The action of tyramine on the rabbit ear artery.

1. The vasoconstrictor potency of extraluminal tyramine on the isolated perfused rabbit ear artery is considerably greater than that of intraluminal tyramine.2. Chronic denervation, which caused the noradrenergic storage structures in the medial-adventitial border of the artery to disappear, reduces the potency of extraluminal tyramine more than that of intraluminal tyramine so that the difference in potency for the two routes of administration tends to disappear. Cocaine exerts a greater inhibitory effect on responses to extraluminal tyramine than on those to intraluminal tyramine.3. It is concluded that the indirect component plays a more prominent part in the responses to extraluminal tyramine than in those to intraluminal tyramine. This conclusion is supported by analysis of the diphasic response of the artery to intraluminal injection of tyramine under perfusion conditions which permit intraluminal fluid to mix with the extraluminal fluid. Evidence is presented that the first phase of the response is mediated by intraluminal tyramine, and the second phase by extraluminal tyramine.