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 μM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F_2α (PGF_2α) 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 PGF_2α 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.