Learning Objective
Identify the major muscarinic receptor antagonists, their pharmacologic effects, clinical uses, and important toxicologic considerations.
Prototype: Atropine
- Tertiary amine → enters CNS
- Prototype M-receptor blocker
- Other agents differ mainly in PK properties (duration, CNS entry)
Pharmacologic Effects of Atropine (Dose-Dependent)
Low → High dose sequence:
- ↓ Secretions (salivary, bronchial, sweat)
- Mydriasis, cycloplegia
- Hyperthermia (↓ sweating → ↑ body temperature)
- Tachycardia
- Sedation
- Urinary retention, constipation
- CNS stimulation → excitation, hallucinations
Drugs With Antimuscarinic Properties (Non-M Blockers)
- Antihistamines
- Tricyclic antidepressants
- Antipsychotics
- Quinidine
- Amantadine
- Meperidine
(These often cause anticholinergic side effects.)
Clinical Uses of Muscarinic Blockers
Important M-Blockers & Uses
| Drug | Key Uses |
|---|---|
| Atropine | Antispasmodic, antisecretory, AChE inhibitor overdose, antidiarrheal, ophthalmology (long action) |
| Tropicamide | Short-acting mydriasis/cycloplegia (ophthalmology) |
| Ipratropium, Tiotropium | Asthma/COPD (inhaled, no CNS entry) |
| Scopolamine | Motion sickness, sedation, short-term memory block |
| Benztropine, Trihexyphenidyl | Parkinsonism, antipsychotic-induced EPS (CNS entry) |
| Oxybutynin, Tolterodine, Trospium | Overactive bladder (urge incontinence) |
Treatment of Antimuscarinic Toxicity
- Supportive care
- ± Physostigmine (only when no contraindications → because it enters CNS)
