Vasoactive responses of a human cystic artery: adrenoceptor characterization.

1. The pharmacology of various agonists and antagonists was studied in the human isolated cystic artery. 2. The estimated pA2s for the alpha 1‐ adrenoceptor antagonist prazosin against the alpha‐adrenoceptor agonists phenylephrine, alpha‐methylnoradrenaline and noradrenaline were not significantly different. Similar results were seen for estimated pA2s of the alpha 2‐adrenoceptor antagonist yohimbine against these same agonists. Equivalent responses to exogenous noradrenaline and to transmural electrical stimulation were blocked to the same degree by an antagonist with alpha 1‐adrenoceptor blocking properties (prazosin). Responses to transmural electrical stimulation, however, tended to be more resistant than equivalent responses to exogenous noradrenaline to blockade by antagonists with alpha 2‐adrenoceptor blocking properties (phentolamine, yohimbine). 3. Relaxation to isoprenaline was observed in partially contracted arterial strips using isoprenaline concentrations of up to 10(‐6) M, but cumulative addition of higher concentrations of isoprenaline sometimes then evoked a contraction from the relaxation nadir. The relaxation effect of isoprenaline was antagonized by propranolol (10(‐5) M). 4. These findings suggest the human cystic artery has almost exclusively alpha 1‐ adrenoceptors postjunctionally, although prejunctional alpha 2‐ adrenoceptors may be present; and, it also has some postjunctional beta‐ adrenoceptors which mediate relaxation.