Endothelial nitric oxide regulates vascular tone by:

• A. NO diffuses to smooth muscle causing vasoconstriction via cGMP degradation.
• B. NO diffuses to smooth muscle causing relaxation via cGMP, with release triggered by shear stress and acetylcholine.
• C. NO increases intracellular calcium in smooth muscle to cause contraction.
• D. NO blocks cGMP signaling and causes vasodilation.

Answer: (B)

The main concept is how endothelial NO relaxes vascular smooth muscle through cGMP signaling. Endothelial cells produce NO in response to shear stress from blood flow and to stimuli like acetylcholine. NO diffuses quickly into adjacent smooth muscle cells and activates soluble guanylate cyclase, which increases cGMP levels. Elevated cGMP then activates protein kinase G, which lowers intracellular calcium and promotes relaxation of the smooth muscle by reducing myosin light-chain phosphorylation, producing vasodilation. This mechanism explains why NO release under flow or acetylcholine stimulation leads to widened vessels.

The other statements don’t fit because NO does not cause vasoconstriction or degrade cGMP; it promotes relaxation by increasing cGMP. It also does not increase calcium to induce contraction, nor does it block cGMP signaling.