A comparison of the beta-adrenoceptor agonist potency of labetalol with those of sympathomimetic drugs, and the role of alpha-receptors in pharmacological actions of these drugs on tracheal smooth muscles in guinea pigs.

Abstract

The β-receptor agonist potency and α-receptor-blocking action of labetalol were investigated in tracheal smooth muscles of guinea pigs. With resting-tone preparations and methacholine-contracted preparations of trachea, labetalol produced relaxation and behaved as a partial agonist. The maximum relaxing response to labetalol of methacholine-contracted preparations was markedly smaller than that of the resting-tone preparations. The results suggest that the relaxing action of labetalol is weaker on the high tracheal tone induced by the cholinergic agonist, methacholine. With resting-tone preparations, norepinephrine and phenylephrine produced relaxation, but after the β-receptors were blocked, these drugs produced contraction. These findings confirm the existence of α-excitatory receptors and the predominance of β-receptors over α-receptors in the trachea. When labetalol was administered first, it showed β-receptor agonist action and inhibition of contraction of the resting-tone preparations due to α-receptors. When labetalol was administered secondly, it did not show β-receptor agonist action but exerted α-adrenoceptor-blocking action. These results suggest that labetalol produces relaxation of the tracheal preparations in guinea pigs through its β-receptor partial agonist action and α-receptor-blocking action. However, it is thought that this relaxing effect of labetalol might be small when the β-adrenoceptor reserve in the trachea is occluded or exhausted by pretreatment with a β-receptor blocker or a cholinergic agonist, methacholine.