OBJECTIVE: Hypercholesterolemia (HC), a major risk factor for onset and progression of renal disease, is associated with increased oxidative stress, potentially causing endothelial dysfunction. One of the sources of superoxide anion is xanthine oxidase (XO), but its contribution to renal endothelial function in HC remains unclear. We tested the hypothesis that XO modulates renal hemodynamics and endothelial function in HC pigs. METHODS: Four groups (n=23) of female domestic pigs were studied 12 weeks after either normal (n=11) or HC diet (n=12). Oxidative stress was assessed by plasma isoprostanes and oxidized LDL, and the XO system by plasma uric acid, urinary xanthine, and renal XO expression (by immunoblotting and immunohistochemistry). Renal hemodynamics and function were studied with electron beam computed tomography before and after endothelium-dependent (acetylcholine) and -independent (sodium-nitroprusside) challenge, during a concurrent intra-renal infusion of either oxypurinol or saline (n=5-6 in each group). RESULTS: HC showed elevated oxidative stress, higher plasma uric acid (23.8±3.8 vs. 6.2±0.8 µM/mM creatinine, p=0.001), lower urinary xanthine, and greater renal XO expression compared to normal. Inhibition of XO in HC significantly improved the blunted responses to acetylcholine of cortical perfusion (13.5±12.1 and 37.2±10.6 %, p=0.01 and p= n.s. vs. baseline, respectively), renal blood flow, and GFR, restored medullary perfusion, and improved the blunted cortical perfusion response to sodium-nitroprusside. CONCLUSIONS: This study demonstrates that the endogenous XO system is activated in swine HC. Furthermore, it suggests an important role for XO in regulation of renal hemodynamics, function, and endothelial function in experimental HC.