An Examination of the Postjunctional Alpha-adrenoceptor Subtypes for (-)-noradrenaline in Several Isolated Blood Vessels from the Rabbit

1. Postjunctional alpha-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 (alpha 1-selective) and UK-14304 (alpha 2-selective), and the potency of antagonists rauwolscine (alpha 2-selective) and corynanthine (alpha 1-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 alpha 1-adrenoceptors since the agonist potency order was NA greater than phenylephrine greater than UK-14304, while corynanthine was 3-10 fold more potent than rauwolscine. 3. The ear vein appears to possess alpha 2-adrenoceptors. The rank order of agonist potency was UK-14304 greater than NA much greater than 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 greater than NA much greater than 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 alpha 2-adrenoceptors, prazosin was either potent (saphenous vein) or relatively inactive (plantaris vein). 5. The characteristics of postjunctional alpha 1- and alpha 2-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.