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Home Books Katzung & Trevor's Pharmacology: Examination & Board Review, 10e

Chapter 7. Cholinoceptor-Activating & Cholinesterase-Inhibiting Drugs

Drugs with acetylcholine-like effects (cholinomimetics) consist of 2 major subgroups on the basis of their mode of action (ie, whether they act directly at the acetylcholine receptor or indirectly through inhibition of cholinesterase). Drugs in the direct-acting subgroup are further subdivided on the basis of their spectrum of action (ie, whether they act on muscarinic or nicotinic cholinoceptors).

Acetylcholine may be considered the prototype that acts directly at both muscarinic and nicotinic receptors. Neostigmine is a prototype for the indirect-acting cholinesterase inhibitors.

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Choline esterA cholinomimetic drug consisting of choline (an alcohol) esterified with an acidic substance, (eg, acetic or carbamic acid); usually poorly lipid-soluble
Cholinergic crisisThe clinical condition of excessive activation of cholinoceptors; it may include skeletal muscle weakness as well as parasympathetic signs
Cholinomimetic alkaloidA drug with weakly basic properties (usually of plant origin) whose effects resemble those of acetylcholine; usually lipid-soluble
CyclospasmMarked contraction of the ciliary muscle; maximum accommodation for close vision
Direct-acting cholinomimeticA drug that binds and activates cholinoceptors; the effects mimic those of acetylcholine
Endothelium-derived relaxing factor (EDRF)A potent vasodilator substance, largely nitric oxide (NO), that is released from vascular endothelial cells
Indirect-acting cholinomimeticA drug that amplifies the effects of endogenous acetylcholine by inhibiting acetylcholinesterase
Muscarinic agonistA cholinomimetic drug that binds muscarinic receptors and has primarily muscarine-like actions
Myasthenic crisisIn patients with myasthenia, an acute worsening of symptoms; usually relieved by increasing cholinomimetic treatment
Nicotinic agonistA cholinomimetic drug that binds nicotinic receptors and has primarily nicotine-like actions
OrganophosphateAn ester of phosphoric acid and an alcohol that inhibits cholinesterase
Organophosphate agingA process whereby the organophosphate, after binding to cholinesterase, is chemically modified and becomes more firmly bound to the enzyme
ParasympathomimeticA drug whose effects resemble those of stimulating the parasympathetic nerves

This class comprises a group of choline esters (acetylcholine, methacholine, carbachol, and bethanechol) and a second group of naturally occurring alkaloids (muscarine, pilocarpine, nicotine, lobeline). Newer drugs are occasionally introduced for special applications. The members differ in their spectrum of action (amount of muscarinic versus nicotinic stimulation) and in their pharmacokinetics (Table 7–1). Both factors influence their clinical use.

Table 7–1 Some Cholinomimetics: Spectrum of Action and Pharmacokinetics.
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Table 7–1 Some Cholinomimetics: Spectrum of Action and Pharmacokinetics.
DrugSpectrum of ActionaPharmacokinetic Features
Direct-acting
AcetylcholineBRapidly hydrolyzed by cholinesterase (ChE); duration of action 5–30 s; poor lipid solubility
BethanecholMResistant to ChE; orally active, poor lipid solubility; duration of action 30 min to 2 h
CarbacholBLike bethanechol
PilocarpineMNot an ester, good lipid solubility; duration of action 30 min to 2 h
NicotineNLike pilocarpine; duration of action 1–6 h; high lipid solubility
VareniclineNPartial agonist at N receptors, high lipid solubility; ...

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