What is the primary mechanism of action of nitroglycerin in treating angina?

• A. Donates nitric oxide to smooth muscle causing venodilation and reduced preload.
• B. Blocks beta-adrenergic receptors to slow heart rate.
• C. Inhibits calcium entry into cardiac muscle to reduce contractility.
• D. Reduces cholesterol synthesis in the liver.

Answer: (A)

Nitroglycerin relieves angina by releasing nitric oxide, which activates guanylate cyclase in vascular smooth muscle and increases cGMP, leading to smooth muscle relaxation. This mainly causes venodilation, which lowers venous return to the heart (preload), reducing left ventricular end-diastolic pressure and wall tension. With less wall stress, myocardial oxygen demand falls, improving the balance between oxygen supply and demand and easing angina. At usual antianginal doses, arterial dilation is less pronounced, so the key effect is decreased preload rather than reducing contractility. Other options describe different drug actions—beta-blockers slow heart rate, calcium channel blockers reduce calcium entry to smooth and cardiac muscle, and statins lower cholesterol—so they’re not the primary mechanism here.