A patient on prazosin develops tachycardia after the first dose. The mechanism responsible is:
- A Direct β1-adrenoceptor stimulation by prazosin
- B Blockade of presynaptic α2-adrenoceptors increasing noradrenaline release
- C Reflex sympathetic activation due to fall in peripheral vascular resistance ✓
- D Inhibition of baroreceptor afferents in the carotid sinus
Correct answer: C. Reflex sympathetic activation due to fall in peripheral vascular resistance
Explanation
Prazosin selectively blocks postsynaptic α1-adrenoceptors, causing vasodilation and a drop in blood pressure. This fall in BP triggers baroreceptor-mediated reflex tachycardia. Unlike non-selective alpha-blockers (e.g., phentolamine), prazosin spares presynaptic α2-receptors, so noradrenaline release is not enhanced — yet reflex tachycardia is still observed.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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