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Antagonism of acetylcholine action in guinea-pig tracheal smooth muscle and epithelium by pirenzepine, 4-DAMP and atropine.

Acetylcholine-induced guinea-pig tracheal smooth muscle contraction and epithelium-derived relaxant factor release were evaluated using guinea-pig open tracheal rings and rat anococcygeus muscle bioassay to get insight into the participation of muscarinic receptor subtypes in these responses. There was a significant difference between the two pA2 values obtained in contraction and relaxation experiments for pirenzepine, but no difference was found either for atropine or for 4-DAMP. This difference seems to be due to the participation of M1-receptors in smooth muscle contraction.