Muscarinic agonist-mediated heterologous desensitization in isolated ileum requires activation of both muscarinic M-2 and M-3 receptors

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 muM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F-2alpha (PGF(2alpha)) 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(2alpha) was prevented in both M-2 and M-3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M-2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M-2- and M-3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M-2 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 M-2 and M-3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.