MECHANISM OF
ACTION
Antimuscarinic drugs are competitive antagonists of the binding of
ACh to muscarinic receptors. The seven trans-membrane helices of these
receptors have a ringlike or-ganization in the cell membrane that forms a
narrow central cleft where ACh binds. At least seven amino acids from four
transmembrane helices have been im-plicated in agonist binding to the
muscarinic receptors. Some of these residues, particularly a negatively charged
aspartate, interact electrostatically with the positively charged quaternary
ammonium moiety of ACh, whereas other residues are required for binding to the
ester moiety. Although the tertiary amine and qua-ternary ammonium groups of
antimuscarinic drugs bind to the same anionic site on the receptor that
agonists occupy, these drugs do not fit into the narrow cleft and consequently
cannot activate the receptor.
Dicyclomine (Bentyl), oxybutynin (Ditropan), and tolterodine (Detrol) are nonselective smooth muscle
re-laxants that produce relatively little antagonism of mus-carinic receptors
at therapeutic concentrations. The mechanism of relaxation is not known.
Finally, some other classes of drugs can act in part as muscarinic
an-tagonists. For example, the antipsychotics and antide-pressants produce
antimuscarinic side effects (e.g., dry mouth).
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