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Background-Despite the importance of endothelial function for coronary regulation, there is little information and virtually no consensus about the causal mechanisms of endothelial dysfunction in myocardial ischemia/reperfusion (I/R) injury. Because tumor necrosis factor-␣ (TNF-␣) is reportedly expressed during ischemia and can induce vascular inflammation leading to endothelial dysfunction, we hypothesized that this inflammatory cytokine may play a pivotal role in I/R injury-induced coronary endothelial dysfunction. Methods and Results-To test this hypothesis, we used a murine model of I/R (30 minutes/90 minutes) in conjunction with neutralizing antibodies to block the actions of TNF-␣. TNF-␣ expression was increased Ͼ4-fold after I/R. To determine whether TNF-␣ abrogates endothelial function after I/R, we assessed endothelial-dependent (ACh) and endothelialindependent (SNP) vasodilation. In sham controls, ACh induced dose-dependent vasodilation that was blocked by the nitric oxide synthase (NOS) inhibitor L-NMMA (10 mol/L), suggesting a key role for NO. In the I/R group, dilation to ACh was blunted, but SNP-induced dilation was preserved. Subsequent incubation of vessels with the superoxide (O 2 ·Ϫ ) scavenger (TEMPOL), or with the inhibitors of xanthine oxidase (allopurinol, oxypurinol), or previous administration of anti-TNF-␣ restored endothelium-dependent dilation in the I/R group and reduced I/R-stimulated O 2 ·Ϫ production in arteriolar endothelial cells. Activation of xanthine oxidase with I/R was prevented by allopurinol or anti-TNF-␣. Conclusions-These results suggest that myocardial I/R initiates expression of TNF-␣, which induces activation of xanthine oxidase and production of O 2 ·Ϫ , leading to coronary endothelial dysfunction. Key Words: coronary artery disease Ⅲ endothelial function Ⅲ nitric oxide Ⅲ microcirculation Ⅲ reactive oxygen species T umor necrosis factor-␣ (TNF-␣) is an inflammatory cytokine 1 that is expressed by macrophages and cardiac tissue early during the myocardial ischemia-reperfusion (I/R) injury. 2,3 Elevations of TNF-␣ expression also appear to cause cardiomyopathy. 4,5 Interestingly, both cardiomyopathy and I/R injury are characterized by endothelial dysfunction, but, a putative role for TNF-␣ in the abnormal vasodilatory responses during I/R has not been elucidated.Studies from our laboratory 6 and others 7,8 have shown that I/R produces vascular endothelial dysfunction as defined by abrogated endothelium-dependent dilation. This adverse effect on endothelial function can occur acutely 6 or chronically (I/R injury in the pig can produce endothelial dysfunction for weeks). 9 However, endothelial dysfunction may be amplified by neutrophil-generated factors including oxygen-derived free radicals, cytokines, proteases, and lipid mediators. 10 I/R generates high levels of free radicals 11 composed of both reactive oxygen intermediates and nitric oxide (NO) via a complex sequence of events. When generated in sufficient concentrations, free radicals directly injure the myocardi...