Inhibition of the creatine kinase reaction decreases the contractile reserve of isolated rat hearts

Maintaining energy provision in the heart: the creatine kinase system in ischaemia-reperfusion injury and chronic heart failure

The non-stop provision of chemical energy is of critical importance to normal cardiac function, requiring the rapid turnover of ATP to power both relaxation and contraction. Central to this is the creatine kinase (CK) phosphagen system, which buffers local ATP levels to optimise the energy available from ATP hydrolysis, to stimulate energy production via the mitochondria and to smooth out mismatches between energy supply and demand. In this review, we discuss the changes that occur in high-energy phosphate metabolism (i.e., in ATP and phosphocreatine) during ischaemia and reperfusion, which represents an acute crisis of energy provision. Evidence is presented from preclinical models that augmentation of the CK system can reduce ischaemia-reperfusion injury and improve functional recovery. Energetic impairment is also a hallmark of chronic heart failure, in particular, down-regulation of the CK system and loss of adenine nucleotides, which may contribute to pathophysiology by limiting ATP supply. Herein, we discuss the evidence for this hypothesis based on preclinical studies and in patients using magnetic resonance spectroscopy. We conclude that the correlative evidence linking impaired energetics to cardiac dysfunction is compelling; however, causal evidence from loss-of-function models remains equivocal. Nevertheless, proof-of-principle studies suggest that augmentation of CK activity is a therapeutic target to improve cardiac function and remodelling in the failing heart. Further work is necessary to translate these findings to the clinic, in particular, a better understanding of the mechanisms by which the CK system is regulated in disease.

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.

Non-invasive investigation of myocardial energetics in cardiac disease using 31P magnetic resonance spectroscopy

Cardiac metabolism and function are intrinsically linked. High-energy phosphates occupy a central and obligate position in cardiac metabolism, coupling oxygen and substrate fuel delivery to the myocardium with external work. This insight underlies the widespread clinical use of ischaemia testing. However, other deficits in high-energy phosphate metabolism (not secondary to supply-demand mismatch of oxygen and substrate fuels) may also be documented, and are of particular interest when found in the context of structural heart disease. This review introduces the scope of deficits in high-energy phosphate metabolism that may be observed in the myocardium, how to assess for them, and how they might be interpreted.

Promising opportunities and potential risk of nanoparticle on the society

The ever-promising opportunities and the uses of NP in our life are increasing but their present and future potential risks on the animals, plants and microorganisms are not well discussed elsewhere. In this review, the authors have systematically discussed the toxic effect of the uses of NP on animals, plants and microorganisms including human health. They have also discussed about the bioaccumulation of these NP in the food chain. Finally, they have provided some possible suggestions for the uses of NP to reduce the detrimental effect on the environment.

Metabolic changes in hypertrophic cardiomyopathies: scientific update from the Working Group of Myocardial Function of the European Society of Cardiology

Disturbed metabolism as a consequence of obesity and diabetes may cause cardiac diseases (recently highlighted in the cardiovascular research spotlight issue on metabolic cardiomyopathies).¹ In turn, the metabolism of the heart may also be disturbed in genetic and acquired forms of hypertrophic cardiac disease. Herein, we provide an overview of recent insights on metabolic changes in genetic hypertrophic cardiomyopathy and discuss several therapies, which may be explored to target disturbed metabolism and prevent onset of cardiac hypertrophy.

This article is part of the Mini Review Series from the Varenna 2017 meeting of the Working Group of Myocardial Function of the European Society of Cardiology.

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.

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.

Dietary nitrate accelerates postexercise muscle metabolic recovery and O2 delivery in hypoxia

We tested the hypothesis that the time constants (τ) of postexercise T2* MRI signal intensity (an index of O2 delivery) and muscle [PCr] (an index of metabolic perturbation, measured by (31)P-MRS) in hypoxia would be accelerated after dietary nitrate (NO3 (-)) supplementation. In a double-blind crossover design, eight moderately trained subjects underwent 5 days of NO3 (-) (beetroot juice, BR; 8.2 mmol/day NO3 (-)) and placebo (PL; 0.003 mmol/day NO3 (-)) supplementation in four conditions: normoxic PL (N-PL), hypoxic PL (H-PL; 13% O2), normoxic NO3 (-) (N-BR), and hypoxic NO3 (-) (H-BR). The single-leg knee-extension protocol consisted of 10 min of steady-state exercise and 24 s of high-intensity exercise. The [PCr] recovery τ was greater in H-PL (30 ± 4 s) than H-BR (22 ± 4 s), N-PL (24 ± 4 s) and N-BR (22 ± 4 s) (P < 0.05) and the maximal rate of mitochondrial ATP resynthesis (Qmax) was lower in the H-PL (1.12 ± 0.16 mM/s) compared with H-BR (1.35 ± 0.26 mM/s), N-PL (1.47 ± 0.28 mM/s), and N-BR (1.40 ± 0.21 mM/s) (P < 0.05). The τ of postexercise T2* signal intensity was greater in H-PL (47 ± 14 s) than H-BR (32 ± 10 s), N-PL (38 ± 9 s), and N-BR (27 ± 6 s) (P < 0.05). The postexercise [PCr] and T2* recovery τ were correlated in hypoxia (r = 0.60; P < 0.05), but not in normoxia (r = 0.28; P > 0.05). These findings suggest that the NO3 (-)-NO2 (-)-NO pathway is a significant modulator of muscle energetics and O2 delivery during hypoxic exercise and subsequent recovery.

Mesenchymal stem cell transplantation for the infarcted heart: therapeutic potential for insulin resistance beyond the heart

Background

This study aimed to evaluate the efficacy of mesenchymal stem cell (MSC) transplantation to mitigate abnormalities in cardiac-specific and systemic metabolism mediated by a combination of a myocardial infarction and diet-induced insulin resistance.

Methods

C57BL/6 mice were high-fat fed for eight weeks prior to induction of a myocardial infarction via chronic ligation of the left anterior descending coronary artery. MSCs were administered directly after myocardial infarction induction through a single intramyocardial injection. Echocardiography was performed prior to the myocardial infarction as well as seven and 28 days post-myocardial infarction. Hyperinsulinemic-euglycemic clamps coupled with 2-[¹⁴C]deoxyglucose were employed 36 days post-myocardial infarction (13 weeks of high-fat feeding) to assess systemic insulin sensitivity and insulin-mediated, tissue-specific glucose uptake in the conscious, unrestrained mouse. High-resolution respirometry was utilized to evaluate cardiac mitochondrial function in saponin-permeabilized cardiac fibers.

Results

MSC administration minimized the decline in ejection fraction following the myocardial infarction. The greater systolic function in MSC-treated mice was associated with increased in vivo cardiac glucose uptake and enhanced mitochondrial oxidative phosphorylation efficiency. MSC therapy promoted reductions in fasting arterial glucose and fatty acid concentrations. Additionally, glucose uptake in peripheral tissues including skeletal muscle and adipose tissue was elevated in MSC-treated mice. Enhanced glucose uptake in these tissues was associated with improved insulin signalling as assessed by Akt phosphorylation and prevention of a decline in GLUT4 often associated with high-fat feeding.

Conclusions

These studies provide insight into the utility of MSC transplantation as a metabolic therapy that extends beyond the heart exerting beneficial systemic effects on insulin action.

Oxidative stress and heart failure in altered thyroid States

Increased or reduced action of thyroid hormone on certain molecular pathways in the heart and vasculature causes relevant cardiovascular derangements. It is well established that hyperthyroidism induces a hyperdynamic cardiovascular state, which is associated with a faster heart rate, enhanced left ventricular systolic and diastolic function whereas hypothyroidism is characterized by the opposite changes. Hyperthyroidism and hypothyroidism represent opposite clinical conditions, albeit not mirror images. Recent experimental and clinical studies have suggested the involvement of ROS tissue damage under altered thyroid status. Altered-thyroid state-linked changes in heart modify their susceptibility to oxidants and the extent of the oxidative damage they suffer following oxidative challenge. Chronic increase in the cellular levels of ROS can lead to a catastrophic cycle of DNA damage, mitochondrial dysfunction, further ROS generation and cellular injury. Thus, these cellular events might play an important role in the development and progression of myocardial remodeling and heart failure in altered thyroid states (hypo- and hyper-thyroidism). The present review aims at elucidating the various signaling pathways mediated via ROS and their modulation under altered thyroid state and the possibility of antioxidant therapy.

3-Methylcrotonylglycine disrupts mitochondrial energy homeostasis and inhibits synaptic Na(+),K (+)-ATPase activity in brain of young rats

Deficiency of 3-methylcrotonyl-CoA carboxylase activity is an inherited metabolic disease biochemically characterized by accumulation and high urinary excretion of 3-methylcrotonylglycine (3MCG), and also of 3-hydroisovalerate in lesser amounts. Affected patients usually have neurologic dysfunction, brain abnormalities and cardiomyopathy, whose pathogenesis is still unknown. The present study investigated the in vitro effects of 3MCG on important parameters of energy metabolism, including CO(2) production from labeled acetate, enzyme activities of the citric acid cycle, as well as of the respiratory chain complexes I-IV (oxidative phosphorylation), creatine kinase (intracellular ATP transfer), and synaptic Na(+),K(+)-ATPase (neurotransmission) in brain cortex of young rats. 3MCG significantly reduced CO(2) production, implying that this compound compromises citric acid cycle activity. Furthermore, 3MCG diminished the activities of complex II-III of the respiratory chain, mitochondrial creatine kinase and synaptic membrane Na(+),K(+)-ATPase. Furthermore, antioxidants were able to attenuate or fully prevent the inhibitory effect of 3MCG on creatine kinase and synaptic membrane Na(+),K(+)-ATPase activities. We also observed that lipid peroxidation was elicited by 3MCG, suggesting the involvement of free radicals on 3MCG-induced effects. Considering the importance of the citric acid cycle and the electron flow through the respiratory chain for brain energy production, creatine kinase for intracellular energy transfer, and Na(+),K(+)-ATPase for the maintenance of the cell membrane potential, the present data indicate that 3MCG potentially impairs mitochondrial brain energy homeostasis and neurotransmission. It is presumed that these pathomechanisms may be involved in the neurological damage found in patients affected by 3-methylcrotonyl-CoA carboxylase deficiency.

Creatine kinase-mediated improvement of function in failing mouse hearts provides causal evidence the failing heart is energy starved

ATP is required for normal cardiac contractile function, and it has long been hypothesized that reduced energy delivery contributes to the contractile dysfunction of heart failure (HF). Despite experimental and clinical HF data showing reduced metabolism through cardiac creatine kinase (CK), the major myocardial energy reserve and temporal ATP buffer, a causal relationship between reduced ATP-CK metabolism and contractile dysfunction in HF has never been demonstrated. Here, we generated mice conditionally overexpressing the myofibrillar isoform of CK (CK-M) to test the hypothesis that augmenting impaired CK-related energy metabolism improves contractile function in HF. CK-M overexpression significantly increased ATP flux through CK ex vivo and in vivo but did not alter contractile function in normal mice. It also led to significantly increased contractile function at baseline and during adrenergic stimulation and increased survival after thoracic aortic constriction (TAC) surgery-induced HF. Withdrawal of CK-M overexpression after TAC resulted in a significant decline in contractile function as compared with animals in which CK-M overexpression was maintained. These observations provide direct evidence that the failing heart is "energy starved" as it relates to CK. In addition, these data identify CK as a promising therapeutic target for preventing and treating HF and possibly diseases involving energy-dependent dysfunction in other organs with temporally varying energy demands.

Evaluation of brain and kidney energy metabolism in an animal model of contrast-induced nephropathy

Contrast-induced nephropathy is a common cause of acute renal failure in hospitalized patients, occurring from 24 to 48 h and up to 5 days after the administration of iodinated contrast media. Encephalopathy may accompany acute renal failure and presents with a complex of symptoms progressing from mild sensorial clouding to delirium and coma. The mechanisms responsible for neurological complications in patients with acute renal failure are still poorly known, but several studies suggest that mitochondrial dysfunction plays a crucial role in the pathogenesis of uremic encephalopathy. Thus, we measured mitochondrial respiratory chain complexes and creatine kinase activities in rat brain and kidney after administration of contrast media. Wistar rats were submitted to 6.0 ml/kg meglumine/sodium diatrizoate administration via the tail vein (acute renal failure induced by contrast media) and saline in an equal volume with the radiocontrast material (control group); 6 days after, the animals were killed and kidney and brain were obtained. The results showed that contrast media administration decreased complexes I and IV activities in cerebral cortex; in prefrontal cortex, complex I activity was inhibited. On the other hand, contrast media administration increased complexes I and II-III activities in hippocampus and striatum and complex IV activity in hippocampus. Moreover, that administration of contrast media also decreased creatine kinase activity in the cerebral cortex. The present findings suggest that the inhibition of mitochondrial respiratory chain complexes and creatine kinase caused by the acute renal failure induced by contrast media administration may be involved in the neurological complications reported in patients and might play a role in the pathogenesis of the encephalopathy caused by acute renal failure.

Reduced in vivo high-energy phosphates precede adriamycin-induced cardiac dysfunction

Adriamycin (ADR) is an established, life-saving antineoplastic agent, the use of which is often limited by cardiotoxicity. ADR-induced cardiomyopathy is often accompanied by depressed myocardial high-energy phosphate (HEP) metabolism. Impaired HEP metabolism has been suggested as a potential mechanism of ADR cardiomyopathy, in which case the bioenergetic decline should precede left ventricular (LV) dysfunction. We tested the hypothesis that murine cardiac energetics decrease before LV dysfunction following ADR (5 mg/kg ip, weekly, 5 injections) in the mouse. As a result, the mean myocardial phosphocreatine-to-ATP ratio (PCr/ATP) by spatially localized (31)P magnetic resonance spectroscopy decreased at 6 wk after first ADR injection (1.79 + or - 0.18 vs. 1.39 + or - 0.30, means + or - SD, control vs. ADR, respectively, P < 0.05) when indices of systolic and diastolic function by magnetic resonance imaging were unchanged from control values. At 8 wk, lower PCr/ATP was accompanied by a reduction in ejection fraction (67.3 + or - 3.9 vs. 55.9 + or - 4.2%, control vs. ADR, respectively, P < 0.002) and peak filling rate (0.56 + or - 0.12 vs. 0.30 + or - 0.13 microl/ms, control vs. ADR, respectively, P < 0.01). PCr/ATP correlated with peak filling rate and ejection fraction, suggesting a relationship between cardiac energetics and both LV systolic and diastolic dysfunction. In conclusion, myocardial in vivo HEP metabolism is impaired following ADR administration, occurring before systolic or diastolic abnormalities and in proportion to the extent of eventual contractile abnormalities. These observations are consistent with the hypothesis that impaired HEP metabolism contributes to ADR-induced myocardial dysfunction.

Role of xanthine oxidoreductase in cardiac nitroso-redox imbalance

Emerging evidence supports the importance of nitroso-redox balance in the cardiovascular system. Xanthine oxidoreductase (XOR) is a major oxidative enzyme and increased XOR activity, leading to both increased production of reactive oxygen species and uric acid, is implicated in heart failure. Within the heart, XOR activity stimulates cardiomyocyte hypertrophy, apoptosis, and impairs matrix structure. The underpinnings of these derangements can be linked not solely to oxidative stress, but may also involve the process of nitroso-redox imbalance. In this regard, XOR interacts with nitric oxide signaling at numerous levels, including a direct protein-protein interaction with neuronal nitric oxide synthase (NOS1) in the sarcoplasmic reticulum. Deficiency or translocation of NOS1 away from this microdomain leads to increased activity of XOR, which in turn impairs excitation-contraction coupling and myofilament calcium sensitivity. There is a mounting abundance of preclinical data supporting beneficial effects of inhibiting XOR, but translation to the clinic continues to be incomplete. A growing understanding of XOR and its role in nitroso-redox imbalance has great potential to lead to improved pathophysiologic insights and possibly therapeutic advances.

Relationships between regional myocardial wall stress and bioenergetics in hearts with left ventricular hypertrophy

This study utilized porcine models of postinfarction left ventricular (LV) remodeling [myocardial infarction (MI); n = 8] and concentric LV hypertrophy secondary to aortic banding (AoB; n = 8) to examine the relationships between regional myocardial contractile function (tagged MRI), wall stress (MRI and LV pressure), and bioenergetics ((31)P-magnetic resonance spectroscopy). Physiological assessments were conducted at a 4-wk time point after MI or AoB surgery. Comparisons were made with size-matched normal animals (normal; n = 8). Both MI and AoB instigated significant LV hypertrophy. Ejection fraction was not significantly altered in the AoB group, but significantly decreased in the MI group (P < 0.01 vs. normal and AoB). Systolic and diastolic wall stresses were approximately two times greater than normal in the infarct region and border zone. Wall stress in the AoB group was not significantly different from that in normal hearts. The infarct border zone demonstrated profound bioenergetic abnormalities, especially in the subendocardium, where the ratio of PCr/ATP decreased from 1.98 +/- 0.16 (normal) to 1.06 +/- 0.30 (MI; P < 0.01). The systolic radial thickening fraction and the circumferential shortening fraction in the anterior wall were severely reduced (MI, P < 0.01 vs. normal). The radial thickening fraction and circumferential shortening fraction in the AoB group were not significantly different from normal. The severely elevated wall stress in the infarct border zone was associated with a significant increase in chemical energy demand and abnormal myocardial energy metabolism. Such severe metabolic perturbations cannot support normal cardiac function, which may explain the observed regional contractile abnormalities in the infarct border zone.

Functional and bioenergetic modulations in the infarct border zone following autologous mesenchymal stem cell transplantation

Preclinical and clinical studies have demonstrated that stem cell transplantation can improve the left ventricular (LV) contractile performance, yet the underlying mechanisms remain unknown. We examined whether mesenchymal stem cell (MSC) transplantation-induced beneficial effects are secondary to paracrine-associated improvements in LV contractile performance, wall stress, and myocardial bioenergetics in hearts with postinfarction LV remodeling. Myocardial contractile function and bioenergetics were compared 4 wk after acute myocardial infarction in normal pigs ( n = 6), untreated pigs with myocardial infarction (MI group; n = 6), and pigs receiving autologous MSC transplantation (MI + MSC group; n = 5). A distal occlusion of the left anterior descending coronary artery instigated significant myocardial hypertrophy. Ejection fraction decreased from 55.3 ± 3.1% (normal) to 30.4 ± 2.3% (MI group; P < 0.01) and to 45.4 ± 3.1% (MI + MSC group; P < 0.01 vs. MI). Hearts in the MI group developed severe contractile dyskinesis in the infarct zone and border zone (BZ). MSC transplantation significantly improved contractile performance from dyskinesis to active contraction ( P < 0.01 vs. MI). BZ systolic wall stress was severely increased in MI hearts but significantly improved after MSC transplantation ( P < 0.01 vs. MI). The BZ demonstrated profound bioenergetic abnormalities in MI pigs; this was significantly improved after MSC transplantation ( P < 0.01 vs. MI). Patchy spared myocytes were found in the infarct zone of hearts receiving MSC transplantation but not in control hearts. These data demonstrate that MSC transplantation into the BZ causes significant improvements in myocardial contractile performance and reduction in wall stress, which ultimately results in significant bioenergetic improvements. Low cell engraftment indicates that MSCs did not provide a structural contribution to the damaged heart and that the observed beneficial effects likely resulted from paracrine repair mechanisms.

Inhibition of cardiac contractility by 5-hydroxydecanoate and tetraphenylphosphonium ion: a possible role of mitoKATP in response to inotropic stress

This study investigates the role of the mitochondrial ATP-sensitive K+ channel (mitoKATP) in response to positive inotropic stress. In Langendorff-perfused rat hearts, inotropy was induced by increasing perfusate calcium to 4 mM, by adding 80 microM ouabain or 0.25 microM dobutamine. Each of these treatments resulted in a sustained increase in rate-pressure product (RPP) of approximately 60%. Inhibition of mitoKATP by perfusion of 5-hydroxydecanoate (5-HD) or tetraphenylphosphonium before induction of inotropic stress resulted in a marked attenuation of RPP. Inhibition of mitoKATP after induction of stress caused the inability of the heart to maintain a high-work state. In human atrial fibers, the increase in contractility induced by dobutamine was inhibited 60% by 5-HD. In permeabilized fibers from the Langendorff-perfused rat hearts, inhibition of mitoKATP resulted, in all cases, in an alteration of adenine nucleotide compartmentation, as reflected by a 60% decrease in the half-saturation constant for ADP [K1/2 (ADP)]. We conclude that opening of cardiac mitoKATP is essential for an appropriate response to positive inotropic stress and propose that its involvement proceeds through the prevention of stress-induced decrease in mitochondrial matrix volume. These results indicate a physiological role for mitoKATP in inotropy and, by extension, in heart failure.

Effects of acute creatine kinase inhibition on metabolism and tension development in isolated single myocytes

This study investigated the effects of acute creatine kinase (CK) inhibition (CKi) on contractile performance, cytosolic Ca2+concentration ([Ca2+]c), and intracellular Po2(Pi[Formula: see text]) in Xenopus laevis isolated myocytes during a 2-min bout of isometric tetanic contractions (0.33-Hz frequency). Peak tension was similar between trials during the first contraction but was significantly ( P < 0.05) attenuated for all subsequent contractions in CKivs. control (Con). The fall in Pi[Formula: see text](ΔPi[Formula: see text]) from resting values was significantly greater in Con (26.0 ± 2.2 Torr) compared with CKi(17.8 ± 1.8 Torr). However, the ratios of Con to CKiend-peak tension (1.53 ± 0.11) and ΔPi[Formula: see text](1.49 ± 0.11) were similar, suggesting an unaltered aerobic cost of contractions. Additionally, the mean response time (MRT) of ΔPi[Formula: see text]was significantly faster in CKivs. Con during both the onset (31.8 ± 5.5 vs. 49.3 ± 5.7 s; P < 0.05) and cessation (21.2 ± 4.1 vs. 68.0 ± 3.2 s; P < 0.001) of contractions. These data demonstrate that initial phosphocreatine hydrolysis in single skeletal muscle fibers is crucial for maintenance of sarcoplasmic reticulum Ca2+release and peak tension during a bout of repetitive tetanic contractions. Furthermore, as Pi[Formula: see text]fell more rapidly at contraction onset in CKicompared with Con, these data suggest that CK activity temporally buffers the initial ATP-to-ADP concentration ratio at the transition to an augmented energetic demand, thereby slowing the initial mitochondrial activation by mitigating the energetic control signal (i.e., ADP concentration, phosphorylation potential, etc.) between sites of ATP supply and demand.

Mapping hypoxia-induced bioenergetic rearrangements and metabolic signaling by 18O-assisted 31P NMR and 1H NMR spectroscopy

Brief hypoxia or ischemia perturbs energy metabolism inducing paradoxically a stress-tolerant state, yet metabolic signals that trigger cytoprotection remain poorly understood. To evaluate bioenergetic rearrangements, control and hypoxic hearts were analyzed with 18O-assisted 31P NMR and 1H NMR spectroscopy. The 18O-induced isotope shift in the 31P NMR spectrum of CrP, betaADP and betaATP was used to quantify phosphotransfer fluxes through creatine kinase and adenylate kinase. This analysis was supplemented with determination of energetically relevant metabolites in the phosphomonoester (PME) region of 31P NMR spectra, and in both aromatic and aliphatic regions of 1H NMR spectra. In control conditions, creatine kinase was the major phosphotransfer pathway processing high-energy phosphoryls between sites of ATP consumption and ATP production. In hypoxia, creatine kinase flux was dramatically reduced with a compensatory increase in adenylate kinase flux, which supported heart energetics by regenerating and transferring beta- and gamma-phosphoryls of ATP. Activation of adenylate kinase led to a build-up of AMP, IMP and adenosine, molecules involved in cardioprotective signaling. 31P and 1H NMR spectral analysis further revealed NADH and H+ scavenging by alpha-glycerophosphate dehydrogenase (alphaGPDH) and lactate dehydrogenase contributing to maintained glycolysis under hypoxia. Hypoxia-induced accumulation of alpha-glycerophosphate and nucleoside 5'-monophosphates, through alphaGPDH and adenylate kinase reactions, respectively, was mapped within the increased PME signal in the 31P NMR spectrum. Thus, 18O-assisted 31P NMR combined with 1H NMR provide a powerful approach in capturing rearrangements in cardiac bioenergetics, and associated metabolic signaling that underlie the cardiac adaptive response to stress.

Inhibition of mitochondrial creatine kinase activity by D-2-hydroxyglutaric acid in cerebellum of young rats

D-2-Hydroxyglutaric aciduria (DHGA) is a neurometabolic disorder biochemically characterized by tissue accumulation and excretion of high amounts of D-2-hydroxyglutaric acid (DGA). Although the affected patients have predominantly severe neurological findings, the underlying mechanisms of brain injury are virtually unknown. In previous studies we have demonstrated that DGA, at concentrations as low as 0.25 mM, significantly decreased creatine kinase activity and other parameters of energy metabolism in cerebral cortex of young rats. In the present study, we investigated the effect of DGA (0.25-5 mM) on total creatine kinase (tCK) activity, as well as on CK activity in cytosolic (Cy-CK) and mitochondrial (Mi-CK) preparations from cerebellum of 30-day-old Wistar rats in order to test whether the inhibitory effect of DGA on CK was tissue specific. We verified that tCK (22% inhibition) and Mi-CK (40% inhibition) activities were moderately inhibited by DGA at concentrations of 2.5 mM and higher, in contrast to Cy-CK, which was not affected by the acid. Kinetic studies revealed that the inhibitory effect of DGA was noncompetitive in relation to phosphocreatine. We also observed that this inhibition was fully prevented by preincubation of the homogenates with reduced glutathione, suggesting that the inhibition of CK activity by DGA is possibly mediated by modification of essential thiol groups of the enzyme. Our present results therefore demonstrate a relatively weak inhibitory effect of DGA on cerebellum Mi-CK activity, as compared to that provoked in cerebral cortex, and may possibly be related to the neuropathology of DHGA, characterized by cerebral cortex abnormalities.

L-2-hydroxyglutaric acid inhibits mitochondrial creatine kinase activity from cerebellum of developing rats

L-2-Hydroxyglutaric acid (LGA) is the biochemical hallmark of patients affected by the neurometabolic disorder known as L-2-hydroxyglutaric aciduria (LHGA). Although this disorder is predominantly characterized by severe neurological findings and pronounced cerebellum atrophy, the neurotoxic mechanisms of brain injury are virtually unknown. In the present study, we investigated the effect of LGA, at 0.25-5mM concentrations, on total creatine kinase (tCK) activity from cerebellum, cerebral cortex, cardiac muscle and skeletal muscle homogenates of 30-day-old Wistar rats. CK activity was measured also in the cytosolic (Cy-CK) and mitochondrial (Mi-CK) fractions from cerebellum. We verified that tCK activity was significantly inhibited by LGA in the cerebellum, but not in cerebral cortex, cardiac muscle and skeletal muscle. Furthermore, CK activity from the mitochondrial fraction was inhibited by LGA, whereas that from the cytosolic fraction of cerebellum was not affected by the acid. Kinetic studies revealed that the inhibitory effect of LGA on Mi-CK was non-competitive in relation to phosphocreatine. Finally, we verified that the inhibitory effect of LGA on tCK was fully prevented by pre-incubation of the homogenates with reduced glutathione (GSH), suggesting that this inhibition is possibly mediated by oxidation of essential thiol groups of the enzyme. Considering the importance of creatine kinase activity for energy homeostasis, our results suggest that the selective inhibition of this enzyme activity by increased levels of LGA could be possibly related to the cerebellar degeneration characteristically found in patients affected by L-2-hydroxyglutaric aciduria.

Phosphocreatine content of freeze-clamped muscle: influence of creatine kinase inhibition

The study of cellular energetics is critically dependent on accurate measurement of high-energy phosphates. Muscle values of phosphocreatine (PCr) vary greatly between in vivo measurements (i.e., by nuclear magnetic resonance) and chemical measurements determined from muscles isolated and quick-frozen. The source of this difference has not been experimentally identified. A likely cause is activation of ATPases and phosphotransfer from PCr to ADP. Therefore, rat hindlimb skeletal muscle was perfused either with or without 2 mM iodoacetamide, a creatine kinase inhibitor, and muscle was freeze-clamped either at rest or after contraction. Creatine kinase inhibition resulted in approximately 6 micromol/g higher PCr and lower creatine in the freeze-clamped soleus, red gastrocnemius, and white gastrocnemius. This PCr content difference was reduced when the initial PCr content was decreased with prior contractions. Therefore, the amount of PCr artifact appears to scale with initial PCr content within a fiber-type section. This artifact directly affects the measurement and, thus, the calculations of muscle energetic parameters from studies using isolated and frozen muscle.

Ethylmalonic acid inhibits mitochondrial creatine kinase activity from cerebral cortex of young rats in vitro

Short-chain acyl-CoA dehydrogenase (SCAD) deficiency is an inherited metabolic disorder biochemically characterized by tissue accumulation of predominantly ethylmalonic acid (EMA) and clinically by neurological dysfunction. In the present study we investigated the in vitro effects of EMA on the activity of the mitochondrial (Mi-CK) and cytosolic (Cy-CK) creatine kinase isoforms from cerebral cortex, skeletal muscle, and cardiac muscle of young rats. CK activities were measured in the mitochondrial and cytosolic fractions prepared from whole-tissue homogenates of 30-day-old Wistar rats. The acid was added to the incubation medium at concentrations ranging from 0.5 to 2.5 mM. EMA had no effect on Cy-CK activity, but significantly inhibited the activity of Mi-CK at 1.0 mM and higher concentrations in the brain. In contrast, both Mi-CK and Cy-CK from skeletal muscle and cardiac muscle were not affected by the metabolite. We also evaluated the effect of the antioxidants glutathione (GSH), ascorbic acid, and alpha-tocopherol and the nitric oxide synthase inhibitor L-NAME on the inhibitory action of EMA on cerebral cortex Mi-CK activity. We observed that the drugs did not modify Mi-CK activity per se, but GSH and ascorbic acid prevented the inhibitory effect of EMA when co-incubated with the acid. In contrast, L-NAME and alpha-tocopherol could not revert the inhibition provoked by EMA on Mi-CK activity. Considering the importance of CK for brain energy homeostasis, it is proposed that the inhibition of Mi-CK activity may be associated to the neurological symptoms characteristic of SCAD deficiency.

Inhibition of creatine kinase activity in vitro by ethylmalonic acid in cerebral cortex of young rats

Short-chain acyl-CoA dehydrogenase deficiency is an inherited metabolic disorder biochemically characterized by tissue accumulation of ethylmalonic (EMA) and methylsuccinic (MSA) acids and clinically by severe neurological symptoms. In the present study we investigated the in vitro effects of EMA and MSA on the activity of creatine kinase (CK) in homogenates from cerebral cortex, skeletal and cardiac muscle of rats. EMA significantly inhibited CK activity from cerebral cortex, but did not affect this activity in skeletal and cardiac muscle. Furthermore, MSA had no effect on this enzyme in all tested tissues. Glutathione (GSH), ascorbic acid and alpha-tocopherol, and the nitric oxide synthase inhibitor L-NAME, did not affect the enzyme activity per se, but GSH fully prevented the inhibitory effect of EMA when co-incubated with EMA. In contrast, alpha-tocopherol, ascorbic acid and L-NAME did not influence the inhibitory effect of the acid. The data suggest that inhibition of brain CK activity by EMA is possibly mediated by oxidation of essential groups of the enzyme, which are protected by the potent intracellular, endogenous, naturally occurring antioxidant GSH.

The effect of Mg2+ on cardiac muscle function: Is CaATP the substrate for priming myofibril cross-bridge formation and Ca2+ reuptake by the sarcoplasmic reticulum?

Kinetics are established for the activation of the myofibril from the relaxed state [Smith, Dixon, Kirschenlohr, Grace, Metcalfe and Vandenberg (2000) Biochem. J. 346, 393-402]. These require two troponin Ca2+-binding sites, one for each myosin head, to act as a single unit in initial cross-bridge formation. This defines the first, or activating, ATPase reaction, as distinct from the further activity of the enzyme that continues when a cross-bridge to actin is already established. The pairing of myosin heads to act as one unit suggests a possible alternating mechanism for muscle action. A large positive inotropic (contraction-intensifying) effect of loading the Mg2+ chelator citrate, via its acetoxymethyl ester, into the heart has confirmed the competitive inhibition of the Ca2+ activation by Mg2+, previously seen in vitro. In the absence of a recognized second Ca2+ binding site on the myofibril, with appropriate binding properties, the bound ATP is proposed as the second activating Ca2+-binding site. As ATP, free or bound to protein, can bind either Mg2+ or Ca2+, this leads to competitive inhibition by Mg2+. Published physico-chemical studies on skeletal muscle have shown that CaATP is potentially a more effective substrate than MgATP for cross-bridge formation. The above considerations allow calculation of the observed variation of fractional activation by Ca2+ as a function of [Mg2+] and in turn reveal simple Michaelis-Menten kinetics for the activation of the ATPase by sub-millimolar [Mg2+]. Furthermore the ability of bound ATP to bind either cation, and the much better promotion of cross-bridge formation by CaATP binding, give rise to the observed variation of the Hill coefficient for Ca2+ activation with altered [Mg2+]. The inclusion of CaADP within the initiating cross-bridge and replacement by MgADP during the second cycle is consistent with the observed fall in the rate of the myofibril ATPase that occurs after two phosphates are released. The similarity of the kinetics of the cardiac sarcoplasmic reticulum ATPase to those of the myofibril, in particular the positive co-operativity of both Mg2+ inhibition and Ca2+ activation, leads to the conclusion that this ATPase also has an initiation step that utilizes CaATP. The first-order activation by sub-millimolar [Mg2+], similar to that of the myofibril, may be explained by Mg2+ involvement in the phosphate-release step of the ATPase. The inhibition of both the myofibril and sarcoplasmic reticulum Ca2+ transporting ATPases by Mg2+ offers an explanation for the specific requirement for phosphocreatine (PCr) for full activity of both enzymes in situ and its effect on their apparent affinities for ATP. This explanation is based on the slow diffusion of Mg2+ within the myofibril and on the contrast of PCr with both ATP and phosphoenolpyruvate, in that PCr does not bind Mg2+ under physiological conditions, whereas both the other two bind it more tightly than the products of their hydrolysis do. The switch to supply of energy by diffusion of MgATP into the myofibril when depletion of PCr raises [ATP]/[PCr] greatly, e.g. during anoxia, results in a local [Mg2+] increase, which inhibits the ATPase. It is possible that mechanisms similar to those described above occur in skeletal muscle but the Ca2+ co-operativity involved would be masked by the presence of two Ca2+ binding sites on each troponin.

Short-term hibernation in adult cardiomyocytes is PO(2) dependent and Ca(2+) mediated

The mechanism of myocardial hibernation, the reversible downregulation of contractile activity on reduction of coronary flow with unchanged cardiac energetics, is presently not understood. The oxygen consumption (VO(2)), shortening fraction (DeltaL), energy status [phosphocreatine (PCr), ATP, and adenosine and lactate release], and free intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured in isolated rat cardiomyocytes at precisely controlled ambient PO(2) (Oxystat). When PO(2) was reduced from 25 to 6 mmHg, VO(2) decreased by 50%, while DeltaL was downregulated from 11.2 +/- 4.1 to 7.6 +/- 4.0%, and energy status was unchanged in the steady state (observation time 12 min). Only transiently PCr decreased, and lactate and adenosine release increased. Further reduction of PO(2) (to 3 mmHg) reduced VO(2) by 80%, decreased PCr by 35%, moderately increased adenosine and lactate release, and progressively reduced DeltaL by 50% (to 5.6 +/- 3.3%). All parameters fully recovered during reoxygenation. PO(2)-dependent downregulation of DeltaL was accompanied by a progressive reduction in systolic [Ca(2+)](i) (from 512 +/- 110 to 357 +/- 91 nmol/l at 6 mmHg and to 251 +/- 69 nmol/l at 3 mmHg), whereas diastolic free [Ca(2+)](i) remained unchanged. Therefore, the mechanism of the reversible, PO(2)-dependent downregulation of contractile activity (myocardial hibernation) involves a substantial reduction of systolic calcium.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Regulation of mitochondrial respiration in heart cells analyzed by reaction-diffusion model of energy transfer

The purpose of this study is to investigate theoretically which intracellular factors may be important for regulation of mitochondrial respiration in working heart cells in vivo. We have developed a model that describes quantitatively the published experimental data on dependence of the rate of oxygen consumption and metabolic state of working isolated perfused rat heart on workload over its physiological range (Williamson JR, Ford G, Illingworth J, Safer B. Circ Res 38, Suppl I, I39-I51, 1976). Analysis of this model shows that for phosphocreatine, creatine, and ATP the equilibrium assumption is an acceptable approximation with respect to their diffusion in the intracellular bulk water phase. However, the ADP concentration changes in the contraction cycle in a nonequilibrium workload-dependent manner, showing the existence of the intracellular concentration gradients. The model shows that workload-dependent alteration of ADP concentration in the compartmentalized creatine kinase system may be taken, together with the changes in P(i) concentration, to be among the major components of the metabolic feedback signal for regulation of respiration in muscle cells.

Myofibrillar disruption in hypocontractile myocardium showing perfusion-contraction matches and mismatches

Chronically instrumented dogs underwent 2- or 5-h regional reductions in coronary flow that were followed, respectively, by balanced reductions in myocardial contraction and O(2) consumption ("hibernation") and persistently reduced contraction despite normal myocardial O(2) consumption ("stunning"). Previously unidentified myofibrillar disruption developed during flow reduction in both experimental models and persisted throughout the duration of reperfusion (2-24 h). Aberrant perinuclear aggregates that resembled thick filaments and stained positively with a monoclonal myosin antibody were present in 34 +/- 3.8% (SE) and 68 +/- 5.9% of "hibernating" and "stunned" subendocardial myocytes in areas subjected to flow reduction and in 16 +/- 2.5% and 44 +/- 7.4% of subendocardial myocytes in remote areas of the same ventricles. Areas of myofibrillar disruption also showed glycogen accretion and unusual heterochromatin clumping adjacent to the inner nuclear envelope. The degrees of flow reduction employed were sufficient to reduce regional myofibrillar creatine kinase activity by 25-35%, but troponin I degradation was not evident. The observed changes may reflect an early, possibly reversible, phase of the myofibrillar loss characteristic of hypocontractile myocardium in patients undergoing revascularization.

Age peculiarities of changes in the contractile function of isolated rat hearts during prolonged perfusion

The experiments on the isolated hearts from adult and old rats have shown that during first hours of prolonged perfusion the contractile function decreased to a greater degree in adult rats compared to the old. Following a 3-h perfusion, there occurred a marked fall of the contractile function in old animals. The creatine phosphokinase activity in a perfusate grew sharply in the old rats and changed insignificantly in the adult. The Na,K-ATPase got activated in adult rats, which was prevented by the preliminary injection of actinomycin D, and declined in the old. It appeared that adult hearts had a better endurance of prolonged perfusion and kept sustaining a definite level of performance almost twice as long as the old. Under prolonged perfusion conditions, there has been synthesized a regulatory factor (invertor) in the heart of adult animals, which reduced the myocardial contractile function. The latter was evidenced from a fact that perfusate from adult hearts 'donors' promoted the decrease of contractility of the adult hearts 'recipients'. The preliminary injection of actinomycin D to adult rats prevented a developing decrease of the contractile function but did not induce any changes in the old. This permits to conclude that synthesis of the regulatory peptide factor promotes an economization and adaptation of the adult animal heart.

Role of MgADP in the development of diastolic dysfunction in the intact beating rat heart

Sarcomere relaxation depends on dissociation of actin and myosin, which is regulated by a number of factors, including intracellular [MgATP] as well as MgATP hydrolysis products [MgADP] and inorganic phosphate [Pi], pHi, and cytosolic calcium concentration ([Ca2+]c). To distinguish the contribution of MgADP from the other regulators in the development of diastolic dysfunction, we used a strategy to increase free [MgADP] without changing [MgATP], [Pi], or pHi. This was achieved by applying a low dose of iodoacetamide to selectively inhibit the creatine kinase activity in isolated perfused rat hearts. [MgATP], [MgADP], [Pi], and [H+] were determined using 31P NMR spectroscopy. The [Ca2+]c and the glycolytic rate were also measured. We observed an approximately threefold increase in left ventricular end diastolic pressure (LVEDP) and 38% increase in the time constant of pressure decay (P < 0.05) in these hearts, indicating a significant impairment of diastolic function. The increase in LVEDP was closely related to the increase in free [MgADP]. Rate of glycolysis was not changed, and [Ca2+]c increased by 16%, which cannot explain the severity of diastolic dysfunction. Thus, our data indicate that MgADP contributes significantly to diastolic dysfunction, possibly by slowing the rate of cross-bridge cycling.

Cardiac phosphocreatine deficiency induced by GPA during postnatal development in rat

The effect of chronic administration of beta-guanidinopropionic acid (GPA) on the protein profiling, energy metabolism and right ventricular (RV) function was studied in the rat heart during the weaning and adolescence period. GPA was given in tap water (1-1.5%) using pair drink controls. The feeding of animals with GPA solution for a six week period resulted in elevation of heart to body weight ratio due to body growth retardation. GPA accumulated in the myocardium up to 67.37 +/- 5.3 mumoles.g dry weight and the tissue content of total creatine, phosphocreatine and ATP was significantly decreased to 15%, 9% and 65% of control values respectively. Total activity of creatine kinase (CK) was not changed, but the proportion of mitochondrial (Mi) CK isoenzyme was decreased; the percentage of MB isoenzyme of CK was significantly higher. GPA treatment resulted in an elevation of the content of cardiac collagenous proteins and decrease of non-collagenous proteins in the heart; in parallel, a decrease of the collagen I to collagen III ratio was detected. The function of the RV was assessed using an isolated perfused heart with RV performing pressure-volume work. As compared to pair-drink controls, RV function was significantly impaired the GPA group: at any given right atrial filling pressure, the RV systolic pressure and the rate of pressure development were decreased by almost a factor of two. Elevation of the RV diastolic pressure with increasing pulmonary artery diastolic pressure was also significantly steeper in the GPA group which also showed decrease of cardiac output, especially at high outflow resistance. It may be assumed that chronic administration of GPA deeply influenced metabolic parameters, protein profiles and contractile function of the developing heart. On the other hand, concentrations of glucose, total lipids and triglycerides in blood plasma were not affected. All these data confirm the concept that the CK system is of central importance both for heart function and for the regulation of normal growth of cardiac myocytes.

Nitric oxide inhibits creatine kinase and regulates rat heart contractile reserve

Cardiac myocytes express both constitutive and cytokine-inducible nitric oxide syntheses (NOS). NO and its congeners have been implicated in the regulation of cardiac contractile function. To determine whether NO could affect myocardial energetics, 31P NMR spectroscopy was used to evaluate high-energy phosphate metabolism in isolated rat hearts perfused with the NO donor S-nitrosoacetylcysteine (SNAC). All hearts were exposed to an initial high Ca2+ (3.5 mM) challenge followed by a recovery period, and then, either in the presence or absence of SNAC, to a second high Ca2+ challenge. This protocol allowed us to monitor simultaneously the effect of SNAC infusion on both contractile reserve (i.e., baseline versus high workload contractile function) and high-energy phosphate metabolism. The initial high Ca2+ challenge caused the rate-pressure product to increase by 74 +/- 5% in all hearts. As expected, ATP was maintained as phosphocreatine (PCr) content briefly dropped and then returned to baseline during the subsequent recovery period. Control hearts responded similarLy to the second high Ca2+ challenge, but SNAC-treated hearts did not demonstrate the expected increase in rate-pressure product. In these hearts, ATP declined significantly during the second high Ca2+ challenge, whereas phosphocreatine did not differ from controls, suggesting that phosphoryl transfer by creatine kinase (CK) was inhibited. CK activity, measured biochemically, was decreased by 61 +/- 13% in SNAC-treated hearts compared to controls. Purified CK in solution was also inhibited by SNAC, and reversal could be accomplished with DTT, a sulfhydryl reducing agent. Thus, NO can regulate contractile reserve, possibly by reversible nitrosothiol modification of CK.

Decreased energy reserve in an animal model of dilated cardiomyopathy. Relationship to contractile performance

An animal model was used to test the hypothesis that in heart failure the decrease in the ability to resynthesize ATP through the creatine kinase (CK) reaction (which we call energy reserve) contributes to the inability of the heart to maintain its normal function and contractile reserve. One-week-old turkey poults were fed furazolidone for 14 days to induce dilated cardiomyopathy. Isolated Langendorff-perfused hearts from these myopathic animals showed a 73% decrease in baseline isovolumic contractile performance. Neither increasing [Ca2+]o nor electrical pacing rate increased isovolumic contractile performance. Measured by 31P nuclear magnetic resonance magnetization transfer and chemical assay, ATP concentration was decreased by 23%, phosphocreatine concentration by 42%, CK enzyme activity by 34%, and the pseudo first-order rate constant for the CK reaction by 50%. Measured CK reaction velocity decreased by 71%. The reduced ability to increase cardiac performance in response to increasing [Ca2+]o in hearts with lower CK reaction velocity was reproduced in part by feeding a separate group of turkey poults beta-guanidino-propionic acid to specifically reduce CK reaction velocity by decreasing guanidino substrate concentration. These hearts had normal baseline performance but blunted contractile reserve. These observations provide further support for the hypothesis that a decrease in energy reserve via the CK system contributes to reduced cardiac function in the failing heart.