INHIBITION OF INDUCIBLE NITRIC-OXIDE SYNTHASE RESTORES ENDOTHELIUM-DEPENDENT RELAXATIONS IN PROINFLAMMATORY MEDIATOR-INDUCED BLOOD-VESSELS

Endothelium-dependent relaxations mediated by nitric oxide (NO) are at tenuated in arteries exposed to proinflammatory mediators. Because pro inflammatory mediators stimulate the expression of the inducible NO sy nthase (iNOS) in vascular cells, the role of iNOS-derived NO in the im paired endothelium-dependent relaxation was examined in arterial ring preparations. Exposure of rabbit carotid arteries to interleukin-1 bet a (IL-1 beta; 100 U/mL for 7 hours) and porcine coronary arteries to a combination of tumor necrosis factor-alpha (1000 U/mL), interferon-ga mma (500 U/mL), and lipopolysaccharide (10 mu g/mL) for 15 hours (cond itions that are associated with iNOS expression) markedly attenuated r elaxations to receptor-dependent agonists, whereas those to the calciu m ionophore A23187 and sodium nitroprusside were virtually unchanged. The impaired relaxation was not associated with a reduced level of the constitutive endothelial NOS (cNOS) but was accompanied by a reduced formation of biologically active NO as assessed in a bioassay system. The attenuated relaxation of carotid arteries to acetylcholine was not affected by superoxide dismutase and was neither found in arteries ex posed to IL-1 beta for only 15 minutes nor in IL-1 beta-treated arteri es for 7 hours followed by a 17-hour incubation period without the cyt okine. Furthermore, no impaired relaxation was found in rings exposed to IL-1 beta in combination with either cycloheximide or N-alpha-tosyl -L-lysine chloromethyl ketone or pyrrolidine dithiocarbamate, treatmen ts that prevent iNOS expression. In addition, selective inhibition of iNOS with S-methylisothiourea (10 mu mol/L) completely restored acetyl choline-induced relaxations. These findings indicate that the continuo us generation of NO induced by proinflammatory mediators plays a major role in the inhibition of endothelium-dependent relaxation, most Like ly by impairing a step in the signal transduction cascade that links a ctivation of endothelial receptors to the calcium-calmodulin-dependent activation of NOS.