The beta-adrenoceptors mediating relaxation of rat oesophageal muscularis mucosae are predominantly of the beta 3-, but also of the beta 2-subtype.

1. beta-Adrenoceptor-mediated relaxation of rat oesophageal smooth muscle was investigated by studying the effects of beta 1- and beta 2-selective antagonists on the relaxation induced by (-)-isoprenaline, the beta 2-selective agonists fenoterol and clenbuterol and the beta 3-agonist, BRL 37344. 2. The highly beta 1-selective antagonist CGP 20721A did not antagonize (-)-isoprenaline- or BRL 37344-induced relaxations in concentrations up to 10 microM. Only at 100 microM of CGP 20712A were clear rightward shifts of the agonist concentration-response curves (CRCs) observed, with pA2 values of 4.70 and 4.97 against (-)-isoprenaline and BRL 37344, respectively. 3. ICI 118,551, a potent and selective beta 2-antagonist, at 100 nM caused moderate rightward shifts of the CRCs of (-)-isoprenaline, fenoterol and clenbuterol; with fenoterol and clenbuterol, this was accompanied by a clear steepening of the curve. Only at the highest concentration (100 microM ICI 118,551) did the shifts to the right further increase substantially. Resulting Schild-plots were clearly biphasic. BRL 37344-induced relaxations were only antagonized at 100 microM ICI 118,551, yielding a pA2 value of 5.48. 4. These results clearly demonstrate that the BRL 37344-induced relaxation of rat oesophageal muscularis mucosae is mediated solely through beta 3-adrenoceptors, whereas (-)-isoprenaline-, fenoterol- and clenbuterol-induced relaxations were shown to involve both beta 2- and, predominantly, beta 3-adrenoceptors.