F2-isoprostane evidence of oxidant stress in the insulin resistant, obese Zucker rat: effects of vitamin E.

We have concurrently investigated oxidant stress, glucose tolerance and glucose-stimulated insulin responses in the obese Zucker rat, a widely used model of insulin resistance. The plasma level of the lipid peroxidation product 8-epi-prostaglandin F2alpha, a sensitive in vivo marker of oxidant stress, was elevated approximately 5-fold in 13-week old obese relative to age-matched, insulin-sensitive lean Zucker rats. Supplementation of the diet with vitamin E (as (+)-alpha-tocopherol acetate, 0.5% w/w) for 4 weeks, reduced plasma 8-epi-prostaglandin F2alpha and concomitantly reversed glucose-stimulated hyperinsulinaemia in the obese Zucker rat without worsening glucose tolerance. We therefore provide evidence of oxidant stress, measured as elevated plasma 8-epi-prostaglandin F2alpha, for the first time in the obese Zucker rat which now provides a rationale for the beneficial effects of antioxidants on insulin action previously reported in this model of insulin resistance.

Investigation of oxidant stress and vasodepression to glyceryl trinitrate in the obese Zucker rat in vivo.

1. We examined the relationship between oxidant stress and the vasodepressor activity of glyceryl trinitrate (GTN) in vivo, including rapid GTN tolerance development, in 13-week old obese and age-matched lean Zucker rats which had been maintained for 4 weeks on either control diet or diets enriched with the lipophilic, chain-breaking antioxidants vitamin E (0.5% w w(-1)) or probucol (0.5% w w(-1)) or the superoxide anion scavenger tiron (1% w v(-1) in drinking water). 2. The basal plasma level of the isoprostane 8-epi-PGF2alpha, an in vivo marker of lipid peroxidation, was elevated by approximately 5 fold in the obese Zucker rat and markedly reduced by dietary lipophilic antioxidants and depressed by dietary tiron. 3. Vasodepression to bolus does GTN (0.1-100 microg kg(-1) i.v.), but not endothelium-dependent vasodepression to bolus dose acetylcholine (ACh, 0.02-2.0 microg kg(-1) i.v.), was impaired in obese animals and completely restored by dietary antioxidants. 4. Nitrate tolerance developed in vivo during a I h infusion of GTN (40 microg kg(-1) min(-1) i.v.) appeared more severe in obese animals. However, rapid nitrate tolerance was not affected by dietary antioxidants in either the obese or lean Zucker rat. 5. We therefore provide evidence that elevated oxidant stress in the obese Zucker rat is associated with an impairment in nitrate vasodepressor activity. However, our data are not consistent with either a role for oxidant stress in rapid nitrate tolerance development in the anaesthetized Zucker rat or the aggravation of this tolerance by pre-existing oxidant stress.

Dietary antioxidant supplementation reduces lipid peroxidation but impairs vascular function in small mesenteric arteries of the streptozotocin-diabetic rat.

Impaired endothelium-dependent relaxation could underlie many of the vascular complications associated with insulin-dependent diabetes mellitus, and may be mediated by increased oxidative stress. The effect of antioxidants on vascular endothelial function and oxidative stress of streptozotocin-diabetic rats was assessed by dietary supplementation with vitamins E and C. Diabetic (i.v. streptozotocin, 45 mg/kg) male Sprague-Dawley rats were fed one of six supplemented diets containing 75.9, 250, or 500 mg vitamin E/kg chow, 250 mg vitamin C/kg H2O, 250 mg vitamin E/kg chow plus 250 mg vitamin C/kg H2O, or chow deficient in vitamin E, and then compared to standard-fed control rats. After 4 weeks, small mesenteric arteries were dissected and mounted on a small vessel myograph, concentration response curves were then constructed to noradrenaline, acetylcholine and sodium nitroprusside. Acetylcholine-mediated relaxation was impaired in arteries from diabetic rats (pEC50 6.701+/-SEM 0.120, n = 8) compared to controls (7.386+/-0.078, n = 6; p < 0.05). The 500 mg/kg vitamin E diet further impaired maximum relaxation to acetylcholine (58.2+/-10.5 vitamin E, n = 7 vs 84.4+/-5.3 % standard, p < 0.05), and the combined vitamin E plus C diet impaired maximum relaxation to sodium nitroprusside (48.5+/-4.1 in vitamin E + C, n = 8 vs 75.6+/-3.9 % standard; p < 0.01). However, plasma 8-epi-prostaglandin (PG)F2alpha (measured as an estimate of oxidative stress) was dose-dependently decreased in rats on vitamin E supplemented diets. Dietary antioxidant supplementation did not reverse impaired endothelial function in this model of uncontrolled diabetes despite a concomitant decrease in oxidative stress.

Endothelial dysfunction in streptozotocin-diabetic rats is not reversed by dietary probucol or simvastatin supplementation.

Oxidative stress and dyslipidaemia are key features of diabetes mellitus and may be involved in mediating the vascular endothelial dysfunction associated with this disease. The aim of this study was to examine the effect of dietary lipid-lowering and antioxidant agents on vascular endothelial function and oxidative stress. Diabetic male Sprague-Dawley rats (i.v. streptozotocin, 45 mg/kg) were fed for 4 weeks on a standard diet or on a diet supplemented with either the lipid-lowering antioxidant probucol (1% w/w in diet) or the 3-hydroxy 3-methylglutaryl coenzyme-A (HMG-CoA) reductase inhibitor simvastatin (0.01% w/w in diet). Responses to noradrenaline, acetylcholine, and sodium nitroprusside were assessed in small mesenteric arteries (mean internal diameter 300+/-5 microm, n = 80) mounted on a small vessel myograph. Plasma concentrations of total cholesterol and triglycerides were significantly raised in standard-fed diabetic rats and significantly reduced in probucol and simvastatin-fed diabetic rats 8-epi-prostaglandin (PG)F2alpha, an indicator of oxidative stress, was raised in liver and aorta from diabetic rats compared to controls. Probucol supplementation reduced 8-epi-PGF2alpha in aorta and liver of diabetic rats but increased 8-epi-PGF2alpha content in plasma and aorta from control animals. The abnormal relaxation to acetylcholine in arteries from the diabetic rats (pEC550 diabetic 6.763+/-0.172 vs control 7.541+/-0.175; p < 0.05) was not improved by probucol or simvastatin. These data, therefore, do not support a role for oxidative stress or dyslipidaemia in mediating the impaired ACh-induced endothelium-dependent relaxation of small mesenteric arteries from the streptozotocin-diabetic rat.