Muscarinic receptor subtypes in canine trachea

Prejunctional and postjunctional muscarinic receptor subtypes were characterized in canine trachealis muscle strips. In vitro contractile responses of muscle strips to acetylcholine or electric field stimulation were determined in the absence and the presence of gallamine, pirenzepine, and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP). Gallamine had no effect on the contractile response to acetylcholine but enhanced the contractile response to electric field stimulation. Pirenzepine and 4-DAMP reduced the contractile response to acetylcholine and electric field stimulation. The pA₂ value for pirenzepine vs. acetylcholine [7.18 ± 0.59 (SD)] was consistent with the affinity of pirenzepine for M₂ or M₃-receptors; whereas the pA₂ value for 4-DAMP vs. acetylcholine (8.92 ± 0.42) and the extremely low affinity of gallamine indicated postjunctional muscarinic receptors of the M₃ subtype. The enhancement of the contractile response to electric field stimulation by gallamine suggested the presence of M₂-prejunctional receptors.