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Some vasodilators correct nerve conduction velocity and endoneurial blood flow deficits in diabetic rats. It is not known whether vasa nervorum has ATP-sensitive K(+) (K(ATP)) channels that mediate vasodilation, or whether K(ATP) channels could modulate peripheral nerve function. Therefore, we examined the effects of 2 weeks treatment with the K(ATP) channel openers, celikalim and WAY135201 (R-4-[3, 4-dioxo-2-(1, 2, 2-trimethyl-propylamino)-cyclobut-1-1-enylamino]-3-methoxy-+ ++benzonitri le), on sciatic nerve blood flow, conduction velocity, Na(+)-K(+) ATPase activity and glutathione content after 6 weeks of untreated streptozotocin-diabetes in rats. Blood flow and motor conduction velocity, 47.6% and 20.3% reduced by diabetes, respectively, were completely restored by both celikalim and WAY135201 treatments. Diabetes diminished sciatic Na(+)-K(+) ATPase activity by 47.6% and this was 80-90% corrected by the K(ATP) channel openers. Sciatic nerve glutathione content, 30.3% reduced by diabetes, was unaffected by celikalim or WAY135201. Thus, K(ATP) channel openers had marked beneficial effects on nerve perfusion and function in experimental diabetic neuropathy, and may be suitable for further study in clinical trials.

Aims/hypothesis: Increased protein kinase C activity has been linked to diabetic vascular complications in the retina and kidney, which were attenuated by protein kinase C antagonist treatment. Neuropathy has a vascular component, therefore, the aim was to assess whether treatment with WAY151 003 or chelerythrine, inhibitors of protein kinase C regulatory and catalytic domains respectively, could correct nerve blood flow, conduction velocity, Na(+),K(+)-ATPase, and glutathione deficits in diabetic rats., Methods: Diabetes was induced by streptozotocin. Sciatic nerve conduction velocity was measured in vivo and sciatic endoneurial perfusion was monitored by microelectrode polarography and hydrogen clearance. Glutathione content and Na(+),K(+)-ATPase activity were measured in extracts from homogenised sciatic nerves., Results: After 8 weeks of diabetes, sciatic blood flow was 50 % reduced. Two weeks of WAY151 003 (3 or 100 mg/kg) treatment completely corrected this deficit and chelerythrine dose-dependently improved nerve perfusion. The inhibitors dose-dependently corrected a 20 % diabetic motor conduction deficit, however, at high doses ( > 3.0 mg/kg WAY151003; > 0.1 mg/kg chelerythrine) conduction velocity was reduced towards the diabetic level. Sciatic Na(+),K(+)-ATPase activity, 42 % reduced by diabetes, was partially corrected by low but not high dose WAY151 003. In contrast, only a very high dose of chelerythrine partially restored Na(+),K(+)-ATPase activity. A 30 % diabetic deficit in sciatic glutathione content was unchanged by protein kinase C inhibition. The benefits of WAY151 003 on blood flow and conduction velocity were blocked by nitric oxide synthase inhibitor co-treatment., Conclusion/interpretation: Protein kinase C contributes to experimental diabetic neuropathy by a neurovascular mechanism rather than through Na(+),K(+)-ATPase defects.

Long-term prospective studies comparing the effects of conventional and intensive insulin therapy have linked diabetic hyperglycemia to the development of diabetic retinopathy, nephropathy, and neuropathy. The mechanisms through which glucose metabolism leads to the development of these secondary complications, however, are incompletely understood. In animal models of diabetic neuropathy, the loss of nerve function in myelinated nerve fibers has been related to a series of biochemical changes. Nerve glucose, which is in equilibrium with plasma glucose levels, rapidly increases during diabetic hyperglycemia because glucose entry is independent of insulin. This excess glucose is metabolized in large part by the polyol pathway. Increased flux through this pathway is accompanied by the depletion of myo-inositol, a loss of Na/K ATPase activity and the accumulation of sodium. Supportive evidence linking these biochemical changes to the loss of nerve function has come from studies in which aldose reductase inhibitors block polyol pathway activity, prevent the depletion of myo-inositol and the accumulation of sodium and preserve Na/K ATPase activity, as well as nerve function. The kidney and red blood cells (RBCs) are two additional sites of diabetic lesions that have been reported to develop biochemical changes similar to those in the nerve. We observed that polyol levels in the kidney cortex, medulla, and RBCs increased two- to ninefold in rats following 10 weeks of untreated diabetes. Polyol accumulation was accompanied by a 30% decrease in myo-inositol levels in the kidney cortex, but no change in RBCs or the kidney medulla. Na/K ATPase activity was decreased by 59% in RBCs but was unaffected in the kidney cortex or medulla. Aldose reductase inhibitor treatment that preserved myo-inositol levels, Na/K ATPase, and conduction velocity in the sciatic nerve also preserved Na/K ATPase activity in RBCs. Our results suggest that the pathophysiologic mechanisms underlying diabetic neuropathy are different from those of diabetic nephropathy. Our results also suggest that RBCs maybe a surrogate tissue for the assessment of diabetes-induced changes in nerve Na/K ATPase activity.

Increased activation of the first half of the polyol pathway, the conversion of glucose to sorbitol by aldose reductase, has been implicated in aldose reductase inhibitor-preventable neurochemical changes that may contribute to the aetiology of diabetic neuropathy. Tolrestat has been used as a standard aldose reductase inhibitor to dissect out polyol pathway-dependent mechanisms in many experimental studies; however, doubt has been cast upon its ability to prevent nerve conduction velocity deficits in diabetic rats. Nerve dysfunction has also been linked to abnormal endoneurial blood flow and oxygenation via increased vasa nervorum polyol pathway flux. The aim of this study was to test whether tolrestat could correct sciatic conduction velocity and perfusion defects in diabetic rats. Sciatic motor conduction velocity, 21% reduced by 1 month of streptozotocin-induced diabetes, was corrected by 23% and 84% with 1 month of tolrestat treatment at doses of 7 and 35 mg/kg/day respectively. Endoneurial blood flow, 44-52% reduced by untreated diabetes, was within the nondiabetic range with high-dose tolrestat treatment and the flow deficit was 39% corrected by the low dose. Sciatic sorbitol and fructose concentrations were approximately 13-fold and approximately 4-fold elevated by untreated diabetes. This was 32-50% attenuated by low-dose tolrestat and sorbitol and fructose content was suppressed below the nondiabetic level by high dose treatment. A 58% nerve myo-inositol deficit was partially (32%) corrected by high-dose tolrestat treatment. We conclude that tolrestat restores defective conduction and blood flow in diabetic rats and is a good pharmacological tool for studies on polyol pathway effects in peripheral nerve.

Impaired omega-6 essential fatty acid metabolism and exaggerated polyol pathway flux contribute to the neurovascular abnormalities in streptozotocin-diabetic rats. The potential interactions between these mechanisms were examined by comparing the effects of threshold doses of aldose reductase inhibitors and evening primrose oil, alone and in combination, on neurovascular deficits. In addition, high-dose aldose reductase inhibitor and evening primrose oil treatment effects were challenged by co-treatment with the cyclo-oxygenase inhibitor, flurbiprofen, or the nitric oxide synthase inhibitor, NG-nitro-L-arginine. Eight weeks of diabetes caused an 18.9% reduction in sciatic motor conduction velocity (p < 0.001). This was only modestly ameliorated by a 0.1% dietary supplement of evening primrose oil or the aldose reductase inhibitors ZD5522 (0.25 mg.kg-1.day-1 and WAY121 509 (0.2 mg.kg-1.day-1 for the final 2 weeks. However, joint treatment with primrose oil and ZD5522 or WAY121 509 caused marked 71.5 and 82.4% corrections, respectively, of the conduction deficit. Sciatic nutritive blood flow was 43.1% reduced by diabetes (p < 0.001) and this was corrected by 67.8% with joint ZD5522 and primrose oil treatment (p < 0.001). High-dose WAY121 509 (10 mg. kg-1.day-1 and primrose oil (10% dietary supplement) prevented sciatic conduction velocity and nutritive blood flow deficits in 1-month diabetic rats (p < 0.001). However, these effects were abolished by flurbiprofen (5 mg.kg(-1).day-1 and NG-nitro-L-arginine (10 mg.kg-1.day-1) co-treatment (p < 0.001). Thus, the data provide evidence for synergistic interactions between polyol pathway/nitric oxide and essential fatty acid/cyclo-oxygenase systems in the control of neurovascular function in diabetic rats, from which a potential therapeutic advantage could be derived.

The main aim was to investigate whether 1 month of aldose reductase inhibitor treatment could correct a deficit in sciatic nerve nutritive blood flow following 1 month of untreated streptozotocin-induced diabetes in rats. Treatment was with two doses of WAY-121,509, both of which completely blocked neuronal sorbitol accumulation. The high dose fully corrected a motor conduction velocity deficit, whereas the low dose caused 51.3% amelioration. Nutritive endoneurial blood flow, monitored by hydrogen clearance, was 43.4% reduced after 1 month of diabetes. This was completely corrected by the high dose of WAY-121,509. In addition, vascular conductance was supranormal and there was a decrease in arteriovenous shunt flow. Low dose treatment caused a 55.6% improvement of the nutritive endoneurial blood flow deficit, paralleling the conduction velocity effect. WAY-121,509 did not alter nerve perfusion in nondiabetic rats. Data from multiple sciatic nerve penetrations by oxygen sensitive microelectrodes revealed a 42.0% deficit in mean endoneurial oxygen tension with diabetes, whereas tensions were in the nondiabetic range for high dose WAY-121,509 treatment. Thus, the data highlight neurovascular actions of aldose reductase inhibition, and suggest that neuronal polyol pathway metabolite levels are a poor predictor of functional efficacy.

The present study was designed to examine the development of structural changes, characteristic of diabetic neuropathy, in chronic galactosemia and their responsiveness to inhibition of the polyol-pathway. Sprague-Dawley rats weighing 70-90 g were given a 50% galactose diet continued for 4 or 8 months. Half of these animals were simultaneously given the aldose reductase inhibitor (ARI) WAY 121-509. ARI-treatment normalized galactitol and myoinositol levels in the sciatic nerve. At 4 months, sciatic nerve conduction velocity (NCV) in galactosemic rats was reduced by 30% which was prevented in ARI-treated rats. At 8 months galactosemia reduced NCV to 58% of control values, while ARI-treatment for 8 months improved NCV to 71% of control values. ARI-treatment prevented in galactosemic rats nodal structural changes characteristic of diabetic neuropathy, whereas axonal atrophy was not affected by ARI-treatment, which may in part account for the only partial prevention of the NCV slowing at 8 months. Nerve fiber regeneration was increased 4-fold in ARI-treated rats compared with untreated galactosemic rats. These data suggest that chronic galactosemia produces a neuropathy structurally similar to diabetic neuropathy. The lack of an ARI-treatment effect on axonal atrophy suggests that this defect is not polyol related in galactosemia.