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

Treatment for Diabetic Peripheral Neuropathy: What have we Learned from Animal Models?

INTRODUCTION

Animal models have been widely used to investigate the etiology and potential treatments for diabetic peripheral neuropathy. What we have learned from these studies and the extent to which this information has been adapted for the human condition will be the subject of this review article.

METHODS

A comprehensive search of the PubMed database was performed, and relevant articles on the topic were included in this review.

RESULTS

Extensive study of diabetic animal models has shown that the etiology of diabetic peripheral neuropathy is complex, with multiple mechanisms affecting neurons, Schwann cells, and the microvasculature, which contribute to the phenotypic nature of this most common complication of diabetes. Moreover, animal studies have demonstrated that the mechanisms related to peripheral neuropathy occurring in type 1 and type 2 diabetes are likely different, with hyperglycemia being the primary factor for neuropathology in type 1 diabetes, which contributes to a lesser extent in type 2 diabetes, whereas insulin resistance, hyperlipidemia, and other factors may have a greater role. Two of the earliest mechanisms described from animal studies as a cause for diabetic peripheral neuropathy were the activation of the aldose reductase pathway and increased non-enzymatic glycation. However, continuing research has identified numerous other potential factors that may contribute to diabetic peripheral neuropathy, including oxidative and inflammatory stress, dysregulation of protein kinase C and hexosamine pathways, and decreased neurotrophic support. In addition, recent studies have demonstrated that peripheral neuropathy-like symptoms are present in animal models, representing pre-diabetes in the absence of hyperglycemia.

CONCLUSION

This complexity complicates the successful treatment of diabetic peripheral neuropathy, and results in the poor outcome of translating successful treatments from animal studies to human clinical trials.

Bioactive Compounds and Their Neuroprotective Effects in Diabetic Complications

Hyperglycemia, hyperlipidemia and impaired insulin signaling during the development of diabetes can cause diabetic complications, such as diabetic neuropathy, resulting in significant morbidity and mortality. Although various therapeutics are available for the treatment of diabetic neuropathy, no absolute cure exists, and additional research is necessary to comprehensively understand the underlying pathophysiological pathways. A number of studies have demonstrated the potential health benefits of bioactive compounds, i.e., flavonoids and vitamins, which may be effective as supplementary treatments for diabetes and its complications. In this review, we highlight the most recent reports about the mechanisms of action of bioactive compounds (flavonoids and vitamins) possessing potential neuroprotective properties in diabetic conditions. Additional clinical studies are required to determine the appropriate dose and duration of bioactive compound supplementation for neuroprotection in diabetic patients.

Molecular mechanisms of diabetic vascular complications

Diabetic complications are the major causes of morbidity and mortality in patients with diabetes. Microvascular complications include retinopathy, nephropathy and neuropathy, which are leading causes of blindness, end‐stage renal disease and various painful neuropathies; whereas macrovascular complications involve atherosclerosis related diseases, such as coronary artery disease, peripheral vascular disease and stroke. Diabetic complications are the result of interactions among systemic metabolic changes, such as hyperglycemia, local tissue responses to toxic metabolites from glucose metabolism, and genetic and epigenetic modulators. Chronic hyperglycemia is recognized as a major initiator of diabetic complications. Multiple molecular mechanisms have been proposed to mediate hyperglycemia’s adverse effects on vascular tissues. These include increased polyol pathway, activation of the diacylglycerol/protein kinase C pathway, increased oxidative stress, overproduction and action of advanced glycation end products, and increased hexosamine pathway. In addition, the alterations of signal transduction pathways induced by hyperglycemia or toxic metabolites can also lead to cellular dysfunctions and damage vascular tissues by altering gene expression and protein function. Less studied than the toxic mechanisms, hyperglycemia might also inhibit the endogenous vascular protective factors such as insulin, vascular endothelial growth factor, platelet‐derived growth factor and activated protein C, which play important roles in maintaining vascular homeostasis. Thus, effective therapies for diabetic complications need to inhibit mechanisms induced by hyperglycemia’s toxic effects and also enhance the endogenous protective factors. The present review summarizes these multiple biochemical pathways activated by hyperglycemia and the potential therapeutic interventions that might prevent diabetic complications. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00018.x, 2010)

Pharmacological treatment of diabetic neuropathic pain

Neuropathic pain continues to be a difficult and challenging clinical issue to deal with effectively. Painful diabetic polyneuropathy is a complex pain condition that occurs with reasonable frequency in the population and it may be extremely difficult for clinicians to provide patients with effective analgesia. Chronic neuropathic pain may occur in approximately one of every four diabetic patients. The pain may be described as burning or a deep-seated ache with sporadic paroxysms of lancinating painful exacerbations. The pain is often constant, moderate to severe in intensity, usually primarily involves the feet and generally tends to worsen at night. Treatment may be multimodal but largely involves pharmacological approaches. Pharmacological therapeutic options include antidepressants (tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors), α2δ ligands and topical (5%) lidocaine patch. Other agents may be different antiepileptic drugs (carbamazepine, lamotrigine, topiramate), topical capsaicin, tramadol and other opioids. Progress continues with respect to understanding various mechanisms that may contribute to painful diabetic neuropathy. Agents that may hold some promise include neurotrophic factors, growth factors, immunomodulators, gene therapy and poly (adenosine diphosphate-ribose) polymerase inhibitors. It is hoped that in the future clinicians will be able to assess patient pathophysiology, which may help them to match optimal therapeutic agents to target individual patient aberrant mechanisms.

Aldose reductase deficiency improves Wallerian degeneration and nerve regeneration in diabetic thy1-YFP mice

This study examined the role of aldose reductase (AR) in diabetes-associated impaired nerve regeneration using thy1-YFP (YFP) mice. Sciatic nerves of nondiabetic and streptozotocin-induced diabetic AR(+/+)YFP and AR(-/-)YFP mice were transected after 4 weeks of diabetes. Wallerian degeneration and nerve regeneration were evaluated at 1 and 2 weeks postaxotomy by fluorescence microscopy. Motor nerve conduction velocity recovery and regenerating nerve morphometric parameters were determined at 10 and 20 weeks, respectively. There was no difference in the extent of Wallerian degeneration, size of regenerating stump, motor nerve conduction velocity recovery, or caliber of regenerating fibers between nondiabetic AR(+/+)YFP and AR(-/-)YFP mice. In diabetic AR(+/+)YFP mice, Wallerian degeneration was delayed, associated with slower macrophage invasion and abnormal vascularization. Those mice had smaller regenerating stumps, slower motor nerve conduction velocity, and smaller regenerating fibers compared with nondiabetic mice. These features of impaired nerve regeneration were largely attenuated in diabetic AR(-/-)YFP mice. Retarded macrophage invasion and vascularization associated with Wallerian degeneration were normalized in diabetic AR(-/-)YFP mice. These results indicate that AR plays an important role in diabetes-associated impaired nerve regeneration, in part by affecting vascularization and macrophage invasion during Wallerian degeneration. The thy1-YFP mice are valuable tools for further investigation of the mechanism of diabetes-associated nerve regeneration.

Diabetic neuropathy: mechanisms to management

Neuropathy is the most common and debilitating complication of diabetes and results in pain, decreased motility, and amputation. Diabetic neuropathy encompasses a variety of forms whose impact ranges from discomfort to death. Hyperglycemia induces oxidative stress in diabetic neurons and results in activation of multiple biochemical pathways. These activated pathways are a major source of damage and are potential therapeutic targets in diabetic neuropathy. Though therapies are available to alleviate the symptoms of diabetic neuropathy, few options are available to eliminate the root causes. The immense physical, psychological, and economic cost of diabetic neuropathy underscore the need for causally targeted therapies. This review covers the pathology, epidemiology, biochemical pathways, and prevention of diabetic neuropathy, as well as discusses current symptomatic and causal therapies and novel approaches to identify therapeutic targets.

Long-term treatment with ranirestat (AS-3201), a potent aldose reductase inhibitor, suppresses diabetic neuropathy and cataract formation in rats

We investigated the chronic functional and histopathological changes in the sciatic nerve and lens of streptozotocin (STZ)-diabetic rats and evaluated the preventive effects of ranirestat (AS-3201), a potent aldose reductase inhibitor, on these changes. Sorbitol levels in the sciatic nerve and lens, motor nerve conduction velocity (MNCV), and development of cataracts were measured in STZ-diabetic rats given a ranirestat-admixed diet (0.0005%) for 35 weeks. Ranirestat reduced sorbitol accumulation in the sciatic nerve and improved the decrease in MNCV of STZ-diabetic rats. Morphological and morphometric examination of changes in sural nerve revealed that treatment with ranirestat prevented both the deformity of myelinated fibers and the decrease in their axonal and myelin areas (atrophy). Ranirestat also averted the changes in the size frequency histogram of myelinated fibers. Finally, STZ-diabetic rats developed early lens opacities 8 weeks after STZ injection and had cataract by the end of the experimental period. However, in the ranirestat-treated diabetic rats, no lens opacity was observed in any rat throughout the entire experimental period. This study suggests that the polyol pathway plays an important role in the progress of diabetic neuropathy and cataract formation in STZ-diabetic rats. Ranirestat should be a promising agent for the treatment of complications associated with diabetes, especially neuropathy.

Treatment with the xanthine oxidase inhibitor, allopurinol, improves nerve and vascular function in diabetic rats

Several putative sources of reactive oxygen species could potentially contribute to diabetic neuropathy and vasculopathy. The aim was to assess the involvement of elevated xanthine oxidase activity. After 6 weeks of streptozotocin-diabetes, groups of rats were given 2 weeks of high-dose allopurinol treatment (50 and 250 mg/kg) to gauge the effect of maximal blockade of xanthine oxidase. In the final experiments, rats were subjected to sensory testing and, under butabarbital anaesthesia, measurements were made on nerve conduction velocities and neural tissue blood flow estimated by hydrogen clearance microelectrode polarography. Further groups were used to study detailed responses of the isolated mesenteric vascular bed after 4 weeks of diabetes and allopurinol (150 mg/kg) treatment. Diabetes caused 20% and 14% reduction in motor and sensory conduction velocity, which were 78% and 81% corrected by allopurinol treatment respectively, both doses giving similar results. Diabetic rats showed tactile allodynia and thermal hyperalgesia, which were completely corrected by allopurinol, whereas mechanical hyperalgesia was only 45% ameliorated. Sciatic nerve and superior cervical ganglion blood flow was halved by diabetes and allopurinol corrected this by approximately 63%. Mesenteric endothelium-dependent vascular responses to acetylcholine, which depend upon nitric oxide and endothelium derived hyperpolarizing factor, were attenuated by diabetes. Allopurinol treatment gave approximately 50% protection for both components. Thus, xanthine oxidase is an important source of reactive oxygen species that contributes to neurovascular dysfunction in experimental diabetes. Inhibition of xanthine oxidase could be a potential therapeutic approach to diabetic neuropathy and vasculopathy.

Restoration of ultrastructural and biochemical changes in alloxan-induced diabetic rat sciatic nerve on treatment with Na3VO4 and Trigonella--a promising antidiabetic agent

Vanadium has been reported to have broad pharmacological activity both in vitro and in vivo. Vanadium compound, sodium orthovanadate, Na3VO4, is well known for its hypoglycaemic effects. However, Na3VO4 exerts these effects at relatively high doses (0.6 mg/ml) and exhibit several toxic effects. In the present study lower doses of Na3VO4 (0.2 mg/ml) are combined with Trigonella foenum graecum seed powder (TSP), another hypoglycaemic agent, to reduce its toxicity without compromising its antidiabetic potential. The efficacy of the lower doses of Na3VO4 has been investigated in restoring the altered glucose metabolism and histological structure in the sciatic nerves in 21 and 60 days alloxan diabetic rats. A portion of the glucose was found to be channelled from the normal glycolytic route to polyol pathway, evident by the reduced hexokinase activity and increased polyol pathway enzymes aldose reductase and sorbitol dehydrogenase activity causing accumulation of sorbitol and fructose in diabetic conditions. Ultrastructural observation of the sciatic nerve showed extensive demylination and axonal loss after eight weeks of diabetes induction. Blood glucose levels increased in diabetic rats were normalized with the lower dose of vanadium and Trigonella treatment. The treatment of the diabetic rats with vanadium and Trigonella prevented the activation of the polyol pathway and sugar accumulations. The sciatic nerves were also protected against the structural abnormalities found in diabetes with Trigonella foenum graecum as well as Na3VO4. Results suggest that lower doses of Na3VO4 may be used in combination with TSP as an efficient antidiabetic agent to effectively control the long-term complications of diabetes in tissues like peripheral nerve.

Inhibitors of Advanced Glycation End Product Formation and Neurovascular Dysfunction in Experimental Diabetes

Advanced glycation and lipoxidation end products (AGEs/ALEs) have been implicated in the pathogenesis of the major microvascular complications of diabetes mellitus: nephropathy, neuropathy, and retinopathy. This article reviews the evidence regarding the peripheral nerve and its vascular supply. Most investigations done to assess the role of AGEs/ALEs in animal models of diabetic neuropathy have used aminoguanidine as a prototypic inhibitor. Preventive or intervention experiments have shown treatment benefits for motor and sensory nerve conduction velocity, autonomic nitrergic neurotransmission, nerve morphometry, and nerve blood flow. The latter depends on improvements in nitric oxide-mediated endothelium-dependent vasodilation and is responsible for conduction velocity improvements. A mechanistic interpretation of aminoguanidine's action in terms of AGE/ALE inhibition is made problematic by the relative lack of specificity. However, other unrelated compounds, such as pyridoxamine and pyridoxamine analogues, have recently been shown to have beneficial effects similar to aminoguanidine, as well as to improve pain-related measures of thermal hyperalgesia and tactile allodynia. These data also stress the importance of redox metal ion-catalyzed AGE/ALE formation. A further approach is to decrease substrate availability by reducing the elevated levels of hexose and triose phosphates found in diabetes. Benfotiamine is a transketolase activator that directs these substrates to the pentose phosphate pathway, thus reducing tissue AGEs. A similar spectrum of improvements in nerve and vascular function were noted when using benfotiamine in diabetic rats. Taken together, the data provide strong support for an important role for AGEs/ALEs in the etiology of diabetic neuropathy.

Aldose reductase gene polymorphisms and susceptibility to microvascular complications in Type 2 diabetes

AIMS

The gene encoding the human aldose reductase, the first and rate-limiting enzyme of the polyol pathway of glucose metabolism, is a promising candidate gene which may contribute to diabetic microvascular complications. We investigated the association of two previously reported DNA sequence variants of this gene, the C-106T polymorphism and the (CA)(n) dinucleotide repeat marker, with the risk of albuminuria and retinopathy in Finnish Type 2 diabetic patients and non-diabetic control subjects.

METHODS

The study population included 85 Finnish, middle-aged, newly diagnosed Type 2 diabetic patients and 126 non-diabetic control subjects. Genetic analyses were performed using the polymerase chain reaction, restriction fragment length polymorphism, and automated laser fluorescence scanning analyses. Microvascular complications were determined using 10-year follow-up data of urinary albumin excretion measurements and ophthalmological examinations.

RESULTS

The C and Z-2 alleles of the C-106T polymorphism and the (CA)(n) repeat marker, respectively, were found to be more frequent in Type 2 diabetic subjects than in non-diabetic subjects. The C and Z-2 alleles were in 60% linkage disequilibrium in diabetic subjects. At the time of diagnosis, diabetic subjects with the T allele of the C-106T polymorphism had significantly higher urinary albumin excretion rate and prevalence of albuminuria than subjects with the C-106C genotype (prevalence of albuminuria: 33.3 vs. 13.8%, P = 0.036, odds ratio = 3.9, 95% confidence interval 1.1, 14.7). The Z-2 allele of the (CA)(n) repeat marker was not consistently associated with the prevalence of albuminuria. No associations were observed between the polymorphisms examined and the prevalence of retinopathy at any point of the follow-up.

CONCLUSIONS

The present study suggests that the C-106T polymorphism of the aldose reductase gene could be involved in the early development of microalbuminuria in Finnish Type 2 diabetic patients.

Biochemical Pathways for Microvascular Complications of Diabetes Mellitus

OBJECTIVE

To review the current biochemical theories on how diabetes contributes to microvascular disease.

DATA SOURCES

MEDLINE search (1980-June 2003) and bibliographies of articles obtained on this topic.

STUDY SELECTION AND DATA EXTRACTION

Articles identified from the data sources were evaluated and those deemed relevant to this review were incorporated.

DATA SYNTHESIS

The prevailing biochemical theories on how diabetes leads to microvascular disease include increased polyol (sorbitol/aldose reductase) pathway flux, production of advanced glycation end-products, generation of reactive oxygen species, and activation of diacylglycerol and protein kinase C isoforms. These pathways contribute to endothelial damage and dysfunction and may alter gene functioning.

CONCLUSIONS

Each pathway, via varied and often overlapping mechanisms, contributes to altered microvascular function that leads to the development of retinopathy, neuropathy, and nephropathy, the major microvascular complications associated with diabetes.

An aldose reductase inhibitor reverses early diabetes-induced changes in peripheral nerve function, metabolism, and antioxidative defense

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.

Clinical efficacy of fidarestat, a novel aldose reductase inhibitor, for diabetic peripheral neuropathy: a 52-week multicenter placebo-controlled double-blind parallel group study

OBJECTIVE

The purpose of this study was to evaluate the efficacy of fidarestat, a novel aldose reductase (AR) inhibitor, in a double-blind placebo controlled study in patients with type 1 and type 2 diabetes and associated peripheral neuropathy.

RESEARCH DESIGN AND METHODS

A total of 279 patients with diabetic neuropathy were treated with placebo or fidarestat at a daily dose of 1 mg for 52 weeks. The efficacy evaluation was based on change in electrophysiological measurements of median and tibial motor nerve conduction velocity, F-wave minimum latency, F-wave conduction velocity (FCV), and median sensory nerve conduction velocity (forearm and distal), as well as an assessment of subjective symptoms.

RESULTS

Over the course of the study, five of the eight electrophysiological measures assessed showed significant improvement from baseline in the fidarestat-treated group, whereas no measure showed significant deterioration. In contrast, in the placebo group, no electrophysiological measure was improved, and one measure significantly deteriorated (i.e., median nerve FCV). At the study conclusion, the fidarestat-treated group was significantly improved compared with the placebo group in two electrophysiological measures (i.e., median nerve FCV and minimal latency). Subjective symptoms (including numbness, spontaneous pain, sensation of rigidity, paresthesia in the sole upon walking, heaviness in the foot, and hypesthesia) benefited from fidarestat treatment, and all were significantly improved in the treated versus placebo group at the study conclusion. At the dose used, fidarestat was well tolerated, with an adverse event profile that did not significantly differ from that seen in the placebo group.

CONCLUSIONS

The effects of fidarestat-treatment on nerve conduction and the subjective symptoms of diabetic neuropathy provide evidence that this treatment alters the progression of diabetic neuropathy.

Effects of 15-month aldose reductase inhibition with fidarestat on the experimental diabetic neuropathy in rats

We examined the effects of long-term treatment with an aldose reductase inhibitor (ARI) fidarestat on functional, morphological and metabolic changes in the peripheral nerve of 15-month diabetic rats induced by streptozotocin (STZ). Slowed F-wave, motor nerve and sensory nerve conduction velocities were corrected dose-dependently in fidarestat-treated diabetic rats. Morphometric analysis of myelinated fibers demonstrated that frequencies of abnormal fibers such as paranodal demyelination and axonal degeneration were reduced to the extent of normal levels by fidarestat-treatment. Axonal atrophy, distorted axon circularity and reduction of myelin sheath thickness were also inhibited. These effects were associated with normalization of increased levels of sorbitol and fructose and decreased level of myo-inositol in the peripheral nerve by fidarestat. Thus, the results demonstrated that long-term treatment with fidarestat substantially inhibited the functional and structural progression of diabetic neuropathy with inhibition of increased polyol pathway flux in diabetic rats.

Administration of an aldose reductase inhibitor, ONO-2235, to streptozotocin-diabetic mice restores reductions of DRG neuronal attachment to extracellular matrix proteins in vitro

Attachments of cultured dorsal root ganglion (DRG) neurons to the extracellular matrix (ECM) proteins (type I and IV collagens, laminin and fibronectin) and the adhesion ligand arginine-glycine-aspartic acid (RGD) were impaired in mice 2 weeks after the induction of diabetes by streptozotocin (STZ). However, administration of the aldose reductase inhibitor, ONO-2235, to the STZ-diabetic mice for 1 week restored DRG neuronal attachment to the ECM proteins and RGD to a level close to normal mice. These results suggest that activation of the aldose reductase and subsequent metabolic disorders in diabetic animals may play an important role in detrimental alterations of the neuronal cell-surface receptors for the ECM proteins.

Effects of the peptide HP228 on nerve disorders in diabetic rats

Motor and sensory nerve conduction velocities (MNCV and SNCV) were reduced in the sciatic nerve of rats after 4 weeks of untreated streptozotocin-induced diabetes, and declined further during the following 4 weeks. Treating diabetic rats with the novel peptide HP228 had no effect on the decline of MNCV after the first 4 weeks of diabetes but attenuated the decline in SNCV. HP228 treatment also prevented any further decline in MNCV or SNCV between weeks 4 and 8 of diabetes. Consequently, at the conclusion of the study, the nerve conduction velocities (NCVs) in treated rats were significantly (both P < .001) higher than in untreated diabetic rats. Reduced nerve homogenate Na+,K+-adenosine triphosphatase (ATPase) activity in diabetic rats was significantly (P < .05) increased by HP228 but remained significantly (P < .05) lower than in untreated controls. HP228 treatment also reduced nerve Na+,K+-ATPase activity of control rats compared with untreated controls (P < .05). There was no effect of HP228 on the hyperglycemia, nerve polyol accumulation, myo-inositol depletion, reduced nerve laser Doppler blood flow, thermal hypoalgesia, or reduced mean axonal caliber in diabetic rats or on any of these parameters in control rats. These data demonstrate that a novel peptide may protect against the slowing of nerve conduction in prolonged diabetes and that the mechanism of action is unrelated to aldose reductase inhibition, prevention of nerve ischemia, or axonal atrophy. HP228 may prove a potential therapeutic agent for the treatment of prolonged diabetic neuropathy.

Effects of dietary supplementation with arachidonic acid rich oils on nerve conduction and blood flow in streptozotocin-diabetic rats

Diabetes mellitus is associated with defective essential fatty acid desaturation. In experimental models this contributes to characteristic reductions in peripheral nerve conduction velocity (NCV) and blood flow, which may be corrected by dietary supplementation with gamma-linolenic acid (GLA) rich oils to bypass the delta-6 desaturation deficit. There is debate about the mechanism of this improvement, including whether it depends on synthesis of series 1 prostanoids derived from di-homo GLA or series 2 prostanoids from arachidonic acid (ARA). The aim was to assess the efficacy of two ARA-rich (approximately 39% content) oils in correcting neurovascular dysfunction in streptozotocin-induced diabetic rats. After 6 weeks of untreated diabetes, rats were treated for a further 2 weeks with 1% dietary oil supplements before assessment of sciatic motor NCV and endoneurial blood flow. NCV was 19% reduced in diabetic rats and this was largely (approximately 86%) corrected by both oil treatments. A 48% deficit in endoneurial nutritive blood flow with diabetes was approximately 70% reversed by the two oils, vascular conductance being in the non-diabetic range. Thus, nerve conduction and perfusion deficits in diabetic rats are corrected by ARA-rich oil treatment. The magnitudes of these changes were similar to expectations based on previous studies of GLA-rich oils, therefore it is likely that the neurovascular effect of increased synthesis of series 2 prostanoids makes a major contribution to the beneficial action of n-6 essential fatty acids in experimental diabetic neuropathy.

The aldose reductase inhibitor sorbinil does not prevent the impairment in nitric oxide-mediated neurotransmission in anococcygeus muscle from diabetic rats

This study investigated whether increased polyol pathway activity could contribute to alterations in nitrergic neurotransmission in anococcygeus muscles from 8-week diabetic rats. In the presence of guanethidine (10-30 microM) and clonidine (0.01-0.05 microM), relaxations obtained to nitrergic nerve stimulation (0.5-5 Hz, 10-s train), to sodium nitroprusside (5-500 nM) and to nitric oxide (0.1-3 microM) were significantly reduced in muscles from diabetic rats compared to responses from control rats. Treatment of diabetic rats with the aldose reductase inhibitor sorbinil (42 mg/kg per day via feed for 8 weeks) did not affect impaired reactivity to nitrergic nerve stimulation, sodium nitroprusside or nitric oxide. The results suggest increased polyol pathway activity does not contribute to the alterations in nitrergic neurotransmission in anococcygeus muscles from diabetic rats.

Nerve function and regeneration in diabetic and galactosaemic rats: antioxidant and metal chelator effects

Immature rats were made diabetic with streptozotocin or were fed a 40% galactose diet to stimulate the polyol pathway. Separate diabetic and galactosaemic groups were treated with butylated hydroxytoluene or trientine. After 4 weeks the sciatic nerve was freeze-lesioned. Two weeks later, the degree of myelinated fibre regeneration was assessed electrophysiologically and nerve conduction velocity was measured in the contralateral leg. Similar sciatic motor and saphenous sensory nerve conduction velocity deficits of approximately 18% and 19%, respectively, compared to age-matched control rats were found in both models. They were partially prevented by treatment (approximately 68% for butylated hydroxytoluene and 63% for trientine). There were 12% and 10% deficits in nerve regeneration distance with diabetes and galactosaemia respectively, which were markedly attenuated (approximately 80%) by both treatments. The data emphasise the importance of elevated, free radical activity for the aetiology of neural/neurovascular deficits in experimental diabetes and galactosaemia.

Comparison of the effects of aminoguanidine and L-carnitine treatments on somatosensorial evoked potentials in alloxan-diabetic rats

The effects of aminoguanidine (AG) and L-carnitine (LC) on somatosensorial evoked potential (SEP) latency and neural levels of thiobarbituric acid reactive substances (TBARS), products of lipid peroxidation, were compared in alloxan-diabetic rats. AG and LC were given to diabetic rats starting from the 3rd week after the induction of diabetes and lasting for 4 weeks. SEP latency was measured by stimulating via caudal nerve and recording via cortex, once weekly during the treatments. Diabetes caused deficits in SEP (P < 0.05 vs non-diabetic control rats, respectively). AG and LC restored SEP latencies slightly but not significantly, with the exception of the prominent effect of AG at the first week and both treatments at the 4th week of the treatments (P < 0.05 vs untreated diabetic rats, respectively). Diabetes caused elevation in neural TBARS levels (P < 0.05 vs non-diabetic group), which was prevented by both AG and LC (P < 0.05 vs untreated diabetic rats, respectively). Weight and the glucose levels were not influenced by the treatments. Our results suggest that AG improves SEP latencies better than LC. Our results also suggest that the beneficial effects of both AG and LC on diabetic neuropathy are not associated with the regulation of glycemia, but these effects may be related in part with prevention of lipid peroxidation.

Comparison of the effects of ascorbyl gamma-linolenic acid and gamma-linolenic acid in the correction of neurovascular deficits in diabetic rats

Essential fatty acid metabolism is impaired by diabetes mellitus and gamma-linolenic acid rich treatments such as evening primrose oil correct deficits in nerve conduction and endoneurial blood flow in diabetic rats. Other mechanistically unrelated treatments, such as antioxidants and aldose reductase inhibitors have a similar effect and there may be positive interactions with multiple treatments. Our aim was to compare the efficacy of a novel essential fatty acid derivative, ascorbyl gamma-linolenic acid, with that of gamma-linolenic acid in correcting diabetic neurovascular deficits. Eight weeks of diabetes caused 20.4 and 48.2% reductions in sciatic motor conduction velocity and nutritive endoneurial blood flow, respectively. Treatment was given for the last 2 weeks with gamma-linolenic acid (100 mg.kg-1.day-1) either in pure form or as ascorbyl gamma-linolenic acid, an equivalent dose of ascorbate (21 mg.kg-1.day-1) or jointly with ascorbate and gamma-linolenic acid. Conduction velocity was corrected by 39.8, 87.4, 13.2 and 66.8% with gamma-linolenic acid, ascorbyl gamma-linolenic acid, ascorbate and gamma-linolenic acid plus ascorbate, respectively. Corresponding ameliorations of the nutritive blood flow deficit were 44.0, 87.4, 87.4, 13.2 and 65.7%. For the gamma-linolenic acid plus ascorbate combinatin, and especially for ascorbyl gamma-linolenic acid, the magnitude of correction for conduction velocity and blood flow was greater than expected for simple addition of ascorbate and gamma-linolenic acid, indicating a synergistic interaction. Thus, with an efficacy 40 times that of evening primrose oil in rats, ascorbyl gamma-linolenic acid may be a suitable candidate for clinical trials of diabetic neuropathy.

Influence of aldose reductase inhibition on the microvascular reactivity in experimental diabetes

1. To verify if tolrestat, an aldose reductase inhibitor, corrects the impaired responses of microvessels to histamine and bradykinin in alloxan-diabetic rats, the mesenteric microcirculation was studied in vivo in anaesthetised animals. 2. The impaired responses were corrected by tolrestat 5 mg/kg/day for 7 days p.o. Similar responses to acetylcholine and sodium nitroprusside were obtained in preparations of diabetic and control rats and were not altered by tolrestat treatment. 3. As in diabetes, galctosemia induced impaired responses to histamine and bradykinin; these altered responses were corrected by tolrestat treatment. 4. These data allow us to suggest that the polyol pathway activity might be involved in the altered responses of microvessels observed in diabetic rats. It is possible that polyol activation may play an important role in the development of vascular dysfunction in diabetes mellitus.

An aldose reductase inhibitor but not myo-inositol blocks enhanced polyphosphoinositide turnover in peripheral nerve from diabetic rats

Experimental diabetic neuropathy, whether chemically induced or present in several spontaneously diabetic animal models, is characterized by sorbitol accumulation and myo-inositol depletion and usually also by enhanced turnover of the monoesterified moieties of polyphosphoinositides, particularly phosphatidylinositol-4,5-bisphosphate (PIP2). This study examined the relationship of these alterations by assessing the effects of myo-inositol and the aldose reductase inhibitor, sorbinil, supplied as dietary supplements, on sorbitol and myo-inositol concentrations and incorporation of 32P into polyphosphoinositides in sciatic nerve from rats killed 8 weeks after induction of diabetes with streptozotocin. Nerves from diabetic rats killed after 8 weeks of disease exhibited 52% to 76% greater PIP2 labeling, markedly elevated sorbitol levels, and 30% less myo-inositol when compared with age-matched normal rats. Incorporation of isotope into PIP2 in nerves from animals fed a myo-inositol supplement, added to either a high-sucrose diet or standard rat chow beginning immediately after induction of diabetes, remained substantially elevated, whereas myo-inositol levels were corrected to normal. Essentially the same results were obtained when rats were fed the myo-inositol-containing diet beginning 4 weeks after streptozotocin injection. In contrast, PIP2 labeling in nerves from diabetic rats that received the sorbinil-supplemented diet for either 4 or 8 weeks was not different from that in controls. myo-Inositol levels in these animals were also restored to normal, whereas sorbitol levels remained elevated, albeit reduced by approximately 30%. These results indicate that myo-inositol administration is unable to completely counteract the impact of diabetes on the turnover of monoesterified phosphate groups in PIP2. In contrast, sorbinil can correct this abnormality, but this beneficial effect is not dependent on the presence of normal sorbitol concentrations.

Reversal of defective peripheral nerve conduction velocity, nutritive endoneurial blood flow, and oxygenation by a novel aldose reductase inhibitor, WAY-121,509, in streptozotocin-induced diabetic rats

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.

Impaired myelinated fiber regeneration following freeze-injury in rats with streptozotocin-induced diabetes: involvement of the polyol pathway

This study examined the effects of streptozotocin-induced diabetes mellitus and aldose reductase inhibitor (ZD5522) treatment on myelinated fiber regeneration in rats. After 2 months of diabetes, with or without ZD5522 treatment (10 mg.kg-1.day-1) from induction, sciatic nerve degeneration was initiated by a punctate lesion using a liquid N2 cooled probe. Regeneration was studied over a subsequent 14-day period using in vitro electrophysiological techniques. There was a 21.4% (P < 0.001) deficit in the maximum fiber regeneration distance in diabetic rats, 14 days postlesion. This was partially (64.9%, P < 0.01) prevented by aldose reductase inhibitor treatment, the resultant regeneration distance being not significantly different from that of age-matched nondiabetic control rats. The regeneration rate, assessed from data collected 4, 9 and 14 days postlesion, was 23.7% (P < 0.001) reduced by diabetes and ZD5522 treatment provided 73.1% protection (P < 0.01). We conclude that polyol pathway activity is involved in impaired regeneration in experimental diabetes, potential pathophysiological mechanisms involving a reduction in neurotrophic support and impaired endoneurial blood supply.

Quantitative analysis of myelinated nerve fibers of peripheral nerve in streptozotocin-induced diabetes mellitus

The purpose of this study was to examine morphometric changes of myelinated fibers in early stages of experimental diabetes mellitus. Adult male Wistar rats aged 17 wk were used in this study. Diabetes mellitus was induced by streptozotocin. Samples of common peroneal nerve from diabetic rats (4 and 8 wk after induction of diabetes mellitus) and age-matched control animals were removed and processed. The semithin cross sections were stained with toluidine blue and used for myelinated fiber computer-aided morphometric analysis. There were no significant changes in diabetic animals after 4 wk duration of the disease. There was significant reduction in myelinated nerve fiber caliber in diabetic rats 8 wk after induction of diabetes as compared to age-matched controls. There was no significant reduction of axonal area in this group of diabetic rats, so diminution of fiber area was caused predominantly by reduction of myelin sheath area. The study demonstrates that the induction of diabetes mellitus in rat by streptozotocin is accompanied by early changes of the morphometric indices of myelinated nerve fibers of peripheral nerve.

Effects of an aldose reductase inhibitor on erythrocyte fructose 3-phosphate and sorbitol 3-phosphate levels in diabetic patients

Fructose 3-phosphate and sorbitol 3-phosphate are novel metabolites that have been shown to associate with the polyol pathway in animal experiments. Fructose 3-phosphate is of particular interest because of its potent glycation capability as compared with other glycolytic intermediates, e.g., fructose. We observed the effects of treatment with epalrestat, an aldose reductase inhibitor, on their concentrations in erythrocytes from diabetic patients. The levels of both metabolites were significantly higher in diabetic patients than in non-diabetic subjects. A group of patients who had been treated with epalrestat showed significantly lower levels of both metabolites as compared with those untreated. A treatment of three patients with epalrestat for one month resulted in obvious decreases in their concentrations. The results suggest a possible explanation for the preventive effect of an aldose reductase inhibitor on nonenzymatic glycation.

Diabetic late complications: will aldose reductase inhibitors or inhibitors of advanced glycosylation endproduct formation hold promise?

Patients suffering from the severe complications associated with both insulin- (IDDM) and non-insulin-dependent diabetes mellitus (NIDDM): nephropathy, retinopathy, neuropathy, and atherosclerosis are still largely left without a prospect of an efficient treatment. This is the case even if it has been assumed for decades and now finally proved by the results from the Diabetes Control and Complications Trial (DCCT) that hyperglycemia is the single main cause of these complications. Improved glycemic control as a result of intensive insulin treatment has the potential to reduce the incidence and progression of complications, but implementation and monitoring of improved glycemic control in all groups of IDDM and NIDDM patients in different communities will be difficult and expensive. Results from the recently terminated DCCT have shown that even with intensive insulin treatment, there will be a significant burden of complications on the diabetic population. It will, therefore, still be of immense importance for the long-term quality of life for the diabetic patient that additional possibilities are developed for prevention and intervention against diabetic complications. Almost two decades of research, animal model testing, and clinical trials have been conducted on various efficient aldose reductase inhibitors. Now the concept of inhibition of formation of advanced glycosylation endproducts on proteins and lipids resulting from extra- and intracellular hyperglycemia is entering the scene as an alternative or perhaps supplementary approach to reduce the occurrence of diabetic complications. An overview of the results from these two fields of research and associated drug-development programs will be presented along with thoughts on possible future developments.

The relative roles of advanced glycation, oxidation and aldose reductase inhibition in the development of experimental diabetic nephropathy in the Sprague-Dawley rat

Advanced glycation is an important pathogenic mechanism in the development of diabetic complications. However, other biochemical processes, such as the polyol pathway or lipid and protein oxidation which can interact with advanced glycation can also yield tissue fluorescence and may also be implicated in the genesis of diabetic microangiopathy. Aminoguanidine is an inhibitor of advanced glycation, but it is not known if all of its effects are mediated by this mechanism. The present study explores the relative contributions of aldose reductase, oxidative stress and advanced glycation on the development of aortic and renal fluorescence and urinary albumin excretion in streptozotocin diabetic rats. The study groups included non-diabetic (control), streptozotocin diabetic rats and diabetic rats receiving aminoguanidine, the anti-oxidants butylated hydroxytoluene and probucol and the aldose reductase inhibitor, ponalrestat. Serial measurements of glycaemic control and urinary albumin excretion were performed every 8 weeks. At 32 weeks, animals were killed, tissues removed and collagen extracted for measurement of fluorescence. Diabetic rats had increased fluorescence in aorta, glomeruli and renal tubules. Aminoguanidine prevented an increase in fluorescence at all three sites suggesting that diabetes-related tissue fluorescence is predominantly due to advanced glycation. Ponalrestat retarded fluorescence in aorta only and butylated hydroxytoluene attenuated fluorescence at the renal sites but not in the aorta. Diabetic rats had increased renal cortical sorbitol levels. Ponalrestat normalized renal cortical sorbitol levels but aminoguanidine did not affect this parameter. The only agent to decrease plasma thiobarbituric acid reactive substances was butylated hydroxytoluene. Diabetic rats developed albuminuria over the 32-week period.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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.

Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

L-Fucose is a potent, competitive inhibitor of myo-inositol transport by cultured mammalian cells. Chronic exposure of neuroblastoma cells to L-fucose causes a concentration-dependent decrease in myo-inositol content, accumulation, and incorporation into phosphoinositides. In these studies, L-fucose supplementation of culture medium was used to assess the effect of decreased myo-inositol metabolism and content on bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol production. Chronic exposure of cells to 30 mM L-fucose caused a sustained decrease in bradykinin-stimulated, but not basal, 3H-inositol phosphate release and 32P incorporation into phosphatidylinositol in cells incubated in serum-free, unsupplemented medium. In addition, 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was not altered in L-fucose-conditioned cells. Acute exposure of cells to serum-free medium containing 30 mM L-fucose did not affect either basal or bradykinin-stimulated 32P incorporation into phosphatidylinositol. Basal diacylglycerol content was decreased by 20% in cells chronically exposed to 30 mM L-fucose, although analysis of the molecular species profile revealed no compositional change. Bradykinin stimulated diacylglycerol production in neuroblastoma cells by increasing the hydrolysis of both phosphoinositides and phosphatidylcholine. Bradykinin-stimulated production of total diacylglycerol was similar for control and L-fucose-conditioned cells. However, there was a decrease in the bradykinin-induced generation of the 1-stearoyl-2-arachidonoyl diacylglycerol molecular species in the cells chronically exposed to 30 mM L-fucose. This molecular species accounts for about 70% of the composition of phosphoinositides, but only 10% of phosphatidylcholine. The results suggest that a decrease in myo-inositol uptake results in diminished agonist-induced phosphatidylinositol synthesis and phosphoinositide hydrolysis in cultured neuroblastoma cells grown in L-fucose-containing medium.

Effect of long-term treatment with a new aldose reductase inhibitor, (2S,4S)-6-fluoro-2',5'-dioxospiro-[chroman-4,4'-imidazolidine]-2-carbox amide (SNK-860), on peripheral neuropathy in streptozotocin-induced diabetic rats

We studied the long-term effects of a new aldose reductase inhibitor, (2S,4S)-6-fluoro-2',5'-dioxospiro-[chroman-4,4'-imidazolidine]-2- carboxamide (SNK-860), on functional, biochemical, and structural changes in peripheral nerve of streptozotocin (STZ)-induced diabetic rats. During the experimental period of 26 weeks, the delayed motor-nerve conduction in diabetic rats was significantly prevented by SNK-860 treatment, and elevated sorbitol levels and reduced myo-inositol levels were normalized to 100% and 71% of control levels, respectively. Teased nerve fiber studies demonstrated that the frequency of abnormal fibers was significantly reduced in treated diabetic rats. Morphometric analysis of myelinated fibers also disclosed prevention of axonal atrophy, distorted axonal circularity and preservation of large-sized fibers following SNK-860 treatment. These results suggest that long-term treatment with SNK-860 has a beneficial preventive effect on the development of experimental diabetic neuropathy.

Angiotensin II receptor blockade improves nerve function, modulates nerve blood flow and stimulates endoneurial angiogenesis in streptozotocin-diabetic rats and nerve function

We examined the effect of the angiotensin II receptor blocker, ZD 8731, on nerve function, capillary density, and blood flow in streptozotocin-diabetic rats. Deficits in sciatic motor and saphenous sensory nerve conduction velocity of 21% and 15%, respectively, were observed after 1 month of diabetes mellitus (p < 0.001). These were completely ameliorated by a further month of ZD 8731 treatment (p < 0.001). Treatment of non-diabetic rats for 1 month with ZD 8731 had no effect on motor or sensory conduction velocity. Sciatic nerve capillary density was not significantly affected by 1- or 2-month untreated diabetes, however, there was a 15% increase in density with ZD 8731 treatment (p < 0.001). Treatment of non-diabetic rats for 1 month had no effect on capillary density. Diabetes prolonged the time taken for 80% conduction failure by 19% (p < 0.05) and 49% (p < 0.001) for 1 and 2 months of diabetes, respectively, when sciatic nerve was exposed to hypoxia in vitro. ZD 8731 treatment during the second month of diabetes limited the prolongation to 22%, not significantly different from 1 month of untreated diabetes but less than for the 2-month diabetic group (p < 0.001). Concentrations of sciatic nerve polyol pathway metabolites were elevated six-fold and myo-inositol was reduced 40% by diabetes; ZD 8731 treatment was without effect. Acute experiments examined the effect of ZD 8731 on sciatic nerve blood flow using laser-Doppler flowmetry. In non-diabetic rats, blood flow changes followed the dose-dependent reductions in systemic arterial pressure and there were no significant variations in sciatic vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting effects of treatment with omega-3 and omega-6 essential fatty acids on peripheral nerve function and capillarization in streptozotocin-diabetic rats

Essential fatty acid metabolism is impaired by diabetes mellitus and this may be important in the aetiology of peripheral nerve dysfunction. The effects of gamma-linolenic acid (omega-6) and fish oil (omega-3) alone, and in combination, on nerve function and capillarization were examined in 2-month streptozotocin-diabetic rats. Diabetes resulted in approximately 15% and 23% decreases in saphenous sensory and sciatic motor nerve conduction velocities, respectively (p < 0.001). Motor and sensory conduction velocities were in the non-diabetic range after both preventive and reversal omega-6 treatment of diabetic rats (p < 0.001). No significant changes occurred in omega-6 treated non-diabetic rats. Preventive omega-3 treatment was largely ineffective. Reversal treatment with a combination of omega-6 and omega-3 fatty acids was marginally effective and improved motor (p < 0.05), but not sensory conduction velocity. In vitro measurement of sciatic nerve resistance to hypoxic conduction failure in diabetic rats revealed a 56% increase in the time taken for the compound action potential amplitude to be reduced by 80% (p < 0.01) compared to non-diabetic rats. This was partially prevented by omega-6 treatment (29% increase, p < 0.01). Reversal omega-6 treatment had a lesser effect (37% increase, p < 0.05 compared to untreated diabetic rats). omega-3 treatment had no significant effect on conduction failure time. Sciatic endoneurial capillary density increased by 11% with preventive omega-6 treatment (p < 0.05), but was unaffected by reversal omega-6 and by omega-3 treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction and relaxation of aortas from galactosaemic rats and the effects of aldose reductase inhibition

Rats were fed for 10 days with a 40% galactose diet, in order to chronically stimulate the polyol pathway. Thoracic aorta contraction and relaxation were studied. Compared to controls, galactosaemia did not influence contractions to phenylephrine or serotonin. Acetylcholine produced concentration-dependent relaxation of aortic rectangles precontracted with phenylephrine; galactosaemia caused a 25% deficit in maximum relaxation to acetylcholine (P < 0.01) and a 168% increase in EC50. There was a similar 25% reduction in relaxation to 3 microM calcium ionophore A23187 (P < 0.05). By contrast, there were no significant differences in endothelium-independent relaxation to nitroglycerine or cromakalim. The abnormalities in endothelium-dependent relaxation were completely prevented by treating galactosaemic rats with the aldose reductase inhibitor, ponalrestat. Thus, the data demonstrate that elevated polyol pathway activity contributes to reduced endothelium production, release or the action of nitric oxide in experimental galactosaemia, and suggest that this mechanism could also contribute to the vascular defects seen in diabetes mellitus.

Reversal of hyperglycemic-induced defects in myo-inositol metabolism and Na+/K+ pump activity in cultured neuroblastoma cells by normalizing glucose levels

myo-Inositol accumulation and incorporation into phosphoinositides was decreased in neuroblastoma cells chronically exposed to medium containing 30 mmol/L glucose or 30 mmol/L galactose. In addition, the intracellular content of myo-inositol and phosphatidylinositol was decreased and the sorbitol or galactitol content increased in cells cultured for 2 weeks in medium containing 30 mmol/L glucose or 30 mmol/L galactose, respectively. Na+/K+ adenosine triphosphatase (ATPase) transport activity was also significantly decreased by long-term exposure of neuroblastoma cells to medium containing 30 mmol/L glucose or 30 mmol/L galactose. When glucose-conditioned cells were placed in medium containing a normal glucose concentration for 24 hours, myo-inositol metabolism and content, phosphatidylinositol levels, and Na+/K+ pump activity were restored or completely returned to normal values. These functions were also significantly improved, except for the phosphatidylinositol content, which was increased by 55%, when galactose-conditioned cells were incubated for 24 hours in unsupplemented medium. The polyol content of the glucose- or galactose-conditioned cells was also significantly reduced. Returning the cells to normal glucose levels for 1 to 3 hours did not completely restore myo-inositol metabolism. Improved myo-inositol metabolism and content, sorbitol levels, and Na+/K+ ATPase transport activity were also obtained within 24 hours when cells chronically exposed to medium supplemented with 30 mmol/L glucose were placed in medium containing 30 mmol/L glucose and 0.4 mmol/L sorbinil. The phosphatidylinositol content of these cells was improved by approximately 30%. Cells prelabeled for 24 hours with [U-14C]sorbitol metabolize more than 50% of the [U-14C]sorbitol during a 24-hour incubation in unsupplemented medium. These studies conducted at the cellular level suggest that restoration of normal myo-inositol metabolism, polyol content, and Na+/K+ pump activity altered by hyperglycemic conditions occurs rapidly following normalization of glucose concentration.

The effects of evening primrose oil on nerve function and capillarization in streptozotocin-diabetic rats: modulation by the cyclo-oxygenase inhibitor flurbiprofen

1. The aims of this study were first, to examine whether deficits in nerve conduction in streptozotocin-diabetic rats could be reversed by a 10% dietary supplement of evening primrose oil. Second, to determine the time-course of reversal, and third, to assess whether the effects could be blocked by the cyclo-oxygenase inhibitor flurbiprofen (5 mg kg-1 day-1). 2. One-month diabetes produced 20% and 15% deficits in sciatic motor and saphenous sensory conduction velocity respectively, which were maintained over 2 months diabetes. 3. The effect of 1-month evening primrose oil treatment on abnormalities caused by an initial month of untreated diabetes was examined. Motor and sensory nerve conduction velocity were restored to the non-diabetic level. 4. Resistance to hypoxic conduction failure was investigated for sciatic nerve trunk in vitro. The 80% conduction failure times were 29% and 55% prolonged by 1- and 2-month diabetes respectively. Evening primrose oil did not reverse the increased hypoxic resistance following 1-month untreated diabetes. 5. Sciatic nerve endoneurial capillary density was not significantly affected by diabetes, but was 16% increased in diabetic rats with reversal by evening primrose oil treatment for 1 month compared to 2-month untreated diabetes. 6. Serial motor conduction velocity measurement after 3-month untreated diabetes revealed complete normalization by evening primrose oil within 4 days. Cessation of treatment resulted in a rapid decline in conduction velocity over 24 h. 7. In a preventive study of 2-month duration, 6 groups of rats were used. These comprised non-diabetic controls, diabetic rats, and evening primrose oil-treated diabetic rats, both with and without flurbiprofen treatment. Flurbiprofen had no significant effect in non-diabetic rats, but produced an 11% worsening of motor conduction velocity and a 21% reduction of sciatic capillary density in diabetic rats. Evening primrose oil prevented the decreases in conduction velocity and increased hypoxic resistance with diabetes, and caused a 23% increase in capillary density. Flurbiprofen completely blocked the effect of evening primrose oil on conduction velocity, resistance to hypoxia, and capillarization.8. Six main conclusions were reached. First, evening primrose oil rapidly reverses conduction deficits in diabetic rats. Second, the effects of treatment may be very short-lived, suggesting a primary metabolic action. Third, evening primrose oil cannot reverse established changes in hypoxic resistance over 1-month treatment. Fourth, long-term treatment causes angiogenesis, suggesting a vascular action. Fifth,products of cyclo-oxygenase-mediated metabolism are necessary for maintaining vasa nervorum integrity in diabetic rats. Sixth, evening primrose oil probably acts by providing substrate for vasodilator prostanoid synthesis by vasa nervorum.

Potential therapeutic approaches to the treatment or prevention of diabetic neuropathy: evidence from experimental studies

Recent investigations using experimental models of diabetes mellitus have emphasized the importance of impaired blood flow for the development of nerve dysfunction. Other observations suggest that this may also be the case for patients. A number of studies have revealed that several types of vasodilators can prevent or successfully treat early conduction abnormalities in diabetic rodents. These include alpha 1-adrenoreceptor antagonists, calcium channel blockers, agents that inhibit the renin-angiotensin system, and vasomodulator prostanoids. Other treatments applied to animal models, such as omega-6 essential fatty acids, aldose reductase inhibitors, aminoguanidine which prevents the formation of advanced glycation end-products, and anti-oxidants all appear to have vascular-related effects that lead to improvements in nerve conduction. These findings suggest that endothelial dysfunction and oxidative stress could be important factors in the aetiology of diabetic neuropathy. Studies have also focused on deficits in axon growth and regeneration, their relation to impaired neuronal synthesis and transport of growth-related chemicals, and neuronotrophic abnormalities. Taken together, the data give rise to the notion that an optimal therapeutic strategy could consist of improving the microenvironment of damaged nerve fibres by manipulating nerve blood flow while concurrently encouraging repair with trophic agents.

Prevention and reversal of motor and sensory peripheral nerve conduction abnormalities in streptozotocin-diabetic rats by the prostacyclin analogue iloprost

The effects of the prostacyclin analogue iloprost on nerve function were examined in streptozotocin-diabetic rats. Rats were treated either with iloprost from induction of diabetes over 2 months in a preventive experiment, or for 1 month following a 1 month untreated period of diabetes in a reversal experiment. One and 2 months untreated diabetic control, non-diabetic control, and iloprost-treated non-diabetic groups were also used. Diabetes of 1 month duration caused a 21% (P < 0.001) reduction in sciatic motor conduction velocity and a 14% (P < 0.001) deficit in saphenous sensory conduction. This was not significantly changed over a subsequent month without treatment. Diabetic rats given iloprost treatment in both preventive and reversal studies had motor and sensory conduction velocities not significantly different from those of non-diabetic controls, but greater than for untreated diabetes (P < 0.01). Iloprost treatment did not have a significant effect on nerve conduction in non-diabetic rats. The time taken for sciatic nerve compound action potential amplitude to be reduced by 80% under hypoxic conditions in vitro was progressively elevated by 19% and 57% after 1 and 2 months diabetes respectively. Iloprost treatment significantly attenuated this for both preventive (47%, P < 0.001) and reversal (50%, P < 0.001) studies. There was no effect on hypoxic resistance for non-diabetic rats. In the preventive group there was a 28% increase in sciatic nerve endoneurial capillary density (P < 0.001), a lesser effect (16%, P < 0.05) in the reversal group, and no effect in non-diabetic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid reversal of a motor nerve conduction deficit in streptozotocin-diabetic rats by the angiotensin converting enzyme inhibitor lisinopril

The effect of treatment of rats with the angiotensin converting enzyme inhibitor lisinopril after 5 weeks of untreated streptozotocin-diabetes was examined by daily monitoring of sciatic motor conduction velocity to tibialis anterior muscle. Diabetes produced a 31.5% decrease in conduction velocity (P < 0.001). Lisinopril treatment caused a progressive improvement which was significant after 3 days (P = 0.002), full normalization being achieved by 6 days (P < 0.0001). After 7 days of treatment there followed a 7-day washout period in which no lisinopril was given. During this time conduction velocity declined to untreated diabetic levels over 3 days. A subsequent treatment period resulted in complete normalization of conduction velocity within 2 days (P < 0.0001). Thus, the marked functional effects seen for vasodilator treatment with lisinopril suggest that angiotension converting enzyme inhibitors may have potential therapeutic value in the treatment of diabetic neuropathy.

Dissociation between biochemical and functional effects of the aldose reductase inhibitor, ponalrestat, on peripheral nerve in diabetic rats

1. The aim of the study was to examine the effects in rats of two different doses of the aldose reductase inhibitor, ponalrestat, on functional measures of nerve conduction and sciatic nerve biochemistry. 2. After 1 month, streptozotocin-induced diabetes produced 22%, 23% and 15% deficits in conduction velocity of sciatic nerves supplying gastrocnemius and tibialis anterior muscles and saphenous sensory nerve respectively compared to controls. These deficits were maintained over 2 months diabetes. 3. Slower-conducting motor fibres supplying the interosseus muscles of the foot did not show a diabetic deficit compared to onset controls, however, there was a 13% reduction in conduction velocity after 2 months diabetes relative to age-matched controls, indicating a maturation deficit. 4. Resistance to hypoxic conduction failure was investigated for sciatic nerve trunks in vitro. There was an increase in the duration of hypoxia necessary for an 80% reduction in compound action potential amplitude with diabetes. This was progressive; after 1 month, hypoxia time was increased by 22% and after 2 months by 57%. 5. The effect of 1-month treatment with the aldose reductase inhibitor, ponalrestat, on the abnormalities caused by an initial month of untreated diabetes was examined. Two doses of ponalrestat were employed, 8 mg kg-1 day-1 (which is equivalent to, or greater than, the blockade employed in clinical trials), and 100 mg kg-1 day-1. 6. Sciatic nerve sorbitol content was increased 7 fold by diabetes. Both doses were effective in reducing this; 70% for 8 mg kg-1 day-1, and to within the control range for 100 mg kg-1 day-1.