Role of L-Ascorbic acid in renal pathophysiology in streptozotocin-induced diabetic rats

Abstract

The present study was performed to investigate whether L-ascorbic acid supplementation is able to ameliorate the renal dysfunction and pathology in streptozotocin-induced diabetic rats. Male Sprague-Dawley rats were randomly induced diabetes mellitus by intravenous injection of streptozotocin (STZ) (55 mg/kg BW) while the control rats were injected with citrate buffer as placebo. Hyperglycemic stage (blood glucose concentration>200 mg/dl) was verified two days after the diabetic induction. L-ascorbic acid (AA) I g/L in drinking water was given to either a control group (CON-AA) or a diabetic group(STZ- AA) in ad libilum. Another control group (CON) or diabetic group (STZ) was given with water. The experimental studies were carried out for 4, 8, 16 and 24 week of the adminstrations. At the end of specified period, blood glucose was measured and renal clearance studies of inulin and para-aminopippuric acid were performed to determine the glomerular filtration rate (GFR) and the effective renal plasma flow (ERPE), respectively. Systemic blood pressure was measured throughtout the clearance study. Urine flow rate (V),urinary. excretion (UE) of Na+ , K+ and Cl and plasma concentration of the elctrolytes were determined. Filtration fraction (FF), V/GFR, fraction excretion (FE) of the electrolytes, renal vascular resistance (RVR) and percentage of kidney weight to body weight (KW/BW) were calculated. In addition, mitochondrial activity was studied. The concentrations of malondialdehyde (MDA), transforming growth factor-B l (TGF-B l ) and glucose transporter-l (Glut l) expression in renal cortex were determined. A number of abnormal glomeruli (glomerulosclerosis) was examined by the periodic acid-Schiff staining under light microscope for the renal pathological study. GFR and ERPF of STZ-AA were significantly increased (p<0.05 ) and RVR was decreased (p<0.05 ) as compared with STZ at week 16. As compared with STZ, FE of Na+ and K+ were decreased (p<0.05 ) in STZ-AA at week 8. The significant decreases in UE of NA+, K+ and plasma concentration of Na+ were seen in STZ-AA at week 24. In addition, the blood glucose concentration and the percentage of KW/BW were significantly decreased (p <0.05 ) at week 16. On the study of renal mitochondrial activity, both STZ and STZ-AA demonstrated the increased oxygen consumption rates in both the resting stage and the stimulated stage (p <0.05 ) as compared with CON at week 24, using glutamate and malate as the substrates. In the case of using succinate as the substrate, STZ showed the significant increase in the value of P/O (p<0.05 ) as compared with that of CON. AA could normalize the mitochondrial activity in STZ-AA STZ rats showed the increases in the MDA concentration in the renal cortex at week 8 and 24 as compared with that of CON. The MDA concentration of STZ-AA was significantly decreased (p<0.05 ) as compared with that of STZ at week 16. The TGF- B l concentrations in the renal cortex of STZ were significantly increased (p<0.05) as compared with those of CON since at week 8 to week 24. STZ-AA showed the significant decrease in the TGF- B l concentration, which were not different from CON at week 16 and 24. The TGF- B l concentration of STZ-AA was significant decreased (p<0.05 ) as compared with that of STZ at week 16. In the present study, STZ-AA showed the significant increase (p <0.001) in Glut I expression in the renal cortex as compared with that of STZ at week 16, while STZ showed the expression of Glut l as same as that of CON. The renal pathological study demonstrated the significant increases in a number of abnormal glomeruli of STZ as compared with that of CON at week 16 and 24. AA could significant decrease in (p<0.05 ) the numbers of abnormal glomeruli in STZ-AA as compared with that of STZ at week 16 and 24. In conclusion the present study indicates that AA is able to ameliorate the renal pathophysiology in STZ-induced diabetic rats. The mechanism of AA on the renal pathophysiology may be the inhibition of TGF-B l production via the suppression of oxidative stress by Glut l upregulation.