Defective sarcoplasmic reticular calcium transport in diabetic cardiomyopathy

Effects of non-coding RNAs and RNA-binding proteins on mitochondrial dysfunction in diabetic cardiomyopathy

Vascular complications are the main cause of diabetes mellitus-associated morbidity and mortality. Oxidative stress and metabolic dysfunction underly injury to the vascular endothelium and myocardium, resulting in diabetic angiopathy and cardiomyopathy. Mitochondrial dysfunction has been shown to play an important role in cardiomyopathic disruptions of key cellular functions, including energy metabolism and oxidative balance. Both non-coding RNAs and RNA-binding proteins are implicated in diabetic cardiomyopathy, however, their impact on mitochondrial dysfunction in the context of this disease is largely unknown. Elucidating the effects of non-coding RNAs and RNA-binding proteins on mitochondrial pathways in diabetic cardiomyopathy would allow further insights into the pathophysiological mechanisms underlying diabetic vascular complications and could facilitate the development of new therapeutic strategies. Stem cell-based models can facilitate the study of non-coding RNAs and RNA-binding proteins and their unique characteristics make them a promising tool to improve our understanding of mitochondrial dysfunction and vascular complications in diabetes.

The Diabetic Cardiorenal Nexus

The end-stage of the clinical combination of heart failure and kidney disease has become known as cardiorenal syndrome. Adverse consequences related to diabetes, hyperlipidemia, obesity, hypertension and renal impairment on cardiovascular function, morbidity and mortality are well known. Guidelines for the treatment of these risk factors have led to the improved prognosis of patients with coronary artery disease and reduced ejection fraction. Heart failure hospital admissions and readmission often occur, however, in the presence of metabolic, renal dysfunction and relatively preserved systolic function. In this domain, few advances have been described. Diabetes, kidney and cardiac dysfunction act synergistically to magnify healthcare costs. Current therapy relies on improving hemodynamic factors destructive to both the heart and kidney. We consider that additional hemodynamic solutions may be limited without the use of animal models focusing on the cardiomyocyte, nephron and extracellular matrices. We review herein potential common pathophysiologic targets for treatment to prevent and ameliorate this syndrome.

An Emerging Role of Defective Copper Metabolism in Heart Disease

Copper is an essential trace metal element that significantly affects human physiology and pathology by regulating various important biological processes, including mitochondrial oxidative phosphorylation, iron mobilization, connective tissue crosslinking, antioxidant defense, melanin synthesis, blood clotting, and neuron peptide maturation. Increasing lines of evidence obtained from studies of cell culture, animals, and human genetics have demonstrated that dysregulation of copper metabolism causes heart disease, which is the leading cause of mortality in the US. Defects of copper homeostasis caused by perturbed regulation of copper chaperones or copper transporters or by copper deficiency resulted in various types of heart disease, including cardiac hypertrophy, heart failure, ischemic heart disease, and diabetes mellitus cardiomyopathy. This review aims to provide a timely summary of the effects of defective copper homeostasis on heart disease and discuss potential underlying molecular mechanisms.

Structure-Function Relationships and Modifications of Cardiac Sarcoplasmic Reticulum Ca2+-Transport

Sarcoplasmic reticulum (SR) is a specialized tubular network, which not only maintains the intracellular concentration of Ca2+ at a low level but is also known to release and accumulate Ca2+ for the occurrence of cardiac contraction and relaxation, respectively. This subcellular organelle is composed of several phospholipids and different Ca2+-cycling, Ca2+-binding and regulatory proteins, which work in a coordinated manner to determine its function in cardiomyocytes. Some of the major proteins in the cardiac SR membrane include Ca2+-pump ATPase (SERCA2), Ca2+-release protein (ryanodine receptor), calsequestrin (Ca2+-binding protein) and phospholamban (regulatory protein). The phosphorylation of SR Ca2+-cycling proteins by protein kinase A or Ca2+-calmodulin kinase (directly or indirectly) has been demonstrated to augment SR Ca2+-release and Ca2+-uptake activities and promote cardiac contraction and relaxation functions. The activation of phospholipases and proteases as well as changes in different gene expressions under different pathological conditions have been shown to alter the SR composition and produce Ca2+-handling abnormalities in cardiomyocytes for the development of cardiac dysfunction. The post-translational modifications of SR Ca2+ cycling proteins by processes such as oxidation, nitrosylation, glycosylation, lipidation, acetylation, sumoylation, and O GlcNacylation have also been reported to affect the SR Ca2+ release and uptake activities as well as cardiac contractile activity. The SR function in the heart is also influenced in association with changes in cardiac performance by several hormones including thyroid hormones and adiponectin as well as by exercise-training. On the basis of such observations, it is suggested that both Ca2+-cycling and regulatory proteins in the SR membranes are intimately involved in determining the status of cardiac function and are thus excellent targets for drug development for the treatment of heart disease.

Current Status and Potential Therapeutic Strategies for Using Non-coding RNA to Treat Diabetic Cardiomyopathy

Diabetic cardiomyopathy (DMCM) is the leading cause of mortality and morbidity among diabetic patients. DMCM is characterized by an increase in oxidative stress with systemic inflammation that leads to cardiac fibrosis, ultimately causing diastolic and systolic dysfunction. Even though DMCM pathophysiology is well studied, the approach to limit this condition is not met with success. This highlights the need for more knowledge of underlying mechanisms and innovative therapies. In this regard, emerging evidence suggests a potential role of non-coding RNAs (ncRNAs), including micro-RNAs (miRNAs), long non-coding RNAs (lncRNAs), and circular RNAs (circRNAs) as novel diagnostics, mechanisms, and therapeutics in the context of DMCM. However, our understanding of ncRNAs’ role in diabetic heart disease is still in its infancy. This review provides a comprehensive update on pre-clinical and clinical studies that might develop therapeutic strategies to limit/prevent DMCM.

The Effects of Valsartan on Cardiac Function and Pro-Oxidative Parameters in the Streptozotocin-Induced Diabetic Rat Heart

Diabetes mellitus is a major risk factor for cardiovascular diseases, while cardiovascular diseases are a leading cause of morbidity and mortality worldwide. The renin–angiotensin– aldosterone system controls renal, cardiovascular, adrenal function and regulates fluid and electrolyte balance as well as blood pressure. Because of his role, inhibition of reninangiotensin-aldosteron system is another therapy approach that reduces the risk of diabetes and cardiovascular disease. In this study, our goal was to evaluate effect of valsartan,as inhibitor of angiotensin II receptor type 1, on cardiac tissue and function, with focus on cardiodynamic and oxidative stress. The present study was carried out on 20 adult male Wistar albino rats (8 week old and with body masses of 180-200 g). Rats were divided randomly into 2 groups (10 animals per group). Healthy animals treated with 1 μM of valsartan and streptozotocin-induced diabetic animals perfused with 1 μM of valsartan 4 weeks after the induction of diabetes. Our results demonstrated that acute application of valsartan has different effect on cardiodynamics in rat heart of diabetic and healthy animals but did not improve cardiac function in hyperglycemia-induced changes. A challenge for further investigations are studies with chronic or acute administration, alone or in combination with other angiotensin-converting-enzyme inhibitor in various models of diabetes.

Calcium Homeostasis in Ventricular Myocytes of Diabetic Cardiomyopathy

Diabetes mellitus (DM) is a chronic metabolic disorder commonly characterized by high blood glucose levels, resulting from defects in insulin production or insulin resistance, or both. DM is a leading cause of mortality and morbidity worldwide, with diabetic cardiomyopathy as one of its main complications. It is well established that cardiovascular complications are common in both types of diabetes. Electrical and mechanical problems, resulting in cardiac contractile dysfunction, are considered as the major complications present in diabetic hearts. Inevitably, disturbances in the mechanism(s) of Ca²⁺ signaling in diabetes have implications for cardiac myocyte contraction. Over the last decade, significant progress has been made in outlining the mechanisms responsible for the diminished cardiac contractile function in diabetes using different animal models of type I diabetes mellitus (TIDM) and type II diabetes mellitus (TIIDM). The aim of this review is to evaluate our current understanding of the disturbances of Ca²⁺ transport and the role of main cardiac proteins involved in Ca²⁺ homeostasis in the diabetic rat ventricular cardiomyocytes. Exploring the molecular mechanism(s) of altered Ca²⁺ signaling in diabetes will provide an insight for the identification of novel therapeutic approaches to improve the heart function in diabetic patients.

Role of catecholamines in the pathogenesis of diabetic cardiomyopathy 1

Although the sympathetic nervous system plays an important role in the regulation of cardiac function, the overactivation of the sympathetic nervous system under stressful conditions including diabetes has been shown to result in the excessive production of circulating catecholamines as well as an increase in the myocardial concentration of catecholamines. In this brief review, we provide some evidence to suggest that the oxidation products of catecholamines such as aminochrome and oxyradicals, lead to metabolic derangements, Ca²⁺-handling abnormalities, increase in the availability of intracellular free Ca²⁺, as well as activation of proteases and changes in myocardial gene expression. These alterations due to elevated levels of circulatory catecholamines are associated with oxidative stress, subcellular remodeling, and the development of cardiac dysfunction in chronic diabetes.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Proarrhythmic Remodeling of Calcium Homeostasis in Cardiac Disease; Implications for Diabetes and Obesity

A rapid growth in the incidence of diabetes and obesity has transpired to a major heath issue and economic burden in the postindustrial world, with more than 29 million patients affected in the United States alone. Cardiovascular defects have been established as the leading cause of mortality and morbidity of diabetic patients. Over the last decade, significant progress has been made in delineating mechanisms responsible for the diminished cardiac contractile function and enhanced propensity for malignant cardiac arrhythmias characteristic of diabetic disease. Rhythmic cardiac contractility relies upon the precise interplay between several cellular Ca²⁺ transport protein complexes including plasmalemmal L-type Ca²⁺ channels (LTCC), Na⁺-Ca²⁺ exchanger (NCX1), Sarco/endoplasmic Reticulum (SR) Ca²⁺-ATPase (SERCa2a) and ryanodine receptors (RyR2s), the SR Ca²⁺ release channels. Here we provide an overview of changes in Ca²⁺ homeostasis in diabetic ventricular myocytes and discuss the therapeutic potential of targeting Ca²⁺ handling proteins in the prevention of diabetes-associated cardiomyopathy and arrhythmogenesis.

Calcium Signaling in the Ventricular Myocardium of the Goto-Kakizaki Type 2 Diabetic Rat

The association between diabetes mellitus (DM) and high mortality linked to cardiovascular disease (CVD) is a major concern worldwide. Clinical and preclinical studies have demonstrated a variety of diastolic and systolic dysfunctions in patients with type 2 diabetes mellitus (T2DM) with the severity of abnormalities depending on the patients' age and duration of diabetes. The cellular basis of hemodynamic dysfunction in a type 2 diabetic heart is still not well understood. The aim of this review is to evaluate our current understanding of contractile dysfunction and disturbances of Ca²⁺ transport in the Goto-Kakizaki (GK) diabetic rat heart. The GK rat is a widely used nonobese, nonhypertensive genetic model of T2DM which is characterized by insulin resistance, elevated blood glucose, alterations in blood lipid profile, and cardiac dysfunction.

Altered profile of mRNA expression in atrioventricular node of streptozotocin‑induced diabetic rats

Prolonged action potential duration, reduced action potential firing rate, upstroke velocity and rate of diastolic depolarization have been demonstrated in atrioventricular node (AVN) cells from streptozotocin (STZ)-induced diabetic rats. To further clarify the molecular basis of these electrical disturbances, the mRNA profiles encoding a variety of proteins associated with the generation and conduction of electrical activity in the AVN, were evaluated in the STZ-induced diabetic rat heart. Expression of mRNA was measured in AVN biopsies using reverse transcription-quantitative polymerase chain reaction techniques. Notable differences in mRNA expression included upregulation of genes encoding membrane and intracellular Ca²⁺ transport, including solute carrier family 8 member A1, transient receptor potential channel 1, ryanodine receptor 2/3, hyperpolarization-activated cyclic-nucleotide 2 and 3, calcium channel voltage-dependent, β2 subunit and sodium channels 3a, 4a, 7a and 3b. In addition to this, potassium channels potassium voltage-gated channel subfamily A member 4, potassium channel calcium activated intermediate/small conductance subfamily N α member 2, potassium voltage-gated channel subfamily J members 3, 5, and 11, potassium channel subfamily K members 1, 2, 3 and natriuretic peptide B (BNP) were upregulated in AVN of STZ heart, compared with controls. Alterations in gene expression were associated with upregulation of various proteins including the inwardly rectifying, potassium channel K_ir3.4, NCX1 and BNP. The present study demonstrated notable differences in the profile of mRNA encoding proteins associated with the generation, conduction and regulation of electrical signals in the AVN of the STZ-induced diabetic rat heart. These data will provide a basis for a substantial range of future studies to investigate whether variations in mRNA translate into alterations in electrophysiological function.

Ischemic preconditioning: Interruption of various disorders

Ischemic heart diseases are the leading cause of morbidity and mortality worldwide. Reperfusion of an ischemic heart is necessary to regain the normal functioning of the heart. However, abrupt reperfusion of an ischemic heart elicits a cascade of adverse events that leads to injury of the myocardium, i.e., ischemia-reperfusion injury. An endogenous powerful strategy to protect the ischemic heart is ischemic preconditioning, in which the myocardium is subjected to short periods of sublethal ischemia and reperfusion before the prolonged ischemic insult. However, it should be noted that the cardioprotective effect of preconditioning is attenuated in some pathological conditions. The aim of this article is to review present knowledge on how menopause and some metabolic disorders such as diabetes and hyperlipidemia affect myocardial ischemic preconditioning and the mechanisms involved.

The effects of zofenopril on cardiac function and pro-oxidative parameters in the streptozotocin-induced diabetic rat heart

Diabetes mellitus is a chronic condition that continues to increase in both incidence and prevalence. Renin-Angiotensin-Aldosterone System is one of the main modulators of chronic hyperglycaemia and, thus, its influence on tissues. Hyperglycaemia-induced oxidative stress is an important factor in diabetic cardiomyopathy. The present study was carried out on 24 adult male Wistar albino rats (8-week-old and with body masses of 190 ± 10 g). We evaluated the influence of acute administration of zofenopril on ex vivo myocardial function from rats with streptozotocin-induced diabetes mellitus, with a special emphasis on cardiodynamic and oxidative stress parameters in diabetic rat hearts. Rats were divided randomly into two groups (12 animals per group): control non-diabetic animals (C) were healthy rats perfused with 1.5 µM of zofenopril, and STZ-treated diabetic animals (DM) were diabetic animals perfused with 1.5 µM of zofenopril 4 weeks after the induction of diabetes. Our results demonstrated that diabetic rats are characterized by a depressed cardiac performance and that oxidative markers are related to alterations in cardiac function in rats with 4 weeks of STZ-induced diabetes. Additionally, the use of zofenopril as a monotherapy slightly diminished cardiac damage induced by chronic hyperglycaemia. However, long-term follow-up intervention trials are necessary to fully demonstrate the benefit of zofenopril in this context. A challenge for future investigations will be to identify the effects of chronic administration or combination therapy with angiotensin-converting enzyme inhibitors in various models of diabetes.

Inhibiting Insulin-Mediated β2-Adrenergic Receptor Activation Prevents Diabetes-Associated Cardiac Dysfunction

BACKGROUND

Type 2 diabetes mellitus (DM) and obesity independently increase the risk of heart failure by incompletely understood mechanisms. We propose that hyperinsulinemia might promote adverse consequences in the hearts of subjects with type-2 DM and obesity.

METHODS

High-fat diet feeding was used to induce obesity and DM in wild-type mice or mice lacking β₂-adrenergic receptor (β₂AR) or β-arrestin2. Wild-type mice fed with high-fat diet were treated with a β-blocker carvedilol or a GRK2 (G-protein-coupled receptor kinase 2) inhibitor. We examined signaling and cardiac contractile function.

RESULTS

High-fat diet feeding selectively increases the expression of phosphodiesterase 4D (PDE4D) in mouse hearts, in concert with reduced protein kinase A phosphorylation of phospholamban, which contributes to systolic and diastolic dysfunction. The expression of PDE4D is also elevated in human hearts with DM. The induction of PDE4D expression is mediated by an insulin receptor, insulin receptor substrate, and GRK2 and β-arrestin2-dependent transactivation of a β₂AR-extracellular regulated protein kinase signaling cascade. Thus, pharmacological inhibition of β₂AR or GRK2, or genetic deletion of β₂AR or β-arrestin2, all significantly attenuate insulin-induced phosphorylation of extracellular regulated protein kinase and PDE4D induction to prevent DM-related contractile dysfunction.

CONCLUSIONS

These studies elucidate a novel mechanism by which hyperinsulinemia contributes to heart failure by increasing PDE4D expression and identify β₂AR or GRK2 as plausible therapeutic targets for preventing or treating heart failure in subjects with type 2 DM.

High Intensity Aerobic Exercise Training Improves Deficits of Cardiovascular Autonomic Function in a Rat Model of Type 1 Diabetes Mellitus with Moderate Hyperglycemia

Indices of cardiovascular autonomic neuropathy (CAN) in experimental models of Type 1 diabetes mellitus (T1DM) are often contrary to clinical data. Here, we investigated whether a relatable insulin-treated model of T1DM would induce deficits in cardiovascular (CV) autonomic function more reflective of clinical results and if exercise training could prevent those deficits. Sixty-four rats were divided into four groups: sedentary control (C), sedentary T1DM (D), control exercise (CX), or T1DM exercise (DX). Diabetes was induced via multiple low-dose injections of streptozotocin and blood glucose was maintained at moderate hyperglycemia (9-17 mM) through insulin supplementation. Exercise training consisted of daily treadmill running for 10 weeks. Compared to C, D had blunted baroreflex sensitivity, increased vascular sympathetic tone, increased serum neuropeptide Y (NPY), and decreased intrinsic heart rate. In contrast, DX differed from D in all measures of CAN (except NPY), including heart rate variability. These findings demonstrate that this T1DM model elicits deficits and exercise-mediated improvements to CV autonomic function which are reflective of clinical T1DM.

Characterization of L-type calcium channel activity in atrioventricular nodal myocytes from rats with streptozotocin-induced Diabetes mellitus

Cardiovascular complications are common in patients with Diabetes mellitus (DM). In addition to changes in cardiac muscle inotropy, electrical abnormalities are also commonly observed in these patients. We have previously shown that spontaneous cellular electrical activity is altered in atrioventricular nodal (AVN) myocytes, isolated from the streptozotocin (STZ) rat model of type-1 DM. In this study, utilizing the same model, we have characterized the changes in L-type calcium channel activity in single AVN myocytes. Ionic currents were recorded from AVN myocytes isolated from the hearts of control rats and from those with STZ-induced diabetes. Patch-clamp recordings were used to assess the changes in cellular electrical activity in individual myocytes. Type-1 DM significantly altered the cellular characteristics of L-type calcium current. A reduction in peak I_CaL density was observed, with no corresponding changes in the activation parameters of the current. L-type calcium channel current also exhibited faster time-dependent inactivation in AVN myocytes from diabetic rats. A negative shift in the voltage dependence of inactivation was also evident, and a slowing of restitution parameters. These findings demonstrate that experimentally induced type-1 DM significantly alters AVN L-type calcium channel cellular electrophysiology. These changes in ion channel activity may contribute to the abnormalities in cardiac electrical function that are associated with high mortality levels in patients with DM.

Role of protein phosphorylation in excitation-contraction coupling in taurine deficient hearts

Taurine is a beta-amino acid found in very high concentration in the heart. Depletion of these intracellular stores results in the development of cardiomyopathy, thought to be mediated by abnormal sarcoplasmic reticular (SR) Ca(2+) transport. There is also evidence that taurine directly alters the Ca(2+) sensitivity of myofibrillar proteins. Major regulators of SR Ca(2+) ATPase (SERCA2a) are the phosphorylation status of a regulatory protein, phospholamban, and SERCA2a expression, which are diminished in the failing heart. The failing heart also exhibits reductions in myofibrillar Ca(2+) sensitivity, a property regulated by the phosphorylation of the muscle protein, troponin I. Therefore, we tested the hypothesis that taurine deficiency leads to alterations in SR Ca(2+) ATPase activity related to reduced phospholamban phosphorylation and expression of SERCA2a. We found that a sequence of events, which included elevated protein phosphatase 1 activity, reduced autophosphorylation of CaMKII, and reduced phospholamban phosphorylation, supports the reduction in SR Ca(2+) ATPase activity. However, the reduction in SR Ca(2+) ATPase activity was not caused by reduced SERCA2a expression. Taurine transporter knockout (TauTKO) hearts also exhibited a rightward shift in the Ca(2+) dependence of the myofibrillar Ca(2+) ATPase, a property that is associated with an elevation in phosphorylated troponin I. The findings support the observation that taurine deficient hearts develop systolic and diastolic defects related to reduced SR Ca(2+) ATPase activity, a change mediated in part by reduced phospholamban phosphorylation.

Mechanisms of subcellular remodeling in heart failure due to diabetes

Diabetic cardiomyopathy is not only associated with heart failure but there also occurs a loss of the positive inotropic effect of different agents. It is now becoming clear that cardiac dysfunction in chronic diabetes is intimately involved with Ca(2+)-handling abnormalities, metabolic defects and impaired sensitivity of myofibrils to Ca(2+) in cardiomyocytes. On the other hand, loss of the inotropic effect in diabetic myocardium is elicited by changes in signal transduction mechanisms involving hormone receptors and depressions in phosphorylation of various membrane proteins. Ca(2+)-handling abnormalities in the diabetic heart occur mainly due to defects in sarcolemmal Na(+)-K(+) ATPase, Na(+)-Ca(2+) exchange, Na(+)-H(+) exchange, Ca(2+)-channels and Ca(2+)-pump activities as well as changes in sarcoplasmic reticular Ca(2+)-uptake and Ca(2+)-release processes; these alterations may lead to the occurrence of intracellular Ca(2+) overload. Metabolic defects due to insulin deficiency or ineffectiveness as well as hormone imbalance in diabetes are primarily associated with a shift in substrate utilization and changes in the oxidation of fatty acids in cardiomyocytes. Mitochondria initially seem to play an adaptive role in serving as a Ca(2+) sink, but the excessive utilization of long-chain fatty acids for a prolonged period results in the generation of oxidative stress and impairment of their function in the diabetic heart. In view of the activation of sympathetic nervous system and renin-angiotensin system as well as platelet aggregation, endothelial dysfunction and generation of oxidative stress in diabetes and blockade of their effects have been shown to attenuate subcellular remodeling, metabolic derangements and signal transduction abnormalities in the diabetic heart. On the basis of these observations, it is suggested that oxidative stress and subcellular remodeling due to hormonal imbalance and metabolic defects play a critical role in the genesis of heart failure during the development of diabetic cardiomyopathy.

Role of microangiopathy in diabetic cardiomyopathy

Although heart disease due to diabetes is mainly associated with complications of the large vessels, microvascular abnormalities are also considered to be involved in altering cardiac structure and function. Three major defects, such as endothelial dysfunction, alteration in the production/release of hormones, and shift in metabolism of smooth muscle cells, have been suggested to produce damage to the small arteries and capillaries (microangiopathy) due to hyperglycemia, and promote the development of diabetic cardiomyopathy. These factors may either act alone or in combination to produce oxidative stress as well as changes in cellular signaling and gene transcription, which in turn cause vasoconstriction and structural remodeling of the coronary vessels. Such alterations in microvasculature produce hypoperfusion of the myocardium and thereby lower the energy status resulting in changes in Ca(2+)-handling, apoptosis, and decreased cardiac contractile force. This article discusses diabetes-induced mechanisms of microvascular damage leading to cardiac dysfunction that is characterized by myocardial dilatation, cardiac hypertrophy as well as early diastolic and late systolic defects. Metabolic defects and changes in neurohumoral system due to diabetes, which promote disturbances in vascular homeostasis, are highlighted. In addition, increase in the vulnerability of the diabetic heart to the development of heart failure and the signaling pathways integrating nuclear factor κB and protein kinase C in diabetic cardiomyopathy are also described for comparison.

Advanced glycation end products: role in pathology of diabetic cardiomyopathy

Increasing evidence demonstrates that advanced glycation end products (AGEs) play a pivotal role in the development and progression of diabetic heart failure, although there are numerous other factors that mediate the disease response. AGEs are generated intra- and extracellularly as a result of chronic hyperglycemia. Then, following the interaction with receptors for advanced glycation end products (RAGEs), a series of events leading to vascular and myocardial damage are elicited and sustained, which include oxidative stress, increased inflammation, and enhanced extracellular matrix accumulation resulting in diastolic and systolic dysfunction. Whereas targeting glycemic control and treating additional risk factors, such as obesity, dyslipidemia, and hypertension, are mandatory to reduce chronic complications and prolong life expectancy in diabetic patients, drug therapy tailored to reducing the deleterious effects of the AGE-RAGE interactions is being actively investigated and showing signs of promise in treating diabetic cardiomyopathy and associated heart failure. This review shall discuss the formation of AGEs in diabetic heart tissue, potential targets of glycation in the myocardium, and underlying mechanisms that lead to diabetic cardiomyopathy and heart failure along with the use of AGE inhibitors and breakers in mitigating myocardial injury.

Functional and structural impact of pirfenidone on the alterations of cardiac disease and diabetes mellitus

A synthetic compound, termed pirfenidone (PFD), is considered promising for the treatment of cardiac disease. It leads to beneficial effects in animal models of diabetes mellitus (DM); as well as in heart attack, atrial fibrillation, muscular dystrophy, and diabetic cardiomyopathy (DC). The latter is a result of alterations linked to metabolic syndrome as they promote cardiac hypertrophy, fibrosis and contractile dysfunction. Although reduced level of fibrosis and stiffness represent an essential step in the mechanism of PFD action, a wide range of functional effects might also contribute to the therapeutic benefits. For example, PFD stimulates L-type voltage-gated Ca(2+) channels (LTCCs), which are pivotal for a process known as excitation-contraction coupling (ECC). Recent evidence suggests that these two types of actions - namely structural and functional - aid in treating both cardiac disease and DM. This view is supported by the fact that in DC, for example, systolic dysfunction arises from both cardiac stiffness linked to fibrosis and down-regulation of ECC. Thus, not surprisingly, clinical trials have been conducted with PFD in the settings of DM, for treating not only cardiac but also renal disease. This review presents all these concepts, along with the possible mechanisms and pathophysiological consequences.

Streptozotocin-induced diabetes prolongs twitch duration without affecting the energetics of isolated ventricular trabeculae

BACKGROUND

Diabetes induces numerous electrical, ionic and biochemical defects in the heart. A general feature of diabetic myocardium is its low rate of activity, commonly characterised by prolonged twitch duration. This diabetes-induced mechanical change, however, seems to have no effect on contractile performance (i.e., force production) at the tissue level. Hence, we hypothesise that diabetes has no effect on either myocardial work output or heat production and, consequently, the dependence of myocardial efficiency on afterload of diabetic tissue is the same as that of healthy tissue.

METHODS

We used isolated left ventricular trabeculae (streptozotocin-induced diabetes versus control) as our experimental tissue preparations. We measured a number of indices of mechanical (stress production, twitch duration, extent of shortening, shortening velocity, shortening power, stiffness, and work output) and energetic (heat production, change of enthalpy, and efficiency) performance. We calculated efficiency as the ratio of work output to change of enthalpy (the sum of work and heat).

RESULTS

Consistent with literature results, we showed that peak twitch stress of diabetic tissue was normal despite suffering prolonged duration. We report, for the first time, the effect of diabetes on mechanoenergetic performance. We found that the indices of performance listed above were unaffected by diabetes. Hence, since neither work output nor change of enthalpy was affected, the efficiency-afterload relation of diabetic tissue was unaffected, as hypothesised.

CONCLUSIONS

Diabetes prolongs twitch duration without having an effect on work output or heat production, and hence efficiency, of isolated ventricular trabeculae. Collectively, our results, arising from isolated trabeculae, reconcile the discrepancy between the mechanical performance of the whole heart and its tissues.

Conundrum of pathogenesis of diabetic cardiomyopathy: role of vascular endothelial dysfunction, reactive oxygen species, and mitochondria

Diabetic cardiomyopathy and heart failure have been recognized as the leading causes of mortality among diabetics. Diabetic cardiomyopathy has been characterized primarily by the manifestation of left ventricular dysfunction that is independent of coronary artery disease and hypertension among the patients affected by diabetes mellitus. A complex array of contributing factors including the hypertrophy of left ventricle, alterations of metabolism, microvascular pathology, insulin resistance, fibrosis, apoptotic cell death, and oxidative stress have been implicated in the pathogenesis of diabetic cardiomyopathy. Nevertheless, the exact mechanisms underlying the pathogenesis of diabetic cardiomyopathy are yet to be established. The critical involvement of multifarious factors including the vascular endothelial dysfunction, microangiopathy, reactive oxygen species (ROS), oxidative stress, mitochondrial dysfunction has been identified in the mechanism of pathogenesis of diabetic cardiomyopathy. Although it is difficult to establish how each factor contributes to disease, the involvement of ROS and mitochondrial dysfunction are emerging as front-runners in the mechanism of pathogenesis of diabetic cardiomyopathy. This review highlights the role of vascular endothelial dysfunction, ROS, oxidative stress, and mitochondriopathy in the pathogenesis of diabetic cardiomyopathy. Furthermore, the review emphasizes that the puzzle has to be solved to firmly establish the mitochondrial and/or ROS mechanism(s) by identifying their most critical molecular players involved at both spatial and temporal levels in diabetic cardiomyopathy as targets for specific and effective pharmacological/therapeutic interventions.

ß-blocker timolol prevents arrhythmogenic Ca²⁺ release and normalizes Ca²⁺ and Zn²⁺ dyshomeostasis in hyperglycemic rat heart

Defective cardiac mechanical activity in diabetes results from alterations in intracellular Ca(2+) handling, in part, due to increased oxidative stress. Beta-blockers demonstrate marked beneficial effects in heart dysfunction with scavenging free radicals and/or acting as an antioxidant. The aim of this study was to address how β-blocker timolol-treatment of diabetic rats exerts cardioprotection. Timolol-treatment (12-week), one-week following diabetes induction, prevented diabetes-induced depressed left ventricular basal contractile activity, prolonged cellular electrical activity, and attenuated the increase in isolated-cardiomyocyte size without hyperglycemic effect. Both in vivo and in vitro timolol-treatment of diabetic cardiomyocytes prevented the altered kinetic parameters of Ca(2+) transients and reduced Ca(2+) loading of sarcoplasmic reticulum (SR), basal intracellular free Ca(2+) and Zn(2+) ([Ca(2+)]i and [Zn(2+)]i), and spatio-temporal properties of the Ca(2+) sparks, significantly. Timolol also antagonized hyperphosphorylation of cardiac ryanodine receptor (RyR2), and significantly restored depleted protein levels of both RyR2 and calstabin2. Western blot analysis demonstrated that timolol-treatment also significantly normalized depressed levels of some [Ca(2+)]i-handling regulators, such as Na(+)/Ca(2+) exchanger (NCX) and phospho-phospholamban (pPLN) to PLN ratio. Incubation of diabetic cardiomyocytes with 4-mM glutathione exerted similar beneficial effects on RyR2-macromolecular complex and basal levels of both [Ca(2+)]i and [Zn(2+)]i, increased intracellular Zn(2+) hyperphosphorylated RyR2 in a concentration-dependent manner. Timolol also led to a balanced oxidant/antioxidant level in both heart and circulation and prevented altered cellular redox state of the heart. We thus report, for the first time, that the preventing effect of timolol, directly targeting heart, seems to be associated with a normalization of macromolecular complex of RyR2 and some Ca(2+) handling regulators, and prevention of Ca(2+) leak, and thereby normalization of both [Ca(2+)]i and [Zn(2+)]i homeostasis in diabetic rat heart, at least in part by controlling the cellular redox status of hyperglycemic cardiomyocytes.

Diabetic cardiomyopathy: understanding the molecular and cellular basis to progress in diagnosis and treatment

Diabetes mellitus is an important and prevalent risk factor for congestive heart failure. Diabetic cardiomyopathy has been defined as ventricular dysfunction that occurs in diabetic patients independent of a recognized cause such as coronary artery disease or hypertension. The disease course consists of a hidden subclinical period, during which cellular structural insults and abnormalities lead initially to diastolic dysfunction, later to systolic dysfunction, and eventually to heart failure. Left ventricular hypertrophy, metabolic abnormalities, extracellular matrix changes, small vessel disease, cardiac autonomic neuropathy, insulin resistance, oxidative stress, and apoptosis are the most important contributors to diabetic cardiomyopathy onset and progression. Hyperglycemia is a major etiological factor in the development of diabetic cardiomyopathy. It increases the levels of free fatty acids and growth factors and causes abnormalities in substrate supply and utilization, calcium homeostasis, and lipid metabolism. Furthermore, it promotes excessive production and release of reactive oxygen species, which induces oxidative stress leading to abnormal gene expression, faulty signal transduction, and cardiomyocytes apoptosis. Stimulation of connective tissue growth factor, fibrosis, and the formation of advanced glycation end-products increase the stiffness of the diabetic hearts. Despite all the current information on diabetic cardiomyopathy, translational research is still scarce due to limited human myocardial tissue and most of our knowledge is extrapolated from animals. This paper aims to elucidate some of the molecular and cellular pathophysiologic mechanisms, structural changes, and therapeutic strategies that may help struggle against diabetic cardiomyopathy.

Effects of human immunodeficiency virus and metabolic complications on myocardial nutrient metabolism, blood flow, and oxygen consumption: a cross-sectional analysis

BACKGROUND

In the general population, peripheral metabolic complications (MC) increase the risk for left ventricular dysfunction. Human immunodeficiency virus infection (HIV) and combination anti-retroviral therapy (cART) are associated with MC, left ventricular dysfunction, and a higher incidence of cardiovascular events than the general population. We examined whether myocardial nutrient metabolism and left ventricular dysfunction are related to one another and worse in HIV infected men treated with cART vs. HIV-negative men with or without MC.

METHODS

Prospective, cross-sectional study of myocardial glucose and fatty acid metabolism and left ventricular function in HIV+ and HIV-negative men with and without MC. Myocardial glucose utilization (GLUT), and fatty acid oxidation and utilization rates were quantified using 11C-glucose and 11C-palmitate and myocardial positron emission tomography (PET) imaging in four groups of men: 23 HIV+ men with MC+ (HIV+/MC+, 42 ± 6 yrs), 15 HIV+ men without MC (HIV+/MC-, 41 ± 6 yrs), 9 HIV-negative men with MC (HIV-/MC+, 33 ± 5 yrs), and 22 HIV-negative men without MC (HIV-/MC-, 25 ± 6 yrs). Left ventricular function parameters were quantified using echocardiography.

RESULTS

Myocardial glucose utilization was similar among groups, however when normalized to fasting plasma insulin concentration (GLUT/INS) was lower (p < 0.01) in men with metabolic complications (HIV+: 9.2 ± 6.2 vs. HIV-: 10.4 ± 8.1 nmol/g/min/μU/mL) than men without metabolic complications (HIV+: 45.0 ± 33.3 vs. HIV-: 60.3 ± 53.0 nmol/g/min/μU/mL). Lower GLUT/INS was associated with lower myocardial relaxation velocity during early diastole (r = 0.39, p < 0.001).

CONCLUSION

Men with metabolic complications, irrespective of HIV infection, had lower basal myocardial glucose utilization rates per unit insulin that were related to left ventricular diastolic impairments, indicating that well-controlled HIV infection is not an independent risk factor for blunted myocardial glucose utilization per unit of insulin.

TRIAL REGISTRATION

NIH Clinical Trials NCT00656851.

Azelnidipine prevents cardiac dysfunction in streptozotocin-diabetic rats by reducing intracellular calcium accumulation, oxidative stress and apoptosis

BACKGROUND

Numerous evidences suggest that diabetic heart is characterized by compromised ventricular contraction and prolonged relaxation attributable to multiple causative factors including calcium accumulation, oxidative stress and apoptosis. Therapeutic interventions to prevent calcium accumulation and oxidative stress could be therefore helpful in improving the cardiac function under diabetic condition.

METHODS

This study was designed to examine the effect of long-acting calcium channel blocker (CCB), Azelnidipine (AZL) on contractile dysfunction, intracellular calcium (Ca2+) cycling proteins, stress-activated signaling molecules and apoptosis on cardiomyocytes in diabetes. Adult male Wistar rats were made diabetic by a single intraperitoneal (IP) injection of streptozotocin (STZ). Contractile functions were traced from live diabetic rats to isolated individual cardiomyocytes including peak shortening (PS), time-to-PS (TPS), time-to-relengthening (TR90), maximal velocity of shortening/relengthening (± dL/dt) and intracellular Ca2+ fluorescence.

RESULTS

Diabetic heart showed significantly depressed PS, ± dL/dt, prolonged TPS, TR90 and intracellular Ca2+ clearing and showed an elevated resting intracellular Ca2+. AZL itself exhibited little effect on myocyte mechanics but it significantly alleviated STZ-induced myocyte contractile dysfunction. Diabetes increased the levels of superoxide, enhanced expression of the cardiac damage markers like troponin I, p67phox NADPH oxidase subunit, restored the levels of the mitochondrial superoxide dismutase (Mn-SOD), calcium regulatory proteins RyR2 and SERCA2a, and suppressed the levels of the anti-apoptotic Bcl-2 protein. All of these STZ-induced alterations were reconciled by AZL treatment.

CONCLUSION

Collectively, the data suggest beneficial effect of AZL in diabetic cardiomyopathy via altering intracellular Ca2+ handling proteins and preventing apoptosis by its antioxidant property.

Fatty acid synthase modulates homeostatic responses to myocardial stress

Fatty acid synthase (FAS) promotes energy storage through de novo lipogenesis and participates in signaling by the nuclear receptor PPARα in noncardiac tissues. To determine if de novo lipogenesis is relevant to cardiac physiology, we generated and characterized FAS knockout in the myocardium (FASKard) mice. FASKard mice develop normally, manifest normal resting heart function, and have normal cardiac PPARα signaling as well as fatty acid oxidation. However, they decompensate with stress. Most die within 1 h of transverse aortic constriction, probably due to arrhythmia. Voltage clamp measurements of FASKard cardiomyocytes show hyperactivation of L-type calcium channel current that could not be reversed with palmitate supplementation. Of the classic regulators of this current, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) but not protein kinase A signaling is activated in FASKard hearts, and knockdown of FAS in cultured cells activates CaMKII. In addition to being intolerant of the stress of acute pressure, FASKard hearts were also intolerant of the stress of aging, reflected as persistent CaMKII hyperactivation, progression to dilatation, and premature death by ∼1 year of age. CaMKII signaling appears to be pathogenic in FASKard hearts because inhibition of its signaling in vivo rescues mice from early mortality after transverse aortic constriction. FAS was also increased in two mechanistically distinct mouse models of heart failure and in the hearts of humans with end stage cardiomyopathy. These data implicate a novel relationship between FAS and calcium signaling in the heart and suggest that FAS induction in stressed myocardium represents a compensatory response to protect cardiomyocytes from pathological calcium flux.

Age-related regulation of excitation-contraction coupling in rat heart

Hearts from subjects with different ages have different Ca(2+) signaling. Release of Ca(2+) from intracellular stores in response to an action potential initiates cardiac contraction. Both depolarization-stimulated and spontaneous Ca(2+) releases, Ca(2+) transients and Ca(2+) sparks, demonstrate the main events of excitation-contraction coupling (ECC). Global increase in free Ca(2+) concentration ([Ca(2+)]( i )) consists of summation of Ca(2+) release events in cardiomyocytes. Since the Ca(2+) flux induced by Ca(2+) sparks reports a summation of ryanodine-sensitive Ca(2+) release channels (RyR2s)'s behavior in a spark cluster, evaluation of the properties of Ca(2+) sparks and Ca(2+) transients may provide insight into the role of RyR2s on altered heart function between 3-month-old (young adult) and 6-month-old (mature adult) rats. Basal [Ca(2+)]( i ) and Ca(2+) sparks frequency were significantly higher in mature adult rats compared to those of young adults. Moreover, amplitudes of Ca(2+) sparks and Ca(2+) transients were significantly smaller in mature adults than those of young adults with longer time courses. A smaller L-type Ca(2+) current density and decreased SR Ca(2+) load was observed in mature adult rats. In addition, RyR2s were markedly hyperphosphorylated, and phosphorylation levels of PKA and CaMKII were higher in heart from mature adults compared to those of young adults, whereas their SERCA protein levels were similar. Our data demonstrate that hearts from rats with different ages have different Ca(2+) signaling including hyperphosphorylation of RyR2s and higher basal [Ca(2+)]( i ) together with increased oxidized protein-thiols in mature adult rats compared to those of young adults, which play important roles in ECC. Finally, we report that ECC efficiency changes with age during maturation, partially related with an increased cellular oxidation level leading to reduced free protein-thiols in cardiomyocytes.

Decreased cardiac SERCA2 expression, SR Ca uptake, and contractile function in hypothyroidism are attenuated in SERCA2 overexpressing transgenic rats

The sarco/endoplasmic reticulum (SR) Ca(2+)-ATPase SERCA2a has a key role in controlling cardiac contraction and relaxation. In hypothyroidism, decreased expression of the thyroid hormone (TH)-responsive SERCA2 gene contributes to slowed SR Ca(2+) reuptake and relaxation. We investigated whether cardiac expression of a TH-insensitive SERCA2a cDNA minigene can rescue SR Ca(2+) handling and contractile function in female SERCA2a-transgenic rats (TG) with experimental hypothyroidism. Wild-type rats (WT) and TG were rendered hypothyroid by 6-N-propyl-2-thiouracil treatment for 6 wk; control rats received no treatment. In vivo measured left ventricular (LV) hemodynamic parameters were compared with SERCA2a expression and function in LV tissue. Hypothyroidism decreased LV peak systolic pressure, dP/dt(max), and dP/dt(min) in both WT and TG. However, loss of function was less in TG. Thus slowed relaxation in hypothyroidism was found to be 1.5-fold faster in TG compared with WT (P < 0.05). In parallel, a 1.4-fold higher V(max) value of homogenate SR Ca(2+) uptake was observed in hypothyroid TG (P < 0.05 vs. hypothyroid WT), and the hypothyroidism-caused decline of LV SERCA2a mRNA expression in TG by -24% was markedly less than the decrease of -49% in WT (P < 0.05). A linear relationship was observed between the SERCA2a/PLB mRNA ratio values and the V(max) values of SR Ca(2+) uptake when the respective data of all experimental groups were plotted together (r = 0.90). The data show that expression of the TH-insensitive SERCA2a minigene compensates for loss of expressional activity of the TH-responsive native SERCA2a gene in the female hypothyroid rat heart. However, SR Ca(2+) uptake and in vivo heart function were only partially rescued.

Neuropeptide Y level in paraventricular nucleus of experimental diabetic rats: correlation with sympathetic activity and body weight

Neuropeptide Y (NPY), colocalized with norepinephrine neuron, is known to modulate sympathetic activity and feeding behavior. Although experimental type 1 diabetes has increased sympathetic activity at the early part of the disease process, little effort was made so far to understand the correlation between NPY level in the hypothalamus and sympathetic activity in diabetes. Male Sprague Dawley rats were made diabetic by a single injection of streptozotocin (65 mg/kg body weight, IV). The animals were then studied after 2, 4, and 8 weeks. Control animals received only citrate vehicle. In an effort to clarify the modulatory effect of NPY at the early stage of diabetes, the paraventricular nucleus (PVN) of hypothalamus was sampled by microdialysis for NPY and norepinephrine level. While NPY level was increased immediately within 2 weeks (along with feeding behavior), norepinephrine level was increased only after 8 weeks following injection of streptozotocin. The animals lost significant weight. These results are interpreted to mean that a strong correlation exists between the feeding behavior and NPY level in PVN. Since NPY is known to inhibit sympathetic activity it is possible that NPY receptors are down-regulated following diabetes. The higher level of norepinephrine indicating higher sympathetic activity did not allow the animals to gain weight. In addition, controversy exists regarding pleiotropic activities of NPY related to the feeding behavior of these animals.

Mitochondria in the diabetic heart

Diabetes mellitus increases the risk of developing cardiovascular diseases such as coronary artery disease and heart failure. Studies have shown that the heart failure risk is increased in diabetic patients even after adjusting for coronary artery disease and hypertension. Although the cause of this increased heart failure risk is multifactorial, increasing evidence suggests that derangements in cardiac energy metabolism play an important role. In particular, abnormalities in cardiomyocyte mitochondrial energetics appear to contribute substantially to the development of cardiac dysfunction in diabetes. This review will summarize these abnormalities in mitochondrial function and discuss potential underlying mechanisms.

Morphological characteristics of cardiac calcium release units in animals with metabolic and circulatory disorders

Metabolic and circulatory disorders such as diabetes and hypertension are associated with cardiac dysfunction. Research on these types of experimental animals has observed abnormal calcium (Ca(2+)) sparks and waves in cells; a potential mechanism altering excitation-contraction (e-c) coupling in the myocardium. The e-c coupling depends on the intricate spatial relationship between the sarcolemma and sarcoplasmic reticulum calcium release units (CRU's). The objective of this study was to assess for a presence or absence of abnormalities in CRU's from type II diabetic and hypertensive rat models. Myocardial tissue underwent perfusion fixation followed by selective staining of the CRU's and the features observed using a high voltage electron microscope. Results revealed both diabetic groups had significant increases in body weight, a tendency toward an enlarged heart, and a significant disruption of the CRU's and displacement of transverse (t)-tubules in a longitudinal direction. The hypertensive model characteristically showed a dramatic increase in heart size, a significant increase in disrupted CRU's and a tendency towards longitudinal t-tubule orientation. We propose the two disorders of diabetes and hypertension have a similar etiology of cardiomyopathy resulting, in part, from an increase in the number of incomplete CRU's, due to a morphological change in the architecture and orientation of the t-tubules. These architectural changes could potentially explain the impaired calcium signaling previously observed in diabetic and hypertensive cardiomyopathy.

Prevention of incipient diabetic cardiomyopathy by high-dose thiamine

Diabetic cardiomyopathy can progress toward overt heart failure with increased mortality. The hexosamine biosynthesis pathway has been implicated in signaling for fibrosis by the kidney. Thiamine (vitamin B(1)) is an indispensable coenzyme and required at intracellular glucose metabolism. In this study, we assessed if decrease of flux through the hexosamine biosynthesis pathway induced by high-dose thiamine therapy counteracts diabetes-induced cardiac fibrosis. The diabetes model used was the streptozotocin-induced diabetic rat. Normal control and diabetic rats were studied for 2 weeks with and without thiamine, and followings were analyzed; plasma biochemicals (total cholesterol and triglycerides), morphological changes, mRNA abundance relevant to cardiac failure (brain natriuretic peptide) and fibrosis (transforming growth factor-beta1, thrombospondine, fibronectin, plasminogen activator-I and connective tissue growth factor) as well as and matrix metalloproteinase activity were investigated. Thiamine repletion prevented diabetes-induced cardiac fibrosis without changes in plasma glucose concentration. This was achieved by prevention of thiamine depletion, increased pro-fibrotic mRNA abundance and decreased metalloproteinase activity in the heart of diabetic rats. O-glycosylated protein was significantly higher in the left ventricular of diabetic rats compared to control rats, which was decreased by thiamine administration. Thiamine repletion prevented diabetes-induced cardiac fibrosis in experimental diabetes, probably by suppression of hexosamine biosynthesis pathway.

Alterations in mitochondrial function and cytosolic calcium induced by hyperglycemia are restored by mitochondrial transcription factor A in cardiomyocytes

Mitochondrial transcription factor A (TFAM) is essential for mitochondrial DNA transcription and replication. TFAM transcriptional activity is decreased in diabetic cardiomyopathy; however, the functional implications are unknown. We hypothesized that a reduced TFAM activity may be responsible for some of the alterations caused by hyperglycemia. Therefore, we investigated the effect of TFAM overexpression on hyperglycemia-induced cytosolic calcium handling and mitochondrial abnormalities. Neonatal rat cardiomyocytes were exposed to high glucose (30 mM) for 48 h, and we examined whether TFAM overexpression, by protecting mitochondrial DNA, could reestablish calcium fluxes and mitochondrial alterations toward normal. Our results shown that TFAM overexpression increased to more than twofold mitochondria copy number in cells treated either with normal (5.5 mM) or high glucose. ATP content was reduced by 30% and mitochondrial calcium decreased by 40% after high glucose. TFAM overexpression returned these parameters to even higher than control values. Calcium transients were prolonged by 70% after high glucose, which was associated with diminished sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a and cytochrome-c oxidase subunit 1 expression. These parameters were returned to control values after TFAM overexpression. High glucose-induced protein oxidation was reduced by TFAM overexpression, indicating a reduction of the high glucose-induced oxidative stress. In addition, we found that TFAM activity can be modulated by O-linked beta-N-acetylglucosamine glycosylation. In conclusion, TFAM overexpression protected cell function against the damage induced by high glucose in cardiomyocytes.

Conditional increase in SERCA2a protein is able to reverse contractile dysfunction and abnormal calcium flux in established diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by reduced cardiac contractility independent of vascular disease. A contributor to contractile dysfunction in the diabetic heart is impaired sarcoplasmic reticulum function with reduced sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) pump activity, leading to disturbed intracellular calcium handling. It is currently unclear whether increasing SERCA2a activity in hearts with existing diabetic cardiomyopathy could still improve calcium flux and contractile performance. To test this hypothesis, we generated a cardiac-specific tetracycline-inducible double transgenic mouse, which allows for doxycycline (DOX)-based inducible SERCA2a expression in which DOX exposure turns on SERCA2a expression. Isolated cardiomyocytes and Langendorff perfused hearts from streptozotocin-induced diabetic mice were studied. Our results show that total SERCA2a protein levels were decreased in the diabetic mice by 60% compared with control. SERCA2a increased above control values in the diabetic mice after DOX. Dysfunctional contractility in the diabetic cardiomyocyte was restored to normal by induction of SERCA2a expression. Calcium transients from diabetic cardiomyocytes showed a delayed rate of diastolic calcium decay of 66%, which was reverted toward normal after SERCA2a expression induced by DOX. Global cardiac function assessed in the diabetic perfused heart showed diminished left ventricular pressure, rate of contraction, and relaxation. These parameters were returned to control values by SERCA2a expression. In conclusion, we have used mice allowing for inducible expression of SERCA2a and could demonstrate that increased expression of SERCA2a leads to improved cardiac function in mice with an already established diabetic cardiomyopathy in absence of detrimental effects.

Physical coupling supports the local Ca2+ transfer between sarcoplasmic reticulum subdomains and the mitochondria in heart muscle

In many cell types, transfer of Ca(2+) released via ryanodine receptors (RyR) to the mitochondrial matrix is locally supported by high [Ca(2+)] microdomains at close contacts between the sarcoplasmic reticulum (SR) and mitochondria. Here we studied whether the close contacts were secured via direct physical coupling in cardiac muscle using isolated rat heart mitochondria (RHMs). "Immuno-organelle chemistry" revealed RyR2 and calsequestrin-positive SR particles associated with mitochondria in both crude and Percoll-purified "heavy" mitochondrial fractions (cRHM and pRHM), to a smaller extent in the latter one. Mitochondria-associated vesicles were also visualized by electron microscopy in the RHMs. Western blot analysis detected greatly reduced presence of SR markers (calsequestrin, SERCA2a, and phospholamban) in pRHM, suggesting that the mitochondria-associated particles represented a small subfraction of the SR. Fluorescence calcium imaging in rhod2-loaded cRHM revealed mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)) responses to caffeine-induced Ca(2+) release that were prevented when thapsigargin was added to predeplete the SR or by mitochondrial Ca(2+) uptake inhibitors. Importantly, caffeine failed to increase [Ca(2+)] in the large volume of the incubation medium, suggesting that local Ca(2+) transfer between the SR particles and mitochondria mediated the [Ca(2+)](m) signal. Despite the substantially reduced SR presence, pRHM still displayed a caffeine-induced [Ca(2+)](m) rise comparable with the one recorded in cRHM. Thus, a relatively small fraction of the total SR is physically coupled and transfers Ca(2+) locally to the mitochondria in cardiac muscle. The transferred Ca(2+) stimulates dehydrogenase activity and affects mitochondrial membrane permeabilization, indicating the broad significance of the physical coupling in mitochondrial function.

Dietary supplementation with vitamin E ameliorates cardiac failure in type I diabetic cardiomyopathy by suppressing myocardial generation of 8-iso-prostaglandin F2alpha and oxidized glutathione

BACKGROUND

Diabetic cardiomyopathy has been documented as an underlying cause of heart failure in diabetic patients. Although oxidative stress has been implicated in diabetic cardiomyopathy, much of the current evidence lacks specificity. Furthermore, studies investigating antioxidant protection with vitamin E in this unique cardiac phenomenon have yet to be performed. In the present study, we sought to determine whether vitamin E supplementation can confer cardioprotective effects against diabetic cardiomyopathy in relation to specific and quantitative markers of myocardial oxidative stress.

METHODS AND RESULTS

Diabetes was induced in rats by a single injection of streptozotocin. Animals were fed either a basal diet or a diet enriched with 2000 IU of vitamin E per kilogram beginning immediately after induction of diabetes and continued for 8 weeks. Rats were examined for diabetic cardiomyopathy by left ventricular (LV) hemodynamic analysis. Myocardial oxidative stress was assessed by measuring the formation of 8-iso-prostaglandin F2alpha and oxidized glutathione. In the unsupplemented streptozotocin-diabetic rats, LV systolic pressure, rate of pressure increase (+dP/dt), and rate of pressure decay (-dP/dt) were depressed, whereas LV end-diastolic pressure was increased, indicating reduced LV contractility and slowing of LV relaxation. These hemodynamic alterations were accompanied by increased myocardial formation of 8-iso-prostaglandin F2alpha and oxidized glutathione. Vitamin E supplementation improved LV function and significantly attenuated myocardial 8-iso-prostaglandin F2alpha and oxidized glutathione accumulation in streptozotocin-diabetic rats.

CONCLUSIONS

These findings demonstrate the usefulness of vitamin E supplementation during the early phases of type I diabetes for the prophylaxis of cardiomyopathy and subsequent heart failure.

Inhibition of nitric oxide synthase enhances contractile response of ventricular myocytes from streptozotocin-diabetic rats

The contractile hyporesponsiveness of the streptozotocin diabetic rat heart in vitro to beta-adrenergic agonists is eliminated when the heart is perfused with N(G)-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of nitric oxide synthase (NOS). The following study evaluated the hypothesis that an increased production of NO/cGMP within the diabetic myocyte inhibits the beta-adrenergic-stimulated increase in calcium current and contractile response. Male Sprague-Dawley rats were given an intravenous injection of streptozotocin (60 mg/kg). After 8 weeks, L-type calcium currents were recorded in ventricular myocytes using the whole cell voltage-clamp method. Shortening of isolated myocytes was determined using a video edge detection system. cAMP and cGMP were measured using radioimmunoassay. Nitric oxide production was determined using the Griess assay kit. Basal cGMP levels and nitric oxide production were elevated in diabetic myocytes. Shortening of the diabetic myocytes in response to isoproterenol (1 microM) was markedly diminished. However, there was no detectable difference in the isoproterenol-stimulated L-type calcium current or cAMP levels between control and diabetic myocytes. Acute superfusion of the diabetic myocyte with L-NAME (1 mM) decreased basal cGMP and markedly enhanced the shortening response to isoproterenol but did not alter isoproterenol-stimulated calcium current. These data suggest that increased production of NO/cGMP within the diabetic myocyte suppressed beta-adrenergic stimulated shortening of the myocyte. However, NO/cGMP apparently does not suppress shortening of the myocyte by inhibition of the beta-stimulated calcium current.

Alterations in cardiac contractile performance and sarcoplasmic reticulum function in sucrose-fed rats is associated with insulin resistance

Diabetes mellitus (DM) causes the development of a specific cardiomyopathy that results from the metabolic derangements present in DM and manifests as cardiac contractile dysfunction. Although myocardial dysfunction in Type 1 DM has been associated with defects in the function and regulation of the sarcoplasmic reticulum (SR), very little is known about SR function in Type 2 DM. Accordingly, this study examined whether abnormalities in cardiac contractile performance and SR function occur in the prestage of Type 2 DM (i.e., during insulin resistance). Sucrose feeding was used to induce whole body insulin resistance, whereas cardiac contractile performance was assessed by echocardiography and SR function was measured by SR calcium (Ca2+) uptake. Sucrose-fed rats exhibited hyperinsulinemia, hyperglycemia, and hyperlipidemia relative to control rats. Serial echocardiographic assessments in the sucrose-fed rats revealed early abnormalities in diastolic function followed by late systolic dysfunction and concurrent alterations in myocardial structure. The hearts of the 10-wk sucrose-fed rats showed depressed SR function demonstrated by a significant reduction in SR Ca2+uptake. The decline in SR Ca2+uptake was associated with a significant decrease in the cAMP-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase II-mediated phosphorylation of phospholamban. The results show that abnormalities in cardiac contractile performance and SR function occur at an insulin-resistant stage before the manifestation of overt Type 2 DM.

Restoration of diabetes-induced abnormal local Ca2+ release in cardiomyocytes by angiotensin II receptor blockade

Stimulation of local renin-angiotensin system and increased levels of oxidants characterize the diabetic heart. Downregulation of ANG II type 1 receptors (AT(1)) and enhancement in PKC activity in the heart point out the role of AT(1) blockers in diabetes. The purpose of this study was to evaluate a potential role of an AT(1) blocker, candesartan, on abnormal Ca(2+) release mechanisms and its relationship with PKC in the cardiomyocytes from streptozotocin-induced diabetic rats. Cardiomyocytes were isolated enzymatically and then incubated with either candesartan or a nonspecific PKC inhibitor bisindolylmaleimide I (BIM) for 6-8 h at 37 degrees C. Both candesartan and BIM applied on diabetic cardiomyocytes significantly restored the altered kinetic parameters of Ca(2+) transients, as well as depressed Ca(2+) loading of sarcoplasmic reticulum, basal Ca(2+) level, and spatiotemporal properties of the Ca(2+) sparks. In addition, candesartan and BIM significantly antagonized the hyperphosphorylation of cardiac ryanodine receptor (RyR2) and restored the depleted protein levels of both RyR2 and FK506 binding protein 12.6 (FKBP12.6). Furthermore, candesartan and BIM also reduced the increased PKC levels and oxidized protein thiol level in membrane fraction of diabetic rat cardiomyocytes. Taken together, these data demonstrate that AT(1) receptor blockade protects cardiomyocytes from development of cellular alterations typically associated with Ca(2+) release mechanisms in diabetes mellitus. Prevention of these alterations by candesartan may present a useful pharmacological strategy for the treatment of diabetic cardiomyopathy.

Effects of diabetes on ryanodine receptor Ca release channel (RyR2) and Ca2+ homeostasis in rat heart

The defects identified in the mechanical activity of the hearts from type 1 diabetic animals include alteration of Ca2+ signaling via changes in critical processes that regulate intracellular Ca2+ concentration. These defects result partially from a dysfunction of cardiac ryanodine receptor calcium release channel (RyR2). The present study was designed to determine whether the properties of the Ca2+ sparks might provide insight into the role of RyR2 in the altered Ca2+ signaling in cardiomyocytes from diabetic animals when they were analyzed together with Ca2+ transients. Basal Ca2+ level as well as Ca2+-spark frequency of cardiomyoctes isolated from 5-week streptozotocin (STZ)-induced diabetic rats significantly increased with respect to aged-matched control rats. Ca2+ transients exhibited significantly reduced amplitude and prolonged time courses as well as depressed Ca2+ loading of sarcoplasmic reticulum in diabetic rats. Spatio-temporal properties of the Ca2+ sparks in cardiomyocytes isolated from diabetic rats were also significantly altered to being almost parallel to the changes of Ca2+ transients. In addition, RyR2 from diabetic rat hearts were hyperphosphorylated and protein levels of both RyR2 and FKBP12.6 depleted. These data show that STZ-induced diabetic rat hearts exhibit altered local Ca2+ signaling with increased basal Ca2+ level.

Abnormal myocardial perfusion and contractile recruitment during exercise in type 1 diabetic patients

BACKGROUND

No data are available on the relationship between myocardial perfusion and left ventricular (LV) function in type 1 diabetes mellitus (T1DM), which may constitute a factor explaining the progressive contractile dysfunction to the overt phase of diabetic cardiomyopathy.

HYPOTHESIS

This study was undertaken to test whether myocardial perfusion abnormalities are present at rest and during exercise and whether they are related to contractile dysfunction in T1DM.

METHODS

Twenty-two patients with T1DM, aged 32 +/- 8.3 years, without macro- or microvascular complications, and 10 controls, aged 31 +/- 3 years, were studied. Left ventricular function and myocardial perfusion were assessed by two-dimensional and myocardial contrast echocardiography at rest and during handgrip (HG).

RESULTS

Fourteen patients with T1DM showed a decline in LV ejection fraction (LVEF) during HG (Group 1) while 8 had a normal response (Group 2). Both basal myocardial blood volume (MBV) and velocity (beta) were normal in T1DM. During exercise, MBV and beta increased and were associated with an increase in myocardial blood flow (MBF) in controls. In T1DM, beta did not change and MBV increased only in Group 2, while this increase was not observed in Group 1 (controls: 14.9 +/- 2.3 vs. Group 1: 7.6 +/- 1.6, p < 0.001; and vs. Group 2: 10.2 +/- 2.8, p < 0.001), beta (0.86 +/- 0.12 vs. 0.68 +/- 0.14, p < 0.001; and vs. 0.67 +/- 0.15, p < 0.001). A correlation between the ratio exercise MBF/resting MBF and LVEF at peak exercise in T1DM was observed (r = 0.805, p < 0.001).

CONCLUSIONS

A large proportion of patients with T1DM exhibit abnormalities in myocardial adaptable capacity to match an acute overload, which are related to a defective increase in myocardial perfusion.

Renin-angiotensin blockade attenuates cardiac myofibrillar remodelling in chronic diabetes

Previous studies have shown that the renin-angiotensin system (RAS) is activated in diabetes and this may contribute to the subcellular remodelling and heart dysfunction in this disease. Therefore, we examined the effects of RAS blockade by enalapril, an angiotensin-converting enzyme inhibitor, and losartan, an angiotensin receptor AT1 antagonist, on cardiac function, myofibrillar and myosin ATPase activity as well as myosin heavy chain (MHC) isozyme expression in diabetic hearts. Diabetes was induced in rats by a single injection of streptozotocin (65 mg/kg; i.v.) and these animals were treated with and without enalapril (10 mg/kg/day; oral) or losartan (20 mg/kg/day; oral) for 8 weeks. Enalapril or losartan prevented the depressions in left ventricular rate of pressure development, rate of pressure decay and ventricular weight seen in diabetic animals. Both drugs also attenuated the decrease in myofibrillar Ca2+-ATPase, Mg2+-ATPase and myosin ATPase activity seen in diabetic rats. The diabetes-induced increase in beta-MHC content and gene expression as well as the decrease in alpha-MHC content and mRNA levels were also prevented by enalapril and losartan. These results suggest the occurrence of myofibrillar remodelling in diabetic cardiomyopathy and provide evidence that the beneficial effects of RAS blockade in diabetes may be associated with attenuation of myofibrillar remodelling in the heart.

Role of dual-site phospholamban phosphorylation in intermittent hypoxia-induced cardioprotection against ischemia-reperfusion injury

Cardioprotection by intermittent high-altitude (IHA) hypoxia against ischemia-reperfusion (I/R) injury is associated with Ca(2+) overload reduction. Phospholamban (PLB) phosphorylation relieves cardiac sarcoplasmic reticulum (SR) Ca(2+)-pump ATPase, a critical regulator in intracellular Ca(2+) cycling, from inhibition. To test the hypothesis that IHA hypoxia increases PLB phosphorylation and that such an effect plays a role in cardioprotection, we compared the time-dependent changes in the PLB phosphorylation at Ser(16) (PKA site) and Thr(17) (CaMKII site) in perfused normoxic rat hearts with those in IHA hypoxic rat hearts submitted to 30-min ischemia (I30) followed by 30-min reperfusion (R30). IHA hypoxia improved postischemic contractile recovery, reduced the maximum extent of ischemic contracture, and attenuated I/R-induced depression in Ca(2+)-pump ATPase activity. Although the PLB protein levels remained constant during I/R in both groups, Ser(16) phosphorylation increased at I30 and 1 min of reperfusion (R1) but decreased at R30 in normoxic hearts. IHA hypoxia upregulated the increase further at I30 and R1. Thr(17) phosphorylation decreased at I30, R1, and R30 in normoxic hearts, but IHA hypoxia attenuated the depression at R1 and R30. Moreover, PKA inhibitor H89 abolished IHA hypoxia-induced increase in Ser(16) phosphorylation, Ca(2+)-pump ATPase activity, and the recovery of cardiac performance after ischemia. CaMKII inhibitor KN-93 also abolished the beneficial effects of IHA hypoxia on Thr(17) phosphorylation, Ca(2+)-pump ATPase activity, and the postischemic contractile recovery. These findings indicate that IHA hypoxia mitigates I/R-induced depression in SR Ca(2+)-pump ATPase activity by upregulating dual-site PLB phosphorylation, which may consequently contribute to IHA hypoxia-induced cardioprotection against I/R injury.