beta 1-Adrenoceptors mediating relaxation of the guinea-pig trachea: experiments with prenalterol, a beta 1-selective adrenoceptor agonist.

In the presence of 17 beta-oestradiol, prenalterol, a beta-selective adrenoceptor agonist, caused a dose-dependent relaxation of the isolated, pilocarpine-contracted guinea-pig trachea. This effect was blocked by the antagonists propranolol (non-selective) and practolol (beta 1-selective) but not by IPS 339 [(t-butylamino-3-ol-2-propyl)oximino-9-fluorene HCl] (beta 2-selective). The relaxing effect of terbutaline, a beta 2-selective adrenoceptor agonist, was more efficiently blocked by IPS 339 than by practolol. These data support the hypothesis that the guinea-pig trachea contains both beta 1- and beta 2-adrenoceptors mediating relaxation and that the beta 1-adrenoceptors are selectively stimulated by prenalterol. The efficacy of prenalterol was less than that of terbutaline, thus confirming its partial agonistic activity. In the absence of 17 beta-oestradiol, the ability of prenalterol to relax the pilocarpine-contracted trachea was lost. It is suggested that 17 beta-oestradiol may act as a functional antagonist to pilocarpine as it caused a partial relaxation itself.