Aldose reductase inhibitors (ARIs) prevent peripheral nerve dysfunction and morphological abnormalities in diabetic animal models. However, some experimental intervention studies and clinical trials of ARIs on diabetic neuropathy appeared disappointing because of either 1) their inadequate design and, in particular, insufficient correction of the sorbitol pathway activity or 2) the inability to reverse established functional and metabolic deficits of diabetic neuropathy by AR inhibition in general. We evaluated whether diabetes-induced changes in nerve function, metabolism, and antioxidative defense are corrected by the dose of ARI (sorbinil, 65 mg/kg/d in the diet), resulting in complete inhibition of increased sorbitol pathway activity. The groups included control rats and streptozotocin-diabetic rats treated with/without ARI for 2 weeks after 4 weeks of untreated diabetes. ARI treatment corrected diabetes-induced nerve functional changes; that is, decrease in endoneurial nutritive blood flow, motor and sensory nerve conduction velocities, and metabolic abnormalities (i.e., mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiency). ARI restored nerve concentrations of two major non-enzymatic antioxidants, reduced glutathione (GSH) and ascorbate, and completely arrested diabetes-induced lipid peroxidation. In conclusion, treatment with adequate doses of ARIs (that is, doses that completely inhibit increased sorbitol pathway activity) is an effective approach for reversal of, at least, early diabetic neuropathy.

Aims/hypothesis. A strong positive correlation has been found between lipid peroxidation product and vascular endothelial growth factor concentrations in the vitreous of patients with proliferative diabetic retinopathy. To establish a causal relation between diabetes-associated enhanced oxidative stress and vascular endothelial growth factor production, we evaluated two antioxidants, dl-a-lipoic acid and taurine, on retinal vascular endothelial growth factor protein and mRNA expression and on parameters of oxidative stress in streptozotocin-diabetic rats. Methods. Our experiments were on control rats and streptozotocin-diabetic rats with a 6-week duration of diabetes, treated with or without dl-a-lipoic acid (100 mg´kg ±1´d±1 , i. p.) or taurine (1 % in the diet) starting from induction of diabetes. Vascular endothelial growth factor protein in retinal homogenates was assessed by sandwich ELISA with an affinity-purified polyclonal antibody and vascular endothelial growth factor mRNA by ribonuclease protection assay. Retinal lipid peroxidation products i. e. malondialdehyde plus 4-hydroxyalkenals were quantified with n-methyl-2-phenylindole. Retinal reduced and oxidized glutathione, ascorbate, dehydroascorbate, and sorbitol pathway intermediates were measured spectrofluorometrically, and taurine by reverse-phase HPLC.Results. Vascular endothelial growth factor protein concentration (means SD) was increased in diabetic rats compared with control rats (33 7 vs 19 5 pg/mg total protein, p < 0.01) This increase was attenuated by taurine (26 8, p < 0.05) and prevented by dla-lipoic acid (21 4, p < 0.01). Vascular endothelial growth factor mRNA abundance was reduced by 1.4-fold in diabetic rats compared with control rats and this decrease was attenuated but not completely prevented by both antioxidants. Malondialdehyde plus 4-hydroxyalkenal concentration was increased in diabetic rats compared with control rats, and both antioxidants arrested accumulation of lipid peroxidation products. Taurine, reduced glutathione, oxidized glutathione, ascorbate, dehydroascorbate and sorbitol pathway intermediate concentrations as well as oxidized glutathione/reduced glutathione and dehydroascorbate/ascorbate ratios were similar in control and diabetic rats treated with or without taurine. Conclusion/interpretation. Oxidative stress is directly involved in up regulation of vascular endothelial growth factor protein in the retina during early diabetes. [Diabetologia (2001

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Numerous studies in experimental models of diabetes [1±5] as well as a few clinical trials [6±9] have shown a reduction of peripheral nerve conduction deficit and morphological abnormalities by structurally different aldose reductase inhibitors thus implicating increased activity of the sorbitol pathway in the pathogenesis of diabetic neuropathy. The mechanisms leading from increased sorbitol pathway activity to complex functional, metabolic and morphological changes in the peripheral nervous system in diabetes are not com- Diabetologia (1999) AbstractAims/hypothesis. Studies of the role of sorbitol dehydrogenase in nerve functional deficits induced by diabetes reported contradictory results. We evaluated whether sorbitol dehydrogenase inhibition reduces metabolic abnormalities and enhances oxidative stress characteristic of experimental diabetic neuropathy. Methods. Control and streptozotocin-diabetic rats were treated with or without sorbitol dehydrogenase inhibitor (SDI)-157 (100 mg × kg ±1 × day ±1 , in the drinking water, for 3 weeks). Sciatic nerve free mitochondrial (cristae and matrix) and cytosolic NAD + : NADH ratios were calculated from the b-hydroxybutyrate, glutamate and lactate dehydrogenase systems. Concentrations of metabolites, e. g. sorbitol pathway intermediates and variables of energy state were measured in individual nerves spectrofluorometrically by enzymatic procedures. Results. The flux through sorbitol dehydrogenase (manifested by nerve fructose concentrations) was inhibited by 53 % and 74 % in control and diabetic rats treated with SDI compared with untreated control and diabetic groups. Free NAD + :NADH ratios in mitochondrial cristae, matrix and cytosol were decreased in diabetic rats compared with controls and reduction in either of the three variables was not prevented by sorbitol dehydrogenase inhibitor. Phosphocreatine concentrations and phosphocreatine:creatine ratios were decreased in diabetic rats compared with controls and were further reduced by the inhibitor. Malondialdehyde plus 4-hydroxyalkenals concentration was increased and reduced gluthathione concentration was reduced in diabetic rats compared with the control group, and changes in both variables were further exacerbated by sorbitol dehydrogenase inhibitor. Neither NAD-redox and energy states nor lipid aldehyde and reduced gluthathione concentrations were affected by treatment with the inhibitor in control rats. Conclusion/interpretation. Inhibition of sorbitol dehydrogenase does not offer an effective approach for prevention of oxidation and metabolic imbalances in the peripheral nerve that is induced by diabetes and is adverse rather than beneficial. [Diabetologia (1999

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Oxidative stress has a key role in the pathogenesis of diabetic complications. We have previously reported that taurine (T), which is known to counteract oxidative stress in tissues (lens, kidney, retina) of diabetic rats, attenuates nerve blood flow and conduction deficits in early experimental diabetic neuropathy (EDN). The purpose of this study was to evaluate whether dietary T supplementation counteracts oxidative stress and the nerve growth factor (NGF) deficit in the diabetic peripheral nerve. The experiments were performed in control rats and streptozotocin‐diabetic rats fed standard or 1% T‐supplemented diets for 6 weeks. All measurements were performed in the sciatic nerve. Malondialdehyde (MDA) plus 4‐hydroxyalkenals (4‐HA) were quantified with N‐methyl‐2‐phenylindole. GSH, GSSG, dehydroascorbate (DHAA), and ascorbate (AA) were assayed spectrofluorometrically, T by reverse‐phase HPLC, and NGF by ELISA. MDA plus 4‐HA concentration (mean +/− SEM) was increased in diabetic rats (0.127 +/−0.006 vs 0.053 +/−0.003 mu mol/g in controls, P<0.01), and this increase was partially prevented by T (0.0960.004, P<0.01 vs untreated diabetic group). GSH levels were similarly decreased in diabetic rats treated with or without taurine vs controls. GSSG levels were similar in control and diabetic rats but were lower in diabetic rats treated with T (P<0.05 vs controls). AA levels were decreased in diabetic rats (0.133+0.015 vs 0.219 +/−0.023 mu mol/g in controls, P<0.05), and this deficit was prevented by T. DHAA/AA ratio was increased in diabetic rats vs controls (P<0.05), and this increase was prevented by T. T levels were decreased in diabetic rats (2.7 +/−0.16 vs 3.8 +/−0.1 mu mol/g in controls, P<0.05) and were repleted by T supplementation (4.20.3). NGF levels were decreased in diabetic rats (2.35 +/−0.20 vs 3.57 +/−0.20 ng/g in controls, P<0.01), and this decrease was attenuated by T treatment (3.160.28, P<0.05 vs diabetic group). In conclusion, T counteracts oxidative stress and the NGF deficit in early EDN. Antioxidant effects of T in peripheral nerve are, at least in part, mediated through the ascorbate system of antioxidative defense. The findings are consistent with the important role for oxidative stress in impaired neurotrophic support in EDN.

All biochemical changes assessed in our study are known to be prevented by ARIs. Despite the essential normalization of the sorbitol pathway activity, only part of them were, however, reversed by the ARI treatment introduced at the very early, i.e. precataractous, stage of diabetes. Therefore, intervention studies can easily underestimate the importance of aldose reductase in the pathogenesis of diabetic complications and should be interpreted with caution.