Decrease in tetrahydrobiopterin as a possible cause of nephropathy in type II diabetic rats

A decrease in renal synthesis of nitric oxide (NO) in the progression of diabetic nephropathy has been documented. As (6 R )-5,6,7,8-tetrahydrobiopterin (BH 4 ) is an essential cofactor of NO synthase, we investigated whether BH 4 deficiency is involved in the pathogenesis of nephropathy. Ten-week-old Otsuka Long–Evans Tokushima Fatty (OLETF) rats were used as a type II diabetic model, and Long–Evans Tokushima Otsuka (LETO) rats as the healthy controls. OLETF rats were orally treated with BH 4 (10 mg/kg daily) or with water from 10 to 61 weeks of age. In another experiment, OLETF rats were treated orally with a calcium channel blocker, benidipine (5 mg/kg daily), or with 0.3% carboxymethyl cellulose (nontreated) from 10 to 52 weeks of age. Proteinuria was observed periodically, and at the end of the study, BH 4 level and GTP cyclohydrolase I (GTPCH) activity in the kidney were measured. Proteinuria was observed at 13 weeks of age in the OLETF rats, and deteriorated until 61 weeks of age. Supplemental BH 4 reduced the proteinuria. At 52 weeks of age, GTPCH activity and the BH 4 level were decreased in the plasma and kidneys of OLETF rats, whereas they were significantly higher in the benidipine group than in the nontreated group. Proteinuria was milder in the benidipine group than in the nontreated group, without a concomitant decrease in blood pressure. Histologically observed glomerulosclerosis was mild in the BH 4 and benidipine groups. In type II diabetic rats, renal BH 4 is considered to play a crucial role in the pathogenesis of diabetic nephropathy. Benidipine was found to preserve BH 4 levels, suggesting therapeutic renoprotective effects.