A 45-year-old with Raynaud's phenomenon is given a drug that selectively blocks alpha-1A and alpha-1B adrenoceptors without blocking alpha-2 receptors. Which advantage does this selectivity confer compared with a non-selective alpha-blocker?
- A It eliminates reflex tachycardia because alpha-2 blockade on cardiac sympathetic terminals is avoided
- B Alpha-2 receptors mediate vasodilation; sparing them augments the vasodilatory effect
- C Alpha-2 receptors are presynaptic autoreceptors that inhibit noradrenaline release; preserving their function limits the reflex noradrenaline surge that would occur with non-selective alpha blockade ✓
- D Alpha-1A and alpha-1B blockade causes only venodilation, whereas alpha-2 blockade would cause arteriolar constriction that counteracts the benefit
Correct answer: C. Alpha-2 receptors are presynaptic autoreceptors that inhibit noradrenaline release; preserving their function limits the reflex noradrenaline surge that would occur with non-selective alpha blockade
Explanation
Presynaptic alpha-2 autoreceptors on sympathetic nerve terminals provide negative feedback, inhibiting noradrenaline (NA) release when synaptic NA rises. Non-selective alpha-blockers (phentolamine) also block these presynaptic alpha-2 receptors, removing the feedback brake and causing a reflex NA surge, which produces tachycardia, palpitations, and first-dose hypotension. Selective alpha-1 blockers (prazosin, doxazosin) spare alpha-2, preserving this feedback and minimising reflex tachycardia.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.