Cell Biology/Signaling Inhibition of Nitric Oxide–Stimulated Vasorelaxation by Carbon Monoxide-Releasing Molecules

Objective—Carbon monoxide (CO) is a weak soluble guanylyl cyclase stimulator, leading to transient increases in cGMP and vasodilation. The aim of the present work was to measure the effect of CO-releasing molecules (CORMs) on the cGMP/nitric oxide (NO) pathway and to evaluate how selected CORMs affect NO-induced vasorelaxation. Methods and Results—Incubation of smooth muscle cells with some but not all of the CORMs caused a minor increase in cGMP levels. Concentration-response curves were bell-shaped, with higher CORMs concentrations producing lower increases in cGMP levels. Although exposure of cells to CORM-2 enhanced cGMP formation, we observed that the compound inhibited NO-stimulated cGMP accumulation in cells and NO-stimulated soluble guanylyl cyclase activity that could be reversed by superoxide anion scavengers. Reactive oxygen species generation from CORMs was confirmed using luminol-induced chemiluminescence and electron spin resonance. Furthermore, we observed that NO is scavenged by CORM-2. When used alone CORM-2 relaxed vessels through a cGMP-mediated pathway but attenuated NO donor-stimulated vasorelaxation. Conclusion—We conclude that the CORMs examined have context-dependent effects on vessel tone, as they can directly dilate blood vessels, but also block NO-induced vasorelaxation. Key Words: nitric oxide Ⅲ pharmacology Ⅲ vascular biology Ⅲ cGMP Ⅲ carbon monoxide C arbon monoxide (CO), which was previously considered only as a highly toxic and life-threatening pollutant, has been recognized as a signaling molecule having regulatory roles in many physiological and pathophysiological processes within the cardiovascular, immune, and nervous systems. 1,2 It is generated in mammalian tissues via the endogenous degradation of heme by a family of constitutive (heme oxygenase-2 and heme oxygenase-3) and inducible (heme oxygenase-1) heme oxygenase enzymes. 2 CO gas binds to heme-containing proteins and exerts antiinflammatory and antioxidant effects. 2,3 In the cardiovascular system, CO exhibits vasodilatory properties, prevents endothelial cell apo-ptosis, controls vascular smooth muscle cell proliferation, and inhibits platelet aggregation. 3–5 It has been suggested that CO can be used as a therapeutic agent; testing the beneficial actions of CO has been facilitated by the discovery of molecules that release CO under appropriate conditions. 1,6 Initial CO-releasing molecules (CORMs) were transition metal carbonyls soluble only in organic solvents and required the use of physical or strong chemical stimuli to release CO. 7,8 More recently, water-soluble CORMs have been reported that liberate CO in a controlled fashion in biological systems and have the potential to deliver it to tissues and organs. 9,10 Several uses have been proposed for CORMs, ranging from …