Differential Coupling of Muscarinic M1, M2, and M3 Receptors to Phosphoinositide Hydrolysis in Urinary Bladder and Longitudinal Muscle of the Ileum of the Mouse

We investigated the coupling of muscarinic receptor (M) subtypes to phosphoinositide hydrolysis in ileum and urinary bladder using muscarinic receptor knockout mice. In urinary bladder from wild-type mice, the muscarinic agonist oxotremorine-M, elicited a robust phosphoinositide response characterized by an EC 50 value of 0.22 μM and a maximal response ( E max ) of 32.8% conversion of [ 3 H]inositol-labeled phosphoinositides into [ 3 H]inositol phosphates. A similar response was observed in urinary bladder from M 2 knockout mice, whereas no measurable response was observed in urinary bladder from M 3 and M 2 /M 3 knockout mice. In ilea from wild-type and M 2 knockout mice, substantial phosphoinositide responses to oxotremorine-M were measured, characterized by EC 50 values of 0.37 and 0.52 μM and E max values of 35.8 and 34.7%, respectively. Oxotremorine-M also elicited phosphoinositide hydrolysis in ilea from M 3 and M 2 /M 3 knockout mice, although these responses were less sensitive (EC 50 values of 1.6 and 1.4 μM; E max values of 31.2 and 20.8%, respectively). The response in ileum from the M 2 /M 3 knockout was significantly smaller than that from the M 3 knockout. The muscarinic phosphoinositide response in ilea from M 2 /M 3 knockout mice originated in the smooth muscle and exhibited a profile for competitive antagonism consistent with an M 1 mechanism. These data suggest a major role for the M 3 receptor in eliciting phosphoinositide hydrolysis in the ileum and urinary bladder and minor roles for the M 1 and M 2 in ileum.