Nerve conduction and aldose reductase inhibition during 5 years of diabetes or galactosaemia in dogs

Summary To evaluate the role of excessive polyol pathway activity in the pathogenesis of nerve disorders in diabetes mellitus, nerve conduction velocity was measured in motor nerves of diabetic dogs given an aldose reductase inhibitor (Sorbinil) or placebo, and also in non-diabetic dogs made experimentally galactosaemic. The nerve conduction velocity slowly declined in the diabetic placebo group, becoming significantly less than normal by the fifth year of the study, and the decline was prevented by administration of the aldose reductase inhibitor. Non-diabetic dogs made galactosaemic by consuming a 30 % galactose diet developed erythrocyte and nerve polyol concentrations many times greater than that of diabetic or normal animals, but the nerve conduction velocity remained normal throughout 5 years of study. These results in dogs suggest that aldose reductase inhibitors may prevent defective nerve conduction in long-term diabetes, and raise the possibility that excessive accumulation of polyol itself is not sufficient to produce the nerve defect in the absence of excessive polyol utilization. [Diabetologia (1994) 37: 141–144] Received: 8 June 1993 and in revised form: 26 August 1993     

Sorbitol metabolism in the retina, optic nerve, and sural nerve of diabetic rats treated with an aldose reductase inhibitor

Sorbitol concentration has been measured in retina, optic, and sural nerve of normal, diabetic, and aldose reductase inhibitor-treated diabetic rats. The sural nerve displayed significantly higher sorbitol content than the retina and the optic nerve both in control animals and in diabetic animals. In the sural nerve the response to treatment with an aldose reductase inhibitor was more marked than in the two other tissues. The activities of aldose reductase and sorbitol dehydrogenase were not influenced by diabetes. It is suggested that aldose reductase inhibition may be of greater use for alleviating peripheral nervous system accumulation of sorbitol than for hindering CNS accumulation of the polyol.     

Aldose reductase inhibition,nerve perfusion,oxygenation and function in streptozotocin-diabetic rats: Dose-response considerations and independence from a myo-inositol mechanism

Summary We examined the effects of aldose reductase inhibition on nerve biochemistry and function, blood flow and endoneurial oxygenation in experimental diabetes mellitus. After 1 month untreated diabetes in rats, treatment with the novel sulphonylnitromethane aldose reductase inhibitor, ZENECA ZD5522, prevented a progressive increase in sciatic nerve resistance to hypoxic conduction failure (p<0.05). Motor conduction velocity deficits after 4 months untreated diabetes were rapidly returned to normal within 12 days (p<0.0001) by ZD5522 treatment. Following 2-months untreated diabetes, examination of 1 month ZD5522 treatment dose-response relationships for correction of nerve sorbitol and fructose accumulations and reduction in myo-inositol concentration, sciatic motor and saphenous sensory conduction velocity and sciatic blood flow by laser-Doppler flowmetry revealed poor agreement between nerve function and biochemical indices. In addition, polyol accumulation differed between sciatic and saphenous nerves, the latter showing ten-fold lower sorbitol concentrations. Laser-Doppler blood flow was 60% decreased by untreated diabetes (p<0.001) and there was a strong correlation between ZD5522-mediated increases in blood flow and conduction velocity (p<0.0001). Measurement of nutritive endoneurial blood flow by microelectrode polarography and hydrogen clearance showed 44% and 45% deficits for 1 and 2 months untreated diabetes (p<0.001) that were prevented by ponalrestat and ZD5522 treatments, respectively. In contrast, 2 months myo-inositol treatment from diabetes induction did not prevent reduction in blood flow or sciatic motor conduction velocity. A 37% reduction in endoneurial oxygen tension after 2 months diabetes (p<0.001) was completely prevented by ZD5522 treatment (p< 0.001). The data show that a very high degree of polyol pathway blockade is necessary to correct nerve functional deficits and that aldose reductase inhibitors have a neurovascular action that does not depend on restoration of nerve myo-inositol.Abbreviations ARI aldose reductase inhibitor - ED₅₀ 50% effective dose - EMG electromyogram - Na⁺-K⁺-ATPase sodium potassium adenosine triphosphatase - NCV nerve conduction velocity - NO nitric oxide - RHCF resistance to hypoxic conduction failure     

Redox state-dependent and sorbitol accumulation-independent diabetic albuminuria in mice with transgene-derived human aldose reductase and sorbitol dehydrogenase deficiency

Aims/hypothesis We investigated the role played by sorbitol accumulation in the kidney in the development of diabetic albuminuria.Methods We created mice (hAR-Tg:SDH null) with transgene-derived human aldose reductase and sorbitol dehydrogenase (SDH) deficiency, and analysed (i) the contribution of accumulated sorbitol to urinary albumin excretion rate, and (ii) the effect of the aldose reductase inhibitor, epalrestat, on the diabetic redox state, including decreased renal reduced glutathione concentrations or increased lactate to pyruvate ratios in the diabetic kidney.Results Compared to littermates, non-diabetic transgenic mice had a 2.6-fold increase in aldose reductase mRNA. In a diabetic group, aldose reductase mRNA in hAR-Tg mice was 2.7-fold higher than in littermates. In the diabetic and non-diabetic groups, hAR-Tg:SDH null mice had the highest sorbitol content among all four genetic types including hAR-Tg:SDH null, SDH null, hAR-Tg and littermates. The urinary albumin excretion rate in non-diabetic groups was similar in the four genetic types of mouse. In diabetic groups it was greater than in non-diabetic groups, but did not correlate with the sorbitol content among the four genetic types of mouse. When aldose reductase inhibitor and streptozotocin were given simultaneously at 6 weeks of age, epalrestat prevented diabetic increases in urinary albumin excretion rate and completely prevented diabetic decreases in reduced glutathione concentrations and diabetic increases in lactate to pyruvate ratios, even in the presence of transgenic aldose reductase.Conclusions/interpretation The degree of diabetic albuminuria in genetically modified mice is dependent on the redox state and independent of polyol accumulation; aldose reductase inhibitor can prevent diabetic albuminuria by normalising diabetic redox changes.Abbreviations AR Aldose reductase - SDH sorbitol dehydrogenase - UAE urinary albumin excretion rate - GSH reduced glutathione - hAR-Tg human aldose reductase-transgenic mouse - L/P lactate/pyruvate     

Effects of a fructose-rich diet and the aldose reductase inhibitor,ONO-2235, on the development of diabetic neuropathy in streptozotocin-treated rats

Summary Streptozotocin-diabetic rats were maintained on a 72% fructose diet for 4 weeks and some were treated with an aldose reductase inhibitor (either alrestatin: 0.9 g · kg^–1 · day^–1 or ONO-2235: 50 mg · kg^–1 · day^–1). Fructose feeding significantly influenced the development of impaired motor nerve conduction velocity in the diabetic rats and this effect was positively correlated with sorbitol accumulation in the sciatic nerve of diabetic rats maintained on a fructose-rich diet. Treatment with ONO-2235, a new aldose reductase inhibitor, prevented both slowing of motor nerve conduction velocity and elevation of nerve sorbitol concentration. On the other hand, erythrocyte sorbitol levels were significantly correlated to those of the sciatic nerve (r=0.86, p<0.001) and the retina (r=0.91, p<0.001) in these animals. Thus, our findings suggest that increased polyol pathway activity may be related to the pathogenesis of diabetic neuropathy and erythrocyte sorbitol concentrations may prove a useful indicator for the presence of diabetic complications.     

Evaluation of a sorbitol dehydrogenase inhibitor on diabetic peripheral nerve metabolism: a prevention study

Abstract Aims/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 β-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) 42: 1187–1194] Received: 19 April 1999 and in revised form: 28 May 1999     

Reduced nerve blood flow in diabetic rats: relationship to nitric oxide production and inhibition of aldose reductase

This study examined links between impaired nitric oxide production in the sciatic endoneurium, nerve blood flow, and polyol pathway flux, to test the hypothesis that reduced nerve blood flow might be compromised by competition for NADPH between aldose reductase and nitric oxide synthase. Sciatic nerves of streptozotocin-diabetic rats showed reduced laser Doppler flux (by 51 % or 63 %; both p<0.05)—indicative of reduced nerve blood flow—and reduced motor nerve conduction velocity (17 % in two experiments; p<0.05). Acute interruption of nitric oxide production in the sciatic nerves of control rats, via endoneurial injection of Nω-nitro-D -arginine methyl ester ( L -NAME), caused a local reduction (of 64 %; p<0.001) in nerve Doppler flux. This was reversed by either L -arginine or sodium nitroprusside. The response to L -NAME was greatly reduced in diabetic rats (only 22 % reduction; p<0.01), though both L -arginine and SNP caused marked increases in flux. Chronic inhibition of aldose reductase in diabetic rats (with either sorbinil or imirestat at a range of doses) had little effect on resting sciatic nerve Doppler flux, though both inhibitors normalized conduction velocity. Both aldose reductase inhibitors reduced sorbitol pathway intermediates in a dose-related manner. These findings do not support the proposition that aldose reductase inhibitors normalise conduction velocity by mechanisms dependent upon either normalization of endoneurial nitric oxide or nerve blood flow. Instead, a mechanism based upon more direct effects on axon or Schwann cell function is favoured. © 1998 John Wiley & Sons, Ltd.     

Measurements of tissue sorbitol in diabetes mellitus: enzyme method versus gas-liquid chromatography.

Two methods are commonly used to measure sorbitol in mammalian tissues. The first uses sorbitol dehydrogenase for a coupled enzymatic reaction; unfortunately, other polyols are also substrates for this enzyme. The second uses gas-liquid chromatography (GLC) for separation of polyols and mass quantitation of sorbitol. A comparison of these two methods for the measurement of sorbitol in duplicate samples of lens, nerve, and erythrocytes indicates that GLC of polyol acetates consistently finds less sorbitol than measured by sorbitol dehydrogenase. Erythritol, threitol, ribitol, arabitol, and galactitol are polyols found in variable quantities in these tissues, which have a variable influence on the activity of sorbitol dehydrogenase and therefore alter sorbitol quantitation with this enzyme. Moreover, there is an unidentified substance(s) that reacts with sorbitol dehydrogenase which seems to increase in association with hyperglycemia in the lens and nerve, but not in erythrocytes. The quantity of this unknown substance(s) seems to be reduced by the aldose reductase inhibitor sorbinil in erythrocytes and to a lesser extent sciatic nerve and lens. Since enzymatic sorbitol quantitation in the lens, nerve, and erythrocytes is influenced by many known and unknown factors other than sorbitol, we recommend that GLC of polyol acetates be used to measure sorbitol in biologic tissues.     

The role of cyclic adenosine 3',5'-monophosphate and polyol metabolism in diabetic neuropathy.

The effects of a stable prostacyclin analog, Iloprost, and aldose reductase inhibitors (ONO-2235 and isoliquiritigenin) were studied to elucidate the role of cAMP in diabetic neuropathy in relation to polyol metabolism. In in vivo experiments, the cAMP and myoinositol contents in sciatic nerves and motor nerve conduction velocity were significantly reduced in diabetic rats. Iloprost significantly restored the reduced cAMP content in sciatic nerves and improved motor nerve conduction velocity in diabetic rats. However, the contents of sorbitol or myoinositol in sciatic nerves were not affected by Iloprost in diabetic rats. On the other hand, aldose reductase inhibitors significantly reduced the sorbitol content and increased the cAMP and myoinositol contents in the sciatic nerves of diabetic rats. The motor nerve conduction velocity was also slightly but significantly improved by treatment with aldose reductase inhibitors. There was a negative correlation between cAMP and sorbitol in the sciatic nerves of diabetic rats treated with aldose reductase inhibitors and a positive correlation between cAMP and motor nerve conduction velocity. In in vitro experiments, Iloprost significantly increased cAMP, but did not affect the sorbitol content in sciatic nerves. Aldose reductase inhibitors inhibited sorbitol accumulation and increased cAMP in sciatic nerves. Our data suggest that polyol pathway activation somehow results in cAMP reduction in sciatic nerves and that the reduction of cAMP in peripheral nerves may be closely related to the pathogenesis of diabetic neuropathy.     

Effects of aminoguanidine on peripheral nerve function and polyol pathway metabolites in streptozotocin-diabetic rats

Summary The effect of 2 months aminoguanidine treatment on nerve conduction abnormalities was studied in streptozotocin-diabetic rats. Treatment with aminoguanidine from the induction of diabetes mellitus prevented a 22% decrease in sciatic motor nerve conduction velocity (p <0.001), and a 10% deficit in sensory saphenous conduction velocity (p <0.01). There was a 49% increase in resistance of sciatic nerve to hypoxic conduction failure in vitro. This was not significantly affected by aminoguanidine treatment. Sciatic nerve polyol pathway metabolites, sorbitol and fructose, were elevated 10-fold by diabetes (p <0.001). Myo-inositol levels were 18% decreased by diabetes. Aminoguanidine treatment had no significant effect on sorbitol, fructose or myo-inositol levels. Aminoguanidine has been identified as both an inhibitor of the formation of advanced glycation end products, and an aldose reductase inhibitor. The data suggest that beneficial actions on nerve function do not depend on the latter property. They support the notion that advanced glycation end products contribute to the aetiology of early diabetic neuropathy, possibly acting via a vascular mechanism, and that aminoguanidine treatment may have therapeutic applications.     

Prevention of urinary albumin excretion in 6 month streptozocin-diabetic rats with the aldose reductase inhibitor tolrestat

Recent clinical data strongly suggest that elevated urinary albumin excretion (UAE) identifies diabetic subjects at risk of developing nephropathy. Elevated UAE is attributed to increased transglomerular pressure, which is associated with poor metabolic control in rats. Because excess glucose in diabetes is metabolized via the polyol pathway, we were interested in whether the diabetes-induced elevation in UAE in rats could be prevented by inhibiting aldose reductase (AR), the first enzyme in the polyol pathway, with the AR inhibitor tolrestat. In fact, in rats made diabetic with streptozocin (35 mg/kg IV), treatment for 6 months with tolrestat (25 mg/kg/day in the diet) prevented both sorbitol accumulation in the kidney and the increase in UAE. Sorbitol accumulation and the increased UAE were not associated with statistically significant mesangial expansion, and the thickening of glomerular basement membranes was not affected by tolrestat treatment. The authors conclude that the 4.7-fold elevation in UAE in chronically diabetic rats is linked to the increased flux of glucose through the polyol pathway since it was prevented by inhibiting aldose reductase with tolrestat.     

Neutrophil aldose reductase activity and its association with established diabetic microvascular complications.

Direct investigation of the polyol pathway is rarely possible in studies of human diabetes. A spectrophotometric assay has been developed for the measurement of aldose reductase and sorbitol dehydrogenase activity in the neutrophil. Neutrophil aldose reductase activity was increased in patients with Type 1 diabetes with complications (median 40 (interquartile range 28-48) u, where 1 unit of enzyme activity = nmol NADPH min-1 10(8)-cells-1) compared with those without complications (20 (16-36) u, p less than 0.01) and normal control subjects (20 (8-36) u, p less than 0.01). In Type 2 diabetes, patients with complications also had higher aldose reductase activity (40 (28-52) u) than those without complications (24 (16-36) u, p less than 0.01). There were no differences between patients without complications and normal control subjects. Sorbitol dehydrogenase activity was decreased in diabetic patients (p less than 0.02) but not significantly different between diabetic patients with and without complications.     

Ⅱ型糖尿病患者红细胞的醛糖还原酶活性与细胞损伤

应用荧光法测定了15例无并发症的Ⅱ型糖尿病患者红细胞内醛糖还原酶的活性,患者的酶活性明显高于正常对照。同时用气相色谱法测量了红细胞内糖醇的浓度,患者的山梨醇和果糖的浓度明显高于对照,而两者的肌醇浓度差异无显著性。患者全血还原型谷胱甘肽和氧化型谷胱甘肽的浓度明显低于正常对照。实验结果提示:醛糖还原酶的激活加速了多元醇代谢途径,但山梨醇在细胞内的蓄积加速它向果糖转化,从而缓解山梨醇的堆积和细胞损伤。     

Pharmacological manipulation of vascular endothelium function in non-diabetic and streptozotocin-diabetic rats: effects on nerve conduction,hypoxic resistance and endoneurial capillarization

Summary We examined the potential for some of the abnormalities of vascular endothelium found in diabetes mellitus to cause neuropathic changes. Non-diabetic rats were treated for 2 months with the cyclo-oxygenase inhibitor flurbiprofen (5 mg·kg⁻¹·day⁻¹) to reduce prostacyclin production, the nitric oxide synthase inhibitor N^G-nitro-L-arginine (5 or 25 mg·kg⁻¹·day⁻¹), or combined treatment. There were dose-dependent reductions in sciatic motor and saphenous sensory conduction velocity. The two inhibitors acted synergistically, thus, the 5–6% motor conduction deficits (p<0.01) produced by either flurbiprofen or N^G-nitro-l-arginine (5 mg·kg⁻¹·day⁻¹) increased to 17% (p<0.001) for combined treatment. With N^G-nitro-l-arginine (25 mg·kg⁻¹·day⁻¹) and flurbiprofen, motor and sensory conduction velocity were reduced by 23% (p<0.001) and 12% (p<0.001), respectively, matching the deficits following 2-month streptozotocin diabetes. N^G-nitro-l-arginine (25 mg·kg⁻¹·day⁻¹) and flurbiprofen together produced a 13% prolongation of the time taken for 80% hypoxic conduction failure in vitro (p<0.05) and a 10% reduction in sciatic capillary density. A second investigation tested an alternative hypothesis that overproduction of nitric oxide was responsible for vascular-related complications in diabetes, the prediction being that N^G-nitro-L-arginine (5 mg·kg⁻¹·day⁻¹) would prevent nerve dysfunction. However, rather than prophylaxis during 2-month streptozotocin diabetes, treatment exacerbated nerve abnormalities. Thus, N^G-nitro-L-arginine worsened (8%,p<0.001) the motor conduction deficit, there was an 11% increase in hypoxic conduction failure time (p<0.01) and an 11% reduction in endoneurial capillary density (p<0.01). We conclude that overproduction of nitric oxide is unlikely to be involved in the aetiology of experimental diabetic neuropathy. However, endothelial dysfunction resulting in impaired nitric oxide and prostacyclin synthesis could make a substantial contribution.     

Evaluation of an aldose reductase inhibitor on lens metabolism, ATPases and antioxidative defense in streptozotocin-diabetic rats: an intervention study

Aims/hypothesis. Aldose reductase inhibitors (ARIs) prevent biochemical abnormalities associated with diabetic complications. We evaluated whether a short-term intervention with an adequate dose of ARI, introduced at the very early, precataractous stage, reversed diabetes-induced metabolic imbalances, down-regulation of ATPases and oxidative stress in the lens. Methods. The groups included mature control and streptozotocin-diabetic rats treated with or without ARI sorbinil (65 mg · kg^–1· day^–1, in the diet, for 2 weeks after 4 weeks of untreated diabetes). Free cytosolic NAD⁺:NADH and NADP⁺:NADPH ratios were calculated from the lactate dehydrogenase and malic enzyme systems. Concentrations of metabolites and adenine nucleotides, Na⁺/K⁺-ATPase, H⁺-ATPase and Ca⁺⁺-independent Mg⁺⁺-ATPase activities and variables of oxidative stress were measured in individual lenses. Results. Sorbinil treatment essentially corrected diabetes-induced sorbitol and fructose accumulation, myo-inositol depletion, decrease in free cytosolic NAD⁺:NADH ratio and energy deficiency. Malondialdehyde accumulation, reduced glutathione depletion and the increase in oxidized glutathione:reduced glutathione ratio were partially corrected. Free cytosolic NADP⁺:NADPH ratio and 4-hydroxyalkenal concentrations were similarly increased in diabetic rats treated with or without ARI. Sorbinil did not counteract diabetes-induced down-regulation of the three ATPase activities. Conclusion/interpretation. 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. [Diabetologia (2000) 43: 1048–1055] Received: 24 February 2000 and in revised form: 6 April 2000     

Sorbitol pathway of glucose metabolism in streptozotocin diabetes of varying duration and severity

The degree of accumulation of sorbitol pathway metabolites by the lens, sciatic nerve and aorta is determined by the duration and gravity of experimental diabetes. As the blood glucose content (indicating experimental diabetes gravity) rises, the aldose reductase activity in the tissues increases, the content of sorbitol and fructose ascends. The sorbitol-dehydrogenase activity does not depend on diabetes gravity. An increase of aldose reductase and a decrease in the sorbitol-dehydrogenase activity occur within the first two weeks after diabetes induction, which correlates with the maximal rate of sorbitol and fructose accumulation. The conclusion is made on the necessity of a strict compensation for diabetes to prevent the development of chronic complications.     

The effects of Sorbinil on peripheral nerve conduction velocity,polyol concentrations and morphology in the streptozotocin-diabetic rat

Summary This study examined the effects of an aldose reductase inhibitor, Sorbinil, on neuropathy over a 6-month period in streptozotocin-diabetic rats. Sorbinil treatment prevented the 10-fold increase in nerve sorbitol found with diabetes. It produced a 60% improvement in tibial nerve motor conduction velocity after 6 months. Morphometric profiles of nerves were also normalised. Axon area was reduced by 14% in untreated diabetic rats compared to age-matched controls, whereas Sorbinil-treated animals showed normal age-related axon growth. Myelin area was increased by 28% in untreated diabetic animals, but was the same as age-matched controls with Sorbinil treatment. Nervemyo-inositol levels were reduced by 45% after three months of untreated diabetes, but were normal after ix months. Sorbinil treatmend tended to restore myo-inositol levels toward normal over the shorter time period. It was concluded that axon growth retardation is the most likely cause of the conduction deficit seen in longterm experimental diabetes.     

Impaired contraction and relaxation in aorta from streptozotocin-diabetic rats: Role of polyol pathway

Summary The effects of 3 months streptozotocin-induced diabetes mellitus on contraction and relaxation of aorta were examined in vitro. A further diabetic group was treated with a novel sulphonylnitromethane-based aldose reductase inhibitor for 3 months following diabetes induction. Diabetes resulted in reduced maximal tension production, particularly for responses to phenylephrine (p < 0.001) and serotonin (p < 0.001). However, with aldose reductase inhibitor treatment, responses were in the non-diabetic range. The ratio of maximum contractions to noradrenaline and phenylephrine were 28 % elevated by diabetes (p < 0.01), which may suggest increased ₂-adrenoreceptor-mediated responses. Endothelium-independent relaxation to glyceryl trinitrate was unaffected by diabetes or treatment. By contrast, there were 38 % deficits in endothelium-dependent relaxation to acetylcholine (p < 0.001) and Ca²⁺ ionophore A23187 (p < 0.001) with diabetes which were prevented by aldose reductase inhibitor treatment (p < 0.001). A 121 % shift in the concentration giving a 50% maximum effect for acetylcholine towards lower sensitivity with diabetes (p < 0.001) was also largely corrected by treatment (p < 0.001). A non-diabetic group treated with aldose reductase inhibitor showed a 30 % decrease in the 50 % effective concentration for acetylcholine (p < 0.05). A 15 % deficit in maximum relaxation to the ATP-sensitive K⁺ channel opener cromakalim for the diabetic group (p < 0.001) was prevented by aldose reductase inhibitor treatment (p < 0.01). We conclude that there are polyol pathway related abnormalities for contraction, some aspects of endothelium-independent relaxation, but particularly for endothelium-dependent relaxation in aorta from chronic streptozotocin-diabetic rats. If found in the appropriate circulatory beds, these could potentially contribute to the putative vascular basis of some of the complications of diabetes. Their amelioration could account for many of the beneficial effects of aldose reductase inhibitors.     

Alteration of urinary sorbitol excretion in WBN-kob diabetic rats - treatment with an aldose reductase inhibitor

An accelerated polyol pathway in diabetes contributes to the development of diabetic complications. To elucidate diabetic nephropathy involving also renal tubular damage, we measured urinary sorbitol concentration concomitantly with urinary N-acetyl-D-glucosaminidase (NAG) excretion in WBN-kob diabetic rats.Twenty-four-hour urinary sorbitol concentrations increased in the diabetic rats in parallel with whole blood sorbitol concentrations. An increase in 24-h urinary NAG excretion coincided with the elevated urinary sorbitol levels in the diabetic rats. The administration of epalrestat, an aldose reductase inhibitor, reduced the increased whole blood and urinary sorbitol concentrations and urinary NAG excretion concomitantly with renal aldose reductase inhibition in the diabetic rats.These results indicate that diabetic nephropathy involves distorted cell function of renal tubules, and that treatment with epalrestat may prevent at least the progress of the nephropathy.     

Erythrocytic sorbitol contents in diabetic patients correlate with blood aldose reductase protein contents and plasma glucose levels, and are normalized by the potent aldose reductase inhibitor fidarestat (SNK-860)

The accumulation of sorbitol by the activated polyol pathway is considered to be a major cause of diabetic neuropathy. Because the erythrocytic sorbitol contents reportedly reflects that in nerves, erythrocytic sorbitol measurement would be useful for confirming the effect of an aldose reductase inhibitor (ARI). In this study, we examined erythrocytic sorbitol contents in healthy subjects and diabetic patients under fasting and postprandial conditions. Then, the contributions of blood aldose reductase (AR) contents and plasma glucose levels to the accumulated erythrocytic sorbitol contents were also analyzed. Erythrocytic sorbitol contents in the healthy subjects were 11.7 and 12.5–12.6 nmol/g Hb in fasting and postprandial status, respectively. In contrast, the erythrocytic sorbitol contents in diabetic patients were apparently higher (approximately 2.5-fold), but fidarestat treatment restored the elevated erythrocytic sorbitol contents to normal. In the diabetic patients, erythrocytic sorbitol contents were highly correlated with blood AR contents multiplied by the plasma glucose levels, whereas in the normal and fidarestat-treated diabetic patients no such correlation was observed. Taken together, these results suggest both the blood AR contents and the plasma glucose levels are factors determining erythrocytic sorbitol contents in diabetic patients. Notably, the potent ARI fidarestat was shown to normalize elevated erythrocytic sorbitol contents.