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INTRODUCTION

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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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DIRECT-ACTING CHOLINOMIMETIC AGONISTS

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This class comprises a group of choline esters (acetylcholine, methacholine, carbachol, 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.

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High-Yield Terms to Learn
Choline esters A cholinomimetic drug consisting of choline (an alcohol) or a choline derivative, esterified with an acidic substance (eg, acetic or carbamic acid); usually poorly lipid-soluble
Cholinergic crisis The clinical condition of excessive activation of cholinoceptors; it may include skeletal muscle weakness as well as parasympathetic effects, usually caused by cholinesterase inhibitors; cf myasthenic crisis
Cholinomimetic alkaloids A drug with weakly alkaline properties (usually an amine of plant origin) whose effects resemble those of acetylcholine; usually lipid-soluble
Cyclospasm Marked contraction of the ciliary muscle; maximum accommodation for close vision
Direct-acting cholinomimetic A 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 cholinomimetic A drug that amplifies the effects of endogenous acetylcholine by inhibiting acetylcholinesterase
Muscarinic agonist A cholinomimetic drug that binds muscarinic receptors and has primarily muscarine-like actions
Myasthenic crisis In patients with myasthenia, an acute worsening of symptoms; usually relieved by increasing cholinesterase inhibitor treatment; cf cholinergic crisis
Nicotinic agonist A cholinomimetic drug that binds nicotinic receptors and has primarily nicotine-like actions
Organophosphate An ester of phosphoric acid and an alcohol that inhibits cholinesterase
Organophosphate aging A process whereby the organophosphate, after binding to cholinesterase, is chemically modified and becomes more firmly bound to the enzyme
Parasympathomimetic A drug whose effects resemble those of stimulating the parasympathetic nerves

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Table Graphic Jump Location
TABLE 7–1Some cholinomimetics: spectrum of action and pharmacokinetics.

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