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Hydrogen sulfide (H₂S) is a unique gasotransmitter, with regulatory roles in the cardiovascular, nervous, and immune systems. Some of the vascular actions of H₂S (stimulation of angiogenesis, relaxation of vascular smooth muscle) resemble those of nitric oxide (NO). Although it was generally assumed that H₂S and NO exert their effects via separate pathways, the results of the current study show that H₂S and NO are mutually required to elicit angiogenesis and vasodilatation. Exposure of endothelial cells to H₂S increases intracellular cyclic guanosine 5′-monophosphate (cGMP) in a NO-dependent manner, and activated protein kinase G (PKG) and its downstream effector, the vasodilator-stimulated phosphoprotein (VASP). Inhibition of endothelial isoform of NO synthase (eNOS) or PKG-I abolishes the H₂S-stimulated angiogenic response, and attenuated H₂S-stimulated vasorelaxation, demonstrating the …