The effect of tetrahydrobiopterin (BH4) on diabetic nephropathy

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Nitric oxide synthase (NOS) requires tetrahydrobiopterin (BH4) as a coenzyme. Recent works showed that NOS activity is regulated in a BH4-dependent manner. Nitric oxide (NO) synthesis and metabolism are impaired in blood vessels of diabetic subjects. The concentration of BH4 is low in diabetic rats, probably contributing to endothelial dysfunction. If diabetic kidneys had such deficiency, it might cause nephropathy. The object of this research was to clarify anti-nephropathic effects of BH4 in diabetes meritus.We studied in streptozotocin (STZ) induced diabetic rats as a model of type 1 diabetes. Spargue-Dawley rats were divided by three groups, Control, STZ, STZ+BH4. BH4 was administered orally at 20mg/kg BW daily. STZ group had albuminuria at 9-week, and developed diabetic nephropathy until 14-week. But STZ+BH4 group did not develop during experiment. The renal expression of eNOS was decreased in STZ group, and this decrease was restored in STZ+BH4 group.We set out to find if the BH4 … More level in kidneys is low and, if so, whether it is involved in proteinuria. We used Otsuka Long-Evans Tokushima Fatty (OLETF) rats, with a model of type 2 diabetes, and used Long-Evans Tokushima Otsuka (LETO) rats as healthy controls. OLETF rats were put into groups with or without daily oral doses of BH4 (10 mg/kg) during the study. We obtained plasma, red blood cells, and kidneys from 62-week-old rats and measured BH4, total biopterin, and GTP cyclohydrolase I activity, the key enzyme of de novo BH4 synthesis. We measured urinary protein excretion in the rats at 13, 37, or 61 weeks of age as a marker of nephropathy and stained their kidneys with anti-eNOS antibodies with an immunofluorescent method. Plasma and renal levels of BH4 and biopterin were lower in diabetic rats than in controls. GTP cyclohydrolase activity of red blood cells and renal tissue from diabetic rats was less than that of controls. The BH4/biopterin ratio was smaller in the diabetic rats, suggesting that dihydropteridine reductase, which recycles BH4 from inactive BH2, has low activity in diabetes. Urinary protein was greater in diabetic rats at 13 weeks than in controls ; the difference increased with age. BH4 suppressed proteinuria in diabetic rats at 37 and 61 weeks without affecting body weight, plasma glucose, or blood pressure. Diabetic kidneys had less fluorescence from anti-eNOS antibodies than control kidneys, unless BH4 was given. In this model of diabetes, little BH4 was synthesized, apparently because of GTP cyclohydrolase and dihydropteridine reductase activities were low. Deficiencies of BH4 and of the eNOS-NO system may contribute to the progression of diabetic nephropathy. These results suggest that BH4 seems to have anti-nephropathic effects and may preYent progress of diabetic nephropathy. Less