Expression of muscarinic receptor subtypes and M2-muscarinic inhibition of adenylyl cyclase in lung

The relative distribution and absolute quantities of muscarinic receptor subtypes m₁, m₂, m₃, and m₄ were determined in membranes of canine trachealis muscle, bronchi, and lung parenchyma by immunoprecipitation with receptor subtype-specific rabbit polyclonal antisera. Additionally, the functional coupling of muscarinic receptors to the inhibition of adenylyl cyclase was related to the presence of m₂-muscarinic receptors in each region. Immunoprecipitation identified more total muscarinic receptors in crachealis muscle than in bronchi or lung. m₂-Muscarinic receptor predominated in tracheal muscle (372 ± 85 fmol/mg protein) with fewer m₃ receptors (48 ± 5 fmol/mg protein). Bronchi contained 6.6 ± 2.0 and 9.2 ± 1.8 fmol/mg protein of m₂ and m₃ receptors, respectively. Lung parenchyma contained 13.9 ± 3.9 fmol/mg protein of m₃ receptors. Adenylyl cyclase activity increased in response to guanosine triphosphate and isoproterenol in membranes from all three lung regions, but muscarinic-mediated inhibition of adenylyl cyclase occurred only in trachealis membranes. These studies provide the first quantitative assessment of muscarinic receptor subtypes in different regions of the lung and relate the ability to measure muscarinic inhibition of adenylyl cyclase to the presence of m₂ receptors.