EFFECT OF LOW CONCENTRATIONS OF K- ON THE NA+-DEPENDENT NEURONAL UPTAKE OF [H-3] DOPAMINE( AND CL)

The specific uptake of [H-3] dopamine (DA) was studied using a crude s ynaptosomal fraction obtained from rat striatum. In a medium containin g a 10 mM NaHCO3/NaH2PO4 buffer and no added K+ ions, addition of NaCl elicited an increase in DA uptake for Na+ concentrations from 10 to 6 0 mM, and then a decrease of uptake for Na+ concentrations up to 130 m M. These data confirm that rather low NaCl concentrations produce a ma ximal DA uptake. This biphasic curve of uptake resulted from significa nt changes in the V-max of the DA uptake. Except for 10 mM Na+, this c urve was not significantly modified when 9 mM NaHCO3/NaH2PO4 were repl aced by 9 mM NaCl. This result indicates that the Cl- dependence of th e DA uptake is mainly secondary to the Na+ dependence. Addition of KCl up to 3 mM did not modify the ascending part of the NaCl-dependent up take curve. In contrast, the reduction in uptake produced by high Naconcentrations was prevented in a concentration-dependent manner by KC l; this effect resulted from a decrease in the K-m, and an increase in the V-max for the uptake. Measurements of membrane potential, with th e help of the fluorescent probe 3,3'-diethylthiadicarbocyanine iodide [DiSC(2)(5)] and purified synaptosomes prepared from rat striatum and cerebral cortex, revealed that addition of 3 mM KCl to a medium contai ning a high Na+ concentration and no K+ ions produced a marked and sta ble decrease in the fluorescence level. This decrease which correspond s to an increase in membrane polarization was blocked by 0.1 mM ouabai n. These data suggest that low K+ concentrations are likely to prevent the decrease in uptake elicited by high Na+ concentrations by restora tion, via a Na+/K+ ATPase-mediated mechanism, of the membrane potentia l and/or a transmembrane electrochemical Na+ gradient more favourable to DA uptake.