Nitric oxide is a ______ factor produced by endothelial cells while endothelin is a ______ factor.

• A. Relaxing, contracting
• B. Contracting, relaxing
• C. Vasodilating, vasoconstricting
• D. Procoagulant, anticoagulant

Answer: (A)

Endothelial cells regulate vascular tone by releasing factors that either promote relaxation or promote contraction. Nitric oxide is an endothelium-derived relaxing factor: it is produced by endothelial nitric oxide synthase and diffuses into surrounding smooth muscle, where it activates guanylate cyclase to raise cGMP, leading to smooth muscle relaxation and vasodilation. Endothelin-1, on the other hand, is a potent endothelium-derived vasoconstrictor: it binds to receptors on smooth muscle, increasing intracellular calcium and causing contraction. This balance between NO-driven relaxation and endothelin-driven contraction explains why nitric oxide is described as a relaxing factor and endothelin as a contracting (vasoconstricting) factor.