It has been suggested that increased production of nitric oxide (NO), a potent endothelium-derived vasodilator, may be responsible for increased blood flow in the retinal and renal vascular beds in early diabetes. However, NO-mediated vasodilation has been reported as impaired in diabetes, and there is evidence that the synthesis and release of NO by the vascular endothelium may be flawed in this condition. We examined the effect of high ambient glucose and exposure to exogenous glycated proteins on NO synthesis in cultured retinal microvascular endothelial cells (RMECs), using a polarographic sensor to measure released NO gas. Nitrite (the stable end product of the reaction between NO and molecular oxygen) was measured in tissue culture supernatants. The expression of vascular endothelial constitutive nitric oxide synthase (eNOS), which is responsible for NO synthesis in endothelial cells, was studied by Western blot analysis and Northern hybridization experiments. A dose-dependent reduction of NO synthesis by RMECs occurred 5 days after exposure to 15 and 25 mmol/l glucose, and concomitantly we found that accumulation of nitrite in culture supernatants of high-glucose exposed cells was also reduced. Coincubation of endothelial cells with inhibitors of protein kinase C (PKC) increased the accumulation of nitrite but did not restore it to the levels obtained when cells were cultured in 5 mmol/l glucose. The expression of eNOS by RMECs was markedly reduced by 5 days of exposure to 25 mmol/l glucose and glycated albumin. This study implicates the PKC pathway, which is known to be upregulated on exposure to high ambient glucose concentrations, as a possible factor in the inhibition of eNOS expression in RMECs. This study also suggests that glycated proteins may be involved in the pathogenesis of vascular endothelial dysfunction by modulating the nitric oxide synthase (NOS)/NO pathway in retinal vascular endothelial cells.