The M2 muscarinic receptor mediates contraction through indirect mechanisms in mouse urinary bladder

We investigated the contractile role of M-2 muscarinic receptors in mouse urinary bladder. When measured in the absence of other agents, contractions elicited to the muscarinic agonist oxotremorine-M exhibited properties consistent with that expected for an M-3 response in urinary bladder from wild-type and M-2 knockout (KO) mice. Evidence for a minor M-2 receptor-mediated contraction was revealed by a comparison of responses in M-3 knockout and M-2/M-3 double knockout mice. Treatment of wild-type and M-2 knockout urinary bladder with N-2-chloroethyl-4-piperidinyl diphenylacetate (4-DAMP mustard) caused a large inhibition of the muscarinic contractile response. The residual contractions were much smaller in M-2 knockout bladder compared with wild type, suggesting that M-2 receptors rescue the muscarinic contractile response in wildtype bladder following inactivation of M-3 receptors with 4-DAMP mustard. When measured in the presence of prostaglandin F-2 alpha and isoproterenol or forskolin, oxotremorine-M mediated a potent contractile response in urinary bladder from M-3 KO mice. This response exhibited an M-2 profile in competitive antagonism studies and was completely absent in M-2/M-3 KO mice. Following 4-DAMP mustard treatment, oxotremorine-M elicited a contractile response in wild-type urinary bladder in the presence of KCl and isoproterenol or forskolin, and this response was diminished in M-2 KO mice. Our results show that the M-2 receptor mediates contractions indirectly in the urinary bladder by enhancing M-3 receptor-mediated contractions and inhibiting relaxation. We also show that it is difficult to detect M-2 receptor function in competitive antagonism studies under conditions where a simultaneous activation of M-2 and M-3 receptors occurs.