The Na(+)-dependent release of endogenous dopamine and noradrenaline from rat brain synaptosomes

The involvement of intrasynaptosomal-free Ca++ concentration [( Ca++]i) in Na(+)-dependent release of endogenous dopamine and noradrenaline from rat brain synaptosomes was studied. The release of endogenous dopamine and noradrenaline from rat whole brain synaptosomes were measured using high-performance liquid chromatography with electrochemical detector. The change of [Ca++]i was measured fluorometrically using a Ca++ indicator, Quin-2. Whether extracellular Ca++ was present or not, 30 microM veratridine, a Na(+)-ionophore, increased the release of endogenous dopamine and noradrenaline. In the presence of 1.25 mM Ca++, 30 microM veratridine increased [Ca++]i. In contrast, in the absence of extracellular Ca++, veratridine did not affect [Ca++]i. Ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) increased the release of dopamine and noradrenaline in Ca-Mg-free medium. This stimulatory effect of EGTA seemed to be the result of an increase in the influx of Na+ through Ca++ channels in the absence of divalent cation. In Ca-Mg-free medium, EGTA caused a slight decrease in [Ca++]i. The EGTA-stimulated release of dopamine and noradrenaline was blocked by La which also significantly blocked the decrease in [Ca++]i observed after the addition of EGTA. These results suggest that the Na(+)-dependent release of dopamine and noradrenaline may not depend on a change in [Ca++]i. Veratridine (30 microM)-induced release of dopamine and noradrenaline were detected simultaneously. However, the time needed to induce the maximal stimulatory effect of veratridine on the release of dopamine was apparently shorter than that of noradrenaline. This delay might suggest that the Na(+)-dependent release process of dopamine is not similar to that of noradrenaline.