Role of Nitric Oxide in the Pathogenesis of Diabetic Nephropathy in Streptozotocin-Induced Diabetic Rats

OBJECTIVES: Several reports suggest that enhanced generation or actions of nitric oxide (NO) have been implicated in the pathogenesis of glomerular hyperfiltration and hyperperfusion that occurs in early diabetes. However, the precise role of altered NO generation in the pathogenesis of diabetic nephropathy is unclear. The present study was aimed at investigating the role of nitric oxide in the pathogenesis of glomerular hyperfiltration and hyperperfusion in streptozotocin-induced diabetic rats. METHODS: To evaluate the role of NO in diabetic hyperfiltration, we measured plasma and urine concentrations of NO(2)(−)/NO(3)(−), stable metabolic products of NO and protein expressions of three isoforms of nitric oxide synthase (NOS) in streptozotocin-induced diabetic rats. We also investigated renal hemodynamic changes, such as glomerular filtration rate (GFR) and renal plasma flow (RPF), in responses to acute and chronic administration of NO synthesis inhibitor, nitro-L-arginine methyl ester (L-NAME), in diabetic and control rats. RESULTS: Diabetic rats exhibited significantly elevated plasma and urinary NO(2)(−)/NO(3)(−) levels at 28 days after streptozotocin injection, and total excretion of NO(2)(−)/NO(3)(−) was approximately five-fold higher in diabetic rats than controls. Insulin and L-NAME treatment prevented the increases in plasma and urinary NO(2)(−)/NO(3)(−) concentrations in diabetic rats, respectively. The three isoforms of NOS (bNOS, iNOS, and ecNOS) were all increased in the renal cortex, whereas they remained unaltered in the renal medulla at day 28. GFR and RPF were significantly elevated in diabetic rats, and acute and chronic inhibition of NO synthesis by L-NAME attenuated the renal hemodynamic changes (increases in GFR and RPF) in diabetic rats, respectively. CONCLUSIONS: NO synthesis was increased due to enhanced NOS expression in diabetic rats, and chronic NO blockade attenuated renal hyperfiltration and hyperperfusion in diabetic rats. In addition, diabetic rats exhibited enhanced renal hemodynamic responses to acute NO inhibition and excreted increased urinary NO(2)(−)/NO(3)(−). These results suggest that excessive NO production may contribute to renal hyperfiltration and hyperperfusion in early diabetes.