The muscarinic receptors of airway smooth muscle: their characterization in vitro.

We have used radioligand binding and in vitro muscle contraction techniques to characterize the muscarinic cholinergic receptors of canine tracheal smooth muscle. The tritiated muscarinic antagonist, quinuclidinyl benzilate ([3H]QNB), bound no particulate preparations of tracheal smooth muscle with high affinity, saturability, pharmacologic specificity, and stereoselectivity. The equilibrium dissociation constant for the binding reaction was 33 +/- 3 pM (mean +/- SE). The concentration of binding sites was 410 +/- 34 pmol/g protein. The ability of muscarinic agents to inhibit [3H]QNB binding paralleled their relative effects on tracheal smooth muscle contraction in vitro. We found the anesthetic agents, lidocaine and tetracaine, and the nicotonic cholinergic antagonist d-tubocurarine, were competitive inhibitors of [3H]QNB binding to the tracheal smooth muscle preparations. Histamine, aminophylline, and adrenergic agonists did not act as competitive inhibitors. The guanine nucleotide, guanyl-5'-imidodiphosphate, reduced the ability of the muscarinic agonist, acetylcholine, to compete with high affinity for [3H]QNB binding site(s) in tracheal smooth muscle but had minimal effects on the binding of the muscarinic antagonist, atropine. THe radioligand binding and in vitro contraction techniques described are being used to study the possible alteration of muscarinic receptors of airway smooth muscle in disease or from treatment.