PHARMACOLOGICAL CHARACTERIZATION OF α-ADRENORECEPTOR SUBTYPES IN RAT ISOLATED THORACIC AORTA

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Abstract
The subtype of .alpha.-adrenoreceptor mediating contraction in rat isolated thoracic aorta was classified pharmacologically using preferential agonists and antagonists and by using mixed agonist and antagonist interactions. Noradrenaline [norepinephrine, NE] was 8 to 10-times more potent at contracting the aorta than phenylephrine [PE] and both agonists were .apprx. 1000 and 10,000-fold, respectively, more potent than azepexole [A] (a preferential .alpha.2-agonist). Prazosin (a preferential .alpha.1-antagonist) inhibited the dose-response curves to NE and PE 100 and 1000-times, respectively, more effectively than either phentolamine or rauwolscine (a preferential .alpha.2-antagonist). Prazosin (5 .times. 10-9 M) completely abolished contractions elicited by a single concentration of A (3 .times. 10-4 M). In mixed antagonist studies, rauwolscine (5 .times. 10-7 M) failed to shift the dose-response curves to NE and PE obtained in the presence of prazosin (5 .times. 10-9 M). In mixed agonist experiments, A (3 .times. 10-4 M) acted as a partial antagonist toward PE-induced contractions. Apparently, the .alpha.-adrenoreceptor of the rat thoracic aorta is predominantly, if not exclusively, of the .alpha.1-subtype.