Bioenergetic consequences of cardiac phosphocreatine depletion induced by creatine analogue feeding

Myocardial Metabolism in Heart Failure

PURPOSE OF REVIEW

Myocardial metabolism is intricately linked to cardiac function. Perturbations of cardiac energy metabolism result in an energy-starved heart and the development of contractile dysfunction. In this review, we discuss alterations in myocardial energy supply, transcriptional changes in response to different energy demands, and mitochondrial function in the development of heart failure.

RECENT FINDINGS

Recent studies on substrate modulation through modifying energy substrate supply have shown cardioprotective properties. In addition, large cardiovascular outcome trials of anti-diabetic agents have demonstrated prognostic benefit, suggesting the importance of myocardial metabolism in cardiac function. Understanding molecular and transcriptional controls of cardiac metabolism promises new research avenues for metabolic treatment targets. Future studies assessing the impact of substrate modulation on cardiac energetic status and function will better inform development of metabolic therapies.

Metabolomic profiling in dogs with dilated cardiomyopathy eating non-traditional or traditional diets and in healthy controls

Dilated cardiomyopathy (DCM), caused by genetic and environmental factors, usually progresses to heart failure, a major cause of death in elderly people. A diet-associated form of DCM was recently identified in pet dogs eating non-traditional (NT) diets. To identify potential dietary causes, we analyzed metabolomic signatures and gene set/pathway enrichment in (1) all dogs based on disease, diet, and their interactions and (2) dogs with DCM based on diet. Metabolomic analysis was performed in 38 dogs with DCM eating NT diets (DCM-NT), 8 dogs with DCM eating traditional diets, 12 healthy controls eating NT diets, and 17 healthy controls eating traditional diets. Overall, 153 and 63 metabolites differed significantly between dogs with DCM versus healthy controls and dogs eating NT versus traditional diets, respectively, with 12 metabolites overlapping both analyses. Protein-protein interaction networks and gene set enrichment analysis identified 105 significant pathways and gene sets including aging-related pathways (e.g., nuclear factor-kappa B, oxidative damage, inflammation). Seventeen metabolites differed significantly in dogs with DCM eating NT versus traditional diets (e.g., fatty acids, amino acids, legume biomarkers), suggesting different mechanisms for primary versus diet-associated DCM. Our multifaceted metabolomic assessment of DCM in dogs highlighted diet's role in some forms of DCM.

Creatine deficiency and heart failure

Impaired cardiac energy metabolism has been proposed as a mechanism common to different heart failure aetiologies. The energy-depletion hypothesis was pursued by several researchers, and is still a topic of considerable interest. Unlike most organs, in the heart, the creatine kinase system represents a major component of the metabolic machinery, as it functions as an energy shuttle between mitochondria and cytosol. In heart failure, the decrease in creatine level anticipates the reduction in adenosine triphosphate, and the degree of myocardial phosphocreatine/adenosine triphosphate ratio reduction correlates with disease severity, contractile dysfunction, and myocardial structural remodelling. However, it remains to be elucidated whether an impairment of phosphocreatine buffer activity contributes to the pathophysiology of heart failure and whether correcting this energy deficit might prove beneficial. The effects of creatine deficiency and the potential utility of creatine supplementation have been investigated in experimental and clinical models, showing controversial findings. The goal of this article is to provide a comprehensive overview on the role of creatine in cardiac energy metabolism, the assessment and clinical value of creatine deficiency in heart failure, and the possible options for the specific metabolic therapy.

Impaired cardiac contractile function in arginine:glycine amidinotransferase knockout mice devoid of creatine is rescued by homoarginine but not creatine

Aims

Creatine buffers cellular adenosine triphosphate (ATP) via the creatine kinase reaction. Creatine levels are reduced in heart failure, but their contribution to pathophysiology is unclear. Arginine:glycine amidinotransferase (AGAT) in the kidney catalyses both the first step in creatine biosynthesis as well as homoarginine (HA) synthesis. AGAT-/- mice fed a creatine-free diet have a whole body creatine-deficiency. We hypothesized that AGAT-/- mice would develop cardiac dysfunction and rescue by dietary creatine would imply causality.

Methods and results

Withdrawal of dietary creatine in AGAT-/- mice provided an estimate of myocardial creatine efflux of ∼2.7%/day; however, in vivo cardiac function was maintained despite low levels of myocardial creatine. Using AGAT-/- mice naïve to dietary creatine we confirmed absence of phosphocreatine in the heart, but crucially, ATP levels were unchanged. Potential compensatory adaptations were absent, AMPK was not activated and respiration in isolated mitochondria was normal. AGAT-/- mice had rescuable changes in body water and organ weights suggesting a role for creatine as a compatible osmolyte. Creatine-naïve AGAT-/- mice had haemodynamic impairment with low LV systolic pressure and reduced inotropy, lusitropy, and contractile reserve. Creatine supplementation only corrected systolic pressure despite normalization of myocardial creatine. AGAT-/- mice had low plasma HA and supplementation completely rescued all other haemodynamic parameters. Contractile dysfunction in AGAT-/- was confirmed in Langendorff perfused hearts and in creatine-replete isolated cardiomyocytes, indicating that HA is necessary for normal cardiac function.

Conclusions

Our findings argue against low myocardial creatine per se as a major contributor to cardiac dysfunction. Conversely, we show that HA deficiency can impair cardiac function, which may explain why low HA is an independent risk factor for multiple cardiovascular diseases.

Does young age really put the heart at risk?

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

The location of energetic compartments affects energetic communication in cardiomyocytes

The heart relies on accurate regulation of mitochondrial energy supply to match energy demand. The main regulators are Ca²⁺ and feedback of ADP and P_i. Regulation via feedback has intrigued for decades. First, the heart exhibits a remarkable metabolic stability. Second, diffusion of ADP and other molecules is restricted specifically in heart and red muscle, where a fast feedback is needed the most. To explain the regulation by feedback, compartmentalization must be taken into account. Experiments and theoretical approaches suggest that cardiomyocyte energetic compartmentalization is elaborate with barriers obstructing diffusion in the cytosol and at the level of the mitochondrial outer membrane (MOM). A recent study suggests the barriers are organized in a lattice with dimensions in agreement with those of intracellular structures. Here, we discuss the possible location of these barriers. The more plausible scenario includes a barrier at the level of MOM. Much research has focused on how the permeability of MOM itself is regulated, and the importance of the creatine kinase system to facilitate energetic communication. We hypothesize that at least part of the diffusion restriction at the MOM level is not by MOM itself, but due to the close physical association between the sarcoplasmic reticulum (SR) and mitochondria. This will explain why animals with a disabled creatine kinase system exhibit rather mild phenotype modifications. Mitochondria are hubs of energetics, but also ROS production and signaling. The close association between SR and mitochondria may form a diffusion barrier to ADP added outside a permeabilized cardiomyocyte. But in vivo, it is the structural basis for the mitochondrial-SR coupling that is crucial for the regulation of mitochondrial Ca²⁺-transients to regulate energetics, and for avoiding Ca²⁺-overload and irreversible opening of the mitochondrial permeability transition pore.

The effect of the creatine analogue beta-guanidinopropionic acid on energy metabolism: a systematic review

Background

Creatine kinase plays a key role in cellular energy transport. The enzyme transfers high-energy phosphoryl groups from mitochondria to subcellular sites of ATP hydrolysis, where it buffers ADP concentration by catalyzing the reversible transfer of the high-energy phosphate moiety (P) between creatine and ADP. Cellular creatine uptake is competitively inhibited by beta-guanidinopropionic acid. This substance is marked as safe for human use, but the effects are unclear. Therefore, we systematically reviewed the effect of beta-guanidinopropionic acid on energy metabolism and function of tissues with high energy demands.

Methods

We performed a systematic review and searched the electronic databases Pubmed, EMBASE, the Cochrane Library, and LILACS from their inception through March 2011. Furthermore, we searched the internet and explored references from textbooks and reviews.

Results

After applying the inclusion criteria, we retrieved 131 publications, mainly considering the effect of chronic oral administration of beta-guanidinopropionic acid (0.5 to 3.5%) on skeletal muscle, the cardiovascular system, and brain tissue in animals. Beta-guanidinopropionic acid decreased intracellular creatine and phosphocreatine in all tissues studied. In skeletal muscle, this effect induced a shift from glycolytic to oxidative metabolism, increased cellular glucose uptake and increased fatigue tolerance. In heart tissue this shift to mitochondrial metabolism was less pronounced. Myocardial contractility was modestly reduced, including a decreased ventricular developed pressure, albeit with unchanged cardiac output. In brain tissue adaptations in energy metabolism resulted in enhanced ATP stability and survival during hypoxia.

Conclusion

Chronic beta-guanidinopropionic acid increases fatigue tolerance of skeletal muscle and survival during ischaemia in animal studies, with modestly reduced myocardial contractility. Because it is marked as safe for human use, there is a need for human data.

Energy metabolism in heart failure and remodelling

Myocytes of the failing heart undergo impressive metabolic remodelling. The time line for changes in the pathways for ATP synthesis in compensated hypertrophy is: flux through the creatine kinase (CK) reaction falls as both creatine concentration ([Cr]) and CK activity fall; increases in [ADP] and [AMP] lead to increases in glucose uptake and utilization; fatty acid oxidation either remains the same or decreases. In uncompensated hypertrophy and in other forms of heart failure, CK flux and fatty acid oxidation are both lower; any increases in glucose uptake and utilization are not sufficient to compensate for overall decreases in the capacity for ATP supply and [ATP] falls. Metabolic remodelling is under transcriptional and post-transcriptional control. The lower metabolic reserve of the failing heart contributes to impaired contractile reserve.

Creatine kinase inhibitor iodoacetamide antagonizes calcium-stimulated inotropy in cardiomyocytes

1. Inhibition of creatine kinase is known to suppress cardiac contractile reserve in intact hearts, although the underlying mechanism has not been elucidated. 2. The present study was designed to examine whether cardiac depression induced by creatine kinase inhibition was due to action at the level of the essential contractile element, namely cardiomyocytes. Adult rat cardiomyocytes were perfused with the creatine kinase inhibitor iodoacetamide (90 micromol/L) for 90 min. Mechanical and intracellular Ca(2+) properties were evaluated using edge-detection and fluorescence microscopy, respectively. Myocytes were superfused with normal (1.3 mmol/L) or high (3.3 mmol/L) extracellular Ca(2+) contractile buffer. Mechanical function was examined, including peak shortening (PS), maximal velocity of shortening/relengthening (+/-dL/dt), time to 90% PS (TPS(90)), time to 90% relengthening (TR(90)) and integration of shortening/relengthening (normalized to PS). Intracellular Ca(2+) transients were evaluated using the following indices: resting and rise of fura-2 fluorescence intensity (Delta FFI) and intracellular Ca(2+) decay time constant. 3. The results indicate that elevated extracellular Ca(2+) stimulated cardiomyocyte positive inotrope, manifested as increased PS, +/-dL/dt, area of shortening, resting FFI and Delta FFI associated with a shortened TR(90) and intracellular Ca(2+) decay time constant. High extracellular Ca(2+) did not affect TPS(90) and area of relengthening. Iodoacetamide ablated high Ca(2+)-induced increases in PS, +/-dL/dt, area of shortening, resting FFI, Delta FFI and shortened TR(90) and intracellular Ca(2+) decay time constant. Iodoacetamide itself significantly enhanced the area of relengthening and TR(90) without affecting other indices. 4. Collectively, these data demonstrate that inhibition of creatine kinase blunts high extracellular Ca(2+)-induced increases in cardiomyocyte contractile response (i.e. cardiac contractile reserve).

Mitochondrial complex II in the post-ischemic heart: oxidative injury and the role of protein S-glutathionylation

Mitochondrial superoxide (O2.) is an important mediator of ischemia/reperfusion (I/R) injury. The O2. generated in mitochondria also acts as a redox signal triggering cellular apoptosis. The enzyme succinate ubiquinone reductase (SQR or complex II) is one of the major mitochondrial components hosting regulatory thiols. Here the intrinsic protein S-glutathionylation (PrSSG) at the 70-kDa FAD-binding subunit of SQR was detected in rat heart and in isolated SQR using an anti-GSH monoclonal antibody. When rats were subjected to 30 min of coronary ligation followed by 24 h of reperfusion, the electron transfer activity (ETA) of SQR in post-ischemic myocardium was significantly decreased by 41.5 +/- 2.9%. The PrSSGs of SQR-70 kDa were partially or completely eliminated in post-ischemic myocardium obtained from in vivo regional I/R hearts or isolated global I/R hearts, respectively. These results were further confirmed by using isolated succinate cytochrome c reductase (complex II + complex III). In the presence of succinate, O2. was generated and oxidized the SQR portion of SCR, leading to a 60-70% decrease in its ETA. The gel band of the S-glutathionylated SQR 70-kDa polypeptide was cut out and digested with trypsin, and the digests were subjected to liquid chromatography/tandem mass spectrometry analysis. One cysteine residue, Cys(90), was involved in S-glutathionylation. These results indicate that the glutathione-binding domain, (77)AAFGLSEAGFNTACVTK(93) (where underline indicates Cys(90)), is susceptible to redox change induced by oxidative stress. Furthermore, in vitro S-glutathionylation of purified SQR resulted in enhanced SQR-derived electron transfer efficiency and decreased formation of the 70-kDa-derived protein thiyl radical induced by O2. . Thus, the decreasing S-glutathionylation and ETA in mitochondrial complex II are marked during myocardial ischemia/reperfusion. This redox-triggered impairment of complex II occurs in the post-ischemic heart and should be useful to identify disease pathogenesis related to reactive oxygen species-induced mitochondrial dysfunction.

Altered creatine kinase adenosine triphosphate kinetics in failing hypertrophied human myocardium

BACKGROUND

The progression of pressure-overload left ventricular hypertrophy (LVH) to chronic heart failure (CHF) may involve a relative deficit in energy supply and/or delivery.

METHODS AND RESULTS

We measured myocardial creatine kinase (CK) metabolite concentrations and adenosine triphosphate (ATP) synthesis through CK, the primary energy reserve of the heart, to test the hypothesis that ATP flux through CK is impaired in patients with LVH and CHF. Myocardial ATP levels were normal, but creatine phosphate levels were 35% lower in LVH patients (n = 10) than in normal subjects (n = 14, P < 0.006). Left ventricular mass and CK metabolite levels in LVH were not different from those in patients with LVH and heart failure (LVH+CHF, n = 10); however, the myocardial CK pseudo first-order rate constant was normal in LVH (0.36 +/- 0.04 s(-1) in LVH versus 0.32 +/- 0.06 s(-1) in normal subjects) but halved in LVH+CHF (0.17 +/- 0.06 s(-1), P < 0.001). The net ATP flux through CK was significantly reduced by 30% in LVH (2.2 +/- 0.7 micromol x g(-1) x s(-1), P = 0.011) and by a dramatic 65% in LVH+CHF (1.1 +/- 0.4 micromol x g(-1) x s(-1), P < 0.001) compared with normal subjects (3.1 +/- 0.8 micromol x g(-1) x s(-1)).

CONCLUSIONS

These first observations in human LVH demonstrate that it is not the relative or absolute CK metabolite pool sizes but rather the kinetics of ATP turnover through CK that distinguish failing from nonfailing hypertrophic hearts. Moreover, the deficit in ATP kinetics is similar in systolic and nonsystolic heart failure and is not related to the severity of hypertrophy but to the presence of CHF. Because CK temporally buffers ATP, these observations support the hypothesis that a deficit in myofibrillar energy delivery contributes to CHF pathophysiology in human LVH.

Is the failing heart energy starved? On using chemical energy to support cardiac function

The requirement of chemical energy in the form of ATP to support systolic and diastolic work of the heart is absolute. Because of its central role in cardiac metabolism and performance, the subject of this review on energetics in the failing heart is ATP. We briefly review the basics of myocardial ATP metabolism and describe how this changes in the failing heart. We present an analysis of what is now known about the causes and consequences of these energetic changes and conclude by commenting on unsolved problems and opportunities for future basic and clinical research.

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.

Effect of creatine monohydrate on cardiac function in a rat model of endotoxemia

BACKGROUND

Reports have attributed cardiac failure during acute models of endotoxemia to a lack of high-energy phosphates. This study was undertaken to investigate whether creatine (Cr) administered during perfusion could enhance myocardial protection and improve recovery of cardiac function in a rat model of endotoxemia.

METHODS

Acute endotoxemia was induced in rats by a bolus injection of Escherichia coli endotoxin (LPS: 4 mg/kg, ip) while control rats were injected with an equal volume of 0.9% normal saline. To assess the adequacy of energy metabolism, freeze-clamped hearts were obtained from animals to study the concentrations of endogenous ATP, phosphocreatine (PCr), inorganic phosphate (P(i)), and intracellular pH by (31)P-cryomagnetic resonance spectroscopy. In a separate experiment, isolated hearts were perfused via a Langendorff column with Krebs-Henseleit buffer containing different concentrations of creatine monohydrate (1, 3, or 10 mM). Cardiac performance was evaluated via a paced (300 bpm) isovolumetric balloon preparation. Measurements of cardiac function including left ventricular developed pressure (LVDP), the maximum rates of ventricular pressure rise (LV +dP/dt) and fall (LV -dP/dt), and coronary flow were made for both LPS and saline-treated animals.

RESULTS

High-energy phosphate ratios of PCr/ATP and PCr/P(i) in hearts declined significantly at 4 h after endotoxin treatment. As anticipated, LVDP and LV +dP/dt(max) at a given preload and heart rate were significantly (P < 0.05) lower at 4 h when measured at the same time point. The functional recovery of these parameters was not improved by the addition of creatine monohydrate to the perfusion buffer. Creatine produced a significant (P < 0.05) negative inotropic effect in hearts from saline-treated animals. The LVDP was reduced by 30% at the lowest concentration and by 50% at the highest concentration of creatine monohydrate. Furthermore, creatine significantly (P < 0.05) reduced LV -dP/dt(max) in both saline and LPS-treated rats. These data demonstrate that exogenous creatine does not contribute to myocardial preservation in endotoxemia.

CONCLUSIONS

Energy stores in the rat heart decline early in endotoxemia accompanied by reduced myocardial performance, suggesting that the ability of the heart to perform mechanical work is impaired. Cardiac dysfunction in an acute model of endotoxemia was not improved with exogenous creatine during perfusion. Creatine's effects were primarily lusitropic by delaying the onset of myocardial relaxation in all hearts. The deleterious effects of exogenous creatine monohydrate in normal hearts should be examined in future experimental studies.

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.

Deficiency in parvalbumin increases fatigue resistance in fast-twitch muscle and upregulates mitochondria

The soluble Ca2+-binding protein parvalbumin (PV) is expressed at high levels in fast-twitch muscles of mice. Deficiency of PV in knockout mice (PV -/-) slows down the speed of twitch relaxation, while maximum force generated during tetanic contraction is unaltered. We observed that PV-deficient fast-twitch muscles were significantly more resistant to fatigue than were the wild type. Thus components involved in Ca2+ homeostasis during the contraction-relaxation cycle were analyzed. No upregulation of another cytosolic Ca2+-binding protein was found. Mitochondria are thought to play a physiological role during muscle relaxation and were thus analyzed. The fractional volume of mitochondria in the fast-twitch muscle extensor digitorum longus (EDL) was almost doubled in PV -/- mice, and this was reflected in an increase of cytochrome c oxidase. A faster removal of intracellular Ca2+ concentration ([Ca2+]i) 200-700 ms after fast-twitch muscle stimulation observed in PV -/- muscles supports the role for mitochondria in late [Ca2+]i removal. The present results also show a significant increase of the density of capillaries in EDL muscles of PV -/- mice. Thus alterations in the dynamics of Ca2+ transients detected in fast-twitch muscles of PV -/- mice might be linked to the increase in mitochondria volume and capillary density, which contribute to the greater fatigue resistance of these muscles.

Is greater tissue activity of creatine kinase the genetic factor increasing hypertension risk in black people of sub-Saharan African descent?

We postulate that the genetic factor increasing the propensity of black people of sub-Saharan African descent to develop high blood pressure is the relatively high activity of creatine kinase, predominantly in vascular and cardiac muscle tissue. Such greater activity of creatine kinase has been reported in skeletal muscle of black untrained subjects has been reported to be almost twice the activity found in white subjects. Creatine kinase, a key enzyme of cellular energy metabolism, increases the capacity of the cell to function under high demands. The enzyme regulates, buffers and transports, via phosphocreatine and creatine, energy produced by glycolysis and oxidative phosphorylation to sites of energy consumption such as myofibrils and membrane ion pumps. At these cellular locations, it is involved in the contraction process and active trans- membranous transport by readily providing the ATP needed for these processes. In addition, creatine kinase is increasingly reported to be involved in trophic responses. Furthermore, by using H+ and ADP to synthesize ATP, creatine kinase prevents acidification of the cell, providing relative protection against the effects of ischaemia. Greater creatine kinase activity in cardiovascular muscle and other tissues with high energy demands could increase cardiovascular contractile reserve, enhance trophic responses and increase renal tubular ability to retain salt. This could facilitate the development of arterial hypertension under chronic provocative circumstances, with higher mean blood pressures, more left ventricular hypertrophy and relatively fewer ischaemic events. Therefore, greater cellular activity of creatine kinase might explain the greater hypertension risk and the clinical characteristics of hypertensive disease observed in the black population.

Metabolic control of contractile performance in isolated perfused rat heart. Analysis of experimental data by reaction:diffusion mathematical model

The intracellular mechanisms of regulation of energy fluxes and respiration in contracting heart cells were studied. For this, we investigated the workload dependencies of the rate of oxygen consumption and metabolic parameters in Langendorff-perfused isolated rat hearts.(31)P NMR spectroscopy was used to study the metabolic changes during transition from perfusion with glucose to that with pyruvate with and without active creatine kinase system. The experimental results showed that transition from perfusion with glucose to that with pyruvate increased the phosphocreatine content and stability of its level at increased workloads. Inhibition of creatine kinase reaction by 15-min infusion of iodoacetamide decreased the maximal developed tension and respiration rates by a factor of two.(31)P NMR data were analyzed by a mathematical model of compartmentalized energy transfer, which is independent from the restrictions of the classical concept of creatine kinase equilibrium. The analysis of experimental data by this model shows that metabolic stability-constant levels of phosphocreatine, ATP and inorganic phosphate-at increased energy fluxes is an inherent property of the compartmentalized system. This explains the observed substrate specificity by changes in mitochondrial membrane potential. The decreased maximal respiration rate and maximal work output of the heart with inhibited creatine kinase is well explained by the rise in myoplasmic ADP concentration. This activates the adenylate kinase reaction in the myofibrillar space and in the mitochondria to fulfil the energy transfer and signal transmission functions, usually performed by creatine kinase. The activity of this system, however, is not sufficient to maintain high enough energy fluxes. Therefore, there is a kinetic explanation for the decreased maximal respiration rate of the heart with inhibited creatine kinase: i.e. a kinetically induced switch from an efficient energy transfer pathway (PCr-CK system) to a non-efficient one (myokinase pathway) within the energy transfer network of the cell under conditions of low apparent affinity of mitochondria to ADP in vivo. This may result in a significant decrease in the thermodynamic affinity of compartmentalized ATPase systems and finally in heart failure.

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.

Inhibition of neutrophil superoxide production by uremic concentrations of guanidino compounds

In uremia, diminished reactive oxygen intermediate production is an important consequence of impaired neutrophil function. The effects of guanidino compounds, which are known uremic toxins, on neutrophil reactive oxygen intermediate production in vitro were studied. Neutrophils from healthy volunteers were exposed for 3 h to individual guanidino compounds or mixed guanidino compounds (GCmix), at concentrations observed in uremic plasma. After removal of the guanidino compounds, the neutrophils were activated by adhesion, N-formylmethionylleucylphenylalanine, phorbol myristate acetate, or opsonized zymosan, and superoxide production was measured by monitoring lucigenin chemiluminescence. The direct effects of guanidino compounds on superoxide production in activated neutrophils were also measured. The energy status (ATP and creatine phosphate), antioxidant status (total glutathione), and glycolytic flux (lactate production) were measured. GCmix pretreatment decreased superoxide production in activated neutrophils (activated by N-formylmethionylleucylphenylalanine or zymosan) by 50% (P < 0.01), decreased ATP concentrations by 60% (P < 0.05), and inhibited glycolytic flux (lactate production) by 45% (P < 0.01) but did not alter glutathione concentrations. Simultaneous GCmix exposure and activation did not inhibit NADPH oxidase activity in cell lysates but inhibited superoxide formation in zymosan-activated intact neutrophils; this inhibition was reversed after removal of the guanidino compounds. Guanidinosuccinic acid, guanidinopropionic acid, and guanidinobutyric acid, when tested individually, were each as potent as GCmix. The inhibition of neutrophil superoxide generation by guanidino compounds results from decreased energy status. Micromolar concentrations of guanidino compounds significantly inhibit neutrophil metabolism, with serious implications for the functions of neutrophils in host defenses.

Calcium transients in single fibers of low-frequency stimulated fast-twitch muscle of rat

Ca(2+) transients were investigated in single fibers isolated from rat extensor digitorum longus muscles exposed to chronic low-frequency stimulation for different time periods up to 10 days. Approximately 2.5-fold increases in resting Ca(2+) concentration ([Ca(2+)]) were observed 2 h after stimulation onset and persisted throughout the stimulation period. The elevated [Ca(2+)] levels were in the range characteristic of slow-twitch fibers from soleus muscle. In addition, we noticed a transitory elevation of the integral [Ca(2+)] per pulse with a maximum ( approximately 5-fold) after 1 day. Steep decreases in rate constant of [Ca(2+)] decay could be explained by an immediate impairment of Ca(2+) uptake and, with longer stimulation periods, by an additional loss of cytosolic Ca(2+) binding capacity resulting from a decay in parvalbumin content. A partial recovery of the rate constant of [Ca(2+)] decay in 10-day stimulated muscle could be explained by an increasing mitochondrial contribution to Ca(2+) sequestration.

Functional and energetic consequences of chronic myocardial creatine depletion by beta-guanidinopropionate in perfused hearts and in intact rats

Oral feeding with the creatine analogue beta-guanidinopropionate (beta-GP) reduces myocardial phosphocreatine and creatine concentrations by about 80%in vitro, this is accompanied by reduced contractile performance. We hypothesized, thus, that beta-GP feeding leads to hemodynamic changes in vivo characteristic of heart failure. beta-GP was fed to Wistar rats for up to 8 weeks. In isolated hearts, function was measured isovolumically, myocardial energetics were followed with (31)P-NMR spectroscopy. In vivo hemodynamics were measured with Millar-Tip-catheters and an electromagnetic flow probe. Beta-GP feeding did not alter heart weight. In vitro, diastolic pressure-volume curves indicated structural left ventricular dilatation, and a 36% reduction of left ventricular developed pressure was found; phosphocreatine was reduced by approximately 80%, ATP unchanged and creatine kinase reaction velocity ((31)P-MR saturation transfer) decreased by approximately 90%. The total creatine pool (high-pressure liquid chromatography) was reduced by up to approximately 70%. In contrast to in vitro findings, in vivo cardiac hemodynamics (including left ventricular developed pressure, d P/d t(max), cardiac output and peripheral vascular resistance) at rest and during acute volume loading showed no alterations after beta-GP feeding. The only functional impairment observed in vivo was a 14% reduction of maximum left ventricular developed pressure during brief aortic occlusion. In the intact rat, cardiac and/or humoral compensatory mechanisms are sufficient to maintain normal hemodynamics in spite of a 90% reduction of creatine kinase reaction velocity. However, chronic beta-GP feeding leads to structural left ventricular dilatation.

What does chronic electrical stimulation teach us about muscle plasticity?

The model of chronic low-frequency stimulation for the study of muscle plasticity was developed over 30 years ago. This protocol leads to a transformation of fast, fatigable muscles toward slower, fatigue-resistant ones. It involves qualitative and quantitative changes of all elements of the muscle fiber studied so far. The multitude of stimulation-induced changes makes it possible to establish the full adaptive potential of skeletal muscle. Both functional and structural alterations are caused by orchestrated exchanges of fast protein isoforms with their slow counterparts, as well as by altered levels of expression. This remodeling of the muscle fiber encompasses the major, myofibrillar proteins, membrane-bound and soluble proteins involved in Ca2+ dynamics, and mitochondrial and cytosolic enzymes of energy metabolism. Most transitions occur in a coordinated, time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. This review summarizes the advantages of chronic low-frequency stimulation for studying activity-induced changes in phenotype, and its potential for investigating regulatory mechanisms of gene expression. The potential clinical relevance or utility of the technique is also considered.

Overexpression of human copper, zinc-superoxide dismutase (SOD1) prevents postischemic injury

Superoxide and superoxide-derived oxidants have been hypothesized to be important mediators of postischemic injury. Whereas copper, zinc-superoxide dismutase, SOD1, efficiently dismutates superoxide, there has been controversy regarding whether increasing intracellular SOD1 expression would protect against or potentiate cellular injury. To determine whether increased SOD1 protects the heart from ischemia and reperfusion, studies were performed in a newly developed transgenic mouse model in which direct measurement of superoxide, contractile function, bioenergetics, and cell death could be performed. Transgenic mice with overexpression of human SOD1 were studied along with matched nontransgenic controls. Immunoblotting and immunohistology demonstrated that total SOD1 expression was increased 10-fold in hearts from transgenic mice compared with nontransgenic controls, with increased expression in both myocytes and endothelial cells. In nontransgenic hearts following 30 min of global ischemia a reperfusion-associated burst of superoxide generation was demonstrated by electron paramagnetic resonance spin trapping. However, in the transgenic hearts with overexpression of SOD1 the burst of superoxide generation was almost totally quenched, and this was accompanied by a 2-fold increase in the recovery of contractile function, a 2.2-fold decrease in infarct size, and a greatly improved recovery of high energy phosphates compared with that in nontransgenic controls. These results demonstrate that superoxide is an important mediator of postischemic injury and that increasing intracellular SOD1 dramatically protects the heart from this injury. Thus, increasing intracellular SOD1 expression may be a highly effective approach to decrease the cellular injury that occurs following reperfusion of ischemic tissues.

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

BACKGROUND

In patients with heart failure due to dilated cardiomyopathy, cardiac energy metabolism is impaired, as indicated by a reduction of the myocardial phosphocreatine-to-ATP ratio, measured noninvasively by 31P-MR spectroscopy. The purpose of this study was to test whether the phosphocreatine-to-ATP ratio also offers prognostic information in terms of mortality prediction as well as how this index compares with well-known mortality predictors such as left ventricular ejection fraction (LVEF) or New York Heart Association (NYHA) class.

METHODS AND RESULTS

Thirty-nine patients with dilated cardiomyopathy were followed up for 928+/-85 days (2.5 years). At study entry, LVEF and NYHA class were determined, and the cardiac phosphocreatine-to-ATP ratio was measured by localized 31P-MR spectroscopy of the anterior myocardium. During the study period, total mortality was 26%. Patients were divided into two groups, one with a normal phosphocreatine-to-ATP ratio (>1.60; mean+/-SE, 1.98+/-0.07; n=19; healthy volunteers: 1.94+/-0.11, n=30) and one with a reduced phosphocreatine-to-ATP ratio (<1.60; 1.30+/-0.05; n=20). At re-evaluation (mean, 2.5 years), 8 of 20 patients with reduced phosphocreatine-to-ATP ratios had died, all of cardiovascular causes (total and cardiovascular mortality, 40%). Of the 19 patients with normal phosphocreatine-to-ATP ratios, 2 had died (total mortality, 11%), one of cardiovascular causes (cardiovascular mortality, 5%). Kaplan-Meier analysis showed significantly reduced total (P=.036) and cardiovascular (P=.016) mortality for patients with normal versus patients with low phosphocreatine-to-ATP ratios. A Cox model for multivariate analysis showed that the phosphocreatine-to-ATP ratio and NYHA class offered significant independent prognostic information on cardiovascular mortality.

CONCLUSIONS

The myocardial phosphocreatine-to-ATP ratio, measured noninvasively with 31P-MR spectroscopy, is a predictor of both total and cardiovascular mortality in patients with dilated cardiomyopathy.

In vivo brain phosphocreatine and ATP regulation in mice fed a creatine analog

Mitochondrial and cytosolic creatine kinase (CK) isozymes are active in cells with high and variable ATP metabolic rates. beta-Guanidinopropionic acid (GPA), a competitive inhibitor of creatine transport, was used to study the hypothesis that the creatine-CK-phosphocreatine (PCr) system is important in regulating brain ATP metabolism. The CK-catalyzed reaction rate and reactant concentrations were measured in vivo with 31P nuclear magnetic resonance spectroscopy during energy deficit (hypoxia) or high-energy turnover (seizures) states in urethane-anesthetized mice fed GPA, creatine, or standard chow (controls). Brain phosphagen (i.e., cellular energy reserves) or PCr plus phosphorylated GPA (GPAP) concentrations were equal. The phosphagen-to-NTP ratio was lower than in controls. In vivo CK reaction rate decreased fourfold, whereas ex vivo CK activity that was biochemically measured was doubled. During seizures, CK-catalyzed fluxes increased only in GPA-fed mice. Phosphagen increased in GPA-fed mice, whereas PCr decreased in controls. Survival was higher and brain phosphagen and ATP losses were less for hypoxic GPA-fed mice than for controls. In contrast to mice fed GPA, hypoxic survival and CK reactant concentrations during hypoxia and seizures were the same in creatine-fed mice and controls. Thus GPA, GPAP, or adaptive changes in ATP metabolism stabilize brain ATP and enhance survival during hypoxia in mice.

Creatine kinase system in failing and nonfailing human myocardium

BACKGROUND

The creatine kinase (CK) reaction is important for rapid resynthesis of ATP when the heart increases its work. Studies defining the CK system in human failing and nonfailing myocardium are limited and in conflict. To resolve this conflict, we measured the activities of CK and its isoenzymes and the contents of creatine and CK-B in homogenates of human myocardium.

METHODS AND RESULTS

Myocardium was sampled from 23 subjects who underwent heart transplant, 36 subjects maintained in an intensive care unit before heart harvesting, 13 accident victims, and 2 patients undergoing heart surgery. Since the characteristics of myocardium of potential organ donors differed from those of myocardium of accident victims, data are presented for three groups: failing, donor, and control. CK activity was 7.7 +/- 1.9 and 6.0 +/- 1.4 IU/mg protein in left (LV) and right (RV) ventricles of failing, 9.4 +/- 2.5 and 10.7 +/- 2 IU/mg protein in LV and RV of donor, and 11.6 +/- 2.4 IU/mg protein in LV of control hearts. CK-MM and the mitochondrial isoenzyme activities were lower in failing and donor LV, and CK-MB activity and CK-B content were higher in failing and donor hearts. Creatine contents were 64 +/- 25 and 56 +/- 18.6 nmol/mg protein in LV and RV of failing, 96 +/- 30 and 110 +/- 24 nmol/mg protein in LV and RV of donor, and 131 +/- 28 nmol/mg protein in LV of control hearts.

CONCLUSIONS

In failing and nonfailing donor human myocardium, there is a combined decrease of CK activity and creatine that may impair the ability to deliver ATP to energy-consuming systems.

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.

The utilisation of creatine and its analogues by cytosolic and mitochondrial creatine kinase

We have investigated the utilisation of four analogues of creatine by cytosolic Creatine Kinase (CK), using 31P-NMR in the porcine carotid artery, and by mitochondrial CK (Mt-CK), using oxygen consumption studies in isolated heart mitochondria and skinned fibers. Porcine carotid arteries were superfused for 12 h with Krebs-Henseleit buffer at 22 degrees C, containing 11 mM glucose as substrate, and supplemented with either 20 mM beta-guanidinopropionic acid (beta-GPA), methyl-guanidinopropionic acid (m-GPA), guanidinoacetic acid (GA) or cyclocreatine (cCr). All four analogues entered the tissue and became phosphorylated by CK as seen by 31 P-NMR, Inhibition of oxidative metabolism by 1 mM cyanide after accumulation of the phosphorylated analogue resulted in the utilisation of PCr, beta-GPA-P, GA-P and GA-P over a similar time course (approximately 2 h), despite very different kinetic properties of these analogues in vitro. cCr-P was utilised at a significantly slower rate, but was rapidly dephosphorylated in the presence of both 1 mM iodoacetate and cyanide (to inhibit both glycolysis and oxidative metabolism respectively). The technique of creatine stimulated respiration was used to investigate the phosphorylation of the analogues by Mt-CK, Isolated mitochondria were subjected to increasing [ATP], whereas skinned fibres received a similar protocol with increasing [ADP]. There was a significant stimulation of respiration by creatine and cCr in isolated mitochondria (decreased K(m) and increased Vmax vs control), but none by GA, mGPA or beta-GPA (also in skinned fibres), indicating that these latter analogues were not utilised by Mt-CK. These results demonstrate differences in the phosphorylation and dephosphorylation of creatine and its analogues by cytosolic CK and Mt-CK in vivo and in vitro.

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.

Effect of left ventricular hypertrophy secondary to chronic pressure overload on transmural myocardial 2-deoxyglucose uptake. A 31P NMR spectroscopic study

BACKGROUND

This study tested the hypothesis that 31P nuclear magnetic resonance (NMR)-detectable 2-deoxyglucose (2DG) uptake is increased in chronically pressure-overloaded hypertrophied left ventricular myocardium.

METHODS AND RESULTS

31P NMR spectroscopy was used to determine the transmural distribution of high-energy phosphate levels and 2-deoxyglucose-6-phosphate (2DGP) accumulation during intracoronary infusion of 2DG (15 mumol.kg body wt-1.min-1) in eight normal dogs and in eight dogs with severe left ventricular hypertrophy (LVH) produced by ascending aortic banding. The ratio of LV weight to body weight was 8.25 +/- 0.65 g/kg in the LVH group compared with 4.35 +/- 0.11 g/kg in the normal group (P < .01). Myocardial ATP content was decreased by approximately 40% and phosphocreatine (PCr) by approximately 60% in LVH hearts. ATP values were transmurally uniform in LVH and normal hearts, whereas PCr was lower in the subendocardium (Endo) than the subepicardium (Epi) of both groups. The PCr/ATP ratio was lower in LVH hearts (1.72 +/- 0.05, 1.64 +/- 0.07, and 1.53 +/- 0.10 in Epi, midwall, and Endo, respectively) compared with normal hearts (2.36 +/- 0.05, 2.09 +/- 0.06, and 1.96 +/- 0.06; each P < .01 normal versus LVH). Arterial blood levels of glucose, insulin, and free fatty acids were comparable between groups, whereas arterial lactate and norepinephrine levels were significantly higher in the LVH group. 2DG infusion did not affect systemic hemodynamics or myocardial high-energy phosphate or inorganic phosphate levels in either group. At the end of 60 minutes of 2DG infusion, there was no detectable accumulation of 2DGP in the normal hearts. However, seven of the eight LVH hearts showed time-dependent accumulation of 2DGP, which was linearly related to the severity of hypertrophy (r = .90 for subendocardial 2DGP versus LV weight/body weight). A transmural gradient of 2DGP was present, with greatest accumulation in the subendocardium (3.3 +/- 1.6, 5.8 +/- 2.3, and 7.9 +/- 2.2 mumol/g in Epi, midwall, and Endo of the LVH hearts, respectively; P < .05 Epi versus Endo).

CONCLUSIONS

The pressure-overloaded hypertrophied left ventricle demonstrated increased accumulation of 2DGP detected with 31P NMR spectroscopy. Accumulation of 2DGP was positively correlated with the degree of hypertrophy and was most marked in the subendocardium.

Muscle creatine kinase-deficient mice. I. Alterations in myofibrillar function

The regulation of contractile activity in mice bearing a null mutation of the M-isoform of creatine kinase gene, has been investigated in tissue extracts and Triton X-100-treated preparations of ventricular, soleus, and gastrocnemius muscles of control and transgenic mice. Skinned fiber experiments did not evidence any statistical difference in the maximal force or the calcium sensitivity of either muscle type. Rigor tension development at a low MgATP concentration was greatly influenced by phosphocreatine in control but not in transgenic mice as should be expected. In calcium-activated ventricular preparations, although the force developed by each cross-bridge was the same in control and transgenic animals, the rate constant of tension changes appeared to be markedly slowed in transgenic animals. As the ventricular isomyosin pattern was not altered, we suggested that, in transgenic animals, cross-bridge cycling was hindered by a local decrease in the MgATP to MgADP ratio, due to lack of a local MgATP regenerating system. Myokinase activity was not significantly changed while activities of pyruvate kinase or glyceraldehyde-3-phosphate dehydrogenase were found to be increased in transgenic animals. These results show that no fundamental remodelling occurs in myofibrils of transgenic animals but that important adaptations modify the bioenergetic pathways including glycolytic metabolism.

Functional coupling between glycolysis and sarcoplasmic reticulum Ca2+ transport

To investigate whether the energy derived from glycolysis is functionally coupled to Ca2+ active transport in sarcoplasmic reticulum (SR), we determined whether glycolytic enzymes were associated with SR membranes and whether metabolism through these enzymes was capable of supporting 45Ca transport. Sealed right-side-out SR vesicles were isolated by step sucrose gradient from rabbit skeletal and cardiac muscle. Intravesicular 45Ca transport was measured after the addition of glycolytic substrates and cofactors specific for each of the glycolytic reactions being studied or after the addition of exogenous ATP and was expressed as transport sensitive to the specific Ca(2+)-ATPase inhibitor thapsigargin. We found that the entire chain of glycolytic enzymes from aldolase onward, including aldolase, GAPDH, phosphoglycerate kinase (PGK), phosphoglyceromutase, enolase, and pyruvate kinase (PK), was associated with SR vesicles from both cardiac and skeletal muscle. Iodoacetic acid, an inhibitor of GAPDH, eliminated 45Ca transport supported by fructose-1,6-diphosphate, the substrate for aldolase, but transport was completely restored by phosphoenolpyruvate (the substrate for PK), indicating that both of the ATP-producing glycolytic enzymes, GAPDH/PGK and PK, were associated with the SR and functionally capable of providing ATP for the Ca2+ pump. Addition of a soluble hexokinase ATP trap eliminated 45Ca transport fueled by exogenous ATP but had markedly less effect on 45Ca transport supported by endogenously produced ATP (via glycolysis). Similarly, at very low concentrations of ATP and ADP (10 to 50 nmol/L), ATP that was produced endogenously from ADP and phosphoenolpyruvate supported 15-fold more 45Ca transport than ATP that was supplied exogenously at the same concentration. These results are consistent with functional coupling of glycolytic ATP to Ca2+ transport and support the hypothesis that ATP generated by SR-associated glycolytic enzymes may play an important role in cellular Ca2+ homeostasis by driving the SR Ca2+ pump.

Enalapril treatment increases cardiac performance and energy reserve via the creatine kinase reaction in myocardium of Syrian myopathic hamsters with advanced heart failure

BACKGROUND

Converting enzyme inhibitor treatment of congestive heart failure slows progression to failure and reduces mortality rate. It is known whether these benefits are due solely to improved hemodynamics or also to improved myocyte energetics. This study examines the effect of enalapril treatment on both isovolumic contractile performance and its biochemical correlate, flux through the creatine kinase (CK) system, in an animal model of severely failing myocardium.

METHODS AND RESULTS

Seven-month-old Syrian cardiomyopathic (TO-2 strain) and normal golden Syrian (FIB strain) hamsters were each randomly assigned to one of three groups supplied daily with either no, low (25 mg/kg body wt), or high (100 mg/kg body wt) doses of enalapril for 12 to 14 weeks. At 10 months of age, all substrates and products and flux through the CK reaction were measured in isolated perfused hearts by 31P magnetization transfer and chemical assay. Compared with normal hamsters, the myopathic hamsters exhibited significantly lower body weights and higher biventricular heart weights, which were partially reversed by drug treatment. The Langendorff-perfused hearts showed decreased isovolumic contractile performance with identical load conditions. This was partially reversed by drug treatment. In the failing hearts, the following substrate and product concentrations and enzyme activities were decreased compared with nonfailing hearts but were unchanged by drug treatment: ATP (-28%), phosphocreatine (-48%), free creatine (-64%), ADP (-51%), and CK (-34%, primarily MM isoenzyme). Flux through the CK reaction for the untreated cardiomyopathic hamster hearts was decreased by 67%, and this decrease was almost completely reversed by enalapril treatment. The increased CK flux is due to an increase in the rate constant for the reaction, since substrate concentrations are unchanged, and is not predicted by the rate equation. In enalapril-treated failing hearts, phosphoryl transfer via the CK reaction increased with contractile performance. This was not observed in the nonfailing hearts, in which energy reserve is adequate to support changes in contractile performance.

CONCLUSIONS

Decreased flux through CK reaction leads to decreased capacity for ATP synthesis and may contribute to decreased contractile performance in cardiomyopathic hamster hearts. Enalapril treatment results in increased phosphoryl transfer through the CK reaction in failing myocardium, and this increase is coupled to improved cardiac performance. Decreased CK flux in failing hearts is due to a combination of decreased Vmax and lower guanidino pool; this mechanism fails to explain changes in CK flux in enalapril-treated failing hearts.

Contractile economy and aerobic recovery metabolism in skeletal muscle adapted to creatine depletion

Mice were treated for 7-12 wk with the creatine analogue beta-guanidinopropionic acid (beta-GPA). Treatment reduced total creatine to approximately 5% of control values in soleus (SOL) and extensor digitorum longus (EDL) muscles. In both muscles from treated mice, phosphorylated beta-GPA accumulated and resting [ATP] decreased by approximately 50%. Relative to controls, cytochrome oxidase and citrate synthase activities increased significantly in EDL from treated mice, but not in SOL; creatine kinase activity decreased significantly in SOL, but not in EDL. Measurements of poststimulation energy metabolism show that the energy cost to maintain tension in SOL and EDL from treated mice was approximately 50% of that in control muscle. Relative to controls, first-order rate constants of poststimulation O2 demand were 2- and 3.6-fold greater in SOL and EDL, respectively, from treated mice. Increased economy of SOL and EDL from treated mice is consistent with previously reported changes in myosin isoenzymes. Increases in rate constants of O2 utilization in creatine-depleted muscle are inconsistent with the hypothesis that cytoplasmic or mitochondrial creatine kinase is rate limiting for cellular respiration.

Myofibrillar creatine kinase and cardiac contraction

This article is a review on the organization and function of myofibrillar creatine kinase in striated muscle. The first part describes myofibrillar creatine kinase as an integral structural part of the complex organization of myofibrils in striated muscle. The second part considers the intrinsic biochemical and mechanical properties of myofibrils and the functional coupling between myofibrillar CK and myosin ATPase. Skinned fiber studies have been developed to evidence this functional coupling and the consequences for cardiac contraction. The data show that creatine kinase in myofibrils is effective enough to sustain normal tension and relaxation, normal Ca sensitivity and kinetic characteristics. Moreover, the results suggest that myofibrillar creatine kinase is essential in maintaining adequate ATP/ADP ratio in the vicinity of myosin ATPase active site to prevent dysfunctioning of this enzyme. Implications for the physiology and physiopathology of cardiac muscle are discussed.

Metabolic compartmentation and substrate channelling in muscle cells. Role of coupled creatine kinases in in vivo regulation of cellular respiration--a synthesis

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotides in vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cells in vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.

Creatine metabolism and the consequences of creatine depletion in muscle

Currently, considerable research activities are focussing on biochemical, physiological and pathological aspects of the creatine kinase (CK)-phosphorylcreatine (PCr)-creatine (Cr) system (for reviews see [1,2]), but only little effort is directed towards a thorough investigation of Cr metabolism as a whole. However, a detailed knowledge of Cr metabolism is essential for a deeper understanding of bioenergetics in general and, for example, of the effects of muscular dystrophies, atrophies, CK deficiencies (e.g. in transgenic animals) or Cr analogues on the energy metabolism of the tissues involved. Therefore, the present article provides a short overview on the reactions and enzymes involved in Cr biosynthesis and degradation, on the organization and regulation of Cr metabolism within the body, as well as on the metabolic consequences of 3-guanidinopropionate (GPA) feeding which is known to induce a Cr deficiency in muscle. In addition, the phenotype of muscles depleted of Cr and PCr by GPA feeding is put into context with recent investigations on the muscle phenotype of 'gene knockout' mice deficient in the cytosolic muscle-type M-CK.

Skeletal muscles of mice deficient in muscle creatine kinase lack burst activity

To understand the physiological role of the creatine kinase-phosphocreatine (CK-PCr) system in muscle bioenergetics, a null mutation of the muscle CK (M-CK) gene was introduced into the germline of mice. Mutant mice show no alterations in absolute muscle force, but lack the ability to perform burst activity. Their fast-twitch fibers have an increased intermyofibrillar mitochondrial volume and an increased glycogenolytic/glycolytic potential. PCr and ATP levels are normal in resting M-CK-deficient muscles, but rates of high energy phosphate exchange between PCr and ATP are at least 20-fold reduced. Strikingly, PCr levels decline normally during muscle exercise, suggesting that M-CK-mediated conversion is not the only route for PCr utilization in active muscle.

Functional coupling between sarcoplasmic-reticulum-bound creatine kinase and Ca(2+)-ATPase

We investigated the role of creatine kinase bound to sarcoplasmic reticulum membranes of fast skeletal muscle in the local regeneration of ATP and the possible physiological significance of this regeneration for calcium pump function. Our results indicate that ADP produced by sarcoplasmic reticulum Ca(2+)-ATPase is effectively phosphorylated by creatine kinase in the presence of creatine phosphate. This phosphorylation is an important function of the membrane-bound creatine kinase because accumulation of ADP has a depressive effect on Ca(2+)-uptake by sarcoplasmic reticulum vesicles. The concentration-dependent depression of Ca(2+)-uptake by ADP was especially pronounced when there was strong back inhibition by high intravesicular [Ca2+]. ATP regenerated by endogenous creatine kinase was not in free equilibrium with the ATP in the surrounding medium, but was used preferentially by Ca(2+)-ATPase for Ca(2+)-uptake. Efficient translocation of ATP from creatine kinase to Ca(2+)-ATPase, despite the presence of an ATP trap in the surrounding medium, can be explained by close localization of creatine kinase and Ca(2+)-ATPase on the sarcoplasmic reticulum membranes. These results suggest the existence of functional coupling between creatine kinase and Ca(2+)-ATPase on skeletal muscle sarcoplasmic reticulum membranes. Several factors (amount of membrane-bound creatine kinase, oxidation of SH groups of creatine kinase, decrease in [phosphocreatine]) can influence the ability of creatine kinase/phosphocreatine system to support a low ADP/ATP ratio and fuel the Ca(2+)-pump with ATP. These factors may become operative in the living cells, influencing functional coupling between creatine kinase and Ca(2+)-ATPase and may have an indirect effect on Ca(2+)-pump function before Ca(2+)-ATPase itself is affected.

Altering creatine kinase isoenzymes in transgenic mouse muscle by overexpression of the B subunit

To change the levels of expression and isoenzyme distribution of creatine kinase (CK) in muscle, transgenic technology was used to express the B subunit of CK in mouse muscle. Normally, mammalian skeletal muscle contains the MM dimer of CK. The BB dimer and MB heterodimer of CK can be found in brain and heart, respectively. Heterologous genes consisting of skeletal and cardiac muscle-specific actin promoters fused to the genomic coding region of the B form of CK were used to create transgenic mice. Lines were established from the three highest expressing founders. Analysis of skeletal muscle extracts revealed that all three lines had an increase in total CK activity measured under maximal velocity conditions. The highest expressing line, 7001, had a CK activity 150% that of control muscle. Nuclear magnetic resonance saturation transfer was used to measure the in vivo rate of the CK reaction. In 7001 hindlimb muscles, the CK catalyzed reaction was 200% that of control muscle. The elevation in CK activity in transgenic muscle was accompanied by significant changes in the composition of the cytosolic isoenzyme ratio of CK. In control, 100% of CK was MM, whereas 7001 had 60 +/- 18% MM, 32 +/- 10% MB, and 8 +/- 2% BB. There were no changes in ATP, phosphocreatine, Pi, or creatine levels in transgenic muscle compared with control. Immunofluorescence of myofibrils isolated from control and transgenic muscle revealed specific association of CK to the M line. Small amounts of MB CK were detected on myofibrils from transgenic mice. Transgenic mice expressing the B subunit of CK in muscle represent a first step toward altering CK isoforms so as to elucidate the specific roles of these isoforms in energy metabolism.