In an experiment, intravenous epinephrine raises mean arterial pressure in a normal subject. After pretreatment with phenoxybenzamine, the same epinephrine dose now lowers arterial pressure. This change in response is explained by:

• A Blockade of beta-2 mediated vasodilation leaving alpha effects unopposed
• B Unmasking of beta-2 mediated vasodilation after alpha-1 blockade ✓
• C Baroreflex-mediated withdrawal of sympathetic tone
• D Conversion of phenoxybenzamine into a vasodilating metabolite

Explanation

Epinephrine stimulates both alpha-1 (vasoconstriction) and beta-2 (vasodilation) receptors. Phenoxybenzamine irreversibly blocks alpha-1, removing the pressor component and revealing the beta-2 vasodilatory effect, so pressure falls. This is classical epinephrine reversal. The same manoeuvre with norepinephrine does not cause reversal because norepinephrine has negligible beta-2 agonist activity, only loss of the pressor effect.

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

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