Background/Aims: Reactive oxygen species, and especially superoxide (O-2(radical anion)), have been implicated in diabetic nephropathy. O-2(radical anion) accumulation in cells is dependent on O-2(radical anion) production (by NADH/NADPH oxidase) as well as scavenging by superoxide dismutase ( SOD) activity. This study was designed to investigate the effects of high glucose (HG) on O-2(radical anion) accumulation and SOD activity in human mesangial cells (HMC) and to determine if these effects are mediated by angiotensin II (Ang II). Methods: HMC were incubated in media containing 10 m M glucose ( control, C), 30 m M glucose ( HG), 10 m M glucose + either 20 m M 2-deoxy-D-glucose (2-DG) or 20 m M mannitol ( high mannitol, HM) ( osmotic controls), or Ang II (10(-5) M). Ang II action was antagonized by employing 10(-4) M of Ang II receptor antagonists (losartan or irbesartan) or 10(-4) M of NADH/NADPH oxidase inhibitors [ diphenyleneiodonium chloride (DPI) or apocynin]. Superoxide and total SOD activity were assayed using chemiluminescence of lucigenin. Results: Incubation of HMC in HG resulted in a 1.6-fold increase in Ang I ( p < 0.05) and a 1.4-fold increase in Ang II levels ( p < 0.05) in cell lysates. These changes were accompanied by a >2-fold increase in O-2(radical anion) accumulation ( p < 0.01), which was inhibited by losartan and irbesartan. Exogenous Ang II increased net O-2(radical anion) accumulation by 2.7- fold ( p < 0.01), which was normalized by losartan and irbesartan. DPI and apocynin blocked the HG and Ang II- induced increases in O-2(radical anion) ( p < 0.01). HG but not exogenous Ang II inhibited total SOD activity by 30%, which was not affected by losartan. Conclusion: High glucose increases O-2(radical anion) accumulation in HMC primarily by increasing its production via the Ang II- NADH/NADPH oxidase system. Copyright (C) 2005 S. Karger AG, Basel.