General comments—Muscarinic receptor antagonists reduce the effects of ACh by competitively inhibiting its binding to muscarinic cholinergic receptors. In general, muscarinic antagonists cause little blockade at nicotinic receptors; however, the quaternary ammonium derivatives of atropine are generally more potent at muscarinic receptors and exhibit a greater degree of nicotinic blocking activity, and consequently are more likely to interfere with ganglionic or neuromuscular transmission. At high or toxic doses, central effects of atropine and related drugs are observed, generally CNS stimulation followed by depression; since quaternary compounds penetrate the blood-brain barrier poorly, they have little or no effect on the CNS.
Parasympathetic neuroeffector junctions in different organs vary in their sensitivity to muscarinic receptor antagonists (Table 7-2). Effects such as reduction of gastric secretions occur only at doses that produce severe undesirable effects. This hierarchy of relative sensitivities is not a consequence of differences in the affinity of atropine for the muscarinic receptors at these sites; atropine lacks receptor subtype selectivity. More likely determinants include the degree to which the functions of various end organs are regulated by parasympathetic tone and the involvement of intramural neurons and reflexes. Actions of most clinically available muscarinic receptor antagonists differ only quantitatively from those of atropine. No antagonist in the receptor-selective category, including pirenzepine, is completely selective; in fact, clinical efficacy may arise from a balance of antagonistic actions on two or more receptor subtypes.
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