Activity of creatine kinase in a contracting mammalian muscle of uniform fiber type

Energy metabolism design of the striated muscle cell

The design of the energy metabolism system in striated muscle remains a major area of investigation. Here, we review our current understanding and emerging hypotheses regarding the metabolic support of muscle contraction. Maintenance of ATP free energy, so called energy homeostasis, via mitochondrial oxidative phosphorylation is critical to sustained contractile activity, and this major design criterion is the focus of this review. Cell volume invested in mitochondria reduces the space available for generating contractile force, and this spatial balance between mitochondria acontractile elements to meet the varying sustained power demands across muscle types is another important design criterion. This is accomplished with remarkably similar mass-specific mitochondrial protein composition across muscle types, implying that it is the organization of mitochondria within the muscle cell that is critical to supporting sustained muscle function. Beyond the production of ATP, ubiquitous distribution of ATPases throughout the muscle requires rapid distribution of potential energy across these large cells. Distribution of potential energy has long been thought to occur primarily through facilitated metabolite diffusion, but recent analysis has questioned the importance of this process under normal physiological conditions. Recent structural and functional studies have supported the hypothesis that the mitochondrial reticulum provides a rapid energy distribution system via the conduction of the mitochondrial membrane potential to maintain metabolic homeostasis during contractile activity. We extensively review this aspect of the energy metabolism design contrasting it with metabolite diffusion models and how mitochondrial structure can play a role in the delivery of energy in the striated muscle.

Metabolic Basis of Creatine in Health and Disease: A Bioinformatics-Assisted Review

Creatine (Cr) is a ubiquitous molecule that is synthesized mainly in the liver, kidneys, and pancreas. Most of the Cr pool is found in tissues with high-energy demands. Cr enters target cells through a specific symporter called Na⁺/Cl⁻-dependent Cr transporter (CRT). Once within cells, creatine kinase (CK) catalyzes the reversible transphosphorylation reaction between [Mg²⁺:ATP⁴⁻]²⁻ and Cr to produce phosphocreatine (PCr) and [Mg²⁺:ADP³⁻]⁻. We aimed to perform a comprehensive and bioinformatics-assisted review of the most recent research findings regarding Cr metabolism. Specifically, several public databases, repositories, and bioinformatics tools were utilized for this endeavor. Topics of biological complexity ranging from structural biology to cellular dynamics were addressed herein. In this sense, we sought to address certain pre-specified questions including: (i) What happens when creatine is transported into cells? (ii) How is the CK/PCr system involved in cellular bioenergetics? (iii) How is the CK/PCr system compartmentalized throughout the cell? (iv) What is the role of creatine amongst different tissues? and (v) What is the basis of creatine transport? Under the cellular allostasis paradigm, the CK/PCr system is physiologically essential for life (cell survival, growth, proliferation, differentiation, and migration/motility) by providing an evolutionary advantage for rapid, local, and temporal support of energy- and mechanical-dependent processes. Thus, we suggest the CK/PCr system acts as a dynamic biosensor based on chemo-mechanical energy transduction, which might explain why dysregulation in Cr metabolism contributes to a wide range of diseases besides the mitigating effect that Cr supplementation may have in some of these disease states.

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

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

Test-retest analysis of multiple 31 P magnetization exchange pathways using asymmetric adiabatic inversion

PURPOSE

A ³¹ P-MR inversion transfer (IT) method with a short adiabatic inversion pulse is proposed and its test-retest reliability was evaluated for two spectral fitting strategies.

METHODS

Assessment in a test-retest design (3 Tesla, vastus muscles, 12 healthy volunteers, 14 inversion times, 22 ms asymmetric adiabatic inversion pulse, adiabatic excitation); spectral fitting in Fitting Tool for Interrelated Arrays of Datasets (FitAID) and Java Magnetic Resonance User Interface (jMRUI); least squares solution of the Bloch-McConnell-Solomon matrix formalism including all 14 measured time-points with equal weighting.

RESULTS

The cohort averages of k[PCr→γ-ATP] (phosphocreatine, PCr; adenosine triphosphate, ATP) are 0.246 ± 0.050s^-1 versus 0.254 ± 0.050s^-1 , and k[Pi→γ-ATP] 0.086 ± 0.033s^-1 versus 0.066 ± 0.034s^-1 (average ± standard deviation, jMRUI versus FitAID). Coefficients of variation of the differences between test and retest are lowest (9.5%) for k[PCr→γ-ATP] fitted in FitAID, larger (15.2%) for the fit in jMRUI, and considerably larger for k[Pi→γ-ATP] fitted in FitAID (43.4%) or jMRUI (47.9%). The beginning of the IT effect can be observed with magnetizations above 92% for noninverted lines while inversion of the ATP resonances is better than -72%.

CONCLUSION

The performance of the asymmetric adiabatic pulse allows an accurate observation of IT effects even in the early phase; the least squares fit of the Bloch-McConnell-Solomon matrix formalism is robust; and the type of spectral fitting can influence the results significantly. Magn Reson Med 78:33-39, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

(31)P-MRS of healthy human brain: ATP synthesis, metabolite concentrations, pH, and T1 relaxation times

The conventional method for measuring brain ATP synthesis is (31)P saturation transfer (ST), a technique typically dependent on prolonged pre-saturation with γ-ATP. In this study, ATP synthesis rate in resting human brain is evaluated using EBIT (exchange kinetics by band inversion transfer), a technique based on slow recovery of γ-ATP magnetization in the absence of B1 field following co-inversion of PCr and ATP resonances with a short adiabatic pulse. The unidirectional rate constant for the Pi → γ-ATP reaction is 0.21 ± 0.04 s(-1) and the ATP synthesis rate is 9.9 ± 2.1 mmol min(-1)  kg(-1) in human brain (n = 12 subjects), consistent with the results by ST. Therefore, EBIT could be a useful alternative to ST in studying brain energy metabolism in normal physiology and under pathological conditions. In addition to ATP synthesis, all detectable (31)P signals are analyzed to determine the brain concentration of phosphorus metabolites, including UDPG at around 10 ppm, a previously reported resonance in liver tissues and now confirmed in human brain. Inversion recovery measurements indicate that UDPG, like its diphosphate analogue NAD, has apparent T1 shorter than that of monophosphates (Pi, PMEs, and PDEs) but longer than that of triphosphate ATP, highlighting the significance of the (31)P-(31)P dipolar mechanism in T1 relaxation of polyphosphates. Another interesting finding is the observation of approximately 40% shorter T1 for intracellular Pi relative to extracellular Pi, attributed to the modulation by the intracellular phosphoryl exchange reaction Pi ↔ γ-ATP. The sufficiently separated intra- and extracellular Pi signals also permit the distinction of pH between intra- and extracellular environments (pH 7.0 versus pH 7.4). In summary, quantitative (31)P MRS in combination with ATP synthesis, pH, and T1 relaxation measurements may offer a promising tool to detect biochemical alterations at early stages of brain dysfunctions and diseases.

Comparison of (31)P saturation and inversion magnetization transfer in human liver and skeletal muscle using a clinical MR system and surface coils

(31)P MRS magnetization transfer ((31)P-MT) experiments allow the estimation of exchange rates of biochemical reactions, such as the creatine kinase equilibrium and adenosine triphosphate (ATP) synthesis. Although various (31)P-MT methods have been successfully used on isolated organs or animals, their application on humans in clinical scanners poses specific challenges. This study compared two major (31)P-MT methods on a clinical MR system using heteronuclear surface coils. Although saturation transfer (ST) is the most commonly used (31)P-MT method, sequences such as inversion transfer (IT) with short pulses might be better suited for the specific hardware and software limitations of a clinical scanner. In addition, small NMR-undetectable metabolite pools can transfer MT to NMR-visible pools during long saturation pulses, which is prevented with short pulses. (31)P-MT sequences were adapted for limited pulse length, for heteronuclear transmit-receive surface coils with inhomogeneous B1 , for the need for volume selection and for the inherently low signal-to-noise ratio (SNR) on a clinical 3-T MR system. The ST and IT sequences were applied to skeletal muscle and liver in 10 healthy volunteers. Monte-Carlo simulations were used to evaluate the behavior of the IT measurements with increasing imperfections. In skeletal muscle of the thigh, ATP synthesis resulted in forward reaction constants (k) of 0.074 ± 0.022 s(-1) (ST) and 0.137 ± 0.042 s(-1) (IT), whereas the creatine kinase reaction yielded 0.459 ± 0.089 s(-1) (IT). In the liver, ATP synthesis resulted in k = 0.267 ± 0.106 s(-1) (ST), whereas the IT experiment yielded no consistent results. ST results were close to literature values; however, the IT results were either much larger than the corresponding ST values and/or were widely scattered. To summarize, ST and IT experiments can both be implemented on a clinical body scanner with heteronuclear transmit-receive surface coils; however, ST results are much more robust against experimental imperfections than the current implementation of IT.

Sprint exercise performance: does metabolic power matter?

Prevailing physiological paradigms explain both sprint and endurance exercise performance in terms of the availability of metabolic energy. However, for all-out efforts of 60 s or less, the prevailing view is no longer viable. Contemporary evidence indicates that sprinting performance is determined by musculoskeletal force application, with a duration dependency explained by the intrinsically rapid rates at which skeletal muscle fatigues in vivo.

Simulating the physiology of athletes during endurance sports events: modelling human energy conversion and metabolism

The human physiological system is stressed to its limits during endurance sports competition events. We describe a whole body computational model for energy conversion during bicycle racing. About 23 per cent of the metabolic energy is used for muscle work, the rest is converted to heat. We calculated heat transfer by conduction and blood flow inside the body, and heat transfer from the skin by radiation, convection and sweat evaporation, resulting in temperature changes in 25 body compartments. We simulated a mountain time trial to Alpe d'Huez during the Tour de France. To approach the time realized by Lance Armstrong in 2004, very high oxygen uptake must be sustained by the simulated cyclist. Temperature was predicted to reach 39°C in the brain, and 39.7°C in leg muscle. In addition to the macroscopic simulation, we analysed the buffering of bursts of high adenosine triphosphate hydrolysis by creatine kinase during cyclical muscle activity at the biochemical pathway level. To investigate the low oxygen to carbohydrate ratio for the brain, which takes up lactate during exercise, we calculated the flux distribution in cerebral energy metabolism. Computational modelling of the human body, describing heat exchange and energy metabolism, makes simulation of endurance sports events feasible.

Application of the principles of systems biology and Wiener's cybernetics for analysis of regulation of energy fluxes in muscle cells in vivo

The mechanisms of regulation of respiration and energy fluxes in the cells are analyzed based on the concepts of systems biology, non-equilibrium steady state kinetics and applications of Wiener’s cybernetic principles of feedback regulation. Under physiological conditions cardiac function is governed by the Frank-Starling law and the main metabolic characteristic of cardiac muscle cells is metabolic homeostasis, when both workload and respiration rate can be changed manifold at constant intracellular level of phosphocreatine and ATP in the cells. This is not observed in skeletal muscles. Controversies in theoretical explanations of these observations are analyzed. Experimental studies of permeabilized fibers from human skeletal muscle vastus lateralis and adult rat cardiomyocytes showed that the respiration rate is always an apparent hyperbolic but not a sigmoid function of ADP concentration. It is our conclusion that realistic explanations of regulation of energy fluxes in muscle cells require systemic approaches including application of the feedback theory of Wiener’s cybernetics in combination with detailed experimental research. Such an analysis reveals the importance of limited permeability of mitochondrial outer membrane for ADP due to interactions of mitochondria with cytoskeleton resulting in quasi-linear dependence of respiration rate on amplitude of cyclic changes in cytoplasmic ADP concentrations. The system of compartmentalized creatine kinase (CK) isoenzymes functionally coupled to ANT and ATPases, and mitochondrial-cytoskeletal interactions separate energy fluxes (mass and energy transfer) from signalling (information transfer) within dissipative metabolic structures – intracellular energetic units (ICEU). Due to the non-equilibrium state of CK reactions, intracellular ATP utilization and mitochondrial ATP regeneration are interconnected by the PCr flux from mitochondria. The feedback regulation of respiration occurring via cyclic fluctuations of cytosolic ADP, Pi and Cr/PCr ensures metabolic stability necessary for normal function of cardiac cells.

Regulation of respiration controlled by mitochondrial creatine kinase in permeabilized cardiac cells in situ. Importance of system level properties

The main focus of this investigation is steady state kinetics of regulation of mitochondrial respiration in permeabilized cardiomyocytes in situ. Complete kinetic analysis of the regulation of respiration by mitochondrial creatine kinase was performed in the presence of pyruvate kinase and phosphoenolpyruvate to simulate interaction of mitochondria with glycolytic enzymes. Such a system analysis revealed striking differences in kinetic behaviour of the MtCK-activated mitochondrial respiration in situ and in vitro. Apparent dissociation constants of MgATP from its binary and ternary complexes with MtCK, Kia and Ka (1.94+/-0.86 mM and 2.04+/-0.14 mM, correspondingly) were increased by several orders of magnitude in situ in comparison with same constants in vitro (0.44+/-0.08 mM and 0.016+/-0.01 mM, respectively). Apparent dissociation constants of creatine, Kib and Kb (2.12+/-0.21 mM 2.17+/-0.40 Mm, correspondingly) were significantly decreased in situ in comparison with in vitro mitochondria (28+/-7 mM and 5+/-1.2 mM, respectively). Dissociation constant for phosphocreatine was not changed. These data may indicate selective restriction of metabolites' diffusion at the level of mitochondrial outer membrane. It is concluded that mechanisms of the regulation of respiration and energy fluxes in vivo are system level properties which depend on intracellular interactions of mitochondria with cytoskeleton, intracellular MgATPases and cytoplasmic glycolytic system.

Metabolic compartmentation - a system level property of muscle cells: real problems of diffusion in living cells

Problems of quantitative investigation of intracellular diffusion and compartmentation of metabolites are analyzed. Principal controversies in recently published analyses of these problems for the living cells are discussed. It is shown that the formal theoretical analysis of diffusion of metabolites based on Fick's equation and using fixed diffusion coefficients for diluted homogenous aqueous solutions, but applied for biological systems in vivo without any comparison with experimental results, may lead to misleading conclusions, which are contradictory to most biological observations. However, if the same theoretical methods are used for analysis of actual experimental data, the apparent diffusion constants obtained are orders of magnitude lower than those in diluted aqueous solutions. Thus, it can be concluded that local restrictions of diffusion of metabolites in a cell are a system-level properties caused by complex structural organization of the cells, macromolecular crowding, cytoskeletal networks and organization of metabolic pathways into multienzyme complexes and metabolons. This results in microcompartmentation of metabolites, their channeling between enzymes and in modular organization of cellular metabolic networks. The perspectives of further studies of these complex intracellular interactions in the framework of Systems Biology are discussed.

Musculoskeletal spectroscopy

Magnetic resonance spectroscopy (MRS) of skeletal muscle has been successfully applied by physiologists over several decades, particularly for studies of high-energy phosphates (by (31)P-MRS) and glycogen (by (13)C-MRS). Unfortunately, the observation of these heteronuclei requires equipment that is typically not available on clinical MR scanners, such as broadband capability and a second channel for decoupling and nuclear Overhauser enhancement (NOE). On the other hand, (1)H-MR spectra of skeletal muscle can be acquired on many routine MR systems and also provide a wealth of physiological information. In particular, studies of intramyocellular lipids (IMCL) attract physiologists and endocrinologists because IMCL levels are related to insulin resistance and thus can lead to a better understanding of major health problems in industrial countries. The combination of (1)H-, (13)C-, and (31)P-MRS gives access to the major long- and short-term energy sources of skeletal muscle. This review summarizes the technical aspects and unique MR-methodological features of the different nuclei. It reviews clinical studies that employed MRS of one or more nuclei, or combinations of MRS with other MR modalities. It also illustrates that MR spectra contain additional physiological information that is not yet used in routine clinical applications.

Skeletal muscle energetics with PNMR: personal views and historic perspectives

This article reviews historical and current NMR approaches to describing in vivo bioenergetics of skeletal muscles in normal and diseased populations. It draws upon the first author's more than 70 years of personal experience in enzyme kinetics and the last author's physiological approaches. The development of in vivo PNMR jointly with researchers around the world is described. It is explained how non-invasive PNMR has advanced human exercise biochemistry, physiology and pathology. Further, after a brief explanation of bioenergetics with PNMR on creatine kinase, anerobic glycolysis and mitochondrial oxidative phosphorylation, some basic and controversial subjects are focused upon, and the authors' view of the subjects are offered, with questions and answers. Some of the research has been introduced in exercise physiology. Future directions of NMR on bioenergetics, as a part of system biological approaches, are indicated.

CK flux or direct ATP transfer: versatility of energy transfer pathways evidenced by NMR in the perfused heart

How the myocardium is able to permanently coordinate its intracellular fluxes of ATP synthesis, transfer and utilization is difficult to investigate in the whole organ due to the cellular complexity. The adult myocardium represents a paradigm of an energetically compartmented cell since 50% of total CK activity is bound in the vicinity of other enzymes (myofibrillar sarcolemmal and sarcoplasmic reticulum ATPases as well as mitochondrial adenine nucleotide translocator, ANT). Such vicinity of enzymes is well known in vitro as well as in preparations of skinned fibers to influence the kinetic properties of these enzymes and thus the functioning of the subcellular organelles. Intracellular compartmentation has often been neglected in the NMR analysis of CK kinetics in the whole organ. It is indeed a methodological challenge to reveal subcellular kinetics in a working organ by a global approach such as NMR. To get insight in the energy transfer pathway in the perfused rat heart, we developed a combined analysis of several protocols of magnetization transfer associated with biochemical data and quantitatively evaluated which scheme of energetic exchange best describes the NMR data. This allows to show the kinetic compartmentation of subcellular CKs and to quantify their fluxes. Interestingly, we could show that the energy transfer pathway shifts from the phosphocreatine shuttle in the oxygenated perfused heart to a direct ATP diffusion from mitochondria to cytosol under moderate inhibition of ATP synthesis. Furthermore using NMR measured fluxes and the known kinetic properties of the enzymes, it is possible to model the system, estimate local ADP concentrations and propose hypothesis for the versatility of energy transfer pathway. In the normoxic heart, a 3-fold ADP gradient was found between mitochondrial intermembrane space, cytosol and ADP in the vicinity of ATPases. The shift from PCr to ATP transport observed when ATP synthesis decreases might result from a balance in the activity of two populations of ANT, either coupled or uncoupled to CK. We believe this NMR approach could be a valuable tool to reinvestigate the control of respiration by ADP in the whole heart reconciling the biochemical knowledge of mitochondrial obtained in vitro or in skinned fibers with data on the whole heart as well as to identify the implication of bioenergetics in the pathological heart.

Identification of subcellular energy fluxes by P NMR spectroscopy in the perfused heart: contractility induced modifications of energy transfer pathways

The identification of subcellular fluxes of exchange of ATP, phosphocreatine (PCr) and Pi between mitochondria, cytosol and ATPases and pathways of energy transfer in a whole organ is a challenge specially in the myocardium where 50% of creatine kinases (CK) are found in close vicinity of ATP producing (mito-CK) and utilizing (MM-bound CK) reactions. To dissect their contribution in cardiac energy transfer we recently developed a new experimental 31P NMR spectroscopy approach. This led to identify three kinetically different subcellular CKs and to evidence experimentally the CK shuttle in a rat heart perfused in isovolumy. Here we show that a decreased energy demand alters energetic pathways : two CKs (cytosolic and MM-bound) functioning at equilibrium and a non mitochondrial ATP<-->Pi exchange was sufficient to describe NMR data. Mito-CK fluxes was not detected anymore. This confirms the dependence of energy pathways upon cardiac activity. Indeed the subcellular localization and activity of CKs may have important bioenergetic consequences for the in vivo control of respiration at high work: free ADP estimated from global CK equilibrium might not always adequately reflect its concentration at the ANT.

The effects of rapid salinity change on in vivo arginine kinase flux in the juvenile blue crab, Callinectes sapidus

The effect of acclimation salinity and salinity changes on the concentration of high-energy phosphate metabolites and arginine kinase (AK) flux was examined in vivo in juvenile blue crabs using 31P-nuclear magnetic resonance (NMR). Crabs were acclimated for 7 days to a salinity of 5 or 35 per thousand and then placed in a flow apparatus that could sustain the animals while NMR spectra were acquired. Crabs were subjected to either hyperosmotic salinity changes, where an animal acclimated to 5 per thousand was exposed to a salinity of 35 per thousand, or hyposmotic changes, which involved the reciprocal exchange. Neither acclimation salinity nor salinity change had a significant effect on the concentrations of arginine phosphate, inorganic phosphate or ATP. 31P-NMR saturation transfer experiments were used to determine the effect of salinity on the forward and reverse flux of the AK reaction. There was no significant effect of acclimation salinity or salinity change on the flux rate through this reaction. This is in contrast to previous results, which showed that AK flux in isolated muscle was sensitive to prevailing osmotic conditions (Holt and Kinsey, J. Exp. Biol. 205 (2002) 1775-1785). The present study indicates that the integrated osmoregulatory capacity of the intact animal is sufficient to preserve cellular energy status and enzyme function during acute salinity changes.

Shaken and stirred: muscle structure and metabolism

Muscles are ideal models with which to examine the relationship between structure and metabolism because they are some of the most highly structured cells, and are capable of the largest and most rapid metabolic transitions as well as the highest metabolic rates known. Studies of metabolism have traditionally been conducted within what can considered as the kinetic paradigm provided by 'solution biochemistry'; i.e. the rates of enzymatic reactions are studied in terms of their regulation by mass-action and allosteric effectors and, most recently, metabolic control analysis of pathways. This approach has served biology well and continues to be useful. Here, we consider the diffusion of small and large molecules in muscles and energy metabolism in the context of intracellular space. We find that in attempting to explain certain phenomena, a purely kinetic paradigm appears insufficient. Instead, phenomena such as the 'shuttling' of high-energy phosphate donors and acceptors and the binding of metabolic enzymes to intracellular structures or to each other are better understood when metabolic rates and their regulation are considered in the context of intracellular compartments, distances, gradients and diffusion. As in all of biology, however, complexity dominates, and to such a degree that one pathway may consist of several reactions that each behave according to different rules. 'Soluble' creatine kinase operates at or near equilibrium, while mitochondrial and myofibrillar creatine kinases directly channel substrate to (or from) the adenine nucleotide translocase and actomyosin-ATPase, their operation being thus displaced from equilibrium. Hexose 6-phosphate metabolism appears to obey the rules of solution biochemistry, e.g. phosphoglucoisomerase behaves as Haldane would have predicted in 1930. In contrast, given low steady-state substrate and product concentrations and high flux rates, a number of glycolytic reactions further downstream must be catalyzed by enzymes localized in close proximity to each other. Metabolites may be channeled within these complexes. When observed, mechanistic differences between species in the same steps or processes should not be surprising, considering how animals vary so much in structures, mechanical properties, mitochondrial contents and metabolic rates. This analysis suggests that declarations of the triumph of one mechanism or paradigm over all others, as well as calls for the abandonment of solution biochemistry, are unwarranted. Rather, metabolic biochemistry would seem better served by reconciling the old and the new.

Presence of (phospho)creatine in developing and adult skeletal muscle of mice without mitochondrial and cytosolic muscle creatine kinase isoforms

We assessed the relationship between phosphocreatine (PCr) and creatine (Cr) content and creatine kinase (CK) activity in skeletal muscle of mice. The PCr and total Cr (tCr) concentrations, as well as CK activity, in hindlimb muscles of mice, with or without the cytosolic and mitochondrial isoforms of muscle creatine kinase (wild-type or CK--/-- mice), were determined by in vivo magnetic resonance (MR) spectroscopy and by biochemical means during postnatal growth and adulthood. In wild-type muscle the [tCr], PCr/ATP ratio and CK activity increased rapidly in the first 4-7 weeks. Remarkably, CK--/-- mice showed a similar increase in the PCr/ATP ratio during the first month in the presence of only minor brain-type BB-CK activity. Uptake of Cr in muscle was seemingly unrelated to CK activity as tCr increased in the same way in the muscles of both mouse types. At older ages the PCr/ATP ratio decreased in CK--/-- muscles, in contrast to wild-type where it still slowly increased, whereas [tCr] was similar for muscle of both mouse types. Using a new in vivo MR approach with application of [4-13C]Cr, a lower PCr/tCr ratio was also observed in CK--/-- muscle. From these data it follows that in vivo global ATP levels at rest are similar in the presence or absence of CK. Although Cr could still be converted to PCr in mature CK--/-- muscle, the immediate availability of PCr decreased, and PCr became partly inconvertible at older ages. Apparently, catalysis of the CK reaction by BB-CK, although significant in muscles of newborn mice, gradually declines to very low levels in adulthood. Part or all of this BB-CK may arise from satellite cells fusing with myotubes, a process that is most active during the first months of life. Finally, our observation that the MR and chemical assessment of muscle [tCr] and PCr/tCr ratio were similar for all mice does not support the existence of a significant MR-invisible or immobile pool of Cr, with a role for CK in this phenomenon.

Issues of exploration: human health and wellbeing during a mission to Mars

Today, the tools are in our hands to enable us to travel away from our home planet and become citizens of the solar system. Even now, we are seriously beginning to develop the robust infrastructure that will make the 21st century the Century of Space Travel. But this bold step must be taken with due concern for the health, safety and wellbeing of future space explorers. Our long experience with space biomedical research convinces us that, if we are to deal effectively with the medical and biomedical issues of exploration, then dramatic and bold steps are also necessary in this field. We can no longer treat the human body as if it were composed of muscles, bones, heart and brain acting independently. Instead, we must lead the effort to develop a fully integrated view of the body, with all parts connected and fully interacting in a realistic way. This paper will present the status of current (2000) plans by the National Space Biomedical Research Institute to initiate research in this area of integrative physiology and medicine. Specifically, three example projects are discussed as potential stepping stones towards the ultimate goal of producing a digital human. These projects relate to developing a functional model of the human musculoskeletal system and the heart.

Osmotic effects on arginine kinase function in living muscle of the blue crab Callinectes sapidus

Flux was examined through the reaction catalyzed by arginine kinase in intact blue crab (Callinectes sapidus) muscle during simulated changes in salinity. Isolated dark levator muscles from the swimming leg were superfused with a saline solution that had an osmolarity equivalent to that of the hemolymph under different salinity regimes. Animals were acclimated for 7 days to a salinity of 5, 17 or 35‰, which corresponds to a hemolymph osmolarity of 640, 720 or 960 mosmoll-1, respectively. Experiments were conducted under control conditions, in which the osmolarity of the superfusion medium matched that of the acclimated hemolymph, as well as under hypo- and hyperosmotic conditions. These latter treatments were meant to simulate a rapid change in environmental salinity. Pseudo-first-order unidirectional rate constants and flux rates were measured for arginine kinase in the forward and reverse directions using a 31P-nuclear magnetic resonance saturation transfer method. There were no differences in the rate constants or flux rates among the controls, indicating that arginine kinase function is not modulated by salinity if the animal has had sufficient acclimation time. However, the rate constants and flux rates of arginine kinase varied over a modest 1.7-fold range across the three types of osmotic treatments, although the range for the flux data was reduced when cell volume changes were taken into account. The hyperosmotic treatments led to a reduction in arginine kinase flux, while the hypo-osmotic treatments led to an enhanced arginine kinase flux. We propose that this effect is mediated by an increase in the concentration of perturbing inorganic ions under hyperosmotic conditions and a decrease in the concentration of such ions during the hypo-osmotic treatments.

31P NMR detection of subcellular creatine kinase fluxes in the perfused rat heart: contractility modifies energy transfer pathways

The subcellular fluxes of exchange of ATP and phosphocreatine (PCr) between mitochondria, cytosol, and ATPases were assessed by (31)P NMR spectroscopy to investigate the pathways of energy transfer in a steady state beating heart. Using a combined analysis of four protocols of inversion magnetization transfer associated with biochemical data, three different creatine kinase (CK) activities were resolved in the rat heart perfused in isovolumic control conditions: (i) a cytosolic CK functioning at equilibrium (forward, F(f) = PCr --> ATP, and reverse flux, F(r) = ATP --> PCr = 3.3 mm.s(-1)), (ii) a CK localized in the vicinity of ATPases (MM-CK bound isoform) favoring ATP synthesis (F(f) = 1.7 x F(r)), and (iii) a mitochondrial CK displaced toward PCr synthesis (F(f) = 0.3 and F(r) = 2.6 mm.s(-1)). This study thus provides the first experimental evidence that the energy is carried from mitochondria to ATPases by PCr (i.e. CK shuttle) in the whole heart. In contrast, a single CK functioning at equilibrium was sufficient to describe the data when ATP synthesis was partly inhibited by cyanide (0.15 mm). In this case, ATP was directly transferred from mitochondria to cytosol suggesting that cardiac activity modified energy transfer pathways. Bioenergetic implications of the localization and activity of enzymes within myocardial cells are discussed.

Transgenic livers expressing mitochondrial and cytosolic CK: mitochondrial CK modulates free ADP levels

The function of creatine kinase (CK) and its effect on phosphorus metabolites was studied in livers of transgenic mice expressing human ubiquitous mitochondrial CK (CK-Mit) and rat brain CK (CK-B) isoenzymes and their combination. (31)P NMR spectroscopy and saturation transfer were recorded in livers of anesthetized mice to measure high-energy phosphates and hepatic CK activity. CK reaction velocity was related to total enzyme activity irrespective of the isoenzyme expressed, and it increased with increasing concentrations of creatine (Cr). The fluxes mediated by both isoenzymes in both directions (phosphocreatine or ATP synthesis) were equal. Over a 20-fold increase in CK-Mit activity (28-560 micromol. g wet wt(-1). min(-1)), the fraction of phosphorylated Cr increased 1.6-fold. Hepatic free ADP concentrations calculated by assuming equilibrium of the CK-catalyzed reaction in vivo decreased from 84 +/- 9 to 38 +/- 4 nmol/g wet wt. Calculated free ADP levels in mice expressing high levels of CK-B (920-1,635 micromol. g wet wt(-1). min(-1)) were 52 +/- 6 nmol/g wet wt. Mice expressing both isoenzymes had calculated free ADP levels of 36 +/- 4 nmol/g wet wt. These findings indicate that CK-Mit catalyzes its reaction equally well in both directions and can lower hepatic apparent free ADP concentrations.

Discrimination of cardiac subcellular creatine kinase fluxes by NMR spectroscopy: a new method of analysis

A challenge in the understanding of creatine kinase (CK) fluxes reflected by NMR magnetization transfer in the perfused rat heart is the choice of a kinetic model of analysis. The complexity of the energetic pathways, due to the presence of adenosine triphosphate (ATP)-inorganic phosphate (Pi) exchange, of metabolite compartmentation and of subcellular localization of CK isozymes cannot be resolve from the sole information obtained from a single NMR protocol. To analyze multicompartment exchanges, we propose a new strategy based on the simultaneous analysis of four inversion transfer protocols. The time course of ATP and Phosphocreatine (PCr) magnetizations computed from the McConnell equations were adjusted to their experimental value for exchange networks of increasing complexity (up to six metabolite pools). Exchange schemes were selected by the quality of their fit and their consistency with data from other sources: the size of mitochondrial pools and the ATP synthesis flux. The consideration of ATP-Pi exchange and of ATP compartmentation were insufficient to describe the data. The most appropriate exchange scheme in our normoxic heart involved the discrimination of three specific CK activities (cytosolic, mitochondrial, and close to ATPases). At the present level of heart contractility, the energy is transferred from mitochondria to myofibrils mainly by PCr.

Creatine loading and resting skeletal muscle phosphocreatine flux: a saturation-transfer NMR study

31P saturation-transfer nuclear magnetic resonance spectroscopy was used to study skeletal muscle phosphocreatine (PCr) flux in healthy male volunteers. Data analysis included consideration of effects from incomplete saturation and radiofrequency spillover. Spectra were recorded from the resting gastrocnemius muscle before and after 6 days of creatine monohydrate (Cr-H2O) intake (20 g/day). Parallel to an improved muscle performance during maximal intermittent exercise following Cr-H2O supplementation, the concentration of PCr increased (P=0.01) by 23% (34.9+/-2.8 mmol/l vs. 28.6+/-2.7 mmol/l), whereas other metabolites were unaffected (inorganic phosphate: 4.3+/-1.4 mmol/l, free intracellular Mg(2+): 1.1+/-0.7 mmol/l, cytosolic pH: 7.04+/-0.02). Forward and reverse fluxes through the creatine kinase (CK) reaction did not change significantly from their baseline levels (v(for): 11.8+/-5.4 mmol/l per second vs. 15.3+/-6.8 mmol/l per second, (v(rev): 9.5+/-3.4 mmol/l per second vs. 10.9+/-3.7 mmol/l per second). The rate of PCr resynthesis in resting muscle is not limited by the CK reaction, which is near equilibrium. Consequently, the post-load increase in total creatine has no effect on the unidirectional CK reaction rates.

A model of oxidative phosphorylation in mammalian skeletal muscle

A dynamic computer model of oxidative phosphorylation in oxidative mammalian skeletal muscle was developed. The previously published model of oxidative phosphorylation in isolated skeletal muscle mitochondria was extended by incorporation of the creatine kinase system (creatine kinase plus phosphocreatine/creatine pair), cytosolic proton production/consumption system (proton production/consumption by the creatine kinase-catalysed reaction, efflux/influx of protons), physiological size of the adenine nucleotide pool and some additional minor changes. Theoretical studies performed by means of the extended model demonstrated that the CK system, which allows for large changes in P(i) in relation to isolated mitochondria system, has no significant influence on the kinetic properties of oxidative phosphorylation, as inorganic phosphate only slightly modifies the relationship between the respiration rate and [ADP]. Computer simulations also suggested that the second-order dependence of oxidative phosphorylation on [ADP] proposed in the literature refers only to the ATP synthesis flux, but not to the oxygen consumption flux (the difference between these two fluxes being due to the proton leak). Next, time courses of changes in fluxes and metabolite concentrations during transition between different steady-states were simulated. The model suggests, in accordance with previous theoretical predictions, that activation of oxidative phosphorylation by an increase in [ADP] can (roughly) explain the behaviour of the system only at low work intensities, while at higher work intensities parallel activation of different steps of oxidative phosphorylation is involved.

Evolution and physiological roles of phosphagen systems

Phosphagens are phosphorylated guanidino compounds that are linked to energy state and ATP hydrolysis by corresponding phosphagen kinase reactions: phosphagen + MgADP + H(+) <--> guanidine acceptor + MgATP. Eight different phosphagens (and corresponding phosphagen kinases) are found in the animal kingdom distributed along distinct phylogenetic lines. By far, the creatine phosphate/creatine kinase (CP/CK) system, which is found in the vertebrates and is widely distributed throughout the lower chordates and invertebrates, is the most extensively studied phosphagen system. Phosphagen kinase reactions function in temporal ATP buffering, in regulating inorganic phosphate (Pi) levels, which impacts glycogenolysis and proton buffering, and in intracellular energy transport. Phosphagen kinase reactions show differences in thermodynamic poise, and the phosphagens themselves differ in terms of certain physical properties including intrinsic diffusivity. This review evaluates the distribution of phosphagen systems and tissue-specific expression of certain phosphagens in an evolutionary and functional context. The role of phosphagens in regulation of intracellular Pi levels likely evolved early. Thermodynamic poise of the phosphagen kinase reaction profoundly impacts this capacity. Furthermore, it is hypothesized that the capacity for intracellular targeting of CK evolved early as a means of facilitating energy transport in highly polarized cells and was subsequently exploited for temporal ATP buffering and dynamic roles in metabolic regulation in cells displaying high and variable rates of aerobic energy production.

Control of oxidative phosphorylation in skeletal muscle

The classical concept of ATP-demand control of energy metabolism in skeletal muscle has to be modified on the basis of studies showing the influence of additional controlling parameters (reducing equivalent supply, oxygen availability, proton leak, diffusion restrictions and the creatine kinase system) and on the basis of applications of metabolic control analysis showing very clearly multistep control. This concept of multistep control allows to quantify the individual influence of any parameter on mitochondrial oxidative phosphorylation and is extremely helpful to analyze the metabolic consequences of enzyme deficiencies in skeletal muscle occurring in mitochondrial myopathies.

Cellular energetics analysis by a mathematical model of energy balance: estimation of parameters in human skeletal muscle

Cellular energy balance requires that the physiological demands by ATP-utilizing functions be matched by ATP synthesis to sustain muscle activity. We devised a new method of analysis of these processes in data from single individuals. Our approach is based on the logic of current information on the major mechanisms involved in this energy balance and can quantify not directly measurable parameters that govern those mechanisms. We use a mathematical model that simulates by ordinary, nonlinear differential equations three components of cellular bioenergetics (cellular ATP flux, mitochondrial oxidative phosphorylation, and creatine kinase kinetics). We incorporate data under resting conditions, during the transition toward a steady state of stimulation and during the transition during recovery back to the original resting state. Making use of prior information about the kinetic parameters, we fitted the model to previously published dynamic phosphocreatine (PCr) and inorganic phosphate (P(i)) data obtained in normal subjects with an activity-recovery protocol using (31)P nuclear magnetic resonance spectroscopy. The experiment consisted of a baseline phase, an ischemic phase (during which muscle stimulation and PCr utilization occurred), and an aerobic recovery phase. The model described satisfactorily the kinetics of the changes in PCr and P(i) and allowed estimation of the maximal velocity of oxidative phosphorylation and of the net ATP flux in individuals both at rest and during stimulation. This work lays the foundation for a quantitative, model-based approach to the study of in vivo muscle energy balance in intact muscle systems, including human muscle.

Evidence for myocardial ATP compartmentation from NMR inversion transfer analysis of creatine kinase fluxes

The interpretation of creatine kinase (CK) flux measured by (31)P NMR magnetization transfer in vivo is complex because of the presence of competing reactions, metabolite compartmentation, and CK isozyme localization. In the isovolumic perfused rat heart, we considered the influence of both ATP compartmentation and ATP-P(i) exchange on the forward (F(f): PCr --> ATP) and reverse (F(r)) CK fluxes derived from complete analysis of inversion transfer. Although F(f) should equal F(r) because of the steady state, in both protocols when PCr (inv-PCr) or ATP (inv-ATP) was inverted and the contribution of ATP-P(i) was masked by saturation of P(i) (sat-P(i)), F(f)/F(r) significantly differed from 1 (0.80 +/- 0.06 or 1.32 +/- 0.06, respectively, n = 5). These discrepancies could be explained by a compartment of ATP (f(ATP)) not involved in CK. Consistently, neglecting ATP compartmentation in the analysis of CK in vitro results in an underestimation of F(f)/F(r) for inv-PCr and its overestimation for inv-ATP. Both protocols gave access to f(ATP) if the system was adequately analyzed. The fraction of ATP not involved in CK reaction in a heart performing medium work amounts to 20-33% of cellular ATP. Finally, the data suggest that the effect of sat-P(i) might not result only from the masking of ATP-P(i) exchange.

Temperature dependence of human gastrocnemius pH and high-energy phosphate concentration by noninvasive techniques

It is well established that ADP is an important regulator of the oxidative phosphorylation in the mitochondria. Thus, by means of noninvasive techniques it is demonstrated that the relationship between O(2) consumption of the human gastrocnemius at rest and its temperature is likely determined by at least two factors: 1) the modulation of the rate of the chemical reactions imposed by the "physical" temperature-effect; 2) the influence of temperature-induced ADP concentration changes ( approximately 0.83 microM degrees C(-1)) on oxidative phosphorylation. ADP was assessed by applying the temperature-corrected Lohmann equilibrium equation. PCr and ATP were found to increase, with decreasing temperature (-0.54+/-0.05 and -0.17+/-mM degrees C(-1), respectively), while pH varies following the alpha-stat hypothesis (-0.016+/-0.001 pH degrees C(-1)). These findings should be of value when dealing with muscle physiology in extreme environments or clinical applications of hypothermia.

Myofibrillar or mitochondrial creatine kinase deficiency alone does not impair mouse diaphragm isotonic function

Creatine kinase (CK) provides ATP buffering in skeletal muscle and is expressed as 1) cytosolic myofibrillar CK (M-CK) and 2) sarcomeric mitochondrial CK (ScCKmit) isoforms that differ in their subcellular localization. The diaphragm (Dia) expresses both M-CK and ScCKmit in abundance. We compared the power and work output of 1) control CK-sufficient (Ctl), 2) M-CK-deficient [M-CK(-/-)], 3) ScCKmit-deficient [ScCKmit(-/-)], and 4) combined M-CK/ScCKmit-deficient null mutant [CK(-/-)] Dia during repetitive isotonic activations to determine the effect of CK phenotype on Dia function. Maximum power was obtained at approximately 0.4 tetanic force in all groups. M-CK(-/-) and ScCKmit(-/-) Dia were able to sustain power and work output at Ctl levels during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load), and the duration of sustained Dia shortening was 67 +/- 4 s in M-CK(-/-), 60 +/- 4 s in ScCKmit(-/-), and 62 +/- 5 s in Ctl Dia. In contrast, CK(-/-) Dia power and work declined acutely and failed to sustain shortening altogether by 40 +/- 6 s. We conclude that Dia power and work output are not absolutely dependent on the presence of either M-CK or ScCKmit, whereas the complete absence of CK acutely impairs Dia shortening capacity during repetitive activation.

Regulation of ATP supply in mammalian skeletal muscle during resting state-->intensive work transition

In the present debating paper, the problem how the rate of ATP supply by oxidative phosphorylation in mitochondria is adjusted to meet a greatly increased demand for ATP during intensive exercise of skeletal muscle is discussed. Different experimental results are collected from different positions of the literature and confronted with five conceptual models of the regulation of the oxidative phosphorylation system. The previously performed computer simulations using a dynamic model of oxidative phosphorylation are also discussed in this context. The possible regulatory mechanisms considered in the present article are: (A) output activation: an external effector activates directly only the output of the system (ATP turnover); (B) input/output activation: an external effector activates directly the output (ATP usage) and input (substrate dehydrogenation) of the system; (C) removal of substrate shortage: only ATP consumption and substrate supply by blood are directly activated; (D) removal of oxygen shortage: only ATP consumption and oxygen supply by blood are directly activated; (E) each step activation: an external effector activates both the ATP-consuming subsystem and all the steps in the ATP-producing subsystem (particular enzymes/carriers/blocks of oxidative phosphorylation, substrate supply, oxygen supply). The performed confrontation of the considered mechanisms with the presented results leads to the conclusion that only the each step activation model is quantitatively consistent with the whole set of experimental data discussed. It is therefore postulated that a universal effector/regulatory mechanism of a still unknown nature which activates all steps of oxidative phosphorylation should exist and be discovered. A possible nature of such an effector is shortly discussed.

Kinetics of PCr to ATP and beta-ATP to beta-ADP phosphoryl conversion are modified in working rat skeletal muscle after training

Kinetics of phosphoryl transfers from PCr to gamma-ATP and from beta-ATP to beta-ADP were measured by magnetization transfer in an in vivo 31P NMR experiment in working rat skeletal hind leg muscles. Two groups were examined. One group was submitted to a 6-week training program of treadmill running. The other group was composed of sedentary animals. Metabolic oxidative capacity and mechanical performance were improved greatly by training as shown previously. Phosphoryl transfer of PCr-->gamma-ATP or beta-ATP-->beta-ADP total fluxes were identical in resting trained and untrained muscles. Under stimulation, the flux of creatine kinase transfer was significantly inhibited by 23% compared with resting level in untrained muscles; by contrast, it was not inhibited and maintained at the high resting level in trained muscles. Thus physiological changes probably linked to a decrease of the production of anions, which could inhibit creatine kinase, were able to maintain creatine kinase flux. The flux of beta-ATP to beta-ADP transfer were enhanced largely in working muscles from 1.4+/-0.8 and 2+/-0.8 at rest to 4+/-1.6 and 6.6+/-2.7 mM s(-1) for untrained and trained muscles respectively; the effect was more pronounced in trained than in untrained muscles. These results showed an acceleration of phosphoryl turnover in working muscles after training, which could contribute to improve oxidative and mechanical performances. Such kinetic measurements of phosphoryl conversion may provide information on ATP turnover in pathophysiologic situations where ADP accumulates because of impaired ATP synthesis (mitochondrial myopathies, lower perfusion level).

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

The Cardiome Project. An integrated view of cardiac metabolism and regional mechanical function

The goal, to develop a functioning three-dimensional computational model of the excitation, metabolism and contraction of the heart within three years, is one of the beginnings for the Cardiome Project. Our first stage will not be likely to provide highly accurate prediction of physiological behavior in general, but will be focussed so that it is adequate for at least three specific purposes: response to regional flow reduction, response to heart rate changes, and response to increased metabolic drive. We would like to make the model visualizable by three-dimensional viewing, with cross-sectional and transparency viewing approaches, illustrate the fiber directions, the arteries, the deformation with contraction and images of regional functions such as oxygen consumption, preejection strain, or lactate concentration. The display techniques developed by Hunter et al. and by McCulloch et al. would be excellent for such demonstration and teaching purposes, and should be attractive enough for public display. The Physiome Project is underway now, with growing government and private support. Now we are going from the era of molecular biology, led by the Genome Project, into a new era of integrative biology. The goal is to understand biology so deeply and so broadly that predictions about interventions can be made. Methods of experimentation and of diagnosis are critical to acquiring the data, and therefore in making the prediction, and so all aspects of our Society's efforts and interests are relevant to undertaking this grand challenge.

Recovery of free ADP, Pi, and free energy of ATP hydrolysis in human skeletal muscle

We measured significant undershoots of the concentrations of free ADP ([ADP]) and Pi ([Pi]) and the free energy of ATP hydrolysis (DeltaGATP) below initial resting levels during recovery from severe ischemic exercise with 31P-nuclear magnetic resonance spectroscopy in 11 healthy sports students. Undershoots of the rate of oxidative phosphorylation would be predicted if the rate of oxidative phosphorylation would depend solely on free [ADP], [Pi], or DeltaGATP. However, undershoots of the rate of oxidative phosphorylation have not been reported in the literature. Furthermore, undershoots of the rate of oxidative phosphorylation are unlikely because there is evidence that a balance between ATP production and consumption cannot be achieved if an undershoot of the rate of oxidative phosphorylation actually occurs. Therefore, oxidative phosphorylation seems to depend not only on free [ADP], [Pi], or DeltaGATP. An explanation is that acidosis-related or other factors control oxidative phosphorylation additionally, at least under some conditions.

Rate of phosphate release after photoliberation of adenosine 5'-triphosphate in slow and fast skeletal muscle fibers

Inorganic phosphate (Pi) release was determined by means of a fluorescent Pi-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of Pi release followed photoliberation of approximately 1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence and presence of Ca2+ at 15 degrees C. In the absence of Ca2+, Pi release occurred with a slow rate of 11 +/- 3 microM . s-1 (n = 3) in soleus fibers and 23 +/- 1 microM . s-1 (n = 10) in psoas fibers. At saturating Ca2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of Pi release was 0.57 +/- 0.05 mM . s-1 in soleus (n = 13) and 4.7 +/- 0.2 mM . s-1 in psoas (n = 23), corresponding to a rate of Pi release per myosin head of 3.8 s-1 in soleus and 31.5 s-1 in psoas. Pi release declined at a rate of 0.48 s-1 in soleus and of 5.2 s-1 in psoas. Pi release in soleus was slightly faster in the presence of an ATP regenerating system but slower when 0.5 mM ADP was added. The reduction in the rate of Pi release results from an initial redistribution of cross-bridges over different states and a subsequent ADP-sensitive slowing of cross-bridge detachment.

Functional aspects of the X-ray structure of mitochondrial creatine kinase: a molecular physiology approach

Mitochondrial creatine kinase (Mi-CK) is a central enzyme in energy metabolism of tissues with high and fluctuating energy requirements. In this review, recent progress in the functional and structural characterization of Mi-CK is summarized with special emphasis on the solved X-ray structure of chicken Mib-CK octamer (Fritz-Wolf et al., Nature 381, 341-345, 1996). The new results are discussed in a historical context and related to the characteristics of CK isoforms as known from a large number of biophysical and biochemical studies. Finally, two hypothetical functional aspects of the Mi-CK structure are proposed: (i) putative membrane binding motifs at the top and bottom faces of the octamer and (ii) a possible functional role of the central 20 A channel.

Theoretical modelling of some spatial and temporal aspects of the mitochondrion/creatine kinase/myofibril system in muscle

After discussing approaches to the modelling of mitochondrial regulation in muscle, we describe a model that takes account, in a simplified way, of some aspects of the metabolic and physical structure of the energy production/usage system. In this model, high-energy phosphates (ATP and phosphocreatine) and low energy metabolites (ADP and creatine) diffuse between the mitochondrion and the myofibrillar ATPase, and can be exchanged at any point by creatine kinase. Creatine kinase is not assumed to be at equilibrium, so explicit account can be taken of substantial changes in its activity of the sort that can now be achieved by transgenic technology in vivo. The ATPase rate is the input function. Oxidative ATP synthesis is controlled by juxtamitochondrial ADP concentration. To allow for possible functional 'coupling' between the components of creatine kinase associated with the mitochondrial adenine nucleotide translocase and the myofibrillar ATPase, we define parameters phi and psi that set the fraction of the total flux carried by ATP rather than phosphocreatine out of the mitochondrial unit and into the ATPase unit, respectively. This simplification is justified by a detailed analysis of the interplay between the mitochondrial outer membrane porin proteins, mitochondrial creatine kinase and the adenine nucleotide translocase. As both processes of possible 'coupling' are incorporated into the model as quantitative parameters, their effect on the energetics of the whole cell model can be explicitly assessed. The main findings are as follows: (1) At high creatine kinase activity, the hyperbolic relationship of oxidative ATP synthesis rate to spatially averaged ADP concentration at steady state implies also a near-linear relationship to creatine concentration, and a sigmoid relation to free energy of ATP hydrolysis. At high creatine kinase activity, the degree of functional coupling at either the mitochondrial or ATPase end has little effect on these relationships. However, lowering the creatine kinase activity raises the mean steady state ADP and creatine concentrations, and this is exaggerated when phi or psi is near unity (i.e. little coupling). (2) At high creatine kinase activity, the fraction of flow at steady state carried in the middle of the model by ATP is small, unaffected by the degree of functional coupling, but increases with ADP concentration and rate of ATP turnover. Lowering the creatine kinase activity raises this fraction, and this is exaggerated when psi or psi is near unity. (3) Both creatine and ADP concentrations show small gradients decreasing towards the mitochondrion (in the direction of their net flux), while ATP and phosphocreatine concentration show small gradients decreasing towards the myosin ATPase. Unless phi = psi = 0 (i.e. complete coupling), there is a gradient of net creatine kinase flux that results from the need to transform some of the 'adenine nucleotide flux' at the ends of the model into 'creatine flux' in the middle; the overall net flux is small, but only zero if phi = psi. A reduction in cytosolic creatine kinase activity decreases ADP concentration at the mitochondrial end and increases it at the ATPase end. (4) During work-jump transitions, spatial average responses exhibit exponential kinetics similar to those of models of mitochondrial control that assume equilibrium conditions for creatine kinase. (5) In response to a step increase in ATPase activity, concentration changes start at the ATPase end and propagate towards the mitochondrion, damped in time and space. This simplified model embodies many important features of muscle in vivo, and accommodates a range of current theories as special cases. We end by discussing its relationship to other approaches to mitochondrial regulation in muscle, and some possible extensions of the model.

In situ measurements of creatine kinase flux by NMR. The lessons from bioengineered mice

P-31 nuclear magnetic resonance (NMR) is uniquely suited to measure the kinetics of the phosphoryl-exchange reaction catalyzed by creatine kinase in intact mammalian tissue, especially striated muscle. Recently developed transgenic mouse models of the creatine kinase iso-enzyme system open novel opportunities to assess the functional importance of the individual iso-enzymes and their relative contribution to the total in situ flux through the CK reaction. This chapter reviews the most recent findings from NMR flux measurements on such genetic models of CK function. Findings in intact mouse skeletal and cardiac muscle in vivo are compared to data from purified mitochondrial and cytosolic creatine kinase in vitro. The relevance of findings in transgenic animals for the function of CK in wild-type tissue is described and the perspectives of transgenic techniques in future quantitative studies on the creatine kinase iso-enzyme system are indicated.

Functional coupling of creatine kinases in muscles: species and tissue specificity

Creatine kinase (CK) isoenzymes are present in all vertebrates. An important property of the creatine kinase system is that its total activity, its isoform distribution, and the concentration of guanidino substrates are highly variable among species and tissues. In the highly organized structure of adult muscles, it has been shown that specific CK isoenzymes are bound to intracellular compartments, and are functionally coupled to enzymes and transport systems involved in energy production and utilization. It is however, not established whether functional coupling and intracellular compartmentation are present in all vertebrates. Furthermore, these characteristics seem to be different among different muscle types within a given species. This study will review some of these aspects. It has been observed that: (1) In heart ventricle, CK compartmentation and coupling characterize adult mammalian cells. It is almost absent in frogs, and is weakly present in birds. (2) Efficient coupling of MM-CK to myosin ATPase is seen in adult mammalian striated muscles but not in frog and bird heart where B-CK is expressed instead of M-CK. Thus, the functional efficacy of bound MM-CK to regulate adenine nucleotide turnover within the myofibrillar compartment seems to be specific for muscles expressing M-CK as an integral part of the sarcomere. (3) Mi-CK expression and/or functional coupling are highly tissue and species specific; moreover, they are subject to short term and long term adaptations, and are present late in development. The mitochondrial form of CK (mi-CK) can function in two modes depending on the tissue: (i) in an <<ADP regeneration mode>> and (ii) in an <<ADP amplification mode>>. The mode of action of mi-CK seems to be related to its precise localization within the mitochondrial intermembrane space, whereas its amount might control the quantitative aspects of the coupling. Mi-CK is highly plastic, making it a strong candidate for fine regulation of excitation-contraction coupling in muscles and for energy transfer in cells with large and fluctuating energy demands in general. (4) Although CK isoforms show a binding specificity, the presence of a given isoform within a tissue or a species only, does not predict its functional role. For example, M-CK is expressed before it is functionally compartmentalized within myofibrils during development. Similarly, the presence of ubiquitous or sarcomeric mi-CK isoforms, is not an index of functional coupling of mi-CK to oxidative phosphorylation. (5) Amongst species or muscles, it appears that a large buffering action of the CK system is associated with rapid contraction and high glycolytic activity. On the other hand, an oxidative metabolism is associated with isoform diversity, increased compartmentation, a subsequent low buffering action and efficient phosphotransfer between mitochondria and energy utilization sites. It can be concluded that, in addition to a high variation of total activity and isoform expression, the role of the CK system also critically depends on its intracellular organization and interaction with energy producing and utilizing pathways. This compartmentation will determine the high cellular efficiency and fine specialization of highly organized and differentiated muscle cells.

Energy balance in muscle activity: simulations of ATPase coupled to oxidative phosphorylation and to creatine kinase

Energy balance refers to the dynamic homeostasis of ATP and related forms of chemical potential within cells. This regulation is accomplished mainly by oxidative metabolism in most mammals. This homeostasis matches dynamically the energy demands of cellular ATPases (net decrease in chemical potential energy) with the energy supply by mitochondrial oxidative phosphorylation (net increase in chemical potential energy). Muscle cells are distinguished from most other cell types in their ability to attain energy balance with more than a 10-fold range of ATPase demand. Creatine kinase maintains a near to equilibrium flux: PCr + ADP<-->ATP + Cr. One important function of creatine kinase is to buffer ATP and ADP concentrations. A system of differential equations describe the coupled operation of cellular ATPase, creatine kinase and oxidative phosphorylation. These equations used experimentally measured concentrations of relevant metabolites and enzyme activities to simulate energy balance in muscle cells. The principle of energy balance is adequately illustrated by simulations with only a three component system.

Absence of myofibrillar creatine kinase and diaphragm isometric function during repetitive activation

Creatine kinase (CK) provides ATP buffering in skeletal muscle and is expressed as 1) cytosolic myofibrillar CK (M-CK) and 2) sarcomeric mitochondrial CK (ScCKmit) isoforms that differ in their subcellular localization. We compared the isometric contractile and fatigue properties of 1) control CK-sufficient (Ctl), 2) M-CK-deficient (M-CK[-/-]), and 3) combined M-CK/ScCKmit-deficient null mutant (CK[-/-]) diaphragm (Dia) to determine the effect of the absence of M-CK activity on Dia performance in vitro. Baseline contractile properties were comparable across groups except for specific force, which was approximately 16% lower in CK[-/-] Dia compared with M-CK[-/-] and Ctl Dia. During repetitive activation (40 Hz, (1)/(3) duty cycle), force declined in all three groups. This decline was significantly greater in CK[-/-] Dia compared with Ctl and M-CK[-/-] Dia. The pattern of force decline did not differ between M-CK[-/-] and Ctl Dia. We conclude that Dia isometric muscle function is not absolutely dependent on the presence of M-CK, whereas the complete absence of CK acutely impairs isometric force generation during repetitive activation.

In vivo phosphocreatine and ATP in piglet cerebral gray and white matter during seizures

The creatine kinase (CK) reaction is thought to be important in coupling ATP metabolism and regulating ADP concentration in tissues with high and variable ATP turnover, including cerebral gray matter (GM). There is low phosphocreatine (PCr), low CK reaction rates, and high mitochondrial CK (MiCK) isoenzyme activity in GM compared to white matter (WM). To compare the CK reaction in GM and WM when ATP metabolism is high, CK reactants and reaction rates were measured in predominantly GM and WM slices in vivo in 2 and 14-day old piglets during pentylenetetrazole (PTZ) seizures using 31P nuclear magnetic resonance (NMR) 1-dimensional chemical shift imaging (CSI). Arterial pressure, temperature, and blood gasses were stable at both ages. Before seizures, the PCr/nucleoside triphosphate (NTP) ratio was higher in WM than GM at both ages with a developmental increase seen in WM. The CK reaction rate constant increased in both regions between 2 and 14 days. During seizures, PCr/NTP increased in GM at 14 days due to increased PCr while the ratio and PCr decreased in WM. The NTP was more stable in WM and GM at both ages. The CK reaction rate decreased in both regions more at 2 than at 14 days. Thus, brain ATP, deduced from NTP, is stable during seizures in the piglet. In GM stable ATP is associated with a unique increase in PCR concentration.

Some new aspects of creatine kinase (CK): compartmentation, structure, function and regulation for cellular and mitochondrial bioenergetics and physiology

Creatine kinase (CK) isoenzymes, specifically located at places of energy demand and energy production, are linked by a phosphocreatine/creatine (PCr/Cr) circuit, found in cells with intermittently high energy demands. Cytosolic CKs, in close conjunction with Ca(2+)-pumps, play a crucial role for the energetics of Ca(2+)-homeostasis. Mitochondrial Mi-CK, a cuboidal-shaped octamer with a central channel, binds and crosslinks mitochondrial membranes and forms a functionally coupled microcompartment with porin and adenine nucleotide translocase for vectorial export of PCr into the cytosol. The CK system is regulated by AMP-activated protein kinase via PCr/Cr and ATP/AMP ratios. Mi-CK stabilizes and cross-links cristae- or inner/outer membranes to form parallel membrane stacks and, if overexpressed due to creatine depletion or cellular energy stress, forms those crystalline intramitochondrial inclusions seen in some mitochondrial cytopathy patients. Mi-CK is a prime target for free radical damage by peroxynitrite. Mi-CK octamers, together with CK substrates have a marked stabilizing and protective effect against mitochondrial permeability transition pore opening, thus providing a rationale for creatine supplementation of patients with neuromuscular and neurodegenerative diseases.