Mechanism of the indirect adrenergic effect of histamine in cat cerebral arteries

KCl (50 mM), tyramine (10(-7) M), and histamine (10(-4) M) induced an increase in tritium release from cat cerebral arteries preincubated with 3H]noradrenaline, this increase being due in part to noradrenaline. When calcium was absent from the superfusion medium, only tyramine (10(-7) M) enhanced the tritium outflow. Colchicine (10(-3) M) partially inhibited the increase in radioactivity brought about by 10(-4) M histamine. KCl (50 mM) also evoked release of radioactivity from cerebral arteries preloaded with 3H]histamine; this release was unaffected by reserpine pretreatment or removal of both superior cervical sympathetic ganglia. Neither tyramine (10(-7) M) nor compound 48/80 (300 micrograms ml-1) altered the spontaneous tritium outflow from cerebral blood vessels preincubated with 3H]histamine. These results suggest that histamine is not accumulated by sympathetic nerve endings and elicits its noradrenaline-releasing effect by means of an exocytotic process.