Synthesis of pyridinium analogues of acetylcholine and their interactions with intestinal muscarinic receptors.

N-(beta-Acetoxyethyl)pyridinium salts were synthesized and tested for muscarinic receptor interactions by the guinea pig ileum assay. Agonist activity indicates that receptor binding is substantially retained when the ammonium group of acetylcholine is formally replaced by a pyridinium ring. Introduction of alkyl groups into the ring yields antagonists. The 4-tert-butylpyridinium derivative is proved to have an activity superior to that of the 4-methylpyridinium salt. Competitive antagonism is favored by the more hydrophobic property of the tert-butyl group. A nonpolar area is suggested to be situated in the direct vicinity of the anionic binding sites of muscarinic receptors. The interaction of hydrophobic substituents with this area determines the antimuscarinic properties of pyridinium salts.