Classification of beta-adrenoceptors in isolated bronchus of the pig.

1 (+/-)-Isoprenaline (Iso), (-)-adrenaline (Ad), (-)-noradrenaline (NA), the beta 2-selective adrenoceptor agonist (+/-)-fenoterol (Fen) and the beta 1-selective adrenoceptor agonist (+/-)-RO363 caused concentration-dependent relaxation of preparations of pig bronchus pre-contracted with carbachol 40-ng/ml (0.22 microM). Iso, Ad, NA and Fen caused complete relaxation of carbachol-induced tone, but RO363 caused relaxation equivalent to only 59% of the maximal response to Iso. 2 When relaxation responses to these amines were plotted as a % of their maximal effects, comparison of EC50 values showed that the order of potency was RO363 greater than Iso greater than NA greater than Fen greater than Ad (14.4:4.6:1:0.4:0.3). 3 pA2 values determined for the beta-adrenoceptor antagonists propranolol (non-selective) and atenolol (beta 1-selective), or the partial agonist salbutamol (beta 2-selective) using Iso as agonist were 8.3, 7.3 and 4.4 respectively. The pA2 value for atenolol using RO363 as the agonist was 7.6. 4 These results indicate that porcine bronchus contains a homogeneous population of beta 1-adrenoceptors.