Energy metabolism in the normal and failing heart: potential for therapeutic interventions

An in silico study of energy metabolism in cardiac excitation-contraction coupling

The heart produces and uses ATP at a high rate. Each step involved in ATP metabolism has been extensively studied. However, functional coupling between ATP production and membrane excitation-contraction coupling, which is the main ATP consumption process, is not yet fully understood because of complicated interactions and the lack of quantitative data obtained in vivo. Computer simulation is a powerful tool for integrating experimental data and for solving their complicated interactions. To investigate the mechanisms underlying cardiac excitation-contraction-energy metabolism coupling, we have developed a computer model of cardiac excitation-contraction coupling (Kyoto model) that includes the major processes of ATP production, such as oxidative phosphorylation that was originally developed for skeletal muscle by Korzeniewski and Zoladz [Biophys Chem 92: 17-34, 2001], creatine kinase, and adenylate kinase. In this review, we briefly summarize cardiac energy metabolism and discuss the regulation of mitochondrial ATP synthesis, using the Kyoto model.

Mitochondrial tissue specificity of substrates utilization in rat cardiac and skeletal muscles

As energetic metabolism is crucial for muscles, they develop different adaptations to respond to fluctuating demand among muscle types. Whereas quantitative characteristics are known, no study described simultaneously quantitative and qualitative differences among muscle types in terms of substrates utilization patterns. This study thus defined the pattern of substrates preferential utilization by mitochondria from glycolytic gastrocnemius (GAS) and oxidative soleus (SOL) skeletal muscles and from heart left ventrical (LV) in rats. We measured in situ, ADP (2 mM)-stimulated, mitochondrial respiration rates from skinned fibers in presence of increasing concentrations of pyruvate (Pyr) + malate (Mal), palmitoyl-carnitine (Palm-C) + Mal, glutamate (Glut) + Mal, glycerol-3-phosphate (G3-P), lactate (Lact) + Mal. Because the fibers oxygen uptake (Vs) followed Michaelis-Menten kinetics in function of substrates level we determined the Vs and Km, representing maximal oxidative capacity and the mitochondrial sensibility for each substrate, respectively. Vs were in the order GAS < SOL < LV for Pyr, Glu, and Palm-C substrates, whereas in the order SOL = LV < GAS with G3-P. Moreover, the relative capacity to oxidize Palm-C is extremely higher in LV than in SOL. Vs was not stimulated by the Lact substrate. The Km was equal for Pyr among muscles, but much lower for G3-P in GAS and lower for Palm-C in LV. These results demonstrate qualitative mitochondrial tissue specificity for metabolic pathways. Mitochondria of glycolytic muscle fibers are well adapted to play a central role for maintaining a satisfactory cytosolic redox state in these fibers, whereas mitochondria of LV developed important capacities to use fatty acids.

Ranolazine*

Ranolazine is a novel new antianginal agent currently under investigation as monotherapy and adjunct therapy for the treatment of chronic stable angina. While the mechanism of action of ranolazine is not completely understood, it is believed to involve a reduction in fatty acid oxidation, ultimately leading to a shift in myocardial energy production from fatty acid oxidation to glucose oxidation. Since the oxidation of glucose requires less oxygen than the oxidation of fatty acids, ranolazine can help maintain myocardial function in times of ischemia. In addition, ranolazine has minimal effect on blood pressure and heart rate. Ranolazine, by inhibiting cellular ionic channels, prolongs the corrected QT interval. However, ranolazine has not yet been associated with any incidences of ventricular arrhythmia. The clinical data with ranolazine focuses on its use in chronic stable angina, where it has been shown to increase exercise tolerance and decrease angina compared with placebo, as well as in combination with beta-blockers and calcium channel blockers. The use of ranolazine for other cardiac conditions and the effect of ranolazine on morbidity and mortality remains to be determined. Ongoing clinical trials will help further establish the role of ranolazine in the treatment of cardiovascular disorders.

Myocardial energy metabolism during ischemia and the mechanisms of metabolic therapies

The primary effect of ischemia is reduced aerobic adenosine triphosphate (ATP) formation in mitochondria. This triggers accelerated glycolysis and reduced cell pH, Ca(2+) accumulation, K(+) efflux, adenosine formation, and the clinical signs of ischemia: chest pain and a shift in the ST segment. Traditional therapies for angina are aimed at either decreasing the need for ATP by suppressing heart rate, blood pressure, and cardiac contractility, or at increasing oxygen delivery to the mitochondria, or both. An additional approach to treating angina is to suppress myocardial fatty acid oxidation, increase pyruvate oxidation, and reduce anaerobic glycolysis. High fatty acid levels result in oxygen wasting and inhibit the oxidation of pyruvate in the mitochondria. In experimental models, the partial inhibition of myocardial fatty acid oxidation with agents such as oxfenicine, ranolazine, and trimetazidine stimulates glucose oxidation and reduces lactate production during ischemia. Clinical studies demonstrate that this approach is as effective as traditional hemodynamic therapies at improving exercise tolerance and reducing the frequency of angina. Moreover, because these agents do not suppress heart rate, blood pressure, or contractility, they are effective as add-on therapy to Ca(2+)-channel and beta-adrenergic receptor antagonists.

Lactate concentration gradient from right atrium to pulmonary artery

INTRODUCTION

We compared simultaneous measurements of blood lactate concentration ([Lac]) in the right atrium (RA) and in the pulmonary artery (PA). Our aim was to determine if the mixing of right atrial with coronary venous blood, having substantially lower [Lac], results in detectable decreases in [Lac] from the RA to the PA.

METHODS

A prospective, sequential, observational study was conducted in a medical-surgical intensive care unit. We enrolled 45 critically ill adult individuals of either sex requiring pulmonary artery catheters (PACs) to guide fluid therapy. Immediately following the insertion of the PAC, one paired set of blood samples per patient was drawn in random order from the PAC's proximal and distal ports for measurement of hemoglobin concentration, O2 saturation (SO2) and [Lac]. We defined Delta[Lac] as ([Lac]ra - [Lac]pa), DeltaSO2 as (SraO2 - SpaO2) and the change in O2 consumption (DeltaVO2) as the difference in systemic VO2 calculated using Fick's equation with either SraO2 or SpaO2 in place of mixed venous SO2. Data were compared by paired Student's t-test, Spearman's correlation analysis and by the method of Bland and Altman.

RESULTS

We found SraO2 > SpaO2 (74.2 +/- 9.1 versus 69.0 +/- 10.4%; p < 0.001) and [Lac]ra > [Lac]pa (3.9 +/- 3.0 versus 3.7 +/- 3.0 mmol x l-1; p < 0.001). Delta[Lac] correlated with DeltaVO2 (r2 = 0.34; p < 0.001).

CONCLUSION

We found decreases in [Lac] from the RA to PA in this sample of critically ill individuals. We conclude that parallel decreases in SO2 and [Lac] from the RA to PA support the hypothesis that these gradients are produced by mixing RA with coronary venous blood of lower SO2 and [Lac]. The present study is a preliminary observation of this phenomenon and further work is needed to define the physiological and clinical significance of Delta[Lac].

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.

Fatty acid metabolism is enhanced in type 2 diabetic hearts

The metabolic phenotype of hearts has been investigated using rodent models of type 2 diabetes which exhibit obesity and insulin resistance: db/db and ob/ob mice, and Zucker fatty and ZDF rats. In general, cardiac fatty acid (FA) utilization is enhanced in type 2 diabetic hearts, with increased rates of FA oxidation (db/db, ob/ob and ZDF models) and increased FA esterification into cellular triacylglycerols (db/db hearts). Hearts from db/db and ob/ob mice and ZDF rat hearts all have elevated levels of myocardial triacylglycerols, consistent with enhanced FA utilization. A number of mechanisms may be responsible for enhanced FA utilization in type 2 diabetic hearts: (i) increased FA uptake into cardiac myocytes and into mitochondria; (ii) altered mitochondrial function, with up-regulation of uncoupling proteins; and (iii) stimulation of peroxisome proliferator-activated receptor-alpha. Enhanced cardiac FA utilization in rodent type 2 diabetic models is associated with reduced cardiac contractile function, perhaps as a consequence of lipotoxicity and/or reduced cardiac efficiency. Similar results have been obtained with human type 2 diabetic hearts, suggesting that pharmacological interventions that can reduce cardiac FA utilization may have beneficial effects on contractile function.

Peroxisome proliferator-activated receptor alpha and hypertensive heart disease

Peroxisome proliferator-activated receptor alpha (PPARalpha) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily. It is expressed by cardiomyocytes and regulates gene expression of key proteins involved in myocardial lipid and energy metabolism. Accordingly, the activitity of PPARalpha is an important determinant of cardiomyocyte lipid homeostasis and ATP production. Currently, animal and human data suggest that deactivation of PPARalpha may contribute substantially to phenotypic changes that accompany cardiac growth in conditions of pressure overload, and the hypothesis emerges that a compromised PPARalpha activity may participate in the transition from compensated left ventricular hypertrophy to heart failure in hypertensive heart disease. The availability of PPARalpha activators (e.g. fibric acid derivates and statins) must stimulate investigation into the potential cardioprotective actions of these compounds beyond their hypolipidaemic effects and via restoration of PPARalpha activity in the hypertrophied and failing heart.

Effects of chronic administration of clenbuterol on function and metabolism of adult rat cardiac muscle

Clenbuterol (Clen), a β2-agonist, is known to produce skeletal and myocardial hypertrophy. This compound has recently been used in combination with left ventricular assist devices for the treatment of end-stage heart failure to reverse or prevent the adverse effects of unloading-induced myocardial atrophy. However, the mechanisms of action of Clen on myocardial cells have not been fully elucidated. In an attempt to clarify this issue, we examined the effects of chronic administration of Clen on Ca2+ handling and substrate preference in cardiac muscle. Rats were treated with either 2 mg·kg−1·day−1 Clen or saline (Sal) for 4 wk with the use of osmotic minipumps. Ventricular myocytes were enzymatically dissociated. Cells were field stimulated at 0.5, 1, and 2 Hz, and cytoplasmic Ca2+ transients were monitored with the use of the fluorescent indicator indo-1 acetoxymethyl ester. Two-dimensional surface area and action potentials in current clamp were also measured. We found that in the Clen group there was significant hypertrophy at the organ and cellular levels compared with Sal. In Clen myocytes, the amplitude of the indo-1 ratio transients was significantly increased. Sarcoplasmic reticulum Ca2+ content, estimated by rapid application of 20 mM caffeine, was significantly increased in the Clen group. The action potential was prolonged in the Clen group compared with Sal. Carbohydrate contribution to the tricarboxylic cycle (Krebs cycle) flux was increased several times in the Clen group. This increase was associated with decreased expression of peroxisome proliferator-activated receptor-α. This study shows that chronic administration of Clen induces cellular hypertrophy and increases oxidative carbohydrate utilization together with an increase in sarcoplasmic reticulum Ca2+ content, which results in increased amplitude of the Ca2+ transients. These effects could be important when Clen is used in conjunction with left ventricular assist devices treatment.

Fatty acid oxidation and its impact on response of spontaneously hypertensive rat hearts to an adrenergic stress: benefits of a medium-chain fatty acid

The spontaneously hypertensive rat (SHR) is a model of cardiomyopathy characterized by a restricted use of exogenous long-chain fatty acid (LCFA) for energy production. The aims of the present study were to document the functional and metabolic response of the SHR heart under conditions of increased energy demand and the effects of a medium-chain fatty acid (MCFA; octanoate) supplementation in this situation. Hearts were perfused ex vivo in a working mode with physiological concentrations of substrates and hormones and subjected to an adrenergic stimulation (epinephrine, 10 μM).13C-labeled substrates were used to assess substrate selection for energy production. Compared with control Wistar rat hearts, SHR hearts showed an impaired response to the adrenergic stimulation as reflected by 1) a smaller increase in contractility and developed pressure, 2) a faster decline in the aortic flow, and 3) greater cardiac tissue damage (lactate dehydrogenase release: 1,577 ± 118 vs. 825 ± 44 mU/min, P < 0.01). At the metabolic level, SHR hearts presented 1) a reduced exogenous LCFA contribution to the citric acid cycle flux (16 ± 1 vs. 44 ± 4%, P < 0.001) and an enhanced contribution of endogenous substrates (20 ± 4 vs. 1 ± 4%, P < 0.01); and 2) an increased lactate production from glycolysis, with a greater lactate-to-pyruvate production ratio. Addition of 0.2 mM octanoate reduced lactate dehydrogenase release (1,145 ± 155 vs. 1,890 ± 89 mU/min, P < 0.001) and increased exogenous fatty acid contribution to energy metabolism (23.7 ± 1.3 vs. 15.8 ± 0.8%, P < 0.01), which was accompanied by an equivalent decrease in unlabeled endogenous substrate contribution, possibly triglycerides (11.6 ± 1.5 vs. 19.0 ± 1.2%, P < 0.01). Taken altogether, these results demonstrate that the SHR heart shows an impaired capacity to withstand an acute adrenergic stress, which can be improved by increasing the contribution of exogenous fatty acid oxidation to energy production by MCFA supplementation.

Cardiac gene expression profile in rats with terminal heart failure and cachexia

About one-half of double transgenic rats (dTGR) overexpressing the human renin and angiotensinogen genes die by age 7 wk of terminal heart failure (THF); the other (preterminal) one-half develop cardiac damage but survive. Our study’s aim was to elucidate cardiac gene expression differences in dTGR-THF compared with dTGR showing compensated cardiac hypertrophy but not yet THF. dTGR treated with losartan (LOS) and nontransgenic rats (SD) served as controls. THF-dTGR body weight was significantly lower than for all other groups. At death, THF-dTGR had blood pressures of 228 ± 7 mmHg (cardiac hypertrophy index 6.2 ± 0.1 mg/g). Tissue Doppler showed reduced peak early (Ea) to late (Aa) diastolic expansion in THF-dTGR, indicating diastolic function. Preterminal dTGR had blood pressures of 197 ± 5 mmHg (cardiac hypertrophy index 5.1 ± 0.1 mg/g); Ea < Aa compared with LOS-dTGR (141 ± 6 mmHg; 3.7±0.1 mg/g; Ea > Aa) and SD (112 ± 4 mmHg; 3.6 ± 0.1 mg/g; Ea > Aa). Left ventricular RNA was isolated for the Affymetrix system and TaqMan RT-PCR. THF-dTGR and dTGR showed upregulation of hypertrophy markers and α/β-myosin heavy chain switch to the fetal isoform. THF-dTGR (vs. dTGR) showed upregulation of 239 and downregulation of 150 genes. Various genes of mitochodrial respiratory chain and lipid catabolism were reduced. In addition, genes encoding transcription factors (CEBP-β, c-fos, Fra-1), coagulation, remodeling/repair components (HSP70, HSP27, heme oxygenase), immune system (complement components, IL-6), and metabolic pathway were differentially expressed. In contrast, LOS-dTGR and SD had similar expression profiles. These data demonstrate that THF-dTGR show an altered expression profile compared with preterminal dTGR.

Energy substrate metabolism in cardiac hypertrophy

Cardiac hypertrophy is a response to long-term pathologic (eg, hypertension) or physiologic (eg, exercise) hemodynamic overload accompanied by changes in energy substrate utilization. The pattern of substrate utilization (or metabolic phenotype) differs dramatically between pathologic and physiologic cardiac hypertrophy with directionally opposite changes in oxidation of fatty acids and glucose and glycolysis. These findings indicate that the metabolic response to long-term alterations in hemodynamic workload is not stereotypical, but is influenced by the nature of the stimulus leading to cardiac hypertrophy. Although the changes in substrate utilization are adaptive, in the case of pathologic stimuli, the changes in metabolism interfere with functional resiliency of the heart to metabolic stress, as occurs during ischemia-reperfusion. The distinct metabolic phenotypes of hearts hypertrophied in response to pathologic or physiologic stimuli are due not only to alteration in expression of metabolic enzymes and proteins, but also to post-translational modulation of metabolic enzymes and proteins.

Fatty acid oxidation inhibitors in the management of chronic complications of atherosclerosis

Ischemic heart disease is characterized by a modification of the normal energy balance of the heart. During and following an ischemic event, circulating fatty acids are elevated, resulting in the acceleration of fatty acid oxidation at the expense of glucose oxidation. Despite the reduction in glucose oxidation, the rate of glycolysis increases, leading to an uncoupling of glucose metabolism. This results in the accumulation of metabolic byproducts, which leads to a decrease in cardiac efficiency. A novel therapeutic strategy involves improving the efficiency of oxygen utilization by the ischemic heart by the modulation of energy metabolism. This can be achieved by a reduction in the levels of circulating fatty acids using beta-blockers, glucose-insulin-potassium infusions, and nicotinic acid. Alternatively, fatty acid oxidation can be directly inhibited using trimetazidine, ranolazine, or glucose oxidation directly activated using dichloroacetate, which significantly improves the efficiency of the heart.

Peroxisomal and mitochondrial oxidation of fatty acids in the heart, assessed from the 13C labeling of malonyl-CoA and the acetyl moiety of citrate

We previously showed that a fraction of the acetyls used to synthesize malonyl-CoA in rat heart derives from partial peroxisomal oxidation of very long and long-chain fatty acids. The 13C labeling ratio (malonyl-CoA)/(acetyl moiety of citrate) was >1.0 with 13C-fatty acids, which yields [13C]acetyl-CoA in both mitochondria and peroxisomes and < 1.0 with substrates, which yields [13C]acetyl-CoA only in mitochondria. In this study, we tested the influence of 13C-fatty acid concentration and chain length on the labeling of acetyl-CoA formed in mitochondria and/or peroxisomes. Hearts were perfused with increasing concentrations of labeled docosanoate, oleate, octanoate, hexanoate, butyrate, acetate, or dodecanedioate. In contrast to the liver, peroxisomal oxidation of 1-13C-fatty acids in heart does not form [1-13C]acetate. With [1-13C]docosanoate and [1,12-13C2]dodecanedioate, malonyl-CoA enrichment plateaued at 11 and 9%, respectively, with no detectable labeling of the acetyl moiety of citrate. Thus, in the intact rat heart, docosanoate and dodecanedioate appear to be oxidized only in peroxisomes. With [1-13C]oleate or [1-13C]octanoate, the labeling ratio >1 indicates the partial peroxisomal oxidation of oleate and octanoate. In contrast, with [3-13C]octanoate, [1-13C]hexanoate, [1-13C]butyrate, or [1,2-13C2]acetate, the labeling ratio was <0.7 at all concentrations. Therefore, in rat heart, (i) n-fatty acids shorter than 8 carbons do not undergo peroxisomal oxidation, (ii) octanoate undergoes only one cycle of peroxisomal beta-oxidation, (iii) there is no detectable transfer to the mitochondria of acetyl-CoA from the cytosol or the peroxisomes, and (iv) the capacity of C2-C18 fatty acids to generate mitochondrial acetyl-CoA decreases with chain length.

Inhibition of ErbB2 causes mitochondrial dysfunction in cardiomyocytes

OBJECTIVES

We investigated the effects of erbB2 inhibition by anti-erbB2 antibody on cardiomyocyte survival and mitochondrial function.

BACKGROUND

ErbB2 is an important signal integrator for the epidermal growth factor family of receptor tyrosine kinases. Herceptin, an inhibitory antibody to the erbB2 receptor, is a potent chemotherapeutic but causes cardiac toxicity.

METHODS

Primary cultures of neonatal rat ventricular myocytes were exposed to anti-erbB2 antibody (Ab) (7.5 mug/ml) for up to 24 h. Cell viability, mitochondrial function, and apoptosis were measured using multiple complementary techniques.

RESULTS

ErbB2 inhibition was associated with a dramatic increase in expression of the pro-apoptotic Bcl-2 family protein Bcl-xS and decreased levels of anti-apoptotic Bcl-xL. There was a time-dependent increase in mitochondrial translocation and oligomerization of bcl-associated protein (BAX), as indicated by 1,6-bismaleimidohexane crosslinking. The BAX oligomerization was associated with cytochrome c release and caspase activation. These alterations induced mitochondrial dysfunction, a loss of mitochondrial membrane potential (psi) (76.9 +/- 2.4 vs. 51.7 +/- 0.1; p < 0.05; n = 4), a 35% decline in adenosine triphosphate levels (p < 0.05), and a loss of redox capacity (0.72 +/- 0.04 vs. 0.64 +/- 0.02; p< 0.01). Restoration of Bcl-xL levels through transactivating regulatory protein-mediated protein transduction prevented the decline in psi mitochondrial reductase activity and cytosolic adenosine triphosphate.

CONCLUSIONS

Anti-erbB2 activates the mitochondrial apoptosis pathway through a previously undescribed modulation of Bcl-xL and -xS, causing impairment of mitochondrial function and integrity and disruption of cellular energetics.

Uncoupling proteins in human heart

Abnormal energetic activity in heart failure correlates inversely with plasma free-fatty-acid concentrations. However, the link between energetic and metabolic abnormalities is unknown. To investigate this association, we obtained blood samples from 39 patients undergoing coronary artery bypass graft surgery. Patients fasted overnight before samples were taken. When plasma free-fatty-acid concentrations were raised, cardiac mitochondrial uncoupling proteins (UCP) increased (isoform UCP2, p<0.0001; isoform UCP3, p=0.0036) and those of glucose transporter (GLUT4) protein decreased (cardiac, p=0.0001; skeletal muscle, p=0.0006). Consequently, energy deficiency in heart failure might result from increased mitochondrial UCPs (ie, less efficient ATP synthesis) and depleted GLUT4 (ie, reduced glucose uptake). New treatment to correct these energy defects would be to simultaneously lower plasma free fatty acids and provide an alternative energy source.

Moderate severity heart failure does not involve a downregulation of myocardial fatty acid oxidation

Recent human and animal studies have demonstrated that in severe end-stage heart failure (HF), the cardiac muscle switches to a more fetal metabolic phenotype, characterized by downregulation of free fatty acid (FFA) oxidation and an enhancement of glucose oxidation. The goal of this study was to examine myocardial substrate metabolism in a model of moderate coronary microembolization-induced HF. We hypothesized that during well-compensated HF, FFA oxidation would predominate as opposed to a more fetal metabolic phenotype of greater glucose oxidation. Cardiac substrate uptake and oxidation were measured in normal dogs ( n = 8) and in dogs with microembolization-induced HF ( n = 18, ejection fraction = 28%) by infusing three isotopic tracers ([9,10-3H]oleate, [U-14C]glucose, and [1-13C]lactate) in anesthetized open-chest animals. There were no differences in myocardial substrate metabolism between the two groups. The total activity of pyruvate dehydrogenase, the key enzyme regulating myocardial pyruvate oxidation (and hence glucose and lactate oxidation) was not affected by HF. We did not observe any difference in the activity of carnitine palmitoyl transferase I (CPT-I) and its sensitivity to inhibition by malonyl-CoA between groups; however, malonyl-CoA content was decreased by 22% with HF, suggesting less in vivo inhibition of CPT-I activity. The differences in malonyl-CoA content cannot be explained by changes in the Michaelis-Menten constant and maximal velocity for malonyl-CoA decarboxylase because neither were affected by HF. These results support the concept that there is no decrease in fatty acid oxidation during compensated HF and that the downregulation of fatty acid oxidation enzymes and the switch to carbohydrate oxidation observed in end-stage HF is only a late-stage phenomemon.

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.

“Energenetics” of Heart Failure

It has been postulated that the failing heart suffers from chronic energy starvation, and that the derangements in cardiac energy production contribute to the inevitable transition from compensated hypertrophy to decompensated heart failure. Although the existence of metabolic alterations is hardly disputed anymore, the molecular mechanisms driving this "metabolic remodeling" process and its significance for the development of cardiac failure are still open to discussion. Next to changes in mitochondrial function, the hypertrophied heart is characterized by a marked change in substrate preference away from fatty acids toward glucose. Several lines of evidence suggest that these metabolic adaptations are brought about, at least in part, by alterations in the rate of transcription of genes encoding for proteins involved in substrate transport and metabolism. Here, we present an overview of the principal metabolic changes and discuss the various mechanisms that are likely to play a role, with special emphasis on gene regulatory mechanisms. In addition, the significance of these changes for the etiology of heart failure is discussed.

Effects of amino acid supplementation on left ventricular remodeling in patients with chronic heart failure with decreased systolic function and diabetes mellitus: rationale and design of a magnetic resonance imaging study

A considerable number of patients with reduced systolic function caused by primary or ischemic cardiomyopathy have viable and noncontractile myocardium. This may be related to numerous and perhaps overlapping factors, such as chronic ischemia (stunning/hibernation), neurohormonal abnormalities, oxidative stress, metabolic imbalances, and/or nutritional depletion. Changes in myocardial substrate utilization have adverse effects on the metabolism of the viable but noncontractile myocardium. Shifting the energy substrate preference away from fatty acids and replenishing the tricarboxylic acid cycle components via amino acids rather than via fatty acids would increase adenosine triphosphate production, with positive effects on cellular metabolism. A proposed study design is described and will be piloted through the Effects of Diatrofen on Myocardial Function in Patients with Chronic Heart Failure trial (D-CHF), an evaluation of an oral amino acid supplementation treatment in outpatients with heart failure.

Diabetic cardiomyopathy: a metabolic perspective

Patients with diabetes mellitus have a >3-fold increased risk of coronary ischemic events and congestive heart failure. Several hypotheses have been provided to explain the increased cardiac vulnerability in individuals with diabetes; among these are the metabolic abnormalities. Diabetes is associated with profound changes in cardiac metabolism, characterized by diminished glucose utilization, diminished rates of lactate oxidation, and increased use of fatty acids. Very few investigations have focused on amino acid disturbances at the level of the heart. This area of research is potentially relevant because cardiac amino acid alterations may not only result in a reduced energy reserve but could also lead to quantitative and qualitative abnormalities of contractile protein present in the diabetic heart.

Differential modulation of citrate synthesis and release by fatty acids in perfused working rat hearts

The objective of this study was to test the effect of increasing fatty acid concentrations on substrate fluxes through pathways leading to citrate synthesis and release in the heart. This was accomplished using semirecirculating work-performing rat hearts perfused with substrate mixtures mimicking the in situ milieu (5.5 mM glucose, 8 nM insulin, 1 mM lactate, 0.2 mM pyruvate, and 0.4 mM oleate-albumin) and 13C methods. Raising the fatty acid concentration from 0.4 to 1 mM with long-chain oleate or medium-chain octanoate resulted in a lowering ( approximately 20%) of cardiac output and efficiency with unaltered O2 consumption. At the metabolic level, beyond the expected effects of high fatty acid levels on the contribution of pyruvate decarboxylation (reduced >3-fold) and beta-oxidation (enhanced approximately 3-fold) to citrate synthesis, there was also a 2.4-fold lowering of anaplerotic pyruvate carboxylation. Despite the dual inhibitory effect of high fatty acids on pyruvate decarboxylation and carboxylation, tissue citrate levels were twofold higher, but citrate release rates remained unchanged at 11-14 nmol/min, representing <0.5% of citric acid cycle flux. A similar trend was observed for most metabolic parameters after oleate or octanoate addition. Together, these results emphasize a differential modulation of anaplerotic pyruvate carboxylation and citrate release in the heart by fatty acids. We interpret the lack of effects of high fatty acid concentrations on citrate release rates as suggesting that, under physiological conditions, this process is maximal, probably limited by the activity of its mitochondrial or plasma membrane transporter. Limited citrate release at high fatty acid concentrations may have important consequences for the heart's fuel metabolism and function.

Carnitine palmitoyltransferase-I, a new target for the treatment of heart failure: perspectives on a shift in myocardial metabolism as a therapeutic intervention

Although the heart is capable of extracting energy from different types of substrates such as fatty acids and carbohydrates, fatty acids are the preferred fuel under physiological conditions. In view of the presence of diverse defects in myocardial metabolism in the failing heart, changes in metabolism of glucose and fatty acids are considered as viable targets for therapeutic modification in the treatment of heart failure. One of these changes involves the carnitine palmitoyltransferase (CPT) enzymes, which are required for the transfer of long chain fatty acids into the mitochondrial matrix for oxidation. Since CPT inhibitors have been shown to prevent the undesirable effects induced by mechanical overload, e.g. cardiac hypertrophy and heart failure, it was considered of interest to examine whether the inhibition of CPT enzymes represents a novel approach for the treatment of heart disease. A shift from fatty acid metabolism to glucose metabolism due to CPT-I inhibition has been reported to exert beneficial effects in both cardiac hypertrophy and heart failure. Since the inhibition of fatty acid oxidation is effective in controlling abnormalities in diabetes mellitus, CPT-I inhibitors may also prove useful in the treatment of diabetic cardiomyopathy. Accordingly, it is suggested that CPT-I may be a potential target for drug development for the therapy of heart disease in general and heart failure in particular.

Transcriptional regulation of energy substrate metabolism in normal and hypertrophied heart

Impaired myocardial energy metabolism in cardiac hypertrophy and failure is characterized by decreased fatty-acid oxidation and increased glucose utilization. Mechanisms involving deactivation of peroxisome proliferator-activated receptor alpha/relinoid X receptor alpha (PPARalpha/RXRalpha),and activation of chicken ovalbumin upstream promoter transcription factor (COUP-TF), and transcription factors Sp1 and Sp3, lead to decreased capacity for fatty acid utilization in hypertrophied hearts. Furthermore, impaired myocardial energetic status stimulates glucose uptake and glycolysis, which, in combination with the permissive effect due to decreased fatty acid oxidation, promotes increases in glucose utilization in hypertrophied hearts. Finally, shifting substrate utilization toward glucose is likely adaptive and has the potential to delay transition to heart failure.

Regulation of hypertrophic and apoptotic signaling pathways by reactive oxygen species in cardiac myocytes

Increasing evidence suggests that oxidative and nitrosative stress play an important role in regulation of cardiac myocyte growth and survival. The cardiovascular system is continuously exposed to both reactive oxygen species (ROS) and nitrogen species (RNS), collectively termed reactive inflammatory species (RIS), and imbalances between the enzymes that regulate their bioavailability are associated with cardiac hypertrophy and the pathogenesis of cardiomyopathies, myocardial infarction and heart failure. It is now clear that RIS act as critical regulators of cardiac myocyte hypertrophy and apoptosis through control of redox-sensitive signaling cascades, such as tyrosine kinases and phosphatases, protein kinase C, and mitogen-activated protein kinases. This review will focus on the mechanisms by which ROS/RNS modulate cardiac myocyte growth and apoptosis induced by neurohormones and cytokines, and will discuss evidence for a role in the pathophysiology of heart failure.

The molecular basis of myocardial hypertrophy and heart failure

Heart failure (HF) is the inability of the heart to cope with the metabolic demands of the periphery. It is the common end-stage of many frequent cardiac diseases and is characterized by relentless progression. Mechanisms of progression include renal sodium and water retention, neurohumoral activation and alterations of the protein composition (gene programme) of the heart itself. In this review, we explain the often confusing terminology in the subject, briefly touch upon the peripheral mechanisms of HF, and then focus on the changes in the gene programme of the failing heart and the molecular mechanisms leading to them. Understanding the basic processes underlying HF will help uninitiated readers to gain insight into recent novel approaches to its treatment.

Myocardial recovery from ischemia is impaired in CD36-null mice and restored by myocyte CD36 expression or medium-chain fatty acids

Long-chain fatty acid uptake, which provides a large part of myocardial energy, is impaired in human and murine hearts deficient in the membrane fatty acid translocase, FAT/CD36. We examined myocardial function in CD36-null mice using the working heart. Fatty acid oxidation and stores of glycogen, triglycerides, and ATP were reduced in CD36-deficient hearts and were restored to WT levels by rescue of myocyte CD36. Under normal perfusion conditions, CD36-null hearts had similar cardiac outputs and end-diastolic pressures as WT or transgenic hearts. After 6 min of ischemia, cardiac output decreased by 41% and end diastolic pressure tripled for CD36-null hearts, with no significant changes in WT or transgenic hearts. Null hearts also failed more frequently after ischemia as compared with WT or transgenics. To dissect out contribution of fatty acid uptake, a perfusate-lacking fatty acids was used. This decreased cardiac output after ischemia by 30% in WT hearts as compared with 50% for CD36-deficient hearts. End diastolic pressure, a negative index of myocardial performance, increased after ischemia in all heart types. Addition to the perfusate of a medium-chain fatty acid (caprylic acid) that does not require CD36 for uptake alleviated poor ischemic tolerance of CD36-null hearts. In summary, recovery from ischemia is compromised in CD36-deficient hearts and can be restored by CD36 rescue or by supplying medium-chain fatty acids. It would be important to determine whether the findings apply to the human situation where polymorphisms of the CD36 gene are relatively common.

Targeting signaling pathways in heart failure by gene transfer

Congestive heart failure represents an enormous clinical problem demanding effective therapeutic approaches. The varied etiologies of heart failure include abnormalities of ion handling, cellular signaling, neurohormonal control, and apoptosis, all of which are potentially amenable to genetic manipulation. Gene therapy holds the promise of retarding the progression, preventing, and perhaps reversing heart failure. Advances in our knowledge of possible targets, vectors, and delivery techniques have revolutionized this field in recent years, bringing us close to clinical application.

Metabolic therapy in the treatment of ischaemic heart disease: the pharmacology of trimetazidine

The primary result of myocardial ischaemia is reduced oxygen consumption and adenosine triphosphate (ATP) formation in the mitochondria, and accelerated anaerobic glycolysis, lactate accumulation and cell acidosis. Classic pharmacotherapy for demand-induced ischaemia is aimed at restoring the balance between ATP synthesis and breakdown by increasing the oxygen delivery (i.e. with long acting nitrates or Ca2+ channel antagonist) or by decreasing cardiac power by reducing blood pressure and heart rate (i.e. with beta-blocker or Ca2+ channel antagonist). Animal studies show that fatty acids are the primary mitochondrial substrate during moderate severity myocardial ischaemia, and that they inhibit the oxidation of carbohydrate and drive the conversion of pyruvate to lactate. Drugs that partially inhibit myocardial fatty acid oxidation increase carbohydrate oxidation, which results in reduced lactate production and a higher cell pH during ischaemia. Trimetazidine (1-[2,3,4-trimethoxibenzyl]-piperazine) is the first and only registered drug in this class, and is available in over 90 countries world-wide. Trimetazidine selectively inhibits the fatty acid beta-oxidation enzyme 3-keto-acyl-CoA dehydrogenase (3-KAT), and is devoid of any direct haemodynamic effects. In double-blind placebo-controlled trials trimetazidine significantly improved symptom-limited exercise performance in stable angina patients when used either as monotherapy or in combination with beta-blockers or Ca2+ channel antagonists. Given available evidence, trimetazidine is an excellent alternative to classic haemodynamic agents, and is unique in its ability to reduce symptoms of angina when used in patients resistant to a haemodynamic treatment as vasodilators, beta-blockers or Ca2+ channel antagonists.

Regulation of peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1 alpha ) and mitochondrial function by MEF2 and HDAC5

The myocyte enhancer factor-2 (MEF2) transcription factor regulates muscle development and calcium-dependent gene expression. MEF2 activity is repressed by class II histone deacetylases (HDACs), which dissociate from MEF2 when phosphorylated on two serine residues in response to calcium signaling. To explore the potential importance of MEF2/HDAC interactions in the heart, we generated transgenic mice expressing a signal-resistant form of HDAC5 under cardiac-specific and doxycycline-inducible regulation. Transgene expression resulted in sudden death in male mice accompanied by loss and morphologic changes of cardiac mitochondria and down-regulation of mitochondrial enzymes. The transcriptional coactivator PGC-1 alpha, a master regulator of mitochondrial biogenesis and fatty acid oxidation, was also down-regulated in response to HDAC5 expression. Examination of the PGC-1 alpha promoter revealed two MEF2-binding sites that mediate transcriptional activation by MEF2 and repression by HDAC5. These findings identify PGC-1 alpha as a key target of the MEF2/HDAC regulatory pathway and demonstrate this pathway's importance in maintenance of cardiac mitochondrial function.

Clinical Value of Iodine-123 Beta-Methyliodophenyl Pentadecanoic Acid (BMIPP) Myocardial Single Photon Emission Computed Tomography for Predicting Cardiac Death Among Patients With Chronic Heart Failure

In the present study, the effectiveness of 123I-beta-methyliodophenyl pentadecanoic acid (BMIPP) single photon emission computed tomography (SPECT) for predicting cardiac death of patients with chronic heart failure was evaluated. Abnormalities of fatty acid metabolism are found in patients with chronic heart failure and BMIPP was developed as a tracer for scintigraphic assessment of myocardial fatty acid utilization. The study group comprised 74 patients with chronic heart failure with a left ventricular ejection fraction (LVEF) <45% on left ventriculography or radionuclide angiocardiography. They underwent both 201Tl SPECT and BMIPP SPECT. The uptake of tracer was scored semiquantitatively from 0 (normal) to 4 (defect) in 20 segments and a total defect score (TDS) for all 20 segments was calculated. On planar images the mediastinum to heart count ratio (H/M) was calculated for the BMIPP and Tl studies, and the H/M(BMIPP):H/M(Tl) (H/M(BMIPP) divided by H/M(Tl)) was also calculated. The mean follow-up period was 660 days and there were 17 cases of cardiac death. Multivariate analysis identified H/M(BMIPP):H/M(Tl) (p<0.05) and LVEF (p<0.05) as independent predictors of cardiac death. The receiver-operating characteristic curve of H/M(BMIPP):H/M (Tl) was situated to the left relative to LVEF. Analysis of the myocardial metabolism by BMIPP SPECT can predict the high-risk patients with chronic heart failure.

Short-term treatment with ranolazine improves mechanical efficiency in dogs with chronic heart failure

The present study assesses whether ranolazine increases left ventricular (LV) function without an increase in myocardial oxygen consumption (MVO2) and thus improves LV mechanical efficiency in dogs with heart failure (HF). Ranolazine did not change MVO2 and LV mechanical efficiency increased (22.4+/-2.8% to 30.9+/-3.4% (P<0.05). In contrast, dobutamine significantly increased MVO2 and did not improve mechanical efficiency. Thus, short-term treatment with ranolazine improved LV function without an increase in MO2, resulting in an increased myocardial mechanical efficiency in dogs with HF.

Metabolic approaches to the treatment of ischemic heart disease: the clinicians' perspective

This review article discusses pharmacological approaches to optimizing myocardial metabolism during ischemia. Fatty acids are the main fuel for the healthy heart, with a lesser contribution coming from the oxidation of glucose and lactate. Myocardial ischaemia dramatically alters fuel metabolism, causing an accelerated rate of glucose conversion to lactate and a switch from lactate uptake by the heart to lactate production. This causes a dramatic disruption in cell homeostasis (e.g. lactate accumulation and a decrease in pH and ATP). Paradoxically, moderately ischemic tissue (approximately 50% of normal flow) continues to derive most of its energy (50-70%) from the oxidation of fatty acids despite a high rate of lactate production. This ischaemia-induced disruption in cardiac metabolism can be minimized by metabolic agents that reduce fatty acid oxidation and increase the combustion of glucose and lactate, resulting in clinical benefit to the ischemic patient. Agents that inhibit fatty acid beta-oxidation, such as ranolazine and trimetazidine, have proven to be effective in the treatment of stable angina. Treatment of acute myocardial infarction patients with an infusion of the glucose-insulin-potassium, which results in suppression of myocardial fatty acid oxidation and greater glucose combustion, has proven effective in reducing mortality. These metabolic therapies are free of direct hemodynamic or chronotropic effects, and thus are well positioned for use alongside traditional agents such as beta-adrenergic receptor antagonists or calcium channel antagonists.