 136 mM Na+, [H-3]DA uptake was optimal with 2 mM Mg2+, 1 mM K+, or 1 mM Ca2+. In contrast, higher concentrations of divalent cations competitively blocked the uptake process. K+ concentrations >50 mM impaired the specific binding, whereas in the millimolar range of concentrations, K+ noncompetitively inhibited the uptake. Decreasing the Na+ concentration increased the inhibitory effect of K+, Ca2+, and Mg2+ on the specific uptake. An increase in NaCl concentration from 0 to 120 mM elicited a significant decline in the affinity of some substrates for the [H-3]GBR 12783 binding site. An uptake study performed using optimal experimental conditions defined in the present study revealed that decreasing Na+ concentration reduces the affinity of DA for the neuronal transport. We propose a hypothetical model for the neuronal transport of DA in which both Na+ and K+ membrane gradients are involved.