Cholinergic Antagonists and Their Types

Study Notes

Cholinergic Antagonists

Cholinergic antagonists bind to cholinoceptors and prevent the effects of acetylcholine (ACh) and other cholinergic agonists.
There are three types of cholinergic antagonist drugs: antimuscarinic agents, ganglionic blockers, and neuromuscular-blocking agents.

Antimuscarinic Agents

Antimuscarinic agents block muscarinic receptors, causing inhibition of muscarinic functions.
They do not block nicotinic receptors, and therefore have little or no action at skeletal neuromuscular junctions (NMJs).

Atropine (Antimuscarinic Agent)

Atropine is an alkaloid with a high affinity for muscarinic receptors.
It binds competitively and prevents ACh from binding to muscarinic receptors.
Atropine acts both centrally and peripherally, with general actions lasting about 4 hours.
Effects of atropine:
- CNS: confusion, delirium
- GI: decrease motility and acid secretions (without interfering with hydrochloric secretion)
- Heart rate: increase at high doses, decrease at low doses
- Body secretions: decrease saliva, bronchial secretions, and sweat (leading to elevated body temperature)
- Eye: mydriasis (cycloplegic), allowing for measurement of refractive errors without interference by the accommodative capacity of the eye

Therapeutic Uses of Atropine

As a mydriatic and cycloplegic: used topically for producing mydriasis and cycloplegia, lasting 7-10 days
As a preanaesthetic medication:
- Preventing vagal bradycardia during anaesthesia
- Preventing laryngospasm by decreasing respiratory secretions
- Acting as an antisecretory agent to block secretions in the upper and lower respiratory tracts before surgery
As an antispasmodic: useful in dysmenorrhoea, intestinal and renal colic
In poisoning:
- Life-saving drug in organophosphorous poisoning
- Drug of choice in some types of mushroom poisoning
- Used in curare poisoning with neostigmine to counteract the muscarinic effects of neostigmine
As a vagolytic: used to treat sinus bradycardia and partial heart block due to increased vagal activity

Side Effects of Atropine

Blurred vision
Decreased secretions
Hyperthermia
Constipation
Urinary retention
Delirium
Hallucinations

Description

Learn about cholinergic antagonists, their effects on acetylcholine, and their three types, including antimuscarinic agents. Understand their role in blocking muscarinic receptors and inhibiting muscarinic functions.