A patient with a pheochromocytoma has paroxysmal hypertension, palpitations, and sweating. Which adrenoceptor mediates the vasoconstriction causing acute hypertensive episodes?

• A Alpha-1 adrenoceptors on vascular smooth muscle ✓
• B Beta-1 adrenoceptors on the SA node
• C Beta-2 adrenoceptors causing paradoxical vasoconstriction
• D Alpha-2 adrenoceptors on postsynaptic smooth muscle

Explanation

Catecholamine surge from pheochromocytoma (predominantly norepinephrine, though some tumors secrete predominantly epinephrine) causes vasoconstriction primarily via alpha-1 adrenoceptors on vascular smooth muscle, increasing peripheral vascular resistance and causing acute hypertension. Alpha-1 receptors couple to Gq → phospholipase C → IP3 → intracellular Ca2+ release → smooth muscle contraction. Beta-1 receptors mediate tachycardia; beta-2 receptors on muscle vessels cause vasodilation. Alpha-2 presynaptic receptors reduce NE release (not the primary hypertensive mechanism).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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