Autoradiographic localization of β‐adrenoceptors in pig lung using [125I]‐iodocyanopindolol

Abstract

1 The binding of the β-adrenoceptor radioligand [¹²⁵I]-iodocyanopindolol (I-CYP) has been studied in pig lung parenchyma and the distribution of binding sites visualised by light microscopic autoradiography. 2 I-CYP binding was saturable (maximum binding capacity B_max = 51 ± 3 fmol mg⁻¹ protein), involving sites with high affinity (dissociation constant K_D = 73 ± 10 pM). 3 Specific I-CYP binding was displaceable both by β-adrenoceptor agonists ((−)-isoprenaline > (−)-adrenaline > (±)-fenoterol > (−)-noradrenaline > (+)-isoprenaline > (±)-RO363) and antagonists ((±)-propranolol > ICI-118551 > atenolol), indicating a predominance of β₂-adrenoceptors. Further analysis showed that displacement data for the β₁-selective antagonist atenolol and the β₂-selective antagonist ICI-118551 were fitted best to a 2 binding site model and that both β₁- and β₂-adrenoceptors were present in pig lung in the ratio 28:72 respectively. 4 Autoradiographic grains were localized over tissue and were most dense over alveolar walls > vascular endothelium > vascular smooth muscle > bronchial smooth muscle = bronchial epithelium. Atenolol (10⁻⁵ M) caused a 31 % reduction in specific grain density over alveolar wall tissue, while a 10 fold lower concentration of ICI-118551 (10⁻⁶ M) caused a 50% decrease. These results are consistent with binding data in pig lung parenchyma demonstrating a mixed population of β-adrenoceptors with a predominance of the β₂ subtype. 5 Approximately 95 % of the parenchymal β-adrenoceptors were associated with the alveolar wall as a mixed population of the β₁ and β₂ subtypes in the ratio 30:70 respectively. A greater proportion of the β-adrenoceptors associated with bronchial and vascular smooth muscle seemed to be of the β₂ subtype. 6 It is possible that the previously described relaxant responses of the pig lung parenchyma strip to β-agonists, mediated via β₂-adrenoceptors, resulted from the sum of reactivities in airway and vascular smooth muscle together with relaxation of alveolar interstitial cells.