Responses of isolated dog coronary arteries to tyramine.

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Abstract
In isolated dog coronary arteries contracted with prostaglandin F2.alpha., tyramine in concentrations of 10-6 and 5 .times. 10-6 M caused relaxations, but it produced contractions at 2 .times. 10-5 M or higher. The relaxant response to tyramine was attenuated, but the contractile response was enhanced at the 2nd trial as compared with the responses at the 1st. Relaxations induced by low concentrations of tyramine were reversed to contractions by treatment with propranolol (10-6 M) or sotalol (10-5 M), and were abolished by cocaine (3 .times. 10-6 M) or bretylium (2 .times. 10-5 M). In coronary arteries isolated from reserpine (0.5 mg/kg)-pretreated dogs, tyramine produced only a contraction. Under resting conditions, contractions induced by tyramine (5 .times. 10-6 to 2 .times. 10-3 M) were potentiated by cocaine and propranolol and were inhibited by phentolamine. Norepinephrine [NE] produced a dose-dependent relaxation in the arteries contracted with prostaglandin F2.alpha.. In the presence of propranolol, the arteries under resting conditions were contracted by NE the contraction being suppressed by treatment with phentolamine. Relaxations of dog coronary arteries induced by tyramine were apparently mediated by liberation of NE from adrenergic nerves which stimulates .beta.-adrenoceptors in the smooth muscle. The tyramine (2 .times. 10-5 M or higher)-induced contraction was evidently not mediated by NE released but was partly due to a direct action on .alpha.-adrenoceptors.