Which molecule is a key endothelium-derived vasodilator?

• A. Endothelin
• B. Thromboxane A2
• C. Nitric Oxide
• D. Prostacyclin

Answer: (C)

Endothelial cells regulate blood vessel tone by releasing factors that either relax or constrict the smooth muscle. The standout endothelium-derived vasodilator is nitric oxide. It is produced by endothelial nitric oxide synthase from L-arginine in response to stimuli like shear stress and certain agonists. Once formed, NO quickly diffuses into neighboring vascular smooth muscle and activates soluble guanylate cyclase, increasing cGMP levels. This leads to a decrease in intracellular calcium and relaxation of the muscle, causing vasodilation. NO works rapidly and locally, which is why it’s considered the principal endothelium-derived vasodilator. Other molecules on the list include endothelin and thromboxane A2, which promote constriction, while prostacyclin is a vasodilator that also inhibits platelet aggregation but is not the primary endothelium-derived relaxing factor.