A 55-year-old woman with hypertension is prescribed hydralazine. The primary mechanism by which hydralazine lowers blood pressure is:
- A Selective arteriolar smooth muscle relaxation via opening of ATP-sensitive potassium channels ✓
- B Inhibition of angiotensin-converting enzyme
- C Central alpha-2 agonism reducing sympathetic outflow
- D Blockade of L-type calcium channels in vascular smooth muscle
Correct answer: A. Selective arteriolar smooth muscle relaxation via opening of ATP-sensitive potassium channels
Explanation
Hydralazine directly relaxes vascular smooth muscle of arterioles by opening ATP-sensitive potassium channels (KATP), causing membrane hyperpolarisation and reduced intracellular calcium—leading to decreased peripheral resistance. It does not act on veins, does not inhibit ACE, and does not act centrally. A reflex sympathetic activation (tachycardia) and fluid retention necessitate co-prescription with a beta-blocker and diuretic. This mechanism differs from calcium channel blockers (which block ICaL).
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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