The guinea pig ileum lacks the direct, high-potency, M2- muscarinic, contractile mechanism characteristic of the mouse ileum

We explored whether the M2muscarinic receptor in the guinea pig ileum elicits a highly potent, directcontractile response, like that from the M3muscarinic receptor knockout mouse. First, we characterized the irreversible receptor-blocking activity of 4-DAMP mustard in ileum from muscarinic receptor knockout mice to verify its M3selectivity. Then, we used 4-DAMP mustard to inactivate M3responses in the guinea pig ileum to attempt to reveal direct, M2receptor-mediated contractions. The muscarinic agonist, oxotremorine-M, elicited potent contractions in ileum from wild-type, M2receptor knockout, and M3receptor knockout mice characterized by negative log EC50(pEC50) values ± SEM of 6.75±0.03, 6.26±0.05, and 6.99±0.08, respectively. The corresponding Emaxvalues in wild-type and M2receptor knockout mice were approximately the same, but that in the M3receptor knockout mouse was only 36% of wild type. Following 4-DAMP mustard treatment, the concentration-response curve of oxotremorine-M in wild-type ileum resembled that of the M3knockout mouse in terms of its pEC50, Emax, and inhibition by selective muscarinic antagonists. Thus, 4-DAMP mustard treatment appears to inactivate M3responses selectively and renders the muscarinic contractile behavior of the wild-type ileum similar to that of the M3knockout mouse. Following 4-DAMP mustard treatment, the contractile response of the guinea pig ileum to oxotremorine-M exhibited low potency and a competitive-antagonism profile consistent with an M3response. The guinea pig ileum, therefore, lacks a direct, highly potent, M2-contractile component but may have a direct, lower potency M2component. © The Author(s) 2009