Influence of calcium on noradrenaline release evoked by 5-hydroxytryptamine, tyramine and potassium from goat pial arteries

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Abstract
The release of tritium (3 H) evoked by tyramine, potassium (K+) and 5-hydroxytryptamine (5-HT) from goat pial arteries preloaded with [3 H]noradrenaline (3 H-NA) was studied. In normal Krebs-bicarbonate solution (KBS) all these agents caused a transient increase in radioactivity release over the basal spontaneous outflow. The pattern of release evoked by 5-HT was similar to that induced by tyramine with a slow onset and decline, but different from that induced by K+ which produced a rapid peak of 3 H release followed by a quick fall. The removal of Ca2+ from the medium did not modify the efflux of radioactivity caused by tyramine, but the 3 H efflux produced by K+ was markedly reduced. Nevertheless, in this Ca2+ -free medium the 3 H release evoked by 5-HT was partially, but significantly, decreased. These results indicate that K+ evokes NA release by a Ca2+ -dependent process, probably of an exocytotic nature, while tyramine mediates NA release by means of a Ca2+ -independent mechanism. However, 5-HT possesses a Ca2+ -dependent and a tyramine-like component.