A healthy volunteer receives an intravenous infusion of epinephrine and his mean arterial pressure rises. The infusion is repeated after pretreatment with an alpha-adrenergic antagonist. What happens to his blood pressure now?

• A It falls below baseline because unopposed beta-2 vasodilation dominates ✓
• B It rises further because beta-1 responses are unopposed
• C It remains unchanged because all adrenergic receptors are blocked
• D It rises initially and then falls due to baroreceptor reflex

Explanation

Epinephrine stimulates both alpha-1 vasoconstrictor and beta-2 vasodilator receptors. Alpha blockade removes the pressor component, leaving beta-2 mediated vasodilation dominant, so blood pressure falls. This phenomenon is called epinephrine reversal and is the classic demonstration distinguishing epinephrine from norepinephrine, whose pure alpha effect is simply attenuated rather than reversed by alpha blockers.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

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