Intravenous injection of epinephrine raises mean arterial pressure. After pretreatment with an adequate dose of phentolamine, the same dose of epinephrine now produces a fall in mean arterial pressure. What is this phenomenon called and what mediates it?
- A Epinephrine reversal, mediated by unopposed beta-2 mediated vasodilation ✓
- B Tachyphylaxis, mediated by receptor desensitisation
- C Baroreceptor failure, mediated by autonomic denervation
- D Anaphylactoid reaction, mediated by histamine release from phentolamine
Correct answer: A. Epinephrine reversal, mediated by unopposed beta-2 mediated vasodilation
Explanation
Epinephrine activates both alpha-1 vasoconstrictor and beta-2 vasodilator receptors. Phentolamine competitively blocks the alpha receptors, unmasking pure beta-2 vasodilation, so blood pressure falls instead of rising; heart rate still rises through cardiac beta-1 stimulation. This response distinguishes alpha antagonists experimentally and clinically. Receptor desensitisation takes repeated dosing over time and does not appear after a single pretreatment, so tachyphylaxis is not the answer here.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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