title: Contractile roles of the M2 and M3 muscarinic receptors in the guinea pig colon.
abstract: The contractile roles of the M2 and M3 muscarinic receptors were investigated in guinea pig longitudinal colonic smooth muscle. Prior treatment of the colon with N-(2-chloroethyl)-4-piperidinyl diphenylacetate (4-DAMP mustard) (40 nM) in combination with [[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11- dihydro-6H-pyrido[2,3b][1,4]benzodiazepine-6-one (AF-DX 116) (1.0 microM) caused a subsequent, irreversible inhibition of oxotremorine-M-induced contractions when measured after extensive washing. The estimate of the degree of receptor inactivation after 2 hr (97%) was not much greater than that measured after 1 hr (95%), which suggests that both 4-DAMP mustard-sensitive and -insensitive muscarinic subtypes contribute to the contractile response. Pertussis toxin treatment had no significant inhibitory effect on the control contractile response to oxotremorine-M, but caused an 8.8-fold increase in the EC50 value measured after a 2-hr treatment with 4-DAMP mustard. These results suggest that, after elimination of most of the M3 receptors with 4-DAMP mustard, the contractile response can be mediated by the pertussis toxin-sensitive M2 receptor. After pertussis toxin treatment, the kinetics of alkylation of muscarinic receptors in the colon were consistent with a single, 4-DAMP mustard-sensitive, M3 receptor subtype mediating the contractile response. When measured after a 2-hr treatment with 4-DAMP mustard and in the presence of histamine (0.30 microM) and either forskolin (10 microM) or isoproterenol (0.60 microM), the contractile responses to oxotremorine-M were pertussis toxin-sensitive and potently antagonized by the M2 selective antagonist, AF-DX 116. Collectively, our results indicate that the M2 receptor elicits contraction through two mechanisms, a direct contraction and an indirect contraction by preventing the relaxant effects of cAMP-generating agents.
authors.affiliation: Department of Pharmacology, College of Medicine, University of California, Irvine 92717, USA.
authors.forename: G W
authors.initials: GW
authors.lastname: Sawyer
authors.forename: F J
authors.initials: FJ
authors.lastname: Ehlert
mesh_terms: D000818:Animals; D005576:Colforsin / Q000494:pharmacology; D003106:Colon / Q000502:physiology*; D000242:Cyclic AMP / Q000378:metabolism; D006168:Guinea Pigs; D006632:Histamine / Q000494:pharmacology; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects*; D010095:Oxotremorine / Q000031:analogs & derivatives / Q000494:pharmacology; D037342:Pertussis Toxin; D010880:Piperidines / Q000494:pharmacology; D010890:Pirenzepine / Q000031:analogs & derivatives / Q000494:pharmacology; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic / Q000502:physiology*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D010880:Piperidines; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D010890:Pirenzepine; D010095:Oxotremorine; C042743:oxotremorine M; C042375:4-diphenylacetoxy-1,1-dimethylpiperidinium; D006632:Histamine; D000242:Cyclic AMP; D037342:Pertussis Toxin; D007545:Isoproterenol; C048593:otenzepad