β1‐Adrenoceptors mediating relaxation of the guinea‐pig trachea: experiments with prenalterol, a β1‐selective adrenoceptor agonist

In the presence of 17 β‐oestradiol, prenalterol, a β1‐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 (β1‐selective) but not by IPS 339 [(t‐butylamino‐3‐ol‐2‐propyl)oximino‐9‐fluorene HCl] (β2‐selective). The relaxing effect of terbutaline, a β2‐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 β1‐ and β2‐adrenoceptors mediating relaxation and that the β1‐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 β‐oestradiol, the ability of prenalterol to relax the pilocarpine‐contracted trachea was lost. It is suggested that 17 β‐oestradiol may act as a functional antagonist to pilocarpine as it caused a partial relaxation itself.