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

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Abstract
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 unafffected by reserpine pretreatment or removal of both superior cervical sympathetic ganglia. Neither tyramine (10−7 m) nor compound 48/80 (300 μg 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.