Influence of calcium on noradrenaline release evoked by 5-hydroxytrypamine and potassium from goat pial arteries.

The release of tritium (3H) evoked by tyramine, potassium (K+) and 5-hydroxytryptamine (5-HT) from goat pial arteries preloaded with [3H]noradrenaline (3H-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 3H 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 3H efflux produced by K+ was markedly reduced. Nevertheless, in this Ca2+-free medium the 3H release evoked by 5-HT was partially, but significantly, decreased. These results indicate that K+ evokes NA release by a Ca/+-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.