Comparison of muscarinic and beta adrenergic receptors between bovine peripheral lung and tracheal smooth muscles: A striking difference in the receptor concentration

This study was undertaken to compare the activity of muscarinic and beta adrenergic receptors in bovine peripheral lung to the corresponding receptor activity in tracheal smooth muscle. We used [³H] quinuclidinyl benzilate (QNB) and [³H]dihydroalprenolol (DHA) to measure muscarinic and beta receptor activity, respectively. Binding to QNB and DHA at 25°C was rapid, reversible, saturable and of high affinity. The order of potency for cholinergic and adrenergic agents competing for binding was compatible with muscarinic and beta₂ adrenergic potencies. We found that the concentration of muscarinic receptor binding sites was 37-fold greater in the tracheal muscle preparation (2805 ± 309 fmol/mg protein) than in the peripheral lung preparation (76 ± 28 fmol/mg protein). Unlike muscarinic receptors, the lung contained 8-fold higher concentration of the beta adrenergic receptors than did the tracheal muscle (1588 ± 417 vs. 199 ± 42 fmol/mg protein). The dissociation constant or the agonist's inhibitory constant (Ki) for either receptor binding sites, however, was not significantly different between the two tissues. Furthermore, in vitro contraction studies showed that the response of tracheal muscle strips to methacholine was markedly greater than the response of peripheral lung strips, a finding consistent with the QNB binding result. The muscle but not the peripheral lung strip exhibited a relaxing response to epinephrine. Our data indicate a striking quantitative difference in muscarinic and beta adrenergic receptors between lung tissue and tracheal muscle, and that each receptor in the lung is qualitatively similar to the corresponding receptor in the muscle.