Antioxidant effect of vitamin C on endothelial function in streptozotocin-induced diabetic rats

Abstract

To examine the efficacy of long-term supplemented vitamin C, a natural antioxidant, on the prevention of endothelial dysfunction in diabetic cerebral microcirculation, tire animal model of streptozotocin (STZ)-induced diabetic rats (a single intravenous mjection of STZ; 55 mg/kg bw) was used. Male Wistar Furth rats weighing 200-250 g were divided randomly into two major groups of diabetes (STZ) and nondiabetes (CON). In each group, two subgroups were further randomly divided as followings: CON with supplemented and CON without supplemented by vitamin C, and STZ with supplemented and STZ without supplemented by vitamin C. The treatment of vitamin C was performed by allowing the animals freely assessed to drinking water added lg/L of vitamin C. The intravital fluorescent microscopy technique was performed at 12, 24 and 36 weeks (wks) after the mjection of STZ by using a craniotomy and a closed cramai window technique for cerebral microcirculatory visualization. At the end of each experiment, blood sample and cerebral tissues from the same fluorescent studied area were taken for metabolic and transmission electron microscopic studies, respectively. The experimental results showed that at 12, 24, and 36 wks after the STZ injection, hyperglycemia, hypercholesterolemia and hypertriglyceridemia were significantly developed and markedly severe at 36 weeks. Simultaneously, the decreased plasma vitamin C level was also demonstrated significantly in STZ-rats. Interestingly, the present study demonstrated that the level of blood glucose (BG) in STZ-Vit C rats was significantly less than the value of STZ-rats, however, the significant difference was only monitored at 36 wks after STZ injection (BG ; STZ-rats = 398.12 ± 17.12 mg/dl, BG ; STZ-Vit C rats = 317.28 ± 29.58 mg/dl; p<0.05). Moreover, at 36 wks, the results of plasma cholesterol (Chol) and triglyceride (TG) levels in STZ-Vit C rats were significantly different than those values of STZ-rats (Chol ; STZ-rats = 157.83 ± 25.21 mg/dl; cholesterol (Chol) level of STZ-Vit C rats = 71.33 ± 4.72 mg/dl, triglyceride (TG) level of STZ-rats = 154.17 ± 37.09 mg/dl; triglyceride level of STZ-Vit C rats = 53.56 ± 8.13 mg/dl). Besides, the results also showed that mean arterial pressure (MAP) was elevated for all three monitored time point whereas the mean value of cerebral arteriolar flow rate was significantly reduced in 36 wks STZ- rats. Vitamin C has the benefit effects to reduce these abnormalities of both MAP and the arteriolar flow rate (arteriolar flow rate : 36 wks STZ-rats = 0.40 ± 0.04 nl/sec, 36 wks STZ-Vit C rats = 1.92 ± 0.09 nl/sec ). Especially, to examine the antioxidant effept-of vitamin C directly on endothelial function, the number of leukocytes adhesion were counted. And the cerebral arteriolar responses to acetylcholine (Ach), adenosine-5’ diphosphate (ADP) and nitroglycerine (NTG) were respectively examined. The results demonstrated that there were the significantly increased leukocytes adhesion on the endothelial cells of postcapillaiy venules in STZ-rats for all three monitored time point as compared to the CON-rats (36 wks STZ-rats = 4.63 ± 0.33 cells/100um; 36 wks CON-rats = 0.62 ± 0.16 cells/100um). Interestingly, the leukocytes adhesion were almost entirely prevented by vitamin C supplementation for all three monitored time points. Moreover, for all three monitored time points, the responses of cerebral arterioles (20-30 um) demonstrated that the endothelium-dependent vasodilation to Ach and ADP were significantly decreased in STZ-rats as compared to the CON-rats (p<0.01). The supplementation of vitamin C could significantly prevent these impairment of endothelium-dependent vasodilation to Ach and ADP. However, the responses of endotheliumindependent vasodilation to NTG were evaluated, no significant differences were found between control and diabetic rats both with and without vitamin c supplementation. Moreover, the vitamin C supplementation could also prevent the thickening of basement membrane that were occurred in the cerebral arterioles and capillaries of STZ-rats as compared to those of CON-rats (p<0.01). In conclusion, the present study demonstrated that long-term supplementation of vitamin C, a natural antioxidant, could markedly prevent the diabetic induced endothelial dysfunctions including the ultrastructural changes of cerebralmicrocirculation. Therefore, vitamin c might be possibly important for the therapeutic prevention of diabetic cerebrovasculardiseases in the near future.