Which statement best describes endothelium-derived control of vascular tone with NO and endothelin?

• A. Endothelin causes vasodilation; NO causes vasoconstriction; both balance tone.
• B. NO causes vasodilation by smooth muscle relaxation; endothelin causes vasoconstriction; both balance tone.
• C. NO causes vasoconstriction; endothelin causes vasodilation
• D. Endothelin has no effect on vascular tone

Answer: (B)

Endothelium-derived control of vascular tone is a push-pull system: a relaxing signal and a constrictive signal keep blood vessel diameter and flow in balance. Nitric oxide, produced by endothelial cells, diffuses into smooth muscle and activates soluble guanylate cyclase, increasing cGMP. That cascade leads to relaxation of the smooth muscle and dilation of the vessel, allowing more blood to flow when needed. Endothelin-1, a potent peptide released by the endothelium, binds to receptors on smooth muscle and triggers calcium-driven contraction, causing vasoconstriction. Together, these opposing actions help maintain basal tone and finely tune responses to physiological stimuli. The statement that NO causes vasodilation by smooth muscle relaxation and endothelin causes vasoconstriction, with both factors balancing tone, best captures this relationship.