It is generally considered that the anticholinergic molecules have a primary point of attachment to cholinergic sites through the cationic head (i.e., the positively charged nitrogen). For quaternary ammonium compounds, there is no question of what is implied, but for tertiary amines, one assumes, with good reason, that the cationic head is achieved by protonation of the amine at physiological pH. The nature of the substituents on this cationic head is critical insofar as a parasympathomimetic response is concerned. Steric factors that cause diffusion of the onium charge or produce a less-than-optimal drug-receptor interaction result in a decrease of parasympathomimetic properties and allow the drug to act as an antagonist because of other bonding interactions. Ariens68 has shown that carbocholines (e.g., benzilylcarbocholine) engage in a typical competitive action with ACh, although they are less effective than the corresponding compounds possessing a cationic head, suggesting
that hydrophobic bonding may play an important role in these drug-receptor interactions.
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