Review of Pharmacology - 9E (2015)

Review of Pharmacology
Parasympathomimetic Drugs
Autonomic Nervous System
Saxitoxin (obtained from
dinoflagellates) is a sodium
channel blocker whereas
α-bungarotoxin (component
of venom of banded krait) is
irreversible antagonist at N M
receptors.
These drugs may directly activate the muscarinic receptors (directly acting) or may act by
increasing the availability of ACh at the synaptic cleft (indirectly acting).
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DIRECTLY ACTING DRUGS
These are the esters of choline and may be natural alkaloids (ACh, muscarine, nicotine,
pilocarpine and arecoline) or synthetic derivatives (methacholine, carbachol and bethanechol).
• Acetylcholine is not used clinically because it is metabolized very quickly by
cholinesterases in the plasma and is not effective even by i.v. route.
• Methacholine has maximum action on myocardium. It can be given inhalationally
for the diagnosis of bronchial hyperreactivity in patients who do not have clinically
apparent asthma methacholine challenge test.
• Bethanechol is mainly used for its action on urinary bladder and has no nicotinic
activity.
• Pilocarpine is used in glaucoma due to its pupillary constrictor (miotic) action.
However because of its very short duration of action, intraocular tension may increase
even if one or two doses are missed.
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