Effects Of Elevated Glucose Levels Research Articles

Diabetes-induced glomerular dysfunction: Links to a more reduced cytosolic ratio of NADH/NAD+

These studies were undertaken to examine effects of elevated glucose levels on glycolysis, sorbitol pathway activity, and the cytosolic redox state of NADH/NAD+ in isolated glomeruli. Blood-free glomeruli were isolated from kidneys of male, Sprague-Dawley rats using standard sieving techniques, then incubated for one hour at 37 degrees C, pH 7.4, pO2 approximately 500 torr, in Krebs bicarbonate/Hepes buffer containing 5 or 30 mM glucose. Elevated glucose levels increased glucose 6-phosphate, fructose 6-phosphate, total triose phosphates, lactate, the lactate/pyruvate ratio, sorbitol, and fructose, but did not affect sn-glycerol 3-phosphate, pyruvate, or myo-inositol levels. The more reduced glomerular cytosolic redox state (manifested by the tissue lactate/pyruvate ratio) induced by 30 mM glucose was completely abrogated by aldose reductase inhibitors added to the diet two to seven days prior to glomerular isolation. These observations, coupled with evidence linking glucose- and diabetes-induced glomerular dysfunction to increased sorbitol pathway metabolism, support the hypothesis that metabolic imbalances associated with a more reduced ratio of cytosolic NADH/NAD+ (resulting from increased glucose metabolism via the sorbitol pathway) play an important role in mediating glucose- and diabetes-induced glomerular dysfunction.

Hyperglycemia Can Cause Membrane Lipid Peroxidation and Osmotic Fragility in Human Red Blood Cells

The present study has examined the effect of elevated glucose levels on membrane lipid peroxidation and osmotic fragility in human red blood cells (RBC). Defibrinated whole blood or RBC were incubated with varying concentrations of glucose at 37 degrees C for 24 h. RBC incubated with elevated levels of glucose showed a significantly increased membrane lipid peroxidation when compared with control RBC. A significant positive correlation was observed between the extent of glucose-induced membrane lipid peroxidation and the osmotic fragility of treated RBC. Glucose-induced membrane lipid peroxidation and osmotic fragility were blocked when RBC were pretreated with fluoride, an inhibitor of glucose metabolism; with vitamin E, an antioxidant; with para-chloromercurobenzoate and metyrapone, inhibitors of the cytochrome P-450 system; or with dimethylfurane, diphenylamine, and thiourea, scavengers of oxygen radicals. RBC treated with elevated glucose concentrations also showed an increase in NADPH levels. Exogenous addition of NADPH to normal RBC lysate induced membrane lipid peroxidation similar to that observed in the glucose-treated RBC. These data suggest that elevated glucose levels can cause the peroxidation of membrane lipids in human RBC.

The effect of elevated glucose levels on myo-inositol metabolism in cultured bovine aortic endothelial cells

Bovine aorta endothelial cells were used to determine the effect of high ambient glucose concentrations on myo-inositol metabolism. Culturing the cells for a minimum of 1 week in elevated glucose concentrations caused an increase in the intracellular sorbitol content and a decrease in myo-inositol levels. The accumulation of myo-inositol from the medium and incorporation into phospholipids was reduced 25% to 50% in cells grown in the presence of 30 to 50 mmol/L glucose. This effect was not observed following a short-term exposure of the cells to elevated glucose levels. Kinetic analysis of high-affinity myo-inositol uptake showed that the K'm was significantly increased in cells grown in 30 mmol/L glucose compared to those cultured in 5.6 mmol/L glucose. This would suggest that exposing endothelial cells to high ambient glucose levels for a minimum of 1 week leads to a competitive type of inhibition of high-affinity myo-inositol uptake. The changes in myo-inositol metabolism and content and sorbitol levels mediated by glucose exposure were blocked by addition of the aldose reductase inhibitor, sorbinil, to the media, suggesting that these changes are caused by the accumulation of sorbitol by the cells. Exposure of bovine aorta endothelial cells to high ambient levels of glucose leads to accumulation of sorbitol in the cells, which is responsible for alterations in myo-inositol metabolism. These changes could result in alteration of endothelial cell membrane function and contribute to the pathology of diabetes mellitus.

Effect of sorbinil on myo-inositol metabolism in cultured neuroblastoma cells exposed to increased glucose levels.

Neuroblastoma cells were used to determine the effect of sorbinil on myo-inositol metabolism in cells exposed to elevated levels of glucose in culture. Exposing cells to elevated levels of glucose led to an increase in levels of intracellular sorbitol. The increase in sorbitol levels was dependent on the extracellular glucose concentration. In contrast, the myo-inositol content of cells was decreased in the presence of increasing concentrations of extracellular glucose. Increasing the concentration of glucose in the culture medium caused a decrease in myo-inositol uptake and in the incorporation of extracellular myo-inositol into phospholipid. The effect of elevated glucose levels on myo-inositol metabolism and sorbitol accumulation was blocked by addition of 0.4 mM sorbinil. The ability of sorbinil to block the decrease in myo-inositol metabolism and sorbitol accumulation caused by 30 mM extracellular glucose was dependent on its concentration. Maximal effects were obtained with 0.4 mM sorbinil. However, there was some variation in the degree of effectiveness among batches of sorbinil. These results at the cellular level suggest that the intracellular accumulation of sorbitol is responsible for the alteration of myo-inositol metabolism observed in neuroblastoma cells exposed to elevated glucose concentrations.

Effect of increased glucose levels on Na+/K+-pump activity in cultured neuroblastoma cells.

Neuroblastoma cells were used to analyze the effect of elevated glucose levels on myo-inositol metabolism and Na+/K+-pump activity. The activity of the Na+/K+ pump in neuroblastoma cells is almost totally sensitive to ouabain inhibition. Culturing neuroblastoma cells in 30 mM glucose caused a significant decrease in Na+/K+-pump activity, myo-inositol metabolism, and myo-inositol content, compared to cells grown in the presence of 30 mM fructose. Glucose supplementation also caused a large intracellular accumulation of sorbitol. The aldose reductase inhibitor sorbinil prevented the abnormalities in myo-inositol metabolism and partially restored Na+/K+-pump activity in neuroblastoma cells cultured in the presence of elevated glucose levels. These results suggest that the accumulation of sorbitol by neuroblastoma cells exposed to elevated concentrations of extracellular glucose causes a decrease in myo-inositol metabolism and these abnormalities are associated with a reduction in Na+/K+-pump activity.

Glucose-induced Increases in Renal Hemodynamic Function: Possible Modulation by Renal Prostaglandins

Increased glomerular filtration rate and kidney size early in the course of experimental and human diabetes may be important in the pathogenesis of diabetic nephropathy. Factors causing these renal functional changes are unknown. The isolated, perfused rat kidney (IPRK) was used to study the effects of elevated glucose levels on kidneys from normal and diabetic rats in the absence of complex systemic effects of in vivo hyperglycemia. It was found that acute increases in perfusate glucose levels caused sustained dose-dependent vasodilatation in both normal and diabetic isolated kidneys. Furthermore, in normal kidneys, raising perfusate glucose to levels seen in moderately severe diabetes caused increased inulin clearance (Cln). In contrast, equal osmolar concentrations of mannitol caused sustained vasoconstriction and a slight decrease in Cln. Prostaglandin synthetase inhibitors reduced glucose-induced vasodilatation by 50% and prevented the increase in Cln that followed the addition of glucose to normal kidneys. Thus, these studies demonstrated that elevated glucose levels caused significant vasodilatation and increased Cln in the IPRK, and these glucose-induced hemodynamic changes were attenuated by prostaglandin synthetase inhibitors. It is possible that these glucose-induced effects may be important determinants of increased glomerular function in early diabetes.