Diabetic vascular dysfunction is associated with an increase in reactive oxygen species (ROS). In this study, we hypothesized that hyperglycemia-induced ROS generation would impair the function of large conductance Ca(2+)-activated K(+) (BK) channels, which are major determinants in vasorelaxation. We found that when cultured in high glucose (HG) (22 mmol/L), HEK293 cells showed a reduction in expressed hSlo current densities, as well as slowed activation and deactivation kinetics. When human coronary smooth muscle cells were cultured in HG, similar findings were observed for the BK currents. HG enhanced superoxide dismutase and suppressed catalase (CAT) expression in HEK293 cells, leading to a significant increase in intracellular ROS. The effects of HG were mimicked by hydrogen peroxide (H(2)O(2)), and hSlo functions were restored by CAT gene transfer. Peroxynitrite inhibited hSlo current density but did not change channel kinetics. The hSloC911A mutant was insensitive to the effects of HG and H(2)O(2). Hence, imbalance of antioxidant enzymes plays a critical role in ROS generation in HG, impairing hSlo functions through H(2)O(2)-dependent oxidation at cysteine 911. This may represent an important fundamental mechanism that contributes to the impairment of vasodilation in diabetes.