Classification of β‐adrenoceptors in human isolated bronchus

Abstract

1 (±)-Isoprenaline (Iso), (−)−adrenaline (Ad), (−)−noradrenaline (NA), (±)-phenylephrine (Phe) and the β₂-selective adrenoceptor agonist (±)-fenoterol (Fen) caused a concentration-dependent relaxation of human isolated bronchial preparations. Iso, Ad and NA caused complete relaxation of both spontaneous and carbachol-induced bronchial tone. Fen, which was only tested in preparations where tone was induced with carbachol, also caused complete relaxation. However, Phe was a partial agonist in all preparations tested. 2 When relaxation responses to these amines were calculated as a % of their maximal effects, comparison of EC₅₀ values showed that the order of potency was Iso > Ad = Fen > NA > Phe (92:27:25:1:0.2) in preparations with carbachol-induced tone and Iso > Ad > NA > Phe (112:38:1:0.3) in preparations with spontaneous tone. 3 pA₂ values determined for the β-adrenoceptor antagonists propranolol (non-selective), atenolol (β-selective) and ICI-118, 551 (β₂-selective), using Iso as an agonist were, 9.3, 5.3 and 9.1 respectively. 4 These results indicate that β₂-adrenoceptors mediate relaxation of human isolated bronchus to sympathomimetic amines in preparations obtained 4–14 h post-mortem from non-diseased lung. α-Adrenoceptors were apparently sparse or absent in this tissue.