Cholinoceptor antagonists

Question 1

What are the consequences of cholinoceptor blockade?

Answer 1

• Nicotinic receptor antagonists are used as ganglion blocking drugs - interfere with both parasympathetic and sympathetic action - widespread effects
• Hypotension due to vasodilation
• Reduced blood volume
• Pupil dilation
• Bronchodilation
• Bladder dysfunction
• Loss of GI motility, tone and secretions
• Exocrine secretions reduced overall, e.g. saliva, sweating, GI secretions

Question 2

What are the main clinical uses of muscarinic cholinoceptor antagonists?

Answer 2

• Atropine - pupil dilation, antidote for organophosphate (anticholinesterase) poisoning
• Tropicamide - pupil dilation for retinal exams
• Hyoscine - good sedative, anti motion sickness, Parkinson’s, IBS, anaesthetic premeds esp. when intubation required - airway dilation, dry throat, lungs and mouth - reduces risk of aspiration
• Ipratropium bromide - asthma, COPD

Question 3

What are the unwanted effects of muscarinic cholinoceptor antagonists?

Answer 3

• Hot as hell - decreased sweating
• Dry as a bone - reduced secretions everywhere
• Blind as a bat - cyclopegia - effects on accommodation ability
• Mad as a hatter - high dose affects CNS - agitation, restlessness, confusion

Question 4

What are the signs and symptoms of atropine poisoning?

Answer 4

• Hot as hell - warm, dry skin from decreased sweating
• Dry as a bone - reduced secretions everywhere
• Red as a beet - vasodilation
• Blind as a bat - blurred vision
• Mad as a hatter - high dose affects CNS - agitation, restlessness, confusion

Question 5

How is atropine poisoning treated?

Answer 5

• Anticholinesterase - indirectly acting cholinomimetic (physostigmine)
• Prevents ACh breakdown in synapse, so [ACh] increases and outcompetes atropine
• Slowly over time the body will clear atropine