Neurovascular interactions between aldose reductase and angiotensin-converting enzyme inhibition in diabetic rats

Metanx alleviates multiple manifestations of peripheral neuropathy and increases intraepidermal nerve fiber density in Zucker diabetic fatty rats

Metanx is a product containing L-methylfolate, pyridoxal 5'-phosphate, and methylcobalamin for management of endothelial dysfunction. Metanx ingredients counteract endothelial nitric oxide synthase uncoupling and oxidative stress in vascular endothelium and peripheral nerve. This study evaluates Metanx on diabetic peripheral neuropathy in ZDF rats, a model of type 2 diabetes. Metanx was administered to 15-week-old ZDF and ZDF lean rats at either 4.87 mg ⋅ kg(-1) ⋅ day(-1) (a body weight-based equivalent of human dose) or 24.35 mg ⋅ kg(-1) ⋅ day(-1) by oral gavage two times a day for 4 weeks. Both doses alleviated hind limb digital sensory, but not sciatic motor, nerve conduction slowing and thermal and mechanical hypoalgesia in the absence of any reduction of hyperglycemia. Low-dose Metanx increased intraepidermal nerve fiber density but did not prevent morphometric changes in distal tibial nerve myelinated fibers. Metanx treatment counteracted endothelial nitric oxide synthase uncoupling, inducible nitric oxide synthase upregulation, and methylglyoxal-derived advanced glycation end product, nitrotyrosine, and nitrite/nitrate accumulation in the peripheral nerve. In conclusion, Metanx, at a body weight-based equivalent of human dose, increased intraepidermal nerve fiber density and improved multiple parameters of peripheral nerve function in ZDF rats. Clinical studies are needed to determine if Metanx finds use in management of diabetic peripheral neuropathy.

Baicalein alleviates diabetic peripheral neuropathy through inhibition of oxidative-nitrosative stress and p38 MAPK activation

With the consideration of the multifactorial etiology of diabetic peripheral neuropathy, an ideal drug or drug combination should target at least several key pathogenetic mechanisms. The flavonoid baicalein (5,6,7-trihydroxyflavone) has been reported to counteract sorbitol accumulation, activation of 12/15-lipoxygenase, oxidative-nitrosative stress, inflammation, and impaired signaling in models of chronic disease. This study evaluated baicalein on diabetic peripheral neuropathy. Control and streptozotocin-diabetic C57Bl6/J mice were maintained with or without baicalein treatment (30 mg kg(-1) d(-1), i.p., for 4 weeks after 12 weeks without treatment). Neuropathy was evaluated by sciatic motor and hind-limb digital sensory nerve conduction velocities, thermal algesia (Hargreaves test), tactile response threshold (flexible von Frey filament test), and intraepidermal nerve fiber density (fluorescent immunohistochemistry with confocal microscopy). Sciatic nerve and spinal cord 12/15-lipoxygenase and total and phosphorylated p38 mitogen-activated protein kinase expression and nitrated protein levels were evaluated by Western blot analysis, 12(S)hydroxyeicosatetraenoic acid concentration (a measure of 12/15-lipoxygenase activity) by ELISA, and glucose and sorbitol pathway intermediate concentrations by enzymatic spectrofluorometric assays. Baicalein did not affect diabetic hyperglycemia, and alleviated nerve conduction deficit and small sensory nerve fiber dysfunction, but not intraepidermal nerve fiber loss. It counteracted diabetes-associated p38 mitogen-activated protein kinase phosphorylation, oxidative-nitrosative stress, and 12/15-lipoxygenase overexpression and activation, but not glucose or sorbitol pathway intermediate accumulation. In conclusion, baicalein targets several mechanisms implicated in diabetic peripheral neuropathy. The findings provide rationale for studying hydroxyflavones with an improved pharmacological profile as potential treatments for diabetic neuropathy and other diabetic complications.

Low-dose poly(ADP-ribose) polymerase inhibitor-containing combination therapies reverse early peripheral diabetic neuropathy

Poly(ADP-ribose) polymerase (PARP) inhibition has recently been identified as a novel approach to treatment of experimental peripheral diabetic neuropathy (PDN). However, long-term inhibition of PARP, an enzyme involved in DNA repair, can potentially result in premature aging, loss of genome stability, and other side effects. This study evaluated potential synergistic interactions between low doses of the potent and specific PARP inhibitor 1,5-isoquinolinediol (ISO) and one of two vasodilators, the ACE inhibitor lisinopril (LIS) and the beta2-adrenoceptor agonist salbutamol (SAL) in the model of early PDN. Control and streptozotocin (STZ)-induced diabetic rats were treated with either ISO plus LIS or ISO plus SAL for 2 weeks after an initial 2 weeks without treatment. ISO (intraperitoneally) and LIS and SAL (both in the drinking water) were used in subtherapeutic doses, resulting in a minor correction of diabetes-associated sciatic motor and hind-limb digital sensory nerve conduction deficits when administered as monotherapies. Both combination treatments corrected endoneurial blood flow and vascular conductance deficits in STZ-induced diabetic rats. ISO plus SAL corrected all other changes of PDN, i.e., motor nerve conduction velocity (MNCV) and sensory nerve conduction velocity (SNCV) deficits as well as thermal and mechanical hyperalgesia. With ISO plus LIS, no significant correction of MNCV was observed, and the effect on thermal hyperalgesia was quite modest. SNCV and mechanical hyperalgesia were corrected. In vitro studies in human endothelial and Schwann cells showed early accumulation of poly(ADP-ribosyl)ated proteins (Western blot analysis) in response to high glucose, thus suggesting the importance of PARP activation in human PDN. In conclusion, low-dose PARP inhibitor-containing combination therapies may constitute a new approach for treatment of PDN.

Cardiac abnormalities in diabetic patients with neuropathy: effects of aldose reductase inhibitor administration

OBJECTIVE

The goal of this study was to determine whether treatment with an aldose reductase inhibitor (ARI) has beneficial effects on asymptomatic cardiac abnormalities in diabetic patients with neuropathy.

RESEARCH DESIGN AND METHODS

Diabetic subjects with neuropathy (n = 81) with either a low diastolic peak filling rate or impaired augmentation of left ventricular (LV) ejection fraction (LVEF) during maximal bicycle exercise were identified by gated radionuclide ventriculography. Coronary artery disease, left ventricular hypertrophy, and valvular heart disease were excluded by clinical evaluation, myocardial perfusion imaging, and echocardiography. Subjects were randomized to receive blinded treatment with either the placebo or the ARI zopolrestat 500 or 1,000 mg daily for 1 year.

RESULTS

After 1 year of ARI treatment, there were increases in resting LVEF (P < 0.02), cardiac output (P < 0.03), LV stroke volume (P < 0.004), and exercise LVEF (P < 0.001). In placebo-treated subjects, there were decreases in exercise cardiac output (P < 0.03), stroke volume (P < 0.02), and end diastolic volume (P < 0.04). Exercise LVEF increased with ARI treatment independent of blood pressure, insulin use, or the presence of baseline abnormal heart rate variability. There was no change in resting diastolic filling rates in either group.

CONCLUSIONS

Diabetic patients with neuropathy have LV abnormalities that can be stabilized and partially reversed by ARI treatment.

Nerve conduction velocity and evoked potential latencies in streptozotocin-diabetic rats: effects of treatment with an angiotensin converting enzyme inhibitor

BACKGROUND

Diabetes mellitus is associated with deficits in cerebral function. Vascular disorders may play a role in the pathogenesis and provide a potential target for treatment. The present study examined if prevention and intervention treatment with the angiotensin converting enzyme inhibitor enalapril could improve peripheral and central neurophysiological deficits in streptozotocin-diabetic rats.

METHODS

Sciatic nerve conduction velocities were measured prior to diabetes induction and again every three weeks. In the prevention study, the final nerve conduction measurements were performed at 15 weeks and in the intervention study at 24 weeks. Brain stem auditory and visual evoked potential latencies were measured every 3 weeks from 10 weeks after diabetes induction. In the prevention study, the final evoked potential measurements were performed at 16 weeks and in the intervention study at 25 weeks. Treatment with the angiotensin converting enzyme inhibitor enalapril was started directly after diabetes induction (prevention treatment) and after 15 weeks of diabetes (intervention treatment).

RESULTS

Nerve conduction velocity, brain stem auditory and visual evoked potential latencies were impaired in diabetic rats. Enalapril prevented deficits in nerve conduction velocity (p < 0.001), brain stem auditory evoked potential latencies (p < 0.01) and visual evoked potential latencies (p < 0.005). Enalapril intervention treatment had no effect on nerve conduction velocity and on visual evoked potential latencies, but improved brain stem auditory evoked potential latencies (p < 0.05) after 10 weeks of treatment.

CONCLUSION

Enalapril partially prevents the development of neurophysiological alterations in the peripheral and central nervous system and partially reverses deficits in brain stem auditory evoked potential latencies in STZ-diabetic rats.

The effects of 5-hydroxytryptamine 5-HT2 receptor antagonists on nerve conduction velocity and endoneurial perfusion in diabetic rats

Reduced peripheral nerve perfusion participates in the aetiology of diabetic neuropathy. 5-Hydroxtryptamine causes vasa nervorum vasoconstriction and platelet aggregation, which are enhanced by diabetes. To assess whether these mechanisms could contribute to neuropathy, the effects of 5-hydroxytryptamine 5-HT2 receptor antagonist treatment were examined in streptozotocin-induced diabetic rats. One study determined the dose-response relationship for AT1015 (N-[2-[4-(5H-dibenzo[a,d]cyclohepten-5-ylidene)piperidino]ethyl]-1-formyl-4-piperidinecarboxamide monohydrochloride monohydrate). Two weeks AT1015 treatment after 6 weeks of diabetes dose-dependently corrected 19.7%, 54.1%, and 15.7% deficits in sciatic nerve motor conduction velocity and blood flow, and saphenous nerve sensory conduction: ED50 values were 0.52, 0.74 and 0.15 mg/kg(-1)/day(-1), respectively. In a second study, high-dose AT1015 (3 mg/kg(-1)/day(-1)) actions were compared with those of the 5HT2 receptor antagonists, ritanserin (10 mg/kg(-1)/day(-1)) and sarpogrelate (100 mg/kg(-1)/day(-1)), and the anti-platelet phosphodiesterase III inhibitor, cilostazol (100 mg/kg(-1)/day(-1)). Two weeks treatment with these drugs produced a marked correction (82.6-99.7%) of a 19.8% sciatic motor conduction deficit in diabetic rats. Similarly, 44.7% and 14.9% reductions in sciatic endoneurial blood flow and saphenous sensory conduction velocity were completely reversed. Thus, 5-HT2 receptor antagonists had marked beneficial effects in experimental diabetic neuropathy, and AT1015 appears suitable for further evaluation in clinical trials.