Bert Bravenboer

Cerebral function in diabetes mellitus

SummaryDiabetes mellitus is a common metabolic disorder associated with chronic complications such as nephropathy, angiopathy, retinopathy and peripheral neuropathy. Diabetes is not often considered to have deleterious effects on the brain. However, long-term diabetes results in a variety of subtle cerebral disorders, which occur more frequently than is commonly believed. Diabetic cerebral disorders have been demonstrated at a neurochemical, electrophysiological, structural and cognitive level; however, the pathogenesis is still not clear. Probably alterations in cerebral blood supply and metabolic derangements play a role, as they do in the pathogenesis of diabetic neuropathy. Furthermore, the brain is also affected by recurrent episodes of hypoglycaemia and poor metabolic control. We describe herein the cerebral manifestations of diabetes and discuss the putative pathogenetic mechanisms.

Iron Metabolism Is Associated With Adipocyte Insulin Resistance and Plasma Adiponectin The Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study

OBJECTIVE Adipocyte insulin resistance (IR) is a key feature early in the pathogenesis of type 2 diabetes mellitus (T2DM), and although scarce, data in the literature suggest a direct role for iron and iron metabolism–related factors in adipose tissue function and metabolism. Serum ferritin and transferrin were shown to be associated with muscle insulin resistance (IR) and T2DM, but little is known about the role of iron metabolism on adipose tissue. We therefore investigated whether markers of iron metabolism were associated with adipocyte IR and plasma adiponectin. RESEARCH DESIGN AND METHODS Serum ferritin, transferrin, total iron, non–transferrin-bound iron (NTBI), transferrin saturation, and plasma adiponectin were determined in 492 individuals. Adipocyte IR was defined by the product of fasting insulin and nonesterified fatty acids (NEFAs). Using linear regression analyses, we investigated the difference in adipocyte IR or adiponectin (in %) according to differences in iron metabolism markers. RESULTS Serum ferritin (β = 1.00% increase in adipocyte IR per 10 μg/L [95% CI 0.66–1.34]), transferrin (4.18% per 0.1 g/L [2.88–5.50]), total iron (1.36% per μmol/L [0.61–2.12]), and NTBI (5.14% per μmol/L [1.88–8.52]) were associated with adipocyte IR after adjustment for several covariates, including inflammatory markers. All markers of iron metabolism were also associated with NEFAs (all P < 0.01). In addition, ferritin and transferrin were inversely associated with adiponectin (both P < 0.01). CONCLUSIONS The observed associations of several markers of iron metabolism with adipocyte IR and adiponectin suggest that factors related to iron and iron metabolism may contribute to adipocyte IR early in the pathogenesis of T2DM.

Pathogenesis of diabetic neuropathy: Focus on neurovascular mechanisms

Neuropathies of the peripheral and autonomic nervous systems affect up to half of all people with diabetes, and are major risk factors for foot ulceration and amputation. The aetiology is multifactorial: metabolic changes in diabetes may directly affect neural tissue, but importantly, neurodegenerative changes are precipitated by compromised nerve vascular supply. Experiments in animal models of diabetic neuropathy suggest that similar metabolic sequelae affect neurons and vasa nervorum endothelium. These include elevated polyol pathway activity, oxidative stress, the formation of advanced glycation and lipoxidation end products, and various pro-inflammatory changes such as elevated protein kinase C, nuclear factor κB and p38 mitogen activated protein kinase signalling. These mechanisms do not work in isolation but strongly interact in a mutually facilitatory fashion. Nitrosative stress and the induction of the enzyme poly (ADP-ribose) polymerase form one important link between physiological stressors such as reactive oxygen species and the pro-inflammatory mechanisms. Recently, evidence points to endoplasmic stress and the unfolded protein response as forming another crucial link. This review focuses on the aetiopathogenesis of neurovascular changes in diabetic neuropathy, elucidated in animal studies, and on putative therapeutic targets the majority of which have yet to be tested for efficacy in clinical trials.

Beneficial effect of the Ca2+ antagonist, nimodipine, on existing diabetic neuropathy in the BB/Wor rat

1 Neuropathy is a frequently diagnosed complication of diabetes mellitus. Effective pharmacotherapy is not available. 2 The spontaneously diabetic BB/Wor rats develop secondary complications like neuropathy as do human diabetic patients. 3 BB/Wor rats treated with insulin via a subcutaneous implant show a significant impairment of sensory and motor nerve conduction velocity 6 weeks after the onset of diabetes mellitus. 4 Intraperitoneal treatment of diabetic BB/Wor rats with the Ca2+ antagonist, nimodipine (20 mg kg−1), from week 6 onwards every 48 h for a period of 6 weeks resulted in a significant increase of sensory and motor nerve conduction velocity. 5 Twelve weeks after the onset of diabetes mellitus BB/Wor rats show a 40% impairment of sciatic nerve blood flow as compared to the non‐diabetic age‐matched controls. Treatment with nimodipine (20 mg kg−1) from week 6 onwards significantly increased the sciatic nerve blood flow as compared to placebo‐treated diabetic BB/Wor rats. 6 The adrenergic responsiveness of the vasa nervorum of the sciatic nerve to tyramine and phenylephrine was investigated as a parameter for autonomic neuropathy. 7 The fact that nimodipine treatment restored the reduced response to tyramine independently of the reduced postsynaptic phenylephrine responsiveness indicates that nimodipine improves adrenergic responsiveness mainly at the presynaptic level.

Oxidative stress and antioxidant treatment in diabetic neuropathy

Oxidative stress is one of the factors contributing to the development of diabetic neuropathy. Several mechanisms, among which glucose autooxidation, glycation of antioxidant enzymes like superoxide dismutase, catalase and glutathione peroxidase, and increased polyol pathway activity, lead to increased production of reactive oxygen species. Both direct toxicity of oxygen free radicals to the peripheral nerve as well as changes in endothelial function and vascular reactivity, possibly by the quenching of nitric oxide, may lead to nerve dysfunction. Antioxidant drugs like iron chelators, N-acetylcysteine, probucol, α-lipoic acid and vitamin E can prevent nerve dysfunction in experimental diabetes. The present review focuses on the potential mechanisms explaining the association between oxidative stress and diabetic neuropathy, and summarizes the studies in which the effects of antioxidant treatment on the diabetic peripheral nerve have been evaluated.

Cisplatin-induced neuropathy in mature rats: Effects of the melanocortin-derived peptide ORG 2766

A major side effect of cisplatin treatment is peripheral neuropathy. In the past few years we have provided evidence that the ACTH4–9 analogue ORG 2766 provides protection against this neuropathy in rats and man. In this study we investigated the development of a cisplatin-induced neuropathy and the protective and therapeutic actions of ORG 2766 in mature rats. We also studied the effects of the peptide and of growth impairment caused by food restriction on nerve conduction velocities in healthy young adult rats (not subjected to any other treatment). In the neuropathy experiment, cisplatin induced a significant decrease in sensory nerve conduction velocity (SNCV), which could be prevented by concomitant administration of ORG 2766. The SNCV of the neuropathic animals recovered to control values within 10 weeks of discontinuation of cisplatin treatment. ORG 2766 did not enhance the rate of recovery. In the young adult rats neither ORG 2766 administration nor restricted weight gain significantly influenced either the motor or the sensory nerve conduction velocity. These results validate the animal model of cisplatin-induced neuropathy.

Nerve function and oxidative stress in diabetic and vitamin E deficient rats

Nerve dysfunction in diabetes is associated with increased oxidative stress. Vitamin E depletion also leads to enhanced presence of reactive oxygen species (ROS). We compared systemic and endoneurial ROS activity and nerve conduction in vitamin E-depleted control and streptozotocin-diabetic rats (CE- and DE-), and in normally fed control and diabetic animals (CE+ and DE+). Nerve conduction was reduced in both diabetic groups. Vitamin E depletion caused a small further nerve conduction deficit in the diabetic, but not in the control animals. The combination of vitamin E deficiency and streptozotocin-diabetes (group DE-) appeared to be lethal. In the remaining groups, an important rise in sciatic nerve malondialdehyde (MDA) was observed in the vitamin E-depleted control rats. In contrast, plasma MDA levels were elevated in group DE+ only, whereas hydrogen peroxide levels were increased in group CE-. Endoneurial total and oxidized glutathione and catalase were predominantly elevated in group DE+. These data show that nerve lipid peroxidation induced by vitamin E depletion does not lead to reduced nerve conduction or changes in antioxidant concentrations as observed in STZ-diabetes. The marked systemic changes in MDA and antioxidants suggest that nerve dysfunction in experimental hyperglycemia is rather a consequence of systemic than direct nerve damage.

Body Composition as Determinant of Thrombin Generation in Plasma: The Hoorn Study

Objective—The association between obesity and cardiovascular disease and venous thromboembolism might, at least partially, be explained by a hypercoagulable state. The extent to which body fat mass and its distribution contribute to a hypercoagulable state is unknown. In this study, we investigated the association between body composition and thrombin generation and evaluated the potential mediating role of low-grade inflammation. Methods and Results—We studied 586 individuals from the Hoorn Study (mean age, 69.7±6.5 years, 298 women) in whom body composition was assessed by whole body dual-energy absorptiometry. Thrombin generation was measured using the Calibrated Automated Thrombogram. Multiple regression analyses showed a positive association between total body fat and thrombin generation in women but not in men. In addition, detailed analyses of regional body composition showed that central but not peripheral fat mass was associated with greater thrombin generation and that there was a trend toward an inverse association with peripheral lean mass. The reported positive associations were partially attenuated by low-grade inflammation, however. Conclusion—Body fat mass, in particular a central pattern of fat distribution, is associated with higher levels of thrombin generation in elderly women but not in men. This association may partially be explained by adiposity-related low-grade inflammation, but this hypothesis needs to be further investigated in mechanistic/prospective studies.

High rat food vitamin E content improves nerve function in streptozotocin-diabetic rats

Antioxidants can improve nerve dysfunction in hyperglycaemic rats. We evaluated whether the standard supplementation of rat food with vitamin E (normally added for preservation purposes) or high-dose vitamin E treatment improves nerve conduction in maturing streptozotocin-diabetic rats, a model widely used to study diabetic neuropathy. Hyperglycaemic rats received food containing 25 mg/kg (non-supplemented), 70 mg/kg (standard food) or 12 g/kg (high-dose) vitamin E. Non-diabetic controls received non-supplemented food. Sciatic and tibial sensory and motor nerve conduction velocity were decreased in all diabetic animals. In comparison with standard feeding, the non-supplemented diabetic rats showed lower plasma vitamin E levels but no significant change in nerve conduction. High-dose treatment prevented nerve dysfunction by 50%, and led to attenuated endoneurial lipid peroxidation (measured as malondialdehyde). We conclude that high doses of vitamin E, but not standard vitamin E supplementation of rat food partially prevent nerve dysfunction in young adult streptozotocin-diabetic rats.

Nerve conduction and antioxidant levels in experimentally diabetic rats: effects of streptozotocin dose and diabetes duration

Oxidative stress supposedly plays a role in the pathogenesis of diabetic neuropathy. We have studied whether a variation in the streptozotocin (STZ) dose or diabetes duration affects the outcome of measurements of oxidative damage in relation to nerve conduction. In experiment 1, we induced diabetes in rats using 40 or 60 mg/kg STZ intravenously and assessed sciatic nerve conduction velocity. After 18 weeks, we measured plasma malondialdehyde (MDA) and red blood cell (RBC) and nerve glutathione levels. We observed a dose-dependent effect of STZ on body weight, and to a lesser extent on nerve conduction, but not on RBC or nerve glutathione and plasma MDA. In experiment 2, we administered a fixed dose of STZ (40 mg/kg) and measured antioxidants and MDA in RBCs, plasma, and sciatic nerve after 2, 4, 8, and 18 weeks in diabetic and control rats. RBC glutathione decreased in diabetic animals initially, but did not differ from control values after week 4. Plasma total glutathione increased until week 8. The ratio of total to oxidized glutathione in the sciatic nerve from diabetic animals paralleled the decrease observed in RBCs, and subsequently increased compared with controls. Nerve catalase increased in diabetic animals. Endoneurial MDA remained unchanged, whereas plasma MDA increased and RBC superoxide dismutase (SOD) decreased in the diabetic group. We conclude that differences in antioxidant levels between STZ-diabetic and control rats depend on the duration of hyperglycemia. Furthermore, dose-related effects of STZ on nerve conduction are not reflected in endoneurial lipid peroxidation or glutathione.