Abstract

We investigated the contractile role of M₂ 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 M₃ response in urinary bladder from wild-type and M₂ knockout (KO) mice. Evidence for a minor M₂ receptor-mediated contraction was revealed by a comparison of responses in M₃ knockout and M₂/M₃ double knockout mice. Treatment of wild-type and M₂ 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 M₂ knockout bladder compared with wild type, suggesting that M₂ receptors rescue the muscarinic contractile response in wild-type bladder following inactivation of M₃ receptors with 4-DAMP mustard. When measured in the presence of prostaglandin F_2α and isoproterenol or forskolin, oxotremorine-M mediated a potent contractile response in urinary bladder from M₃ KO mice. This response exhibited an M₂ profile in competitive antagonism studies and was completely absent in M₂/M₃ 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 M₂ KO mice. Our results show that the M₂ receptor mediates contractions indirectly in the urinary bladder by enhancing M₃ receptor-mediated contractions and inhibiting relaxation. We also show that it is difficult to detect M₂ receptor function in competitive antagonism studies under conditions where a simultaneous activation of M₂ and M₃ receptors occurs.