VITAMIN-E PREVENTS DIABETES-INDUCED ABNORMAL RETINAL BLOOD-FLOW VIA THE DIACYLGLYCEROL-PROTEIN KINASE-C PATHWAY

We have characterized effects of d-alpha-tocopherol (vitamin E) on activation of protein kinase C (PKC) and diacylglycerol (DAG) levels in retinal tissues of diabetic rats and correlated its effects to diabetes-induced changes in retinal hemodynamics. Membrane PKC specific activities were increased by 71% in streptozocin-induced diabetic rats compared with controls (P < 0.05). Western blot analysis showed that membrane PKC-P-II was increased by 133 +/- 5% (P < 0.05). Injection of d-alpha-tocopherol (40 mg/kg ip) every other day prevented the increases in membrane PKC specific activity and PKC-beta(II) protein by immunoblots. Diabetes-induced increases in DAG levels were also normalized by d-alpha-tocopherol treatment of 2 wk duration. Physiologically, angiographic abnormalities of retinal hemodynamics based on computerized video-based fluorescein angiography and associated with increases of DAG and membranous PKC levels were also prevented by d-alpha-tocopherol treatment in diabetic rats. The effect of d-alpha-tocopherol on retinal vascular cells was also studied. Exposure of retinal endothelial cells to 22 mM glucose for 3 days increased total DAG and [H-3]palmitate-labeled DAG levels by 35 +/- 8 and 50 +/- 8% (P < 0.05), respectively, compared with exposure to 5.5 mM glucose. The presence of d-alpha-tocopherol (50 mu g/ml) prevented the increases in total DAG and [H-3]palmitate-labeled DAG levels in cells exposed to 22 mM glucose. These findings suggested that treatment with d-alpha-tocopherol can prevent diabetes-induced abnormalities in rat retinal blood flow. The mechanism of d-alpha-tocopherol's effect appears to be mediated by normalization of hyperglycemia-induced activation of the DAG-PKC pathway in the retina of diabetic rats.