NA+/K+ ATPase impairment and experimental glycation: the role of glucose autoxidation

Syringic Acid Ameliorates Cardiac, Hepatic, Renal and Neuronal Damage Induced by Chronic Hyperglycaemia in Wistar Rats: A Behavioural, Biochemical and Histological Analysis

This study investigated the effects of syringic acid (SA) on renal, cardiac, hepatic, and neuronal diabetic complications in streptozotocin-induced neonatal (nSTZ) diabetic rats. STZ (110 mg/kg i.p) was injected into Wistar rat neonates as a split dose (second and third postnatal day). Diabetes mellitus was diagnosed in adults by measuring fasting blood glucose levels, urine volume, and food and water intake. The treatment of SA (25 mg/kg, 50 mg/kg p.o) was given from the 8th to 18th postnatal week. To assess the development of diabetic complications and the effect of therapy, biochemical indicators in serum and behavioural parameters were recorded at specific intervals during the study period. SA (25 mg/kg, 50 mg/kg p.o) treatment reduced hyperglycaemia, polydipsia, polyphagia, polyuria, relative organ weight, cardiac hypertrophic indices, inflammatory markers, cell injury markers, glycated haemoglobin, histopathological score, and oxidative stress, and increased Na/K ATPase activity. These findings suggest that SA might significantly alleviate diabetic complications and/or renal, neuronal, cardiac, and hepatic damage in nSTZ diabetic rats.

The relationship between endogenous secretory RAGE and cardiac autonomic function in prediabetes

AIMS

The putative protective role of esRAGE for cardiac autonomic function (CAF) remains unclear. To address this question, the present study has assessed the relationship of serum AGEs, sRAGE and esRAGE, and tissue AGEs with CAF in a high-risk population without diabetes.

MATERIAL AND METHODS

This study enrolled 48 subjects of mean age 52.7 ± 11.2 years and mean BMI 28.4 ± 6.3 kg/m² , divided into two groups according to glucose tolerance: 16 with normal glucose tolerance (NGT) and 24 with prediabetes. A standard oral glucose tolerance test (OGTT) was performed. The glucose tolerance was defined according to 2006 WHO criteria. Fasting, 120-minutes glucose, lipids, creatinine, and HbA1c were measured. eGFR was calculated (CKD-EPI). Fasting, 120-minutes insulin (ECLIA method), advanced glycation end products (AGEs), plasma-soluble receptor for AGE (sRAGE), and endogenous secreted isoform of the receptor for AGE (esRAGE), (ELISA method) were assessed. HOMA-IR was calculated. Tissue AGEs were assessed by skin autofluorescence (AGE-Reader, DiagnOpticsTM). CAF was evaluated with ANX 3.0 autonomic nervous-monitoring system (ANSAR), applying deep breathing, Valsalva, and standing.

RESULTS

There was a significant decline in CAF in prediabetes in comparison with NGT. Serum and tissue AGEs, sRAGE, and esRAGE levels were similar between groups. On the matrix analysis, both sympathetic and parasympathetic activities at baseline and after standing and sympathetic tone during Valsalva were positively related to esRAGE in prediabetes. Multivariate regression analysis showed that esRAGE is an independent contributor to sympathetic, parasympathetic, and total autonomic tone in prediabetes accounting for about 28%, 34%, and 35% of their variances, respectively.

CONCLUSION

Our results have demonstrated that CAF is decreased in prediabetes. esRAGE, but not sRAGE, is reciprocally related to CAF, probably opposing the negative effects of glycation.

Advanced glycation end products and diabetic complications

During long standing hyperglycaemic state in diabetes mellitus, glucose forms covalent adducts with the plasma proteins through a non-enzymatic process known as glycation. Protein glycation and formation of advanced glycation end products (AGEs) play an important role in the pathogenesis of diabetic complications like retinopathy, nephropathy, neuropathy, cardiomyopathy along with some other diseases such as rheumatoid arthritis, osteoporosis and aging. Glycation of proteins interferes with their normal functions by disrupting molecular conformation, altering enzymatic activity, and interfering with receptor functioning. AGEs form intra- and extracellular cross linking not only with proteins, but with some other endogenous key molecules including lipids and nucleic acids to contribute in the development of diabetic complications. Recent studies suggest that AGEs interact with plasma membrane localized receptors for AGEs (RAGE) to alter intracellular signaling, gene expression, release of pro-inflammatory molecules and free radicals. The present review discusses the glycation of plasma proteins such as albumin, fibrinogen, globulins and collagen to form different types of AGEs. Furthermore, the role of AGEs in the pathogenesis of diabetic complications including retinopathy, cataract, neuropathy, nephropathy and cardiomyopathy is also discussed.

Advanced glycation end products and diabetic complications

During long standing hyperglycaemic state in diabetes mellitus, glucose forms covalent adducts with the plasma proteins through a non-enzymatic process known as glycation. Protein glycation and formation of advanced glycation end products (AGEs) play an important role in the pathogenesis of diabetic complications like retinopathy, nephropathy, neuropathy, cardiomyopathy along with some other diseases such as rheumatoid arthritis, osteoporosis and aging. Glycation of proteins interferes with their normal functions by disrupting molecular conformation, altering enzymatic activity, and interfering with receptor functioning. AGEs form intra- and extracellular cross linking not only with proteins, but with some other endogenous key molecules including lipids and nucleic acids to contribute in the development of diabetic complications. Recent studies suggest that AGEs interact with plasma membrane localized receptors for AGEs (RAGE) to alter intracellular signaling, gene expression, release of pro-inflammatory molecules and free radicals. The present review discusses the glycation of plasma proteins such as albumin, fibrinogen, globulins and collagen to form different types of AGEs. Furthermore, the role of AGEs in the pathogenesis of diabetic complications including retinopathy, cataract, neuropathy, nephropathy and cardiomyopathy is also discussed.

Progressive axonal dysfunction precedes development of neuropathy in type 2 diabetes

To evaluate the development of diabetic neuropathy, the current study examined changes in peripheral axonal function. Nerve excitability techniques were undertaken in 108 type 2 diabetic patients with nerve conduction studies (NCS), HbA(1c) levels, and total neuropathy score (TNS). Patients were categorized into two cohorts: patients with diabetes without neuropathy (DWN group [n = 56]) and patients with diabetes with neuropathy (DN group [n = 52]) and further into severity grade 0 (TNS 0-1 [n = 35]), grade 1 (TNS 2-8 [n = 42]), and grade 2/3 (TNS 9-24 [n = 31]). Results revealed that the DWN group had a significantly increased threshold, prolonged latency, and changes in excitability parameters compared with age-matched control subjects. Patients with neuropathy demonstrated significant alteration in recovery cycle parameters and depolarizing threshold electrotonus. Within the DWN cohort, there were significant correlations between HbA(1c) level and latency and subexcitability, whereas the estimated glomerular filtration rate correlated with superexcitability in patients with neuropathy. Furthermore, excitability parameters became progressively more abnormal with increasing clinical severity. These results suggest a spectrum of excitability abnormalities in patients with diabetes and that early axonal dysfunction may be detected prior to the development of neuropathy. As progressive changes in excitability parameters correlated to neuropathy severity, excitability testing may provide a biomarker of the early development and severity of diabetic neuropathy, providing insights into the pathophysiological mechanisms producing axonal dysfunction.

Glucose autoxidation induces functional damage to proteins via modification of critical arginine residues

Nonenzymatic modification of proteins in hyperglycemia is a major mechanism causing diabetic complications. These modifications can have pathogenic consequences when they target active site residues, thus affecting protein function. In the present study, we examined the role of glucose autoxidation in functional protein damage using lysozyme and RGD-α3NC1 domain of collagen IV as model proteins in vitro. We demonstrated that glucose autoxidation induced inhibition of lysozyme activity as well as NC1 domain binding to α(V)β(3) integrin receptor via modification of critical arginine residues by reactive carbonyl species (RCS) glyoxal (GO) and methylglyoxal while nonoxidative glucose adduction to the protein did not affect protein function. The role of RCS in protein damage was confirmed using pyridoxamine which blocked glucose autoxidation and RCS production, thus protecting protein function, even in the presence of high concentrations of glucose. Glucose autoxidation may cause protein damage in vivo since increased levels of GO-derived modifications of arginine residues were detected within the assembly interface of collagen IV NC1 domains isolated from renal ECM of diabetic rats. Since arginine residues are frequently present within protein active sites, glucose autoxidation may be a common mechanism contributing to ECM protein functional damage in hyperglycemia and oxidative environment. Our data also point out the pitfalls in functional studies, particularly in cell culture experiments, that involve glucose treatment but do not take into account toxic effects of RCS derived from glucose autoxidation.

Effect of hyperosmolality and cations on iodinated contrast medium-induced potassium release from human blood cells

PURPOSE

Potassium release from blood cells is a contrast medium-induced phenomenon. The purposes of the study were to (1) assess the effect of hyperosmolality and of adding sodium ions and calcium ions to a solution on potassium release from human blood cells and (2) reevaluate the possibility of hemolysis as a cause of potassium elevation.

MATERIALS AND METHODS

Fresh human blood was mixed with a test solution to examine the temporal changes in the whole blood potassium levels and to calculate the potassium release rate. Test solutions included 5%, 20%, and 50% glucose; 0.9% and 10% NaCl; and 50% glucose mixed with various amounts of sodium and calcium ions. We also measured serum glutamine oxaloacetic acid transaminase (GOT) and serum lactate dehydrogenase (LDH) to evaluate the possibility of hemolysis.

RESULTS

Hyperosmolality using glucose solutions promoted higher potassium release. The average +/- SD potassium release rates were 7.3 +/- 2.4 micromol/min with 5% glucose, 13.5 +/- 2.3 micromol/min with 20% glucose, and 128.4 +/- 44.9 micromol/min with 50% glucose. The solutions including sodium ions showed lower release rates. The addition of sodium and calcium ions into 50% glucose significantly lowered the potassium release rates. No significant elevation of GOT or LDH was observed, and the possibility of hemolysis was eliminated.

CONCLUSION

Hyperosmolar glucose solution promoted potassium release, but the presence of sodium ions in the hypertonic solution inhibited it. In addition, there is no possibility of hemolysis as a cause of potassium release.

Role of advanced glycation end products and their receptors in development of diabetic neuropathy

Diabetic neuropathy is a life-threatening complication involving both peripheral and autonomic nerves. The hyperglycemia-induced polyol pathway as well as enhanced oxidative stress are among the factors implicated in the pathogenesis of diabetic neuropathy. Their effects are possibly exerted by direct nerve tissue damage or mediated by endothelial injury or vascular dysfunction. Formation of advanced glycation end product (AGE) is another important candidate for the cause of peripheral neuropathy. Indeed, the levels of AGEs were increased in the serum and also in the peripheral nerves obtained from diabetic patients. Structural and functional proteins of those nerves are also glycated, resulting in impaired nerve function and characteristic pathologic alterations. In addition, interaction between AGEs and their receptors induce biological effects on the target tissues for diabetic complications. In the peripheral nerve, the receptor for AGE (RAGE) is expressed in endothelial and Schwann cells. It is thus anticipated that interactions between AGEs and RAGE facilitate endoneural vascular dysfunction, leading to microangiopathy in the peripheral nerve. The roles of these mechanisms, in particular on the molecular mechanisms of AGE-RAGE interactions in the development of diabetic neuropathy are largely still speculative and yet to be explored.

Altered nerve excitability properties in established diabetic neuropathy

The underlying cause of diabetic neuropathy remains unclear, although pathological studies have suggested an ischaemic basis related to microangiopathy, possibly mediated through effects on the energy-dependent Na+/K+ pump. To investigate the pathophysiology of diabetic neuropathy, axonal excitability techniques were undertaken in 20 diabetic patients with neuropathy severity graded through a combination of quantitative sensory testing (QST) using a vibratory stimulus, assessment of symptom severity using the Total Neuropathy Symptom Score (T-NSS) and measurement of glycosylated haemoglobin as a marker of disease control. To assess axonal excitability, compound muscle action potentials were recorded at rest from abductor pollicis brevis following stimulation of the median nerve, and stimulus-response behaviour, threshold electrotonus, a current-threshold relationship and the recovery of excitability were recorded in each patient. All patients had established neuropathy, with abnormalities of T-NSS present in all patients and QST abnormalities present in 65%. Compared with controls, diabetic neuropathy patients had significant reduction in maximal CMAP amplitude (P < 0.0005), accompanied by a 'fanning in' of threshold electrotonus. In addition, the strength-duration time constant was decreased in diabetic neuropathy patients and recovery cycles were altered with reductions in refractoriness, the duration of the relative refractory period, superexcitability and subexcitability. It is proposed that while the changes in threshold electrotonus with supportive findings in the current-threshold relationship are consistent with axonal depolarization, possibly mediated by a decrease in Na+/K+ pump activity, the alterations in the recovery cycle of excitability could be explained on the basis of a smaller action potential, reflecting a limitation on the nodal driving current imposed by a reduction in Na+ conductances.

Diabetes-induced metabolic abnormalities in myocardium: effect of antioxidant therapy

Effects of hyperglycemia (both diabetes and experimental galactosemia) on cardiac metabolism have been determined. In addition, the effect of supplemental antioxidants on these hyperglycemia-induced abnormalities of cardiac metabolism has been investigated. Diabetes or experimental galactosemia of 2 months duration in rats significantly increased oxidative stress in myocardium, as demonstrated by elevation of thiobarbituric acid reactive substances (TBARS) and lipid fluorescent products in left ventricle. Activity of protein kinase C (PKC) was elevated in the myocardium, and the activities of (Na,K)-ATPase and calcium ATPases were subnormal. Administration of supplemental antioxidants containing a mixture of ascorbic acid, Trolox; alpha-tocopherol acetate, N-acetyl cysteine, beta-carotene, and selenium prevented both the diabetes-induced and galactosemia-induced elevation of oxidative stress and PKC activity, and inhibited the decreases of myocardial (Na,K)-ATPase and calcium ATPases. The results show that these metabolic abnormalities are not unique to diabetes per se, but are secondary to elevated blood hexose levels, and supplemental antioxidants inhibit these metabolic abnormalities. Our findings suggest that antioxidants inhibit abnormal metabolic processes that may contribute to the development of cardiac disease in diabetes, and offer a potential clinical means to inhibit cardiac abnormalities in diabetes.

Abnormalities of retinal metabolism in diabetes or experimental galactosemia. VI. Comparison of retinal and cerebral cortex metabolism, and effects of antioxidant therapy

Metabolic abnormalities observed in retina and in cerebral cortex were compared in diabetic rats and experimentally galactosemic rats. Diabetes or experimental galactosemia of 2 months duration significantly increased oxidative stress in retina, as shown by elevation of retinal thiobarbituric acid reactive substances (TBARS) and subnormal activities of antioxidant defense enzymes, but had no such effect in the cerebral cortex. Activities of sodium potassium adenosine triphosphatase [(Na,K)-ATPase] and calcium ATPase became subnormal in retina as well as in cerebral cortex. In contrast, protein kinase C (PKC) activity was elevated in retina but not in cerebral cortex in the same hyperglycemic rats. Dietary supplementation with an antioxidant mixture (containing ascorbic acid, Trolox, alpha-tocopherol acetate, N-acetyl cysteine, beta-carotene, and selenium) prevented the diabetes-induced and galactosemia-induced elevation of retinal oxidative stress, the elevation of retinal PKC activity and the decrease of retinal ATPases. In cerebral cortex, administration of the antioxidant diet also prevented the diabetes-induced decreases in (Na,K)-ATPase and calcium ATPases, but had no effect on TBARS and activities of PKC and antioxidant-defense enzymes. The results indicate that retina and cerebral cortex differ distinctly in their response to elevation of tissue hexose, and that cerebral cortex is more resistant than retina to diabetes-induced oxidative stress. The greater resistance to oxidative stress in cerebral cortex, as compared to retina, is consistent with the resistance of cerebral cortex to microvascular disease in diabetes, and with a hypothesis that oxidative stress contributes to microvascular disease in diabetes. Dietary supplementation with these antioxidants offers a means to inhibit multiple hyperglycemia-induced retinal metabolic abnormalities.