The action of tyramine on the dog isolated atrium.

It has been confirmed that tyramine has positive inotropic and chronotropic actions on the dog isolated atrium. These responses were incompletely and reversibly inhibited by cocaine, but completely and irreversibly blocked by phenoxybenzamine. Blockade of the atrial beta-receptors by dichloroisoprenaline could be overcome by noradrenaline and by larger doses of tyramine. With the aortic strip of the reserpinized rabbit for assay, tyramine was shown to release a vasoactive material from the dog atrium whose receptors were blocked by dichloroisoprenaline. The use of an antihistamine and an anti-5-hydroxytryptamine agent (cyproheptadine) appeared to exclude the possibility that the effect was due to the release of histamine or 5-hydroxytryptamine from the atrium by tyramine. Further observations of the action of the vasoactive material on the guinea-pig ileum and on the rat fundal strip strongly suggested that the material was a catechol amine. It was concluded that under these conditions tyramine acts by liberating catechol amines from storage sites so that the amines are free to act at receptor sites. The behaviour of the atrium to tyramine in the presence of cocaine or of phenoxybenzamine suggests that the liberation of catechol amines by tyramine differs from the release due to adrenergic nerve stimulation. It is suggested that, after an infusion of tyramine, there is a much slower release of catechol amines than after stimulation of adrenergic nerves.