Comparative Cardiovascular Effects of Tyramine, Ephedrine, and Norepinephrine in Man

The systemic hemodynamic and forearm vascular effects of tyramine, ephedrine, and norepinephrine were studied in normal male subjects. The pressor effect of tyramine was associated usually with bradycardia, unchanged cardiac output, and moderate arteriolar constriction in the forearm. A similar pressor effect of ephedrine was characterized by an increase of heart rate and output, with a slight fall in forearm vascular resistance. Occasional subjects responded to tyramine with an increase of cardiac output and a fall in forearm vascular resistance, and to ephedrine, with a marked increase of vascular resistance. The pressor effect of norepinephrine was associated always with bradycardia and with marked arteriolar and venous constriction in the forearm. Brachial arterial infusions of the drugs in nonpressor doses confirmed the vascular effects. Norepinephrine produced dose-dependent forearm constriction in all subjects, while vasodilatation was observed occasionally with tyramine and frequently with ephedrine. The demonstrated differences in local vascular and systemic hemodynamic effects of these drugs suggest some inconsistency in the hypothesis that the effect of either ephedrine or tyramine is mediated exclusively by norepinephrine release.