Hydralazine is a direct-acting smooth muscle relaxant used to treat hypertension by acting as a vasodilator primarily in arteries and arterioles. By relaxing vascular smooth muscle, vasodilators act to decrease peripheral resistance, thereby lowering blood pressure and decreasing afterload. However, this only has a short term effect on blood pressure, as the system will reset to the previous, high blood pressure necessary to maintain pressure in the kidney necessary for natriuresis. The long term effect of antihypertensive drugs comes from their effects on the pressure natriuresis curve. Hydralazine binds to and activates gated potassium channels on vascular smooth muscle. The result is an efflux of potassium and a subsequent hyperpolarization of the cell. This prevents calcium-mediated activation and constriction of the smooth muscle, resulting in vasodilation.
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