The effect of ion pair formation on the antimuscarinic activity of methantheline.

The quaternary ammonium compound, methantheline, was found to antagonize acetylcholine induced contractions in isolated guinea pig ileum by a mechanism which did not conform to the criteria for either competitive or non-competitive inhibition. Enhancement of the lipid solubility of methantheline by formation of an ion pair with trichloracetate failed to influence its cholinergic inhibitory activity. The results suggest that in the guinea pig ileum a) an intracellular site of action does not exist for methantheline and b) the membrane receptors for methantheline most likely are located in an aqueous environment.