Which pairing accurately describes the roles of nitric oxide and endothelin in regulating vascular tone?

• A. Nitric oxide relaxing; endothelin contracting
• B. Nitric oxide contracting; endothelin relaxing
• C. Nitric oxide vasodilating; endothelin vasoconstricting
• D. Nitric oxide procoagulant; endothelin anticoagulant

Answer: (A)

Nitric oxide and endothelin act as opposing regulators of vascular tone. Nitric oxide is produced by endothelial cells and diffuses into vascular smooth muscle, where it activates soluble guanylate cyclase to raise cGMP. This lowers intracellular calcium and promotes smooth muscle relaxation, leading to vasodilation. Endothelin-1, also produced by the endothelium, binds to endothelin receptors on smooth muscle and triggers calcium-raising signaling that causes sustained contraction, i.e., vasoconstriction. In a healthy vessel, there is a balance between these factors, with nitric oxide promoting dilation and endothelin promoting constriction. The pairing described in the options—nitric oxide relaxing and endothelin contracting—best captures their classic roles.