Neuronally released acetylcholine acts on the M2 muscarinic receptor to oppose the relaxant effect of isoproterenol on cholinergic contractions in mouse urinary bladder

We investigated whether M-2 muscarinic receptor activation opposes isoproterenol-induced relaxation in mouse urinary bladder and whether endogenous acetylcholine acts through a similar M-2 mechanism. When measured in urinary bladder from M-3 receptor knockout mice, the muscarinic agonist oxotremorine. elicited only very weak contractions. In the presence of alpha,beta-methylene ATP (30 mu M) and isoproterenol (1 mu M), however, oxotremorine-M elicited a robust contractile response. This response was completely absent in bladder from M-2/M-3 double knockout mice, indicating that activation of the M2 receptor inhibits the relaxant effect of isoproterenol on the contraction to alpha, beta-methylene ATP. Similar results were obtained when prostaglandin F-2 alpha (5 mu M) 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 alpha,beta- methylene ATP. When measured in the presence of alpha, beta-methylene ATP to desensitize the purinergic component of contraction, isoproterenol exhibited moderately greater relaxant activity in field-stimulated bladder from the M-2 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 M-3 knockout and M-2/M-3 double knockout mice in the presence of physostigmine. Our results suggest that neuronally released acetylcholine acts on the M-2 muscarinic receptor to inhibit the relaxant effect of isoproterenol on the minor, cholinergic component of contraction in the field- stimulated mouse urinary bladder.