TY - JOUR T1 - Neuronally Released Acetylcholine Acts on the M<sub>2</sub> Muscarinic Receptor to Oppose the Relaxant Effect of Isoproterenol on Cholinergic Contractions in Mouse Urinary Bladder JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 631 LP - 637 DO - 10.1124/jpet.107.121756 VL - 322 IS - 2 AU - Frederick J. Ehlert AU - Simon Ahn AU - Kirk J. Pak AU - Grace J. Park AU - Marline S. Sangnil AU - John A. Tran AU - Minoru Matsui Y1 - 2007/08/01 UR - http://jpet.aspetjournals.org/content/322/2/631.abstract N2 - We investigated whether M2 muscarinic receptor activation opposes isoproterenol-induced relaxation in mouse urinary bladder and whether endogenous acetylcholine acts through a similar M2 mechanism. When measured in urinary bladder from M3 receptor knockout mice, the muscarinic agonist oxotremorine-M elicited only very weak contractions. In the presence of α,β-methylene ATP (30 μM) and isoproterenol (1 μM), however, oxotremorine-M elicited a robust contractile response. This response was completely absent in bladder from M2/M3 double knockout mice, indicating that activation of the M2 receptor inhibits the relaxant effect of isoproterenol on the contraction to α,β-methylene ATP. Similar results were obtained when prostaglandin F2α (5 μM) was used as the contractile agent but not when serotonin was used. Electrical field stimulation of the urinary bladder from wild-type mouse elicited contractions that were inhibited 20% by atropine and 40% by desensitization with α,β-methylene ATP. When measured in the presence of α,β-methylene ATP to desensitize the purinergic component of contraction, isoproterenol exhibited moderately greater relaxant activity in field-stimulated bladder from the M2 knockout mouse compared with that observed in wild-type bladder. This differential relaxant effect of isoproterenol was greatly increased in the presence of physostigmine. In contrast, no differential effects were noted for isoproterenol in similar experiments on bladders from M3 knockout and M2/M3 double knockout mice in the presence of physostigmine. Our results suggest that neuronally released acetylcholine acts on the M2 muscarinic receptor to inhibit the relaxant effect of isoproterenol on the minor, cholinergic component of contraction in the field-stimulated mouse urinary bladder. The American Society for Pharmacology and Experimental Therapeutics ER -