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

Abnormal cardiac morphology, function and energy metabolism in the dystrophic mdx mouse: an MRI and MRS study

Patients with muscular dystrophy have abnormal cardiac function and decreased high-energy phosphate metabolism. Here, we have determined whether the 8 month old mdx mouse, an animal model of muscular dystrophy, also has abnormal cardiac function and energetics. In vivo cardiac MRI revealed 33% and 104% larger right ventricular end-diastolic and end-systolic volumes, respectively, and 17% lower right ventricular ejection fractions in mdx mice compared with controls. Evidence of left ventricular diastolic dysfunction included 18% lower peak filling rates in mdx mouse hearts. Abnormal cardiac function was accompanied by necrosis and lower citrate synthase activity in the mdx mouse heart, suggesting decreased mitochondrial content. Decreased mitochondrial numbers were associated with 38% lower phosphocreatine concentration, 22% lower total creatine, 36% higher cytosolic free ADP concentration and 1.3 kJ/mol lower free-energy available from ATP hydrolysis in whole isolated, perfused mdx mouse hearts than in controls. Transsarcolemmal creatine uptake was 12% lower in mdx mouse hearts. We conclude that the absence of dystrophin in adult mdx mouse heart, as in the heart of human patient, is associated with right ventricular dilatation, left ventricular diastolic dysfunction and abnormal energy metabolism.

Cardiac phenotype of mitochondrial creatine kinase knockout mice is modified on a pure C57BL/6 genetic background

Discrepant results for the phenotype of mitochondrial creatine kinase knockout mice (Mt-CK(-/-)) could be due to mixed genetic background and use of non-littermate controls. We therefore backcrossed with C57BL/6J for >8 generations, followed by extensive in vivo cardiac phenotyping. Echocardiography and in vivo LV haemodynamics were performed in independent cohorts at 20-40 weeks and 1 year. No significant differences were observed for ECG, LV volumes, pressures, and systolic or diastolic function compared to littermate controls. Furthermore, responses to dobutamine were not different, indicating preserved contractile reserve. Contrary to published reports using Mt-CK(-/-) on a mixed background, we observed normal LV weights even in year old mice, and gene expression of common hypertrophic markers were not elevated. However, previously undetected adaptations were observed: an increase in activity of the cytosolic MM-CK isoenzyme (+20% vs WT, P=0.0009), and of citrate synthase (+18% vs WT, P=0.0007), a marker for mitochondrial volume. In a 3-week voluntary wheel running protocol, Mt-CK(-/-) ran significantly less per day (P=0.009) and attained lower maximum speed compared to controls (P=0.0003), suggesting impaired skeletal muscle function. MM-CK isoenzyme activity was significantly elevated in soleus but not gastrocnemius muscle of KO mice, and citrate synthase activities were normal in both, suggesting compensatory mechanisms are incomplete in skeletal muscle.

CONCLUSIONS

in contrast to previous reports using a mixed genetic background, Mt-CK(-/-) on a C57BL/6 background do not develop LV hypertrophy or dysfunction even up to 1 year, and this may be explained by a compensatory increase in MM-CK activity and mitochondrial volume.

Creatine uptake in mouse hearts with genetically altered creatine levels

Creatine plays an important role in energy metabolism in the heart. Cardiomyocytes accumulate creatine via a specific creatine transporter (CrT), the capacity of which is reduced in the failing heart, resulting in lower myocardial creatine concentration. Therefore, to gain insight into how the CrT is regulated, we studied two mouse models of severely altered myocardial creatine levels. Cardiac creatine uptake levels were measured in isolated hearts from creatine-free guanidinoacetate-N-methyl transferase knock out (GAMT^−/−) mice and from mice overexpressing the myocardial CrT (CrT-OE) using ¹⁴C-radiolabeled creatine. CrT mRNA levels were measured using real time RT-PCR and creatine levels with HPLC. Hearts from GAMT^−/− mice showed a 7-fold increase in V_max of creatine uptake and a 1.4-fold increase in CrT mRNA levels. The increase in Cr uptake and in CrT mRNA levels, however, was almost completely prevented when mice were fed a creatine supplemented diet, indicating that creatine uptake is subject to negative feedback regulation. Cardiac creatine uptake levels in CrT-OE mice were increased on average 2.7-fold, showing a considerable variation, in line with a similar variation in creatine content. Total CrT mRNA levels correlated well with myocardial creatine content (r = 0.67; p < 0.0001) but endogenous CrT mRNA levels did not correlate at all with myocardial creatine content (r = 0.01; p = 0.96). This study shows that creatine uptake can be massively upregulated in the heart, by almost an order of magnitude and that this upregulation is subject to feedback inhibition. In addition, our results strongly suggest that CrT activity is predominantly regulated by mechanisms other than alterations in gene expression.

Quantitation of myocardial borderzone using reconstructive 3-D echocardiography after chronic infarction in rats: incremental value of low-dose dobutamine

Myocardial remodeling determines the degree of left ventricular dysfunction and mortality after transmural chronic myocardial infarction (CMI). Noninvasive characterization and quantitation of myocardial borderzone and collagenous scar are therefore parameters of clinical interest. The aims of this study were (i) to measure accuracy of reconstructive 3-D echocardiography (3DE) in scar and myocardial borderzone size assessment and (ii) to investigate the incremental value of low-dose dobutamine stress. 3DE was performed in 14 immunodeficient rats (rnu-rnu, 180-200 g) with anterior CMI 25 d after coronary ligation. Briefly, consecutive parallel short-axis cineloops were obtained electrocardiogram-gated starting from base to the apex. Morphology (mass, surface) and function (contractility, contractile reserve) of different compartments were assessed and correlated with 3-D histomorphometry. Histology was done using picrosirius red for collagen staining. 3DE left ventricular mass correlated closely with histomorphometry (y = 0.89x + 155, p < 0.0001, r = 0.80). Hypo- and akinetic myocardial surface correlated well with borderzone myocardium (y = 0.34x + 17, p = 0.009, r = 0.62) and collagenous scar (y = 1.9x + 4.4, p < 0.0001, r = 0.79), respectively. Extent of abnormal wall motion was closely related to borderzone and scar tissue area (y = 0.82x + 7, p < 0.0001, r = 0.77). 3DE quantitation of borderzone myocardium, but not collagenous scar, was more closely correlated to histomorphometry during inotropic stimulation. Global contractile reserve is positively associated with the size of myocardial borderzone. Regional contractile reserve of borderzone myocardium is not negatively associated with its collagen content. 3DE allows precise quantitation of myocardial borderzone and identification of transmural scar tissue noninvasively. Assessment of contractile reserve improves characterization and estimation of myocardial borderzone after CMI.

Human cardiac-specific cDNA array for idiopathic dilated cardiomyopathy: sex-related differences

Idiopathic dilated cardiomyopathy (IDCM) constitutes a large portion of patients with heart failure of unknown etiology. Up to 50% of all transplant recipients carry this clinical diagnosis. Female-specific gene expression in IDCM has not been explored. We report sex-related differences in the gene expression profile of ventricular myocardium from patients undergoing cardiac transplantation. We produced and sequenced subtractive cDNA libraries, using human left ventricular myocardium obtained from male transplant recipients with IDCM and nonfailing human heart donors. With the resulting sequence data, we generated a custom human heart failure microarray for IDCM containing 1,145 cardiac-specific oligonucleotide probes. This array was used to characterize RNA samples from female IDCM transplant recipients. We identified a female gene expression pattern that consists of 37 upregulated genes and 18 downregulated genes associated with IDCM. Upon functional analysis of the gene expression pattern, deregulated genes unique to female IDCM were those that are involved in energy metabolism and regulation of transcription and translation. For male patients we found deregulation of genes related to muscular contraction. These data suggest that 1) the gene expression pattern we have detected for IDCM may be specific for this disease and 2) there is a sex-specific profile to IDCM. Our observations further suggest for the first time ever novel targets for treatment of IDCM in women and men.

Increased mitochondrial uncoupling proteins, respiratory uncoupling and decreased efficiency in the chronically infarcted rat heart

Heart failure patients have abnormal cardiac high energy phosphate metabolism, the explanation for which is unknown. Patients with heart failure also have elevated plasma free fatty acid (FFA) concentrations. Elevated FFA levels are associated with increased cardiac mitochondrial uncoupling proteins (UCPs), which, in turn, are associated with decreased mitochondrial respiratory coupling and low cardiac efficiency. Here, we determined whether increased mitochondrial UCP levels contribute to decreased energetics in the failing heart by measuring UCPs and respiration in mitochondria isolated from the viable myocardium of chronically infarcted rat hearts and measuring efficiency (hydraulic work/O(2) consumption) in the isolated, working rat heart. Ten weeks after infarction, cardiac levels of UCP3 were increased by 53% in infarcted, failing hearts that had ejection fractions less than 45%. Cardiac UCP3 levels correlated positively with non-fasting plasma FFAs (r=0.81; p<0.01). Mitochondria from failing hearts were less coupled than those from control hearts, as demonstrated by the lower ADP/O ratio of 1.9+/-0.1 compared with 2.5+/-0.2 in controls (p<0.05). The decreased ADP/O ratio was reflected in an efficiency of 14+/-2% in the failing hearts when perfused with 1 mM palmitate, compared with 20+/-1% in controls (p<0.05). We conclude that failing hearts have increased UCP3 levels that are associated with high circulating FFA concentrations, mitochondrial uncoupling, and decreased cardiac efficiency. Thus, respiratory uncoupling may underlie the abnormal energetics and low efficiency in the failing heart, although whether this is maladaptive or adaptive would require direct investigation.

Mitochondria and heart failure

PURPOSE OF REVIEW

Energetic abnormalities in cardiac and skeletal muscle occur in heart failure and correlate with clinical symptoms and mortality. It is likely that the cellular mechanism leading to energetic failure involves mitochondrial dysfunction. Therefore, it is crucial to elucidate the causes of mitochondrial myopathy, in order to improve cardiac and skeletal muscle function, and hence quality of life, in heart failure patients.

RECENT FINDINGS

Recent studies identified several potential stresses that lead to mitochondrial dysfunction in heart failure. Chronically elevated plasma free fatty acid levels in heart failure are associated with decreased metabolic efficiency and cellular insulin resistance. Tissue hypoxia, resulting from low cardiac output and endothelial impairment, can lead to oxidative stress and mitochondrial DNA damage, which in turn causes dysfunction and loss of mitochondrial mass. Therapies aimed at protecting mitochondrial function have shown promise in patients and animal models with heart failure.

SUMMARY

Despite current therapies, which provide substantial benefit to patients, heart failure remains a relentlessly progressive disease, and new approaches to treatment are necessary. Novel pharmacological agents are needed that optimize substrate metabolism and maintain mitochondrial integrity, improve oxidative capacity in heart and skeletal muscle, and alleviate many of the clinical symptoms associated with heart failure.

MR spectroscopy in heart failure--clinical and experimental findings

Magnetic resonance spectroscopy (MRS) allows for the non-invasive detection of a wide variety of metabolites in the heart. To study the metabolic changes that occur in heart failure, (31)P- and (1)H-MRS have been applied in both patients and experimental animal studies. (31)P-MRS allows for the detection of phosphocreatine (PCr), ATP, inorganic phosphate (Pi) and intracellular pH, while (1)H-MRS allows for the detection of total creatine. All these compounds are involved in the regulation of the available energy from ATP hydrolysis via the creatine kinase (CK) reaction. Using cardiac MRS, it has been found that the PCr/CK system is impaired in the failing heart. In both, patients and experimental models, PCr levels as well as total creatine levels are reduced, and in severe heart failure ATP is also reduced. PCr/ATP ratios correlate with the clinical severity of heart failure and, importantly, are a prognostic indicator of mortality in patients. In addition, the chemical flux through the CK reaction, measured with (31)P saturation transfer MRS, is reduced more than the steady-state levels of high-energy phosphates in failing myocardium in both experimental models and in patients. Experimental studies suggest that these changes can result in increased free ADP levels when the failing heart is stressed. Increased free ADP levels, in turn, result in a reduction in the available free energy of ATP hydrolysis, which may directly contribute to contractile dysfunction. Data from transgenic mouse models also suggest that an intact creatine/CK system is critical for situations of cardiac stress.

The creatine kinase energy transport system in the failing mouse heart

Characteristic alterations of the creatine kinase (CK) system occur in heart failure and may contribute to contractile dysfunction. We examined two mouse models of chronic cardiac stress, transverse aortic constriction (TAC) and coronary artery ligation (CAL), and examined the relationship of CK system changes with hypertrophy and heart failure development. C57Bl/6 mice were subjected to TAC or sham surgery and sacrificed after 2-10 weeks according to echocardiographic criteria of myocardial hypertrophy and function to create four groups representing progressive dysfunction from normal, through compensated hypertrophy, to heart failure. Only mice with congestive heart failure had LV total creatine concentration and total CK activity significantly lower than sham values (11% and 30% lower, respectively). However for all aortic banded mice, a linear relationship was observed between ejection fraction and estimated maximal CK reaction velocity. Mice with heart failure also had corresponding decreases in the activities of the Mito-, MM-, and MB-CK isoenzymes, while the BB isoform remained unchanged. To determine whether these changes were model specific, mice were subjected to CAL or sham operation and followed for 7 weeks. Quantitative changes in total creatine, total CK activity, Mito-CK and MM-CK activities were similar for CAL and TAC mice. We conclude that alterations in the creatine kinase system occur during heart failure in mice qualitatively similar to those occurring in larger animals and humans, suggesting that mice are a suitable model for studying the role of such changes in the pathogenesis of heart failure.

In vivo mouse imaging and spectroscopy in drug discovery

Imaging modalities such as micro-computed tomography (micro-CT), micro-positron emission tomography (micro-PET), high-resolution MRI, optical imaging, and high-resolution ultrasound have become invaluable tools in preclinical pharmaceutical research. They can be used to non-invasively investigate, in vivo, rodent biology and metabolism, disease models, and pharmacokinetics and pharmacodynamics of drugs. The advantages and limitations of each approach usually determine its application, and therefore a small-rodent imaging laboratory in a pharmaceutical environment should ideally provide access to several techniques. In this paper we aim to illustrate how these techniques may be used to obtain meaningful information for the phenotyping of transgenic mice and for the analysis of compounds in murine models of disease.

MR in mouse models of cardiac disease

Transgenic and knockout mice can be used to study the genes and basic mechanisms involved in heart disease, and have therefore assumed a central role in modern cardiac research. MRI and MRS techniques have recently been developed for mice that enable the quantitative or semi-quantitative in vivo assessment of cardiac anatomy, function, perfusion, infarction, Ca(2+) influx, and metabolism. With these techniques, the normal mouse heart has been shown to be well suited as a model of human cardiac disease. The roles of individual genes in normal cardiac physiology have recently been studied by MR, including the role of neuronal nitric oxide synthase in beta-adrenergic stimulation, the roles of the inducible nitric oxide synthase and myoglobin in function, dilation, and energetics, and the role of cardiac troponin I in contractility. Furthermore, with a mouse model of myocardial infarction, the roles of the angiotensin II type 2 receptor, xanthine oxidase inhibitors, blood coagulation factor XIII, and inducible nitric oxide synthase in post-infarct function and remodeling have been further elucidated. Non-invasive in vivo MRI and MRS in mice provide a unique and powerful means for phenotyping genetically engineered mice and can improve our understanding of the roles of specific genes and proteins in cardiac physiology and pathophysiology.

Energetic differences between viable and non-viable myocardium in patients with recent myocardial infarction are not an effect of differences in wall thinning- a multivoxel (31)P-MR-spectroscopy and MRI study

To evaluate multivoxel (31)P-MR spectroscopy (MRS) for assessment of energy metabolism in patients with myocardial infarction (MI) in correlation to left ventricular (LV) wall thickness and the outcome of revascularization. Thirty patients with subacute anterior myocardial infarction and planned revascularization were enrolled. 3D-chemical shift imaging was applied to determine PCr/ATP ratios in two areas: infarcted/anterior and noninfarcted/septal myocardium. MRI was used to evaluate LV function and wall thickness, and was repeated 6 months after revascularization to assess myocardial viability. Fifteen volunteers were controls. Fifteen patients showed normalization of wall motion abnormalities after revascularization (Group 1; viable), 15 not (Group 2; non-viable). Regarding infarcted/anterior myocardium, Group 2 had lower PCr/ATP ratios (0.81 +/- 0.60 vs 1.17 +/- 0.25), and PCr/ATP ratios were reduced in both groups compared to controls (1.45 +/- 0.29). Regarding noninfarcted/septal myocardium, again Group 2 had lower ratios (0.93 +/- 0.53 vs 1.31 +/- 0.38); however, compared to controls (1.51 +/- 0.32) a reduction of PCr/ATP ratios was only found in Group 2. For both myocardial regions, no correlations between PCr/ATP ratios and LV wall thickness were detected. The more severe energetic alteration in irreversibly damaged myocardium is not an effect of differences of wall thinning. Additional alterations of noninfarcted, adjacent myocardium can be detected.

High-dose 17beta-estradiol treatment prevents development of heart failure post-myocardial infarction in the rat

OBJECTIVES

Prognosis of heart failure remains poor despite therapeutic advances, such as angiotensin converting enzyme inhibition or beta-receptor blockade. Thus, more effective forms of treatment are urgently needed. Since estrogens have been shown to modulate migration and proliferation of cardiac fibroblasts and to modulate the expression of estrogen receptors of cardiomyocytes we examined whether high-dose estrogen treatment can affect post-myocardial infarction left ventricular remodeling.

METHODS

Female rats were treated with 17beta-estradiol (7.5 mg/90 d) or placebo for ten weeks, starting two weeks prior to experimental myocardial infarction. Eight weeks after infarction, in vivo echocardiographic and hemodynamic measurements as well as isolated heart perfusion were performed.

RESULTS

In vivo, chronic estrogen treatment almost completely prevented the development of all signs of heart failure that occur in untreated infarcted hearts, such as increased left ventricular diameters (dilatation), reduced fractional shortening (systolic dysfunction) or increased left ventricular end-diastolic pressure (diastolic dysfunction). In vitro, the right- (indicating structural dilatation) and downward (indicating left ventricular dysfunction) shift of left ventricular pressure-volume curves occurring in untreated infarcted hearts was completely prevented by estrogen.

CONCLUSIONS

High dose estradiol treatment prevented development of post-MI remodeling, as assessed by in vivo and in vitro parameters of LV dysfunction. Estrogen may hold the potential of becoming a new form of heart failure treatment.However, the mechanisms responsible for this striking and unexpected beneficial action of estrogen in heart failure remain to be elucidated.

Multimodal functional cardiac MRI in creatine kinase-deficient mice reveals subtle abnormalities in myocardial perfusion and mechanics

A decrease in the supply of ATP from the creatine kinase (CK) system is thought to contribute to the evolution of heart failure. However, previous studies on mice with a combined knockout of the mitochondrial and cytosolic CK (CK−/−) have not revealed overt left ventricular dysfunction. The aim of this study was to employ novel MRI techniques to measure maximal myocardial velocity ( Vmax) and myocardial perfusion and thus determine whether abnormalities in the myocardial phenotype existed in CK−/− mice, both at baseline and 4 wk after myocardial infarction (MI). As a result, myocardial hypertrophy was seen in all CK−/− mice, but ejection fraction (EF) remained normal. Vmax, however, was significantly reduced in the CK−/− mice [wild-type, 2.32 ± 0.09 vs. CK−/−, 1.43 ± 0.16 cm/s, P < 0.05; and wild-type MI, 1.53 ± 0.11 vs. CK−/− MI, 1.26 ± 0.11 cm/s, P = not significant (NS), P < 0.05 vs. baseline]. Myocardial perfusion was also lower in the CK−/− mice (wild-type, 6.68 ± 0.27 vs. CK−/−, 4.12 ± 0.63 ml/g·min, P < 0.05; and wild-type MI, 3.97 ± 0.65 vs. CK−/− MI, 3.71 ± 0.57 ml/g·min, P = NS, P < 0.05 vs. baseline), paralleled by a significantly reduced capillary density (histology). In conclusion, myocardial function in transgenic mice may appear normal when only gross indexes of performance such as EF are assessed. However, the use of a combination of novel MRI techniques to measure myocardial perfusion and mechanics allowed the abnormalities in the CK−/− phenotype to be detected. The myocardium in CK-deficient mice is characterized by reduced perfusion and reduced maximal contraction velocity, suggesting that the myocardial hypertrophy seen in these mice cannot fully compensate for the absence of the CK system.

Xanthine oxidase inhibitors improve energetics and function after infarction in failing mouse hearts

After myocardial infarction, ventricular geometry and function, as well as energy metabolism, change markedly. In nonischemic heart failure, inhibition of xanthine oxidase (XO) improves mechanoenergetic coupling by improving contractile performance relative to a reduced energetic demand. However, the metabolic and contractile effects of XO inhibitors (XOIs) have not been characterized in failing hearts after infarction. After undergoing permanent coronary ligation, mice received a XOI (allopurinol or oxypurinol) or matching placebo in the daily drinking water. Four weeks later, 1H MRI and 31P magnetic resonance spectroscopy (MRS) were used to quantify in vivo functional and metabolic changes in postinfarction remodeled mouse myocardium and the effects of XOIs on that process. End-systolic (ESV) and end-diastolic volumes (EDV) were increased by more than sixfold after infarction, left ventricle (LV) mass doubled ( P < 0.005), and the LV ejection fraction (EF) decreased (14 ± 9%) compared with control hearts (59 ± 8%, P < 0.005) at 1 mo. The myocardial phosphocreatine (PCr)-to-ATP ratio (PCr/ATP) was also significantly decreased in infarct remodeled hearts (1.4 ± 0.6) compared with control animals (2.1 ± 0.5, P < 0.02), in agreement with prior studies in larger animals. The XOIs allopurinol and oxypurinol did not change LV mass but limited the increase in ESV and EDV of infarct hearts by 50%, increased EF (23 ± 9%, P = 0.01), and normalized cardiac PCr/ATP (2.0 ± 0.5, P < 0.04). We conclude that XOIs improve ventricular function after infarction and normalize high-energy phosphate ratio in heart failure. Thus XOI therapy offers a new and potentially complementary approach to limit the adverse contractile and metabolic consequences after infarction.

Growth hormone- and pressure overload-induced cardiac hypertrophy evoke different responses to ischemia-reperfusion and mechanical stretch

OBJECTIVE

To compare the molecular, histological, and functional characteristics of growth hormone (GH)- and pressure overload-induced cardiac hypertrophy, and their responses to ischemia-reperfusion and mechanical stretch.

DESIGN

Four groups of male Wistar rats were studied: aortic banding (n=24, AB) or sham (n=24, controls) for 10 weeks, and GH treatment (n=24; 3.5mg/kg/day, GH) or placebo (n=24, controls) for 4 weeks. At 13 weeks, the rats were randomly subjected to: (i) assessment of basal left ventricular mRNA expression of sarcoplasmic reticulum calcium-ATPase (SERCA-2), phospholamban (PLB), and Na(+)-Ca(2+) exchanger (NCX) and collagen volume fraction (CVF) (Protocol A, 8 rats in each group); (ii) left ventricular no-flow ischemia with simultaneous evaluation of intracellular Ca(2+) handling and ATP, phosphocreatine (PCr) and inorganic phosphate (Pi) content (Protocol B, 12 rats in each group); or (iii) left ventricular mechanical stretch for 40 min with assessment of tumor necrosis-alpha (TNF-alpha) mRNA (Protocol C, 4 rats in each group). Protocol B and C were carried out in a Langendorff apparatus.

RESULTS

In Protocol A, no difference was found as to myocardial mRNA content of Ca(2+) regulating proteins and CVF in GH animals vs controls. In contrast, in the AB group, myocardial mRNA expression of SERCA-2 and PLB was downregulated while that of NCX and CVF were increased vs. controls (p<0.05). In Protocol B, recovery of left ventricular function was significantly decreased in AB vs GH groups and controls and this was associated with 1.6-fold increase in intracellular Ca(2+) overload during reperfusion (p<0.05). Baseline ATP content was similar in the four study groups, whereas PCr and Pi was lower in AB vs GH rats and controls. However, the time courses of high-energy phosphate metabolic changes did not differ during ischemia and reperfusion in the four study groups. In Protocol C, no detectable TNF-alpha mRNA level was found in the left ventricular myocardium of GH treated rats and controls at baseline, while a modest expression was noted in AB animals. Mechanical stretch resulted in de novo myocardial TNF-alpha mRNA expression in GH group and controls, which was dramatically increased in AB animals ( approximately 5-fold above baseline, p<0.001).

CONCLUSIONS

The data show that cardiac hypertrophy activated by short-term GH treatment confers cardioprotection compared with pressure overload with regard to molecular and histological characteristics, and responses to ischemia-reperfusion and mechanical stretch.

Bioenergetic protection of failing atrial and ventricular myocardium by vasopeptidase inhibitor omapatrilat

Deficient bioenergetic signaling contributes to myocardial dysfunction and electrical instability in both atrial and ventricular cardiac chambers. Yet, approaches capable to prevent metabolic distress are only partially established. Here, in a canine model of tachycardia-induced congestive heart failure, we compared atrial and ventricular bioenergetics and tested the efficacy of metabolic rescue with the vasopeptidase inhibitor omapatrilat. Despite intrinsic differences in energy metabolism, failing atria and ventricles demonstrated profound bioenergetic deficiency with reduced ATP and creatine phosphate levels and compromised adenylate kinase and creatine kinase catalysis. Depressed phosphotransfer enzyme activities correlated with reduced tissue ATP levels, whereas creatine phosphate inversely related with atrial and ventricular load. Chronic treatment with omapatrilat maintained myocardial ATP, the high-energy currency, and protected adenylate and creatine kinase phosphotransfer capacity. Omapatrilat-induced bioenergetic protection was associated with maintained atrial and ventricular structural integrity, albeit without full recovery of the creatine phosphate pool. Thus therapy with omapatrilat demonstrates the benefit in protecting phosphotransfer enzyme activities and in preventing impairment of atrial and ventricular bioenergetics in heart failure.

Supranormal myocardial creatine and phosphocreatine concentrations lead to cardiac hypertrophy and heart failure: insights from creatine transporter-overexpressing transgenic mice

BACKGROUND

Heart failure is associated with deranged cardiac energy metabolism, including reductions of creatine and phosphocreatine. Interventions that increase myocardial high-energy phosphate stores have been proposed as a strategy for treatment of heart failure. Previously, it has not been possible to increase myocardial creatine and phosphocreatine concentrations to supranormal levels because they are subject to tight regulation by the sarcolemmal creatine transporter (CrT).

METHODS AND RESULTS

We therefore created 2 transgenic mouse lines overexpressing the myocardial creatine transporter (CrT-OE). Compared with wild-type (WT) littermate controls, total creatine (by high-performance liquid chromatography) was increased in CrT-OE hearts (66+/-6 nmol/mg protein in WT versus 133+/-52 nmol/mg protein in CrT-OE). Phosphocreatine levels (by 31P magnetic resonance spectroscopy) were also increased but to a lesser extent. Surprisingly, CrT-OE mice developed left ventricular (LV) dilatation (LV end-diastolic volume: 21.5+/-4.3 microL in WT versus 33.1+/-9.6 microL in CrT-OE; P=0.002), substantial LV dysfunction (ejection fraction: 64+/-9% in WT versus 49+/-13% in CrT-OE; range, 22% to 70%; P=0.003), and LV hypertrophy (by 3-dimensional echocardiography and magnetic resonance imaging). Myocardial creatine content correlated closely with LV end-diastolic volume (r=0.51, P=0.02), ejection fraction (r=-0.74, P=0.0002), LV weight (r=0.59, P=0.006), LV end-diastolic pressure (r=0.52, P=0.02), and dP/dt(max) (r=-0.69, P=0.0008). Despite increased creatine and phosphocreatine levels, CrT-OE hearts showed energetic impairment, with increased free ADP concentrations and reduced free-energy change levels.

CONCLUSIONS

Overexpression of the CrT leads to supranormal levels of myocardial creatine and phosphocreatine, but the heart is incapable of keeping the augmented creatine pool adequately phosphorylated, resulting in increased free ADP levels, LV hypertrophy, and dysfunction. Our data demonstrate that a disturbance of the CrT-mediated tight regulation of cardiac energy metabolism has deleterious functional consequences. These findings caution against the uncritical use of creatine as a therapeutic agent in heart disease.

Reduced inotropic reserve and increased susceptibility to cardiac ischemia/reperfusion injury in phosphocreatine-deficient guanidinoacetate-N-methyltransferase-knockout mice

BACKGROUND

The role of the creatine kinase (CK)/phosphocreatine (PCr) energy buffer and transport system in heart remains unclear. Guanidinoacetate-N-methyltransferase-knockout (GAMT-/-) mice represent a new model of profoundly altered cardiac energetics, showing undetectable levels of PCr and creatine and accumulation of the precursor (phospho-)guanidinoacetate (P-GA). To characterize the role of a substantially impaired CK/PCr system in heart, we studied the cardiac phenotype of wild-type (WT) and GAMT-/- mice.

METHODS AND RESULTS

GAMT-/- mice did not show cardiac hypertrophy (myocyte cross-sectional areas, hypertrophy markers atrial natriuretic factor and beta-myosin heavy chain). Systolic and diastolic function, measured invasively (left ventricular conductance catheter) and noninvasively (MRI), were similar for WT and GAMT-/- mice. However, during inotropic stimulation with dobutamine, preload-recruitable stroke work failed to reach maximal levels of performance in GAMT-/- hearts (101+/-8 mm Hg in WT versus 59+/-7 mm Hg in GAMT-/-; P<0.05). (31)P-MR spectroscopy experiments showed that during inotropic stimulation, isolated WT hearts utilized PCr, whereas isolated GAMT-/- hearts utilized P-GA. During ischemia/reperfusion, GAMT-/- hearts showed markedly impaired recovery of systolic (24% versus 53% rate pressure product recovery; P<0.05) and diastolic function (eg, left ventricular end-diastolic pressure 23+/-9 in WT and 51+/-5 mm Hg in GAMT-/- during reperfusion; P<0.05) and incomplete resynthesis of P-GA.

CONCLUSIONS

GAMT-/- mice do not develop hypertrophy and show normal cardiac function at low workload, suggesting that a fully functional CK/PCr system is not essential under resting conditions. However, when acutely stressed by inotropic stimulation or ischemia/reperfusion, GAMT-/- mice exhibit a markedly abnormal phenotype, demonstrating that an intact, high-capacity CK/PCr system is required for situations of increased cardiac work or acute stress.

Detection of myocardial infarctions by acquisition-weighted 31P-MR spectroscopy in humans

PURPOSE

To determine whether the recently applied technique of acquisition-weighted 31P-MR spectroscopy (AW-MRS) allows for the detection of depressed energy metabolism in patients with inferior wall myocardial infarctions.

MATERIALS AND METHODS

Eight patients with subacute myocardial infarction and wall motion abnormalities restricted to the inferior wall were examined with a 1.5-T MR scanner. Global and regional left ventricular (LV) function was assessed by cine MRI, and the size and extent of myocardial infarction was assessed by late enhancement (LE). MRS was performed with an AW three-dimensional chemical shift imaging sequence. Phosphocreatine/ATP ratios were determined with the postprocessing model AMARES for four voxels positioned in the anterior, lateral, inferior, and septal parts of the LV.

RESULTS

The LV ejection fraction (EF) was reduced to 37.5%+/-9.0%. Seven of eight patients had transmural LE in the inferior wall, and one patient showed subendocardial enhancement in the inferior-lateral parts. Phosphocreatine/ATP ratios of the inferior wall were significantly reduced (P <0.05) compared to all other parts of the LV (1.03 +/- 0.39 (inferior), 1.67 +/- 0.81 (lateral), 1.73 +/- 0.29 (anterior), and 1.49 +/- 0.31 (septal)). The ratios in five of seven patients with transmural enhancement were <1.00 in the inferior wall.

CONCLUSION

Acquisition weighting allows for the detection of inferior wall infarctions in patients. Transmural signal enhancement is associated with significant depression of phosphocreatine/ATP ratios.

Myocardial contractile efficiency increases in proportion to a fetal enzyme shift in chronically infarcted rat hearts

Changes in creatine kinase (CK) and lactate dehydrogenase (LDH) isoform expression occur in residual tissue after myocardial infarction. It is unknown how these changes correlate with cardiac remodeling, contractile performance and efficiency. Rats were subjected to left coronary artery ligation (MI) or sham operation (sham). Left ventricular end-diastolic pressure (EDP) was measured in vivo 8 weeks later. Hearts were isolated, buffer-perfused (Langendorff) at constant pressure and isovolumetric left ventricular (LV) pressure-volume (PV) curves were recorded. LV PV areas (PVA) were calculated and related to oxygen consumption. Biopsies of intact left ventricular tissue were taken for biochemical measurements. Correlations between in vivo EDP and biochemical parameters were found: Total CK activity (r = -.47, p = .022), CK isoenzyme percentage for BB (r = +.57, p = .004), MB (r = +.54, p = .006) and CK-mito (r = -.51, p = .012), total creatine content (r = -.61, p = .002) and the ratio of LDH5/LDH1 (r = .49, p = .016). Correlations were also detected for left ventricular volume and PVAs at in vivo EDP demonstrating that the extent of CK and LDH system alterations correlate with the extent of LV dilatation and mechanical energy requirements. The slope of the MVO(2)-PVA relation decreased significantly with increasing values of in vivo EDP (r = -.68, p = 0.0003) indicating increased contractile ef.ciency. Improved efficiency correlated with the increase in fetal CK isoenzyme expression. Thus, contractile efficiency increases parallel to the extent of left ventricular dilatation and dysfunction. CK and LDH system changes in residual intact myocardium also occur proportional to LV dysfunction.

ATP flux through creatine kinase in the normal, stressed, and failing human heart

The heart consumes more energy per gram than any other organ, and the creatine kinase (CK) reaction serves as its prime energy reserve. Because chemical energy is required to fuel systolic and diastolic function, the question of whether the failing heart is "energy starved" has been debated for decades. Despite the central role of the CK reaction in cardiac energy metabolism, direct measures of CK flux in the beating human heart were not previously possible. Using an image-guided molecular assessment of endogenous ATP turnover, we directly measured ATP flux through CK in normal, stressed, and failing human hearts. We show that cardiac CK flux in healthy humans is faster than that estimated through oxidative phosphorylation and that CK flux does not increase during a doubling of the heart rate-blood pressure product by dobutamine. Furthermore, cardiac ATP flux through CK is reduced by 50% in mild-to-moderate human heart failure (1.6 +/- 0.6 vs. 3.2 +/- 0.9 micromol/g of wet weight per sec, P <0.0005). We conclude that magnetic resonance strategies can now directly assess human myocardial CK energy flux. The deficit in ATP supplied by CK in the failing heart is cardiac-specific and potentially of sufficient magnitude, even in the absence of a significant reduction in ATP stores, to contribute to the pathophysiology of human heart failure. These findings support the pursuit of new therapies that reduce energy demand and/or augment energy transfer in heart failure and indicate that cardiac magnetic resonance can be used to assess their effectiveness.

Quantitative 3-Dimensional Echocardiography for Accurate and Rapid Cardiac Phenotype Characterization in Mice

Background— Insufficient techniques exist for rapid and reliable phenotype characterization of genetically manipulated mouse models of cardiac dysfunction. We developed a new, robust, 3-dimensional echocardiography (3D-echo) technique and hypothesized that this 3D-echo technique is as accurate as magnetic resonance imaging (MRI) and histology for assessment of left ventricular (LV) volume, ejection fraction, mass, and infarct size in normal and chronically infarcted mice.

Methods and Results— Using a high-frequency, 7/15-MHz, linear-array ultrasound transducer, we acquired ECG and respiratory-gated, 500-μm consecutive short-axis slices of the murine heart within 4 minutes. The short-axis movies were reassembled off-line in a 3D matrix by using the measured platform locations to position each slice in 3D. Epicardial and endocardial heart contours were manually traced, and a B-spline surface was fitted to the delineated image curves to reconstruct the heart volumes. Excellent correlations were obtained between 3D-echo and MRI for LV end-systolic volumes ( r =0.99, P <0.0001), LV end-diastolic volumes ( r =0.99, P <0.0001), ejection fraction ( r =0.99, P <0.0001), LV mass ( r =0.94, P <0.0019), and infarct size ( r =0.98, P <0.0001). Also, excellent correlations were found between the 3D-echo–derived LV mass and necropsy LV mass in normal mice ( r =0.99, P <0.0001), as well as for 3D-echo–derived infarct size and histologically determined infarct size ( r =0.99, P <0.0001) in mice with chronic heart failure. Bland-Altman analysis showed excellent limits of agreement between techniques for all measured parameters.

Conclusion— This new, fast, and highly reproducible 3D-echo technique should be of widespread applicability for high-throughput murine cardiac phenotyping studies.

Expression of MHC-beta and MCT1 in cardiac muscle after exercise training in myocardial-infarcted rats

To evaluate the hypothesis that increasing the potential for glycolytic metabolism would benefit the functioning of infarcted myocardium, we investigated whether mild exercise training would increase the activities of oxidative enzymes, expression of carbohydrate-related transport proteins (monocarboxylate transporter MCT1 and glucose transporter GLUT4), and myosin heavy chain (MHC) isoforms. Myocardial infarction (MI) was produced by occluding the proximal left coronary artery in rat hearts for 30 min. After the rats performed 6 wk of run training on a treadmill, the wall of the left ventricle was dissected and divided into the anterior wall (AW; infarcted region) and posterior wall (PW; noninfarcted region). MI impaired citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities in the AW (P < 0.01) but not in the noninfarcted PW. No differences in the expression of MCT1 were found in either tissues of AW and PW after MI, whereas exercise training significantly increased the MCT1 expression in all conditions, except AW in the MI rats. Exercise training resulted in an increased expression of GLUT4 protein in the AW in the sham rats and in the PW in the MI rats. The relative amount of MHC-beta was significantly increased in the AW and PW in MI rats compared with sham rats. However, exercise training resulted in a significant increase of MHC-alpha expression in both AW and PW in both sham and MI rats (P < 0.01). These findings suggest that mild exercise training enhanced the potential for glycolytic metabolism and ATPase activity of the myocardium, even in the MI rats, ensuring a beneficial role in the remodeling of the heart.

“Energenetics” of Heart Failure

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

Heart rate reduction by zatebradine reduces infarct size and mortality but promotes remodeling in rats with experimental myocardial infarction

The importance of heart rate for left ventricular remodeling and prognosis after myocardial infarction is not known. We examined the contribution of heart rate reduction by zatebradine, a direct sinus node inhibitor without negative inotropic effects on left ventricular function and dilatation, on mortality, energy metabolism, and neurohormonal changes in rats with experimental myocardial infarction (MI). Thirty minutes after left coronary artery ligation or sham operation, the rats were randomized to receive either placebo or zatebradine (100 mg·kg–1·day–1per gavage) continued for 8 wk. Mortality during 8 wk was 33.3% in the placebo and 23.0% in the zatebradine group ( P < 0.05); MI size was 36 ± 2% and 30 ± 1% (means ± SE, P < 0.05), respectively. Zatebradine improved stroke volume index in all treated rats but increased left ventricular volume in rats with small MI (2.43 ± 0.10 vs. 1.81 ± 0.10 ml/kg, P < 0.05) but not in rats with large MI (2.34 ± 0.09 vs. 2.35 ± 0.11 ml/kg, not significant). Zatebradine reduced left and right ventricular norepinephrine and increased left and right ventricular 3,4-dihydroxyphenyl ethylene glycol-to-norepinephrine ratio suggesting aggravation of cardiac sympathetic activation by zatebradine after MI. Creatine kinase and lactate dehydrogenase isoenzymes in rats with MI remained unchanged by zatebradine. Lowering heart rate per se reduces mortality and MI size in this model but induces adverse effects on left ventricular remodeling in rats with small MI.

Chronic coronary artery stenosis induces impaired function of remote myocardium: MRI and spectroscopy study in rat

Our purpose was to study morphological, functional, and metabolic changes induced by chronic ischemia in myocardium supplied by the stenotic vessel and in the remote area by MR techniques. A new technique of image fusion is proposed for analysis of coronary artery stenosis involving coronary MR angiography and spectroscopic imaging. Cine-MRI was performed 2 wk after induction of coronary stenosis. Global heart function and regional wall thickening were determined in 11 Wistar rats with stenosis and compared with 7 control rats. Two weeks after stenosis was induced, spin-labeling MRI for measurement of perfusion was performed in 14 isolated hearts. In eight isolated hearts with coronary stenosis, MR spectroscopy was performed, followed by angiography. 31P metabolite maps were fused with three-dimensional coronary angiograms. Induction of stenosis led to reduced segmental wall thickening (control: 75 +/- 9%, ischemic region: 9 +/- 3%, P < 0.05 vs. control) but also to impaired function of the remote region and lower cardiac output. Perfusion was reduced by 74.9 +/- 4.0% within ischemic segments compared with a septal control region. The phosphocreatine (PCr)/ATP ratio as a marker of ischemia was reduced in the region associated with stenosis (1.09 +/- 0.09) compared with remote (1.27 +/- 0.08) and control hearts (1.43 +/- 0.08; P < 0.05). The histological fraction of fibrosis within the ischemic region (12.8 +/- 1.4%) correlated to ATP signal reduction from remote to the ischemic region (r = 0.71, P < 0.05), but not to reduced wall thickening. Coronary narrowing caused declining function accompanied by diminished PCr/ATP, indicating impaired energy metabolism. Neither decline of function nor PCr signal decline correlated to fraction of fibrosis in histology. In contrast, reduction of ATP correlated to fibrosis and therefore to loss of viability. Impaired function within the ischemic region is associated with decreased PCr. Function of the remote region was affected as well. The fusion of PCr metabolite maps and the coronary angiogram may help to assess coronary morphology and resulting metabolic changes simultaneously.

Interstitial purine metabolites in hearts with LV remodeling

The myocardial ATP concentration is significantly decreased in failing hearts, which may be related to the progressive loss of the myocardial total adenine nucleotide pool. The total myocardial interstitial purine metabolites (IPM) in the dialysate of interstitial fluid could reflect the tissue ATP depletion. In rats, postmyocardial infarction (MI) left ventricular (LV) remodeling was induced by ligation of the coronary artery. Cardiac microdialysis was employed to assess changes of IPM in response to graded beta-adrenergic stimulation with isoproterenol (Iso) in myocardium of hearts with post-MI LV remodeling (MI group) or hearts with sham operation (sham group). The dialysate samples were analyzed for adenosine, inosine, hypoxanthine, xanthine, and uric acid. LV volume was greater in the MI group (2.2 +/- 0.2 ml/kg) compared with the sham group (1.3 +/- 0.2 ml/kg, P < 0.05). Infarct size was 28 +/- 4%. The baseline dialysate level of uric acid was higher in the MI group (18.9 +/- 3.4 micromol) compared with the sham group (4.6 +/- 0.7 micromol, P < 0.01). During and after Iso infusion, the dialysate levels of adenosine, xanthine, and uric acid were all significantly higher in the MI group. Thus the level of IPM is increased in hearts with postinfarction LV remodeling both at baseline and during Iso infusion. These results suggest that the decreased myocardial ATP level in hearts with post-MI LV remodeling may be caused by the chronic depletion of the total adenine nucleotide pool.

Growth hormone induces myocardial expression of creatine transporter and decreases plasma levels of IL-1beta in rats during early postinfarct cardiac remodeling

Growth hormone has been proposed as a potential new therapeutic agent for treatment of myocardial infarction (MI) and congestive heart failure (CHF). The purpose of this study was to evaluate the effects of GH on: (a) myocardial expression of creatine transporter (CreaT) during early postinfarct remodeling, (b) myocardial levels of total creatine (TCr) and adenine pool (TAN) and (c) plasma levels of inflammatory cytokines interleukin-1beta (IL-1beta), tumor-necrosis-factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in rat model of postinfarct cardiac remodeling. Myocardial infarction (MI) was induced by ligation of the left coronary artery in male Sprague-Dawley rats (200-250 g). Three different groups were studied: MI rats treated with GH (n=11) (3 mg/kg/day), MI rats treated with saline (n=10), and sham operated rats (n=7). In the myocardium from GH treated rats the level of mRNA CreaT expression was significantly increased (p<0.01). There was no difference in TCr between the rats with MI and sham-operated rats. Treatment with GH had no effect on TCr. GH had no effect on TAN in left ventricle. All three groups had similar levels of IL-6 and TNF-alpha in plasma. In the rats with MI, treatment with GH normalized the levels of IL-1beta (p<0.05). In conclusion GH increased the expression of CreaT and decreased levels of plasma IL-1beta during postinfarct remodeling in rats. These mechanisms may be responsible for the previously reported beneficial effects of GH on myocardial energy metabolism and preservation of cardiac function in the settings of postinfarct remodeling and CHF.

Oxidative capacity in failing hearts

Although high-energy phosphate metabolism is abnormal in failing hearts [congestive heart failure (CHF)], it is unclear whether oxidative capacity is impaired. This study used the mitochondrial uncoupling agent 2,4-dinitrophenol (DNP) to determine whether reserve oxidative capacity exists during the high workload produced by catecholamine infusion in hypertrophied and failing hearts. Left ventricular hypertrophy (LVH) was produced by ascending aortic banding in 21 swine; 9 animals developed CHF. Basal myocardial phosphocreatine (PCr)/ATP measured with 31P NMR spectroscopy was decreased in both LVH and CHF hearts (corresponding to an increase in free [ADP]), whereas ATP was decreased in hearts with CHF. Infusion of dobutamine and dopamine (each 20 microg. kg-1. min-1 iv) caused an approximate doubling of myocardial oxygen consumption (MVO2) in all groups and decreased PCr/ATP in the normal and LVH groups. During continuing catecholamine infusion, DNP (2-8 mg/kg iv) caused further increases of MVO2 in normal and LVH hearts with no change in PCr/ATP. In contrast, DNP caused no increase in MVO2 in the failing hearts; the associated decrease of PCr/ATP suggests that DNP decreased the mitochondrial proton gradient, thereby causing ADP to increase to maintain adequate ATP synthesis.

Effects of angiotensin-converting enzyme inhibition on changes in left ventricular myocardial creatine kinase system after myocardial infarction: their relation to ventricular remodeling and function

We assessed the effects of angiotensin-converting enzyme (ACE) inhibition on changes in the myocardial intracellular creatine kinase (CK) system in relation to left ventricular (LV) remodeling and function in heart failure after myocardial infarction (MI) in rats. We compared the findings at 4 weeks after MI to those at 12 weeks after MI. LV weight and chamber size were significantly increased and percent fractional shortening (%FS) was decreased in untreated MI rats compared with normal control animals both at 4 and 12 weeks after MI. Animals with MI and treated with the ACE inhibitor temocapril showed significantly reduced LV weight and chamber size and increased %FS compared with untreated MI rats at 12 weeks after MI, but not at 4 weeks after MI. At 4 weeks after MI, no significant changes were found in the total creatine and relative distribution of each CK isoenzyme in either the temocapril-treated or untreated animals with MI compared with the normal controls. In contrast, at 12 weeks after MI, untreated MI rats showed significant reductions in the total creatine and mitochondrial and MM-CK fractions and increases in the MB- and BB-CK fractions compared with the controls. The alterations in the mitochondrial and MB-CK fractions were significantly attenuated after 12 weeks of ACE inhibition. Thus, LV myocardial energy metabolism is progressively impaired and its alteration is not related to the magnitude of geometric changes and LV dysfunction after MI. Most of the beneficial effects of ACE inhibition were observed at 12 weeks after MI. Our results may provide an insight into the therapeutic strategy of ACE inhibition in chronic heart failure after MI.

Creatine transporter activity and content in the rat heart supplemented by and depleted of creatine

The intracellular creatine concentration is an important bioenergetic parameter in cardiac muscle. Although creatine uptake is known to be via a NaCl-dependent creatine transporter (CrT), its localization and regulation are poorly understood. We investigated CrT kinetics in isolated perfused hearts and, by using cardiomyocytes, measured CrT content at the plasma membrane or in total lysates. Rats were fed control diet or diet supplemented with creatine or the creatine analog beta-guanidinopropionic acid (beta-GPA). Creatine transport in control hearts followed saturation kinetics with a K(m) of 70 +/- 13 mM and a V(max) of 3.7 +/- 0.07 nmol x min(-1) x g wet wt(-1). Creatine supplementation significantly decreased the V(max) of the CrT (2.7 +/- 0.17 nmol x min(-1) x g wet wt(-1)). This was matched by an approximately 35% decrease in the plasma membrane CrT; the total CrT pool was unchanged. Rats fed beta-GPA exhibited a >80% decrease in tissue creatine and increase in beta-GPA(total). The V(max) of the CrT was increased (6.0 +/- 0.25 nmol x min(-1) x g wet wt(-1)) and the K(m) decreased (39.8 +/- 3.0 mM). The plasma membrane CrT increased about fivefold, whereas the total CrT pool remained unchanged. We conclude that, in heart, creatine transport is determined by the content of a plasma membrane isoform of the CrT but not by the total cellular CrT pool.

Cardiac magnetic resonance spectroscopy

This review describes recent advances in cardiac magnetic resonance spectroscopy (MRS). MRS allows noninvasive characterization of the metabolic state of cardiac muscle, in both animal and human models. Recent experimental MRS studies have allowed new insights into the essential role of energetics in heart failure. Various new studies suggest a rapidly growing role of MRS for phenotyping new genetically modified mouse models, and recent methodologic advances include development of absolute quantification of high-energy phosphates, measurement of ATP turnover rates and thermodynamic parameters (such as free ADP and free energy change of ATP hydrolysis), and improved acquisition sequences. New patient studies demonstrate the potential value of MRS as a clinical diagnostic tool in patients with ischemic heart disease, heart failure, cardiac transplantation, valve disease, and genetic cardiomyopathy.

Imaging cardiac metabolism in heart failure: The potential of NMR spectroscopy in the era of metabolism revisited

Nuclear magnetic resonance (NMR) spectroscopy remains an extremely powerful technique for investigating abnormalities in the failing heart. The nondestructive nature of the technique allows the response to physiological, pathophysiological and pharmacological interventions to be studied within the same heart. Phosphorus-31 NMR has provided a gold standard over the past two decades for assessing the myocardial energy status both in vitro and in vivo. Carbon-13 isotopomer analysis is emerging as a direct way to monitor metabolic pathways and, in particular, investigate adaptations in energy provision in pathophysiological conditions. Using models of cardiac hypertrophy and heart failure, we investigated the sequences of changes in substrate oxidation in relation to function using 13C methods. The changes in metabolism modify the balance between energy provision and utilisation, and thus play a deleterious role in the progression towards decompensated heart failure. The application of NMR spectroscopy (phosphorus-31 and carbon-13) to the study of integrated metabolism is an area of research which is now coming into its own. Together with other new technologies, NMR will contribute to our improved understanding of cardiac metabolism in situ, leading to more rapid advances in targeting new therapeutic end points.

Absolute concentrations of high-energy phosphate metabolites in normal, hypertrophied, and failing human myocardium measured noninvasively with (31)P-SLOOP magnetic resonance spectroscopy

OBJECTIVE

The purpose of the present study was to measure absolute concentrations of phosphocreatine (PCr) and adenosine triphosphate (ATP) in normal, hypertrophied, and failing human heart.

BACKGROUND

Conflicting evidence exists on the extent of changes of high-energy phosphate metabolites in hypertrophied and failing human heart. Previous reports using phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) have quantified metabolites in relative terms only. However, this analysis cannot detect simultaneous reductions.

METHODS

Four groups of subjects (n = 10 each), were studied: volunteers and patients with hypertensive heart disease (HHD), aortic stenosis, and dilated cardiomyopathy (DCM). Left ventricular (LV) function and mass were measured by cine magnetic resonance imaging. Absolute and relative concentrations of PCr and ATP were determined by (31)P-MRS with spatial localization with optimum point spread function.

RESULTS

Left ventricular ejection fraction remained normal in HHD and aortic stenosis, but was severely reduced to 18% in DCM; LV mass was increased by 55%, 79%, and 68% respectively. In volunteers, PCr and ATP concentrations were 8.82 +/- 1.30 mmol/kg wet weight and 5.69 +/- 1.02 mmol/kg wet weight, and the PCr/ATP ratio was 1.59 +/- 0.33. High-energy phosphate levels were unaltered in HHD. In aortic stenosis, PCr was decreased by 28%, whereas ATP remained constant. In DCM, PCr was reduced by 51%, ATP by 35%, and reduction of the PCr/ATP ratio by 25% was of borderline significance (p = 0.06). Significant correlations were observed among energetic and functional variables, with the closest relations for PCr.

CONCLUSIONS

In human heart failure due to DCM, both PCr and ATP are significantly reduced. Ratios of PCr to ATP underestimate changes of high-energy phosphate levels.

Peroxynitrite-induced inhibition and nitration of cardiac myofibrillar creatine kinase

Although cardiac peroxynitrite formation and attendant protein nitration is an established event in both acute and chronic settings of cardiac failure, the putative intracellular targets involved remain incompletely defined. We have recently shown that the myofibrillar isoform of creatine kinase (a critical energetic controller of cardiomyocyte contractility) may be a particularly sensitive target of peroxynitrite-induced nitration and inactivation in vivo. However, the kinetic and mechanistic aspects of this interaction remain undefined. Here we tested the hypothesis that myofibrillar creatine kinase is sensitive to inhibition by peroxynitrite, and investigated the mechanistic role for tyrosine nitration in this process. Peroxynitrite potently and irreversibly inhibited myofibrillar creatine kinase capacity (Vmax), at concentrations as low as 100 nM, while substrate affinity (Km) was unaffected. Concentration-dependent nitration of myofibrillar creatine kinase was observed. The extent of nitration was linearly related to peroxynitrite concentration and highly correlated to the extent of myofibrillar creatine kinase inhibition. This inhibition was not reversible by treatment with free cysteine (250 microM), but pre-incubation with substrate (phosphocreatine and/or ATP) provided significant protection of MM-CK from both nitration and inhibition. These results suggest that myofibrillar creatine kinase is a highly sensitive target of peroxynitrite-mediated inhibition, and that nitration may mediate this inhibition.

Susceptibility to cardiac ischemia/reperfusion injury is modulated by chronic estrogen status

The purpose of this study was to test whether the susceptibility of the heart to ischemia/reperfusion injury is modulated by the chronic estrogen status, i.e., increased with estrogen deficiency and attenuated by pharmacologic estrogen supplementation. In addition, the study tested whether estrogen-dependent changes in mechanical function are associated with alterations of cardiac high-energy phosphate metabolism. Rats were ovariectomized, not ovariectomized, or ovariectomized and treated with subcutaneous estrogen pellets (1.5 mg/21 d) (n = 8-11 per group). Three weeks later, hearts were isolated and perfused isovolumically under constant perfusion pressure conditions. Hearts were subjected to 15 min of total global ischemia (37 degrees C) and 30 min of reperfusion. Simultaneous [31P] nuclear magnetic resonance spectra were recorded throughout this protocol to monitor changes in ATP, phosphocreatine, and inorganic phosphate content. Whereas preischemic values for heart rate, end-diastolic pressure, and coronary flow were not different among groups, left ventricular developed pressure was slightly but significantly decreased in the estrogen-treated group (p < 0.05). However, treated hearts showed improved recovery of left ventricular developed pressure on reperfusion (89 +/- 4% in control rats, 70 +/- 8% in ovariectomized hearts, and 114 +/- 9% of preischemic values in estrogen-treated rats). However, changes in ATP, phosphocreatine, and inorganic phosphate during ischemia were as previously described and were unaffected by chronic estrogen status. In conclusion, in the isolated buffer-perfused rat heart, estradiol treatment caused improved functional recovery after ischemia/reperfusion injury. This improvement, however, did not include preservation of high-energy phosphate metabolism. Other potential mechanisms include an anti-oxidant activity of 17beta-estradiol-and estrogen-induced alterations in glucose metabolism.

Mitochondrial creatine kinase is critically necessary for normal myocardial high-energy phosphate metabolism

The individual functional significance of the various creatine kinase (CK) isoenzymes for myocardial energy homeostasis is poorly understood. Whereas transgenic hearts lacking the M subunit of CK (M-CK) show unaltered cardiac energetics and left ventricular (LV) performance, deletion of M-CK in combination with loss of sarcomeric mitochondrial CK (ScCKmit) leads to significant alterations in myocardial high-energy phosphate metabolites. To address the question as to whether this alteration is due to a decrease in total CK activity below a critical threshold or due to the specific loss of ScCKmit, we studied isolated perfused hearts with selective loss of ScCKmit (ScCKmit(-/-), remaining total CK activity approximately 70%) using (31)P NMR spectroscopy at two different workloads. LV performance in ScCKmit(-/-) hearts (n = 11) was similar compared with wild-type hearts (n = 9). Phosphocreatine/ATP, however, was significantly reduced in ScCKmit(-/-) compared with wild-type hearts (1.02 +/- 0.05 vs. 1.54 +/- 0.07, P < 0.05). In parallel, free [ADP] was higher (144 +/- 11 vs. 67 +/- 7 microM, P < 0.01) and free energy release for ATP hydrolysis (DeltaG(ATP)) was lower (-55.8 +/- 0.5 vs. -58.5 +/- 0.5 kJ/mol, P < 0.01) in ScCKmit(-/-) compared with wild-type hearts. These results demonstrate that M- and B-CK containing isoenzymes are unable to fully substitute for the loss of ScCKmit. We conclude that ScCKmit, in contrast to M-CK, is critically necessary to maintain normal high-energy phosphate metabolite levels in the heart.

Myocardial energetics in cardiac hypertrophy

1. This review is presented with the intent of illustrating the representative studies of functional and myocardial energetic consequences of hearts with postinfarction left ventricular (LV) remodelling or with concentric hypertrophy and diastolic LV dysfunction in porcine models. 2. Both eccentric and concentric cardiac hypertrophy are associated with the abnormal myocardial energetics that are most severe in hearts with congestive heart failure (CHF). Presently, these abnormalities cannot be satisfactorily explained to be the cause(s) of the dysfunction of failing hearts or cause the progress from compensated cardiac hypertrophy to CHF. 3. Mechanisms governing abnormal myocardial high-energy phosphate (HEP) metabolism in hearts with cardiac hypertrophy and CHF are unclear. Myocardial energy metabolism studies use both kinetic and thermodynamic models. The thermodynamic studies examine the myocardial steady state levels of high- and low-energy phosphate, which indicate myocardial energy state or phosphorylation potential that is defined by the ratio of [ATP]/([ADP][Pi]). The kinetics studies examine the reaction velocity that is regulated by: (i) quantity and activity of the key enzymes; (ii) the concentrations of all the substrates and products; and (iii) the Michaelis-Menten constants of each substrate of the reaction. 4. Significant alterations in myocardial concentrations of phosphocreatine (PCr), ATP and ADP, myocardial oxidative phosphorylation (OXPHOS) protein expression and substrate preference are found in hearts with postinfarction LV remodelling and CHF. However, to define a causal relationship is a different matter. 5. Future studies of animal models of LV hypertrophy or heart failure using gene manipulation may provide additional insights to answer the persisting question of whether limitations of ATP synthetic or transport capacities contribute to the pathogenesis of LV remodelling or failure.

Chronic phosphocreatine depletion by the creatine analogue beta-guanidinopropionate is associated with increased mortality and loss of ATP in rats after myocardial infarction

BACKGROUND

The failing myocardium is characterized by reductions of phosphocreatine (PCr) and free creatine content and by decreases of energy reserve via creatine kinase (CK), ie, CK reaction velocity (Flux(CK)). It has remained unclear whether these changes contribute directly to contractile dysfunction. In the present study, myocardial PCr stores in a heart failure model were further depleted by feeding of the PCr analogue beta-guanidinopropionate (GP). Functional and metabolic consequences were studied.

METHODS AND RESULTS

Rats were subjected to sham operation or left coronary artery ligation (MI). Surviving rats were assigned to 4 groups and fed with 0% (n=7, Sham; n=5, MI) or 1% (n=7 Sham+GP, n=8 MI+GP) GP. Two additional groups were fed GP for 2 or 4 weeks before MI. After 8 weeks, hearts were isolated and perfused, and left ventricular pressure-volume curves were obtained. High-energy phosphate metabolism was determined with (31)P NMR spectroscopy. After GP feeding or MI, left ventricular pressure-volume curves were depressed by 33% and 32%, respectively, but GP feeding in MI hearts did not further impair mechanical function. Both MI and GP feeding reduced PCr content and Flux(CK), but here, effects were additive. In MI+GP rats, PCr levels and Flux(CK) were reduced by 87% and 94%, respectively. Although ATP levels were maintained in the GP and MI groups, ATP content was reduced by 18% in MI+GP hearts. Furthermore, 24-hour mortality in GP-prefed rats was 100%.

CONCLUSIONS

Rats with an 87% predepletion of myocardial PCr content cannot survive an acute MI. Chronically infarcted hearts subjected to additional PCr depletion cannot maintain ATP homeostasis.

(31)P-MR Spectroscopy for the evaluation of energy metabolism in intact residual myocardium after acute myocardial infarction in humans

OBJECTIVE

Experimental studies have demonstrated that acute myocardial infarction (MI) alters energy metabolism even in non-infarcted adjacent tissue. In patients with subacute MI, the influence of the regional ischemic insult on energy metabolism of intact septal myocardium was analyzed using 31P-Magnetic resonance spectroscopy (MRS).

PATIENTS AND METHODS

In eight patients with wall motion abnormalities in the anterior wall 31P-spectra were obtained from non-infarcted adjacent septal myocardium, as well as infarcted anterior myocardium (voxel size 25 ccm each) 29+/-8 days after MI using a 3D-CSI technique. Additionally, cardiac function was analyzed using breath-hold cine MRI. MRI was repeated 6 months after revascularization to assess viability of infarcted segments. Eight age-matched healthy volunteers served as control group.

RESULTS

According to follow-up MRI 4/8 patients showed regional wall motion recovery. Here, PCr/ATP-ratios were not significantly reduced in intact septal myocardium as well as infarcted anterior myocardium compared to healthy volunteers (1.28+/-0.10 and 1.14+/-0.09 vs. 1.45+/-0.29). No recovery of regional function was detected in 4/8 patients with-therefore-non-viable anterior myocardium. PCr/ATP-ratios were significantly reduced in intact and infarcted myocardium compared with healthy volunteers as well as to patients with wall motion recovery (0.77+/-0.17 and 0.49+/-0.23; P<0.05).

DISCUSSION

These preliminary results indicate that energy metabolism is reduced in patients with persisting wall motion abnormalities after myocardial infarction and revascularization in ischemically injured as well as in adjacent non-injured myocardium.

Functional aspects of creatine kinase isoenzymes in endothelial cells

To characterize the isoenzyme distribution of creatine kinase (CK) in endothelial cells (ECs) and its functional role during substrate depletion, ECs from aorta (AECs) and microvasculature (MVECs) of pig and rat were studied. In addition, high- energy phosphates were continuously monitored by (31)P NMR spectroscopy in pig AECs attached to microcarrier beads. CK activity per milligram of protein in rat AECs and MVECs (0.08 +/- 0.01 and 0.15 +/- 0.08 U/mg, respectively) was <3% of that of cardiomyocytes (6.46 +/- 1.02 U/mg). Rat and pig AECs and MVECs displayed cytosolic BB-CK, but no MM-CK. Gel electrophoresis of mitochondrial fractions of rat and pig ECs indicated the presence of mitochondrial Mi-CK, mostly in dimeric form. The presence of Mi(a)-CK was demonstrated by indirect immunofluorescence staining using Mi(a)-CK antibodies. When perifused with creatine-supplemented medium, phosphocreatine (PCr) continuously increased with time (1.2 +/- 0.6 nmol x h(-1) x mg x protein(-1)), indicating creatine uptake and CK activity. Glucose withdrawal from the medium induced a rapid decrease in PCr, which was fully reversible on glucose addition, demonstrating temporal buffering of an energy deficit. Because both cytosolic and mitochondrial CK isoforms are present in ECs, the CK system may also contribute to energy transduction ("shuttle hypothesis").

Detection of myocardial viability based on measurement of sodium content: A (23)Na-NMR study

MRI of total sodium (Na) content may allow assessment of myocardial viability, but information on Na content in normal myocardium, necrotic/scar tissue, and stunned or hibernating myocardium is lacking. Thus, the aims of the study were to: 1) quantify the temporal changes in myocardial Na content post-myocardial infarction (MI) in a rat model (Protocol 1); 2) compare Na in normally perfused, hibernating, and stunned canine myocardium (Protocol 2); and 3) determine whether, in buffer-perfused rat hearts, infarct scar can be differentiated from intact myocardium by (23)Na-MRI (Protocol 3). In Protocol 1, rats were subjected to LAD ligation. Infarct/scar tissue was excised at control and 1, 3, 7, 28, 56, and 128 days post-MI (N = 6-8 each), Na content was determined by (23)Na-NMR spectroscopy (MRS) and ion chromatography. Na content was persistently increased at all time points post-MI averaging 306*-160*% of control values (*P < 0.0083 vs. control). In Protocol 2, (23)Na-MRS of control (baseline), stunned and hibernating samples revealed no difference in Na. In Protocol 3, (23)Na-MRI revealed a mean increase in signal intensity, to 142 +/- 6% of control values, in scar tissue. A threshold of 2 standard deviations of the image intensity allowed determination of infarct size, correlating with histologically determined infarct size (r = 0.91, P < 0.0001).