INVOLVEMENT OF THE M(2) MUSCARINIC RECEPTOR IN CONTRACTIONS OF THE GUINEA-PIG TRACHEA, GUINEA-PIG ESOPHAGUS, AND RAT FUNDUS

The involvement of the M(2) muscarinic receptor in contractile respons es of the guinea pig trachea, guinea pig esophagus, and rat fundus was investigated. In the standard assay, oxotremorine-M elicited contract ions of the trachea with an EC(50) value of approximately 73 nM. -[(Di ethylamino)methyl]-1-piperidinyl]acetyl]-5,11 -dihydro-6H-pyrido[2,3-b ][1,4]benzodiazepine-6-one (AF-DX 116) at 1 and 10 mu M antagonized th ese contractions by 2.1- and 9.0-fold increases in the EC(50) value fo r oxotremorine-M. These effects are consistent with antagonism of an M (3)-mediated contractile response. In subsequent experiments, the M(3) receptors were first inactivated selectively by incubation with N-(2- chloroethyl)-4-piperidinyl diphenylacetate (4-DAMP mustard) (40 nM) fo r 1 hr in the presence of AF-DX 116 (1 mu M) followed by extensive was hing. In 4-DAMP mustard treated trachea, oxotremorine-M elicited contr actions with an EC(50) value of 0.31 mu M in the presence of histamine (10 mu M) and forskolin (4 mu M). Under these conditions, AF-DX 116 a t 1 and 10 mu M antagonized contractions to oxotremorine-M by 8- and 5 9-fold increases in the EC(50), respectively, while parafluorohexahydr osiladiphenidol (p-F-HHSiD) (0.1 mu M) had no effect. These effects ar e consistent with a contraction being mediated by an M(2) receptor. In the guinea pig esophagus and rat fundus, AF-DX 116 and p-F-HHSiD bloc ked contractions measured under similar conditions with magnitudes int ermediate between what would be expected from an M(2) and an M(3) rece ptor, suggesting that perhaps both subtypes contribute to the overall contractile response under these conditions. In addition, contractions of the guinea pig trachea measured in the presence of histamine and f orskolin were pertussis toxin sensitive. These results indicate that, in the trachea, M(3) receptors can dominate the contractile response a fter a majority of the M(2) receptors have been inactivated, whereas i n the guinea pig esophagus and rat fundus, M(2) receptors may contribu te to, but do not play a dominant role in the overall response.