A patient with pheochromocytoma undergoing surgery develops a hypertensive crisis. The anesthesiologist administers phentolamine. The CORRECT statement about phentolamine's pharmacology is:

• A It selectively blocks alpha-1 receptors and does not affect alpha-2 receptors
• B It is a non-competitive, irreversible blocker of alpha-1 receptors only
• C It blocks alpha-1 receptors and also has intrinsic sympathomimetic activity at beta receptors
• D It is a competitive, reversible blocker of both alpha-1 and alpha-2 receptors ✓

Explanation

Phentolamine is a competitive, short-acting, reversible antagonist at both alpha-1 and alpha-2 adrenergic receptors. Blockade of presynaptic alpha-2 receptors disinhibits norepinephrine release, which can cause reflex tachycardia—a clinically significant side effect during pheochromocytoma surgery. Phenoxybenzamine (not phentolamine) is the irreversible, non-competitive alpha-blocker. Prazosin and doxazosin are selective alpha-1 blockers.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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