Myocardial phosphocreatine-to-ATP ratio is a predictor of mortality in patients with dilated cardiomyopathy

Adaptations in Protein Expression and Regulated Activity of Pyruvate Dehydrogenase Multienzyme Complex in Human Systolic Heart Failure

Pyruvate dehydrogenase (PDH) complex, a multienzyme complex at the nexus of glycolytic and Krebs cycles, provides acetyl-CoA to the Krebs cycle and NADH to complex I thus supporting a critical role in mitochondrial energy production and cellular survival. PDH activity is regulated by pyruvate dehydrogenase phosphatases (PDP1, PDP2), pyruvate dehydrogenase kinases (PDK 1-4), and mitochondrial pyruvate carriers (MPC1, MPC2). As NADH-dependent oxidative phosphorylation is diminished in systolic heart failure, we tested whether the left ventricular myocardium (LV) from end-stage systolic adult heart failure patients (n = 26) exhibits altered expression of PDH complex subunits, PDK, MPC, PDP, and PDH complex activity, compared to LV from nonfailing donor hearts (n = 21). Compared to nonfailing LV, PDH activity and relative expression levels of E2, E3bp, E1α, and E1β subunits were greater in LV failure. PDK4, MPC1, and MPC2 expressions were decreased in failing LV, whereas PDP1, PDP2, PDK1, and PDK2 expressions did not differ between nonfailing and failing LV. In order to examine PDK4 further, donor human LV cardiomyocytes were induced in culture to hypertrophy with 0.1 μM angiotensin II and treated with PDK inhibitors (0.2 mM dichloroacetate, or 5 mM pyruvate) or activators (0.6 mM NADH plus 50 μM acetyl CoA). In isolated hypertrophic cardiomyocytes in vitro, PDK activators and inhibitors increased and decreased PDK4, respectively. In conclusion, in end-stage failing hearts, greater expression of PDH proteins and decreased expression of PDK4, MPC1, and MPC2 were evident with higher rates of PDH activity. These adaptations support sustained capacity for PDH to facilitate glucose metabolism in the face of other failing bioenergetic pathways.

Chemical exchange saturation transfer imaging of phosphocreatine in the muscle

PURPOSE

To determine the exchange parameters for the CEST of phosphocreatine (PCrCEST) in phantoms and to characterize PCrCEST in vivo in the muscle at different saturation powers and magnetic fields.

METHODS

Exchange parameters were measured in PCr solutions using varying saturation power at 15.2 T. Z-spectra were analyzed using multipool Lorentzian fitting in the hindlimb using various powers at 2 different fields: 9.4 T and 15.2 T. Modulation of PCr signal in PCrCEST and phosphorus MRS was observed in the mouse hindlimb before and after euthanasia.

RESULTS

The exchange rate of PCr at physiological pH in phantoms was confirmed to be in a much slower exchange regime compared with Cr: k_ex at pH 7.3 and below was less than 400 s^-1 . There was insufficient signal for detection of PCrCEST in the brain, but PCrCEST in the hindlimb was measured to be 2.98% ± 0.43 at a B₁ of 0.47 μT at 15.2 T, which is 29% higher than 9.4T values. The value of PCrCEST at a B₁ of 0.71 μT was not significantly different than that measured at a B₁ of 0.47 μT. After euthanasia, PCrCEST signal dropped by 82.3% compared with an 85% decrease in PCr in phosphorus MRS, whereas CrCEST signal increased by 90.6%.

CONCLUSION

The PCrCEST technique has viable sensitivity in the muscle at high fields and shows promise for the study of metabolic dysfunction and cardiac systems.

Impaired Energy Production Contributes to Development of Failure in Taurine Deficient Heart

Taurine forms a conjugate in the mitochondria with a uridine residue in the wobble position of tRNALeu(UUR). The resulting product, 5-taurinomethyluridine tRNALeu(UUR), increases the interaction between the UUG codon and AAU anticodon of tRNALeu(UUR), thereby improving the decoding of the UUG codon. We have shown that the protein most affected by the taurine conjugation product is ND6, which is a subunit of complex I of the respiratory chain. Thus, taurine deficiency exhibits reduced respiratory chain function. Based on these findings, we proposed that the taurine deficient heart is energy deficient. To test this idea, hearts were perfused with buffer containing acetate and glucose as substrates. The utilization of both substrates, as well as the utilization of endogenous lipids, was significantly reduced in the taurine deficient heart. This led to a 25% decrease in ATP production, an effect primarily caused by diminished aerobic metabolism and respiratory function. In addition, inefficient oxidative phosphorylation causes a further decrease in ATP generation. The data support the idea that reductions in energy metabolism, including oxidative phosphorylation, ATP generation and high energy phosphate content, contribute to the severity of the cardiomyopathy. The findings are also consistent with the hypothesis that taurine deficiency and reduced myocardial energy content increases mortality of the taurine deficient, failing heart. The clinical implications of these findings are addressed.

Mitochondrial dysfunction in pathophysiology of heart failure

Mitochondrial dysfunction has been implicated in the development of heart failure. Oxidative metabolism in mitochondria is the main energy source of the heart, and the inability to generate and transfer energy has long been considered the primary mechanism linking mitochondrial dysfunction and contractile failure. However, the role of mitochondria in heart failure is now increasingly recognized to be beyond that of a failed power plant. In this Review, we summarize recent evidence demonstrating vicious cycles of pathophysiological mechanisms during the pathological remodeling of the heart that drive mitochondrial contributions from being compensatory to being a suicide mission. These mechanisms include bottlenecks of metabolic flux, redox imbalance, protein modification, ROS-induced ROS generation, impaired mitochondrial Ca2+ homeostasis, and inflammation. The interpretation of these findings will lead us to novel avenues for disease mechanisms and therapy.

Obesity, ectopic fat and cardiac metabolism

INTRODUCTION

Obesity is recognized as a risk factor for cardiovascular disease, expending independent adverse effects on the cardiovascular system. This relationship is complex due to several associations with cardiovascular disease risk factors/markers such as hypertension, dyslipidemia, insulin resistance/dysglycemia, or type 2 diabetes mellitus. Obesity induces a variety of cardiovascular system structural adaptations, from subclinical myocardial dysfunction to severe left ventricular systolic heart failure. Abnormalities in cardiac metabolism and subsequent cardiac energy, have been proposed as major contributors to obesity-related cardiovascular disease. Ectopic fat depots play an important role in several of the hypotheses postulated to explain the association between obesity, cardiac metabolism and cardiac dysfunction.

AREAS COVERED

In this review, we addressed with contemporary studies how obesity-associated metabolic conditions and ectopic cardiac fat accumulation, translate into cardiac energy metabolism disturbances that may lead to adverse effects on the cardiovascular system.

EXPERT COMMENTARY

Obesity and ectopic fat accumulation has long been related to metabolic diseases and adverse cardiovascular outcomes. Recent imaging advances have just started to address the complex interplays between obesity, ectopic fat depots, cardiac metabolism and the risk of obesity-related cardiovascular disease. A better comprehension of these obesity-associated metabolic disturbances will lead to earlier detection of patients at increased risk of cardiovascular disease and to the development of novel therapeutic metabolic targets to treat a wide variety of cardiovascular diseases.

An "Exercise" in Cardiac Metabolism

Research has demonstrated that the high capacity requirements of the heart are satisfied by a preference for oxidation of fatty acids. However, it is well known that a stressed heart, as in pathological hypertrophy, deviates from its inherent profile and relies heavily on glucose metabolism, primarily achieved by an acceleration in glycolysis. Moreover, it has been suggested that the chronically lipid overloaded heart augments fatty acid oxidation and triglyceride synthesis to an even greater degree and, thus, develops a lipotoxic phenotype. In comparison, classic studies in exercise physiology have provided a basis for the acute metabolic changes that occur during physical activity. During an acute bout of exercise, whole body glucose metabolism increases proportionately to intensity while fatty acid metabolism gradually increases throughout the duration of activity, particularly during moderate intensity. However, the studies in chronic exercise training are primarily limited to metabolic adaptations in skeletal muscle or to the mechanisms that govern physiological signaling pathways in the heart. Therefore, the purpose of this review is to discuss the precise changes that chronic exercise training elicits on cardiac metabolism, particularly on substrate utilization. Although conflicting data exists, a pattern of enhanced fatty oxidation and normalization of glycolysis emerges, which may be a therapeutic strategy to prevent or regress the metabolic phenotype of the hypertrophied heart. This review also expands on the metabolic adaptations that chronic exercise training elicits in amino acid and ketone body metabolism, which have become of increased interest recently. Lastly, challenges with exercise training studies, which could relate to several variables including model, training modality, and metabolic parameter assessed, are examined.

Loss of Metabolic Flexibility in the Failing Heart

To maintain its high energy demand the heart is equipped with a highly complex and efficient enzymatic machinery that orchestrates ATP production using multiple energy substrates, namely fatty acids, carbohydrates (glucose and lactate), ketones and amino acids. The contribution of these individual substrates to ATP production can dramatically change, depending on such variables as substrate availability, hormonal status and energy demand. This "metabolic flexibility" is a remarkable virtue of the heart, which allows utilization of different energy substrates at different rates to maintain contractile function. In heart failure, cardiac function is reduced, which is accompanied by discernible energy metabolism perturbations and impaired metabolic flexibility. While it is generally agreed that overall mitochondrial ATP production is impaired in the failing heart, there is less consensus as to what actual switches in energy substrate preference occur. The failing heart shift toward a greater reliance on glycolysis and ketone body oxidation as a source of energy, with a decrease in the contribution of glucose oxidation to mitochondrial oxidative metabolism. The heart also becomes insulin resistant. However, there is less consensus as to what happens to fatty acid oxidation in heart failure. While it is generally believed that fatty acid oxidation decreases, a number of clinical and experimental studies suggest that fatty acid oxidation is either not changed or is increased in heart failure. Of importance, is that any metabolic shift that does occur has the potential to aggravate cardiac dysfunction and the progression of the heart failure. An increasing body of evidence shows that increasing cardiac ATP production and/or modulating cardiac energy substrate preference positively correlates with heart function and can lead to better outcomes. This includes increasing glucose and ketone oxidation and decreasing fatty acid oxidation. In this review we present the physiology of the energy metabolism pathways in the heart and the changes that occur in these pathways in heart failure. We also look at the interventions which are aimed at manipulating the myocardial metabolic pathways toward more efficient substrate utilization which will eventually improve cardiac performance.

Diabetic db/db mice do not develop heart failure upon pressure overload: a longitudinal in vivo PET, MRI, and MRS study on cardiac metabolic, structural, and functional adaptations

Aims

Heart failure is associated with altered myocardial substrate metabolism and impaired cardiac energetics. Comorbidities like diabetes may influence the metabolic adaptations during heart failure development. We quantified to what extent changes in substrate preference, lipid accumulation, and energy status predict the longitudinal development of hypertrophy and failure in the non-diabetic and the diabetic heart.

Methods and results

Transverse aortic constriction (TAC) was performed in non-diabetic (db/+) and diabetic (db/db) mice to induce pressure overload. Magnetic resonance imaging, 31P magnetic resonance spectroscopy (MRS), 1H MRS, and 18F-fluorodeoxyglucose-positron emission tomography (PET) were applied to measure cardiac function, energy status, lipid content, and glucose uptake, respectively. In vivo measurements were complemented with ex vivo techniques of high-resolution respirometry, proteomics, and western blotting to elucidate the underlying molecular pathways. In non-diabetic mice, TAC induced progressive cardiac hypertrophy and dysfunction, which correlated with increased protein kinase D-1 (PKD1) phosphorylation and increased glucose uptake. These changes in glucose utilization preceded a reduction in cardiac energy status. At baseline, compared with non-diabetic mice, diabetic mice showed normal cardiac function, higher lipid content and mitochondrial capacity for fatty acid oxidation, and lower PKD1 phosphorylation, glucose uptake, and energetics. Interestingly, TAC affected cardiac function only mildly in diabetic mice, which was accompanied by normalization of phosphorylated PKD1, glucose uptake, and cardiac energy status.

Conclusion

The cardiac metabolic adaptations in diabetic mice seem to prevent the heart from failing upon pressure overload, suggesting that restoring the balance between glucose and fatty acid utilization is beneficial for cardiac function.

Hyperpolarised magnetic resonance for in vivo real-time metabolic imaging

Although non-invasive perfusion and viability imaging often provide the gateway to coronary revascularisation, current non-invasive imaging methods only report the surrogate markers of inducible hypoperfusion and presence or absence of myocardial scar, rather than actually visualising areas of ischaemia and/or viable myocardium. This may lead to suboptimal revascularisation decisions. Normally respiring (viable) cardiomyocytes convert pyruvate to acetyl-CoA and CO₂/bicarbonate (via pyruvate dehydrogenase), but under ischaemic conditions characteristically shift this conversion to lactate (by lactate dehydrogenase). Imaging pyruvate metabolism thus has the potential to improve upon current imaging techniques. Using the novel hyperpolarisation technique of dynamic nuclear polarisation (DNP), the magnetic resonance signal of injected [1-¹³C]pyruvate can be transiently magnified >10 000 times over that seen in conventional MR spectroscopy, allowing the characteristic metabolic signatures of ischaemia (lactate production) and viability (CO₂/bicarbonate production) to be directly imaged. As such DNP imaging of the downstream metabolism of [1-¹³C]pyruvate could surpass the diagnostic capabilities of contemporary ischaemia and viability testing. Here we review the technique, and with brief reference to the salient biochemistry, discuss its potential applications within cardiology. These include ischaemia and viability testing, and further characterisation of the altered metabolism seen at different stages during the natural history of heart failure.

Polymer Functionalization of Isolated Mitochondria for Cellular Transplantation and Metabolic Phenotype Alteration

Aberrant mitochondrial energy transfer underlies prevalent chronic health conditions, including cancer, cardiovascular, and neurodegenerative diseases. Mitochondrial transplantation represents an innovative strategy aimed at restoring favorable metabolic phenotypes in cells with dysfunctional energy metabolism. While promising, significant barriers to in vivo translation of this approach abound, including limited cellular uptake and recognition of mitochondria as foreign. The objective is to functionalize isolated mitochondria with a biocompatible polymer to enhance cellular transplantation and eventual in vivo applications. Herein, it is demonstrated that grafting of a polymer conjugate composed of dextran with triphenylphosphonium onto isolated mitochondria protects the organelles and facilitates cellular internalization compared with uncoated mitochondria. Importantly, mitochondrial transplantation into cancer and cardiovascular cells has profound effects on respiration, mediating a shift toward improved oxidative phosphorylation, and reduced glycolysis. These findings represent the first demonstration of polymer functionalization of isolated mitochondria, highlighting a viable strategy for enabling clinical applications of mitochondrial transplantation.

Probing the cardiac malate-aspartate shuttle non-invasively using hyperpolarized [1,2-13 C2 ]pyruvate

Previous studies have demonstrated that using hyperpolarized [2-¹³ C]pyruvate as a contrast agent can reveal ¹³ C signals from metabolites associated with the tricarboxylic acid (TCA) cycle. However, the metabolites detectable from TCA cycle-mediated oxidation of [2-¹³ C]pyruvate are the result of several metabolic steps. In the instance of the [5-¹³ C]glutamate signal, the amplitude can be modulated by changes to the rates of pyruvate dehydrogenase (PDH) flux, TCA cycle flux and metabolite pool size. Also key is the malate-aspartate shuttle, which facilitates the transport of cytosolic reducing equivalents into the mitochondria for oxidation via the malate-α-ketoglutarate transporter, a process coupled to the exchange of cytosolic malate for mitochondrial α-ketoglutarate. In this study, we investigated the mechanism driving the observed changes to hyperpolarized [2-¹³ C]pyruvate metabolism. Using hyperpolarized [1,2-¹³ C]pyruvate with magnetic resonance spectroscopy (MRS) in the porcine heart with different workloads, it was possible to probe ¹³ C-glutamate labeling relative to rates of cytosolic metabolism, PDH flux and TCA cycle turnover in a single experiment non-invasively. Via the [1-¹³ C]pyruvate label, we observed more than a five-fold increase in the cytosolic conversion of pyruvate to [1-¹³ C]lactate and [1-¹³ C]alanine with higher workload. ¹³ C-Bicarbonate production by PDH was increased by a factor of 2.2. Cardiac cine imaging measured a two-fold increase in cardiac output, which is known to couple to TCA cycle turnover. Via the [2-¹³ C]pyruvate label, we observed that ¹³ C-acetylcarnitine production increased 2.5-fold in proportion to the ¹³ C-bicarbonate signal, whereas the ¹³ C-glutamate metabolic flux remained constant on adrenergic activation. Thus, the ¹³ C-glutamate signal relative to the amount of ¹³ C-labeled acetyl-coenzyme A (acetyl-CoA) entering the TCA cycle was decreased by 40%. The data strongly suggest that NADH (reduced form of nicotinamide adenine dinucleotide) shuttling from the cytosol to the mitochondria via the malate-aspartate shuttle is limited on adrenergic activation. Changes in [5-¹³ C]glutamate production from [2-¹³ C]pyruvate may play an important future role in non-invasive myocardial assessment in patients with cardiovascular diseases, but careful interpretation of the results is required.

Heat Failure Phenotypes Induced by Knockdown of DAPIT in Zebrafish: A New Insight into Mechanism of Dilated Cardiomyopathy

The pathogenesis of heart failure associated with dilated cardiomyopathy (DCM) may result in part from adenosine triphosphate (ATP) dysregulation in the myocardium. Under these conditions, diabetes-associated protein in insulin-sensitive tissue (DAPIT), which is encoded by the upregulated during skeletal muscle growth 5 (USMG5) gene, plays a crucial role in energy production by mitochondrial ATP synthase. To determine whether USMG5 is related to the development of heart failure, we performed clinical and experimental studies. Microarray analysis showed that the expression levels of USMG5 were positively correlated with those of natriuretic peptide precursor A in the human failed myocardium. When endogenous z-usmg5 in zebrafish was disrupted using morpholino (MO) oligonucleotides, the pericardial sac and atrial areas were larger and ventricular fractional shortening was reduced compared to in the control MO group. The expression levels of natriuretic peptides were upregulated in the z-usmg5 MO group compared to in controls. Further, microarray analysis revealed that genes in the calcium signalling pathway were downregulated in the z-usmg5 MO group. These results demonstrate that DAPIT plays a crucial role in the development of heart failure associated with DCM and thus may be a therapeutic target for heart failure.

Mitochondrial cardiomyopathies feature increased uptake and diminished efflux of mitochondrial calcium

Calcium (Ca2+) influx into the mitochondrial matrix stimulates ATP synthesis. Here, we investigate whether mitochondrial Ca2+ transport pathways are altered in the setting of deficient mitochondrial energy synthesis, as increased matrix Ca2+ may provide a stimulatory boost. We focused on mitochondrial cardiomyopathies, which feature such dysfunction of oxidative phosphorylation. We study a mouse model where the main transcription factor for mitochondrial DNA (transcription factor A, mitochondrial, Tfam) has been disrupted selectively in cardiomyocytes. By the second postnatal week (10-15day old mice), these mice have developed a dilated cardiomyopathy associated with impaired oxidative phosphorylation. We find evidence of increased mitochondrial Ca2+ during this period using imaging, electrophysiology, and biochemistry. The mitochondrial Ca2+ uniporter, the main portal for Ca2+ entry, displays enhanced activity, whereas the mitochondrial sodium-calcium (Na+-Ca2+) exchanger, the main portal for Ca2+ efflux, is inhibited. These changes in activity reflect changes in protein expression of the corresponding transporter subunits. While decreased transcription of Nclx, the gene encoding the Na+-Ca2+ exchanger, explains diminished Na+-Ca2+ exchange, the mechanism for enhanced uniporter expression appears to be post-transcriptional. Notably, such changes allow cardiac mitochondria from Tfam knockout animals to be far more sensitive to Ca2+-induced increases in respiration. In the absence of Ca2+, oxygen consumption declines to less than half of control values in these animals, but rebounds to control levels when incubated with Ca2+. Thus, we demonstrate a phenotype of enhanced mitochondrial Ca2+ in a mitochondrial cardiomyopathy model, and show that such Ca2+ accumulation is capable of rescuing deficits in energy synthesis capacity in vitro.

31 P magnetic resonance fingerprinting for rapid quantification of creatine kinase reaction rate in vivo

The purpose of this work was to develop a 31 P spectroscopic magnetic resonance fingerprinting (MRF) method for fast quantification of the chemical exchange rate between phosphocreatine (PCr) and adenosine triphosphate (ATP) via creatine kinase (CK). A 31 P MRF sequence (CK-MRF) was developed to quantify the forward rate constant of ATP synthesis via CK ( kfCK), the T1 relaxation time of PCr ( T1PCr), and the PCr-to-ATP concentration ratio ( MRPCr). The CK-MRF sequence used a balanced steady-state free precession (bSSFP)-type excitation with ramped flip angles and a unique saturation scheme sensitive to the exchange between PCr and γATP. Parameter estimation was accomplished by matching the acquired signals to a dictionary generated using the Bloch-McConnell equation. Simulation studies were performed to examine the susceptibility of the CK-MRF method to several potential error sources. The accuracy of nonlocalized CK-MRF measurements before and after an ischemia-reperfusion (IR) protocol was compared with the magnetization transfer (MT-MRS) method in rat hindlimb at 9.4 T (n = 14). The reproducibility of CK-MRF was also assessed by comparing CK-MRF measurements with both MT-MRS (n = 17) and four angle saturation transfer (FAST) (n = 7). Simulation results showed that CK-MRF quantification of kfCK was robust, with less than 5% error in the presence of model inaccuracies including dictionary resolution, metabolite T2 values, inorganic phosphate metabolism, and B1 miscalibration. Estimation of kfCK by CK-MRF (0.38 ± 0.02 s-1 at baseline and 0.42 ± 0.03 s-1 post-IR) showed strong agreement with MT-MRS (0.39 ± 0.03 s-1 at baseline and 0.44 ± 0.04 s-1 post-IR). kfCK estimation was also similar between CK-MRF and FAST (0.38 ± 0.02 s-1 for CK-MRF and 0.38 ± 0.11 s-1 for FAST). The coefficient of variation from 20 s CK-MRF quantification of kfCK was 42% of that by 150 s MT-MRS acquisition and was 12% of that by 20 s FAST acquisition. This study demonstrates the potential of a 31 P spectroscopic MRF framework for rapid, accurate and reproducible quantification of chemical exchange rate of CK in vivo.

Optimized CEST cardiovascular magnetic resonance for assessment of metabolic activity in the heart

BACKGROUND

Previous studies have linked cardiac dysfunction to loss of metabolites in the creatine kinase system. Chemical exchange saturation transfer (CEST) is a promising metabolic cardiovascular magnetic resonance (CMR) imaging technique and has been applied in the heart for creatine mapping. However, current limitations include: (a) long scan time, (b) residual cardiac and respiratory motion, and (c) B0 field variations induced by respiratory motion. An improved CEST CMR technique was developed to address these problems.

METHODS

Animals with chronic myocardial infarction (N = 15) were scanned using the proposed CEST CMR technique and a late gadolinium enhancement (LGE)  sequence as reference. The major improvements of the CEST CMR technique are: (a) Images were acquired by single-shot FLASH, significantly increasing the scan efficiency. (b) All images were registered to reduce the residual motion. (c) The acquired Z-spectrum was analyzed using 3-pool-model Lorentzian-line fitting to generate CEST signal, reducing the impact of B0 field shifting due to respiratory motion. Feasibility of the technique was tested in a porcine model with chronic myocardial infarction. CEST signal was measured in the scar, border zone and remote myocardium. Initial studies were performed in one patient.

RESULTS

In all animals, healthy remote myocardial CEST signal was elevated (0.16 ± 0.02) compared to infarct CEST signal (0.09 ± 0.02, P < 0.001) and the border zone (0.12 ± 0.02, P < 0.001). For both animal and patient studies, the hypointense regions in the CEST contrast maps closely match the bright areas in the LGE images.

CONCLUSIONS

The proposed CEST CMR technique was developed to address long scan times, respiratory and cardiac motion, and B0 field variations. Lower CEST signal in bright region of the LGE image is consistent with the fact that myocardial infarction has reduced metabolic activity.

Evaluation of cardiac energetics by non-invasive 31P magnetic resonance spectroscopy

Alterations in myocardial energy metabolism have been implicated in the pathophysiology of cardiac diseases such as heart failure and diabetic cardiomyopathy. ³¹P magnetic resonance spectroscopy (MRS) is a powerful tool to investigate cardiac energetics non-invasively in vivo, by detecting phosphorus (³¹P)-containing metabolites involved in energy supply and buffering. In this article, we review the historical development of cardiac ³¹P MRS, the readouts used to assess cardiac energetics from ³¹P MRS, and how ³¹P MRS studies have contributed to the understanding of cardiac energy metabolism in heart failure and diabetes. This article is part of a Special issue entitled Cardiac adaptations to obesity, diabetes and insulin resistance, edited by Professors Jan F.C. Glatz, Jason R.B. Dyck and Christine Des Rosiers.

Plasma Amino Acid Abnormalities in Chronic Heart Failure. Mechanisms, Potential Risks and Targets in Human Myocardium Metabolism

The goal of this study was to measure arterial amino acid levels in patients with chronic heart failure (CHF), and relate them to left ventricular function and disease severity. Amino acids (AAs) play a crucial role for heart protein-energy metabolism. In heart failure, arterial AAs, which are the major determinant of AA uptake by the myocardium, are rarely measured. Forty-one subjects with clinically stable CHF (New York Heart Association (NYHA) class II to IV) were analyzed. After overnight fasting, blood samples from the radial artery were taken to measure AA concentrations. Calorie (Kcal_I), protein-, fat-, carbohydrate-intake, resting energy expenditure (REE), total daily energy expenditure (REE × 1.3), and cardiac right catheterization variables were all measured. Eight matched controls were compared for all measurements, with the exception of cardiac catheterization. Compared with controls, CHF patients had reduced arterial AA levels, of which both their number and reduced rates are related to Heart Failure (HF) severity. Arterial aspartic acid correlated with stroke volume index (r = 0.6263; p < 0.0001) and cardiac index (r = 0.4243; p = 0.0028). The value of arterial aspartic acid (µmol/L) multiplied by the cardiac index was associated with left ventricular ejection fraction (r = 0.3765; p = 0.0076). All NYHA groups had adequate protein intake (≥1.1 g/kg/day) and inadequate calorie intake (Kcal_I < REE × 1.3) was found only in class IV patients. This study showed that CHF patients had reduced arterial AA levels directly related to clinical disease severity and left ventricular dysfunction.

Using a whole-body 31P birdcage transmit coil and 16-element receive array for human cardiac metabolic imaging at 7T

Purpose

Cardiac phosphorus magnetic resonance spectroscopy (³¹P-MRS) provides unique insight into the mechanisms of heart failure. Yet, clinical applications have been hindered by the restricted sensitivity of the surface radiofrequency-coils normally used. These permit the analysis of spectra only from the interventricular septum, or large volumes of myocardium, which may not be meaningful in focal disease. Löring et al. recently presented a prototype whole-body (52 cm diameter) transmit/receive birdcage coil for ³¹P at 7T. We now present a new, easily-removable, whole-body ³¹P transmit radiofrequency-coil built into a patient-bed extension combined with a 16-element receive array for cardiac ³¹P-MRS.

Materials and methods

A fully-removable (55 cm diameter) birdcage transmit coil was combined with a 16-element receive array on a Magnetom 7T scanner (Siemens, Germany). Electro-magnetic field simulations and phantom tests of the setup were performed. In vivo maps of B₁⁺, metabolite signals, and saturation-band efficiency were acquired across the torsos of eight volunteers.

Results

The combined (volume-transmit, local receive array) setup increased signal-to-noise ratio 2.6-fold 10 cm below the array (depth of the interventricular septum) compared to using the birdcage coil in transceiver mode. The simulated coefficient of variation for B₁⁺ of the whole-body coil across the heart was 46.7% (surface coil 129.0%); and the in vivo measured value was 38.4%. Metabolite images of 2,3-diphosphoglycerate clearly resolved the ventricular blood pools, and muscle tissue was visible in phosphocreatine (PCr) maps. Amplitude-modulated saturation bands achieved 71±4% suppression of phosphocreatine PCr in chest-wall muscles. Subjects reported they were comfortable.

Conclusion

This easy-to-assemble, volume-transmit, local receive array coil combination significantly improves the homogeneity and field-of-view for metabolic imaging of the human heart at 7T.

Mitochondrial Bioenergetics and Dysfunction in Failing Heart

Energy insufficiency has been recognized as a key feature of systolic heart failure. Although mitochondria have long been known to sustain myocardial work energy supply, the capacity to therapeutically target mitochondrial bioenergetics dysfunction is hampered by a complex interplay of multiple perturbations that progressively compound causing myocardial failure and collapse. Compared to non-failing human donor hearts, activity rates of complexes I and IV, nicotinamide nucleotide transhydrogenase (NADPH-transhydrogenase, Nnt) and the Krebs cycle enzymes isocitrate dehydrogenase, malate dehydrogenase and aconitase are markedly decreased in end-stage heart failure. Diminished REDOX capacity with lower total glutathione and coenzyme Q₁₀ levels are also a feature of chronic left ventricular failure. Decreased enzyme activities in part relate to abundant and highly specific oxidative, nitrosylative, and hyperacetylation modifications. In this brief review we highlight that energy deficiency in end-stage failing human left ventricle predominantly involves concomitantly impaired activities of key electron transport chain and Krebs cycle enzymes rather than altered expression of respective genes or proteins. Augmented oxidative modification of these enzyme subunit structures, and the formation of highly reactive secondary metabolites, implicates dysfunction due to diminished capacity for management of mitochondrial reactive oxygen species, which contribute further to progressive decreases in bioenergetic capacity and contractile function in human heart failure.

Mitochondrial Function in Non-ischemic Heart Failure

Provision for the continuous demand for energy from the beating heart relies heavily on efficient mitochondrial activity. Non-ischemic cardiomyopathy in which oxygen supply is not limiting results from etiologies such as pressure overload. It is associated with progressive development of metabolic stress culminating in energy depletion and heart failure. The mitochondria from the ventricular walls undergoing non-ischemic cardiomyopathy are subjected to long periods of adaptation to support the changing metabolic milieu, which has been described as mal-adaptation since it ultimately results in loss of cardiac contractile function. While the chronicity of exposure to metabolic stressors, co-morbidities and thereby adaptive changes in mitochondria maybe different between ischemic and non-ischemic heart failure, the resulting pathology is very similar, especially in late stage heart failure. Understanding of the mitochondrial changes in early-stage heart failure may guide the development of mitochondrial-targeted therapeutic options to prevent progression of non-ischemic heart failure. This chapter reviews findings of mitochondrial functional changes in animal models and humans with non-ischemic heart failure. While most animal models of non-ischemic heart failure exhibit cardiac mitochondrial dysfunction, studies in humans have been inconsistent despite confirmed reduction in ATP production. This chapter also reviews the possibility of impairment of substrate supply processes upstream of the mitochondria in heart failure, and discusses potential metabolism-targeted therapeutic options.

Mitochondrial Transplantation in Myocardial Ischemia and Reperfusion Injury

Ischemic heart disease remains the leading cause of death worldwide. Mitochondria are the power plant of the cardiomyocyte, generating more than 95% of the cardiac ATP. Complex cellular responses to myocardial ischemia converge on mitochondrial malfunction which persists and increases after reperfusion, determining the extent of cellular viability and post-ischemic functional recovery. In a quest to ameliorate various points in pathways from mitochondrial damage to myocardial necrosis, exhaustive pharmacologic and genetic tools have targeted various mediators of ischemia and reperfusion injury and procedural techniques without applicable success. The new concept of replacing damaged mitochondria with healthy mitochondria at the onset of reperfusion by auto-transplantation is emerging not only as potential therapy of myocardial rescue, but as gateway to a deeper understanding of mitochondrial metabolism and function. In this chapter, we explore the mechanisms of mitochondrial dysfunction during ischemia and reperfusion, current developments in the methodology of mitochondrial transplantation, mechanisms of cardioprotection and their clinical implications.

Cardiac metabolism - A promising therapeutic target for heart failure

Both heart failure with reduced ejection fraction (HFrEF) and with preserved ejection fraction (HFpEF) are associated with high morbidity and mortality. Although many established pharmacological interventions exist for HFrEF, hospitalization and death rates remain high, and for those with HFpEF (approximately half of all heart failure patients), there are no effective therapies. Recently, the role of impaired cardiac energetic status in heart failure has gained increasing recognition with the identification of reduced capacity for both fatty acid and carbohydrate oxidation, impaired function of the electron transport chain, reduced capacity to transfer ATP to the cytosol, and inefficient utilization of the energy produced. These nodes in the genesis of cardiac energetic impairment provide potential therapeutic targets, and there is promising data from recent experimental and early-phase clinical studies evaluating modulators such as carnitine palmitoyltransferase 1 inhibitors, partial fatty acid oxidation inhibitors and mitochondrial-targeted antioxidants. Metabolic modulation may provide significant symptomatic and prognostic benefit for patients suffering from heart failure above and beyond guideline-directed therapy, but further clinical trials are needed.

Nicotinamide mononucleotide requires SIRT3 to improve cardiac function and bioenergetics in a Friedreich's ataxia cardiomyopathy model

Increasing NAD+ levels by supplementing with the precursor nicotinamide mononucleotide (NMN) improves cardiac function in multiple mouse models of disease. While NMN influences several aspects of mitochondrial metabolism, the molecular mechanisms by which increased NAD+ enhances cardiac function are poorly understood. A putative mechanism of NAD+ therapeutic action exists via activation of the mitochondrial NAD+-dependent protein deacetylase sirtuin 3 (SIRT3). We assessed the therapeutic efficacy of NMN and the role of SIRT3 in the Friedreich's ataxia cardiomyopathy mouse model (FXN-KO). At baseline, the FXN-KO heart has mitochondrial protein hyperacetylation, reduced Sirt3 mRNA expression, and evidence of increased NAD+ salvage. Remarkably, NMN administered to FXN-KO mice restores cardiac function to near-normal levels. To determine whether SIRT3 is required for NMN therapeutic efficacy, we generated SIRT3-KO and SIRT3-KO/FXN-KO (double KO [dKO]) models. The improvement in cardiac function upon NMN treatment in the FXN-KO is lost in the dKO model, demonstrating that the effects of NMN are dependent upon cardiac SIRT3. Coupled with cardio-protection, SIRT3 mediates NMN-induced improvements in both cardiac and extracardiac metabolic function and energy metabolism. Taken together, these results serve as important preclinical data for NMN supplementation or SIRT3 activator therapy in Friedreich's ataxia patients.

Metabolic remodeling in hypertrophied and failing myocardium: a review

The energy starvation hypothesis proposes that maladaptive metabolic remodeling antedates, initiates, and maintains adverse contractile dysfunction in heart failure (HF). Better understanding of the cardiac metabolic phenotype and metabolic signaling could help identify the role metabolic remodeling plays within HF and the conditions known to transition toward HF, including "pathological" hypertrophy. In this review, we discuss metabolic phenotype and metabolic signaling in the contexts of pathological hypertrophy and HF. We discuss the significance of alterations in energy supply (substrate utilization, oxidative capacity, and phosphotransfer) and energy sensing using observations from human and animal disease models and models of manipulated energy supply/sensing. We aim to provide ways of thinking about metabolic remodeling that center around metabolic flexibility, capacity (reserve), and efficiency rather than around particular substrate preferences or transcriptomic profiles. We show that maladaptive metabolic remodeling takes multiple forms across multiple energy-handling domains. We suggest that lack of metabolic flexibility and reserve (substrate, oxidative, and phosphotransfer) represents a final common denominator ultimately compromising efficiency and contractile reserve in stressful contexts.

Does young age really put the heart at risk?

Despite significant advances in the management and treatment of heart disease in children, there continue to be patients who have worse outcomes than might be expected. A number of risk factors that could be responsible have been identified. Evidence-based findings will be reviewed, including whether young age and (or) reduced body weight exacerbate these responses. For example, newborn children undergoing congenital cardiac surgery are known to have worse outcomes than older children. Evidence exists that newborn hearts do not tolerate ischemia as well as adult hearts, developing irreversible injury sooner and exhibiting at-risk metabolic profiles. As well, in response to the administration of heparin, elevations in free fatty acids occur during congenital heart surgery in children, which can have detrimental effects on the heart. Furthermore, myocardial energetic state has also been suggested to impact outcomes. Unfavourable energetic profiles were correlated to lower body weights in the same age healthy newborn piglet model. Newborn children suffering from congenital heart disease, with lower body weights, also had lower myocardial energetic state and this correlated with longer postoperative ventilatory support as well as a trend to longer intensive care unit stay. These findings imply that unfavourable myocardial metabolic profiles could contribute to postoperative complications.

Left ventricular functional, structural and energetic effects of normal aging: Comparison with hypertension

Objectives

Both aging and hypertension are significant risk factors for heart failure in the elderly. The purpose of this study was to determine how aging, with and without hypertension, affects left ventricular function.

Methods

Cross-sectional study of magnetic resonance imaging and ³¹P spectroscopy-based measurements of left ventricular structure, global function, strains, pulse wave velocity, high energy phosphate metabolism in 48 normal subjects and 40 treated hypertensive patients (though no other cardiovascular disease or diabetes) stratified into 3 age deciles from 50–79 years.

Results

Normal aging was associated with significant increases in systolic blood pressure, vascular stiffness, torsion, and impaired diastolic function (all P<0.05). Age-matched hypertension exacerbated the effects of aging on systolic pressure, and diastolic function. Hypertension alone, and not aging, was associated with increased left ventricular mass index, reduced energetic reserve, reduced longitudinal shortening and increased endocardial circumferential shortening (all P<0.05). Multiple linear regression analysis showed that these unique hypertensive features were significantly related to systolic blood pressure (P<0.05).

Conclusions

1) Hypertension adds to the age-related changes in systolic blood pressure and diastolic function; 2) hypertension is uniquely associated with changes in several aspects of left ventricular structure, function, systolic strains, and energetics; and 3) these uniquely hypertensive-associated parameters are related to the level of systolic blood pressure and so are potentially modifiable.

Cardiac metabolic imaging: current imaging modalities and future perspectives

In this review, current imaging techniques and their future perspectives in the field of cardiac metabolic imaging in humans are discussed. This includes a range of noninvasive imaging techniques, allowing a detailed investigation of cardiac metabolism in health and disease. The main imaging modalities discussed are magnetic resonance spectroscopy techniques for determination of metabolite content (triglycerides, glucose, ATP, phosphocreatine, and so on), MRI for myocardial perfusion, and single-photon emission computed tomography and positron emission tomography for quantitation of perfusion and substrate uptake.

Complex Energy Metabolic Changes in Heart Failure With Preserved Ejection Fraction and Heart Failure With Reduced Ejection Fraction

Alterations in cardiac energy metabolism contribute to the severity of heart failure. However, the energy metabolic changes that occur in heart failure are complex, and are dependent not only on the severity and type of heart failure present, but also on the coexistence of common comorbidities such as obesity and type 2 diabetes. In this article we review the cardiac energy metabolic changes that occur in heart failure. An emphasis is made on distinguishing the differences in cardiac energy metabolism between heart failure with preserved ejection fraction (HFpEF) and heart failure with reduced ejection fraction (HFrEF) and in clarifying the common misconceptions surrounding the fate of fatty acids and glucose in the failing heart. The major key points from this article are: (1) mitochondrial oxidative capacity is reduced in HFpEF and HFrEF; (2) fatty acid oxidation is increased in HFpEF and reduced in HFrEF (however, oxidative metabolism of fatty acids in HFrEF still exceeds that of glucose); (3) glucose oxidation is decreased in HFpEF and HFrEF; (4) there is an uncoupling between glucose uptake and oxidation in HFpEF and HFrEF, resulting in an increased rate of glycolysis; (5) ketone body oxidation is increased in HFrEF, which might further reduce fatty acid and glucose oxidation; and finally, (6) branched chain amino acid oxidation is impaired in HFrEF. The understanding of these changes in cardiac energy metabolism in heart failure are essential to allow the development of metabolic modulators in the treatment of heart failure.

Changes in the mitochondrial function and in the efficiency of energy transfer pathways during cardiomyocyte aging

The role of mitochondria in alterations that take place in the muscle cell during healthy aging is a matter of debate during recent years. Most of the studies in bioenergetics have a focus on the model of isolated mitochondria, while changes in the crosstalk between working myofibrils and mitochondria in senescent cardiomyocytes have been less studied. The aim of our research was to investigate the modifications in the highly regulated ATP production and energy transfer systems in heart cells in old rat cardiomyocytes. The results of our work demonstrated alterations in the diffusion restrictions of energy metabolites, manifested by changes in the apparent Michaelis-Menten constant of mitochondria to exogenous ADP. The creatine kinase (CK) phosphotransfer pathway efficiency declines significantly in senescence. The ability of creatine to stimulate OXPHOS as well as to increase the affinity of mitochondria for ADP is falling and the most critical decline is already in the 1-year group (middle-age model in rats). Also, a moderate decrease in the adenylate kinase phosphotransfer system was detected. The importance of glycolysis increases in senescence, while the hexokinase activity does not change during healthy aging. The main result of our study is that the decline in the heart muscle performance is not caused by the changes in the respiratory chain complexes activity but mainly by the decrease in the energy transfer efficiency, especially by the CK pathway.

Cardiac nuclear receptors: architects of mitochondrial structure and function

The adult heart is uniquely designed and equipped to provide a continuous supply of energy in the form of ATP to support persistent contractile function. This high-capacity energy transduction system is the result of a remarkable surge in mitochondrial biogenesis and maturation during the fetal-to-adult transition in cardiac development. Substantial evidence indicates that nuclear receptor signaling is integral to dynamic changes in the cardiac mitochondrial phenotype in response to developmental cues, in response to diverse postnatal physiologic conditions, and in disease states such as heart failure. A subset of cardiac-enriched nuclear receptors serve to match mitochondrial fuel preferences and capacity for ATP production with changing energy demands of the heart. In this Review, we describe the role of specific nuclear receptors and their coregulators in the dynamic control of mitochondrial biogenesis and energy metabolism in the normal and diseased heart.

Impaired cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with Barth syndrome

Barth syndrome (BTHS) is an X-linked condition characterized by altered cardiolipin metabolism and cardioskeletal myopathy. We sought to compare cardiac and skeletal muscle bioenergetics in children, adolescents, and young adults with BTHS and unaffected controls and examine their relationships with cardiac function and exercise capacity. Children/adolescents and young adults with BTHS (n = 20) and children/adolescent and young adult control participants (n = 23, total n = 43) underwent 31P magnetic resonance spectroscopy (31P-MRS) of the lower extremity (calf) and heart for estimation of skeletal muscle and cardiac bioenergetics. Peak exercise testing (VO2peak) and resting echocardiography were also performed on all participants. Cardiac PCr/ATP ratio was significantly lower in children/adolescents (BTHS: 1.5 ± 0.2 vs.

CONTROL

2.0 ± 0.3, P < 0.01) and adults (BTHS: 1.9 ± 0.2 vs.

CONTROL

2.3 ± 0.2, P < 0.01) with BTHS compared to Control groups. Adults (BTHS: 76.4 ± 31.6 vs.

CONTROL

35.0 ± 7.4 sec, P < 0.01) and children/adolescents (BTHS: 71.5 ± 21.3 vs.

CONTROL

31.4 ± 7.4 sec, P < 0.01) with BTHS had significantly longer calf PCr recovery (τPCr) postexercise compared to controls. Maximal calf ATP production through oxidative phosphorylation (Qmax-lin) was significantly lower in children/adolescents (BTHS: 0.5 ± 0.1 vs.

CONTROL

1.1 ± 0.3 mmol/L per sec, P < 0.01) and adults (BTHS: 0.5 ± 0.2 vs.

CONTROL

1.0 ± 0.2 mmol/L sec, P < 0.01) with BTHS compared to controls. Blunted cardiac and skeletal muscle bioenergetics were associated with lower VO2peak but not resting cardiac function. Cardiac and skeletal muscle bioenergetics are impaired and appear to contribute to exercise intolerance in BTHS.

Receptor-interacting protein 140 overexpression impairs cardiac mitochondrial function and accelerates the transition to heart failure in chronically infarcted rats

Heart failure (HF) is associated with myocardial energy metabolic abnormality. Receptor-interacting protein 140 (RIP140) is an important transcriptional cofactor for maintaining energy balance in high-oxygen consumption tissues. However, the role of RIP140 in the pathologic processes of HF remains to be elucidated. In this study, we investigated the role of RIP140 in mitochondrial and cardiac functions in rodent hearts under myocardial infarction (MI) stress. MI was created by a permanent ligation of left anterior descending coronary artery and exogenous expression of RIP140 by adenovirus (Ad) vector delivery. Four weeks after MI or Ad-RIP140 treatment, cardiac function was assessed by echocardiographic and hemodynamics analyses, and the mitochondrial function was determined by mitochondrial genes expression, biogenesis, and respiration rates. In Ad-RIP140 or MI group, a subset of metabolic genes changed, accompanied with slight reductions in mitochondrial biogenesis and respiration rates but no change in adenosine triphosphate (ATP) content. Cardiac malfunction was compensated. However, under MI stress, rats overexpressing RIP140 exhibited greater repressions in mitochondrial genes, state 3 respiration rates, respiration control ratio, and ATP content and had further deteriorated cardiac malfunction. In conclusion, RIP140 overexpression leads to comparable cardiac function as resulted from MI, but RIP140 aggravates metabolic repression, mitochondrial malfunction, and further accelerates the transition to HF in response to MI stress.

Imaging oxygen metabolism with hyperpolarized magnetic resonance: a novel approach for the examination of cardiac and renal function

Every tissue in the body critically depends on meeting its energetic demands with sufficient oxygen supply. Oxygen supply/demand imbalances underlie the diseases that inflict the greatest socio-economic burden globally. The purpose of this review is to examine how hyperpolarized contrast media, used in combination with MR data acquisition methods, may advance our ability to assess oxygen metabolism non-invasively and thus improve management of clinical disease. We first introduce the concept of hyperpolarization and how hyperpolarized contrast media have been practically implemented to achieve translational and clinical research. We will then analyse how incorporating hyperpolarized contrast media could enable realization of unmet technical needs in clinical practice. We will focus on imaging cardiac and renal oxygen metabolism, as both organs have unique physiological demands to satisfy their requirements for tissue oxygenation, their dysfunction plays a fundamental role in society’s most prevalent diseases, and each organ presents unique imaging challenges. It is our aim that this review attracts a multi-disciplinary audience and sparks collaborations that utilize an exciting, emergent technology to advance our ability to treat patients adversely affected by an oxygen supply/demand mismatch.

Weighted averaging in spectroscopic studies improves statistical power

Purpose

In vivo MRS is often characterized by a spectral signal‐to‐noise ratio (SNR) that varies highly between experiments. A common design for spectroscopic studies is to compare the ratio of two spectral peak amplitudes between groups, e.g. individual PCr/γ‐ATP ratios in ³¹P‐MRS. The uncertainty on this ratio is often neglected. We wished to explore this assumption.

Theory

The canonical theory for the propagation of uncertainty on the ratio of two spectral peaks and its incorporation in the Frequentist hypothesis testing framework by weighted averaging is presented.

Methods

Two retrospective re‐analyses of studies comparing spectral peak ratios and one prospective simulation were performed using both the weighted and unweighted methods.

Results

It was found that propagating uncertainty correctly improved statistical power in all cases considered, which could be used to reduce the number of subjects required to perform an MR study.

Conclusion

The variability of in vivo spectroscopy data is often accounted for by requiring it to meet an SNR threshold. A theoretically sound propagation of the variable uncertainty caused by quantifying spectra of differing SNR is therefore likely to improve the power of in vivo spectroscopy studies. Magn Reson Med 78:2082–2094, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

[Cardiac remodeling after myocardial infarction : Clinical practice update]

Heart failure remains a frequent cause of death and is the leading reason for hospitalization in Germany although therapeutic options have significantly increased over the past years particularly in heart failure with reduced ejection fraction. Clinical symptoms are usually preceded by cardiac remodeling, which was originally defined only by left ventricular dilatation and depressed function but is also associated with typical cellular and molecular processes. Healing after acute myocardial infarction is characterized by inflammation, cellular migration and scar formation. Cardiac remodeling is accompanied by adaptive changes of the peripheral cardiovascular system. Since prevention is the primary goal, rapid diagnosis and treatment of myocardial infarction are mandatory. Early reperfusion therapy limits infarct size and enables the best possible preservation of left ventricular function. Standard pharmacotherapy includes angiotensin-converting enzyme inhibitors, angiotensin-1-receptor blockers and beta blockers. In addition, mineralocorticoid receptor antagonists have proven beneficial. Compounds specifically targeting infarct healing processes are currently under development.

Metabolic Modulators in Heart Disease: Past, Present, and Future

Ischemic heart disease and heart failure are leading causes of mortality and morbidity worldwide. They continue to be major burden on health care systems throughout the world, despite major advances made over the past 40 years in developing new therapeutic approaches to treat these debilitating diseases. A potential therapeutic approach that has been underutilized in treating ischemic heart disease and heart failure is "metabolic modulation." Major alterations in myocardial energy substrate metabolism occur in ischemic heart disease and heart failure, and are associated with an energy deficit in the heart. A metabolic shift from mitochondrial oxidative metabolism to glycolysis, as well as an uncoupling between glycolysis and glucose oxidation, plays a crucial role in the development of cardiac inefficiency (oxygen consumed per work performed) and functional impairment in ischemic heart disease as well as in heart failure. This has led to the concept that optimizing energy substrate use with metabolic modulators can be a potentially promising approach to decrease the severity of ischemic heart disease and heart failure, primarily by improving cardiac efficiency. Two approaches for metabolic modulator therapy are to stimulate myocardial glucose oxidation and/or inhibit fatty acid oxidation. In this review, the past, present, and future of metabolic modulators as an approach to optimizing myocardial energy substrate metabolism and treating ischemic heart disease and heart failure are discussed. This includes a discussion of pharmacological interventions that target enzymes involved in fatty acid uptake, fatty acid oxidation, and glucose oxidation in the heart, as well as enzymes involved in ketone and branched chain amino acid catabolism in the heart.

Adiabatic excitation for 31 P MR spectroscopy in the human heart at 7 T: A feasibility study

Purpose

Phosphorus magnetic resonance spectroscopy (³¹P‐MRS) provides a unique tool for assessing cardiac energy metabolism, often quantified using the phosphocreatine (PCr)/adenosine triphosphate (ATP) ratio. Surface coils are typically used for excitation for ³¹P‐MRS, but they create an inhomogeneous excitation field across the myocardium, producing undesirable, spatially varying partial saturation. Therefore, we implemented adiabatic excitation in a 3D chemical shift imaging (CSI) sequence for cardiac ³¹P‐MRS at 7 Tesla (T).

Methods

We optimized an adiabatic half passage pulse with bandwidth sufficient to excite PCr and γ‐ATP together. In addition, the CSI sequence was modified to allow interleaved excitation of PCr and γ‐ATP, then 2,3‐DPG, to enable PCr/ATP determination with blood correction. Nine volunteers were scanned at 2 transmit voltages to confirm that measured PCr/ATP was independent of B1+ (i.e. over the adiabatic threshold). Six septal voxels were evaluated for each volunteer.

Results

Phantom experiments showed that adiabatic excitation can be reached at the depth of the heart using our pulse. The mean evaluated cardiac PCr/ATP ratio from all 9 volunteers corrected for blood signal was 2.14 ± 0.16. Comparing the two acquisitions with different voltages resulted in a minimal mean difference of −0.005.

Conclusion

Adiabatic excitation is possible in the human heart at 7 T, and gives consistent PCr/ATP ratios. Magn Reson Med 78:1667–1673, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Cardiac energetics, oxygenation, and perfusion during increased workload in patients with type 2 diabetes mellitus

AIMS

Patients with type 2 diabetes mellitus (T2DM) are known to have impaired resting myocardial energetics and impaired myocardial perfusion reserve, even in the absence of obstructive epicardial coronary artery disease (CAD). Whether or not the pre-existing energetic deficit is exacerbated by exercise, and whether the impaired myocardial perfusion causes deoxygenation and further energetic derangement during exercise stress, is uncertain.

METHODS AND RESULTS

Thirty-one T2DM patients, on oral antidiabetic therapies with a mean HBA1c of 7.4 ± 1.3%, and 17 matched controls underwent adenosine stress cardiovascular magnetic resonance for assessment of perfusion [myocardial perfusion reserve index (MPRI)] and oxygenation [blood-oxygen level-dependent (BOLD) signal intensity change (SIΔ)]. Cardiac phosphorus-MR spectroscopy was performed at rest and during leg exercise. Significant CAD (>50% coronary stenosis) was excluded in all patients by coronary computed tomographic angiography. Resting phosphocreatine to ATP (PCr/ATP) was reduced by 17% in patients (1.74 ± 0.26, P = 0.001), compared with controls (2.07 ± 0.35); during exercise, there was a further 12% reduction in PCr/ATP (P = 0.005) in T2DM patients, but no change in controls. Myocardial perfusion and oxygenation were decreased in T2DM (MPRI 1.61 ± 0.43 vs. 2.11 ± 0.68 in controls, P = 0.002; BOLD SIΔ 7.3 ± 7.8 vs. 17.1 ± 7.2% in controls, P < 0.001). Exercise PCr/ATP correlated with MPRI (r = 0.50, P = 0.001) and BOLD SIΔ (r = 0.32, P = 0.025), but there were no correlations between rest PCr/ATP and MPRI or BOLD SIΔ.

CONCLUSION

The pre-existing energetic deficit in diabetic cardiomyopathy is exacerbated by exercise; stress PCr/ATP correlates with impaired perfusion and oxygenation. Our findings suggest that, in diabetes, coronary microvascular dysfunction exacerbates derangement of cardiac energetics under conditions of increased workload.

Novel ketone diet enhances physical and cognitive performance

Ketone bodies are the most energy-efficient fuel and yield more ATP per mole of substrate than pyruvate and increase the free energy released from ATP hydrolysis. Elevation of circulating ketones via high-fat, low-carbohydrate diets has been used for the treatment of drug-refractory epilepsy and for neurodegenerative diseases, such as Parkinson's disease. Ketones may also be beneficial for muscle and brain in times of stress, such as endurance exercise. The challenge has been to raise circulating ketone levels by using a palatable diet without altering lipid levels. We found that blood ketone levels can be increased and cholesterol and triglycerides decreased by feeding rats a novel ketone ester diet: chow that is supplemented with (R)-3-hydroxybutyl (R)-3-hydroxybutyrate as 30% of calories. For 5 d, rats on the ketone diet ran 32% further on a treadmill than did control rats that ate an isocaloric diet that was supplemented with either corn starch or palm oil (P < 0.05). Ketone-fed rats completed an 8-arm radial maze test 38% faster than did those on the other diets, making more correct decisions before making a mistake (P < 0.05). Isolated, perfused hearts from rats that were fed the ketone diet had greater free energy available from ATP hydrolysis during increased work than did hearts from rats on the other diets as shown by using [31P]-NMR spectroscopy. The novel ketone diet, therefore, improved physical performance and cognitive function in rats, and its energy-sparing properties suggest that it may help to treat a range of human conditions with metabolic abnormalities.-Murray, A. J., Knight, N. S., Cole, M. A., Cochlin, L. E., Carter, E., Tchabanenko, K., Pichulik, T., Gulston, M. K., Atherton, H. J., Schroeder, M. A., Deacon, R. M. J., Kashiwaya, Y., King, M. T., Pawlosky, R., Rawlins, J. N. P., Tyler, D. J., Griffin, J. L., Robertson, J., Veech, R. L., Clarke, K. Novel ketone diet enhances physical and cognitive performance.

The Role of Cardiovascular Magnetic Resonance Imaging in Heart Failure

Cardiovascular imaging is key for the assessment of patients with heart failure. Today, cardiovascular magnetic resonance imaging plays an established role in the assessment of patients with suspected and confirmed heart failure syndromes, in particular identifying aetiology. Its role in informing prognosis and guiding decisions around therapy are evolving. Key strengths include its accuracy; reproducibility; unrestricted field of view; lack of radiation; multiple abilities to characterise myocardial tissue, thrombus and scar; as well as unparalleled assessment of left and right ventricular volumes. T2* has an established role in the assessment and follow-up of iron overload cardiomyopathy and a role for T1 in specific therapies for cardiac amyloid and Anderson-Fabry disease is emerging.

The Potential of Metabolic Imaging

Metabolic imaging is a field of molecular imaging that focuses and targets changes in metabolic pathways for the evaluation of different clinical conditions. Targeting and quantifying metabolic changes noninvasively is a powerful approach to facilitate diagnosis and evaluate therapeutic response. This review addresses only techniques targeting metabolic pathways. Other molecular imaging strategies, such as affinity or receptor imaging or microenvironment-dependent methods are beyond the scope of this review. Here we describe the current state of the art in clinically translatable metabolic imaging modalities. Specifically, we focus on PET and MR spectroscopy, including conventional (1)H- and (13)C-MR spectroscopy at thermal equilibrium and hyperpolarized MRI. In this article, we first provide an overview of metabolic pathways that are altered in many pathologic conditions and the corresponding probes and techniques used to study those alterations. We then describe the application of metabolic imaging to several common diseases, including cancer, neurodegeneration, cardiac ischemia, and infection or inflammation.

Lone Atrial Fibrillation Is Associated With Impaired Left Ventricular Energetics That Persists Despite Successful Catheter Ablation

Supplemental Digital Content is available in the text.

Lone atrial fibrillation (AF) may reflect a subclinical cardiomyopathy that persists after sinus rhythm (SR) restoration, providing a substrate for AF recurrence. To test this hypothesis, we investigated the effect of restoring SR by catheter ablation on left ventricular (LV) function and energetics in patients with AF but no significant comorbidities.

Energy Deregulation Precedes Alteration in Heart Energy Balance in Young Spontaneously Hypertensive Rats: A Non Invasive In Vivo31P-MR Spectroscopy Follow-Up Study

Introduction

Gradual alterations in cardiac energy balance, as assessed by the myocardial PCr/ATP-ratio, are frequently associated with the development of cardiac disease. Despite great interest for the follow-up of myocardial PCr and ATP content, cardiac MR-spectroscopy in rat models in vivo is challenged by sensitivity issues and cross-contamination from other organs.

Methods

Here we combined MR-Imaging and MR-Spectroscopy (Bruker BioSpec 9.4T) to follow-up for the first time in vivo the cardiac energy balance in the SHR, a genetic rat model of cardiac hypertrophy known to develop early disturbances in cytosolic calcium dynamics.

Results

We obtained consistent ³¹P-spectra with high signal/noise ratio from the left ventricle in vivo by using a double-tuned (³¹P/¹H) surface coil. Reasonable acquisition time (<3.2min) allowed assessing the PCr/ATP-ratio comparatively in SHR and age-matched control rats (WKY): i) weekly from 12 to 21 weeks of age; ii) in response to a bolus injection of the ß-adrenoreceptor agonist isoproterenol at age 21 weeks.

Discussion

Along weeks, the cardiac PCr/ATP-ratio was highly reproducible, steady and similar (2.35±0.06) in SHR and WKY, in spite of detectable ventricular hypertrophy in SHR.

At the age 21 weeks, PCr/ATP dropped more markedly (-17.1%±0.8% vs. -3,5%±1.4%, P<0.001) after isoproterenol injection in SHR and recovered slowly thereafter (time constant 21.2min vs. 6.6min, P<0.05) despite similar profiles of tachycardia among rats.

Conclusion

The exacerbated PCr/ATP drop under ß-adrenergic stimulation indicates a defect in cardiac energy regulation possibly due to calcium-mediated abnormalities in the SHR heart. Of note, defects in energy regulation were present before detectable abnormalities in cardiac energy balance at rest.

Augmentation of Creatine in the Heart

Creatine is a principle component of the creatine kinase (CK) phosphagen system common to all vertebrates. It is found in excitable cells, such as cardiomyocytes, where it plays an important role in the buffering and transport of chemical energy to ensure that supply meets the dynamic demands of the heart. Multiple components of the CK system, including intracellular creatine levels, are reduced in heart failure, while ischaemia and hypoxia represent acute crises of energy provision. Elevation of myocardial creatine levels has therefore been suggested as potentially beneficial, however, achieving this goal is not trivial. This mini-review outlines the evidence in support of creatine elevation and critically examines the pharmacological approaches that are currently available. In particular, dietary creatine-supplementation does not sufficiently elevate creatine levels in the heart due to subsequent down-regulation of the plasma membrane creatine transporter (CrT). Attempts to increase passive diffusion and bypass the CrT, e.g. via creatine esters, have yet to be tested in the heart. However, studies in mice with genetic overexpression of the CrT demonstrate proof-of-principle that elevated creatine protects the heart from ischaemia-reperfusion injury. This suggests activation of the CrT as a major unmet pharmacological target. However, translation of this finding to the clinic will require a greater understanding of CrT regulation in health and disease and the development of small molecule activators.