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.