Mitochondrial respiratory impairment in streptozotocin-induced diabetic rat heart

Electrophilic Aldehyde 4-Hydroxy-2-Nonenal Mediated Signaling and Mitochondrial Dysfunction

Reactive oxygen species (ROS), a by-product of aerobic life, are highly reactive molecules with unpaired electrons. The excess of ROS leads to oxidative stress, instigating the peroxidation of polyunsaturated fatty acids (PUFA) in the lipid membrane through a free radical chain reaction and the formation of the most bioactive aldehyde, known as 4-hydroxynonenal (4-HNE). 4-HNE functions as a signaling molecule and toxic product and acts mainly by forming covalent adducts with nucleophilic functional groups in proteins, nucleic acids, and lipids. The mitochondria have been implicated as a site for 4-HNE generation and adduction. Several studies clarified how 4-HNE affects the mitochondria's functions, including bioenergetics, calcium homeostasis, and mitochondrial dynamics. Our research group has shown that 4-HNE activates mitochondria apoptosis-inducing factor (AIFM2) translocation and facilitates apoptosis in mice and human heart tissue during anti-cancer treatment. Recently, we demonstrated that a deficiency of SOD2 in the conditional-specific cardiac knockout mouse increases ROS, and subsequent production of 4-HNE inside mitochondria leads to the adduction of several mitochondrial respiratory chain complex proteins. Moreover, we highlighted the physiological functions of HNE and discussed their relevance in human pathophysiology and current discoveries concerning 4-HNE effects on mitochondria.

Temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Mitochondrial complex I impairment, and H2O2 and NO productions as early subcellular events

The aim of this work was to study the early events that occur in heart mitochondria and to analyse the temporal evolution of cardiac mitochondrial dysfunction in a type 1 diabetes model. Male Wistar rats were injected with Streptozotocin (STZ, single dose, 60 mg × kg^-1, i.p.) and hyperglycemic state was confirmed 72 h later. The animals were sacrificed 10 or 14 days after STZ-injection. Heart mitochondrial state 3 O₂ consumption sustained by malate-glutamate (21%) or by succinate (16%), and complexes I-III (27%), II-III (24%) and IV (22%) activities were lower in STZ group, when animals were sacrificed at day 14, i.e. ~11 days of hyperglycemia. In contrast, after 10 days of STZ-injection (~7 days of hyperglycemia), only the state 3 O₂ consumption sustained by malate-glutamate (23%) and its corresponding respiratory control (30%) were lower in diabetic rats, in accordance with complex I-III activity reduction (17%). Therefore, this time (~7 days of hyperglycemia) has been considered as an "early stage" of cardiac mitochondrial dysfunction. At this point, mitochondrial production rates of H₂O₂ (117%), NO (30%) and ONOO^- (~225%), and mtNOS expression (29%) were higher; and mitochondrial SOD activity (15%) and [GSH + GSSG] (28%) were lower in diabetic rats. Linear correlations between the modified mitochondrial parameters and glycemias were observed. PGC-1α expression was similar between groups, suggesting that mitochondrial biogenesis was not triggered in this initial phase of mitochondrial dysfunction. Consequently, complex I, H₂O₂ and NO could be considered early subcellular signals of cardiac mitochondrial dysfunction, with NO and H₂O₂ being located upstream de novo synthesis of mitochondria.

Mitochondrial Fission and Mitophagy Coordinately Restrict High Glucose Toxicity in Cardiomyocytes

Hyperglycemia-induced mitochondrial dysfunction plays a key role in the pathogenesis of diabetic cardiomyopathy. Injured mitochondrial segments are separated by mitochondrial fission and eliminated by autophagic sequestration and subsequent degradation in the lysosome, a process termed mitophagy. However, it remains poorly understood how high glucose affects the activities of, and the relationship between, mitochondrial fission and mitophagy in cardiomyocytes. In this study, we determined the functional roles of mitochondrial fission and mitophagy in hyperglycemia-induced cardiomyocyte injury. High glucose (30 mM, HG) reduced mitochondrial connectivity and particle size and increased mitochondrial number in neonatal rat ventricular cardiomyocytes, suggesting an enhanced mitochondrial fragmentation. SiRNA knockdown of the pro-fission factor dynamin-related protein 1 (DRP1) restored mitochondrial size but did not affect HG toxicity, and Mdivi-1, a DRP1 inhibitor, even increased HG-induced cardiomyocyte injury, as shown by superoxide production, mitochondrial membrane potential and cell death. However, DRP1 overexpression triggered mitochondrial fragmentation and mitigated HG-induced cardiomyocyte injury, suggesting that the increased mitochondrial fission is beneficial, rather than detrimental, to cardiomyocytes cultured under HG conditions. This is in contrast to the prevailing hypothesis that mitochondrial fragmentation mediates or contributes to HG cardiotoxicity. Meanwhile, HG reduced mitophagy flux as determined by the difference in the levels of mitochondria-associated LC3-II or the numbers of mitophagy foci indicated by the novel dual fluorescent reporter mt-Rosella in the absence and presence of the lysosomal inhibitors. The ability of HG to induce mitochondrial fragmentation and inhibit mitophagy was reproduced in adult mouse cardiomyocytes. Overexpression of Parkin, a positive regulator of mitophagy, or treatment with CCCP, a mitochondrial uncoupler, induced mitophagy and attenuated HG-induced cardiomyocyte death, while Parkin knockdown had opposite effects, suggesting an essential role of mitophagy in cardiomyocyte survival under HG conditions. Strikingly, Parkin overexpression increased mitochondrial fragmentation, while DRP1 overexpression accelerated mitophagy flux, demonstrating a reciprocal activation loop that controls mitochondrial fission and mitophagy. Thus, strategies that promote the mutual positive interaction between mitochondrial fission and mitophagy while simultaneously maintain their levels within the physiological range would be expected to improve mitochondrial health, alleviating hyperglycemic cardiotoxicity.

Novel Dual-Fluorescent Mitophagy Reporter Reveals a Reduced Mitophagy Flux in Type 1 Diabetic Mouse Heart

CONTEXT

Patients with diabetes are susceptible to heart failure. Defective mitochondria can cause cardiac damage. Mitochondrial autophagy or mitophagy is a quality control mechanism that eliminates dysfunctional mitochondria through lysosome degradation. Mitophagy is essential for maintaining a pool of healthy mitochondria for normal cardiac function. However, the effect of diabetes on the functional status of cardiac mitophagy remains unclear.

OBJECTIVE

To determine and compare cardiac mitophagy flux between diabetic and nondiabetic mice.

METHODS

Using a novel dual fluorescent mitophagy reporter termed mt-Rosella, we labeled and traced mitochondrial fragments that are sequestered by the autophagosome and delivered to and degraded in the lysosome.

RESULTS

Mitophagic activity was reduced in high-glucose-treated cardiomyocytes and in the heart tissue of type 1 diabetic mice.

CONCLUSIONS

Mitophagy was impaired in the heart of diabetic mice, suggesting that restoring or accelerating mitophagy flux may be a useful strategy to reduce cardiac injury caused by diabetes.

Role of Mitophagy in Cardiovascular Disease

Cardiovascular disease is the leading cause of mortality worldwide, and mitochondrial dysfunction is the primary contributor to these disorders. Recent studies have elaborated on selective autophagy-mitophagy, which eliminates damaged and dysfunctional mitochondria, stabilizes mitochondrial structure and function, and maintains cell survival and growth. Numerous recent studies have reported that mitophagy plays an important role in the pathogenesis of various cardiovascular diseases. This review summarizes the mechanisms underlying mitophagy and advancements in studies on the role of mitophagy in cardiovascular disease.

Exercise enhances cardiac function by improving mitochondrial dysfunction and maintaining energy homoeostasis in the development of diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is a major cause of morbidity and mortality in diabetic patients. Reactive oxygen species (ROS) produced by oxidative stress play an important role in the development of DCM. DCM involves abnormal energy metabolism, thereby reducing energy production. Exercise has been reported to be effective in protecting the heart against ROS accumulation during the development of DCM. We hypothesize that the AMPK/PGC-1α axis may play a crucial role in exercise-induced bioenergetic metabolism and aerobic respiration on oxidative stress parameters in the development of diabetic cardiomyopathy. Using a streptozotocin/high-fat diet mouse to generate a diabetic model, our aim was to evaluate the effects of exercise on the cardiac function, mitochondrial oxidative capacity, mitochondrial function, and cardiac expression of PGC-1α. Mice fed a high-fat diet were given MO-siPGC-1α or treated with AMPK inhibitor. Mitochondrial structure and effects of switching between the Warburg effect and aerobic respiration were analysed. Exercise improved blood pressure and systolic dysfunction in diabetic mouse hearts. The beneficial effects of exercise were also observed in a mitochondrial function study, as reflected by an enhanced oxidative phosphorylation level, increased membrane potential, and decreased ROS level and oxygen consumption. On the other hand, depletion of PGC-1α attenuated the effects of exercise on the enhancement of mitochondrial function. In addition, PGC-1α may be responsible for reversing the Warburg effect to aerobic respiration, thus enhancing mitochondrial metabolism and energy homoeostasis. In this study, we demonstrate the protective effects of exercise on shifting energy metabolism from fatty acid oxidation to glucose oxidation in an established diabetic stage. These data suggest that exercise is effective at ameliorating diabetic cardiomyopathy by improving mitochondrial function and reducing metabolic disturbances.

Cardioprotection induced by remote ischemic preconditioning preserves the mitochondrial respiratory function in acute diabetic myocardium

A 2×2 factorial design was used to evaluate possible preservation of mitochondrial functions in two cardioprotective experimental models, remote ischemic preconditioning and streptozotocin-induced diabetes mellitus, and their interaction during ischemia/reperfusion injury (I/R) of the heart. Male Wistar rats were randomly allocated into four groups: control (C), streptozotocin-induced diabetic (DM), preconditioned (RPC) and preconditioned streptozotocin-induced diabetic (DM+RPC). RPC was conducted by 3 cycles of 5-min hind-limb ischemia and 5-min reperfusion. DM was induced by a single dose of 65 mg/kg streptozotocin. Isolated hearts were exposed to ischemia/reperfusion test according to Langendorff. Thereafter mitochondria were isolated and the mitochondrial respiration was measured. Additionally, the ATP synthase activity measurements on the same preparations were done. Animals of all groups subjected to I/R exhibited a decreased state 3 respiration with the least change noted in DM+RPC group associated with no significant changes in state 2 respiration. In RPC, DM and DM+RPC group, no significant changes in the activity of ATP synthase were observed after I/R injury. These results suggest that the endogenous protective mechanisms of RPC and DM do preserve the mitochondrial function in heart when they act in combination.

Diabetes impairs heart mitochondrial function without changes in resting cardiac performance

Diabetes is a chronic disease associated to a cardiac contractile dysfunction that is not attributable to underlying coronary artery disease or hypertension, and could be consequence of a progressive deterioration of mitochondrial function. We hypothesized that impaired mitochondrial function precedes Diabetic Cardiomyopathy. Thus, the aim of this work was to study the cardiac performance and heart mitochondrial function of diabetic rats, using an experimental model of type I Diabetes. Rats were sacrificed after 28days of Streptozotocin injection (STZ, 60mgkg^-1, ip.). Heart O₂ consumption was declined, mainly due to the impairment of mitochondrial O₂ uptake. The mitochondrial dysfunction observed in diabetic animals included the reduction of state 3 respiration (22%), the decline of ADP/O ratio (∼15%) and the decrease of the respiratory complexes activities (22-26%). An enhancement in mitochondrial H₂O₂ (127%) and NO (23%) production rates and in tyrosine nitration (58%) were observed in heart of diabetic rats, with a decrease in Mn-SOD activity (∼50%). Moreover, a decrease in contractile response (38%), inotropic (37%) and lusitropic (58%) reserves were observed in diabetic rats only after a β-adrenergic stimulus. Therefore, in conditions of sustained hyperglycemia, heart mitochondrial O₂ consumption and oxidative phosphorylation efficiency are decreased, and H₂O₂ and NO productions are increased, leading to a cardiac compromise against a work overload. This mitochondrial impairment was detected in the absence of heart hypertrophy and of resting cardiac performance changes, suggesting that mitochondrial dysfunction could precede the onset of diabetic cardiac failure, being H₂O₂, NO and ATP the molecules probably involved in mitochondrion-cytosol signalling.

Mitochondrial dysfunction and its impact on diabetic heart

Mitochondrial dysfunction and associated oxidative stress are strongly linked to cardiovascular, neurodegenerative, and age associated disorders. More specifically cardiovascular diseases are common in patients with diabetes and significant contributor to the high mortality rates associated with diabetes. Studies have shown that the heart failure risk is increased in diabetic patients even after adjusting for coronary artery disease and hypertension. Although the actual basis of the increased heart failure risk is multifactorial, increasing evidences suggest that imbalances in mitochondrial function and associated oxidative stress play an important role in this process. This review summarizes these abnormalities in mitochondrial function and discusses potential underlying mechanisms. This article is part of a Special Issue entitled: Oxidative Stress and Mitochondrial Quality in Diabetes/Obesity and Critical Illness Spectrum of Diseases - edited by P. Hemachandra Reddy.

Mitochondrial quality control in the diabetic heart

Diabetes is a well-known risk factor for heart failure. Diabetic heart damage is closely related to mitochondrial dysfunction and increased ROS generation. However, clinical trials have shown no effects of antioxidant therapies on heart failure in diabetic patients, suggesting that simply antagonizing existing ROS by antioxidants is not sufficient to reduce diabetic cardiac injury. A potentially more effective treatment strategy may be to enhance the overall capacity of mitochondrial quality control to maintain a pool of healthy mitochondria that are needed for supporting cardiac contractile function in diabetic patients. Mitochondrial quality is controlled by a number of coordinated mechanisms including mitochondrial fission and fusion, mitophagy and biogenesis. The mitochondrial damage consistently observed in the diabetic hearts indicates a failure of the mitochondrial quality control mechanisms. Recent studies have demonstrated a crucial role for each of these mechanisms in cardiac homeostasis and have begun to interrogate the relative contribution of insufficient mitochondrial quality control to diabetic cardiac injury. In this review, we will present currently available literature that links diabetic heart disease to the dysregulation of major mitochondrial quality control mechanisms. We will discuss the functional roles of these mechanisms in the pathogenesis of diabetic heart disease and their potentials for targeted therapeutical manipulation.

Metabolic dysfunction in diabetic cardiomyopathy

Diabetic cardiomyopathy (DCM) is defined as cardiac disease independent of vascular complications during diabetes. The number of new cases of DCM is rising at epidemic rates in proportion to newly diagnosed cases of diabetes mellitus (DM) throughout the world. DCM is a heart failure syndrome found in diabetic patients that is characterized by left ventricular hypertrophy and reduced diastolic function, with or without concurrent systolic dysfunction, occurring in the absence of hypertension and coronary artery disease. DCM and other diabetic complications are caused in part by elevations in blood glucose and lipids, characteristic of DM. Although there are pathological consequences to hyperglycemia and hyperlipidemia, the combination of the two metabolic abnormalities potentiates the severity of diabetic complications. A natural competition exists between glucose and fatty acid metabolism in the heart that is regulated by allosteric and feedback control and transcriptional modulation of key limiting enzymes. Inhibition of these glycolytic enzymes not only controls flux of substrate through the glycolytic pathway, but also leads to the diversion of glycolytic intermediate substrate through pathological pathways, which mediate the onset of diabetic complications. The present review describes the limiting steps involved in the development of these pathological pathways and the factors involved in the regulation of these limiting steps. Additionally, therapeutic options with demonstrated or postulated effects on DCM are described.

Diabetic cardiomyopathy: mechanisms and new treatment strategies targeting antioxidant signaling pathways

Cardiovascular disease is the primary cause of morbidity and mortality among the diabetic population. Both experimental and clinical evidence suggest that diabetic subjects are predisposed to a distinct cardiomyopathy, independent of concomitant macro- and microvascular disorders. 'Diabetic cardiomyopathy' is characterized by early impairments in diastolic function, accompanied by the development of cardiomyocyte hypertrophy, myocardial fibrosis and cardiomyocyte apoptosis. The pathophysiology underlying diabetes-induced cardiac damage is complex and multifactorial, with elevated oxidative stress as a key contributor. We now review the current evidence of molecular disturbances present in the diabetic heart, and their role in the development of diabetes-induced impairments in myocardial function and structure. Our focus incorporates both the contribution of increased reactive oxygen species production and reduced antioxidant defenses to diabetic cardiomyopathy, together with modulation of protein signaling pathways and the emerging role of protein O-GlcNAcylation and miRNA dysregulation in the progression of diabetic heart disease. Lastly, we discuss both conventional and novel therapeutic approaches for the treatment of left ventricular dysfunction in diabetic patients, from inhibition of the renin-angiotensin-aldosterone-system, through recent evidence favoring supplementation of endogenous antioxidants for the treatment of diabetic cardiomyopathy. Novel therapeutic strategies, such as gene therapy targeting the phosphoinositide 3-kinase PI3K(p110α) signaling pathway, and miRNA dysregulation, are also reviewed. Targeting redox stress and protective protein signaling pathways may represent a future strategy for combating the ever-increasing incidence of heart failure in the diabetic population.

Evaluation of the cardiolipin biosynthetic pathway and its interactions in the diabetic heart

AIMS

We have previously reported alterations in cardiolipin content and inner mitochondrial membrane (IMM) proteomic make-up specifically in interfibrillar mitochondria (IFM) in the type 1 diabetic heart; however, the mechanism underlying this alteration is unknown. The goal of this study was to determine how the cardiolipin biosynthetic pathway and cardiolipin-IMM protein interactions are impacted by type 1 diabetes mellitus.

MAIN METHODS

Male FVB mice were made diabetic by multiple low-dose streptozotocin injections and sacrificed five weeks post-diabetic onset. Messenger RNA was measured and cardiac mitochondrial subpopulations were isolated. Further mitochondrial functional experimentation included evaluating the protein expression of the enzymes directly responsible for cardiolipin biosynthesis, as well as ATP synthase activity. Interactions between cardiolipin and ATP synthase subunits were also examined.

KEY FINDINGS

Western blot analysis revealed a significant decrease in cardiolipin synthase (CRLS) protein content in diabetic IFM, with a concomitant decrease in its activity. ATP synthase activity was also significantly decreased. We identified two novel direct interactions between two subunits of the ATP synthase F0 complex (ATP5F1 and ATP5H), both of which were significantly decreased in diabetic IFM.

SIGNIFICANCE

Overall, these results indicate that type 1 diabetes mellitus negatively impacts the cardiolipin biosynthetic pathway specifically at CRLS, contributing to decreased cardiolipin content and loss of interactions with key ATP synthase F0 complex constituents in the IFM.

Effect of Piper sarmentosum Extract on the Cardiovascular System of Diabetic Sprague-Dawley Rats: Electron Microscopic Study

Although Piper sarmentosum (PS) is known to possess the antidiabetic properties, its efficacy towards diabetic cardiovascular tissues is still obscured. The present study aimed to observe the electron microscopic changes on the cardiac tissue and proximal aorta of experimental rats treated with PS extract. Thirty-two male Sprague-Dawley rats were divided into four groups: untreated control group (C), PS-treated control group (CTx), untreated diabetic group (D), and PS-treated diabetic group (DTx). Intramuscular injection of streptozotocin (STZ, 50 mg/kg body weight) was given to induce diabetes. Following 28 days of diabetes induction, PS extract (0.125 g/kg body weight) was administered orally for 28 days. Body weight, fasting blood glucose, and urine glucose levels were measured at 4-week interval. At the end of the study, cardiac tissues and the aorta were viewed under transmission electron microscope (TEM). DTx group showed increase in body weight and decrease in fasting blood glucose and urine glucose level compared to the D group. Under TEM study, DTx group showed lesser ultrastructural degenerative changes in the cardiac tissues and the proximal aorta compared to the D group. The results indicate that PS restores ultrastructural integrity in the diabetic cardiovascular tissues.

Characterization of proteomic changes in cardiac mitochondria in streptozotocin-diabetic rats using iTRAQ™ isobaric tags

Diabetes now affects more than 5% of the world's population and heart failure is the most common cause of death amongst diabetic patients. Accumulating evidence supports a view that myocardial mitochondrial structural and functional changes are central to the onset of diabetic heart failure, but the exact nature of these changes at the proteomic level remains unclear.Here we report on proteomic changes in diabetic rat heart mitochondria following 120 days of streptozotocin-diabetes using the recently developed iTRAQ™ labeling method, which permits quantification of proteins directly from complex mixtures, bypassing the limitations associated with gel-based methods such as 2-DE. Of 252 unique proteins identified, 144 were represented in at least three of six individual paired experiments. Relative amounts of 65 proteins differed significantly between the groups, confirming that the cardiac mitochondrial proteome is indeed impacted by diabetes. The most significant changes were increased protein levels of enzymes involved in mitochondrial oxidation of long-chain fatty acids, which was also confirmed by enzyme assays, and decreased levels of multiple enzymes involved in oxidative phosphorylation and catabolism of short-chain fatty acids and branched-chain amino acids. We also found significant changes in levels of several enzymes linked to oxidative stress.

Role of differential signaling pathways and oxidative stress in diabetic cardiomyopathy

Diabetes mellitus increases the risk of heart failure independently of underlying coronary artery disease, and many believe that diabetes leads to cardiomyopathy. The underlying pathogenesis is partially understood. Several factors may contribute to the development of cardiac dysfunction in the absence of coronary artery disease in diabetes mellitus. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Hyperglycemia-induced oxidative stress is a major risk factor for the development of micro-vascular pathogenesis in the diabetic myocardium, which results in myocardial cell death, hypertrophy, fibrosis, abnormalities of calcium homeostasis and endothelial dysfunction. Diabetes-mediated biochemical changes show cross-interaction and complex interplay culminating in the activation of several intracellular signaling molecules. Diabetic cardiomyopathy is characterized by morphologic and structural changes in the myocardium and coronary vasculature mediated by the activation of various signaling pathways. This review focuses on the oxidative stress and signaling pathways in the pathogenesis of the cardiovascular complications of diabetes, which underlie the development and progression of diabetic cardiomyopathy.

Chronic high glucose downregulates mitochondrial calpain 10 and contributes to renal cell death and diabetes-induced renal injury

Whereas most calpains are cytosolic proteases, calpain 10 is resident in mitochondria and is important in mitochondrial homeostasis. Because calpain 10 has been implicated in type 2 diabetes, we studied its possible role in diabetes-induced renal dysfunction. We treated renal proximal tubular cells with high glucose (17 mmol/l) and found decreased mitochondrial calpain 10 mRNA and protein at 96 h compared with cells incubated with 0 or 5 mmol/l glucose or 17 mmol/l D-mannitol. High glucose increased mitochondrial calpain 10 substrates (NDUFB8 and ATP synthase β), decreased basal and uncoupled respiration, and initiated cell apoptosis as indicated by cleaved caspase 3 and nuclear condensation. Renal calpain 10 protein and mRNA were specifically decreased in streptozotocin-induced diabetic rats with kidney dysfunction, and in diabetic ob/ob mice. In agreement with our in vitro data, the kidneys of streptozotocin-induced diabetic rats had elevated calpain 10 substrates and cleaved caspase 3. Finally, specific siRNA-induced knockdown of calpain 10 in the proximal tubules of control rats resulted in decreased renal function as evidenced by increased serum creatinine, and increased caspase 3 cleavage compared with rats receiving scrambled siRNA. Thus, the glucose-induced loss of calpain 10 in vivo results in renal cell apoptosis and organ failure through accumulation of mitochondrial calpain 10 substrates and mitochondrial dysfunction. Whether this is a major cause of the decreased renal function in diabetic nephropathy will require further studies.

Physiological effects of Type 2 diabetes on mRNA processing and gene expression

Characteristics of Type 2 diabetes include both high blood glucose (hyperglycemia) and raised cholesterol and triglycerides (hyperlipidemia). Several studies have now shown that both hyperglycemia and hyperlipidemia can alter gene expression by disrupting physiological mechanisms of gene regulation, including alternative mRNA splicing, epigenetic gene regulation and miRNA-mediated regulation of gene expression. These processes may also be influenced by intracellular oxidative stress, which is increased in diabetes and in response to hyperglycemia and hyperlipidemia. Many pathways relevant to diabetes are affected by altered gene expression, including lipid and glucose metabolism and oxidative phosphorylation. This article considers how hyperglycemia and hyperlipidemia can alter gene expression in diabetes, which could potentially contribute to the worsening of the diabetic phenotype and diabetic complications.

Transmural differences in respiratory capacity across the rat left ventricle in health, aging, and streptozotocin-induced diabetes mellitus: evidence that mitochondrial dysfunction begins in the subepicardium

In diabetic cardiomyopathy, ventricular dysfunction occurs in the absence of hypertension or atherosclerosis and is accompanied by altered myocardial substrate utilization and depressed mitochondrial respiration. It is not known if mitochondrial function differs across the left ventricular (LV) wall in diabetes. In the healthy heart, the inner subendocardial region demonstrates higher rates of blood flow, oxygen consumption, and ATP turnover compared with the outer subepicardial region, but published transmural respirometric measurements have not demonstrated differences. We aim to measure mitochondrial function in Wistar rat LV to determine the effects of age, streptozotocin-diabetes, and LV layer. High-resolution respirometry measured indexes of respiration in saponin-skinned fibers dissected from the LV subendocardium and subepicardium of 3-mo-old rats after 1 mo of streptozotocin-induced diabetes and 4-mo-old rats following 2 mo of diabetes. Heart rate and heartbeat duration were measured under isoflurane-anesthesia using a fetal-Doppler, and transmission electron microscopy was employed to observe ultrastructural differences. Heart rate decreased with age and diabetes, whereas heartbeat duration increased with diabetes. While there were no transmural respirational differences in young healthy rat hearts, both myocardial layers showed a respiratory depression with age (30-40%). In 1-mo diabetic rat hearts only subepicardial respiration was depressed, whereas after 2 mo diabetes, respiration in subendocardial and subepicardial layers was depressed and showed elevated leak (state 2) respiration. These data provide evidence that mitochondrial dysfunction is first detectable in the subepicardium of diabetic rat LV, whereas there are measureable changes in LV mitochondria after only 4 mo of aging.

Role of 14-3-3 protein and oxidative stress in diabetic cardiomyopathy

Cardiovascular disease is a leading cause of death worldwide. Diabetes mellitus is a well-known and important risk factor for cardiovascular diseases. The occurrence of diabetic cardiomyopathy is independent of hypertension, coronary artery disease, or any other known cardiac diseases. There is growing evidence that excess generation of highly reactive free radicals, largely due to hyperglycemia, causes oxidative stress, which further exacerbates the development and progression of diabetes and its complications. Diabetic cardiomyopathy is characterized by morphologic and structural changes in the myocardium and coronary vasculature mediated by the activation of various signaling pathways. Myocardial apoptosis, hypertrophy and fibrosis are the most frequently proposed mechanisms to explain cardiac changes in diabetic cardiomyopathy. Mammalian 14-3-3 proteins are dimeric phosphoserine-binding proteins that participate in signal transduction and regulate several aspects of cellular biochemistry. 14-3-3 protein regulates diabetic cardiomyopathy via multiple signaling pathways. This review focuses on emerging evidence suggesting that 14-3-3 protein plays a key role in the pathogenesis of the cardiovascular complications of diabetes, which underlie the development and progression of diabetic cardiomyopathy.

In vivo cardioprotection by S-nitroso-2-mercaptopropionyl glycine

The reversible S-nitrosation and inhibition of mitochondrial complex I is a potential mechanism of cardioprotection, recruited by ischemic preconditioning (IPC), S-nitrosothiols, and nitrite. Previously, to exploit this mechanism, the mitochondrial S-nitrosating agent S-nitroso-2-mercaptopropionyl glycine (SNO-MPG) was developed, and protected perfused hearts and isolated cardiomyocytes against ischemia-reperfusion (IR) injury. In the present study, the murine left anterior descending coronary artery (LAD) occlusion model of IR injury was employed, to determine the protective efficacy of SNO-MPG in vivo. Intraperitoneal administration of 1 mg/kg SNO-MPG, 30 min prior to occlusion, significantly reduced myocardial infarction and improved EKG parameters, following 30 min occlusion plus 2 or 24 h reperfusion. SNO-MPG protected to the same degree as IPC, and notably was also protective when administered at reperfusion. Cardioprotection was accompanied by increased mitochondrial protein S-nitrosothiol content, and inhibition of complex I, both of which were reversed after 2 h reperfusion. Finally, hearts from mice harboring a heterozygous mutation in the complex I NDUSF4 subunit were refractory to protection by either SNO-MPG or IPC, suggesting that a fully functional complex I, capable of reversible inhibition is critical for cardioprotection. Overall, these results are consistent with a role for mitochondrial S-nitrosation and complex I inhibition in the cardioprotective mechanism of IPC and SNO-MPG in vivo.

TZDs reduce mitochondrial ROS production and enhance mitochondrial biogenesis

Although it has been reported that thiazolidinediones (TZDs) may reduce cardiovascular events in type 2 diabetic patients, its precise mechanism is unclear. We previously demonstrated that hyperglycemia-induced production of reactive oxygen species from mitochondria (mtROS) contributed to the development of diabetic complications, and metformin normalized mt ROS production by induction of MnSOD and promotion of mitochondrial biogenesis by activating the PGC-1alpha pathway. In this study, we examined whether TZDs could inhibit hyperglycemia-induced mtROS production by activating the PGC-1alpha pathway. We revealed that pioglitazone and ciglitazone attenuated hyperglycemia-induced ROS production in human umbilical vein endothelial cells (HUVECs). Both TZDs increased the expression of NRF-1, TFAM and MnSOD mRNA. Moreover, pioglitazone increased mtDNA and mitochondrial density. These results suggest that TZDs normalize hyperglycemia-induced mtROS production by induction of MnSOD and promotion of mitochondrial biogenesis by activating PGC-1alpha. This phenomenon could contribute to the prevention of diabetic vascular complications.

Amino acid supplementation counteracts metabolic and functional damage in the diabetic rat heart

We aimed to assess whether a specific mixture of amino acid (AA) supplements counteracts the metabolic and functional changes in the streptozotocin (STZ)-induced diabetic rat heart model. Adult male Wistar rats were divided into 6 groups (n = 10 each) and treated for 43 days: nondiabetic controls, nondiabetic rats given an AA mixture (0.1 g/kg per day), diabetic rats (induced with 65 mg/kg STZ given intraperitoneally), diabetic rats given AAs, diabetic rats given insulin (5 IU/day given subcutaneously), and diabetic rats given insulin plus AAs. During treatment, glycemia and insulinemia levels were measured in all groups. Changes in enzyme (reduced nicotinamide adenine dinucleotide-dehydrogenase, cytochrome c oxidase) activities and myosin heavy chain (MHC) composition were measured in the left ventricle. In 5 rats contractile function was assessed by measuring maximal shortening velocity of skinned ventricular trabeculae and the expression of translational regulator mammalian target of rapamycin (mTOR) was also found. STZ-induced diabetes was associated with reduced myocardial contractility, overall loss of oxidative capacity, a shift toward a slower MHC phenotype, and decreased mTOR tissue content. All of these changes appeared to be reversible with insulin. AA supplements partially restored the myocardial and oxidative dysfunction and also increased mTOR tissue content. The combination of insulin and AAs did not have a synergistic effect on either enzymatic or functional profiles. We conclude that AA supplements may contribute to restoring the oxidative and contractile dysfunction of diabetic rat hearts, probably through an mTOR-insulin independent mechanism.

Impact of mitochondrial ROS production in the pathogenesis of diabetes mellitus and its complications

In this review, the impacts of mitochondrial reactive oxygen species (ROS) on diabetes and its complications are described. In endothelial cells, high-glucose treatment increases mitochondrial ROS and normalization of the ROS production by inhibitors of mitochondrial metabolism, or by overexpression of UCP-1 or MnSOD, prevents glucose-induced activation of PKC, formation of AGE, and accumulation of sorbitol, all of which are believed to be the main molecular mechanisms of diabetic complications. Glomerular hyperfiltration, one of the characteristics of early diabetic nephropathy, may be caused by mitochondrial ROS through activation of COX-2 gene transcription, followed by PGE2 overproduction. In pancreatic beta cells, hyperglycemia also increases mitochondrial ROS, which suppresses the first phase of glucose-induced insulin secretion, at least in part, through the suppression of GAPDH activity. In liver cells, similar to that in hyperglycemia, TNF-alpha increases mitochondrial ROS, which in turn activates apoptosis signal-regulating kinase 1 (ASK1) and c-jun NH2-terminal kinases (JNK), increases serine phosphorylation of IRS-1, and decreases insulin-stimulated tyrosine phosphorylation of IRS-1, leading to insulin resistance. These results suggest the importance of mitochondrial ROS in the pathogenesis of diabetes mellitus and its complications through modification of various cellular events in many tissues, including vessels, kidney, pancreatic beta cells, and liver.

Cardiac mitochondrial alterations induced by insulin deficiency and hyperinsulinaemia in rats: targeting membrane homeostasis with trimetazidine

The present study investigated the ability of trimetazidine (TMZ) to maintain cardiac mitochondrial function during the development of insulin deficiency and hyperinsulinaemia. The anti-ischaemic drug TMZ is known to increase phospholipid synthesis in cardiac membranes and to have a cardioprotective effect. Insulin deficiency was obtained by streptozotocin injection and hyperinsulinaemia was achieved via a fructose diet. Trimetazidine was incorporated into the diet (7.8 mg/day) and mitochondrial function was evaluated in skinned cardiac fibres. Insulin deficiency decreased mitochondrial affinity for ADP and the index of creatine kinase functional activity. This last alteration was partially prevented by TMZ treatment. Insulin deficiency strongly decreased n-3 polyunsaturated fatty acids, especially the docosahexaenoic acid (DHA) content, in cardiac and mitochondrial membranes, inducing a strong increase in the n-6/n-3 ratio. Trimetazidine treatment limited the increase in the n-6/n-3 ratio and prevented the decrease in DHA content in mitochondrial membranes. Insulin deficiency decreased glutamate- and palmitoylcarnitine-supported respiration. Hyperinsulinaemia affected neither mitochondrial affinity for ADP nor the index of creatine kinase functional activity. Hyperinsulinaemia slightly and significantly affected mitochondrial fatty acid composition, by a small increase the n-6/n-3 ratio. Trimetazidine did not modify membrane-bound mitochondrial function but increased the n-6/n-3 ratio. Moreover, hyperinsulinaemia decreased glutamate-supported respiration. In conclusion, modification of membrane homeostasis with TMZ partially prevented the alterations in fatty acid composition and function in cardiac mitochondria induced by insulin deficiency. Three months of hyperinsulinaemia did not modify membrane-bound mitochondrial function and had only slight effects on fatty acid composition.

Decreased complex II respiration and HNE-modified SDH subunit in diabetic heart

Several lines of research suggest that mitochondria play a role in the etiopathogenesis of diabetic cardiomyopathy, although the mechanisms involved are still debated. In the present study, we report that State 3 oxygen consumption decreases by approximately 35% with glutamate and by approximately 30% with succinate in mitochondria from diabetic rat hearts compared to controls. In these mitochondria the enzymatic activities of complex I and complex II are also decreased to a comparable extent. Western blot analysis of mitochondrial protein pattern using antibodies recognizing proteins modified by the lipid peroxidation product 4-hydroxynonenal indicates the FAD-containing subunit of succinate dehydrogenase as one of the targets of this highly reactive aldehyde. In rats diabetic for 6 or 12 weeks, insulin supplementation for 2 weeks decreases the level of protein modified by 4-hydroxynonenal and restores mitochondrial respiration and enzyme activity to control level. Taken together, these results: (1) indicate that 4-hydroxynonenal is endogenously produced within diabetic mitochondria and forms an adduct with selective mitochondrial proteins, (2) identify one of these proteins as a subunit of succinate dehydrogenase, and (3) provide strong evidence that insulin treatment can reverse and ameliorate free radical damage and mitochondrial function under diabetic conditions.

Oxidative stress and stress signaling: menace of diabetic cardiomyopathy

Cardiovascular disease is the most common cause of death in the diabetic population and is currently one of the leading causes of death in the United States and other industrialized countries. The health care expenses associated with cardiovascular disease are staggering, reaching more than 350 billion dollars in 2003. The risk factors for cardiovascular disease include high fat/cholesterol levels, alcoholism, smoking, genetics, environmental factors and hypertension, which are commonly used to gauge an individual's risk of cardiovascular disease and to track their progress during therapy. Most recently, these factors have become important in the early prevention of cardiovascular diseases. Oxidative stress, the imbalance between reactive oxygen species production and breakdown by endogenous antioxidants, has been implicated in the onset and progression of cardiovascular diseases such as congestive heart failure and diabetes-associated heart dysfunction (diabetic cardiomyopathy). Antioxidant therapy has shown promise in preventing the development of diabetic heart complications. This review focuses on recent advances in oxidative stress theory and antioxidant therapy in diabetic cardiomyopathy, with an emphasis on the stress signaling pathways hypothesized to be involved. Many of these stress signaling pathways lead to activation of reactive oxygen species, major players in the development and progression of diabetic cardiomyopathy.

Protection of hepatocyte mitochondrial ultrastructure and function by strict blood glucose control with insulin in critically ill patients

BACKGROUND

Maintenance of normoglycaemia by use of insulin reduces morbidity and mortality of patients in surgical intensive care. Studies on mitochondrial function in critical illness or diabetes suggest that effects of intensive insulin therapy on mitochondrial integrity contribute to the clinical benefits.

METHODS

Enzyme activities of the respiratory-chain complexes and oxidative-stress-sensitive glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were measured by spectrophotometry in 36 snap-frozen samples of liver and skeletal muscle obtained after death from patients who had been randomly assigned intensive (normoglycaemia) or conventional (hyperglycaemia) insulin therapy and who were similar in terms of admission diagnoses and causes of death. Mitochondrial ultrastructure was examined by electron microscopy in a random subgroup (n=20).

FINDINGS

In the liver, hypertrophic mitochondria with an increased number of abnormal and irregular cristae and reduced matrix electron density were observed in seven of nine conventionally treated patients. Only one of 11 patients given intensive insulin treatment had these morphological abnormalities (p=0.005). The effect on ultrastructure was associated with higher activities of respiratory-chain complex I (median 1.53 [IQR 1.14-3.01] vs 0.81 [0.54-1.43] U/g liver; p=0.008) and complex IV (1.69 [1.40-1.97] vs 1.16 [0.97-1.40] U/g; p=0.008) in the intensive group than in the conventional group. There was no detectable difference in GAPDH activity. In skeletal muscle, mitochondrial ultrastructure and function were not affected by intensive insulin therapy.

INTERPRETATION

Strict glycaemic control with intensive insulin therapy prevented or reversed ultrastructural and functional abnormalities of hepatocyte mitochondria. The lack of effect on skeletal-muscle mitochondria suggests a direct effect of glucose toxicity and glucose control, rather than of insulin, as the likely explanation.

RELEVANCE TO PRACTICE

Maintenance or restoration of mitochondrial function and cellular energetics is another therapeutic target, in addition to optimisation of cardiac output, systemic oxygen delivery, and regional blood flow, that might improve outcome for critically ill patients. Our findings could help to explain the mechanism underlying the reduction in mortality found when normoglycaemia was maintained with insulin, and further support use of intensive insulin therapy in this setting.

Exercise Attenuates Diabetes-Induced Ultrastructural Changes in Rat Cardiac Tissue

INTRODUCTION/PURPOSE

Exercise is an effective nonpharmacological treatment in the prevention of mortality and morbidity due to cardiovascular disease in Type I diabetes. This study sought to explore the effects of endurance exercise on the ultrastructural changes seen in diabetic cardiomyopathy.

METHODS

Seven-week-old rats were divided into three groups consisting of sedentary nondiabetic control, sedentary diabetic, and exercised diabetic animals. Diabetes was induced using streptozotocin injection, and the exercised animals were run daily on a treadmill for 9 wk. Changes in heart ultrastructure were analyzed using transmission electron microscopy.

RESULTS

Ultrastructural changes in the left ventricle produced by diabetes included changes in myofibrillar arrangements, disrupted mitochondria, and increased cytoplasmic area with an increase in lipid amounts and an increase in individual collagen fiber cross-sectional surface area. Also, an increase in heterochromatin lining the nuclear envelope and an increase in invaginations of the nuclear membrane were observed in cardiomyocytes from diabetic rats when compared with the nuclei from nondiabetic cells. Exercise was found to significantly attenuate the diabetes-induced changes in collagen fibrils, cytoplasmic area, and level of mitochondrial disruption. In contrast, exercise did not appear to significantly influence myofibril volume density, lipid accumulation, or nuclear deformities.

CONCLUSION

These findings indicate that exercise restores specific ultrastructural characteristics of diabetic cardiomyopathy returning them toward nondiabetic phenotypes, particularly in the mitochondria and extracellular matrix proteins.

Cardiac mitochondrial damage and biogenesis in a chronic model of type 1 diabetes

Diabetic cardiomyopathy is a common complication leading to heightened risk of heart failure and death. In the present report, we performed proteomic analysis on total cardiac proteins from the OVE26 mouse model of type 1 diabetes to identify protein changes that may contribute to diabetic cardiomyopathy. This analysis revealed that a surprising high proportion (12 of 20) of the altered proteins that could be identified by mass spectrometry were of mitochondrial origin. All but one of these proteins were upregulated by diabetes. Quantitative RT-PCR, performed for two of these proteins, indicated that part of the upregulation was attributed to increased messenger RNA levels. Morphological study of diabetic hearts showed significantly increased mitochondrial area and number as well as focal regions with severe damage to mitochondria. Diabetic mitochondria also showed reduced respiratory control ratio (9.63 +/- 0.20 vs. 6.13 +/- 0.41, P < 0.0001), apparently due to reduced state 3 rate, and diminished GSH level (5.5 +/- 0.9 vs. 8.2 +/- 2.5 micromol/mg protein, P < 0.05), indicating impaired mitochondrial function and increased oxidative stress. Further examination revealed increased mitochondrial DNA (1.03 +/- 0.18 vs. 0.69 +/- 0.13 relative copy number, P < 0.001) and a tendency to higher protein yield in OVE26 cardiac mitochondria, as well as increased mRNA level for mitochondrial transcription factor A and two mitochondrial encoded proteins. Taken together, these results show that mitochondria are a primary target in the diabetic heart, probably due to oxidative stress, and that this damage coincides with and may stimulate mitochondrial biogenesis.

Mitochondria respiration and susceptibility to ischemia-reperfusion injury in diabetic hearts

Cardiovascular complications are the primary cause of death for diabetic patients. Clinical and experimental observation has showed the development of dysfunctional cardiomyopathy as one of the main complications of diabetes. Whether the cardiomyopathy results from an increased susceptibility of cardiac tissue to ischemic insult or from a specific functional defect of cardiac mitochondria is a controversial issue. The investigation of possible functional defect in cardiac mitochondria from diabetic rats indicates a decline in state 3 respiration only in animals presenting a marked decrease in body weight. Mitochondria from rats presenting a level of hyperglycemia similar to diabetic animals but not the marked weight loss typical of the latter group show no decline in state 3 respiration, the values being indistinguishable from those of control mitochondria. Mitochondria from hyperglycemic rats, however, show a 15-20% increase in state 4 oxygen consumption but only when glutamate is used as energetic substrate, as compared to a 40-50% increase in state 4 respiration in mitochondria from diabetic rats under similar experimental conditions. This phenomenon is unrelated to diabetes duration, as it is observed at 2 as well as 8 weeks after diabetes onset. Taken together, these data argue against hyperglycemia per se being a direct cause of the decline in state 3 oxygen consumption observed in cardiac mitochondria of type-I diabetic rats and indicate that differences exist in cardiac mitochondrial function in rats generically labeled as diabetic. These differences can contribute to explain discrepancies in experimental results reported by various groups in the field and provide an additional parameter to be taken into consideration in evaluating the varying sensitivity of diabetic hearts to ischemia-reperfusion injury.

Docosahexaenoic acid affects insulin deficiency- and insulin resistance-induced alterations in cardiac mitochondria

The effect of docosahexaenoic acid (DHA) intake on cardiac mitochondrial function was evaluated in permeabilized fibers in insulin deficiency and insulin resistance in rats. The insulin-deficient state was obtained by streptozotocin injection 2 mo before investigations. Insulin resistance was obtained by feeding a 62% fructose diet for 3 mo. DHA was incorporated in the diet to modify the fatty acid composition of cardiac membranes, including mitochondria. Insulin deficiency decreased mitochondrial creatine kinase (mi-CK) activity and mitochondrial sensitivity to ADP. DHA intake prevented these alterations. Moreover, the insulin-deficient state significantly decreased n-3 polyunsaturated fatty acids (PUFA) and slightly increased n-6 PUFA in both cardiac and mitochondrial membranes, inducing a significant increase in the n-6-to-n-3 ratio. DHA intake maintained high myocardial and mitochondrial DHA content. Insulin deficiency also decreased glutamate- and palmitoylcarnitine-supported mitochondrial respiration, but DHA intake did not prevent these effects. In contrast, insulin resistance did not affect mi-CK activity or sensitivity to ADP. However, insulin resistance influenced the myocardial fatty acid composition with decreased n-6 and n-3 PUFA contents and increased monounsaturated fatty acid content. Only slight alterations were observed in mitochondrial fatty acid composition, and they were corrected by DHA intake. Moreover, insulin resistance decreased the glutamate-supported respiration, and DHA intake did not influence this effect. In conclusion, the impairment of cardiac mitochondrial function was more pronounced in the insulin-deficient state than in insulin resistance. The modification of fatty acid composition of cardiac and mitochondrial membranes by DHA partially prevented the mitochondrial alterations induced in the two models.

Quantitative protein profiling in heart mitochondria from diabetic rats

Quantitative protein profiling based on in vitro stable isotope labeling, two-dimensional polyacrylamide gel electrophoresis, and mass spectrometry is an accurate and reliable approach to measure simultaneously the relative abundance of many individual proteins within two different samples. In the present study, it was used to define a set of alterations caused by diabetes in heart mitochondria from streptozotocin-treated rats. We demonstrated that the expression of proteins from the myocardial tricarboxylic acid cycle was not altered in diabetes. However, up-regulation of the fatty acid beta-oxidation favored fatty acids over glucose as a source of acetyl CoA for the tricarboxylic acid cycle. Protein levels for several proteins involved in electron transport were modestly decreased. Whether this may depress overall ATP production remains to be established, since the protein level of ATP synthase seems to be unchanged. Other changes include down-regulation of protein levels for creatine kinase, voltage-dependent anion channel 1 (VDAC-1), HSP60, and Grp75. The mitochondria-associated level of albumin was decreased, while the level of catalase was substantially increased. All of the changes were evident as early as 1 week after streptozotocin administration. Taken together, these data point to a rapid and highly coordinated regulation of mitochondrial protein expression that occurs during the heart adaptation to diabetes.

Protein tyrosine nitration in the mitochondria from diabetic mouse heart. Implications to dysfunctional mitochondria in diabetes

Oxidative stress has been implicated in dysfunctional mitochondria in diabetes. Tyrosine nitration of mitochondrial proteins was observed under conditions of oxidative stress. We hypothesize that nitration of mitochondrial proteins is a common mechanism by which oxidative stress causes dysfunctional mitochondria. The putative mechanism of nitration in a diabetic model of oxidative stress and functional changes of nitrated proteins were studied in this work. As a source of mitochondria, alloxan-susceptible and alloxan-resistant mice were used. These inbred strains are distinguished by the differential ability to detoxify free radicals. A proteomic approach revealed significant similarity between patterns of tyrosine-nitrated proteins generated in the heart mitochondria under different in vitro and in vivo conditions of oxidative stress. This observation points to a common nitrating species, which may derive from different nitrating pathways in vivo and may be responsible for the majority of nitrotyrosine formed. Functional studies show that protein nitration has an adverse effect on protein function and that protection against nitration protects functional properties of proteins. Because proteins that undergo nitration are involved in major mitochondrial functions, such as energy production, antioxidant defense, and apoptosis, we concluded that tyrosine nitration of mitochondrial proteins may lead to dysfunctional mitochondria in diabetes.

Antidiabetic effects of chitosan oligosaccharides in neonatal streptozotocin-induced noninsulin-dependent diabetes mellitus in rats

The antidiabetic effect of chitosan oligosaccharide (COS) was investigated in neonatal streptozotocin (STZ)-induced noninsulin-dependent diabetes mellitus rats. The fasting glucose level was reduced by about 19% in diabetic rats after treatment with 0.3% COS. Glucose tolerance was lower in the diabetic group compared with the normal group. After diabetic rats had been treated with 0.3% COS for 4 weeks, glucose tolerance increased significantly versus the diabetic control group, and glucose-inducible insulin expression increased significantly. In addition, fed-triglyceride (TG) levels in diabetic rats drinking 0.3% COS were reduced by 49% compared with those in diabetic control rats. The cholesterol levels of animals treated with COS were reduced by about 10% in fed or fasting conditions versus the corresponding controls, although the difference was not statistically significant. It was found that COS has a TG-lowering effect in diabetic rats, and that COS reduces signs of diabetic cardiomyopathy such as vacuolation of mitochondria and the separation and degeneration of myofibrils. In conclusion, these results indicate that COS can be used as an antidiabetic agent because it increases glucose tolerance and insulin secretion and decreases TG.

Alterations in renal mitochondrial respiration in response to the reactive oxoaldehyde methylglyoxal

Chronic hyperglycemia has been linked to alterations in mitochondrial function, suggesting an important role in the pathophysiology of the complications of diabetes mellitus. In the diabetic kidney, ultrastructural changes in mitochondria are associated with impaired tubular function. The goal of this study was to determine if methylglyoxal (MGO), a dicarbonyl compound reaching high levels in hyperglycemic conditions, has direct toxicity for renal mitochondria. Intact mitochondria isolated from the renal cortex of rats were incubated with MGO to determine 1) its effect on mitochondrial respiration, 2) the conditions under which MGO exerts these effects, and 3) the potential mitochondrial targets of MGO influence. This study demonstrates that MGO has an inhibitory effect on both the tricarboxylic acid cycle and the electron respiratory chain. The modifications appear to be specific to certain mitochondrial proteins. Alterations of these proteins lead to disturbances in mitochondria that may play an important role in renal cellular toxicity and in the development of diabetic nephropathy.

Changes in microsomal membrane phospholipids and fatty acids and in activities of membrane-bound enzyme in diabetic rat heart

Diabetes mellitus is associated with alterations in lipid metabolism and cardiac dysfunction despite an absence of coronary arteriosclerotic changes. To investigate mechanisms of cardiac dysfunction in diabetic cardiomyopathy, we studied the relation between activities of membrane-bound enzymes and surrounding phospholipids in rats with diabetes induced with a single intravenous injection of streptozotocin (65 mg/kg). We found that total phospholipid content of sarcoplasmic reticulum membrane increased significantly 8 weeks after treatment with streptozotocin owing to increases in phosphatidylcholine and phosphatidylethanolamine, a decrease in arachidonic acid, and an increase in docosahexaenoic acid in the early stage of diabetes. Sarcolemmal Na+/K(+)-ATPase activity and the number of receptors decreased in isolated cardiomyocytes of diabetic rats 8 weeks after streptozotocin administration. The Ca2+ uptake of both sarcoplasmic reticulum and mitochondria decreased simultaneously in permeabilized, isolated cardiomyocytes from diabetic rats. The depression of membrane-bound enzyme activities was correlated with alterations in phospholipids, which are closely related to the microenvironment of membrane-bound enzymes and influence intracellular Ca2+ metabolism. Because these changes in phospholipids and fatty acids were reversible with insulin therapy, they are diabetes-specific and might be a cause of cardiac dysfunction in diabetes.