Beta 3-adrenoceptor agonists, BRL 37344 and SR 58611A, do not induce relaxation of human, sheep and guinea-pig airway smooth muscle in vitro.

The existence of atypical- or beta 3-adrenoceptors has now been generally accepted. These receptors have been shown to be abundant in adipose tissue and in a number of gastrointestinal smooth muscle preparations. A recent study reported that beta 3-adrenoceptor stimulation mediated relaxation of isolated canine bronchial smooth muscle. The aim of the present study was to extend this observation to other species. We investigated the in vitro responses of guinea-pig, human and sheep bronchial smooth muscle to isoprenaline, salbutamol (a selective beta 2-adrenoceptor agonist), and BRL 37344 and SR 58611A (two presumably selective beta 3-adrenoceptor agonists). The preparations were precontracted to 60-70% of maximal tension with histamine 10(-6) M for guinea-pig and human bronchi, or acetylcholine 10(-6) M for sheep bronchi. In each species, SR 58611A produced a slight fall in tension of about 10% of the effects of theophylline (3 mM), but this decrease in tension was not significantly different from the spontaneous and weak relaxation observed with saline addition during the same duration of the experiment. These relaxations were not modified by either the nonselective beta-adrenoceptor antagonist propranolol or the selective beta 2-adrenoceptor antagonist ICI 118,551. In contrast, BRL 37344 induced a significant concentration-dependent fall in tension induced by both spasmogens. Its relaxant effects were inhibited both by propranolol and ICI 118,551 in human and guinea-pig airways, whereas on the isolated sheep bronchus BRL 37344-induced relaxations were only slightly, albeit significantly, reduced with either of the beta-adrenoceptor antagonists tested.(ABSTRACT TRUNCATED AT 250 WORDS)