muscarinic antagonist
Summary
Muscarinic antagonists (anticholinergics) competitively block muscarinic acetylcholine receptors (M1-M5), inhibiting parasympathetic nervous system effects. Classic examples include atropine, scopolamine, ipratropium/tiotropium, oxybutynin, and benztropine. Clinical effects follow the mnemonic 'can't see, can't pee, can't spit, can't shit' due to widespread parasympathetic blockade.
Detail
Muscarinic receptors are G-protein coupled receptors activated by acetylcholine at parasympathetic postganglionic synapses (and sweat glands, which are sympathetic cholinergic). Muscarinic antagonists competitively inhibit ACh binding, producing effects opposite to parasympathetic stimulation.
Physiologic effects of blockade: Mydriasis and cycloplegia (blocks M3 in eye - blurred vision, photophobia, can precipitate acute angle-closure glaucoma), decreased secretions (dry mouth, dry eyes - xerostomia, xerophthalmia), tachycardia (blocks vagal M2 tone on SA node), bronchodilation (blocks M3 in airway smooth muscle - therapeutic use), decreased GI motility (constipation, blocks M3 in gut), urinary retention (blocks M3 in detrusor muscle), decreased sweating (anhidrosis - can cause hyperthermia, 'red as a beet, dry as a bone, blind as a bat, mad as a hatter, hot as a hare').
Key drugs and clinical uses: - Atropine: treats bradycardia, organophosphate/cholinesterase inhibitor poisoning, preanesthetic to reduce secretions - Scopolamine: motion sickness (patch) - Ipratropium/tiotropium: COPD, asthma (inhaled, minimal systemic absorption) - Oxybutynin, tolterodine: overactive bladder - Benztropine, trihexyphenidyl: Parkinson's disease (restore ACh-dopamine balance), drug-induced extrapyramidal symptoms - Glycopyrrolate: reduce secretions perioperatively - Dicyclomine, hyoscyamine: IBS
Toxicity/overdose: Anticholinergic toxicity presents with delirium, hyperthermia, dry flushed skin, mydriasis, urinary retention, tachycardia, ileus. Physostigmine (crosses BBB) is the antidote for severe cases; treats both central and peripheral symptoms.
Contraindications: Avoid in narrow-angle glaucoma, BPH (worsens urinary retention), and used cautiously in elderly (can precipitate delirium, especially with combined anticholinergic burden from multiple drugs).
Distinguish from ganglionic blockers (nicotinic antagonists at autonomic ganglia) and neuromuscular blockers (nicotinic antagonists at NMJ) - muscarinic antagonists are selective for muscarinic receptors only.
Sources
- First Aid for the USMLE Step 1
- Katzung's Basic and Clinical Pharmacology
- Goodman & Gilman's Pharmacological Basis of Therapeutics
- Lippincott Illustrated Reviews: Pharmacology
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