Impairment of energy metabolism in intact residual myocardium of rat hearts with chronic myocardial infarction

Hyperpolarized 13C and 31P MRS detects differences in cardiac energetics, metabolism, and function in obesity, and responses following treatment

Obesity is associated with important changes in cardiac energetics and function, and an increased risk of adverse cardiovascular outcomes. Multi-nuclear MRS and MRI techniques have the potential to provide a comprehensive non-invasive assessment of cardiac metabolic perturbation in obesity. A rat model of obesity was created by high-fat diet feeding. This model was characterized using in vivo hyperpolarized [1-13C]pyruvate and [2-13C]pyruvate MRS, echocardiography and perfused heart 31P MRS. Two groups of obese rats were subsequently treated with either caloric restriction or the glucagon-like peptide-1 analogue/agonist liraglutide, prior to reassessment. The model recapitulated cardiovascular consequences of human obesity, including mild left ventricular hypertrophy, and diastolic, but not systolic, dysfunction. Hyperpolarized 13C and 31P MRS demonstrated that obesity was associated with reduced myocardial pyruvate dehydrogenase flux, altered cardiac tricarboxylic acid (TCA) cycle metabolism, and impaired myocardial energetic status (lower phosphocreatine to adenosine triphosphate ratio and impaired cardiac ΔG~ATP). Both caloric restriction and liraglutide treatment were associated with normalization of metabolic changes, alongside improvement in cardiac diastolic function. In this model of obesity, hyperpolarized 13C and 31P MRS demonstrated abnormalities in cardiac metabolism at multiple levels, including myocardial substrate selection, TCA cycle, and high-energy phosphorus metabolism. Metabolic changes were linked with impairment of diastolic function and were reversed in concert following either caloric restriction or liraglutide treatment. With hyperpolarized 13C and 31P techniques now available for human use, the findings support a role for multi-nuclear MRS in the development of new therapies for obesity.

Cardiac 31P MR spectroscopy: development of the past five decades and future vision-will it be of diagnostic use in clinics?

In the past five decades, the use of the magnetic resonance (MR) technique for cardiovascular diseases has engendered much attention and raised the opportunity that the technique could be useful for clinical applications. MR has two arrows in its quiver: One is magnetic resonance imaging (MRI), and the other is magnetic resonance spectroscopy (MRS). Non-invasively, highly advanced MRI provides unique and profound information about the anatomical changes of the heart. Excellently developed MRS provides irreplaceable and insightful evidence of the real-time biochemistry of cardiac metabolism of underpinning diseases. Compared to MRI, which has already been successfully applied in routine clinical practice, MRS still has a long way to travel to be incorporated into routine diagnostics. Considering the exceptional potential of ³¹P MRS to measure the real-time metabolic changes of energetic molecules qualitatively and quantitatively, how far its powerful technique should be waited before a successful transition from "bench-to-bedside" is enticing. The present review highlights the seminal studies on the chronological development of cardiac ³¹P MRS in the past five decades and the future vision and challenges to incorporating it for routine diagnostics of cardiovascular disease.

Synergistic effect on cardiac energetics by targeting the creatine kinase system: in vivo application of high-resolution 31P-CMRS in the mouse

BACKGROUND

Phosphorus cardiovascular magnetic resonance spectroscopy (31P-CMRS) has emerged as an important tool for the preclinical assessment of myocardial energetics in vivo. However, the high rate and diminutive size of the mouse heart is a challenge, resulting in low resolution and poor signal-to-noise. Here we describe a refined high-resolution 31P-CMRS technique and apply it to a novel double transgenic mouse (dTg) with elevated myocardial creatine and creatine kinase (CK) activity. We hypothesised a synergistic effect to augment energetic status, evidenced by an increase in the ratio of phosphocreatine-to-adenosine-triphosphate (PCr/ATP).

METHODS AND RESULTS

Single transgenic Creatine Transporter overexpressing (CrT-OE, n = 7) and dTg mice (CrT-OE and CK, n = 6) mice were anaesthetised with isoflurane to acquire 31P-CMRS measurements of the left ventricle (LV) utilising a two-dimensional (2D), threefold under-sampled density-weighted chemical shift imaging (2D-CSI) sequence, which provided high-resolution data with nominal voxel size of 8.5 µl within 70 min. (1H-) cine-CMR data for cardiac function assessment were obtained in the same imaging session. Under a separate examination, mice received invasive haemodynamic assessment, after which tissue was collected for biochemical analysis. Myocardial creatine levels were elevated in all mouse hearts, but only dTg exhibited significantly elevated CK activity, resulting in a 51% higher PCr/ATP ratio in heart (3.01 ± 0.96 vs. 2.04 ± 0.57-mean ± SD; dTg vs. CrT-OE), that was absent from adjacent skeletal muscle. No significant differences were observed for any parameters of LV structure and function, confirming that augmentation of CK activity does not have unforeseen consequences for the heart.

CONCLUSIONS

We have developed an improved 31P-CMRS methodology for the in vivo assessment of energetics in the murine heart which enabled high-resolution imaging within acceptable scan times. Mice over-expressing both creatine and CK in the heart exhibited a synergistic elevation in PCr/ATP that can now be tested for therapeutic potential in models of chronic heart failure.

Impact of circadian time of dosing on cardiomyocyte-autonomous effects of glucocorticoids

OBJECTIVE

Mitochondrial capacity is critical to adapt the high energy demand of the heart to circadian oscillations and diseased states. Glucocorticoids regulate the circadian cycle of energy metabolism, but little is known about how circadian timing of exogenous glucocorticoid dosing directly regulates heart metabolism through cardiomyocyte-autonomous mechanisms. While chronic once-daily intake of glucocorticoids promotes metabolic stress and heart failure, we recently discovered that intermittent once-weekly dosing of exogenous glucocorticoids promoted muscle metabolism in normal and obese skeletal muscle. However, the effects of glucocorticoid intermittence on heart metabolism and heart failure remain unknown. Here we investigated the extent to which circadian time of dosing regulates the effects of the glucocorticoid prednisone in heart metabolism and function in conditions of single pulse or chronic intermittent dosing.

METHODS AND RESULTS

In WT mice, we found that prednisone improved cardiac content of NAD+ and ATP with light-phase dosing (ZT0), while the effects were blocked by dark-phase dosing (ZT12). The drug effects on mitochondrial function were cardiomyocyte-autonomous, as shown by inducible cardiomyocyte-restricted glucocorticoid receptor (GR) ablation, and depended on an intact cardiomyocyte clock, as shown by inducible cardiomyocyte-restricted ablation of Brain and Muscle ARNT-like 1 (BMAL1). Conjugating time-of-dosing with chronic intermittence, we found that once-weekly prednisone improved metabolism and function in heart after myocardial injury dependent on circadian time of intake, i.e. with light-phase but not dark-phase dosing.

CONCLUSIONS

Our study identifies cardiac-autonomous mechanisms through which circadian-specific intermittent dosing reconverts glucocorticoid drugs to metabolic boosters for the heart.

TakoTsubo Syndrome: First an Acute Coronary Vasculitis and Then Prolonged Myocarditis?

Since its initial description by Japanese investigators 30 years ago, TakoTsubo Syndrome (TTS) has variously been regarded as a form of acute coronary syndrome and also as a form of cardiomyopathy (or more accurately, a myocarditis). There is actually good evidence that TTS embodies both of these concepts, and the main purpose of this review is to present data that they occur sequentially. The initial phase of the disorder (over perhaps the first 48 hours post onset of symptoms) represents a form of vasculitis, with associated damage to the endothelial glycocalyx and associated permeabilization of blood vessels. This is followed by a more prolonged phase of myocardial inflammation and oedema, associated with inflammatory activation and energetic impairment within the entire myocardium. Although this phase subsides after several months, it may be followed by longstanding impairment of myocardial function, reflecting residual fibrosis. Understanding of this gradual transition in TTS pathogenesis from vasculature towards myocardium remains an important limitation of patient management, especially as many patients are still told that their hearts have "recovered" within 1-2 weeks. A number of important uncertainties remain. These include development of specific early and ongoing therapeutic strategies to be used to match the sequential pathogenesis of TTS. "And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong!" From: Six wise men of Hindustan.

LncRNA LncHrt preserves cardiac metabolic homeostasis and heart function by modulating the LKB1-AMPK signaling pathway

Metabolic modulation is a promising therapeutic approach to prevent adverse remodeling of the ischemic heart. Because little is known about the involvement of long non-coding RNAs (lncRNAs) in regulating cardiac metabolism, we used unbiased transcriptome profiling in a mouse model of myocardial infarction (MI). We identified a novel cardiomyocyte-enriched lncRNA, called LncHrt, which regulates metabolism and the pathophysiological processes that lead to heart failure. AAV-based LncHrt overexpression protects the heart from MI as demonstrated by improved contractile function, preserved metabolic homeostasis, and attenuated maladaptive remodeling responses. RNA-pull down followed by mass spectrometry and RNA immunoprecipitation (RIP) identified SIRT2 as a LncHrt-interacting protein involved in cardiac metabolic regulation. Mechanistically, we established that LncHrt interacts with SIRT2 to preserve SIRT2 deacetylase activity by interfering with the CDK5 and SIRT2 interaction. This increases downstream LKB1-AMPK kinase signaling, which ameliorates functional and metabolic deficits. Importantly, we found the expression of the human homolog of mouse LncHrt was decreased in patients with dilated cardiomyopathy. Together, these studies identify LncHrt as a cardiac metabolic regulator that plays an essential role in preserving heart function by regulating downstream metabolic signaling pathways. Consequently, LncHrt is a potentially novel RNA-based therapeutic target for ischemic heart disease.

Subtle Role for Adenylate Kinase 1 in Maintaining Normal Basal Contractile Function and Metabolism in the Murine Heart

Aims

Adenylate kinase 1 (AK1) catalyses the reaction 2ADP ↔ ATP + AMP, extracting extra energy under metabolic stress and promoting energetic homeostasis. We hypothesised that increased AK1 activity would have negligible effects at rest, but protect against ischaemia/reperfusion (I/R) injury.

Methods and Results

Cardiac-specific AK1 overexpressing mice (AK1-OE) had 31% higher AK1 activity (P = 0.009), with unchanged total creatine kinase and citrate synthase activities. Male AK1-OE exhibited mild in vivo dysfunction at baseline with lower LV pressure, impaired relaxation, and contractile reserve. LV weight was 19% higher in AK1-OE males due to higher tissue water content in the absence of hypertrophy or fibrosis. AK1-OE hearts had significantly raised creatine, unaltered total adenine nucleotides, and 20% higher AMP levels (P = 0.05), but AMP-activated protein kinase was not activated (P = 0.85). ¹H-NMR revealed significant differences in LV metabolite levels compared to wild-type, with aspartate, tyrosine, sphingomyelin, cholesterol all elevated, whereas taurine and triglycerides were significantly lower. Ex vivo global no-flow I/R, caused four-of-seven AK1-OE hearts to develop terminal arrhythmia (cf. zero WT), yet surviving AK1-OE hearts had improved functional recovery. However, AK1-OE did not influence infarct size in vivo and arrhythmias were only observed ex vivo, probably as an artefact of adenine nucleotide loss during cannulation.

Conclusion

Modest elevation of AK1 may improve functional recovery following I/R, but has unexpected impact on LV weight, function and metabolite levels under basal resting conditions, suggesting a more nuanced role for AK1 underpinning myocardial energy homeostasis and not just as a response to stress.

Current Knowledge and Future Challenges in Takotsubo Syndrome: Part 1-Pathophysiology and Diagnosis

First recognized in 1990, takotsubo syndrome (TTS) constitutes an acute cardiac condition that mimics acute myocardial infarction commonly in the absence of obstructive coronary artery disease; it is characterized by temporary left ventricular dysfunction, regularly in a circumferential apical, midventricular, or basal distribution. Considering its acute clinical presentation, coronary angiography with left ventriculography constitutes the gold standard diagnostic tool to exclude or confirm TTS. Frequently, TTS is related to severe emotional or physical stress and a subsequent increased adrenergic stimulation affecting cardiac function. Beyond clinical presentation, epidemiology, and novel diagnostic biomarkers, this review draws attention to potential pathophysiological mechanisms for the observed reversible myocardial dysfunction such as sympathetic overdrive-mediated multi-vessel epicardial spasms, microvascular dysfunction, the direct toxicity of catecholamines, lipotoxicity, and inflammation. Considering the long-term prognosis, further experimental and clinical research is indispensable to elucidate further pathophysiological mechanisms underlying TTS before randomized control trials with evidence-based therapeutic management can be performed.

Mechanisms underlying the pathophysiology of heart failure with preserved ejection fraction: the tip of the iceberg

Heart failure with preserved ejection fraction (HFpEF) is a multifaceted syndrome with a complex aetiology often associated with several comorbidities, such as left ventricle pressure overload, diabetes mellitus, obesity, and kidney disease. Its pathophysiology remains obscure mainly due to the complex phenotype induced by all these associated comorbidities and to the scarcity of animal models that adequately mimic HFpEF. Increased oxidative stress, inflammation, and endothelial dysfunction are currently accepted as key players in HFpEF pathophysiology. However, we have just started to unveil HFpEF complexity and the role of calcium handling, energetic metabolism, and mitochondrial function remain to clarify. Indeed, the enlightenment of such cellular and molecular mechanisms represents an opportunity to develop novel therapeutic approaches and thus to improve HFpEF treatment options. In the last decades, the number of research groups dedicated to studying HFpEF has increased, denoting the importance and the magnitude achieved by this syndrome. In the current technological and web world, the amount of information is overwhelming, driving us not only to compile the most relevant information about the theme but also to explore beyond the tip of the iceberg. Thus, this review aims to encompass the most recent knowledge related to HFpEF or HFpEF-associated comorbidities, focusing mainly on myocardial metabolism, oxidative stress, and energetic pathways.

Understanding How Phosphorylation and Redox Modifications Regulate Cardiac Ryanodine Receptor Type 2 Activity to Produce an Arrhythmogenic Phenotype in Advanced Heart Failure

Heart failure (HF) is a global pandemic with significant mortality and morbidity. Despite current medications, 50% of individuals die within 5 years of diagnosis. Of these deaths, 30-50% will be a result of sudden cardiac death from ventricular arrhythmias. This review discusses two stress-induced mechanisms, phosphorylation from chronic β-adrenoceptor (β-AR) stimulation and thiol modifications from oxidative stress, and how they modulate the cardiac ryanodine receptor type 2 (RyR2) and foster an arrhythmogenic phenotype. Calcium (Ca²⁺) is the ubiquitous secondary messenger of excitation-contraction coupling and provides a common pathway for contractile dysfunction and arrhythmia genesis. In a healthy heart, Ca²⁺ is released from the sarcoplasmic reticulum (SR) by RyR2. The open probability of RyR2 is under the dynamic influence of co-proteins, ions, and kinases that are in strict balance to ensure normal physiological functioning. In HF, chronic β-AR activity and production of reactive oxygen species and reactive nitrogen species provide two stress-induced mechanisms uncoupling RyR2 control, resulting in pathological diastolic SR Ca²⁺ leak. This increased cytosolic [Ca²⁺] promotes Ca²⁺ extrusion via the local Na⁺/Ca²⁺ exchanger, resulting in net sarcolemmal depolarization, delayed after depolarization and ventricular arrhythmia. Experimental models researching oxidative stress and phosphorylation have aimed to identify how post-translational modifications to the RyR2 macromolecular complex, and the associated Na⁺/Ca²⁺ cycling proteins, result in pathological Ca²⁺ handling and diastolic leak. However, the causative molecular changes remain controversial and undefined. Through understanding the molecular mechanisms that produce an arrhythmic phenotype, novel therapeutic targets to treat HF and prevent its malignant course can be identified.

Re-balancing cellular energy substrate metabolism to mend the failing heart

Fatty acids and glucose are the main substrates for myocardial energy provision. Under physiologic conditions, there is a distinct and finely tuned balance between the utilization of these substrates. Using the non-ischemic heart as an example, we discuss that upon stress this substrate balance is upset resulting in an over-reliance on either fatty acids or glucose, and that chronic fuel shifts towards a single type of substrate appear to be linked with cardiac dysfunction. These observations suggest that interventions aimed at re-balancing a tilted substrate preference towards an appropriate mix of substrates may result in restoration of cardiac contractile performance. Examples of manipulating cellular substrate uptake as a means to re-balance fuel supply, being associated with mended cardiac function underscore this concept. We also address the molecular mechanisms underlying the apparent need for a fatty acid-glucose fuel balance. We propose that re-balancing cellular fuel supply, in particular with respect to fatty acids and glucose, may be an effective strategy to treat the failing heart.

Distinctive patterns of inflammation across the heart failure syndrome

Inflammation has long been known to play a role in heart failure (HF). Earlier studies demonstrated that inflammation contributes to the pathogenesis of HF with reduced ejection fraction (HFrEF), and the knowledge about molecules and cell types specifically involved in inflammatory events has been constantly increased ever since. However, conflicting results of several trials with anti-inflammatory treatments led to the conclusions that inflammation does participate in the progression of HFrEF, but more likely it is not the primary event. Conversely, it has been suggested that inflammation drives the development of HF with preserved ejection fraction (HFpEF). Recently the pharmacological blockade of interleukin-1 has been shown to prevent HF hospitalization and mortality in patients with prior myocardial infarction, lending renewed support to the hypothesis that inflammation is a promising therapeutic target in HF. Inflammation has also been proposed to underlie both HF and commonly associated conditions, such as chronic kidney disease or cancer. Within this last paradigm, an emergent role has been ascribed to clonal hematopoiesis of indeterminate potential. Here, we summarize the recent evidence about the role of inflammation in HF, highlighting the similarities and differences in HFrEF vs. HFpEF, and discuss the diagnostic and therapeutic opportunities raised by antinflammatory-based approaches.

Myocardial and Systemic Inflammation in Acute Stress-Induced (Takotsubo) Cardiomyopathy

BACKGROUND

Acute stress-induced (takotsubo) cardiomyopathy can result in a heart failure phenotype with a prognosis comparable with that of myocardial infarction. In this study, we hypothesized that inflammation is central to the pathophysiology and natural history of takotsubo cardiomyopathy.

METHODS

In a multicenter study, we prospectively recruited 55 patients with takotsubo cardiomyopathy and 51 age-, sex-, and comorbidity-matched control subjects. During the index event and at the 5-month follow-up, patients with takotsubo cardiomyopathy underwent multiparametric cardiac magnetic resonance imaging, including ultrasmall superparamagnetic particles of iron oxide (USPIO) enhancement for detection of inflammatory macrophages in the myocardium. Blood monocyte subpopulations and serum cytokines were assessed as measures of systemic inflammation. Matched control subjects underwent investigation at a single time point.

RESULTS

Subjects were predominantly middle-aged (64±14 years) women (90%). Compared with control subjects, patients with takotsubo cardiomyopathy had greater USPIO enhancement (expressed as the difference between pre-USPIO and post-USPIO T2*) in both ballooning (14.3±0.6 milliseconds versus 10.5±0.9 milliseconds; P<0.001) and nonballooning (12.9±0.6 milliseconds versus 10.5±0.9 milliseconds; P=0.02) left ventricular myocardial segments. Serum interleukin-6 (23.1±4.5 pg/mL versus 6.5±5.8 pg/mL; P<0.001) and chemokine (C-X-C motif) ligand 1 (1903±168 pg/mL versus 1272±177 pg/mL; P=0.01) concentrations and classic CD14⁺⁺CD16^- monocytes (90±0.5% versus 87±0.9%; P=0.01) were also increased whereas intermediate CD14⁺⁺CD16⁺ (5.4±0.3% versus 6.9±0.6%; P=0.01) and nonclassic CD14⁺CD16⁺⁺ (2.7±0.3% versus 4.2±0.5%; P=0.006) monocytes were reduced in patients with takotsubo cardiomyopathy. At 5 months, USPIO enhancement was no longer detectable in the left ventricular myocardium, although persistent elevations in serum interleukin-6 concentrations ( P=0.009) and reductions in intermediate CD14⁺⁺CD16⁺ monocytes (5.6±0.4% versus 6.9±0.6%; P=0.01) remained.

CONCLUSIONS

We demonstrate for the first time that takotsubo cardiomyopathy is characterized by a myocardial macrophage inflammatory infiltrate, changes in the distribution of monocyte subsets, and an increase in systemic proinflammatory cytokines. Many of these changes persisted for at least 5 months, suggesting a low-grade chronic inflammatory state.

CLINICAL TRIAL REGISTRATION

URL: https://www.clinicaltrials.gov . Unique identifier: NCT02897739.

Native T1 time of right ventricular insertion points by cardiac magnetic resonance: relation with invasive haemodynamics and outcome in heart failure with preserved ejection fraction

Aims

Increased afterload to the right ventricle (RV) has been shown to induce myocardial fibrosis at the RV insertion points (RVIPs). Such changes can be discrete but potentially detected by cardiac magnetic resonance (CMR) T1-mapping. Whether RVIP fibrosis is associated with prognosis in heart failure with preserved ejection fraction (HFpEF) is unknown.

Methods and results

We prospectively investigated 167 consecutive HFpEF patients, a population frequently suffering from post-capillary pulmonary hypertension, who underwent CMR including T1-mapping. About 92.8% also underwent right heart catheterization for haemodynamic assessment.

Native T1 times were 995 ± 73 ms at the anterior and 1040 ± 90 ms at the inferior RVIP. By Spearman’s rank order testing, RVIP T1 times were significantly correlated with pulmonary artery pressure (mean PAP, r = 0.313 and 0.311 for anterior and inferior RVIP), pulmonary artery wedge pressure (r = 0.301 and 0.251) and right atrial pressure (r = 0.245 and 0.185; P for all &lt;0.05). During a mean follow-up of 43.2 ± 22.6 months, 30 (18.0%) subjects died. By multivariable Cox regression, NTproBNP [Hazard ratio (HR) 2.105, 95% confidence interval (CI) 1.332–3.328; P = 0.001], systolic PAP (HR 1.618, 95% CI 1.175–2.230; P = 0.003), and native T1 time of the anterior RVIP (HR 1.659, 95% CI 1.125–2.445; P = 0.011) were significantly associated with outcome. Also, by Kaplan–Meier analysis, T1 times at the anterior RVIPs had a significant effect on survival (log-rank, P = 0.002).

Conclusion

Interstitial expansion of the anterior RVIP as detected by CMR T1-mapping reflects haemodynamic alterations, and is independently related with prognosis in HFpEF.

Energetic drain driving hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most common form of hereditary cardiomyopathy and is mainly caused by mutations of genes encoding cardiac sarcomeric proteins. HCM is characterized by hypertrophy of the left ventricle, frequently involving the septum, that is not explained solely by loading conditions. HCM has a heterogeneous clinical profile, but diastolic dysfunction and ventricular arrhythmias represent two dominant features of the disease. Preclinical evidence indicates that the enhanced Calcium (Ca²⁺ ) sensitivity of the myofilaments plays a key role in the pathophysiology of HCM. Notably, this is not always a direct consequence of sarcomeric mutations, but can also result from secondary mutation-driven alterations. Here, we review experimental and clinical evidence indicating that increased myofilament Ca²⁺ sensitivity lies upstream of numerous cellular derangements which potentially contribute to the progression of HCM toward heart failure and sudden cardiac death.

Decreased ATP production and myocardial contractile reserve in metabolic heart disease

Metabolic syndrome is a cluster of obesity-related metabolic abnormalities that lead to metabolic heart disease (MHD) with left ventricular pump dysfunction. Although MHD is thought to be associated with myocardial energetic deficiency, two key questions have not been answered. First, it is not known whether there is a sufficient energy deficit to contribute to pump dysfunction. Second, the basis for the energy deficit is not clear. To address these questions, mice were fed a high fat, high sucrose (HFHS) 'Western' diet to recapitulate the MHD phenotype. In isolated beating hearts, we used 31P NMR spectroscopy with magnetization transfer to determine a) the concentrations of high energy phosphates ([ATP], [ADP], [PCr]), b) the free energy of ATP hydrolysis (∆G~ATP), c) the rate of ATP production and d) flux through the creatine kinase (CK) reaction. At the lowest workload, the diastolic pressure-volume relationship was shifted upward in HFHS hearts, indicative of diastolic dysfunction, whereas systolic function was preserved. At this workload, the rate of ATP synthesis was decreased in HFHS hearts, and was associated with decreases in both [PCr] and ∆G~ATP. Higher work demands unmasked the inability of HFHS hearts to increase systolic function and led to a further decrease in ∆G~ATP to a level that is not sufficient to maintain normal function of sarcoplasmic Ca2+-ATPase (SERCA). While [ATP] was preserved at all work demands in HFHS hearts, the progressive increase in [ADP] led to a decrease in ∆G~ATP with increased work demands. Surprisingly, CK flux, CK activity and total creatine were normal in HFHS hearts. These findings differ from dilated cardiomyopathy, in which the energetic deficiency is associated with decreases in CK flux, CK activity and total creatine. Thus, in HFHS-fed mice with MHD there is a distinct metabolic phenotype of the heart characterized by a decrease in ATP production that leads to a functionally-important energetic deficiency and an elevation of [ADP], with preservation of CK flux.

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.

Persistent Long-Term Structural, Functional, and Metabolic Changes After Stress-Induced (Takotsubo) Cardiomyopathy

Supplemental Digital Content is available in the text.

Background:

Takotsubo cardiomyopathy is an increasingly recognized acute heart failure syndrome precipitated by intense emotional stress. Although there is an apparent rapid and spontaneous recovery of left ventricular ejection fraction, the long-term clinical and functional consequences of takotsubo cardiomyopathy are ill-defined.

Methods:

In an observational case-control study, we recruited 37 patients with prior (>12-month) takotsubo cardiomyopathy, and 37 age-, sex-, and comorbidity-matched control subjects. Patients completed the Minnesota Living with Heart Failure Questionnaire. All participants underwent detailed clinical phenotypic characterization, including serum biomarker analysis, cardiopulmonary exercise testing, echocardiography, and cardiac magnetic resonance including cardiac ³¹P-spectroscopy.

Results:

Participants were predominantly middle-age (64±11 years) women (97%). Although takotsubo cardiomyopathy occurred 20 (range 13–39) months before the study, the majority (88%) of patients had persisting symptoms compatible with heart failure (median of 13 [range 0–76] in the Minnesota Living with Heart Failure Questionnaire) and cardiac limitation on exercise testing (reduced peak oxygen consumption, 24±1.3 versus 31±1.3 mL/kg/min, P<0.001; increased VE/Vco₂ slope, 31±1 versus 26±1, P=0.002). Despite normal left ventricular ejection fraction and serum biomarkers, patients with prior takotsubo cardiomyopathy had impaired cardiac deformation indices (reduced apical circumferential strain, −16±1.0 versus −23±1.5%, P<0.001; global longitudinal strain, −17±1 versus −20±1%, P=0.006), increased native T1 mapping values (1264±10 versus 1184±10 ms, P<0.001), and impaired cardiac energetic status (phosphocreatine/γ-adenosine triphosphate ratio, 1.3±0.1 versus 1.9±0.1, P<0.001).

Conclusions:

In contrast to previous perceptions, takotsubo cardiomyopathy has long-lasting clinical consequences, including demonstrable symptomatic and functional impairment associated with persistent subclinical cardiac dysfunction. Taken together our findings demonstrate that after takotsubo cardiomyopathy, patients develop a persistent, long-term heart failure phenotype.

Clinical Trial Registration:

URL: https://clinicaltrials.gov. Unique identifier: NCT02989454.

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.

[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.

Evaluation of in vivo mitochondrial bioenergetics in skeletal muscle using NMR and optical methods

It is now clear that mitochondria are involved as either a cause or consequence of many chronic diseases. This central role of the mitochondria is due to their position in the cell as important integrators of cellular energetics and signaling. Mitochondrial function affects many aspects of the cellular environment such as redox homeostasis and calcium signaling, which then also exert control over mitochondrial function. This complex dynamic between mitochondrial function and the cellular environment highlights the value of examining mitochondria in vivo in the intact physiological environment. This review discusses NMR and optical approaches used to measure mitochondria ATP and oxygen fluxes that provide in vivo measures of mitochondrial capacity and quality in animal and human models. Combining these in vivo measurements with more traditional ex vivo analyses can lead to new insights into the importance of the cellular environment in controlling mitochondrial function under pathological conditions. Interpretation and underlying assumptions for each technique are discussed with the goal of providing an overview of some of the most common approaches used to measure in vivo mitochondrial function encountered in the literature.

Protective Effect of Creatine Elevation against Ischaemia Reperfusion Injury Is Retained in the Presence of Co-Morbidities and during Cardioplegia

Aims

Ischaemic heart disease is most prevalent in the ageing population and often exists with other comorbidities; however the majority of laboratory research uses young, healthy animal models. Several recent workshops and focus meetings have highlighted the importance of using clinically relevant models to help aid translation to realistic patient populations. We have previously shown that mice over-expressing the creatine transporter (CrT-OE) have elevated intracellular creatine levels and are protected against ischaemia-reperfusion injury. Here we test whether elevating intracellular creatine levels retains a cardioprotective effect in the presence of common comorbidities and whether it is additive to protection afforded by hypothermic cardioplegia.

Methods and Results

CrT-OE mice and wild-type controls were subjected to transverse aortic constriction for two weeks to induce compensated left ventricular hypertrophy (LVH). Hearts were retrogradely perfused in Langendorff mode for 15 minutes, followed by 20 minutes ischaemia and 30 minutes reperfusion. CrT-OE hearts exhibited significantly improved functional recovery (Rate pressure product) during reperfusion compared to WT littermates (76% of baseline vs. 59%, respectively, P = 0.02). Aged CrT-OE mouse hearts (78±5 weeks) also had enhanced recovery following 15 minutes ischaemia (104% of baseline vs. 67%, P = 0.0007). The cardioprotective effect of hypothermic high K⁺ cardioplegic arrest, as used during cardiac surgery and donor heart transplant, was further enhanced in prolonged ischaemia (90 minutes) in CrT-OE Langendorff perfused mouse hearts (76% of baseline vs. 55% of baseline as seen in WT hearts, P = 0.02).

Conclusions

These observations in clinically relevant models further support the development of modulators of intracellular creatine content as a translatable strategy for cardiac protection against ischaemia-reperfusion injury.

Synergistic role of ADP and Ca(2+) in diastolic myocardial stiffness

Diastolic dysfunction in heart failure patients is evident from stiffening of the passive properties of the ventricular wall. Increased actomyosin interactions may significantly limit diastolic capacity, however, direct evidence is absent. From experiments at the cellular and whole organ level, in humans and rats, we show that actomyosin-related force development contributes significantly to high diastolic stiffness in environments where high ADP and increased diastolic [Ca(2+) ] are present, such as the failing myocardium. Our basal study provides a mechanical mechanism which may partly underlie diastolic dysfunction. Heart failure (HF) with diastolic dysfunction has been attributed to increased myocardial stiffness that limits proper filling of the ventricle. Altered cross-bridge interaction may significantly contribute to high diastolic stiffness, but this has not been shown thus far. Cross-bridge interactions are dependent on cytosolic [Ca(2+) ] and the regeneration of ATP from ADP. Depletion of myocardial energy reserve is a hallmark of HF leading to ADP accumulation and disturbed Ca(2+) handling. Here, we investigated if ADP elevation in concert with increased diastolic [Ca(2+) ] promotes diastolic cross-bridge formation and force generation and thereby increases diastolic stiffness. ADP dose-dependently increased force production in the absence of Ca(2+) in membrane-permeabilized cardiomyocytes from human hearts. Moreover, physiological levels of ADP increased actomyosin force generation in the presence of Ca(2+) both in human and rat membrane-permeabilized cardiomyocytes. Diastolic stress measured at physiological lattice spacing and 37°C in the presence of pathological levels of ADP and diastolic [Ca(2+) ] revealed a 76 ± 1% contribution of cross-bridge interaction to total diastolic stress in rat membrane-permeabilized cardiomyocytes. Inhibition of creatine kinase (CK), which increases cytosolic ADP, in enzyme-isolated intact rat cardiomyocytes impaired diastolic re-lengthening associated with diastolic Ca(2+) overload. In isolated Langendorff-perfused rat hearts, CK inhibition increased ventricular stiffness only in the presence of diastolic [Ca(2+) ]. We propose that elevations of intracellular ADP in specific types of cardiac disease, including those where myocardial energy reserve is limited, contribute to diastolic dysfunction by recruiting cross-bridges, even at low Ca(2+) , and thereby increase myocardial stiffness.

Impaired in vivo mitochondrial Krebs cycle activity after myocardial infarction assessed using hyperpolarized magnetic resonance spectroscopy

BACKGROUND

Myocardial infarction (MI) is one of the leading causes of heart failure. An increasing body of evidence links alterations in cardiac metabolism and mitochondrial function with the progression of heart disease. The aim of this work was to, therefore, follow the in vivo mitochondrial metabolic alterations caused by MI, thereby allowing a greater understanding of the interplay between metabolic and functional abnormalities.

METHODS AND RESULTS

Using hyperpolarized carbon-13 ((13)C)-magnetic resonance spectroscopy, in vivo alterations in mitochondrial metabolism were assessed for 22 weeks after surgically induced MI with reperfusion in female Wister rats. One week after MI, there were no detectable alterations in in vivo cardiac mitochondrial metabolism over the range of ejection fractions observed (from 28% to 84%). At 6 weeks after MI, in vivo mitochondrial Krebs cycle activity was impaired, with decreased (13)C-label flux into citrate, glutamate, and acetylcarnitine, which correlated with the degree of cardiac dysfunction. These changes were independent of alterations in pyruvate dehydrogenase flux. By 22 weeks, alterations were also seen in pyruvate dehydrogenase flux, which decreased at lower ejection fractions. These results were confirmed using in vitro analysis of enzyme activities and metabolomic profiles of key intermediates.

CONCLUSIONS

The in vivo decrease in Krebs cycle activity in the 6-week post-MI heart may represent an early maladaptive phase in the metabolic alterations after MI in which reductions in Krebs cycle activity precede a reduction in pyruvate dehydrogenase flux. Changes in mitochondrial metabolism in heart disease are progressive and proportional to the degree of cardiac impairment.

Effect of Chronic Heart Rate Reduction by If Current Inhibitor Ivabradine on Left Ventricular Remodeling and Systolic Performance in Middle-Aged Rats With Postmyocardial Infarction Heart Failure

Background:

A large myocardial infarction (MI) initiates progressive cardiac remodeling that leads to systolic heart failure (HF). Long-term heart rate reduction (HRR) induced by the I f current inhibitor ivabradine (IVA) ameliorates left ventricular (LV) remodeling and improves systolic performance in young post-MI rats. However, the beneficial effects of chronic IVA treatment in middle-aged rats remain to be determined.

Methods:

A large MI was induced in 12-month-old rats by left coronary artery ligation. Rats were treated with IVA via osmotic pumps intraperitoneal in a dose of 10.5 mg/kg/d (MI + IVA) and compared with MI and sham-operated animals 12 weeks after MI.

Results:

Heart rate in MI + IVA rats was on average 29% lower than that of rats in the MI group. Left ventricular remodeling was comparable between post-MI groups, although MI + IVA rats did not show the compensatory thickening of the noninfarcted myocardium. Chronic HRR had no effect on transverse cardiac myocyte size and capillary growth, but it reduced the collagen content in noninfarcted myocardium. Left ventricular systolic performance remained similarly impaired in MI and MI + IVA rats. Moreover, abrupt IVA withdrawal led to worsening HF and reduction of coronary reserve.

Conclusion:

Our data reveal that chronic IVA-induced HRR does not provide sustainable benefits for LV systolic performance in middle-aged rats with post-MI HF.

Experimental model of transthoracic, vascular-targeted, photodynamically induced myocardial infarction

We describe a novel model of myocardial infarction (MI) in rats induced by percutaneous transthoracic low-energy laser-targeted photodynamic irradiation. The procedure does not require thoracotomy and represents a minimally invasive alternative to existing surgical models. Target cardiac area to be photodynamically irradiated was triangulated from the thoracic X-ray scans. The acute phase of MI was histopathologically characterized by the presence of extensive vascular occlusion, hemorrhage, loss of transversal striations, neutrophilic infiltration, and necrotic changes of cardiomyocytes. Consequently, damaged myocardium was replaced with fibrovascular and granulation tissue. The fibrotic scar in the infarcted area was detected by computer tomography imaging. Cardiac troponin I (cTnI), a specific marker of myocardial injury, was significantly elevated at 6 h (41 ± 6 ng/ml, n = 4, P < 0.05 vs. baseline) and returned to baseline after 72 h. Triphenyltetrazolium chloride staining revealed transmural anterolateral infarcts targeting 25 ± 3% of the left ventricle at day 1 with a decrease to 20 ± 3% at day 40 (n = 6 for each group, P < 0.01 vs. day 1). Electrocardiography (ECG) showed significant ST-segment elevation in the acute phase with subsequent development of a pathological Q wave and premature ventricular contractions in the chronic phase of MI. Vectorcardiogram analysis of spatiotemporal electrical signal transduction revealed changes in inscription direction, QRS loop morphology, and redistribution in quadrant areas. The photodynamically induced MI in n = 51 rats was associated with 12% total mortality. Histological findings, ECG abnormalities, and elevated cTnI levels confirmed the photosensitizer-dependent induction of MI after laser irradiation. This novel rodent model of MI might provide a platform to evaluate new diagnostic or therapeutic interventions.

A role for thioredoxin-interacting protein (Txnip) in cellular creatine homeostasis

Creatine is important for energy metabolism, yet excitable cells such as cardiomyocytes do not synthesize creatine and rely on uptake via a specific membrane creatine transporter (CrT; SLC6A8). This process is tightly controlled with downregulation of CrT upon continued exposure to high creatine via mechanisms that are poorly understood. Our aim was to identify candidate endogenous CrT inhibitors. In 3T3 cells overexpressing the CrT, creatine uptake plateaued at 3 h in response to 5 mM creatine but peaked 33% higher (P < 0.01) in the presence of cycloheximide, suggesting CrT regulation depends on new protein synthesis. Global gene expression analysis identified thioredoxin-interacting protein (Txnip) as the only significantly upregulated gene (by 46%) under these conditions (P = 0.036), subsequently verified independently at mRNA and protein levels. There was no change in Txnip expression with exposure to 5 mM taurine, confirming a specific response to creatine rather than osmotic stress. Small-interfering RNA against Txnip prevented Txnip upregulation in response to high creatine, maintained normal levels of creatine uptake, and prevented downregulation of CrT mRNA. These findings were relevant to the in vivo heart since creatine-deficient mice showed 39.71% lower levels of Txnip mRNA, whereas mice overexpressing the CrT had 57.6% higher Txnip mRNA levels and 28.7% higher protein expression compared with wild types (mean myocardial creatine concentration 124 and 74 nmol/mg protein, respectively). In conclusion, we have identified Txnip as a novel negative regulator of creatine levels in vitro and in vivo, responsible for mediating substrate feedback inhibition and a potential target for modulating creatine homeostasis.

Ribose Supplementation Alone or with Elevated Creatine Does Not Preserve High Energy Nucleotides or Cardiac Function in the Failing Mouse Heart

BACKGROUND

Reduced levels of creatine and total adenine nucleotides (sum of ATP, ADP and AMP) are hallmarks of chronic heart failure and restoring these pools is predicted to be beneficial by maintaining the diseased heart in a more favourable energy state. Ribose supplementation is thought to support both salvage and re-synthesis of adenine nucleotides by bypassing the rate-limiting step. We therefore tested whether ribose would be beneficial in chronic heart failure in control mice and in mice with elevated myocardial creatine due to overexpression of the creatine transporter (CrT-OE).

METHODS AND RESULTS

FOUR GROUPS WERE STUDIED: sham; myocardial infarction (MI); MI+ribose; MI+CrT-OE+ribose. In a pilot study, ribose given in drinking water was bioavailable, resulting in a two-fold increase in myocardial ribose-5-phosphate levels. However, 8 weeks post-surgery, total adenine nucleotide (TAN) pool was decreased to a similar amount (8-14%) in all infarcted groups irrespective of the treatment received. All infarcted groups also presented with a similar and substantial degree of left ventricular (LV) dysfunction (3-fold reduction in ejection fraction) and LV hypertrophy (32-47% increased mass). Ejection fraction closely correlated with infarct size independently of treatment (r(2) = 0.63, p<0.0001), but did not correlate with myocardial creatine or TAN levels.

CONCLUSION

Elevating myocardial ribose and creatine levels failed to maintain TAN pool or improve post-infarction LV remodeling and function. This suggests that ribose is not rate-limiting for purine nucleotide biosynthesis in the chronically failing mouse heart and that alternative strategies to preserve TAN pool should be investigated.

Myocardial energetics in heart failure

It has become common sense that the failing heart is an "engine out of fuel". However, undisputable evidence that, indeed, the failing heart is limited by insufficient ATP supply is currently lacking. Over the last couple of years, an increasingly complex picture of mechanisms evolved that suggests that potentially metabolic intermediates and redox state could play the more dominant roles for signaling that eventually results in left ventricular remodeling and contractile dysfunction. In the pathophysiology of heart failure, mitochondria emerge in the crossfire of defective excitation-contraction coupling and increased energetic demand, which may provoke oxidative stress as an important upstream mediator of cardiac remodeling and cell death. Thus, future therapies may be guided towards restoring defective ion homeostasis and mitochondrial redox shifts rather than aiming solely at improving the generation of ATP.

Living without creatine: unchanged exercise capacity and response to chronic myocardial infarction in creatine-deficient mice

RATIONALE

Creatine is thought to be involved in the spatial and temporal buffering of ATP in energetic organs such as heart and skeletal muscle. Creatine depletion affects force generation during maximal stimulation, while reduced levels of myocardial creatine are a hallmark of the failing heart, leading to the widely held view that creatine is important at high workloads and under conditions of pathological stress.

OBJECTIVE

We therefore hypothesised that the consequences of creatine-deficiency in mice would be impaired running capacity, and exacerbation of heart failure following myocardial infarction.

METHODS AND RESULTS

Surprisingly, mice with whole-body creatine deficiency due to knockout of the biosynthetic enzyme (guanidinoacetate N-methyltransferase [GAMT]) voluntarily ran just as fast and as far as controls (>10 km/night) and performed the same level of work when tested to exhaustion on a treadmill. Furthermore, survival following myocardial infarction was not altered, nor was subsequent left ventricular (LV) remodelling and development of chronic heart failure exacerbated, as measured by 3D-echocardiography and invasive hemodynamics. These findings could not be accounted for by compensatory adaptations, with no differences detected between WT and GAMT(-/-) proteomes. Alternative phosphotransfer mechanisms were explored; adenylate kinase activity was unaltered, and although GAMT(-/-) hearts accumulated the creatine precursor guanidinoacetate, this had negligible energy-transfer activity, while mitochondria retained near normal function.

CONCLUSIONS

Creatine-deficient mice show unaltered maximal exercise capacity and response to chronic myocardial infarction, and no obvious metabolic adaptations. Our results question the paradigm that creatine is essential for high workload and chronic stress responses in heart and skeletal muscle.

Myocardial tissue characterization using magnetic resonance noncontrast t1 mapping in hypertrophic and dilated cardiomyopathy

BACKGROUND

Noncontrast magnetic resonance T1 mapping reflects a composite of both intra- and extracellular signal. We hypothesized that noncontrast T1 mapping can characterize the myocardium beyond that achieved by the well-established late gadolinium enhancement (LGE) technique (which detects focal fibrosis) in both hypertrophic (HCM) and dilated (DCM) cardiomyopathy, by detecting both diffuse and focal fibrosis.

METHODS AND RESULTS

Subjects underwent Cardiovascular Magnetic Resonance imaging at 3T (28 HCM, 18 DCM, and 12 normals). Matching short-axis slices were acquired for cine, T1 mapping, and LGE imaging (0.1 mmol/kg). Circumferential strain was measured in the midventricular slice, and (31)P magnetic resonance spectroscopy was acquired for the septum of the midventricular slice. Mean T1 relaxation time was increased in HCM and DCM (HCM 1209±28 ms, DCM 1225±42 ms, normal 1178±13 ms, P<0.05). There was a weak correlation between mean T1 and LGE (r=0.32, P<0.001). T1 values were higher in segments with LGE than in those without (HCM with LGE 1228±41 ms versus no LGE 1192±79 ms, P<0.01; DCM with LGE 1254±73 ms versus no LGE 1217±52 ms, P<0.01). However, in both HCM and DCM, even in segments unaffected by LGE, T1 values were significantly higher than normal (P<0.01). T1 values correlated with disease severity, being increased as wall thickness increased in HCM; conversely, in DCM, T1 values were highest in the thinnest myocardial segments. T1 values also correlated significantly with circumferential strain (r=0.42, P<0.01). Interestingly, this correlation remained statistically significant even for the slices without LGE (r=0.56, P=0.04). Finally, there was also a statistically significant negative correlation between T1 values and phosphocreatine/adenosine triphosphate ratios (r=-0.59, P<0.0001).

CONCLUSIONS

In HCM and DCM, noncontrast T1 mapping detects underlying disease processes beyond those assessed by LGE in relatively low-risk individuals.

Moderate elevation of intracellular creatine by targeting the creatine transporter protects mice from acute myocardial infarction

Aims

Increasing energy storage capacity by elevating creatine and phosphocreatine (PCr) levels to increase ATP availability is an attractive concept for protecting against ischaemia and heart failure. However, testing this hypothesis has not been possible since oral creatine supplementation is ineffectual at elevating myocardial creatine levels. We therefore used mice overexpressing creatine transporter in the heart (CrT-OE) to test for the first time whether elevated creatine is beneficial in clinically relevant disease models of heart failure and ischaemia/reperfusion (I/R) injury.

Methods and results

CrT-OE mice were selected for left ventricular (LV) creatine 20–100% above wild-type values and subjected to acute and chronic coronary artery ligation. Increasing myocardial creatine up to 100% was not detrimental even in ageing CrT-OE. In chronic heart failure, creatine elevation was neither beneficial nor detrimental, with no effect on survival, LV remodelling or dysfunction. However, CrT-OE hearts were protected against I/R injury in vivo in a dose-dependent manner (average 27% less myocardial necrosis) and exhibited greatly improved functional recovery following ex vivo I/R (59% of baseline vs. 29%). Mechanisms contributing to ischaemic protection in CrT-OE hearts include elevated PCr and glycogen levels and improved energy reserve. Furthermore, creatine loading in HL-1 cells did not alter antioxidant defences, but delayed mitochondrial permeability transition pore opening in response to oxidative stress, suggesting an additional mechanism to prevent reperfusion injury.

Conclusion

Elevation of myocardial creatine by 20–100% reduced myocardial stunning and I/R injury via pleiotropic mechanisms, suggesting CrT activation as a novel, potentially translatable target for cardiac protection from ischaemia.

Chronic creatine kinase deficiency eventually leads to congestive heart failure, but severity is dependent on genetic background, gender and age

The creatine kinase (CK) energy transport and buffering system supports cardiac function at times of high demand and is impaired in the failing heart. Mice deficient in muscle- and mitochondrial-CK (M/Mt-CK^−/−) have previously been described, but exhibit an unexpectedly mild phenotype of compensated left ventricular (LV) hypertrophy. We hypothesised that heart failure would develop with age and performed echocardiography and LV haemodynamics at 1 year. Since all previous studies have utilised mice with a mixed genetic background, we backcrossed for >10 generations on to C57BL/6, and repeated the in vivo investigations. Male M/Mt-CK^−/− mice on the mixed genetic background developed congestive heart failure as evidenced by significantly elevated end-diastolic pressure, impaired contractility, LV dilatation, hypertrophy and pulmonary congestion. Female mice were less severely affected, only showing trends for these parameters. After backcrossing, M/Mt-CK^−/− mice had LV dysfunction consisting of impaired isovolumetric pressure changes and reduced contractile reserve, but did not develop congestive heart failure. Body weight was lower in knockout mice as a consequence of reduced total body fat. LV weight was not significantly elevated in relation to other internal organs and gene expression of LVH markers was normal, suggesting an absence of hypertrophy. In conclusion, the consequences of CK deficiency are highly dependent on genetic modifiers, gender and age. However, the observation that a primary defect in CK can, under the right conditions, result in heart failure suggests that impaired CK activity in the failing heart could contribute to disease progression.

Exercise training improves age-related myocardial metabolic derangement: proton magnetic resonance spectroscopy study in the rat model

BACKGROUND AND OBJECTIVES

The objective of this study was to determine whether long-term exercise training will improve age-related cardiac metabolic derangement using proton magnetic resonance (MR) spectroscopy.

MATERIALS AND METHODS

Young and old male Fischer 344 rats were assigned to sedentary controls groups {young control (YC) group-3 months of age: YC, n=10; old control (OC) group-22 months of age: OC, n=10}, and an exercise training group (OT, n=5). After 12-week of treadmill exercise training, MR spectroscopy at 4.7 T was performed to assess myocardial energy metabolism: measurements of myocardial creatine-to-water ratio (Scr/Sw) were performed using the XWIN-NMR software.

RESULTS

Exercise capacity was 14.7 minutes greater in OT than that in OC (20.1±1.9 minutes in OT, 5.4±2.3 minutes in OC; p<0.001). The 12-week exercise training rendered the old rats a maximum exercise capacity matching that of untrained YC rats (17.9±1.5 minutes in YC, 20.1±1.9 minutes in OT; p>0.05). The creatine-to-water ratios in the interventricular septa of YC did not differ significantly from that of OT (0.00131±0.00025 vs. 0.00127±0.00031; p=0.37). However, OC showed significant reduction in creatine-to-water ratio compared to OT (0.00096±0.00025 vs. 0.00127±0.00031; p<0.001). Mean total creatine concentrations in the myocardium were similar between YC and OT (13.3±3.6 vs. 11.5±4.1 mmol/kg wet weight; p=0.29). In contrast, the mean total creatine concentration of OC was significantly reduced compared to OT (6.8±3.2 vs. 11.5±4.1 mmol/kg wet weight; p=0.03).

CONCLUSION

Our findings suggest that long-term exercise training in old rats induced prevention of age-related deterioration in myocardial metabolism.

Ongoing dual-angle measurements for the correction of partial saturation in 31P MR spectroscopy

Use of a repetition time similar to, or shorter than, metabolite T(1)s is common in NMR spectroscopy of biological samples to improve the signal-to-noise ratio. Conventionally, the partial saturation that results from this is corrected using saturation factors. However, this can lead to erroneous results in the presence of chemical exchange or nonconstant T(1)s. We describe an alternative approach to correction for saturation, based on ongoing dual-angle T(1) measurement. Using (31)P magnetic resonance spectroscopy of the perfused rat heart undergoing ischemia-reperfusion, we demonstrate that signal alternations in the data acquired by the dual-angle approach are eliminated by the ongoing dual-angle T(1) measurement correction scheme, meaning that metabolite concentration and T(1) measurement can be made throughout the course of the ischemia-reperfusion protocol. Simulations, based on parameters pertinent to the perfused rat heart, demonstrate that accurate saturation correction is possible with this method except at times of rapid concentration change. Additionally, compared to the conventional saturation factor correction method, the ongoing dual-angle T(1) measurement correction scheme results in improved accuracy in determining the [phosphocreatine] recovery time constant. Thus, the ongoing dual-angle T(1) measurements procedure permits accurate monitoring of metabolite concentrations even in the setting of chemical exchange and T(1) changes and allows more accurate analysis of bioenergetic status.

A mutation in the mitochondrial fission gene Dnm1l leads to cardiomyopathy

Mutations in a number of genes have been linked to inherited dilated cardiomyopathy (DCM). However, such mutations account for only a small proportion of the clinical cases emphasising the need for alternative discovery approaches to uncovering novel pathogenic mutations in hitherto unidentified pathways. Accordingly, as part of a large-scale N-ethyl-N-nitrosourea mutagenesis screen, we identified a mouse mutant, Python, which develops DCM. We demonstrate that the Python phenotype is attributable to a dominant fully penetrant mutation in the dynamin-1-like (Dnm1l) gene, which has been shown to be critical for mitochondrial fission. The C452F mutation is in a highly conserved region of the M domain of Dnm1l that alters protein interactions in a yeast two-hybrid system, suggesting that the mutation might alter intramolecular interactions within the Dnm1l monomer. Heterozygous Python fibroblasts exhibit abnormal mitochondria and peroxisomes. Homozygosity for the mutation results in the death of embryos midway though gestation. Heterozygous Python hearts show reduced levels of mitochondria enzyme complexes and suffer from cardiac ATP depletion. The resulting energy deficiency may contribute to cardiomyopathy. This is the first demonstration that a defect in a gene involved in mitochondrial remodelling can result in cardiomyopathy, showing that the function of this gene is needed for the maintenance of normal cellular function in a relatively tissue-specific manner. This disease model attests to the importance of mitochondrial remodelling in the heart; similar defects might underlie human heart muscle disease.

Changes in creatine transporter function during cardiac maturation in the rat

BACKGROUND

It is well established that the immature myocardium preferentially utilises non-oxidative energy-generating pathways. It exhibits low energy-transfer capacity via the creatine kinase (CK) shuttle, reflected in phosphocreatine (PCr), total creatine and CK levels that are much lower than those of adult myocardium. The mechanisms leading to gradually increasing energy transfer capacity during maturation are poorly understood. Creatine is not synthesised in the heart, but taken up exclusively by the action of the creatine transporter protein (CrT). To determine whether this transporter is ontogenically regulated, the present study serially examined CrT gene expression pattern, together with creatine uptake kinetics and resulting myocardial creatine levels, in rats over the first 80 days of age.

RESULTS

Rats were studied during the late prenatal period (-2 days before birth) and 7, 13, 21, 33, 50 and 80 days after birth. Activity of cardiac citrate synthase, creatine kinase and its isoenzymes as well as lactate dehydrogenase (LDH) and its isoenzymes demonstrated the well-described shift from anaerobic towards aerobic metabolism. mRNA levels of CrT in the foetal rat hearts, as determined by real-time PCR, were about 30% of the mRNA levels in the adult rat heart and gradually increased during development. Creatine uptake in isolated perfused rat hearts increased significantly from 3.0 nmol/min/gww at 13 days old to 4.9 nmol/min/gww in 80 day old rats. Accordingly, total creatine content in hearts, measured by HPLC, increased steadily during maturation (30 nmol/mg protein (-2 days) vs 87 nmol/mg protein (80 days)), and correlated closely with CrT gene expression.

CONCLUSIONS

The maturation-dependant alterations of CK and LDH isoenzyme activities and of mitochondrial oxidative capacity were paralleled by a progressive increase of CrT expression, creatine uptake kinetics and creatine content in the heart.

Is MR spectroscopy of the heart ready for humans?

Cardiac magnetic resonance spectroscopy (MRS) is a non-invasive in vivo technique that can be used to measure high-energy phosphate metabolism in heart without harmful radiation or radio-isotopes. Using the property of atomic nuclear spin, this technique provides real-time information on cardiac metabolite composition, including creatine content. Cardiac (31)P MR spectroscopy has shown most promise for the prognosis and treatment of heart failure, but has also been used as a powerful research tool for uncovering energy deficits in cardiomyopathies, ischaemic heart disease and valvular heart disease. Information provided by cardiac (1)H MRS includes myocardial creatine levels, which are decreased in heart failure, and myocardial fat content. Hyperpolarisation is an emerging MRS technique, which allows the (13)C MR signal to be increased many orders of magnitude in studies of substrate metabolism and enzyme kinetics. Cardiac MRS has predominantly been used in research and is not currently ready for routine clinical practice. However, higher MR field strengths, which provide greater signal and spectral resolution, may allow spectroscopy to become more widespread. This article reviews the applications of cardiac MRS, concentrating on the (31)P nucleus, and the current limitations that prevent routine use in research and clinical practice.

Proteomic analysis of mitochondria reveals a metabolic switch from fatty acid oxidation to glycolysis in the failing heart

This work characterizes the mitochondrial proteomic profile in the failing heart and elucidates the molecular basis of mitochondria in heart failure. Heart failure was induced in rats by myocardial infarction, and mitochondria were isolated from hearts by differential centrifugation. Using two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, a system biology approach was employed to investigate differences in mitochondrial proteins between normal and failing hearts. Mass spectrometry identified 27 proteins differentially expressed that involved in energy metabolism. Among those, the up-regulated proteins included tricarboxylic acid cycle enzymes and pyruvate dehydrogenase complex subunits while the down-regulated proteins were involved in fatty acid oxidation and the OXPHOS complex. These results suggest a substantial metabolic switch from free fatty acid oxidation to glycolysis in heart failure and provide molecular evidence for alterations in the structural and functional parameters of mitochondria that may contribute to cardiac dysfunction during ischemic injury.

Mice over-expressing the myocardial creatine transporter develop progressive heart failure and show decreased glycolytic capacity

The metabolic phenotype of the failing heart includes a decrease in phosphocreatine and total creatine concentration [Cr], potentially contributing to contractile dysfunction. Surprisingly, in 32- week-old mice over-expressing the myocardial creatine transporter (CrT-OE), we previously demonstrated that elevated [Cr] correlates with left ventricular (LV) hypertrophy and failure. The aim of this study was to determine the temporal relationship between elevated [Cr] and the onset of cardiac dysfunction and to screen for potential molecular mechanisms. CrT-OE mice were compared with wild-type (WT) littermate controls longitudinally using cine-MRI to measure cardiac function and single-voxel (1)H-MRS to measure [Cr] in vivo at 6, 16, 32, and 52 weeks of age. CrT-OE mice had elevated [Cr] at 6 weeks (mean 1.9-fold), which remained constant throughout life. Despite this increased [Cr], LV dysfunction was not apparent until 16 weeks and became more pronounced with age. Additionally, LV tissue from 12 to 14 week old CrT-OE mice was compared to WT using 2D difference in-gel electrophoresis (DIGE). These analyses detected a majority of the heart's metabolic enzymes and identified seven proteins that were differentially expressed between groups. The most pronounced protein changes were related to energy metabolism: alpha- and beta-enolase were selectively decreased (p<0.05), while the remaining enzymes of glycolysis were unchanged. Consistent with a decrease in enolase content, its activity was significantly lower in CrT-OE hearts (in WT, 0.59+/-0.02 micromol ATP produced/microg protein/min; CrT-OE, 0.31+/-0.06; p<0.01). Additionally, anaerobic lactate production was decreased in CrT-OE mice (in WT, 102+/-3 micromol/g wet myocardium; CrT-OE, 78+/-13; p=0.02), consistent with decreased glycolytic capacity. Finally, we found that enolase may be regulated by increased expression of the beta-enolase repressor transcription factor, which was significantly increased in CrT-OE hearts. This study demonstrates that chronically increased myocardial [Cr] in the CrT-OE model leads to the development of progressive hypertrophy and heart failure, which may be mediated by a compromise in glycolytic capacity at the level of enolase.

Transient alterations of creatine, creatine phosphate, N-acetylaspartate and high-energy phosphates after mild traumatic brain injury in the rat

In this study, the concentrations of creatine (Cr), creatine phosphate (CrP), N-acetylaspartate (NAA), ATP, ADP and phosphatidylcholine (PC) were measured at different time intervals after mild traumatic brain injury (mTBI) in whole brain homogenates of rats. Anaesthetized animals underwent to the closed-head impact acceleration "weight-drop" model (450 g delivered from 1 m height = mild traumatic brain injury) and were killed at 2, 6, 24, 48 and 120 h after the insult (n = 6 for each time point). Sham-operated rats (n = 6) were used as controls. Compounds of interest were synchronously measured by HPLC in organic solvent deproteinized whole brain homogenates. A reversible decrease of all metabolites but PC was observed, with minimal values recorded at 24 h post-injury (minimum of CrP = 48 h after impact). In particular, Cr and NAA showed a decrease of 44.5 and 29.5%, respectively, at this time point. When measuring NAA in relation to other metabolites, as it is commonly carried out in "in vivo" (1)H-magnetic resonance spectroscopy ((1)H-MRS), an increase in the NAA/Cr ratio and a decrease in the NAA/PC ratio was observed. Besides confirming a transient alteration of NAA homeostasis and ATP imbalance, our results clearly show significant changes in the cerebral concentration of Cr and CrP after mTBI. This suggests a careful use of the NAA/Cr ratio to measure NAA by (1)H-MRS in conditions of altered cerebral energy metabolism. Viceversa, the NAA/PC ratio appears to be a better indicator of actual NAA levels during energy metabolism impairment. Furthermore, our data suggest that, under pathological conditions affecting the brain energetic, the Cr-CrP system is not a suitable tool to buffer possible ATP depletion in the brain, thus supporting the growing indications for alternative roles of cerebral Cr.

Role of phospholipase A2 in activation of isolated cardiomyocyte respiration in postinfarction cardiosclerosis

The rate of oxygen consumption by isolated cardiomyocytes was studied in rats with experimental postinfarction cardiosclerosis. The increase in oxygen consumption under these condition was comparable to that in melittin- and arachidonic acid-induced activation of phospholipase A2 in cardiomyocytes of intact animals. Bromophenacyl bromide inhibition of phospholipase A2 in cardiomyocytes of rats with postinfarction cardiosclerosis led to reduction of oxygen consumption rate to values characteristic of intact animal cardiomyocytes. The results confirm the hypothesis according to which high oxygen consumption in postinfarction cardiosclerosis is related to increased activity of phospholipase A2.

Reduced myocardial creatine kinase flux in human myocardial infarction: an in vivo phosphorus magnetic resonance spectroscopy study

BACKGROUND

Energy metabolism is essential for myocellular viability. The high-energy phosphates adenosine triphosphate (ATP) and phosphocreatine (PCr) are reduced in human myocardial infarction (MI), reflecting myocyte loss and/or decreased intracellular ATP generation by creatine kinase (CK), the prime energy reserve of the heart. The pseudo-first-order CK rate constant, k, measures intracellular CK reaction kinetics and is independent of myocyte number within sampled tissue. CK flux is defined as the product of [PCr] and k. CK flux and k have never been measured in human MI.

METHODS AND RESULTS

Myocardial CK metabolite concentrations, k, and CK flux were measured noninvasively in 15 patients 7 weeks to 16 years after anterior MI using phosphorus magnetic resonance spectroscopy. In patients, mean myocardial [ATP] and [PCr] were 39% to 44% lower than in 15 control subjects (PCr=5.4+/-1.2 versus 9.6+/-1.1 micromol/g wet weight in MI versus control subjects, respectively, P<0.001; ATP=3.4+/-1.1 versus 5.5+/-1.3 micromol/g wet weight, P<0.001). The myocardial CK rate constant, k, was normal in MI subjects (0.31+/-0.08 s(-1)) compared with control subjects (0.33+/-0.07 s(-1)), as was PCr/ATP (1.74+/-0.27 in MI versus 1.87+/-0.45). However, CK flux was halved in MI [to 1.7+/-0.5 versus 3.3+/-0.8 micromol(g . s)(-1); P<0.001].

CONCLUSIONS

These first observations of CK kinetics in prior human MI demonstrate that CK ATP supply is significantly reduced as a result of substrate depletion, likely attributable to myocyte loss. That k and PCr/ATP are unchanged in MI is consistent with the preservation of intracellular CK metabolism in surviving myocytes. Importantly, the results support therapies that primarily ameliorate the effects of tissue and substrate loss after MI and those that reduce energy demand rather than those that increase energy transfer or workload in surviving tissue.

Early beneficial effects of bone marrow-derived mesenchymal stem cells overexpressing Akt on cardiac metabolism after myocardial infarction

Administration of mesenchymal stem cells (MSCs) is an effective therapy to repair cardiac damage after myocardial infarction (MI) in experimental models. However, the mechanisms of action still need to be elucidated. Our group has recently suggested that MSCs mediate their therapeutic effects primarily via paracrine cytoprotective action. Furthermore, we have shown that MSCs overexpressing Akt1 (Akt-MSCs) exert even greater cytoprotection than unmodified MSCs. So far, little has been reported on the metabolic characteristics of infarcted hearts treated with stem cells. Here, we hypothesize that Akt-MSC administration may influence the metabolic processes involved in cardiac adaptation and repair after MI. MI was performed in rats randomized in four groups: sham group and animals treated with control MSCs, Akt-MSCs, or phosphate-buffered saline (PBS). High energy metabolism and basal 2-deoxy-glucose (2-DG) uptake were evaluated on isolated hearts using phosphorus-31 nuclear magnetic resonance spectroscopy at 72 hours and 2 weeks after MI. Treatment with Akt-MSCs spared phosphocreatine stores and significantly limited the increase in 2-DG uptake in the residual intact myocardium compared with the PBS- or the MSC-treated animals. Furthermore, Akt-MSC-treated hearts had normal pH, whereas low pH was measured in the PBS and MSC groups. Correlative analysis indicated that functional recovery after MI was inversely related to the rate of 2-DG uptake. We conclude that administration of MSCs overexpressing Akt at the time of infarction results in preservation of normal metabolism and pH in the surviving myocardium.