The contractions induced in rat and guinea-pig aortic strips by the alpha 2-adrenoceptor selective agonists B-HT 920 and UK 14,304 are mediated by alpha 1-adrenoceptors.

The alpha-adrenergic action of the selective alpha 2-adrenoceptor agonists B-HT 920 and UK 14,304 was investigated on helically cut preparations of rat and guinea-pig isolated aorta. The alpha 1-adrenoceptor selective stimulant (-)-phenylephrine was included for comparison. All agonists induced concentration-dependent contractions in both preparations. Calcium entry blockade by D 600 almost abolished the contractions evoked by B-HT 920 and UK 14,304 in rat aorta while those evoked in guinea-pig aorta were less strongly affected. Contractions elicited by (-)-phenylephrine were moderately impaired by D 600 in rat aorta whereas there was only a limited effect in guinea-pig aorta. Analysis of the prazosin and yohimbine antagonism of B-HT 920- and UK 14,304-evoked contractions showed the involvement of alpha 1-like adrenoceptors in rat and guinea-pig aorta, prazosin being approximately 1000 times more potent that yohimbine. The results show that B-HT 920 and UK 14,304 contract rat and guinea-pig aorta via alpha 1-like adrenoceptors which are not identical. It is submitted that rat and guinea-pig alpha 1-adrenoceptors activate different contractile processes.