Intracellular energetic units in red muscle cells

Heterogeneity of mitochondria and mitochondrial function within cells as another level of mitochondrial complexity

Beyond their fundamental role in energy metabolism, mitochondria perform a great variety of other important cellular functions. However, the interplay among these various roles of mitochondria is still poorly understood, and the underlying mechanisms can be related to system level properties. Importantly, mitochondria localized in different regions of a cell may display different morphology, dissimilar biochemical properties, or may differently interact with other intracellular structures. Recent advances in live imaging techniques have also revealed a functional heterogeneity of mitochondria with respect to mitochondrial redox state, membrane potential, respiratory activity, uncoupling proteins, mitochondrial ROS and calcium. An important and still unresolved question is how the heterogeneity of mitochondrial function and the regional specializations of mitochondria are mechanistically realized in the cell and to what extent this could be dependent on environmental aspects. Distinct mitochondrial subsets may also exhibit different responses to substrates and inhibitors and may vary in their sensitivity to pathology, resistance to apoptosis, oxidative stress, thus also demonstrating heterogeneous behavior. All these observations strongly suggest that the intracellular position, organization and the specific surroundings of mitochondria within the cell define their functional features, while also implying that different mitochondrial subpopulations, clusters or even single mitochondrion may execute diverse processes in a cell. The heterogeneity of mitochondrial function demonstrates an additional level of mitochondrial complexity and is a new, challenging area in mitochondrial research that potentially leads to the integration of mitochondrial bioenergetics and cell physiology with various physiological and pathophysiological implications.

Energetic state is a strong regulator of sarcoplasmic reticulum Ca2+ loss in cardiac muscle: different efficiencies of different energy sources

AIMS

Increased diastolic sarcoplasmic reticulum (SR) Ca(2+) loss could depress contractility in heart failure. Since the failing myocardium has impaired energetics, we investigated whether Ca(2+) loss is linked to changes in energetic pathways.

METHODS AND RESULTS

Leakage from SR in mouse permeabilized preparations was assessed using exogenous ATP, ATP + phosphocreatine (activation of bound creatine kinase, CK), ATP + mitochondrial substrates (mitochondrial activation), or with all of these together (optimal energetic conditions) in Ca(2+)-free solution. In ventricular fibres caffeine-induced tension transients under optimal energetic conditions were used to estimate SR [Ca(2+)]. In cardiomyocytes, intra-SR Ca(2+) was monitored by use of the fluorescent marker Mag-fluo 4. In fibres, SR Ca(2+) content after 5 min incubation strongly depended on energy supply (100%-optimal energetic conditions; 27 +/- 5%-exogenous ATP only, 52 +/- 5%-endogenous CK activation; 88 +/- 8%-mitochondrial activation, P < 0.01 vs. CK system). The significant loss with only exogenous ATP was not inhibited by the ryanodine receptor blockers tetracaine or ruthenium red. However, the SR Ca(2+)-ATPase (SERCA) inhibitors cyclopiazonic acid or 2,5-di(tert-butyl)-1,4-benzohydroquinone significantly decreased Ca(2+) loss. At 100 nM external [Ca(2+)], the SR Ca(2+) loss was also energy dependent and was not significantly inhibited by tetracaine. In cardiomyocytes, the decline in SR [Ca(2+)] at zero external [Ca(2+)] was almost two times slower under optimal energetic conditions than in the presence of exogenous ATP only.

CONCLUSION

At low extra-reticular [Ca(2+)], the main leak pathway is an energy-sensitive backward Ca(2+) pump, and direct mitochondrial-SERCA ATP channelling is more effective in leak prevention than local ATP generation by bound CK.

Bidirectionality and compartmentation of metabolic fluxes are revealed in the dynamics of isotopomer networks

Isotope labeling is one of the few methods of revealing the in vivo bidirectionality and compartmentalization of metabolic fluxes within metabolic networks. We argue that a shift from steady state to dynamic isotopomer analysis is required to deal with these cellular complexities and provide a review of dynamic studies of compartmentalized energy fluxes in eukaryotic cells including cardiac muscle, plants, and astrocytes. Knowledge of complex metabolic behaviour on a molecular level is prerequisite for the intelligent design of genetically modified organisms able to realize their potential of revolutionizing food, energy, and pharmaceutical production. We describe techniques to explore the bidirectionality and compartmentalization of metabolic fluxes using information contained in the isotopic transient, and discuss the integration of kinetic models with MFA. The flux parameters of an example metabolic network were optimized to examine the compartmentalization of metabolites and and the bidirectionality of fluxes in the TCA cycle of Saccharomyces uvarum for steady-state respiratory growth.

The cell-type specificity of mitochondrial dynamics

Recent advances in mitochondrial imaging have revealed that in many cells mitochondria can be highly dynamic. They can undergo fission/fusion processes modulated by various mitochondria-associated proteins and also by conformational transitions in the inner mitochondrial membrane. Moreover, precise mitochondrial distribution can be achieved by their movement along the cytoskeleton, recruiting various connector and motor proteins. Such movement is evident in various cell types ranging from yeast to mammalian cells and serves to direct mitochondria to cellular regions of high ATP demand or to transport mitochondria destined for elimination. Existing data also demonstrate that many aspects of mitochondrial dynamics, morphology, regulation and intracellular organization can be cell type-/tissue-specific. In many cells like neurons, pancreatic cells, HL-1 cells, etc., complex dynamics of mitochondria include fission, fusion, small oscillatory movements of mitochondria, larger movements like filament extension, retraction, fast branching in the mitochondrial network and rapid long-distance intracellular translocation of single mitochondria. Alternatively, mitochondria can be rather fixed in other cells and tissues like adult cardiomyocytes or skeletal muscles with a very regular organelle organization between myofibrils, providing the bioenergetic basis for contraction. Adult cardiac cells show no displacement of mitochondria with only very small-amplitude rapid vibrations, demonstrating remarkable, cell type-dependent differences in the dynamics and spatial arrangement of mitochondria. These variations and the cell-type specificity of mitochondrial dynamics could be related to specific cellular functions and demands, also indicating a significant role of integrations of mitochondria with other intracellular systems like the cytoskeleton, nucleus and endoplasmic reticulum (ER).

Complex patterns of mitochondrial dynamics in human pancreatic cells revealed by fluorescent confocal imaging

Mitochondrial morphology and intracellular organization are tightly controlled by the processes of mitochondrial fission-fusion. Moreover, mitochondrial movement and redistribution provide a local ATP supply at cellular sites of particular demands. Here we analysed mitochondrial dynamics in isolated primary human pancreatic cells. Using real time confocal microscopy and mitochondria-specific fluorescent probes tetramethylrhodamine methyl ester and MitoTracker Green we documented complex and novel patterns of spatial and temporal organization of mitochondria, mitochondrial morphology and motility. The most commonly observed types of mitochondrial dynamics were (i) fast fission and fusion; (ii) small oscillating movements of the mitochondrial network; (iii) larger movements, including filament extension, retraction, fast (0.1-0.3 mum/sec.) and frequent oscillating (back and forth) branching in the mitochondrial network; (iv) as well as combinations of these actions and (v) long-distance intracellular translocation of single spherical mitochondria or separated mitochondrial filaments with velocity up to 0.5 mum/sec. Moreover, we show here for the first time, a formation of unusual mitochondrial shapes like rings, loops, and astonishingly even knots created from one or more mitochondrial filaments. These data demonstrate the presence of extensive heterogeneity in mitochondrial morphology and dynamics in living cells under primary culture conditions. In summary, this study reports new patterns of morphological changes and dynamic motion of mitochondria in human pancreatic cells, suggesting an important role of integrations of mitochondria with other intracellular structures and systems.

Philosophical basis and some historical aspects of systems biology: from Hegel to Noble - applications for bioenergetic research

We live in times of paradigmatic changes for the biological sciences. Reductionism, that for the last six decades has been the philosophical basis of biochemistry and molecular biology, is being displaced by Systems Biology, which favors the study of integrated systems. Historically, Systems Biology - defined as the higher level analysis of complex biological systems - was pioneered by Claude Bernard in physiology, Norbert Wiener with the development of cybernetics, and Erwin Schrödinger in his thermodynamic approach to the living. Systems Biology applies methods inspired by cybernetics, network analysis, and non-equilibrium dynamics of open systems. These developments follow very precisely the dialectical principles of development from thesis to antithesis to synthesis discovered by Hegel. Systems Biology opens new perspectives for studies of the integrated processes of energy metabolism in different cells. These integrated systems acquire new, system-level properties due to interaction of cellular components, such as metabolic compartmentation, channeling and functional coupling mechanisms, which are central for regulation of the energy fluxes. State of the art of these studies in the new area of Molecular System Bioenergetics is analyzed.

Mitochondrial energetic metabolism: a simplified model of TCA cycle with ATP production

Mitochondria play a central role in cellular energetic metabolism. The essential parts of this metabolism are the tricarboxylic acid (TCA) cycle, the respiratory chain and the adenosine triphosphate (ATP) synthesis machinery. Here a simplified model of these three metabolic components with a limited set of differential equations is presented. The existence of a steady state is demonstrated and results of numerical simulations are presented. The relevance of a simple model to represent actual in vivo behavior is discussed.

Anisotropic diffusion of fluorescently labeled ATP in rat cardiomyocytes determined by raster image correlation spectroscopy

A series of experimental data points to the existence of profound diffusion restrictions of ADP/ATP in rat cardiomyocytes. This assumption is required to explain the measurements of kinetics of respiration, sarcoplasmic reticulum loading with calcium, and kinetics of ATP-sensitive potassium channels. To be able to analyze and estimate the role of intracellular diffusion restrictions on bioenergetics, the intracellular diffusion coefficients of metabolites have to be determined. The aim of this work was to develop a practical method for determining diffusion coefficients in anisotropic medium and to estimate the overall diffusion coefficients of fluorescently labeled ATP in rat cardiomyocytes. For that, we have extended raster image correlation spectroscopy (RICS) protocols to be able to discriminate the anisotropy in the diffusion coefficient tensor. Using this extended protocol, we estimated diffusion coefficients of ATP labeled with the fluorescent conjugate Alexa Fluor 647 (Alexa-ATP). In the analysis, we assumed that the diffusion tensor can be described by two values: diffusion coefficient along the myofibril and that across it. The average diffusion coefficients found for Alexa-ATP were as follows: 83 ± 14 μm²/s in the longitudinal and 52 ± 16 μm²/s in the transverse directions (n = 8, mean ± SD). Those values are ∼2 (longitudinal) and ∼3.5 (transverse) times smaller than the diffusion coefficient value estimated for the surrounding solution. Such uneven reduction of average diffusion coefficient leads to anisotropic diffusion in rat cardiomyocytes. Although the source for such anisotropy is uncertain, we speculate that it may be induced by the ordered pattern of intracellular structures in rat cardiomyocytes.

Physical coupling supports the local Ca2+ transfer between sarcoplasmic reticulum subdomains and the mitochondria in heart muscle

In many cell types, transfer of Ca(2+) released via ryanodine receptors (RyR) to the mitochondrial matrix is locally supported by high [Ca(2+)] microdomains at close contacts between the sarcoplasmic reticulum (SR) and mitochondria. Here we studied whether the close contacts were secured via direct physical coupling in cardiac muscle using isolated rat heart mitochondria (RHMs). "Immuno-organelle chemistry" revealed RyR2 and calsequestrin-positive SR particles associated with mitochondria in both crude and Percoll-purified "heavy" mitochondrial fractions (cRHM and pRHM), to a smaller extent in the latter one. Mitochondria-associated vesicles were also visualized by electron microscopy in the RHMs. Western blot analysis detected greatly reduced presence of SR markers (calsequestrin, SERCA2a, and phospholamban) in pRHM, suggesting that the mitochondria-associated particles represented a small subfraction of the SR. Fluorescence calcium imaging in rhod2-loaded cRHM revealed mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)) responses to caffeine-induced Ca(2+) release that were prevented when thapsigargin was added to predeplete the SR or by mitochondrial Ca(2+) uptake inhibitors. Importantly, caffeine failed to increase [Ca(2+)] in the large volume of the incubation medium, suggesting that local Ca(2+) transfer between the SR particles and mitochondria mediated the [Ca(2+)](m) signal. Despite the substantially reduced SR presence, pRHM still displayed a caffeine-induced [Ca(2+)](m) rise comparable with the one recorded in cRHM. Thus, a relatively small fraction of the total SR is physically coupled and transfers Ca(2+) locally to the mitochondria in cardiac muscle. The transferred Ca(2+) stimulates dehydrogenase activity and affects mitochondrial membrane permeabilization, indicating the broad significance of the physical coupling in mitochondrial function.

Local energetic regulation of sarcoplasmic and myosin ATPase is differently impaired in rats with heart failure

Local control of ATP/ADP ratio is essential for efficient functioning of cellular ATPases. Since creatine kinase (CK) activity and mitochondrial content are reduced in heart failure (HF), and cardiomyocyte ultrastructure is altered, we hypothesized that these changes may affect the local energetic control of two major cardiac ATPases, the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and the myosin ATPase. Heart failure was induced by aortic stenosis in rats. Electron microscopy confirmed that failing cardiomyocytes had intracellular disorganization, with fewer contacts between mitochondria and myofibrils. Despite normal SERCA protein content, spontaneous Ca2+ release measurements using Fluo-4 on saponin-permeabilized cardiomyocytes showed a lower SR loading in HF even when endogenous CK and mitochondria were fully activated. Similarly, in permeabilized fibres, SR Ca2+ loading supported by SR-bound CK and mitochondria was significantly reduced in HF (by 49% and 40%, respectively, 43% when both systems were activated, P < 0.05). Alkaline phosphatase treatment had no effect, but glycolytic substrates normalized calcium loading in HF to the sham level. The control by CK and mitochondria of the local ATP/ADP ratio close to the myosin ATPase (estimated by rigor tension) was also significantly impaired in HF fibres (by 32% and 46%, respectively). However, while the contributions of mitochondria and CK to local ATP regeneration were equally depressed in HF for the control of SERCA, mitochondrial contribution was more severely impaired than CK (P < 0.05) with respect to myofilament regulation. These data show that local energetic regulation of essential ATPases is severely impaired in heart failure, and undergoes a complex remodelling as a result of a decreased activity of the ATP-generating systems and cytoarchitecture disorganization.

Regulation of respiration in brain mitochondria and synaptosomes: restrictions of ADP diffusion in situ, roles of tubulin, and mitochondrial creatine kinase

The role of ubiquitous mitochondrial creatine kinase (uMtCK) reaction in regulation of mitochondrial respiration was studied in purified preparations of rat brain synaptosomes and mitochondria. In permeabilized synaptosomes, apparent Km for exogenous ADP, Km (ADP), in regulation of respiration in situ was rather high (110 +/- 11 microM) in comparison with isolated brain mitochondria (9 +/- 1 microM). This apparent Km for ADP observed in isolated mitochondria in vitro dramatically increased to 169 +/- 52 microM after their incubation with 1 muM of dimeric tubulin showing that in rat brain, particularly in synaptosomes, mitochondrial outer membrane permeability for ADP, and ATP may be restricted by tubulin binding to voltage dependent anion channel (VDAC). On the other hand, in synaptosomes apparent Km (ADP) decreased to 25 +/- 1 microM in the presence of 20 mM creatine. To fully understand this effect of creatine on kinetics of respiration regulation, complete kinetic analysis of uMtCK reaction in isolated brain mitochondria was carried out. This showed that oxidative phosphorylation specifically altered only the dissociation constants for MgATP, by decreasing that from ternary complex MtCK.Cr.MgATP (K (a)) from 0.13 +/- 0.02 to 0.018 +/- 0.007 mM and that from binary complex MtCK.MgATP (K (ia)) from 1.1 +/- 0.29 mM to 0.17 +/- 0.07 mM. Apparent decrease of dissociation constants for MgATP reflects effective cycling of ATP and ADP between uMtCK and adenine nucleotide translocase (ANT). These results emphasize important role and various pathophysiological implications of the phosphocreatine-creatine kinase system in energy transfer in brain cells, including synaptosomes.

High- and low-calcium-dependent mechanisms of mitochondrial calcium signalling

The Ca(2+) coupling between endoplasmic reticulum (ER) and mitochondria is central to multiple cell survival and cell death mechanisms. Cytoplasmic [Ca(2+)] ([Ca(2+)](c)) spikes and oscillations produced by ER Ca(2+) release are effectively delivered to the mitochondria. Propagation of [Ca(2+)](c) signals to the mitochondria requires the passage of Ca(2+) across three membranes, namely the ER membrane, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM). Strategic positioning of the mitochondria by cytoskeletal transport and interorganellar tethers provides a means to promote the local transfer of Ca(2+) between the ER membrane and OMM. In this setting, even >100 microM [Ca(2+)] may be attained to activate the low affinity mitochondrial Ca(2+) uptake. However, a mitochondrial [Ca(2+)] rise has also been documented during submicromolar [Ca(2+)](c) elevations. Evidence has been emerging that Ca(2+) exerts allosteric control on the Ca(2+) transport sites at each membrane, providing mechanisms that may facilitate the Ca(2+) delivery to the mitochondria. Here we discuss the fundamental mechanisms of ER and mitochondrial Ca(2+) transport, particularly the control of their activity by Ca(2+) and evaluate both high- and low-[Ca(2+)]-activated mitochondrial calcium signals in the context of cell physiology.

In vivo multiphoton microscopy of NADH and FAD redox states, fluorescence lifetimes, and cellular morphology in precancerous epithelia

Metabolic imaging of the relative amounts of reduced NADH and FAD and the microenvironment of these metabolic electron carriers can be used to noninvasively monitor changes in metabolism, which is one of the hallmarks of carcinogenesis. This study combines cellular redox ratio, NADH and FAD lifetime, and subcellular morphology imaging in three dimensions to identify intrinsic sources of metabolic and structural contrast in vivo at the earliest stages of cancer development. There was a significant (P < 0.05) increase in the nuclear to cytoplasmic ratio (NCR) with depth within the epithelium in normal tissues; however, there was no significant change in NCR with depth in precancerous tissues. The redox ratio significantly decreased in the less differentiated basal epithelial cells compared with the more mature cells in the superficial layer of the normal stratified squamous epithelium, indicating an increase in metabolic activity in cells with increased NCR. However, the redox ratio was not significantly different between the superficial and basal cells in precancerous tissues. A significant decrease was observed in the contribution and lifetime of protein-bound NADH (averaged over the entire epithelium) in both low- and high-grade epithelial precancers compared with normal epithelial tissues. In addition, a significant increase in the protein-bound FAD lifetime and a decrease in the contribution of protein-bound FAD are observed in high-grade precancers only. Increased intracellular variability in the redox ratio, NADH, and FAD fluorescence lifetimes were observed in precancerous cells compared with normal cells.

The creatine kinase phosphotransfer network: thermodynamic and kinetic considerations, the impact of the mitochondrial outer membrane and modelling approaches

In this review, we summarize the main structural and functional data on the role of the phosphocreatine (PCr)--creatine kinase (CK) pathway for compartmentalized energy transfer in cardiac cells. Mitochondrial creatine kinase, MtCK, fixed by cardiolipin molecules in the vicinity of the adenine nucleotide translocator, is a key enzyme in this pathway. Direct transfer of ATP and ADP between these proteins has been revealed both in experimental studies on the kinetics of the regulation of mitochondrial respiration and by mathematical modelling as a main mechanism of functional coupling of PCr production to oxidative phosphorylation. In cells in vivo or in permeabilized cells in situ, this coupling is reinforced by limited permeability of the outer membrane of the mitochondria for adenine nucleotides due to the contacts with cytoskeletal proteins. Due to these mechanisms, at least 80% of total energy is exported from mitochondria by PCr molecules. Mathematical modelling of intracellular diffusion and energy transfer shows that the main function of the PCr-CK pathway is to connect different pools (compartments) of ATP and, by this way, to overcome the local restrictions and diffusion limitation of adenine nucleotides due to the high degree of structural organization of cardiac cells.

A wave of reactive oxygen species (ROS)-induced ROS release in a sea of excitable mitochondria

Once considered simply as the main source of ATP, mitochondria are now implicated in the control of many additional aspects of cell physiology, such as calcium signaling, and pathology, as in injury incurred on ischemia and subsequent reperfusion (I/R). Mitochondrial respiration is ordinarily accompanied by low-level ROS production, but they can respond to elevated ROS concentrations by increasing their own ROS production, a phenomenon termed ROS-induced ROS release (RIRR). Two modes of RIRR have been described. In the first mode of RIRR, enhanced ROS leads to mitochondrial depolarization via activation of the MPTP, yielding a short-lived burst of ROS originating from the mitochondrial electron transport chain (ETC). The second mode of RIRR is MPTP independent but is regulated by the mitochondrial benzodiazepine receptor (mBzR). Increased ROS in the mitochondrion triggers opening of the inner mitochondrial membrane anion channel (IMAC), resulting in a brief increase in ETC-derived ROS. Both modes of RIRR have been shown to transmit localized mitochondrial perturbations throughout the cardiac cell in the form of oscillations or waves but are kinetically distinct and may involve different ROS that serve as second messengers. In this review, we discuss the mechanisms of these different modes of RIRR.

Altered energy transfer from mitochondria to sarcoplasmic reticulum after cytoarchitectural perturbations in mice hearts

Sarcoplasmic reticulum (SR) calcium pump function requires a high local ATP/ADP ratio, which can be maintained by direct nucleotide channelling from mitochondria, and by SR-bound creatine kinase (CK)-catalysed phosphate-transfer from phosphocreatine. We hypothesized that SR calcium uptake supported by mitochondrial direct nucleotide channelling, but not bound CK, depends on the juxtaposition of these organelles. To test this, we studied a well-described model of cytoarchitectural disorganization, the muscle LIM protein (MLP)-null mouse heart. Subcellular organization was characterized using electron microscopy, and mitochondrial, SR and myofibrillar function were assessed in saponin-permeabilized fibres by measuring respiration rates and caffeine-induced tension transients. MLP-null hearts had fewer, less-tightly packed intermyofibrillar mitochondria, and more subsarcolemmal mitochondria. The apparent mitochondrial Km for ADP was significantly lower in the MLP-null heart than in control (175 +/- 15 and 270 +/- 33 microM, respectively), indicating greater ADP accessibility, although maximal respiration rate, mitochondrial content and total CK activity were unaltered. Active tension in the myofibres of MLP-null mice was 54% lower than in controls (39 +/- 3 and 18 +/- 1 mN mm(-2), respectively), consistent with cytoarchitectural disorganization. SR calcium loading in the myofibres of MLP-null mice was similar to that in control myofibres when energy support was provided via Bound CK, but approximately 36% lower than controls when energy support was provided by mitochondrial (P < 0.05). Mitochondrial support for SR calcium uptake was also specifically decreased in the desmin-null heart, which is another model of cytoarchitectural perturbation. Thus, despite normal oxidative capacity, direct nucleotide channelling to the SR was impaired in MLP deficiency, concomitant with looser mitochondrial packing and increased nucleotide accessibility to this organelle. Changes in cytoarchitecture may therefore impair subcellular energy transfer and contribute to energetic and contractile dysfunction.

A computational model integrating electrophysiology, contraction, and mitochondrial bioenergetics in the ventricular myocyte

An intricate network of reactions is involved in matching energy supply with demand in the heart. This complexity arises because energy production both modulates and is modulated by the electrophysiological and contractile activity of the cardiac myocyte. Here, we present an integrated mathematical model of the cardiac cell that links excitation-contraction coupling with mitochondrial energy generation. The dynamics of the model are described by a system of 50 ordinary differential equations. The formulation explicitly incorporates cytoplasmic ATP-consuming processes associated with force generation and ion transport, as well as the creatine kinase reaction. Changes in the electrical and contractile activity of the myocyte are coupled to mitochondrial energetics through the ATP, Ca2+, and Na+ concentrations in the myoplasmic and mitochondrial matrix compartments. The pseudo steady-state relationship between force and oxygen consumption at various stimulus frequencies and external Ca2+ concentrations is reproduced in both model simulations and direct experiments in cardiac trabeculae under normoxic conditions, recapitulating the linearity between cardiac work and respiration in the heart. Importantly, the model can also reproduce the rapid time-dependent changes in mitochondrial NADH and Ca2+ in response to abrupt changes in workload. The steady-state and dynamic responses of the model were conferred by ADP-dependent stimulation of mitochondrial oxidative phosphorylation and Ca2+ -dependent regulation of Krebs cycle dehydrogenases, illustrating how the model can be used as a tool for investigating mechanisms underlying metabolic control in the heart.

Cardiac system bioenergetics: metabolic basis of the Frank-Starling law

The fundamental principle of cardiac behaviour is described by the Frank-Starling law relating force of contraction during systole with end-diastolic volume. While both work and respiration rates increase linearly with imposed load, the basis of mechano-energetic coupling in heart muscle has remained a long-standing enigma. Here, we highlight advances made in understanding of complex cellular and molecular mechanisms that orchestrate coupling of mitochondrial oxidative phosphorylation with ATP utilization for muscle contraction. Cardiac system bioenergetics critically depends on an interrelated metabolic infrastructure regulating mitochondrial respiration and energy fluxes throughout cellular compartments. The data reviewed indicate the significance of two interrelated systems regulating mitochondrial respiration and energy fluxes in cells: (1) the creatine kinase, adenylate kinase and glycolytic pathways that communicate flux changes generated by cellular ATPases within structurally organized enzymatic modules and networks; and (2) a secondary system based on mitochondrial participation in cellular calcium cycle, which adjusts substrate oxidation and energy-transducing processes to meet increasing cellular energy demands. By conveying energetic signals to metabolic sensors, coupled phosphotransfer reactions provide a high-fidelity regulation of the excitation-contraction cycle. Such integration of energetics with calcium signalling systems provides the basis for 'metabolic pacing', synchronizing the cellular electrical and mechanical activities with energy supply processes.

Mitochondrial creatine kinase in human health and disease

Mitochondrial creatine kinase (MtCK), together with cytosolic creatine kinase isoenzymes and the highly diffusible CK reaction product, phosphocreatine, provide a temporal and spatial energy buffer to maintain cellular energy homeostasis. Mitochondrial proteolipid complexes containing MtCK form microcompartments that are involved in channeling energy in form of phosphocreatine rather than ATP into the cytosol. Under situations of compromised cellular energy state, which are often linked to ischemia, oxidative stress and calcium overload, two characteristics of mitochondrial creatine kinase are particularly relevant: its exquisite susceptibility to oxidative modifications and the compensatory up-regulation of its gene expression, in some cases leading to accumulation of crystalline MtCK inclusion bodies in mitochondria that are the clinical hallmarks for mitochondrial cytopathies. Both of these events may either impair or reinforce, respectively, the functions of mitochondrial MtCK complexes in cellular energy supply and protection of mitochondria form the so-called permeability transition leading to apoptosis or necrosis.

Mitochondrial contact sites: Their role in energy metabolism and apoptosis

The energy metabolism of the failing heart is characterised by a 30% decrease of the total adenine nucleotides content and what may be more important by a 60% loss of creatine and creatine phosphate [J.S. Ingwall, R.G. Weiss, Is the failing heart energy starved? On using chemical energy to support cardiac function, Circ. Res. 95 (2004) 35-145]. Besides the effect of these changes on the energy supply, failing heart is known to be more vulnerable to Ca2+ overload and apoptosis-inducing processes. Recent studies have pointed to the critical role of mitochondrial contact sites in controlling both the mitochondrial energy metabolism and Ca2+ homeostasis. This review focuses on the structure and function of protein complexes in mitochondrial contact sites and their regulatory role in the cellular bioenergetics, intra- and extra-mitochondrial Ca2+ levels, and release of apoptosis-inducing factors. Firstly, we review the compositions of different contact sites following by the discussion of experimental data obtained with isolated and reconstituted voltage-dependent anion channel-adenine nucleotide translocase complexes and consequences of the complex disassembling. Furthermore, we describe experiments involving the complex-stabilizing conditions in vitro and in intact cells. At the end, we discuss unsolved problems and opportunities for clinical application of the complex-stabilizing factors.

Structure-function relationships in the regulation of energy transfer between mitochondria and ATPases in cardiac cells

The present study discusses the role of structural organization of cardiac cells in determining the mechanisms of regulation of oxidative phosphorylation and interaction between mitochondria and ATPases. In permeabilized adult cardiomyocytes, the apparent K(m) (Michaelis-Menten constant) for ADP in the regulation of respiration is far higher than in mitochondria isolated from the myocardium. Respiration of mitochondria in permeabilized cardiomyocytes is effectively activated by endogenous ADP produced by ATPases from exogenous ATP, and the activation of respiration is associated with a decrease in the apparent K(m) for ATP in the regulation of ATPase activity compared with this parameter in the absence of oxidative phosphorylation. It has also been shown that a large fraction of the endogenous ADP stimulating respiration remains inaccessible for the exogenous ADP trapping system, consisting of pyruvate kinase and phosphoenolpyruvate, unless the mitochondrial structures are modified by controlled proteolysis. These data point to the endogenous cycling of adenine nucleotides between mitochondria and ATPases. Accordingly, the current hypothesis is that in cardiac cells, mitochondria and ATPases are compartmentalized into functional complexes (ie, intracellular energetic units [ICEUs]), which appear to represent a basic pattern of organization of energy metabolism in these cells. Within the ICEUs, the mitochondria and ATPases interact via different routes: creatine kinase-mediated phosphoryltransfer; adenylate kinase-mediated phosphoryltransfer; and direct ATP and ADP channelling. The function of ICEUs changes not only after selective proteolysis, but also during contraction of cardiomyocytes caused by an increase in cytosolic Ca(2+) concentration up to micromolar levels. In these conditions, the apparent K(m) for exogenous ADP and ATP in the regulation of respiration markedly decreases, and more ADP becomes available for the exogenous pyruvate kinase-phosphoenolpyruvate system, which indicates altered barrier functions of the ICEUs. Thus, structural changes transmitted from the contractile apparatus to mitochondria clearly participate in the regulation of mitochondrial function due to alterations in localized restriction of the diffusion of adenine nucleotides. The importance of strict structural organization in cardiac cells emerged drastically from experiments in which the regulation of mitochondrial respiration was assessed in a novel cardiac cell line, that is, beating and nonbeating HL-1 cells. In these cells, the mitochondrial arrangement is irregular and dynamic, whereas the sarcomeric structures are either absent (in nonbeating HL-1 cells) or only rarely present (in beating HL-1 cells). In parallel, the apparent K(m) for exogenous ADP in the regulation of respiration was much lower than that in permeabilized primary cardiomyocytes, and trypsin treatment exerted no impact on the low K(m) value for ADP, in contrast to adult cardiomyocytes where it caused a marked decrease in this parameter. The HL-1 cells were also characterized by the absence of direct exchange of adenine nucleotides. The results further support the concept that the ICEUs in adult cardiomyocytes are products of complex structural organization developed to create the most optimal conditions for effective energy transfer and feedback between mitochondria and ATPases.

Compartmentation of energy metabolism in atrial myocardium of patients undergoing cardiac surgery

The parameters of oxidative phosphorylation and its interaction with creatine kinase (CK)- and adenylate kinase (AK)-phosphotransfer networks in situ were studied in skinned atrial fibers from 59 patients undergoing coronary artery bypass surgery, valve replacement/correction and atrial septal defect correction. In atria, the mitochondrial CK and AK are effectively coupled to oxidative phosphorylation, the MM-CK is coupled to ATPases and there exists a direct transfer of adenine nucleotides between mitochondria and ATPases. Elimination of cytoplasmic ADP with exogenous pyruvate kinase was not associated with a blockade of the stimulatory effects of creatine and AMP on respiration, neither could it abolish the coupling of MM-CK to ATPases and direct transfer of adenine nucleotides. Thus, atrial energy metabolism is compartmentalized so that mitochondria form functional complexes with adjacent ATPases. These complexes isolate a part of cellular adenine nucleotides from their cytoplasmic pool for participating in energy transfer via CK- and AK-networks, and/or direct exchange. Compared to atria in sinus rhythm, the fibrillating atria were larger and exhibited increased succinate-dependent respiration relative to glutamate-dependent respiration and augmented proton leak. Thus, alterations in mitochondrial oxidative phosphorylation may contribute to pathogenesis of atrial fibrillation.

Effects of hibernation on mitochondrial regulation and metabolic capacities in myocardium of painted turtle (Chrysemys picta)

Painted turtles hibernating during winter may endure long-term exposure to low temperature and anoxia. These two conditions may affect the aerobic capacity of a tissue and might be of particular importance to the cardiac muscle normally highly reliant on aerobic energy production. The present study addressed how hibernation affects respiratory characteristics of mitochondria in situ and the metabolic pattern of turtle myocardium. Painted turtles were acclimated to control (25 degrees C), cold (5 degrees C) normoxic and cold anoxic conditions. In saponin-skinned myocardial fibres, cold acclimation increased mitochondrial respiratory capacity and decreased apparent ADP-affinity. Concomitant anoxia did not affect this. Creatine increased the apparent ADP-affinity to similar values in the three acclimation groups, suggesting a functional coupling of creatine kinase to mitochondrial respiration. As to the metabolic pattern, cold acclimation decreased glycolytic capacity in terms of pyruvate kinase activity and increased lactate dehydrogenase (LHD) activity. Concomitant anoxia counteracted the cold-induced decrease in pyruvate kinase activity and increased creatine kinase activity. In conclusion, cold acclimation seems to increase aerobic and decrease anaerobic energy production capacity in painted turtle myocardium. Importantly, anoxia does not affect the mitochondrial functional integrity but seems to increase the capacity for anaerobic energy production and energy buffering.

Impaired voluntary running capacity of creatine kinase-deficient mice

The creatine kinase system (CK) is important for energy delivery in skeletal and cardiac muscles. The two main isoforms of this enzyme, cytosolic MM-CK and mitochondrial mi-CK, are expressed in a developmental and muscle-type specific manner. Mice deficient in one or both of these isoforms are viable and fertile but exhibit profound functional, metabolic and structural muscle remodelling that primarily affects fast skeletal muscles, which show an increased contribution of oxidative metabolism to contractile function. However, the consequences of these alterations in terms of physical capabilities have not yet been characterized. Consequently, we compared the voluntary exercise capacity of 9-month-old male wild-type (WT), M-CK knockout (M-CK(-/-)), and M-CK and mi-CK double knockout (CK(-/-)) mice, using cages equipped with running wheels. Exercise performance, calculated by total distance covered and by work done during the training period, was more than 10-fold lower in CK(-/-) mice than controls, with M-CK(-/-) mice exhibiting intermediate performance. Similarly, the mean distance run per activation was lower in M-CK(-/-) and even lower in CK(-/-) mice. However, the maximal running speed (V(max)) was lower only for CK(-/-) mice. This was accompanied by severe skeletal muscle mass decrease in CK(-/-) mice, with signs of histological damage that included enlarged interstitial areas, aggregations of mononuclear cells in the interstitium, heterogeneity of myofibre size and the presence of very small fibres. No overt sign of cardiac dysfunction was observed by magnetic resonance imaging during dobutamine stimulation. These results show that metabolic failure induced by CK deficiency profoundly affects the ability of mice to engage in chronic bouts of endurance running exercise and that this decrease in performance is also associated with muscle wasting.

Intracellular energetic units in healthy and diseased hearts

BACKGROUND

The present review examines the role of intra-cellular compartmentation of energy metabolism in vivo.

OBJECTIVE

To compare the kinetics of the activation of mitochondrial respiration in skinned cardiac fibres by exogenous and endogenous adenine nucleotides in dependence of the modulation of cellular structure and contraction.

METHODS

Saponin-permeabilized cardiac fibres or cells were analyzed using oxygraphy and confocal microscopy.

RESULTS

Mitochondria respiration in fibres or cells was upregulated by cumulative additions of ADP to the medium with an apparent K(m) of 200 muM to 300 muM. When respiration was stimulated by endogenous ADP produced by intracellular ATPases, a near maximum respiration rate was achieved at an ADP concentration of less than 20 muM in the medium. A powerful ADP-consuming system, consisting of pyruvate kinase and phosphoenolpyruvate, that totally suppressed the activation of respiration by exogenous ADP, failed to abolish the stimulation of respiration by endogenous ADP, but did inhibit respiration after the cells were treated with trypsin. The addition of up to 4 muM of free Ca(2+) to the actively respiring fibres resulted in reversible hypercontraction associated with a decreased apparent K(m) for exogenous ADP. These changes were fully abolished in fibres after the removal of myosin by KCl treatment.

CONCLUSIONS

Mitochondria and ATPases, together with cytoskeletal proteins that establish the structural links between mitochondria and sarcomeres, form complexes - intracellular energetic units (ICEUs) - in cardiac cells. Within the ICEUs, the mitochondria and ATPases interact via specialized energy transfer systems, such as the creatine kinase- and adenylate kinase-phosphotransfer networks, and direct ATP channelling. Disintegration of the structure and function of ICEUs results in dyscompartmentation of adenine nucleotides and may represent a basis for cardiac diseases.

Intracellular diffusion of adenosine phosphates is locally restricted in cardiac muscle

Recent studies have revealed the structural and functional interactions between mitochondria, myofibrils and sarcoplasmic reticulum in cardiac cells. Direct channeling of adenosine phosphates between organelles identified in the experiments indicates that diffusion of adenosine phosphates is limited in cardiac cells due to very specific intracellular structural organization. However, the mode of diffusion restrictions and nature of the intracellular structures in creating the diffusion barriers is still unclear, and, therefore, a subject of active research. The aim of this work is to analyze the possible role of two principally different modes of restriction distribution for adenosine phosphates (a) the uniform diffusion restriction and (b) the localized diffusion limitation in the vicinity of mitochondria, by fitting the experimental data with the mathematical model. The reaction-diffusion model of compartmentalized energy transfer was used to analyze the data obtained from the experiments with the skinned muscle fibers, which described the following processes: mitochondrial respiration rate dependency on exogenous ADP and ATP concentrations; inhibition of endogenous ADP-stimulated respiration by pyruvate kinase (PK) and phosphoenolpyruvate (PEP) system; kinetics of oxygen consumption stabilization after addition of 2 mM MgATP or MgADP; ATPase activity with inhibited mitochondrial respiration; and buildup of MgADP concentration in the medium after addition of MgATP. The analysis revealed that only the second mechanism considered--localization of diffusion restrictions--is able to account for the experimental data. In the case of uniform diffusion restrictions, the model solution was in agreement only with two measurements: the respiration rate as a function of ADP or ATP concentrations and inhibition of respiration by PK + PEP. It was concluded that intracellular diffusion restrictions for adenosine phosphates are not distributed uniformly, but rather are localized in certain compartments of the cardiac cells.

Functional coupling as a basic mechanism of feedback regulation of cardiac energy metabolism

In this review we analyze the concepts and the experimental data on the mechanisms of the regulation of energy metabolism in muscle cells. Muscular energetics is based on the force-length relationship, which in the whole heart is expressed as a Frank-Starling law, by which the alterations of left ventricle diastolic volume change linearly both the cardiac work and oxygen consumption. The second basic characteristics of the heart is the metabolic stability--almost constant levels of high energy phosphates, ATP and phosphocreatine, which are practically independent of the workload and the rate of oxygen consumption, in contrast to the fast-twitch skeletal muscle with no metabolic stability and rapid fatigue. Analysis of the literature shows that an increase in the rate of oxygen consumption by order of magnitude, due to Frank-Starling law, is observed without any significant changes in the intracellular calcium transients. Therefore, parallel activation of contraction and mitochondrial respiration by calcium ions may play only a minor role in regulation of respiration in the cells. The effective regulation of the respiration under the effect of Frank-Starling law and metabolic stability of the heart are explained by the mechanisms of functional coupling within supramolecular complexes in mitochondria, and at the subcellular level within the intracellular energetic units. Such a complex structural and functional organisation of heart energy metabolism can be described quantitatively by mathematical models.

Studies of mitochondrial respiration in muscle cells in situ: use and misuse of experimental evidence in mathematical modelling

Applications of permeabilized cell and skinned fiber techniques in combination with methods of mathematical modelling for studies of mitochondrial function in the cell are critically evaluated. Mathematical models may be useful tools for explaining biological phenomena, but only if they are selected by fitting the computing results with real experimental data. Confocal microscopy has been used in experiments with permeabilized cardiomyocytes and myocardial fibers to determine the maximal diffusion distance from medium to the core of cells, which is shown not to exceed 8-10 microm. This is a principal index for correctly explaining high apparent Km for exogenous ADP (200-300 microM) in regulation of mitochondrial respiration in oxidative muscle cells in situ. The best fitting of the results of in silico studies may be achieved by using of the compartmentalized energy transfer model. From these results, it may be concluded that in cardiac muscle cells the mitochondria and ATPases are organized into intracellular energetic units (ICEUs) separated from the bulk phase of cytoplasm by some barriers which limit the diffusion of adenine nucleotides. In contrast, alternative models based on the concept of the cell as homogenous system do not explain the observed experimental phenomena and have led to misleading conclusions. The various sources of experimental and conceptual errors are analyzed.

IgG from patients with liver diseases inhibit mitochondrial respiration in permeabilized oxidative muscle cells: impaired function of intracellular energetic units?

The effect of IgG purified from the sera of healthy persons and patients with primary biliary cirrhosis (PBC) and chronic hepatitis (CH) on ADP dependent respiration (oxidative phosphorylation) in skinned muscle fibers from rat oxidative muscles (heart and M. soleus) and glycolytic skeletal muscle (M. gastrocnemius) was studied. The results show that IgG from three different sources inhibited the rate of respiration by 13, 44 and 42%, respectively, these effects being equally expressed in both types of oxidative muscles, whereas no inhibition was observed in glycolytic muscle. The following washout of unbound IgG did not abolish the inhibition of respiration suggesting that the specific interaction of IgG with antigens had taken place. Laser confocal analysis revealed binding of IgG predominantly to the sarcomeric structures such as Z-disk and M-lines in the cardiomyocytes. The staining of IgG within Z-disks and intermitochondrial space coincided throughout the muscle cells so that transversally serial spaces, each containing mitochondria and adjacent sarcomere, became clearly visible. When the IgG from a CH patient was incubated with the skinned myocardial fibers of the desmin knockout mice, its binding to Z-disks and the sarcomeric area was found to be similar to that in normal cardiac muscle. However, the transversal staining pattern was disintegrated, because of the slippage of the myofibrils in relation to each other and accumulation of mitochondria between them. These observations support the recent hypothesis that in oxidative muscles the mitochondria and adjacent sarcomeres form complexes, termed as the intracellular energetic units, ICEUs. Moreover, they indicate that human autoantibodies can be useful tools for localizing the proteins responsible for formation of ICEUs and modulation of their function. Thus, it appears that the proteins associated with the Z-disks and M-lines may participate in formation of ICEUs and that binding of IgG to these proteins decreases the access of exogenous adenine nucleotides to mitochondria, which manifests as decreased rate of ADP-dependent respiration.

Structural and functional adaptations of striated muscles to CK deficiency

In adult mammalian muscle cells, energy consuming processes are mainly localized to the sarcolemma, sarcoplasmic reticulum (SR) and myofibrillar compartments, while energy production occurs within mitochondria or glycolytic complexes. Due to the restricted diffusion of adenine nucleotides near the active sites of ATPases involved in contractile activity and calcium homeostasis, there are multiple local systems that can locally rephosphorylate ADP and provide ATP. The creatine kinase (CK) system, with specific isoenzymes localized within each compartment, efficiently controls local adenylate pools and links energy production and utilization. However, mice lacking one or both of the MM-CK and mi-CK isoforms (CK-/-) are viable and develop almost normal cardiac and skeletal muscle function under the conditions of moderate workload, suggesting adaptations or other mechanisms that may ensure efficient energy transfer. While fixed CK is essentially important, other systems could also be involved as well, such as bound glycolytic enzymes or adenylate kinase. We have shown that, additionally, a direct functional interplay exists between mitochondria and sarcoplasmic reticulum, or between mitochondria and myofilaments in muscle cells, that catalyzes direct energy and signal transfer between organelles. In cardiac cells of CK-/- mice, marked cytoarchitectural modifications were observed, and direct adenine nucleotide channeling between mitochondria and organelles was very effective to rescue SR and myofilament functions. In fast skeletal muscles, increased oxidative capacity also indicates compensatory mechanisms. In mutant mice, mitochondrial capacity increases and a direct energy channeling occurs between mitochondria on one hand and ATP consuming sites on the other. However, these systems appear to be insufficient to fully compensate for the lack of CK at high workload. It can be concluded that local rephosphorylation of ADP is a crucial regulatory point in highly differentiated and organized muscle cells to ensure contractile diversity and efficiency and that the CK system is important to control energy fluxes and energy homeostasis.

Nucleotide-gated KATP channels integrated with creatine and adenylate kinases: amplification, tuning and sensing of energetic signals in the compartmentalized cellular environment

Transmission of energetic signals to membrane sensors, such as the ATP-sensitive K+ (KATP) channel, is vital for cellular adaptation to stress. Yet, cell compartmentation implies diffusional hindrances that hamper direct reception of cytosolic energetic signals. With high intracellular ATP levels, KATP channels may sense not bulk cytosolic, but rather local submembrane nucleotide concentrations set by membrane ATPases and phosphotransfer enzymes. Here, we analyzed the role of adenylate kinase and creatine kinase phosphotransfer reactions in energetic signal transmission over the strong diffusional barrier in the submembrane compartment, and translation of such signals into a nucleotide response detectable by KATP channels. Facilitated diffusion provided by creatine kinase and adenylate kinase phosphotransfer dissipated nucleotide gradients imposed by membrane ATPases, and shunted diffusional restrictions. Energetic signals, simulated as deviation of bulk ATP from its basal level, were amplified into an augmented nucleotide response in the submembrane space due to failure under stress of creatine kinase to facilitate nucleotide diffusion. Tuning of creatine kinase-dependent amplification of the nucleotide response was provided by adenylate kinase capable of adjusting the ATP/ADP ratio in the submembrane compartment securing adequate KATP channel response in accord with cellular metabolic demand. Thus, complementation between creatine kinase and adenylate kinase systems, here predicted by modeling and further supported experimentally, provides a mechanistic basis for metabolic sensor function governed by alterations in intracellular phosphotransfer fluxes.

The fractal architecture of cytoplasmic organization: scaling, kinetics and emergence in metabolic networks

In this work, we highlight the links between fractals and scaling in cells and explore the kinetic consequences for biochemical reactions operating in fractal media. Based on the proposal that the cytoskeletal architecture is organized as a percolation lattice, with clusters emerging as fractal forms, the analysis of kinetics in percolation clusters is especially emphasized. A key consequence of this spatiotemporal cytoplasmic organization is that enzyme reactions following Michaelis-Menten or allosteric type kinetics exhibit higher rates in fractal media (for short times and at lower substrate concentrations) at the percolation threshold than in Euclidean media. As a result, considerably faster and higher amplification of enzymatic activity is obtained. Finally, we describe some of the properties bestowed by cytoskeletal organization and dynamics on metabolic networks.

Random walk analysis of restricted metabolite diffusion in skeletal myofibril systems

The purpose of this work was the development of a basal mathematical model for the diffusion of low-molecular metabolites in a skeletal muscle cell. A three-dimension diffusion of low-molecular particles was simulated by a Monte-Carlo method (random walks of diffusing molecules). The model takes into account the following structural elements: (i) a regular lattice of actin and myosin filaments inside a myofibril; (ii) the membranes of sarcoplasmic reticulum and mitochondria surrounding the myofibrils; (iii) a set of myofibrils inside a skeletal muscle cell. We simulated diffusion of particles in the bulk of intracellular water phase and their reflections from the rigid surfaces of intracellular structures. The model allowed to calculate the apparent coefficients of particle diffusion in the axial and radial directions, Dparallel(app) and Dperpendicular(app), respectively. In accordance with experimental data from literature, the coefficient Dparallel(app) was independent of time. The coefficient of radial diffusion Dperpendicular(app) decreased with time to steady state values similar to that determined by the NMR diffusion spectroscopy methods. The interactions of diffusing particles with thin and thick filaments of myofibrils could explain the decrease in the Dperpendicular(app) value by a factor of 20%. The collisions of particles with myofilaments began to reveal themselves as a gradual decrease in the Dperpendicular(app) value at early stages of diffusion (t1/2 approximately equal to 0.05 microsec). The contribution of particle reflections from the membranes of sarcoplasmic reticulum and mitochondria to the retardation of the radial diffusion was about of 20-30%, depending on porosity of a membranous shield around the myofibril. For conventional sizes of a membranous shield (diameter 2 microm), the interactions of particles with the shield caused a decrease in the Dperpendicular(app) value with a half-time t1/2 approximately equal to 0.5 msec. This time is essentially lower by a factor about of 100 than that found in published NMR measurements. When we considered diffusion of particles inside a cell compartment confined to impermeable membranous shield, the reflection of particles from this shield led the drastic decrease in the radial diffusion coefficient (Dperpendicular(app) --> porportional to when t --> porportional to). This pattern of the Dperpendicular(app)(t) time-course might be expected in the NMR measurements on skeletal muscle tissue where a sarcolemma represents an impermeable shield for ATP and PCr molecules.

Method for in situ detection of the mitochondrial function in neurons

Conventional studies of neuronal mitochondria have been limited to the use of purified preparations of isolated mitochondria, neural cell homogenates, living neurons, or brain slices. However, each technique has several drawbacks. Here, we demonstrate that the neuronal cell's membrane can be effectively permeabilized by saponin-treatment and that these permeabilized neurons can be used for qualitative and quantitative assessments of oxygen consumption in combination with registration of mitochondrial membrane potential and free [Ca2+] in the matrix. Under these conditions, the mitochondrial function can be studied without removing the mitochondria from their natural milieu thus avoiding the damage of the associated cytoskeleton and outer membrane. At the same time, the method allows the estimation of the mitochondrial function independently of other processes in the cell, and the easy manipulation of the milieu surrounding the mitochondria. Thus, the presented method offers the opportunity to study the neuronal mitochondrial function in situ and can also be applied to examine the mitochondrial function by other commonly used methods.

Subcellular heterogeneity in mitochondrial red-ox responses to KATP channel agonists in freshly isolated rabbit cardiomyocytes

We have used the technique of fluorescent microscopy imaging supplemented with the refined analysis of temporal cartography of the cell fluorescence to investigate the mechanisms of regulation of mitochondrial function and its red-ox state in cardiac cells in vivo. Autofluorescence of flavoproteins of the respiratory chain in the isolated rabbit cardiomyocytes was registered before and after application of mitochondrial KATP channel opener diazoxide (100 and 400 microM). Diazoxide addition resulted in oxidation of flavoproteins. Detailed analysis of these responses showed that they were heterogeneous over space and time. The local responses show rapid jumps. In a few cells, metabolic oscillations developed and could be recorded for tens of minutes. Under these conditions the cells appeared divided into a small number of regions in which mitochondria function synchronously. Local pattern of oxidation switches again and again from a reduced state to the same level of oxidation. All these phenomena where absent when the cells were permeabilized by saponin giving a direct access to mitochondrial KATP channel opener. Cross-correlation analysis revealed a high degree of homogeneity for cells presenting metabolic oscillations, contrarily to those displaying a smooth increase in fluorescence in response to diazoxide. The results are consistent with the view that mitochondria form independent functional units whose behaviour can be synchronised by some unknown cellular factors or metabolites.

Regulation of mitochondrial energy production in cardiac cells of rainbow trout (Oncorhynchus mykiss)

In skinned rat cardiac fibres, mitochondrial affinity for endogenous ADP generated by creatine kinase and Ca2+-activated ATPases is higher than for exogenous ADP added to the surrounding medium, suggesting that mitochondria are functionally coupled to creatine kinase and ATPases. Such a coupling may be weaker or absent in ectothermic vertebrate cardiac cells, because they typically have less elaborate intracellular membrane structures, higher glycolytic capacity and lower working temperature. Therefore, we examined skinned cardiac fibres from rainbow trout at 10 degrees C. The apparent mitochondrial affinity for endogenous ADP was obtained by stimulation with ATP and recording of the release of ADP into the surrounding medium. The apparent affinity for endogenous ADP was much higher than for exogenous ADP suggesting a functional coupling between mitochondria and ATPases. The apparent affinity for exogenous ADP and ATP was increased by creatine or an increase in Ca2+-activity, which should increase intrafibrillar turnover of ATP to ADP. In conclusion, ADP seems to be channelled from creatine kinase and ATPases to mitochondria without being released to the surrounding medium. Thus, despite difference in structure, temperature and metabolic capacity, trout myocardium resembles that of rat with regard to the regulation of mitochondrial respiration.

Energy metabolism in heart failure

Heart failure (HF) is a syndrome resulting from the inability of the cardiac pump to meet the energy requirements of the body. Despite intensive work, the pathogenesis of the cardiac intracellular abnormalities that result from HF remains incompletely understood. Factors that lead to abnormal contraction and relaxation in the failing heart include metabolic pathway abnormalities that result in decreased energy production, energy transfer and energy utilization. Heart failure also affects the periphery. Patients suffering from heart failure always complain of early muscular fatigue and exercise intolerance. This is linked in part to intrinsic alterations of skeletal muscle, among which decreases in the mitochondrial ATP production and in the transfer of energy through the phosphotransfer kinases play an important role. Alterations in energy metabolism that affect both cardiac and skeletal muscles argue for a generalized metabolic myopathy in heart failure. Recent evidence shows that decreased expression of mitochondrial transcription factors and mitochondrial proteins are involved in mechanisms causing the energy starvation in heart failure. This review will focus on energy metabolism alterations in long-term chronic heart failure with only a few references to compensated hypertrophy when necessary. It will briefly describe the energy metabolism of normal heart and skeletal muscles and their alterations in chronic heart failure. It is beyond the scope of this review to address the metabolic switches occurring in compensated hypertrophy; readers could refer to well-documented reviews on this subject.

Creatine kinase and mitochondrial respiration in hearts of trout, cod and freshwater turtle

The importance of the creatine kinase system in the cardiac muscle of ectothermic vertebrates is unclear. Mammalian cardiac muscle seems to be structurally organized in a manner that compartmentalizes the intracellular environment as evidenced by the substantially higher mitochondrial apparent Km for ADP in skinned fibres compared to isolated mitochondria. A mitochondrial fraction of creatine kinase is functionally coupled to the mitochondrial respiration, and the transport of phosphocreatine and creatine as energy equivalents of ATP and ADP, respectively, increases the mitochondrial apparent ADP affinity, i.e. lowers the Km. This function of creatine kinase seems to be absent in hearts of frog species. To find out whether this applies to hearts of ectothermic vertebrate species in general, we investigated the effect of creatine on the mitochondrial respiration of saponin-skinned fibres from the ventricle of rainbow trout, Atlantic cod and freshwater turtle. For all three species, the apparent Km for ADP appeared to be substantially higher than for isolated mitochondria. Creatine lowered this Km in trout and turtle, thus indicating a functional coupling between mitochondrial creatine kinase and respiration. However, creatine had no effect on Km in cod ventricle. In conclusion, the creatine kinase-system in trout and turtle hearts seems to fulfil the same functions as in the mammalian heart, i.e. facilitating energy transport and communication between cellular compartments. In cod heart, however, this does not seem to be the case.

Diffusion barriers for ADP in the cardiac cell

The regulation of mitochondrial respiration in the intact heart may differ from that of isolated mitochondria if intracellular diffusion is restricted. Here we consider which factors may hinder diffusion in vivo and, based on computational analysis, design a reverse engineering approach to estimate the role of diffusional resistance in mitochondrial regulation from an experiment on the intact heart. Computational analysis of respiration measurements on skinned heart fibers shows that the outer mitochondrial membrane does not hinder diffusion enough to cause ADP gradients of tens of micromolars. A diffusion model further shows that the mesoscale structure of the myofibrillar space also does not hinder diffusion appreciably. However, ADP gradients are suggested by the measured activation time of oxidative phosphorylation and may be caused by diffusion restriction of other intracellular structures or the in vivo microstructure of networks of physically interacting proteins. Based on computational modeling we propose an experiment on the intact heart that allows to estimate the effective diffusion restriction between ATP producing and consuming sites in the cardiac cell.

Heterogeneity of ADP diffusion and regulation of respiration in cardiac cells

Heterogeneity of ADP diffusion and regulation of respiration were studied in permeabilized cardiomyocytes and cardiac fibers in situ and in silico. Regular arrangement of mitochondria in cells was altered by short-time treatment with trypsin and visualized by confocal microscopy. Manipulation of matrix volumes by changing K(+) and sucrose concentrations did not affect the affinity for ADP either in isolated heart mitochondria or in skinned fibers. Pyruvate kinase (PK)-phosphoenolpyruvate (PEP) were used to trap ADP generated in Ca,MgATPase reactions. Inhibition of respiration by PK-PEP increased 2-3 times after disorganization of regular mitochondrial arrangement in cells. ADP produced locally in the mitochondrial creatine kinase reaction was not accessible to PK-PEP in intact permeabilized fibers, but some part of it was released from mitochondria after short proteolysis due to increased permeability of outer mitochondrial membrane. In in silico studies we show by mathematical modeling that these results can be explained by heterogeneity of ADP diffusion due to its restrictions at the outer mitochondrial membrane and in close areas, which is changed after proteolysis. Localized restrictions and heterogeneity of ADP diffusion demonstrate the importance of mitochondrial functional complexes with sarcoplasmic reticulum and myofibrillar structures and creatine kinase in regulation of oxidative phosphorylation.

An integrated model of cardiac mitochondrial energy metabolism and calcium dynamics

We present an integrated thermokinetic model describing control of cardiac mitochondrial bioenergetics. The model describes the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, and mitochondrial Ca(2+) handling. The kinetic component of the model includes effectors of the TCA cycle enzymes regulating production of NADH and FADH(2), which in turn are used by the electron transport chain to establish a proton motive force (Delta mu(H)), driving the F(1)F(0)-ATPase. In addition, mitochondrial matrix Ca(2+), determined by Ca(2+) uniporter and Na(+)/Ca(2+) exchanger activities, regulates activity of the TCA cycle enzymes isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. The model is described by twelve ordinary differential equations for the time rate of change of mitochondrial membrane potential (Delta Psi(m)), and matrix concentrations of Ca(2+), NADH, ADP, and TCA cycle intermediates. The model is used to predict the response of mitochondria to changes in substrate delivery, metabolic inhibition, the rate of adenine nucleotide exchange, and Ca(2+). The model is able to reproduce, qualitatively and semiquantitatively, experimental data concerning mitochondrial bioenergetics, Ca(2+) dynamics, and respiratory control. Significant increases in oxygen consumption (V(O(2))), proton efflux, NADH, and ATP synthesis, in response to an increase in cytoplasmic Ca(2+), are obtained when the Ca(2+)-sensitive dehydrogenases are the main rate-controlling steps of respiratory flux. These responses diminished when control is shifted downstream (e.g., the respiratory chain or adenine nucleotide translocator). The time-dependent behavior of the model, under conditions simulating an increase in workload, closely reproduces experimentally observed mitochondrial NADH dynamics in heart trabeculae subjected to changes in pacing frequency. The steady-state and time-dependent behavior of the model support the hypothesis that mitochondrial matrix Ca(2+) plays an important role in matching energy supply with demand in cardiac myocytes.

Physical activity changes the regulation of mitochondrial respiration in human skeletal muscle

This study explores the importance of creatine kinase (CK) in the regulation of muscle mitochondrial respiration in human subjects depending on their level of physical activity. Volunteers were classified as sedentary, active or athletic according to the total activity index as determined by the Baecke questionnaire in combination with maximal oxygen uptake values (peak V(O2), expressed in ml min(-1) kg(-1)). All volunteers underwent a cyclo-ergometric incremental exercise test to estimate their peak V(O2) and V(O2) at the ventilatory threshold (VT). Muscle biopsy samples were taken from the vastus lateralis and mitochondrial respiration was evaluated in an oxygraph cell on saponin permeabilised muscle fibres in the absence (V(0)) or in the presence (V(max)) of saturating [ADP]. While V(0) was similar, V(max) differed among groups (sedentary, 3.7 +/- 0.3, active, 5.9 +/- 0.9 and athletic, 7.9 +/- 0.5 micromol O2 min(-1) (g dry weight)(-1)). V(max) was correlated with peak V(O2) (P < 0.01, r = 0.63) and with V(T) (P < 0.01, r = 0.57). There was a significantly greater degree of coupling between oxidation and phosphorylation (V(max)/V(0)) in the athletic individuals. The mitochondrial K(m) for ADP was significantly higher in athletic subjects (P < 0.01). Mitochondrial CK (mi-CK) activation by addition of creatine induced a marked decrease in K(m) in athletic individuals only, indicative of an efficient coupling of mi-CK to ADP rephosphorylation in the athletic subjects only. It is suggested that increasing aerobic performance requires an enhancement of both muscle oxidative capacity and mechanisms of respiratory control, attesting to the importance of temporal co-ordination of energy fluxes by CK for higher efficacy.