An examination of the postjunctional α-adrenoceptor subtypes for (−)-noradrenaline in several isolated blood vessels from the rabbit

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Abstract
1 Postjunctional α-adrenoceptors in several isolated blood vessels from the rabbit have been characterized on the basis of the relative potency of the agonists noradrenaline (NA, non-selective), phenylephrine (α1-selective) and UK-14304 (α2-selective), and the potency of antagonists rauwolscine (α2-selective) and corynanthine (α1-selective) against contractions elicited by NA. In addition, the potency of prazosin against NA was also assessed in the venous preparations. 2 The thoracic aorta, ear artery and left renal vein appear to possess α1-adrenoceptors since the agonist potency order was NA > phenylephrine > UK-14304, while corynanthine was 3–10 fold more potent than rauwolscine. 3 The ear vein appears to possess α2-adrenoceptors. The rank order of agonist potency was UK-14304 > NA > phenylephrine and all three agonists elicited responses of similar magnitude. Furthermore, rauwolscine was 30 fold more potent than corynanthine while prazosin failed to produce a concentration-dependent inhibition. 4 The saphenous vein and the plantaris vein appear to possess a mixture of both subtypes since the rank order of agonist potency was UK-14304 > NA ≫ phenylephrine, while responses elicited by UK-14304 were smaller than those to the other agonists. However, although rauwolscine was 20 to 100 fold more potent than corynanthine in both preparations, suggestive of predominantly α2-adrenoceptors, prazosin was either potent (saphenous vein) or relatively inactive (plantaris vein). 5 The characteristics of postjunctional α1- and α2-adrenoceptors on isolated blood vessels from the rabbit are discussed in relation to the value of both the agonists, particularly NA, and the antagonists used in this study.