The current study investigated the potential of green tea (GT) to improve uncoupling of endothelial nitric oxide synthase (eNOS) in diabetic conditions. In rats with streptozotocin-induced diabetes, nitric oxide (NO) bioavailability was reduced by uncoupling eNOS, characterized by a reduction in tetrahydrobiopterin (BH(4)) levels and a decrease in the eNOS dimer-to-monomer ratio. GT treatment ameliorated these abnormalities. Moreover, immortalized human mesangial cells (ihMCs) exposed to high glucose (HG) levels exhibited a rise in reactive oxygen species (ROS) and a decline in NO levels, which were reversed with GT. BH(4) and the activity of guanosine triphosphate cyclohydrolase I decreased in ihMCs exposed to HG and was normalized by GT. Exogenous administration of BH(4) in ihMCs reversed the HG-induced rise in ROS and the decline in NO production. However, coadministration of GT with BH(4) did not result in a further reduction in ROS production, suggesting that reduced ROS with GT was indeed secondary to uncoupled eNOS. In summary, GT reversed the diabetes-induced reduction of BH(4) levels, ameliorating uncoupling eNOS, and thus increasing NO bioavailability and reducing oxidative stress, two abnormalities that are involved in the pathogenesis of diabetic nephropathy.611838-47