Hyperglycemia and heart dysfunction: an oxidant mechanism contributing to heart failure in diabetes

Glycemic variability in hospitalized patients: choosing metrics while awaiting the evidence

Hyperglycemia, hypoglycemia, preexisting diabetes, and glycemic variability each may affect hospital outcomes. Observational findings derived from randomized trials or retrospective studies suggest that independent of hypoglycemia and hyperglycemia, a relationship exists between variability and hospital outcomes. A review of studies conducted in diverse hospital populations is reported here, showing a relationship between measures of variability and nonglycemic outcomes, including ICU and hospital mortality and length of stay. "Glycemic variability" has an intuitive meaning, understood as a propensity of a single patient to develop repeated episodes of excursions of BG over a relatively short period of time that exceed the amplitude expected in normal physiology. It is proposed that each of 3 dimensions of variability should be separately studied: (1) magnitude of glycemic excursions during intervals of relative stability of the moving average of BG, (2) frequency with which a critical magnitude of excursion is exceeded, and (3) presence or absence of fine tuning. Multiple hospital studies have found that the standard deviation (SD) of the data set of blood glucose values (BG) of individual patients predicts outcomes. An appropriate refinement would be to report the "Reverse-transformed group mean of the SD of the logarithmically transformed BG data set of each patient," with confidence intervals. In logarithmic space, group means of the SD of BGs of each patient may be compared, using an appropriate parametric test. Upon reverse transformation, the upper and lower bounds of the confidence intervals become asymmetric about the reverse-transformed group mean of the SD. There is a need to understand what patterns of dispersion of BG over time are captured by SD as a predictor of outcomes. Among the causes of high SD, a subgroup may consist of patients having frequent oscillations of BG. Another subgroup may consist of patients experiencing a major change of overall glycemia during the timeframe of data collection. Appropriate metrics should be developed to recognize both variability in the sense of recurrent large oscillations of BG, and separately to recognize any time-dependent change of overall glycemia during hospitalization. Especially in relation to uncontrolled diabetes, there is a need to know whether rapid correction of chronic hyperglycemia adversely affects hospital outcomes. We have some understanding of how to control or prevent change of overall glycemia, and less understanding of how to control variability. Each may be associated with outcomes, and each may be detected by a high SD, but it remains uncertain whether intervention to prevent either pattern of changing glycemia would affect outcomes.

High-dose insulin in experimental myocardial infarction in rabbits: protection against effects of hyperglycaemia

INTRODUCTION

Hyperglycaemia at the time of acute myocardial infarction (AMI) is a predictor of survival and is associated with increased mortality and morbidity in patients with or without diabetes mellitus. On the other hand, insulin has been shown to reduce myocardial injury in experimental studies but its benefits have not been confirmed in clinical studies.

METHODS

The isolated perfused heart model was used to examine the direct effect of incremental doses of insulin and varying degrees of hyperglycaemia on infarct size and cardiomyocyte apoptosis in rabbit hearts. The rabbit hearts were subjected to 30-min ischaemia and 2.5-h reperfusion.

RESULTS

Insulin, given alone just before reperfusion, dramatically reduced infarct size in a dose-dependent manner (75-300 μU/ml) during experimental myocardial infarction (46%±2% to 10.9%±3%, P<.001). Acutely elevated glucose levels (33 mmol/L) induced a significantly greater infarct size and cardiomyocyte apoptosis compared to hearts subjected to normal glucose levels. On the other hand, high-dose insulin (300 μU/ml) given 5 min before reperfusion attenuated the extent of infarction and reduced apoptosis in hearts that were exposed to high glucose levels.

CONCLUSION

Acutely elevated levels of glucose induced larger infarct area during ischaemia-reperfusion, and this is mediated through proapoptotic pathways. Insulin, when given just before reperfusion, confers cardioprotection in a dose-dependent manner and reverses the detrimental effect of acute hyperglycaemia. High-dose insulin as well as maintaining normoglycaemia remain important factors that improve outcomes following myocardial infarction.

Increased propensity for cell death in diabetic human heart is mediated by mitochondrial-dependent pathways

Progressive energy deficiency and loss of cardiomyocyte numbers are two prominent factors that lead to heart failure in experimental models. Signals that mediate cardiomyocyte cell death have been suggested to come from both extrinsic (e.g., cytokines) and intrinsic (e.g., mitochondria) sources, but the evidence supporting these mechanisms remains unclear, and virtually nonexistent in humans. In this study, we investigated the sensitivity of the mitochondrial permeability transition pore (mPTP) to calcium (Ca(2+)) using permeabilized myofibers of right atrium obtained from diabetic (n = 9) and nondiabetic (n = 12) patients with coronary artery disease undergoing nonemergent coronary revascularization surgery. Under conditions that mimic the energetic state of the heart in vivo (pyruvate, glutamate, malate, and 100 μM ADP), cardiac mitochondria from diabetic patients show an increased sensitivity to Ca(2+)-induced mPTP opening compared with nondiabetic patients. This increased mPTP Ca(2+) sensitivity in diabetic heart mitochondria is accompanied by a substantially greater rate of mitochondrial H(2)O(2) emission under identical conditions, despite no differences in respiratory capacity under these conditions or mitochondrial enzyme content. Activity of the intrinsic apoptosis pathway mediator caspase-9 was greater in diabetic atrial tissue, whereas activity of the extrinsic pathway mediator caspase-8 was unchanged between groups. Furthermore, caspase-3 activity was not significantly increased in diabetic atrial tissue. These data collectively suggest that the myocardium in diabetic patients has a greater overall propensity for mitochondrial-dependent cell death, possibly as a result of metabolic stress-imposed changes that have occurred within the mitochondria, rendering them more susceptible to insults such as Ca(2+) overload. In addition, they lend further support to the notion that mitochondria represent a viable target for future therapies directed at ameliorating heart failure and other comorbidities that come with diabetes.

Insulin regulation of glutathione and contractile phenotype in diabetic rat ventricular myocytes

Cardiovascular complications of diabetes mellitus involve oxidative stress and profound changes in reduced glutathione (GSH), an essential tripeptide that controls many redox-sensitive cell functions. This study examined regulation of GSH by insulin to identify mechanisms controlling cardiac redox state and to define the functional impact of GSH depletion. GSH was measured by fluorescence microscopy in ventricular myocytes isolated from Sprague-Dawley rats made diabetic by streptozotocin, and video and confocal microscopy were used to measure mechanical properties and Ca(2+) transients, respectively. Spectrophotometric assays of tissue extracts were also done to measure the activities of enzymes that control GSH levels. Four weeks after injection of streptozotocin, mean GSH concentration ([GSH]) in isolated diabetic rat myocytes was approximately 36% less than in control, correlating with decreased activities of two major enzymes regulating GSH levels: glutathione reductase and gamma-glutamylcysteine synthetase. Treatment of diabetic rat myocytes with insulin normalized [GSH] after a delay of 3-4 h. A more rapid but transient upregulation of [GSH] occurred in myocytes treated with dichloroacetate, an activator of pyruvate dehydrogenase. Inhibitor experiments indicated that insulin normalized [GSH] via the pentose pathway and gamma-glutamylcysteine synthetase, although the basal activity of glucose-6-phosphate dehydrogenase was not different between diabetic and control hearts. Diabetic rat myocytes were characterized by significant mechanical dysfunction that correlated with diminished and prolonged Ca(2+) transients. This phenotype was reversed by in vitro treatment with insulin and also by exogenous GSH or N-acetylcysteine, a precursor of GSH. Our data suggest that insulin regulates GSH through pathways involving de novo GSH synthesis and reduction of its oxidized form. It is proposed that a key function of glucose metabolism in heart is to supply reducing equivalents required to maintain adequate GSH levels for the redox control of Ca(2+) handling proteins and contraction.

Impairment of human ether-à-go-go-related gene (HERG) K+ channel function by hypoglycemia and hyperglycemia. Similar phenotypes but different mechanisms

Hyperglycemia and hypoglycemia both can cause prolongation of the Q-T interval and ventricular arrhythmias. Here we studied modulation of human ether-à-go-go-related gene (HERG) K(+) channel, the major molecular component of delayed rectifier K(+) current responsible for cardiac repolarization, by glucose in HEK293 cells using whole-cell patch clamp techniques. We found that both hyperglycemia (extracellular glucose concentration [Glu](o) = 10 or 20 mm) and hypoglycemia ([Glu](o) = 2.5, 1, or 0 mm) impaired HERG function by reducing HERG current (I(HERG)) density, as compared with normoglycemia ([Glu](o) = 5 mm). Complete inhibition of glucose metabolism (glycolysis and oxidative phosphorylation) by 2-deoxy-d-glucose mimicked the effects of hypoglycemia, but inhibition of glycolysis or oxidative phosphorylation alone did not cause I(HERG) depression. Depletion of intracellular ATP mimicked the effects of hypoglycemia, and replacement of ATP by GTP or non-hydrolysable ATP failed to prevent the effects. Inhibition of oxidative phosphorylation by NaCN or application of antioxidants vitamin E or superoxide dismutase mimetic (Mn(III) tetrakis(4-benzoic acid) porphyrin chloride) abrogated and incubation with xanthine/xanthine oxidase mimicked the effects of hyperglycemia. Hyperglycemia or xanthine/xanthine oxidase markedly increased intracellular levels of reactive oxygen species, as measured by 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (CM-H(2)DCFDA) fluorescence dye, and this increase was prevented by NaCN, vitamin E, or Mn(III) tetrakis(4-benzoic acid) porphyrin chloride. We conclude that ATP, derived from either glycolysis or oxidative phosphorylation, is critical for normal HERG function; depression of I(HERG) in hypoglycemia results from underproduction of ATP and in hyperglycemia from overproduction of reactive oxygen species. Impairment of HERG function might contribute to Q-T prolongation caused by hypoglycemia and hyperglycemia.