The cholinoceptor antagonists consist of 2 subclasses based on their spectrum of action (ie, block of muscarinic versus nicotinic receptors). These drugs are pharmacologic antagonists or inverse agonists (eg, atropine). A third, special, subgroup, the cholinesterase regenerators, are not receptor blockers but rather are chemical antagonists of organophosphate acetylcholinesterase (AChE) inhibitors.
A. Classification and Pharmacokinetics
Muscarinic antagonists can be subdivided according to their selectivity for specific M receptors or their lack of such selectivity. Although the division of muscarinic receptors into subgroups is well documented (Chapters 6 and 7), only 2 distinctly receptor-selective M1 antagonists have reached clinical trials (pirenzepine and telenzepine, neither of which is used in the United States). However, as noted later, a few agents in use in the United States are somewhat selective for the M3 subtype. Most of the antimuscarinic drugs in use are relatively nonselective. The muscarinic blockers can also be subdivided on the basis of their primary clinical target organs (central nervous system [CNS], eye, bronchi, or gastrointestinal and genitourinary tracts). Drugs used for their effects on the CNS or the eye must be sufficiently lipid-soluble to cross lipid barriers. A major determinant of this property is the presence or absence of a permanently charged (quaternary) amine group in the drug molecule because charged molecules are less lipid-soluble (see Chapter 1).
Atropine is the prototypical nonselective muscarinic blocker. This alkaloid is found in Atropa belladonna and many other plants. Because it is a tertiary amine, atropine is relatively lipid-soluble and readily crosses membrane barriers. The drug is well distributed into the CNS, the eye, and other organs. It is eliminated partially by metabolism in the liver and partially unchanged in the urine; half-life is approximately 2 h; and duration of action of normal doses is 4–8 h except in the eye (see Drug Summary Table).
In ophthalmology, topical activity (the ability to enter the eye after conjunctival administration) and duration of action are important in determining the usefulness of several antimuscarinic drugs (see Clinical Uses). Similar ability to cross lipid barriers is essential for the agents used in parkinsonism. In contrast, the drugs used for their antisecretory or antispastic actions in the gut, bladder, and bronchi are often selected for minimum CNS activity; these drugs may incorporate quaternary amine groups to limit penetration through the blood–brain barrier.
High-Yield Terms to Learn
|Anticholinergic ||A drug that blocks muscarinic or nicotinic receptors, but commonly used to mean antimuscarinic |
|Antimuscarinic ||A drug that blocks muscarinic but not nicotinic receptors |
|Atropine fever ||Hyperthermia induced by antimuscarinic drugs; caused mainly by inhibition of sweating |
|Atropine flush ||Marked cutaneous vasodilation of the arms and upper torso and head by toxic doses of antimuscarinic drugs, especially atropine; mechanism unknown |