PREVENTION OF GLOMERULAR DYSFUNCTION IN DIABETIC RATS BY TREATMENT WITH D-ALPHA-TOCOPHEROL

Because d-alpha-tocopherol (vitamin E) has been shown to decrease diac ylglycerol (DAG) levels and prevent the activation of protein kinase C (PKC), which is associated with retinal and renal dysfunctions in dia betes, the study presented here characterized the effect of d-alpha-to copherol treatment to prevent glomerular hyperfiltration and increased albuminuria as well as PKC activities in streptozotocin (STZ)-induced diabetic rats. Two weeks after the induction of diabetes, total DAG c ontent and PKC activity in glomeruli were significantly increased in d iabetic rats by 106.4 +/- 16.8% and 66.4 +/- 8.4%, respectively, compa red with control rats. Intraperitoneal injection of d-alpha-tocopherol (40 mg/kg of body weight) every other day prevented the increases in total DAG content and PKC activity in glomeruli of diabetic rats. Glom erular filtration rate (GFR) and filtration fraction (FF) were signifi cantly elevated to 4.98 +/- 0.34 mL/min and 0.36 +/- 0.05, respectivel y, in diabetic rats, compared with 2.90 +/- 0.14 mL/min and 0.25 +/- 0 .02, respectively, in control rats. These hemodynamic abnormalities in diabetic rats were normalized to 2.98 +/- 0.09 mL/ min and 0.24 +/- 0 .01, respectively, by d-alpha-tocopherol. Albuminuria in 10-wk diabeti c rats was significantly increased to 9.1 +/- 2.2 mg/day compared with 1.2 +/- 0.3 mg/day in control rats, whereas d-alpha-tocopherol treatm ent improved albumin excretion rate to 2.4 +/- 0.6 mg/day in diabetic rats. To clarify the mechanism of d-alpha-tocopherol's effect on DAG-P KC pathway, the activity and protein levels of DAG kinase alpha and ga mma, which metabolize DAG to phosphatidic acid, were examined. Treatme nt with d-alpha-tocopherol increased DAG kinase activity in the glomer uli of both control and diabetic rats, by 22.6 +/- 3.6% and 28.5 +/- 2 .3% respectively, although no differences were observed in the basal D AG kinase activity between control and diabetic rats. Because immunobl otting studies did not exhibit any difference in the protein levels of DAG kinase alpha and gamma, the effect of d-alpha-tocopherol is proba bly modulating the enzyme kinetics of DAG kinase. These findings sugge st that the increases in DAG-PKC pathway play an important role for th e development of glomerular hyperfiltration and increased albuminuria in diabetes and that d-alpha-tocopherol treatment could be preventing early changes of diabetic renal dysfunctions by normalizing the increa ses in DAG and PKC levels in glomerular cells.