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The perfused isolated rabbit ear artery was used as a vascular model to compare the constrictor responses to exogenous and endogenous norepinephrine. The aim of the study was to estimate the concentrations of norepinephrine reaching the smooth muscle of the artery during sympathetic nerve stimulation. The artery was perfused so that exogenous norepinephrine was introduced exclusively from the adventitial surface; thus, the drug followed a path to the smooth muscle layer similar to that of the endogenous norepinephrine released from the perivascular ring of sympathetic nerves during electrical stimulation of the artery wall. Cocaine was used to remove the influence of neural uptake of norepinephrine on concentrations of the amine reaching the muscle receptor sites. The estimated concentrations of norepinephrine reaching the smooth muscle receptors for each pulse of electrical stimulation ranged from 24.8 ± 5.9 ng/ml (1.47 × 10−7M) to 29.6 ± 7.7 ng/ml (1.75 × 10−7M) over the range of pressure changes from 30 to 100 mm Hg. This finding suggests that the relative sensitivity to endogenous and exogenous norepinephrine is similar at different response levels in this range and that the effect of local factors in the artery on concentrations reaching the smooth muscle is of the same order of magnitude for both endogenous and exogenous norepinephrine.

Comparison of Constrictor Responses of the Rabbit Ear Artery to Norepinephrine and to Sympathetic Nerve Stimulation