Pre- and postjunctional muscarinic receptors in canine bronchi

Cholinergic receptors on canine bronchial smooth muscle (mediating excitation) or on the cholinergic nerve endings (modulating acetylcholine release) have not yet been characterized. In this study, the effects of agents selective for the various muscarinic receptors on mechanical and electrical responses in isolated segments of canine bronchial smooth muscle (3rd-6th order) were investigated. Carbachol (Cch), bethanechol (Bch), oxotremorine (Oxo), and McNeil A343 (McN) produced membrane depolarization and contractions with a ranked potency order of Oxo approximately Cch greater than Bch greater than McN. Oxo-induced contractions were antagonized by pirenzepine (Pir), AF-DX 116 (AF-DX), or hexahydrosiladifenidol (Hexa); Schild analysis yielded pA2 values of 6.61, 6.81, and 7.40, respectively. Contractions induced by field stimulation (FS) were antagonized by Pir, AF-DX, or Hexa with a potency order of Hexa greater than AF-DX approximately Pir. FS responses were potentiated by lower (i.e., M1-selective) concentrations of Pir, and antagonized by McN. The non-M1 agonists and antagonists had no such effects. These findings indicate that the postjunctional receptors are of the M3 subtype, and that the inhibitory receptors on the cholinergic nerve endings are of the M1 subtype.