β‐Adrenoceptor heterogeneity in guinea‐pig airways: comparison of functional and receptor labelling studies

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Abstract
1 The distribution of β-adrenoceptor subtypes in guinea-pig airways has been studied by radioligand binding assays and analysis of mechanical responses. 2 Binding studies with the ligands [3H]-dihydroalprenolol and [125I]-cyanopindolol, revealed that β-adrenoceptors were unevenly distributed throughout the airways with the highest density located in the parenchyma and the lowest density in the trachea. 3 The relative proportion of β1:β2-adrenoceptor binding sites was assessed by computer-assisted analysis of the inhibition curves generated by selective agents. It was virtually identical in each region and in the order of 15:85%. 4 β-Adrenoceptor agonists caused concentration-dependent relaxations of both tracheal spirals and parenchymal lung strips. This response appeared to be mediated by both β1- and β2-adrenoceptors in tracheal spirals as the pA2 value for the β1-selective antagonist, atenolol, varied depending upon which agonist was used, and, in the presence of the β2-adrenoceptor antagonist ICI 118,551, noradrenaline and isoprenaline produced biphasic concentration-effect curves. In parenchymal lung strips only the one subtype was involved as antagonist pA2 values were not dependent on the agonist used and the properties were consistent with those expected for a β2-adrenoceptor.