Muscarinic receptor subtypes in guinea pig airways

The muscarinic receptors present in the guinea pig tracheobronchial tree were characterized using ligand-binding studies and functional approaches. The binding constants of four selective antagonists, pirenzepine,[11-([2-[(diethylamino) methyl]-1-piperidinyl]acetyl)-5,11-dihydro-6H-pyrido(2,3) (1,4) benzodiazepine-6-on] (AF-DX 116), methoctramine, and 4-diphenylacetoxy-Nmethylpiperidine methobromide(4-DAMP) were examined. Heterogeneity in the airway muscarinic receptors population was revealed by competitive binding experiments against [N-methyl3H]scopolamine with the M2 muscarinic antagonists AF-DX 116 and methoctramine. In guinea pig lung and trachea, AF-DX 116 and methoctramine recognized 86-88% and 50-60% of total receptors with high affinity, respectively. These receptors exhibit binding constants for these two compounds similar to those of the M2 subtype. The low-affinity M2 antagonist binding constants were close to those reported for M3 receptors. In lung and trachea, we found no evidence for a high-affinity [N-methyl-3H]pirenzepine binding sites. In functional studies, pirenzepine, methoctramine, and 4-DAMP inhibited the methacholine-induced contraction of lung parenchymal, main bronchial, and tracheal strips with affinities characteristic of smooth muscle M3 receptors. These results are consistent with the presence of M2 and M3 receptors in guinea pig airways. Throughout the airways, the muscarinic receptors mediating smooth muscle contraction are of the M3 subtype.