Altered metabolism causes cardiac dysfunction in perfused hearts from diabetic (db/db) mice

Role of changes in cardiac metabolism in development of diabetic cardiomyopathy

In patients with diabetes, an increased risk of symptomatic heart failure usually develops in the presence of hypertension or ischemic heart disease. However, a predisposition to heart failure might also reflect the effects of underlying abnormalities in diastolic function that can occur in asymptomatic patients with diabetes alone (termed diabetic cardiomyopathy). Evidence of cardiomyopathy has also been demonstrated in animal models of both Type 1 (streptozotocin-induced diabetes) and Type 2 diabetes (Zucker diabetic fatty rats and ob/ob or db/db mice). During insulin resistance or diabetes, the heart rapidly modifies its energy metabolism, resulting in augmented fatty acid and decreased glucose consumption. Accumulating evidence suggests that this alteration of cardiac metabolism plays an important role in the development of cardiomyopathy. Hence, a better understanding of this dysregulation in cardiac substrate utilization during insulin resistance and diabetes could provide information as to potential targets for the treatment of cardiomyopathy. This review is focused on evaluating the acute and chronic regulation and dysregulation of cardiac metabolism in normal and insulin-resistant/diabetic hearts and how these changes could contribute toward the development of cardiomyopathy.

Cardiac PPARα expression in patients with dilated cardiomyopathy

BACKGROUND

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a central regulator of myocardial fatty acid (FA) metabolism implicated in the pathogenesis of heart failure.

AIMS

To characterize PPARalpha regulation in human dilated cardiomyopathy (DCM), we studied the expression of cardiac PPARalpha, cardiac carnitine palmitoyl-transferase I (CPT-1), a major PPARalpha target gene, and of the cardiac glucose transporter GLUT-4 in patients with DCM.

METHODS

Left ventricular biopsies were taken from patients with DCM (n=16) and control subjects (n=15), and mRNA expression was quantitated using real-time PCR (SYBR((R))Green) and protein expression was measured by Western immunoblotting.

RESULTS

Left ventricular PPARalpha mRNA levels were significantly increased in the DCM group compared to the control group (136+/-25.4% vs. control, p<0.01). Consistently, DCM patients had a significantly higher cardiac CPT-1 mRNA expression (147+/-51% vs. control, p<0.05) compared to the control group. Cardiac GLUT-4 expression was similar in both groups.

CONCLUSION

Elevated cardiac PPARalpha levels followed by an induction of cardiac CPT-1 expression may result in increased fatty acid metabolism for cardiac energy production in DCM, suggesting a specific cardiac metabolic program in human DCM compared to other types of cardiomyopathy.

Lipotoxicity in the heart

Cardiomyopathy is associated with both rare genetic metabolic abnormalities and highly prevalent diseases characterized by elevated serum triglycerides and nonesterified fatty acids, such as obesity and type 2 diabetes. In these disorders, an imbalance between fatty acid uptake and utilization leads to the inappropriate accumulation of free fatty acids and neutral lipids within cardiomyocytes. Through the process of lipotoxicity, this lipid overload causes cellular dysfunction, cell death, and eventual organ dysfunction. This review focuses on lipotoxicity in the heart, with an emphasis on the contribution of this process to the pathogenesis of cardiomyopathy associated with obesity, diabetes, and the metabolic syndrome. The magnitude of the current worldwide epidemic of obesity and type 2 diabetes suggests that understanding the pathogenesis of cardiac complications associated with these diseases will contribute substantially to improvements in health care.

Perfused hearts from Type 2 diabetic (db/db) mice show metabolic responsiveness to insulin

Diabetic ( db/db) mice provide an animal model of Type 2 diabetes characterized by marked in vivo insulin resistance. The effect of insulin on myocardial metabolism has not been fully elucidated in this diabetic model. In the present study we tested the hypothesis that the metabolic response to insulin in db/db hearts will be diminished due to cardiac insulin resistance. Insulin-induced changes in glucose oxidation (GLUox) and fatty acid (FA) oxidation (FAox) were measured in isolated hearts from control and diabetic mice, perfused with both low as well as high concentration of glucose and FA: 10 mM glucose/0.5 mM palmitate and 28 mM glucose/1.1 mM palmitate. Both in the absence and presence of insulin, diabetic hearts showed decreased rates of GLUox and elevated rates of FAox. However, the insulin-induced increment in GLUox, as well as the insulin-induced decrement in FAox, was similar or even more pronounced in diabetic that in control hearts. During elevated FA and glucose supply, however, the effect of insulin was blunted in db/db hearts with respect to both FAox and GLUox. Finally, insulin-stimulated deoxyglucose uptake was markedly reduced in isolated cardiomyocytes from db/db mice, whereas glucose uptake in isolated perfused db/db hearts was clearly responsive to insulin. These results show that, despite reduced insulin-stimulated glucose uptake in isolated cardiomyocytes, isolated perfused db/db hearts are responsive to metabolic actions of insulin. These results should advocate the use of insulin therapy (glucose-insulin-potassium) in diabetic patients undergoing cardiac surgery or during reperfusion after an ischemic insult.

Cardiac metabolism in mice: tracer method developments and in vivo application revealing profound metabolic inflexibility in diabetes

Studies of cardiac fuel metabolism in mice have been almost exclusively conducted ex vivo. The major aim of this study was to assess in vivo plasma FFA and glucose utilization by the hearts of healthy control (db/+) and diabetic (db/db) mice, based on cardiac uptake of (R)-2-[9,10-(3)H]bromopalmitate ([3H]R-BrP) and 2-deoxy-D-[U-14C]glucose tracers. To obtain quantitative information about the evaluation of cardiac FFA utilization with [3H]R-BrP, simultaneous comparisons of [3H]R-BrP and [14C]palmitate ([14C]P) uptake were first made in isolated perfused working hearts from db/+ mice. It was found that [3H]R-BrP uptake was closely correlated with [14C]P oxidation (r2 = 0.94, P < 0.001). Then, methods for in vivo application of [3H]R-BrP and [14C]2-DG previously developed for application in the rat were specially adapted for use in the mouse. The method yields indexes of cardiac FFA utilization (R(f)*) and clearance (K(f)*), as well as glucose utilization (R(g)'). Finally, in the main part of the study, the ability of the heart to switch between FFA and glucose fuels (metabolic flexibility) was investigated by studying anesthetized, 8-h-fasted control and db/db mice in either the basal state or during glucose infusion. In control mice, glucose infusion raised plasma levels of glucose and insulin, raised R(g)' (+58%), and lowered plasma FFA level (-48%), K(f)* (-45%), and R(f)* (-70%). This apparent reciprocal regulation of glucose and FFA utilization by control hearts illustrates metabolic flexibility for substrate use. By contrast, in the db/db mice, glucose infusion raised glucose levels with no apparent influence on cardiac FFA or glucose utilization. In conclusion, tracer methodology for assessing in vivo tissue-specific plasma FFA and glucose utilization has been adapted for use in mice and reveals a profound loss of metabolic flexibility in the diabetic db/db heart, suggesting a fixed level of FFA oxidation in fasted and glucose-infused states.

Cardioprotective mechanisms of the kallikrein-kinin system in diabetic cardiopathy

PURPOSE OF REVIEW

Multiple pathogenic mechanisms contribute to the development of diabetic cardiopathy, including intramyocardial inflammation, cardiac fibrosis, abnormal intracellular Ca handling, microangiopathy and endothelial dysfunction. Moreover, the cardiac kallikrein-kinin system is thought to be altered under diabetic conditions and an improvement of this peptide system, e.g. by gene therapeutic approaches, has also been associated with an amelioration of the diabetic heart. In this review, we will discuss the hypothesis that the stimulation of the kallikrein-kinin system could be a promising target for the treatment of diabetic cardiopathy.

RECENT FINDINGS

The kallikrein-kinin system has cardioprotective properties, which may be particularly important under diabetic conditions. For example, its potential for endothelium-dependent vasodilation, and for improvement of glucose transport and utilization, make bradykinin an important mediator for reducing the consequences of diabetes-related oxidative stress on both the myocardium and vessels.

SUMMARY

The different synergistic cardioprotective effects of the kallikrein-kinin system in the diabetic heart suggest that the stimulation of the kallikrein-kinin system might open new avenues for the treatment of diabetic cardiopathy.

Transportadores de glicose na síndrome metabólica

A regulação da homeostasia intra e extra-celular da glicose está diretamente relacionada ao controle preciso da expressão dos genes que codificam as diferentes isoformas de proteínas transportadoras de glicose, as quais se expressam de maneira tecido-específica, em conseqüência do padrão de ativação dos fatores transcricionais reguladores de cada gene, em cada tipo celular. A síndrome metabólica (SM) abrange uma grande variedade de alterações fisiopatológicas, todas de repercussões sistêmicas, acometendo os mais distintos territórios do organismo, nos quais alterações nos transportadores de glicose presentes são observadas em maior ou menor grau. A presente revisão abordará as alterações na expressão de transportadores de glicose claramente demonstradas na literatura, cujas repercussões nos fluxos territoriais de glicose auxiliam na compreensão de mecanismos fisiopatológicos da SM, assim como dos tratamentos propostos para esta entidade.

Diabetic cardiomyopathy: the search for a unifying hypothesis

Although diabetes is recognized as a potent and prevalent risk factor for ischemic heart disease, less is known as to whether diabetes causes an altered cardiac phenotype independent of coronary atherosclerosis. Left ventricular systolic and diastolic dysfunction, left ventricular hypertrophy, and alterations in the coronary microcirculation have all been observed, although not consistently, in diabetic cardiomyopathy and are not fully explained by the cellular effects of hyperglycemia alone. The recent recognition that diabetes involves more than abnormal glucose homeostasis provides important new opportunities to examine and understand the impact of complex metabolic disturbances on cardiac structure and function.

Reduced mitochondrial oxidative capacity and increased mitochondrial uncoupling impair myocardial energetics in obesity

BACKGROUND

Obesity is a risk factor for cardiovascular disease and is strongly associated with insulin resistance and type 2 diabetes. Recent studies in obese humans and animals demonstrated increased myocardial oxygen consumption (MVO2) and reduced cardiac efficiency (CE); however, the underlying mechanisms remain unclear. The present study was performed to determine whether mitochondrial dysfunction and uncoupling are responsible for reduced cardiac performance and efficiency in ob/ob mice.

METHODS AND RESULTS

Cardiac function, MVO2, mitochondrial respiration, and ATP synthesis were measured in 9-week-old ob/ob and control mouse hearts. Contractile function and MVO2 in glucose-perfused ob/ob hearts were similar to controls under basal conditions but were reduced under high workload. Perfusion of ob/ob hearts with glucose and palmitate increased MVO2 and reduced CE by 23% under basal conditions, and CE remained impaired at high workload. In glucose-perfused ob/ob hearts, mitochondrial state 3 respirations were reduced but ATP/O ratios were unchanged. In contrast, state 3 respiration rates were similar in ob/ob and control mitochondria from hearts perfused with palmitate and glucose, but ATP synthesis rates and ATP/O ratios were significantly reduced in ob/ob, which suggests increased mitochondrial uncoupling. Pyruvate dehydrogenase activity and protein levels of complexes I, III, and V were reduced in obese mice.

CONCLUSIONS

These data indicate that reduced mitochondrial oxidative capacity may contribute to cardiac dysfunction in ob/ob mice. Moreover, fatty acid but not glucose-induced mitochondrial uncoupling reduces CE in obese mice by limiting ATP production and increasing MVO2.

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.

Intracellular sodium increase and susceptibility to ischaemia in hearts from type 2 diabetic db/db mice

AIMS/HYPOTHESIS

An important determinant of sensitivity to ischaemia is altered ion homeostasis, especially disturbances in intracellular Na(+) (Na(i)(+)) handling. As no study has so far investigated this in type 2 diabetes, we examined susceptibility to ischaemia-reperfusion in isolated hearts from diabetic db/db and control db/+ mice and determined whether and to what extent the amount of (Na(i)(+)) increase during a transient period of ischaemia could contribute to functional alterations upon reperfusion.

METHODS

Isovolumic hearts were exposed to 30-min global ischaemia and then reperfused. (23)Na nuclear magnetic resonance (NMR) spectroscopy was used to monitor[Formula: see text] and (31)P NMR spectroscopy to monitor intracellular pH (pH(i)).

RESULTS

A higher duration of ventricular tachycardia and the degeneration of ventricular tachycardia into ventricular fibrillation were observed upon reperfusion in db/db hearts. The recovery of left ventricular developed pressure was reduced. The increase in[Formula: see text] induced by ischaemia was higher in db/db hearts than in control hearts, and the rate of pH(i) recovery was increased during reperfusion. The inhibition of Na(+)/H(+) exchange by cariporide significantly reduced (Na(i)(+)) gain at the end of ischaemia. This was associated with a lower incidence of ventricular tachycardia in both heart groups, and with an inhibition of the degeneration of ventricular tachycardia into ventricular fibrillation in db/db hearts.

CONCLUSIONS/INTERPRETATION

These findings strongly support the hypothesis that increased (Na(i)(+)) plays a causative role in the enhanced sensitivity to ischaemia observed in db/db diabetic hearts.

Accelerated triacylglycerol turnover kinetics in hearts of diabetic rats include evidence for compartmented lipid storage

Triacylglycerol (TAG) storage and turnover rates in the intact, beating rat heart were determined for the first time using dynamic mode (13)C- NMR spectroscopy to elucidate profound differences between hearts from diabetic rats (DR, streptozotocin treatment) and normal rats (NR). The incorporation of [2,4,6,8,10,12,14,16-(13)C(8)]palmitate into the TAG pool was monitored in isolated hearts perfused with physiological (0.5 mM palmitate, 5 mM glucose) and elevated substrate levels (1.2 mM palmitate, 11 mM glucose) characteristic of the diabetic condition. Surprisingly, although the normal hearts were enriched at a near-linear profile for >or=2 h before exponential characterization, exponential enrichment of TAG in diabetic hearts reached steady state after only 45 min. Consequently, TAG turnover rate was determined by fitting an exponential model to enrichment data rather than conventional two-point linear analysis. In the high-substrate group, both turnover rate (DR 820+/- 330, NR 190 +/-150 nmol.min(-1).g(-1) dry wt; P< 0.001) and [TAG] content (DR 78 +/-10, NR 32+/- 4 micromol/g dry wt; P< 0.001) were greater in the diabetic group. At lower substrate concentrations, turnover was greater in diabetics (DR 530+/-300, NR 160+/- 30; P<0.05). However, this could not be explained by simple mass action, because [TAG] content was similar between groups [DR 34+/- 7, NR 39+/- 9 micromol/g dry wt; not significant (NS)]. Consistent with exponential enrichment data, (13)C fractional enrichment of TAG was lower in diabetics (low- substrate groups: DR 4+/-1%, NR 10+/- 4%, P<0.05; high-substrate groups: DR 8+/- 3%, NR 14+/- 9%, NS), thereby supporting earlier speculation that TAG is compartmentalized in the diabetic heart.

Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

AIM

The aim of the present study was to compare the coronary flow (CF) reserve of ex vivo perfused hearts from type 2 diabetic (db/db) and non-diabetic (db/+) mice.

METHODS

The hearts were perfused in the Langendorff mode with Krebs-Henseleit bicarbonate buffer (37 degrees C, pH 7.4) containing 11 mmol L(-1) glucose as energy substrate. The coronary reserve was measured in response to three different interventions: (1) administration of nitroprusside (a nitric oxide donor), (2) administration of adenosine and (3) production of reactive hyperaemia by short-term ischaemia.

RESULTS

Basal CF was approximately 15% lower in diabetic when compared with non-diabetic hearts (2.1 +/- 0.1 vs. 2.6 +/- 0.2 mL min(-1)). The maximum increase in CF rate in response to sodium nitroprusside and adenosine was significantly lower in diabetic (0.6 +/- 0.1 and 0.9 +/- 0.1 mL min(-1) respectively) than in non-diabetic hearts (1.2 +/- 0.1 and 1.4 +/- 0.1 mL min(-1) respectively). Also, there was a clear difference in the rate of return to basal CF following short-term ischaemia between diabetic and non-diabetic hearts. Thus, basal tone was restored 1-2 min after the peak hyperaemic response in non-diabetic hearts, whereas it took approximately 5 min in diabetic hearts.

CONCLUSION

These results show that basal CF, as well as the CF reserve, is impaired in hearts from type 2 diabetic mice. As diabetic and non-diabetic hearts were exposed to the same (maximum) concentrations of NO or adenosine, it is suggested that the lower coronary reserve in type 2 diabetic hearts is, in part, because of a defect in the intracellular pathways mediating smooth muscle relaxation.

Mechanisms of [Ca2+]i transient decrease in cardiomyopathy of db/db type 2 diabetic mice

Cardiovascular disease is the leading cause of death in the diabetic population. However, molecular mechanisms underlying diabetic cardiomyopathy remain unclear. We analyzed Ca2+-induced Ca2+ release and excitation-contraction coupling in db/db obese type 2 diabetic mice and their control littermates. Echocardiography showed a systolic dysfunction in db/db mice. Two-photon microscopy identified intracellular calcium concentration ([Ca2+]i) transient decrease in cardiomyocytes within the whole heart, which was also found in isolated myocytes by confocal microscopy. Global [Ca2+]i transients are constituted of individual Ca2+ sparks. Ca2+ sparks in db/db cardiomyocytes were less frequent than in +/+ myocytes, partly because of a depression in sarcoplasmic reticulum Ca2+ load but also because of a reduced expression of ryanodine receptor Ca2+ channels (RyRs), revealed by [3H]ryanodine binding assay. Ca2+ efflux through Na+/Ca2+ exchanger was increased in db/db myocytes. Calcium current, I(Ca), triggers sarcoplasmic reticulum Ca2+ release and is also involved in sarcoplasmic reticulum Ca2+ refilling. Macroscopic I(Ca) was reduced in db/db cells, but single Ca2+ channel activity was similar, suggesting that diabetic myocytes express fewer functional Ca2+ channels, which was confirmed by Western blots. These results demonstrate that db/db mice show depressed cardiac function, at least in part, because of a general reduction in the membrane permeability to Ca2+. As less Ca2+ enters the cell through I(Ca), less Ca2+ is released through RyRs.

Reduced cardiac efficiency and altered substrate metabolism precedes the onset of hyperglycemia and contractile dysfunction in two mouse models of insulin resistance and obesity

Hyperglycemia is associated with altered myocardial substrate use, a condition that has been hypothesized to contribute to impaired cardiac performance. The goals of this study were to determine whether changes in cardiac metabolism, gene expression, and function precede or follow the onset of hyperglycemia in two mouse models of obesity, insulin resistance, and diabetes (ob/ob and db/db mice). Ob/ob and db/db mice were studied at 4, 8, and 15 wk of age. Four-week-old mice of both strains were normoglycemic but hyperinsulinemic. Hyperglycemia develops in db/db mice between 4 and 8 wk of age and in ob/ob mice between 8 and 15 wk. In isolated working hearts, rates of glucose oxidation were reduced by 28-37% at 4 wk and declined no further at 15 wk in both strains. Fatty acid oxidation rates and myocardial oxygen consumption were increased in 4-wk-old mice of both strains. Fatty acid oxidation rates progressively increased in db/db mice in parallel with the earlier onset and greater duration of hyperglycemia. In vivo, cardiac catheterization revealed significantly increased left ventricular contractility and relaxation (positive and negative dP/dt) in both strains at 4 wk of age. dP/dt declined over time in db/db mice but remained elevated in ob/ob mice at 15 wk of age. Increased beta-myosin heavy chain isoform expression was present in 4-wk-old mice and persisted in 15-wk-old mice. Increased expression of peroxisomal proliferator-activated receptor-alpha regulated genes was observed only at 15 wk in both strains. These data indicate that altered myocardial substrate use and reduced myocardial efficiency are early abnormalities in the hearts of obese mice and precede the onset of hyperglycemia. Obesity per se does not cause contractile dysfunction in vivo, but loss of the hypercontractile phenotype of obesity and up-regulation of peroxisomal proliferator-activated receptor-alpha regulated genes occur later and are most pronounced in the presence of longstanding hyperglycemia.

Activation of PPARgamma enhances myocardial glucose oxidation and improves contractile function in isolated working hearts of ZDF rats

It is suggested that insulin resistance and metabolic maladaptation of the heart are causes of contractile dysfunction. We tested the hypothesis whether systemic PPARgamma activation, by changing the metabolic profile in a model of insulin resistance and type 2 diabetes (the ZDF rat) in vivo, improves contractile function of the heart in vitro. Male Zucker diabetic fatty (ZDF) and Zucker lean (ZL) rats, at 53-56 days of age, were treated with either GI-262570 (a nonthiazolidinedione PPARgamma agonist; A) or vehicle (V) for 1 wk. Agonist treatment resulted in correction of hyperglycemia and dyslipidemia, as well as in reduced hyperinsulinemia. The accumulation of triacylglycerols in the myocardium, characteristic of the ZDF rat, disappeared with treatment. Cardiac power and rates of glucose oxidation in the isolated working heart were significantly reduced in ZDF-V rats, but both parameters increased to nondiabetic levels with agonist treatment. In ZDF-V hearts, transcript levels of PPARalpha-regulated genes and of myosin heavy chain-beta were upregulated, whereas GLUT4 was downregulated compared with ZL. Agonist treatment of ZDF rats reduced PPARalpha-regulated genes and increased transcripts of GLUT4 and GLUT1. In conclusion, by changing the metabolic profile, reducing myocardial lipid accumulation, and promoting the downregulation of PPARalpha-regulated genes, PPARgamma activation leads to an increased capacity of the myocardium to oxidize glucose and to a tighter coupling of oxidative metabolism and contraction in the setting of insulin resistance and type 2 diabetes.

Endothelial dysfunction in Type 2 diabetes correlates with deregulated expression of the tail-anchored membrane protein SLMAP

The Type 2 diabetic db/ db mouse experiences vascular dysfunction typified by changes in the contraction and relaxation profiles of small mesenteric arteries (SMAs). Contractions of SMAs from the db/ db mouse to the α1-adrenoceptor agonist phenylephrine (PE) were significantly enhanced, and acetylcholine (ACh)-induced relaxations were significantly depressed. Drug treatment of db/ db mice with a nonthiazolidinedione peroxisome prolifetor-activated receptor-γ agonist and insulin sensitizing agent 2-[2-(4-phenoxy-2-propylphenoxy)ethyl]indole-5-acetic acid (COOH) completely prevented the changes in endothelium-dependent relaxation, but, with the discontinuation of therapy, endothelial dysfunction returned. Dysfunctional SMAs were found to specifically upregulate the expression of a 35-kDa isoform of sarcolemmal membrane-associated protein (SLMAP), which is a component of the excitation-contraction coupling apparatus and implicated in the regulation of membrane function in muscle cells. Real-time PCR revealed high SLMAP mRNA levels in the db/ db microvasculature, which were markedly downregulated during COOH treatment but elevated again when drug therapy was discontinued. These data reveal that the microvasculature in db/ db mice undergoes significant changes in vascular function with the endothelial component of vascular dysfunction specifically correlating with the overexpression of SLMAP. Thus changes in SLMAP expression may be an important indicator for microvascular disease associated with Type 2 diabetes.

Modulation of potassium currents by angiotensin and oxidative stress in cardiac cells from the diabetic rat

Diabetes induces oxidative stress and leads to attenuation of cardiac K+ currents. We investigated the role of superoxide ions and angiotensin II (ANG II) in generating and linking oxidative stress to the modulation of K+ currents under diabetic conditions. K+ currents were measured using patch-clamp methods in ventricular myocytes from streptozotocin (STZ)-induced diabetic rats. Superoxide ion levels, indicating oxidative stress, were measured by fluorescent labelling with dihydroethidium (DHE). ANG II content was measured using enzyme-linked immunosorbent asssay (ELISA). The results showed DHE fluorescence to be significantly higher in cells from diabetic males, compared to controls. Relief of stress by the NADPH oxidase inhibitor apocynin or by superoxide dismutase (SOD) but not by catalase reversed the attenuation of K+ currents and reduced DHE fluorescence. In cells from diabetic females, neither apocynin nor SOD augmented K+ currents, ANG II was not elevated and DHE fluorescence was significantly weaker than in cells from males. Reduced glutathione (GSH) also augmented K+ currents in cells from diabetic males but not females. In ovariectomized diabetic females K+ currents were augmented by GSH and apocynin. Current augmentation and the attenuation of DHE fluorescence by apocynin were significantly blunted by excess ANG II (300 nm). Diabetic male rats pretreated with the angiotensin-converting enzyme (ACE) inhibitor quinapril were hyperglycaemic, but their cellular ANG II levels and DHE fluorescence were significantly decreased. In cells from these rats, K+ currents were insensitive to apocynin. In conclusion, diabetes-related oxidative stress attenuates K+ currents through ANG II-generated increased superoxide ion levels. When ANG II levels are lower, as in diabetic females or following ACE inhibition in males, oxidative stress is reduced, with blunted alterations in K+ currents.

Influence of substrate supply on cardiac efficiency, as measured by pressure-volume analysis in ex vivo mouse hearts

In the present study, we tested the reliability of measurements of pressure-volume area (PVA) and oxygen consumption (MV̇o2) in ex vivo mouse hearts, combining the use of a miniaturized conductance catheter and a fiber-optic oxygen sensor. Second, we tested whether we could reproduce the influence of increased myocardial fatty acid (FA) metabolism on cardiac efficiency in the isolated working mouse heart model, which has already been documented in large animal models. The hearts were perfused with crystalloid buffer containing 11 mM glucose and two different concentrations of FA bound to 3% BSA. The initial concentration was 0.3 ± 0.1 mM, which was subsequently raised to 0.9 ± 0.1 mM. End-systolic and end-diastolic pressure-volume relationships were assessed by temporarily occluding the preload line. Different steady-state PVA-MV̇o2relationships were obtained by changing the loading conditions (pre- and afterload) of the heart. There were no apparent changes in baseline cardiac performance or contractile efficiency (slope of the PVA-MV̇o2regression line) in response to the elevation of the perfusate FA concentration. However, all hearts ( n = 8) showed an increase in the y-intercept of the PVA-MV̇o2regression line after elevation of the palmitate concentration, indicating an FA-induced increase in the unloaded MV̇o2. Therefore, in the present model, unloaded MV̇o2is not independent of metabolic substrate. This is, to our knowledge, the first report of a PVA-MV̇o2relationship in ex vivo perfused murine hearts, using a pressure-volume catheter. The methodology can be an important tool for phenotypic assessment of the relationship among metabolism, contractile performance, and cardiac efficiency in various mouse models.

PPAR-α activation required for decreased glucose uptake and increased susceptibility to injury during ischemia

The transcription of key metabolic regulatory enzymes in the heart is altered in the diabetic state, yet little is known of the underlying mechanisms. The aim of this study was to investigate the role of peroxisome proliferator-activated receptor-α (PPAR-α) in modulating cardiac insulin-sensitive glucose transporter (GLUT-4) protein levels in altered metabolic states and to determine the functional consequences by assessing cardiac ischemic tolerance. Wild-type and PPAR-α-null mouse hearts were isolated and perfused 6 wk after streptozotocin administration or after 14 mo on a high-fat diet or after a 24-h fast. Myocardial d-[2-3H]glucose uptake was measured during low-flow ischemia, and differences in GLUT-4 protein levels were quantified using Western blotting. In wild-type mice in all three metabolic states, elevated plasma free fatty acids were associated with lower total cardiac GLUT-4 protein levels and decreased glucose uptake during ischemia, resulting in poor postischemic functional recovery. Although PPAR-α-null mice also had elevated plasma free fatty acids, they had neither decreased cardiac GLUT-4 levels nor decreased glucose uptake during ischemia and, consequently, did not have poor recovery during reperfusion. We conclude that elevated plasma free fatty acids are associated with increased injury during ischemia due to decreased cardiac glucose uptake resulting from lower cardiac GLUT-4 protein levels, the levels of GLUT-4 being regulated, probably indirectly, through PPAR-α activation.

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

Mitochondrial energy metabolism in heart failure: a question of balance

The mitochondrion serves a critical role as a platform for energy transduction, signaling, and cell death pathways relevant to common diseases of the myocardium such as heart failure. This review focuses on the molecular regulatory events and downstream effector pathways involved in mitochondrial energy metabolic derangements known to occur during the development of heart failure.

Metabolic effects of insulin on cardiomyocytes from control and diabetic db/db mouse hearts

Diabetic db/db mice exhibit profound insulin resistance in vivo, but the specific degree of cardiac insensitivity to insulin has not been assessed. Therefore, the effect of insulin on cardiomyocytes from db/db hearts was assessed by measuring two metabolic responses (deoxyglucose uptake and fatty acid oxidation) and the phosphorylation of two enzymes in the insulin-signaling cascade [Akt and AMP-activated protein kinase (AMPK)]. Maximal insulin-stimulated deoxyglucose transport was reduced to 58 and 40% of control in cardiomyocytes from db/db mice at two ages (6 and 12 wk). Insulin-stimulated deoxyglucose uptake was also reduced in myocytes from transgenic db/db mice overexpressing the insulin-sensitive glucose transporter (db/db-hGLUT4). Treatment of db/db mice for 1 wk with an insulin-sensitizing peroxisome proliferator-activated receptor-gamma agonist (COOH) completely normalized insulin-stimulated deoxyglucose uptake. Insulin had no direct effect on palmitate oxidation by either control or db/db cardiomyocytes, but the combination of insulin and glucose reduced palmitate oxidation, likely an indirect effect secondary to increased glucose uptake. Insulin had no effect on AMPK phosphorylation from either control or db/db cardiomyocytes. Insulin increased the phosphorylation of Akt in all cardiomyocyte preparations (control, db/db, COOH-treated db/db) to the same extent. Thus insulin has selective metabolic actions in mouse cardiomyocytes; deoxyglucose uptake and Akt phosphorylation are increased, but fatty acid oxidation and AMPK phosphorylation are unchanged. Insulin resistance in db/db cardiomyocytes is manifested by reduced insulin-stimulated deoxyglucose uptake.

Metabolic fuel selection: the importance of being flexible

Studies in genetically engineered mice have shown the importance of cross-talk between organs in the regulation of energy metabolism. In this issue, a careful metabolic characterization of mice with genetic deficiency of the GLUT4 glucose transporter in adipocytes and muscle is reported. These mice compensate for decreased peripheral glucose disposal by increasing hepatic glucose uptake and lipid synthesis as well as by increasing lipid utilization in peripheral tissues. These findings are relevant to humans with type 2 diabetes, in whom a key feature is diminished peripheral glucose disposal.

Impact of altered substrate utilization on cardiac function in isolated hearts from Zucker diabetic fatty rats

The goal of this study was to determine whether changes in cardiac metabolism in Type 2 diabetes are associated with contractile dysfunction or impaired response to ischemia. Hearts from Zucker diabetic fatty (ZDF) and lean control rats were isolated and perfused with glucose, lactate, pyruvate, and palmitate. The rates of glucose, lactate, pyruvate, and palmitate oxidation rates and glycolysis were determined during baseline perfusion and low-flow ischemia (LFI; 0.3 ml/min for 30 min) and after LFI and reperfusion. Under all conditions, ATP synthesis from palmitate was increased and synthesis from lactate was decreased in the ZDF group, whereas the contribution from glucose was unchanged. During baseline perfusion, the rate of glycolysis was lower in the ZDF group; however, during LFI and reperfusion, there were no differences between groups. Despite these metabolic shifts, there were no differences in oxygen consumption or ATP production rates between the groups under any perfusion conditions. Cardiac function was slightly depressed before LFI in the ZDF group, but during reperfusion, function was improved relative to the control group despite the increased dependence on fatty acids for energy production. These data suggest that in this model of diabetes, the shift from carbohydrates to fatty acids for oxidative energy production did not increase myocardial oxygen consumption and was not associated with impaired response to ischemia and reperfusion.

Decreased sarcoplasmic reticulum activity and contractility in diabetic db/db mouse heart

Although it is known that insulin-dependent (type 1) diabetes results in depressed contractile performance associated with diminished sarcoendoplasmic reticular Ca2+-ATPase (SERCA2a) activity, findings in insulin-resistant (type 2) diabetes suggest a less clear association. The db/db insulin-resistant mouse model exhibits decreased cardiac performance both in situ and in isolated ex vivo working hearts. In this study, contractile performance and calcium transients were measured in Langendorff-perfused hearts and isolated cardiac myocytes. Diabetic (db/db) mouse hearts demonstrated decreased rates of contraction, relaxation, and pressure development. Calcium transients from isolated myocytes revealed significantly lower diastolic and systolic levels of calcium in diabetic hearts. Furthermore, the decay rate of the calcium transient was significantly reduced in diabetic myocytes, suggesting a diminished capacity for cytosolic calcium removal not associated with a change in sodium-calcium exchanger activity. Calcium leakage from the sarcoplasmic reticulum (SR) measured using tetracaine was significantly increased in diabetic myocytes. Western blot analysis indicated only a small decrease in SERCA2a expression in diabetic mice, but a large increase in phospholamban expression. Expression of the ryanodine receptor did not differ between groups. In conclusion, the decreased contractile function observed in the db/db diabetic mouse model appears to be related to decreased calcium handling by the SR.

Insulin signaling in heart muscle: Lessons from genetically engineered mouse models

The heart is an insulin-responsive organ, and disorders of insulin action, such as diabetes and obesity, can have profound effects on cardiac performance. Insulin signaling influences numerous functions within the heart, such as metabolic substrate preference, cell size, and the response of the heart to ischemia and hypertrophy. Because the systemic consequences of altered insulin action can have significant but indirect effects on the heart, the generation of mice with altered expression of insulin receptors and key components of the insulin-signal transduction pathways in cardiomyocytes have led to interesting and occasionally surprising new insights into the regulation of cardiac biology by insulin.

Enhanced left ventricular systolic function early in type 2 diabetic mice: clinical implications

It is unclear whether the increase in availability of substrates for energy production in diabetes can lead to enhanced systolic function early in the disease, before the onset of structural changes to the myocardium. To examine this issue, BKS.Cg-m +/+ Lepr db (db/db) mice with type 2 diabetes and wild type controls had left ventricular pressure-volume relationships determined in situ. We demonstrated that the db/db mice, when compared to their wild type controls, generated greater left ventricular pressure and an enhancement of left ventricular systolic function based on enhanced power/EDV, positive dP/dt, preload recruitable stroke work, dP/dt--EDV relationship, and curvilinear end-systolic elastance. This enhancement in systolic function occurred despite the db/db mice having greater body weight, but similar preload (end-diastolic volume) and afterload (effective arterial elastance). We postulate that the previously described enhancement in renal glomerular filtration rate seen early in type 2 diabetes may be in part due to enhanced left ventricular systolic function early in this disease.

Nuclear receptor signaling and cardiac energetics

The heart has a tremendous capacity for ATP generation, allowing it to function as an efficient pump throughout the life of the organism. The adult myocardium uses either fatty acid or glucose oxidation as its main energy source. Under normal conditions, the adult heart derives most of its energy through oxidation of fatty acids in mitochondria. However, the myocardium has a remarkable ability to switch between carbohydrate and fat fuel sources so that ATP production is maintained at a constant rate in diverse physiological and dietary conditions. This fuel selection flexibility is important for normal cardiac function. Although cardiac energy conversion capacity and metabolic flux is modulated at many levels, an important mechanism of regulation occurs at the level of gene expression. The expression of genes involved in multiple energy transduction pathways is dynamically regulated in response to developmental, physiological, and pathophysiological cues. This review is focused on gene transcription pathways involved in short- and long-term regulation of myocardial energy metabolism. Much of our knowledge about cardiac metabolic regulation comes from studies focused on mitochondrial fatty acid oxidation. The genes involved in this key energy metabolic pathway are transcriptionally regulated by members of the nuclear receptor superfamily, specifically the fatty acid-activated peroxisome proliferator-activated receptors (PPARs) and the nuclear receptor coactivator, PPARgamma coactivator-1alpha (PGC-1alpha). The dynamic regulation of the cardiac PPAR/PGC-1 complex in accordance with physiological and pathophysiological states will be described.

Impaired cardiac efficiency and increased fatty acid oxidation in insulin-resistant ob/ob mouse hearts

Diabetes alters cardiac substrate metabolism. The cardiac phenotype in insulin-resistant states has not been comprehensively characterized. The goal of these studies was to determine whether the hearts of leptin-deficient 8-week-old ob/ob mice were able to modulate cardiac substrate utilization in response to insulin or to changes in fatty acid delivery. Ob/ob mice were insulin resistant and glucose intolerant. Insulin signal transduction and insulin-stimulated glucose uptake were markedly impaired in ob/ob cardiomyocytes. Insulin-stimulated rates of glycolysis and glucose oxidation were 1.5- and 1.8-fold higher in wild-type hearts, respectively, versus ob/ob, and glucose metabolism in ob/ob hearts was unresponsive to insulin. Increasing concentrations of palmitate from 0.4 mmol/l (low) to 1.2 mmol/l (high) led to a decline in glucose oxidation in wild-type hearts, whereas glucose oxidation remained depressed and did not change in ob/ob mouse hearts. In contrast, fatty acid utilization in ob/ob hearts was 1.5- to 2-fold greater in the absence or presence of 1 nmol/l insulin and rose with increasing palmitate concentrations. Moreover, the ability of insulin to reduce palmitate oxidation rates was blunted in the hearts of ob/ob mice. Under low-palmitate and insulin-free conditions, cardiac performance was significantly greater in wild-type hearts. However, in the presence of high palmitate and 1 nmol/l insulin, cardiac performance in ob/ob mouse hearts was relatively preserved, whereas function in wild-type mouse hearts declined substantially. Under all perfusion conditions, myocardial oxygen consumption was higher in ob/ob hearts, ranging from 30% higher in low-palmitate conditions to greater than twofold higher under high-palmitate conditions. These data indicate that although the hearts of glucose-intolerant ob/ob mice are capable of maintaining their function under conditions of increased fatty acid supply and hyperinsulinemia, they are insulin-resistant, metabolically inefficient, and unable to modulate substrate utilization in response to changes in insulin and fatty acid supply.

Diabetic cardiomyopathy: evidence, mechanisms, and therapeutic implications

The presence of a diabetic cardiomyopathy, independent of hypertension and coronary artery disease, is still controversial. This systematic review seeks to evaluate the evidence for the existence of this condition, to clarify the possible mechanisms responsible, and to consider possible therapeutic implications. The existence of a diabetic cardiomyopathy is supported by epidemiological findings showing the association of diabetes with heart failure; clinical studies confirming the association of diabetes with left ventricular dysfunction independent of hypertension, coronary artery disease, and other heart disease; and experimental evidence of myocardial structural and functional changes. The most important mechanisms of diabetic cardiomyopathy are metabolic disturbances (depletion of glucose transporter 4, increased free fatty acids, carnitine deficiency, changes in calcium homeostasis), myocardial fibrosis (association with increases in angiotensin II, IGF-I, and inflammatory cytokines), small vessel disease (microangiopathy, impaired coronary flow reserve, and endothelial dysfunction), cardiac autonomic neuropathy (denervation and alterations in myocardial catecholamine levels), and insulin resistance (hyperinsulinemia and reduced insulin sensitivity). This review presents evidence that diabetes is associated with a cardiomyopathy, independent of comorbid conditions, and that metabolic disturbances, myocardial fibrosis, small vessel disease, cardiac autonomic neuropathy, and insulin resistance may all contribute to the development of diabetic heart disease.

Brief episode of STZ-induced hyperglycemia produces cardiac abnormalities in rats fed a diet rich in n-6 PUFA

Diabetic patients are particularly susceptible to cardiomyopathy independent of vascular disease, and recent evidence implicates cell death as a contributing factor. Given its protective role against apoptosis, we hypothesized that dietary n-6 polyunsaturated fatty acid (PUFA) may well decrease the incidence of this mode of cardiac cell death after diabetes. Male Wistar rats were first fed a diet rich in n-6 PUFA [20% (wt/wt) sunflower oil] for 4 wk followed by streptozotocin (STZ, 55 mg/kg) to induce diabetes. After a brief period of hyperglycemia (4 days), hearts were excised for functional, morphological, and biochemical analysis. In diabetic rats, n-6 PUFA decreased caspase-3 activity, crucial for myocardial apoptosis. However, cardiac necrosis, an alternative mode of cell death, increased. In these hearts, a rise in linoleic acid and depleted cardiac glutathione could explain this "switch" to necrotic cell death. Additionally, mitochondrial abnormalities, impaired substrate utilization, and enhanced triglyceride accumulation could have also contributed to a decline in cardiac function in these animals. Our study provides evidence that, in contrast to other models of diabetic cardiomyopathy that exhibit cardiac dysfunction only after chronic hyperglycemia, n-6 PUFA feeding coupled with only 4 days of diabetes precipitated metabolic and contractile abnormalities in the heart. Thus, although promoted as being beneficial, excess n-6 PUFA, with its predisposition to induce obesity, insulin resistance, and ultimately diabetes, could accelerate myocardial abnormalities in diabetic patients.

Onset of diabetes in Zucker diabetic fatty (ZDF) rats leads to improved recovery of function after ischemia in the isolated perfused heart

The aim of this study was to determine whether the transition from insulin resistance to hyperglycemia in a model of type 2 diabetes leads to intrinsic changes in the myocardium that increase the sensitivity to ischemic injury. Hearts from 6-, 12-, and 24-wk-old lean (Control) and obese Zucker diabetic fatty (ZDF) rats were isolated, perfused, and subjected to 30 min of low-flow ischemia (LFI) and 60 min of reperfusion. At 6 wk, ZDF animals were insulin resistant but not hyperglycemic. By 12 wk, the ZDF group was hyperglycemic and became progressively worse by 24 wk. In spontaneously beating hearts rate-pressure product (RPP) was depressed in the ZDF groups compared with age-matched Controls, primarily due to lower heart rate. Pacing significantly increased RPP in all ZDF groups; however, this was accompanied by a significant decrease in left ventricular developed pressure. There was also greater contracture during LFI in the ZDF groups compared with the Control group; surprisingly, however, functional recovery upon reperfusion was significantly higher in the diabetic 12- and 24-wk ZDF groups compared with age-matched Control groups and the 6-wk ZDF group. This improvement in recovery in the ZDF diabetic groups was independent of substrate availability, severity of ischemia, and duration of diabetes. These data demonstrate that, although the development of type 2 diabetes leads to progressive contractile and metabolic abnormalities during normoxia and LFI, it was not associated with increased susceptibility to ischemic injury.

Catalase protects cardiomyocyte function in models of type 1 and type 2 diabetes

Many diabetic patients suffer from a cardiomyopathy that cannot be explained by poor coronary perfusion. Reactive oxygen species (ROS) have been proposed to contribute to this cardiomyopathy. Consistent with this we found evidence for induction of the antioxidant genes for catalase in diabetic OVE26 hearts. To determine whether increased antioxidant protection could reduce diabetic cardiomyopathy, we assessed cardiac morphology and contractility, Ca(2+) handling, malondialdehyde (MDA)-modified proteins, and ROS levels in individual cardiomyocytes isolated from control hearts, OVE26 diabetic hearts, and diabetic hearts overexpressing the antioxidant protein catalase. Diabetic hearts showed damaged mitochondria and myofibrils, reduced myocyte contractility, slowed intracellular Ca(2+) decay, and increased MDA-modified proteins compared with control myocytes. Overexpressing catalase preserved normal cardiac morphology, prevented the contractile defects, and reduced MDA protein modification but did not reverse the slowed Ca(2+) decay induced by diabetes. Additionally, high glucose promoted significantly increased generation of ROS in diabetic cardiomyocytes. Chronic overexpression of catalase or acute in vitro treatment with rotenone, an inhibitor of mitochondrial complex I, or thenoyltrifluoroacetone, an inhibitor of mitochondrial complex II, eliminated excess ROS production in diabetic cardiomyocytes. The structural damage to diabetic mitochondria and the efficacy of mitochondrial inhibitors in reducing ROS suggest that mitochondria are a source of oxidative damage in diabetic cardiomyocytes. We also found that catalase overexpression protected cardiomyocyte contractility in the agouti model of type 2 diabetes. These data show that both type 1 and type 2 diabetes induce damage at the level of individual myocytes, and that this damage occurs through mechanisms utilizing ROS.

Treatment of type 2 diabetic db/db mice with a novel PPARgamma agonist improves cardiac metabolism but not contractile function

Hearts from insulin-resistant type 2 diabetic db/db mice exhibit features of a diabetic cardiomyopathy with altered metabolism of exogenous substrates and reduced contractile performance. Therefore, the effect of chronic oral administration of 2-(2-(4-phenoxy-2-propylphenoxy)ethyl)indole-5-acetic acid (COOH), a novel ligand for peroxisome proliferator-activated receptor-gamma that produces insulin sensitization, to db/db mice (30 mg/kg for 6 wk) on cardiac function was assessed. COOH treatment reduced blood glucose from 27 mM in untreated db/db mice to a normal level of 10 mM. Insulin-stimulated glucose uptake was enhanced in cardiomyocytes from COOH-treated db/db hearts. Working perfused hearts from COOH-treated db/db mice demonstrated metabolic changes with enhanced glucose oxidation and decreased palmitate oxidation. However, COOH treatment did not improve contractile performance assessed with ex vivo perfused hearts and in vivo by echocardiography. The reduced outward K+ currents in diabetic cardiomyocytes were still attenuated after COOH. Metabolic changes in COOH-treated db/db hearts are most likely indirect, secondary to changes in supply of exogenous substrates in vivo and insulin sensitization.

Gender-dependent attenuation of cardiac potassium currents in type 2 diabetic db/db mice

Single ventricular myocytes were prepared from control db/+ and insulin-resistant diabetic db/db male mice at 6 and 12 weeks of age. Peak and sustained outward potassium currents were measured using whole-cell voltage clamp methods. At 6 weeks currents were fully developed in control and diabetic mice, with no differences in the density of either current. By 12 weeks both currents were significantly attenuated in the diabetic mice, but could be augmented by in vitro incubation with the angiotensin-converting enzyme (ACE) inhibitor quinapril (1 microM, 5-9 h). In cells from female db/db mice (12 weeks of age), K(+) currents were not attenuated and no effects of quinapril were observed. To investigate whether lack of insulin action accounts for these gender differences, cells were also isolated from cardiomyocyte-specific insulin receptor knockout (CIRKO) mice. Both K(+) currents were significantly attenuated in cells from male and female CIRKO mice, and action potentials were significantly prolonged. Incubation with quinapril did not augment K(+) currents. Our results demonstrate that type 2 diabetes is associated with gender-selective attenuation of K(+) currents in cardiomyocytes, which may underlie gender differences in the development of some cardiac arrhythmias. The mechanism for attenuation of K(+) currents in cells from male mice is due, at least in part, to an autocrine effect resulting from activation of a cardiac renin-angiotensin system. Insulin is not involved in these gender differences, since the absence of insulin action in CIRKO mice diminishes K(+) currents in cells from both males and females.

Left-ventricular diastolic dysfunction may be prevented by chronic treatment with PPAR-alpha or -gamma agonists in a type 2 diabetic animal model

OBJECTIVES

The aim of this study was to determine whether the peroxisome proliferator-activated receptor (PPAR) ligands could prevent left-ventricular diastolic dysfunction (LVDD) in rats with advanced diabetes. In addition, this study examined whether the activity of malonyl-CoA decarboxylase (MCD), which is an enzyme related to the degradation of malonyl-CoA that is known to regulate the fatty acid metabolism, is changed by the diabetic state itself or by treatment with the PPAR ligands.

METHODS

Male Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of type 2 diabetes, aged 28 weeks, were divided into 3 groups: the untreated, pioglitazone-treated (10 mg/kg/d), and fenofibrate-treated (150 mg/kg/d) groups. The rats were treated for 10 weeks. Male Long-Evans Tokushima Otsuka (LETO) rats were used as nondiabetic control. Doppler echocardiography and measurements of the MCD activity at the myocardium were performed.

RESULTS

At the age of 38 weeks, the OLETF rats treated with either pioglitazone or fenofibrate showed an improvement in the plasma glucose levels after glucose loading as well as an improvement in the fasting plasma insulin, triglyceride, and FFA levels compared to the untreated OLETF rats. The untreated OLETF rats showed a prolonged deceleration time of the E-wave (DTE) (74.3 +/- 3.7 vs LETO, 56.3 +/- 3.8 ms, P < 0.05) and a reduced ratio of the peak early diastolic velocity wave to the late diastolic wave (E/A ratio) (1.25 +/- 0.06 vs LETO 1.54 +/- 0.08, P < 0.05). Pioglitazone treatment in the OLETF rats improved the DTE (51.6 +/- 1.7 ms, P < 0.05), and the fenofibrate treatment also improved the DTE (61.4 +/- 4.3 ms, P < 0.05) and E/A ratio (1.57 +/- 0.05, P < 0.05) compared to the untreated OLETF rats. The parameters related to the systolic function did not change among the groups at both pre- and post-treatments. The MCD activity of the myocardium was remarkably lower in the OLETF rats compared to the LETO rats (3.26 +/- 0.38 vs 7.76 +/- 0.84 nmol/min/mg protein, P < 0.05). The pioglitazone and fenofibrate treatments resulted in an increase in the MCD activity compared to that in the untreated rats (7.20 +/- 0.74 and 8.33 +/- 0.83 nmol/min/mg protein, P < 0.05, respectively).

CONCLUSIONS

The PPAR-alpha or -gamma agonists prevented LVDD in the advanced diabetic rat hearts, possibly through an improvement in the fatty acid metabolism in the myocardium or a correction of the hyperglycemia and/or hyperlipidemia.

Diastolic dysfunction is associated with altered myocardial metabolism in asymptomatic normotensive patients with well-controlled type 2 diabetes mellitus

OBJECTIVES

This study evaluated myocardial function in relation to high-energy phosphate (HEP) metabolism in asymptomatic patients with uncomplicated type 2 diabetes mellitus using magnetic resonance (MR) techniques.

BACKGROUND

Myocardial dysfunction may occur in patients with type 2 diabetes mellitus in the absence of coronary artery disease or left ventricular (LV) hypertrophy. The mechanisms underlying this diabetic cardiomyopathy are largely unknown, but may involve altered myocardial energy metabolism.

METHODS

We assessed myocardial systolic and diastolic function and HEP metabolism in 12 asymptomatic normotensive male patients with recently diagnosed, well-controlled type 2 diabetes and 12 controls, using MR imaging and phosphorus-31-nuclear MR spectroscopy (31P-MRS) on a 1.5 T clinical scanner; 31P-MR spectra were quantified, and myocardial HEP metabolism was expressed as phosphocreatine to adenosine-triphosphate (PCr/ATP) ratio.

RESULTS

No differences were found in LV mass and systolic function between patients and controls. However, early (E) acceleration peak, deceleration peak, peak filling rate, and transmitral early-to-late diastolic peak flow (E/A) ratio, all indexes of diastolic function, were significantly decreased in patients compared with controls (p < 0.02). In addition, myocardial PCr/ATP in patients was significantly lower than in controls (1.47 vs. 1.88, p < 0.01). Inverse associations were found between myocardial PCr/ATP and E acceleration peak, E deceleration peak, and E peak filling rate (all, p < 0.05).

CONCLUSIONS

These results indicate that altered myocardial energy metabolism may contribute to LV diastolic functional changes in patients with recently diagnosed, well-controlled and uncomplicated type 2 diabetes.

Stimulation of glucose uptake by chronic vanadate pretreatment in cardiomyocytes requires PI 3-kinase and p38 MAPK activation

Vanadate, an inhibitor of tyrosine phosphatases, has insulin-mimetic properties. It has been shown that acute vanadate administration enhances glucose uptake independently of phosphatidylinositol (PI) 3-kinase and p38 MAPK. However, therapeutic vanadate use requires chronic administration, and this could potentially involve a different signaling pathway(s). Thus, we examined the mechanisms by which chronic vanadate exposure (16 h) stimulates glucose uptake in primary cultures of adult cardiomyocytes. The effect of vanadate on the activation of insulin-signaling molecules was evaluated 60 min after its withdrawal and in the absence of insulin. We therefore evaluated the persistent effect of vanadate on the insulin-signaling cascade. Our results demonstrate that preincubation with low vanadate concentrations (25-75 microM) induces a dose-dependent increase in glucose uptake. The augmentation of this process was not due to alterations in GLUT1 or GLUT4 protein levels, transcription, or de novo protein synthesis. Chronic vanadate exposure was associated with activation of the insulin receptor, insulin receptor substrate-1 (IRS-1), PKB/Akt, and p38 MAPK. Furthermore, inhibition of PI 3-kinase or p38 MAPK by wortmannin and PD-169316, respectively, significantly inhibited vanadate-mediated glucose uptake in cardiomyocytes. Thus, over time, different (albeit overlapping) signaling cascades may be activated by vanadate.

PTG gene deletion causes impaired glycogen synthesis and developmental insulin resistance

Protein targeting to glycogen (PTG) is a scaffolding protein that targets protein phosphatase 1alpha (PP1alpha) to glycogen, and links it to enzymes involved in glycogen synthesis and degradation. We generated mice that possess a heterozygous deletion of the PTG gene. These mice have reduced glycogen stores in adipose tissue, liver, heart, and skeletal muscle, corresponding with decreased glycogen synthase activity and glycogen synthesis rate. Although young PTG heterozygous mice initially demonstrate normal glucose tolerance, progressive glucose intolerance, hyperinsulinemia, and insulin resistance develop with aging. Insulin resistance in older PTG heterozygous mice correlates with a significant increase in muscle triglyceride content, with a corresponding attenuation of insulin receptor signaling. These data suggest that PTG plays a critical role in glycogen synthesis and is necessary to maintain the appropriate metabolic balance for the partitioning of fuel substrates between glycogen and lipid.

Potential mechanisms and consequences of cardiac triacylglycerol accumulation in insulin-resistant rats

The accumulation of intracellular triacylglycerol (TG) is highly correlated with muscle insulin resistance. However, it is controversial whether the accumulation of TG is the result of increased fatty acid supply, decreased fatty acid oxidation, or both. Because abnormal fatty acid metabolism is a key contributor to the pathogenesis of diabetes-related cardiovascular dysfunction, we examined fatty acid and glucose metabolism in hearts of insulin-resistant JCR:LA-cp rats. Isolated working hearts from insulin-resistant rats had glycolytic rates that were reduced to 50% of lean control levels (P < 0.05). Cardiac TG content was increased by 50% (P < 0.05) in the insulin-resistant rats, but palmitate oxidation rates remained similar between the insulin-resistant and lean control rats. However, plasma fatty acids and TG levels, as well as cardiac fatty acid-binding protein (FABP) expression, were significantly increased in the insulin-resistant rats. AMP-activated protein kinase (AMPK) plays a major role in the regulation of cardiac fatty acid and glucose metabolism. When activated, AMPK increases fatty acid oxidation by inhibiting acetyl-CoA carboxylase (ACC) and reducing malonyl-CoA levels, and it decreases TG content by inhibiting glycerol-3-phosphate acyltransferase (GPAT), the rate-limiting step in TG synthesis. The activation of AMPK also stimulates cardiac glucose uptake and glycolysis. We thus investigated whether a decrease in AMPK activity was responsible for the reduced cardiac glycolysis and increased TG content in the insulin-resistant rats. However, we found no significant difference in AMPK activity. We also found no significant difference in various established downstream targets of AMPK: ACC activity, malonyl-CoA levels, carnitine palmitoyltransferase I activity, or GPAT activity. We conclude that hearts from insulin-resistant JCR:LA-cp rats accumulate substantial TG as a result of increased fatty acid supply rather than from reduced fatty acid oxidation. Furthermore, the accumulation of cardiac TG is associated with a reduction in insulin-stimulated glucose metabolism.

Metallothionein prevents diabetes-induced deficits in cardiomyocytes by inhibiting reactive oxygen species production

Many individuals with diabetes experience impaired cardiac contractility that cannot be explained by hypertension and atherosclerosis. This cardiomyopathy may be due to either organ-based damage, such as fibrosis, or to direct damage to cardiomyocytes. Reactive oxygen species (ROS) have been proposed to contribute to such damage. To address these hypotheses, we examined contractility, Ca(2+) handling, and ROS levels in individual cardiomyocytes isolated from control hearts, diabetic OVE26 hearts, and diabetic hearts overexpressing antioxidant protein metallothionein (MT). Our data showed that diabetic myocytes exhibited significantly reduced peak shortening, prolonged duration of shortening/relengthening, and decreased maximal velocities of shortening/relengthening as well as slowed intracellular Ca(2+) decay compared with control myocytes. Overexpressing MT prevented these defects induced by diabetes. In addition, high glucose and angiotensin II promoted significantly increased generation of ROS in diabetic cardiomyocytes. Chronic overexpression of MT or acute in vitro treatment with the flavoprotein inhibitor diphenyleneiodonium or the angiotensin II type I receptor antagonist losartan eliminated excess ROS production in diabetic cardiomyocytes. These data show that diabetes induces damage at the level of individual myocyte. Damage can be attributed to ROS production, and diabetes increases ROS production via angiotensin II and flavoprotein enzyme-dependent pathways.

Molecular identification and characterization of a novel nuclear protein whose expression is up-regulated in insulin-resistant animals

Energy metabolism is the most fundamental capacity for mammals, impairment of which causes a variety of diseases such as type 2 diabetes and insulin resistance. Here, we identified a novel gene, termed diabetes-related ankyrin repeat protein (DARP) that is up-regulated in the heart of KKA(y) mouse, a type 2 diabetes and insulin resistance model animal. DARP contains putative nuclear localization signals and four tandem ankyrin-like repeats. Its expression is restricted in heart, skeletal muscle, and brown adipose. Western blot analysis and immunocytochemistry of DARP-transfected Chinese hamster ovary (CHO) and COS-7 cells reveal that DARP is a nuclear protein. When DARP is expressed in CHO cells, [1-(14)C]palmitate uptake is significantly decreased, whereas the palmitate oxidation does not show significant change. Furthermore, DARP expression is altered by the change of energy supply induced by excess fatty acid treatment of skeletal myotube in vitro and fasting treatment of C57 mouse in vivo. We confirmed that DARP expression is also altered in Zucker fatty rat, another insulin resistance model animal. Taken together, these data suggest that DARP is a novel nuclear protein potentially involved in the energy metabolism. Detailed analysis of DARP may provide new insights in the energy metabolism.

A critical role for PPARalpha-mediated lipotoxicity in the pathogenesis of diabetic cardiomyopathy: modulation by dietary fat content

To explore the role of peroxisome proliferator-activated receptor alpha (PPARalpha)-mediated derangements in myocardial metabolism in the pathogenesis of diabetic cardiomyopathy, insulinopenic mice with PPARalpha deficiency (PPARalpha(-/-)) or cardiac-restricted overexpression [myosin heavy chain (MHC)-PPAR] were characterized. Whereas PPARalpha(-/-) mice were protected from the development of diabetes-induced cardiac hypertrophy, the combination of diabetes and the MHC-PPAR genotype resulted in a more severe cardiomyopathic phenotype than either did alone. Cardiomyopathy in diabetic MHC-PPAR mice was accompanied by myocardial long-chain triglyceride accumulation. The cardiomyopathic phenotype was exacerbated in MHC-PPAR mice fed a diet enriched in triglyceride containing long-chain fatty acid, an effect that was reversed by discontinuing the high-fat diet and absent in mice given a medium-chain triglyceride-enriched diet. Reactive oxygen intermediates were identified as candidate mediators of cardiomyopathic effects in MHC-PPAR mice. These results link dysregulation of the PPARalpha gene regulatory pathway to cardiac dysfunction in the diabetic and provide a rationale for serum lipid-lowering strategies in the treatment of diabetic cardiomyopathy.

Age-dependent changes in metabolism, contractile function, and ischemic sensitivity in hearts from db/db mice

Glucose and palmitate metabolism and contractile function were measured with ex vivo perfused working hearts from control (db/+) and diabetic (db/db) female mice at 6, 10-12, and 16-18 weeks of age. Palmitate oxidation was increased by 2.2-fold in 6-week-old db/db hearts and remained elevated in 10- to 12- and 16- to 18-week-old hearts. Carbohydrate oxidation was normal at 6 weeks but was reduced to 27 and 23% of control at 10-12 and 16-18 weeks, respectively. At 6 weeks, db/db hearts exhibited a slight reduction in mechanical function, whereas marked signs of dysfunction were evident at 10-12 and 16-18 weeks. Mechanical function after ischemia-reperfusion was examined in hearts from male mice; at 6 weeks, db/db hearts showed normal recovery, whereas at 12 weeks it was markedly reduced. Fatty acid oxidation was the predominant substrate used after reperfusion. Thus, diabetic db/db hearts exhibit signs of a progressive cardiomyopathy; increased fatty acid oxidation preceded reductions in carbohydrate oxidation. Postischemic recovery of function was reduced in db/db hearts, in parallel with age-dependent changes in normoxic contractile performance. Finally, peroxisome proliferator-activated receptor-alpha treatment (3 weeks) did not affect sensitivity to ischemia-reperfusion, even though carbohydrate oxidation was increased and palmitate oxidation was decreased.

Chylomicron and palmitate metabolism by perfused hearts from diabetic mice

Hydrolysis of triacylglycerols (TG) in circulating chylomicrons by endothelium-bound lipoprotein lipase (LPL) provides a source of fatty acids (FA) for cardiac metabolism. The effect of diabetes on the metabolism of chylomicrons by perfused mouse hearts was investigated with db/db (type 2) and streptozotocin (STZ)-treated (type 1) diabetic mice. Endothelium-bound heparin-releasable LPL activity was unchanged in both type 1 and type 2 diabetic hearts. The metabolism of LPL-derived FA was examined by perfusing hearts with chylomicrons containing radiolabeled TG and by measuring (3)H(2)O accumulation in the perfusate (oxidation) and incorporation of radioactivity into tissue TG (esterification). Rates of LPL-derived FA oxidation and esterification were increased 2.3-fold and 1.7-fold in db/db hearts. Similarly, LPL-derived FA oxidation and esterification were increased 3.4-fold and 2.5-fold, respectively, in perfused hearts from STZ-treated mice. The oxidation and esterification of [(3)H]palmitate complexed to albumin were also increased in type 1 and type 2 diabetic hearts. Therefore, diabetes may not influence the supply of LPL-derived FA, but total FA utilization (oxidation and esterification) was enhanced.

Echocardiographic assessment of cardiac function in diabetic db/db and transgenic db/db-hGLUT4 mice

Control db/+ and diabetic db/db mice at 6 and 12 wk of age were subjected to echocardiography to determine whether contractile function was reduced in vivo and restored in transgenic db/db-human glucose transporter 4 (hGLUT4) mice (12 wk old) in which cardiac metabolism has been normalized. Systolic function was unchanged in 6-wk-old db/db mice, but fractional shortening and velocity of circumferential fiber shortening were reduced in 12-wk-old db/db mice (43.8 +/- 2.1% and 8.3 +/- 0.5 circs/s, respectively) relative to db/+ control mice (59.5 +/- 2.3% and 11.8 +/- 0.4 circs/s, respectively). Doppler flow measurements were unchanged in 6-wk-old db/db mice. The ratio of E and A transmitral flows was reduced from 3.56 +/- 0.29 in db/+ mice to 2.40 +/- 0.20 in 12-wk-old db/db mice, indicating diastolic dysfunction. Thus a diabetic cardiomyopathy with systolic and diastolic dysfunction was evident in 12-wk-old diabetic db/db mice. Cardiac function was normalized in transgenic db/db-hGLUT4 mice, indicating that altered cardiac metabolism can produce contractile dysfunction in diabetic db/db hearts.

Cardiac function and metabolism in Type 2 diabetic mice after treatment with BM 17.0744, a novel PPAR-alpha activator

Hearts from diabetic db/db mice, a model of Type 2 diabetes, exhibit left ventricular failure and altered metabolism of exogenous substrates. Peroxisome proliferator-activated receptor-alpha (PPAR-alpha) ligands reduce plasma lipid and glucose concentrations and improve insulin sensitivity in db/db mice. Consequently, the effect of 4- to 5-wk treatment of db/db mice with a novel PPAR-alpha ligand (BM 17.0744; 25-38 mg x kg(-1) x day(-1)), commencing at 8 wk of age, on ex vivo cardiac function and metabolism was determined. Elevated plasma concentrations of glucose, fatty acids, and triacylglycerol (34.0 +/- 3.6, 2.0 +/- 0.4, and 0.9 +/- 0.1 mM, respectively) were reduced to normal after treatment with BM 17.0744 (10.8 +/- 0.6, 1.1 +/- 0.1, and 0.6 +/- 0.1 mM). Plasma insulin was also reduced significantly in treated compared with untreated db/db mice. Chronic treatment of db/db mice with the PPAR-alpha agonist resulted in a 50% reduction in rates of fatty acid oxidation, with a concomitant increase in glycolysis (1.7-fold) and glucose oxidation (2.3- fold). Correction of the diabetes-induced abnormalities in systemic and cardiac metabolism after BM 17.0744 treatment did not, however, improve left ventricular contractile function.