title: Functional role of M2 muscarinic receptors in the guinea pig ileum.
abstract: Muscarinic agonists elicit contraction in the standard guinea pig ileum bioassay through activation of M3 muscarinic receptors that are also linked to phosphoinositide hydrolysis. Surprisingly, the most abundant muscarinic receptor in the ileum is the M2 which causes a specific inhibition of cyclic AMP accumulation elicited by the beta-adrenergic receptor. After most of the M3 receptors are inactivated, the ileum still retains high sensitivity to muscarinic agonists provided that the contractile responses are measured in the presence of histamine and forskolin, which together, have no effect on contraction. Under these conditions, the potencies of antagonists for blocking the contractile response are consistent with those expected for an M2 response. Moreover, the muscarinic contractile response measured in the presence of histamine and forskolin after inactivation of M3 receptors is pertussis toxin sensitive. In contrast, muscarinic contractions in the standard bioassay are pertussis toxin insensitive. These results demonstrate that the M2 muscarinic receptor can cause an indirect contraction of the guinea pig ileum by preventing the relaxing effect of agents that increase cAMP.
authors: Department of Pharmacology, College of Medicine, University of California, Irvine 92717, USA.
authors: F J
authors: FJ
authors: Ehlert
authors: E A
authors: EA
authors: Thomas
mesh_terms: D000818:Animals; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000378:metabolism; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects / Q000502:physiology; D006632:Histamine / Q000494:pharmacology; D007082:Ileum / Q000187:drug effects / Q000502:physiology*; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D010095:Oxotremorine / Q000031:analogs & derivatives / Q000494:pharmacology; D037342:Pertussis Toxin; D010880:Piperidines / Q000494:pharmacology; D051381:Rats; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000502:physiology*; D015398:Signal Transduction; D013363:Submandibular Gland / Q000187:drug effects / Q000378:metabolism; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D018727:Muscarinic Antagonists; D010880:Piperidines; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3; D011976:Receptors, Muscarinic; D010566:Virulence Factors, Bordetella; D005576:Colforsin; D010095:Oxotremorine; C042743:oxotremorine M; C042375:4-diphenylacetoxy-1,1-dimethylpiperidinium; D006632:Histamine; D000242:Cyclic AMP; D037342:Pertussis Toxin