A volunteer receives an infusion of a catecholamine that raises mean arterial pressure. Phentolamine is then administered, and the same infusion now produces a fall in blood pressure accompanied by tachycardia. The catecholamine being studied is:
- A Epinephrine ✓
- B Phenylephrine
- C Norepinephrine
- D Dopamine
Correct answer: A. Epinephrine
Explanation
Epinephrine activates both alpha-1 (vasoconstriction) and beta-2 (vasodilation) receptors. When alpha receptors are blocked by phentolamine, unopposed beta-2 vasodilation dominates and blood pressure falls; this is classical epinephrine reversal. Norepinephrine lacks meaningful beta-2 activity, so alpha blockade simply abolishes its pressor effect without reversing it. Phenylephrine is purely alpha-selective and cannot produce reversal.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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