title: Muscarinic agonist-mediated heterologous desensitization in isolated ileum requires activation of both muscarinic M2 and M3 receptors.
abstract: We investigated the subtypes of the muscarinic receptor mediating short-term heterologous desensitization in the isolated ileum. Treatment of the ileum from C57BL/6 mice with acetylcholine (30 microM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F2alpha (PGF2alpha) and the muscarinic agonist, oxotremorine-M. This subsensitivity was characterized by 7- and 3-fold increases in the EC50 values of the agonists, respectively, with no significant effect on the maximal response. The subsensitivity to PGF2alpha was prevented in both M2 and M3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M2- and M3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M2 antagonistic profile. Modeling studies showed that the competitive antagonism of response contingent upon activation of two receptor subtypes should exhibit a pharmacological profile similar to that of the least sensitive signaling pathway. Our results demonstrate that muscarinic agonist-mediated short-term heterologous desensitization of intestinal smooth muscle is contingent upon activation of both M2 and M3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.
authors.affiliation: Department of Physical Sciences, Chapman University, Orange, California, USA.
authors.forename: Michael T
authors.initials: MT
authors.lastname: Griffin
authors.forename: Minoru
authors.initials: M
authors.lastname: Matsui
authors.forename: Darakhshanda
authors.initials: D
authors.lastname: Shehnaz
authors.forename: Khurram Z
authors.initials: KZ
authors.lastname: Ansari
authors.forename: Makoto M
authors.initials: MM
authors.lastname: Taketo
authors.forename: Toshiya
authors.initials: T
authors.lastname: Manabe
authors.forename: Frederick J
authors.initials: FJ
authors.lastname: Ehlert
mesh_terms: D000818:Animals; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008954:Models, Biological; D018721:Muscarinic Agonists / Q000494:pharmacology*; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D011869:Radioligand Assay; D043585:Receptor, Muscarinic M2 / Q000235:genetics / Q000378:metabolism*; D043587:Receptor, Muscarinic M3 / Q000235:genetics / Q000378:metabolism*
chemical_list: D018721:Muscarinic Agonists; D018727:Muscarinic Antagonists; D043585:Receptor, Muscarinic M2; D043587:Receptor, Muscarinic M3