Role of creatine kinase in force development in chemically skinned rat cardiac muscle

Deep Transcriptomic Analysis Reveals the Dynamic Developmental Progression during Early Development of Channel Catfish (Ictalurus punctatus)

The transition from fertilized egg to larva in fish is accompanied with various biological processes. We selected seven early developmental stages in channel catfish, Ictalurus punctatus, for transcriptome analysis, and covered 22,635 genes with 590 million high-quality RNA-sequencing (seq) reads. Differential expression analysis between neighboring developmental timepoints revealed significantly enriched biological categories associated with growth, development and morphogenesis, which was most evident at 2 vs. 5 days post fertilization (dpf) and 5 vs. 6 dpf. A gene co-expression network was constructed using the Weighted Gene Co-expression Network Analysis (WGCNA) approach and four critical modules were identified. Among candidate hub genes, GDF10, FOXA2, HCEA and SYCE3 were involved in head formation, egg development and the transverse central element of synaptonemal complexes. CK1, OAZ2, DARS1 and UBE2V2 were mainly associated with regulation of cell cycle, growth, brain development, differentiation and proliferation of enterocytes. IFI44L and ZIP10 were critical for the regulation of immune activity and ion transport. Additionally, TCK1 and TGFB1 were related to phosphate transport and regulating cell proliferation. All these genes play vital roles in embryogenesis and regulation of early development. These results serve as a rich dataset for functional genomic studies. Our work reveals new insights of the underlying mechanisms in channel catfish early development.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Restrictions in ATP diffusion within sarcomeres can provoke ATP-depleted zones impairing exercise capacity in chronic obstructive pulmonary disease

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is characterized by the inability of patients to sustain a high level of ventilation resulting in perceived exertional discomfort and limited exercise capacity of leg muscles at average intracellular ATP levels sufficient to support contractility.

METHODS

Myosin ATPase activity in biopsy samples from healthy and COPD individuals was implemented as a local nucleotide sensor to determine ATP diffusion coefficients within myofibrils. Ergometric parameters clinically measured during maximal exercise tests in both groups were used to define the rates of myosin ATPase reaction and aerobic ATP re-synthesis. The obtained parameters in combination with AK- and CK-catalyzed reactions were implemented to compute the kinetic and steady-state spatial ATP distributions within control and COPD sarcomeres.

RESULTS

The developed reaction-diffusion model of two-dimensional sarcomeric space identified similar, yet extremely low nucleotide diffusion in normal and COPD myofibrils. The corresponding spatio-temporal ATP distributions, constructed during imposed exercise, predicted in COPD sarcomeres a depletion of ATP in the zones of overlap between actin and myosin filaments along the center axis at average cytosolic ATP levels similar to healthy muscles.

CONCLUSIONS

ATP-depleted zones can induce rigor tension foci impairing muscle contraction and increase a risk for sarcomere damages. Thus, intra-sarcomeric diffusion restrictions at limited aerobic ATP re-synthesis can be an additional risk factor contributing to the muscle contractile deficiency experienced by COPD patients.

GENERAL SIGNIFICANCE

This study demonstrates how restricted substrate mobility within a cellular organelle can provoke an energy imbalance state paradoxically occurring at abounding average metabolic resources.

Decreased creatine kinase is linked to diastolic dysfunction in rats with right heart failure induced by pulmonary artery hypertension

Our objective was to investigate the role of creatine kinase in the contractile dysfunction of right ventricular failure caused by pulmonary artery hypertension. Pulmonary artery hypertension and right ventricular failure were induced in rats by monocrotaline and compared to saline-injected control animals. In vivo right ventricular diastolic pressure-volume relationships were measured in anesthetized animals; diastolic force-length relationships in single enzymatically dissociated myocytes and myocardial creatine kinase levels by Western blot. We observed diastolic dysfunction in right ventricular failure indicated by significantly steeper diastolic pressure-volume relationships in vivo and diastolic force-length relationships in single myocytes. There was a significant reduction in creatine kinase protein expression in failing right ventricle. Dysfunction also manifested as a shorter diastolic sarcomere length in failing myocytes. This was associated with a Ca(2+)-independent mechanism that was sensitive to cross-bridge cycling inhibition. In saponin-skinned failing myocytes, addition of exogenous creatine kinase significantly lengthened sarcomeres, while in intact healthy myocytes, inhibition of creatine kinase significantly shortened sarcomeres. Creatine kinase inhibition also changed the relatively flat contraction amplitude-stimulation frequency relationship of healthy myocytes into a steeply negative, failing phenotype. Decreased creatine kinase expression leads to diastolic dysfunction. We propose that this is via local reduction in ATP:ADP ratio and thus to Ca(2+)-independent force production and diastolic sarcomere shortening. Creatine kinase inhibition also mimics a definitive characteristic of heart failure, the inability to respond to increased demand. Novel therapies for pulmonary artery hypertension are needed. Our data suggest that cardiac energetics would be a potential ventricular therapeutic target.

Submembranous recruitment of creatine kinase B supports formation of dynamic actin-based protrusions of macrophages and relies on its C-terminal flexible loop

Subcellular partitioning of creatine kinase contributes to the formation of patterns in intracellular ATP distribution and the fuelling of cellular processes with a high and sudden energy demand. We have previously shown that brain-type creatine kinase (CK-B) accumulates at the phagocytic cup in macrophages where it is involved in the compartmentalized generation of ATP for actin remodeling. Here, we report that CK-B catalytic activity also helps in the formation of protrusive ruffle structures which are actin-dependent and abundant on the surface of both unstimulated and LPS-activated macrophages. Recruitment of CK-B to these structures occurred transiently and inhibition of the enzyme's catalytic activity with cyclocreatine led to a general smoothening of surface morphology as visualized by scanning electron microscopy. Comparison of the dynamics of distribution of YFP-tagged CK-mutants and isoforms by live imaging revealed that amino acid residues in the C-terminal segment (aa positions 323-330) that forms one of the protein's two mobile loops are involved in partitioning over inner regions of the cytosol and nearby sites where membrane protrusions occur during induction of phagocytic cup formation. Although wt CK-B, muscle-type CK (CK-M), and a catalytically dead CK-B-E232Q mutant with intact loop region were normally recruited from the cytosolic pool, no dynamic transition to the phagocytic cup area was seen for the CK-homologue arginine kinase and a CK-B-D326A mutant protein. Bioinformatics analysis helped us to predict that conformational flexibility of the C-terminal loop, independent of conformational changes induced by substrate binding or catalytic activity, is likely involved in exposing the enzyme for binding at or near the sites of membrane protrusion formation.

The location of energetic compartments affects energetic communication in cardiomyocytes

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

Unchanged mitochondrial organization and compartmentation of high-energy phosphates in creatine-deficient GAMT-/- mouse hearts

Disruption of the creatine kinase (CK) system in hearts of CK-deficient mice leads to changes in the ultrastructure and regulation of mitochondrial respiration. We expected to see similar changes in creatine-deficient mice, which lack the enzyme guanidinoacetate methyltransferase (GAMT) to produce creatine. The aim of this study was to characterize the changes in cardiomyocyte mitochondrial organization, regulation of respiration, and intracellular compartmentation associated with GAMT deficiency. Three-dimensional mitochondrial organization was assessed by confocal microscopy. On populations of permeabilized cardiomyocytes, we recorded ADP and ATP kinetics of respiration, competition between mitochondria and pyruvate kinase for ADP produced by ATPases, ADP kinetics of endogenous pyruvate kinase, and ATP kinetics of ATPases. These data were analyzed by mathematical models to estimate intracellular compartmentation. Quantitative analysis of morphological and kinetic data as well as derived model fits showed no difference between GAMT-deficient and wild-type mice. We conclude that inactivation of the CK system by GAMT deficiency does not alter mitochondrial organization and intracellular compartmentation in relaxed cardiomyocytes. Thus, our results suggest that the healthy heart is able to preserve cardiac function at a basal level in the absence of CK-facilitated energy transfer without compromising intracellular organization and the regulation of mitochondrial energy homeostasis. This raises questions on the importance of the CK system as a spatial energy buffer in unstressed cardiomyocytes.

The creatine kinase system and pleiotropic effects of creatine

The pleiotropic effects of creatine (Cr) are based mostly on the functions of the enzyme creatine kinase (CK) and its high-energy product phosphocreatine (PCr). Multidisciplinary studies have established molecular, cellular, organ and somatic functions of the CK/PCr system, in particular for cells and tissues with high and intermittent energy fluctuations. These studies include tissue-specific expression and subcellular localization of CK isoforms, high-resolution molecular structures and structure–function relationships, transgenic CK abrogation and reverse genetic approaches. Three energy-related physiological principles emerge, namely that the CK/PCr systems functions as (a) an immediately available temporal energy buffer, (b) a spatial energy buffer or intracellular energy transport system (the CK/PCr energy shuttle or circuit) and (c) a metabolic regulator. The CK/PCr energy shuttle connects sites of ATP production (glycolysis and mitochondrial oxidative phosphorylation) with subcellular sites of ATP utilization (ATPases). Thus, diffusion limitations of ADP and ATP are overcome by PCr/Cr shuttling, as most clearly seen in polar cells such as spermatozoa, retina photoreceptor cells and sensory hair bundles of the inner ear. The CK/PCr system relies on the close exchange of substrates and products between CK isoforms and ATP-generating or -consuming processes. Mitochondrial CK in the mitochondrial outer compartment, for example, is tightly coupled to ATP export via adenine nucleotide transporter or carrier (ANT) and thus ATP-synthesis and respiratory chain activity, releasing PCr into the cytosol. This coupling also reduces formation of reactive oxygen species (ROS) and inhibits mitochondrial permeability transition, an early event in apoptosis. Cr itself may also act as a direct and/or indirect anti-oxidant, while PCr can interact with and protect cellular membranes. Collectively, these factors may well explain the beneficial effects of Cr supplementation. The stimulating effects of Cr for muscle and bone growth and maintenance, and especially in neuroprotection, are now recognized and the first clinical studies are underway. Novel socio-economically relevant applications of Cr supplementation are emerging, e.g. for senior people, intensive care units and dialysis patients, who are notoriously Cr-depleted. Also, Cr will likely be beneficial for the healthy development of premature infants, who after separation from the placenta depend on external Cr. Cr supplementation of pregnant and lactating women, as well as of babies and infants are likely to be of benefit for child development. Last but not least, Cr harbours a global ecological potential as an additive for animal feed, replacing meat- and fish meal for animal (poultry and swine) and fish aqua farming. This may help to alleviate human starvation and at the same time prevent over-fishing of oceans.

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.

Dependence of myosin-ATPase on structure bound creatine kinase in cardiac myofibrils from rainbow trout and freshwater turtle

The influence of myofibrillar creatine kinase on the myosin-ATPase activity was examined in cardiac ventricular myofibrils isolated from rainbow trout (Oncorhynchus mykiss) and freshwater turtle (Trachemys scripta). The ATPase rate was assessed by recording the rephosphorylation of ADP by the pyruvate kinase reaction alone or together with the amount of creatine formed, when myofibrillar bound creatine kinase was activated with phosphocreatine. The steady-state concentration of ADP in the solution was varied through the activity of pyruvate kinase added to the solution. For rainbow trout myofibrils at a high pyruvate kinase activity, creatine kinase competed for ADP but did not influence the total ATPase activity. When the ADP concentration was elevated within the physiological range by lowering the pyruvate kinase activity, creatine kinase competed efficiently and increased the ATPase activity twice or more for both trout and turtle. As examined for trout myofibrils, the ATPase activity was reduced about four times by inhibiting the activity of myofibril-bound creatine kinase with iodoacetamide and this reduction was only partially counteracted, when the creatine kinase activity was restored by adding creatine kinase to the solution. Hence, the results suggest that myofibril-bound creatine kinase is needed to fully activate the myosin-ATPase activity in hearts from ectothermic vertebrates despite their low energy turn-over relative to endothermic species.

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 quantitative analysis of cardiac myocyte relaxation: a simulation study

The determinants of relaxation in cardiac muscle are poorly understood, yet compromised relaxation accompanies various pathologies and impaired pump function. In this study, we develop a model of active contraction to elucidate the relative importance of the [Ca2+]i transient magnitude, the unbinding of Ca2+ from troponin C (TnC), and the length-dependence of tension and Ca2+ sensitivity on relaxation. Using the framework proposed by one of our researchers, we extensively reviewed experimental literature, to quantitatively characterize the binding of Ca2+ to TnC, the kinetics of tropomyosin, the availability of binding sites, and the kinetics of crossbridge binding after perturbations in sarcomere length. Model parameters were determined from multiple experimental results and modalities (skinned and intact preparations) and model results were validated against data from length step, caged Ca2+, isometric twitches, and the half-time to relaxation with increasing sarcomere length experiments. A factorial analysis found that the [Ca2+]i transient and the unbinding of Ca2+ from TnC were the primary determinants of relaxation, with a fivefold greater effect than that of length-dependent maximum tension and twice the effect of tension-dependent binding of Ca2+ to TnC and length-dependent Ca2+ sensitivity. The affects of the [Ca2+]i transient and the unbinding rate of Ca2+ from TnC were tightly coupled with the effect of increasing either factor, depending on the reference [Ca2+]i transient and unbinding rate.

Functional compartmentation of glycogen phosphorylase with creatine kinase and Ca2+ATPase in skeletal muscle

This manuscript discusses aspects of functional compartmentation in the regulation of metabolism. The functional consequences of enzymes coupling between creatine kinase, glycogen phosphorylase and sarcoplasmic reticular Ca2+ ATPase is examined. It is proposed that the coupling of creatine kinase and glycogen phosphorylase classifies as a novel class of diazyme complex with an important regulatory role in the inhibition of glycogenolysis at rest. In addition it is suggested that creatine kinase, glycogen phosphorylase and the sarcoplasmic reticular Ca2+ ATPase may couple to form a three-enzyme complex. From a consideration of the structure and chemical catalysis of the putative three-enzyme complex, a novel net reaction for glycogenolysis in the vicinity of the sarcoplasmic reticulum is suggested (Phosphocreatine+Glycogen+H(+)Creatine+Glycogen(n)(-1)+Glucose-1-Phosphate). The three-enzyme complex may also have an important role in inhibiting glycogenolysis at rest as well as improving the efficiency of high-energy phosphate transfer.

The creatine kinase/creatine connection to Alzheimer's disease: CK-inactivation, APP-CK complexes and focal creatine deposits

Cytosolic brain-type creatine kinase (BB-CK), which is coexpressed with ubiquitous mitochondrial uMtCK, is significantly inactivated by oxidation, in Alzheimer's disease (AD) patients. Since CK has been shown to play a fundamental role in cellular energetics of the brain, any disturbance of this enzyme may exasperate the AD disease process. Mutations in amyloid precursor protein (APP) are associated with early onset AD and result in abnormal processing of APP, and accumulation of A beta peptide, the main constituent of amyloid plaques in AD brain. Recent data on a direct interaction between APP and the precursor of uMtCK support an emerging relationship between AD, cellular energy levels and mitochondrial function. In addition, recently discovered creatine (Cr) deposits in the brain of transgenic AD mice, as well as in the hippocampus from AD patients, indicate a direct link between perturbed energy state, Cr metabolism and AD. Here, we review the roles of Cr and Cr-related enzymes and consider the potential value of supplementation with Cr, a potent neuroprotective substance. As a hypothesis, we consider whether Cr, if given at an early time point of the disease, may prevent or delay the course of AD-related neurodegeneration.

A novel mechanism of regulation of cardiac contractility by mitochondrial functional state

It is generally considered that mitochondria regulate cardiac cell contractility by providing ATP for cellular ATPases and by participating in Ca2+ homeostasis. However, other possible mechanisms by which mitochondria can influence contractility have been largely overlooked. Here, we demonstrate that inhibition of the mitochondrial electron transport chain strongly increases Ca2+-dependent and independent isometric force development in rat ventricular fibers with selectively permeabilized sarcolemma. This effect is unrelated to the ATP-generating activity of mitochondria or Ca2+ homeostasis. Furthermore, various conditions that increase K+ accumulation in the mitochondrial matrix (activation of ATP- or Ca2+-dependent K+ channels as well as inhibition of the K+ efflux pathway via the K+/H+ exchanger) induce a similar mechanical response. Modulators of mitochondrial function that augment isometric force also cause swelling of mitochondria in the vicinity of myofibrils in situ, as shown by confocal microscopy. Osmotic compression of intracellular structures abolishes the effect of mitochondria-induced force modulation, suggesting a mechanical basis for the interaction between the organelles. These findings suggest a novel mechanism for cellular regulation of myofibrillar function, whereby increases in mitochondrial volume can impose mechanical constraints inside the cell, leading to an increase in force developed by myofibrils.

SR33805, a Ca2+ antagonist with length-dependent Ca2+ -sensitizing properties in cardiac myocytes

1. This study examined the effects of SR33805, a fantofarone derivative with reported strong Ca(2+) -antagonistic properties, on the contractile properties of intact and skinned rat ventricular myocytes. 2. On intact cells loaded with the Ca(2+)-fluorescent indicator Indo-1, the application of low concentrations of SR33805 enhanced the amplitude of unloaded cell shortening and decreased the duration of cell shortening. Amplitude of the Ca(2+) transient was also decreased. 3. These effects were accompanied with a shortening of the action potential and a dose-dependent blockade of L-type calcium current (IC(50)=2.4 x 10(-8) M). 4. On skinned cardiac cells, the application of a low SR33805 concentration (10(-8) M) induced a significant increase in maximal Ca(2+)-activated force at the two-tested sarcomere lengths (SLs), 1.9 and 2.3 microm. 5. The application of a larger dose of SR33805 (10(-6)-10(-5) M) induced a significant leftward shift of the tension-pCa relation that accounts for Ca(2+)-sensitization of the myofilaments, particularly at 2.3 microm SL. 6. In conclusion, despite its strong Ca(2+)-antagonistic properties SR33805 increases cardiac cell contractile activity as a consequence of its Ca(2+)-sensitizing effects. These effects are attributable to both an increase in the maximal Ca(2+)-activated force and a length-dependent Ca(2+)-sensitization.

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.

Coupling of creatine kinase to glycolytic enzymes at the sarcomeric I-band of skeletal muscle: a biochemical study in situ

The specific interaction of muscle type creatine-kinase (MM-CK) with the myofibrillar M-line was demonstrated by exchanging endogenous MM-CK with an excess of fluorescently labeled MM-CK in situ, using chemically skinned skeletal muscle fibers and confocal microscopy. No binding of labeled MM-CK was noticed at the I-band of skinned fibers, where the enzyme is additionally located in vivo, as shown earlier by immunofluorescence staining of cryosections of intact muscle. However, when rhodamine-labeled MM-CK was diffused into skinned fibers that had been preincubated with phosphofructokinase (PFK), a glycolytic enzyme known to bind to actin, a striking in vivo-like interaction of Rh-MM-CK with the I-band was found, presumably mediated by binding of Rh-MM-CK to the glycolytic enzyme. Aldolase, another actin-binding glycolytic enzyme was also able to bind Rh-MM-CK to the I-band, but formation of the complex occurred preferably at long sarcomere length (> 3.0 microm). Neither pyruvate kinase, although known for its binding to actin, nor phosphoglycerate kinase (PGK), not directly interacting with the I-band itself, did mediate I-band targeting of MM-CK. Anchoring of MM-CK to the I-band via PFK, but not so via aldolase, was strongly pH-dependent and occurred below pH 7.0. Labeling performed at different sarcomere length indicated that the PFK/MM-CK complex bound to thin filaments of the I-band, but not within the actomyosin overlap zones. The physiological consequences of the structural interaction of MM-CK with PFK at the I-band is discussed with respect to functional coupling of MM-CK to glycolysis, metabolic regulation and channeling in multi-enzyme complexes. The in situ binding assay with skinned skeletal muscle fibers described here represents a useful method for further studies of specific protein-protein interactions in a structurally intact contractile system under various precisely controlled conditions.

Glycolysis supports calcium uptake by the sarcoplasmic reticulum in skinned ventricular fibres of mice deficient in mitochondrial and cytosolic creatine kinase

Several works have shown the importance of the creatine kinase (CK) system for cardiac energetics and Ca2+ homeostasis. Nevertheless, CK-deficient mice have cardiac function close to normal, at least under conditions of low or moderate workload. To characterize possible adaptive changes of the sarcoplasmic reticulum (SR) and potential role of glycolytic support in cardiac contractility we used the skinned fibre technique to study properties of the SR and myofibrils, in control and muscle-type homodimer (MM-/mitochondrial-CK)-deficient mice. In control fibres, SR Ca2+ loading with ATP and phosphocreatine (solution PL) was significantly better than loading with ATP alone (solution AL), as determined by analysis of caffeine-induced tension transients. Loading in the presence of ATP and glycolytic intermediates (solution GL) was not significantly different from solution PL. These data indicate that Ca2+ uptake by the SR in situ depends on a local ATP:ADP ratio that is controlled by both CK and glycolytic enzymes. In CK-deficient mice, Ca2+ loading was impaired in solution PL due to the absence of CK. In solution GL, loading was significantly increased, such that calculated Ca2+ release parameters were normalized to those in control fibres in solution PL. In CK-deficient mice, fibre kinetic parameters of tension recovery were impaired after quick stretch in solution PL and were not improved in solution GL. These results show that in CK-deficient mice, at least under basal conditions, glycolysis can replace the CK system in fueling the SR Ca2+ ATPase, but not the myosin ATPase, and may in part explain the limited phenotypic alterations seen in the hearts of these mice.

Subcellular creatine kinase alterations. Implications in heart failure

We have tested the hypothesis that decreased functioning of creatine kinase (CK) at sites of energy production and utilization may contribute to alterations in energy fluxes and calcium homeostasis in congestive heart failure (CHF). Heart failure was induced by aortic banding in 3-week-old rats. Myofilaments, sarcoplasmic reticulum (SR), mitochondrial functions, and CK compartmentation were studied in situ using selective membrane permeabilization of left ventricular fibers with detergents (saponin for mitochondria and SR and Triton X-100 for myofibrils). Seven months after surgery, animals were in CHF. A decrease in total CK activity could be accounted for by a 4-fold decrease in activity and content (Western blots) of mitochondrial CK and a 30% decrease in M isoform of CK (MM-CK) activity. In myofibrils, maximal force, crossbridge kinetics, and alpha-myosin heavy-chain expression decreased, whereas calcium sensitivity of tension development remained unaltered. Myofibrillar CK efficacy was unchanged. Calcium uptake capacities of SR were estimated from the surface of caffeine-induced tension transient (SCa) after loading with different substrates. In CHF, SCa decreased by 23%, and phosphocreatine was 2 times less efficient in enhancing calcium uptake. Oxidative capacities of the failing myocardium measured as oxygen consumption per gram of fiber dry weight decreased by 28%. Moreover, the control of respiration by creatine, ADP, and AMP was severely impaired. Our observations provide evidence that alterations in CK compartmentation may contribute to alterations of energy fluxes and calcium homeostasis in CHF.

Cardiac performance and creatine kinase flux during inhibition of ATP synthesis in the perfused rat heart

To study the relation among mitochondrial energy supply, cardiac performance, and energy transfer through creatine kinase (CK), two acute models of inhibition of ATP synthesis were compared in the isovolumic acetate-perfused rat heart. Similar impairments of mechanical performance (rate-pressure product, RPP) were achieved by various stepwise decreases in O(2) supply (PO(2) down to 20% of control) or by infusing CN (0.15-0.25 mM). The forward CK flux measured by saturation-transfer (31)P NMR spectroscopy was 6.1 +/- 0. 4 mM/s in control hearts. Only after severe hypoxia (PO(2) < 40% of control) did CK flux drop (to 1.9 +/- 0.2 mM/s at PO(2) = 25% of control) together with impaired systolic activity and a rise in end-diastolic pressure. In contrast, in mild hypoxia CK flux remained constant and similar to control (5.3 +/- 0.5 mM/s, not significant) despite a twofold reduction in systolic activity. Similarly in all CN groups, constant CK flux was maintained for a threefold reduction in RPP, showing the absence of a relation between cardiac performance and global NMR-measured CK flux during mild ATP synthesis inhibition.

Intracellular effects of nitric oxide on force production and Ca2+ sensitivity of cardiac myofilaments

The gaseous free radical nitric oxide (NO*) has been implicated in a wide range of physiological functions and also has a role in the pathogenesis of cellular injury. It has been suggested that NO* and its congeners may exert effects on actin-myosin crossbridge cycling by modulating critical thiols on the myosin head. To understand the mode and site of actin of NO* in myofibrils, the effects of the NO* donor 3-(2-hydroxy-1-methyl-2-nitrosohydrazine)-N-methyl-1-propanamine (NOC-7) have been studied in Triton X-100-treated rabbit cardiac fibers, in which isometric force was measured at controlled degrees of activation. Experiments were undertaken after previous exposure of the preparations to NOC-7 (for 30 min). We found that NO* induced several alterations of myofibrillar function, i.e., decrease in Ca2+ sensitivity and Hill coefficient and potentiation of rigor contracture. We attributed the effect on rigor contracture to strong inhibition of myofibrillar creatine kinase (CK) activity, because it could be prevented by exogeneously added CK; such CK inactivation afforded by NO* may result in the myofibrillar ATP-to-ADP ratio. In further experiments, concentration of NO* released from NOC-7 was determined by the electron spin resonance spin-trapping technique; N-(dithiocarboxy)sarcosine-Fe2+ complex was used as the spin-trap. NO* at cumulative concentration of 0.69 microM was effective in producing both enhancement of rigor contracture and decrease of myofibrillar-bound CK activity; however, Ca2+-sensitivity (pCa50) was significantly decreased at >5.6 microM of NO*, suggesting a result from different mechanisms. Thus, the observed decrease in Ca2+ sensitivity seems to be associated with direct modification of the regulatory proteins by a relatively higher concentration of NO*, and possibly not via inhibition of myofibrillar CK activity. The data reported here indicate that CK may be a pathophysiologically main target for increased NO* formation at low molecular range in the disease state in cardiac muscle.

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.

Tumour necrosis factor-alpha, but not septic plasma depresses cardiac myofilament contraction

PURPOSE

In sepsis, myocardial depression may be caused by mediators released as part of the inflammatory reaction, lumour necrosis factor alpha (TNF alpha) is one mediator that may contribute to this depression. In the present study, we contrasted the effects of TNF alpha and septic plasma fraction (SP) obtained from an E. coli model on contractile tension in intact and skinned canine ventricular trabecular (VT) preparations. The objectives were to determine whether SP or TNF alpha could impair contractile tension at the level of the myofilaments, and to determine the extent to which TNF alpha may account for myocardial depression found in E. coli sepsis.

METHODS

Measurements of isometric tensions were made after TNF alpha and SP (10,000 to 30,000 MW fraction) were added to respective intact or skinned canine VT preparations. In the skinned preparation, trabeculae were chemically skinned with Triton X-100.

RESULTS

Septic plasma caused a decrease in contraction in the intact preparation compared with preseptic plasma (50 +/- 7 vs 33 +/- 7%, P < 0.05), but had no effect in the skinned preparation. On the other hand, TNF alpha (30 ng.ml-1) caused an approximately 50% reduction in tension (29 +/- 2 mg vs 16 +/- 5 mg) in the skinned preparation (P < 0.05), but had no effect in the intact preparation.

CONCLUSION

These results suggest that TNF alpha and SP act through different mechanisms. While SP requires an intact membrane, TNF alpha impairs function by a direct effect on the myofilaments.

Rigor tension in single skinned rat cardiac cell: role of myofibrillar creatine kinase

OBJECTIVE

To elucidate the role of bound creatine kinase in adenine nucleotide compartmentation in myofibrils, the effects of this enzyme's substrates and products on rigor tension were studied in using isolated skinned rat cardiomyocytes rather than fibers, to avoid restrictions due to concentration gradients within the multicellular preparations.

METHODS

A new experimental set-up was built to allow continuous and stable measurements of force developed by cells. Triton X-100-treated cardiomyocytes were glued between a glass holder and the needle of a galvanometer. A feedback system allowed the precise measurement of force by recording the coil current necessary to prevent movement of the needle.

RESULTS

At very low [Ca2+] (pCa 7), as MgATP level decreased, rigor tension appeared. In the absence of phosphocreatine (PCr), this tension started to rise at MgATP concentrations several times higher than in the presence of 12 mM PCr. In the absence of PCr, the pMgATP/tension curves of single cells usually had a complicated relationship which could not be analyzed by a simple Hill equation. In the absence of PCr, 250 microM MgADP strongly potentiated rigor tension development in the 1 mM-3 microM range of [MgATP]; at 100 microM MgATP, in the presence of MgADP, the tension was 4.6 times higher than in the absence of MgADP. Addition of 12 mM PCr immediately eliminated rigor. Finally, in the presence of 100 microM MgATP and 250 microM MgADP, a decrease in PCr resulted in rigor; the half-maximal contracture being recorded at 1 mM PCr.

CONCLUSIONS

These results indicate a myofibrillar compartmentation of adenine nucleotides influenced by bound creatine kinase, since at equal MgATP concentrations in extramyofibrillar milieu the response of myofibrils strongly depends on the presence of PCr. Local accumulation of ADP in myofibrils due to a fall in cellular PCr and inability of myofibrillar creatine kinase to rephosphorylate ADP produced by myosin ATPase could be an important mechanism of diastolic tension rise in ischaemic conditions.

Tension response of the cardiotonic agent (+)-EMD-57033 at the single cell level

The effects of the Ca sensitizer (+)-EMD-57033 were tested on single chemically skinned cells isolated from rat ventricle. The present study demonstrates that (+)-EMD-57033 (10 microM) increased maximal force by 20% (at pCa 4.5) and myofilament Ca sensitivity by 0.2 pCa unit. However, the force-length dependency was not affected by the addition of (+)-EMD-57033, since similar Ca-sensitizing effects occurred at different sarcomere lengths. Consequently, the Ca-sensitizing effect of the drug and of the sarcomere length might be additive. Cross-bridge kinetics were also investigated in the presence of the thiadiazinone derivative. (+)-EMD-57033 induced marked increases in the rate of tension redevelopment (ktr) after brief slack release/restretch, particularly at low Ca concentrations. These results suggest that the Ca-sensitizing effects of (+)-EMD-57033 are due, at least in part, to an increased number of attached cross bridges during one cyclo. This observation, together with the increase in peak force, is discussed in relation to the reduction in energy cost induced by such Ca-sensitizing agents.

An estimate of the participation of the sarcoplasmic reticulum in the intracellular Ca2+ regulation in adult and newborn ferret hearts

The aim of the present study was to estimate the participation of the sarcoplasmic reticulum in the Ca2+ regulation of the contraction of newborn ferret heart. Cyclopiazonic acid has been used to block the sarcoplasmic reticulum Ca2+ pump in adult and newborn (1 month and 5-6 day old) ferret ventricles of intact and saponin-treated preparations. Cyclopiazonic acid induced a decrease of the amplitude of the caffeine contractures generated in saponin skinned fibers. The sensitivity of the sarcoplasmic reticulum Ca2+ uptake to cyclopiazonic acid was similar in adult and newborn hearts. In intact preparations, cyclopiazonic acid (1-20 microM) induced a negative inotropic effect on the twitch with a prolongation in its kinetics. The maximal decrease in the amplitude of the twitch was larger in adult (92.4%) than in 1 month old (86.5%) and 5-6 day newborns (72.5%). Contrary to other species, where the sarcoplasmic reticulum Ca2+ pump is not functional in neonatal heart, it is proposed that ferret myocardium shows an early maturation of sarcoplasmic reticulum function.

Cardiac pump function of the isolated rat heart at two modes of energy deprivation and effect of adrenergic stimulation

The contractile function of the isolated rat heart and high energy phosphate content were evaluated under conditions of depressed energy supply caused by disturbances either in mitochondrial ATP production or ATP-phosphocreatine transformation. Amytal (0.3 mM), an inhibitor of mitochondrial respiration, or iodoacetamide (IAA, 0.1 mM) reducing in this dose creatine kinase activity to 19% of the initial level, were used, respectively. Myocardial ATP content remained unaffected in both groups and PCr content decreased to 37% only in amytal-treated group. Very similar alterations in cardiac pump function during volume load were observed in both treated groups; maximal cardiac output was significantly less by 30%, cardiac pressure-volume work by 38-40%, left ventricular (LV) systolic pressure by 24-29%, and LV +dP/dt by 36-39%. In contrast, the extent of decreased LV distensibility was different, a curve relating LV filling volume and end-diastolic pressure was shifted up and to the left much more prominently after IAA treatment. Heart rate was decreased by 24% only in amytal-treated group. Results indicate that a decreased myocardial distensibility is a dominating feature in the acute cardiac pump failure caused by an inhibition of myocardial creatine kinase. Isoproterenol (0.1 microM) substantially increased heart rate and pressure-rate product in IAA-treated hearts but failed to increase cardiac work probably due to its inability to improve myocardial distensibility.

Creatine kinase is the main target of reactive oxygen species in cardiac myofibrils

Reactive oxygen species (ROS) have been reported to alter cardiac myofibrillar function as well as myofibrillar enzymes such as myosin ATPase and creatine kinase (CK). To understand their precise mode and site of action in myofibrils, the effects of the xanthine/xanthine oxidase (X/XO) system or of hydrogen peroxide (H2O2) have been studied in the presence and in the absence of phosphocreatine (PCr) in Triton X-100-treated cardiac fibers. We found that xanthine oxidase (XO), with or without xanthine, induced a decrease in maximal Ca(2+)-activated tension. We attributed this effect to the high contaminating proteolytic activity in commercial XO preparations, since it could be prevented a protease inhibitor, phenylmethylsulfonyl fluoride (PMSF), and it could be mimicked by trypsin. In further experiments, XO was pre-treated with 1 mmo1/L PMSF. Superoxide anion production by the X/XO system, characterized by electron paramagnetic resonance spin-trapping technique, was not altered by PMSF. A slight increase in maximal force was then observed either with X/XO (100 mumol/L per 30 mIU/mL) or H2O2. pMgATP-rigor tension relationships have been established in the presence and in the absence of PCr to separate the effects of ROS on myosin ATPase and myofibrillar-bound CK. In the absence of PCr, pMgATP50, the pMgATP necessary to induce half-maximal rigor tension, was reduced from 5.03 +/- 0.17 (n = 21) to 4.22 +/- 0.22 (n = 4) after 25 minutes of incubation in the presence one of 30 mIU/mL. XO and 100 mumol/L xanthine or to 4.04 +/- 0.1 (n = 11) after incubation in the presence of 2.5 mmol/L H2O2. The ROS effects were partially prevented or antagonized by 1 mmol/L dithiothreitol. No effect was observed on pMgATP50 when PCr was absent. pCa-tension relationships have been evaluated to assess the effects of ROS on active tension development. Incubations with H2O2 induced on increase in Ca2+ sensitivity and resting tension when MgATP was provided through myofibrillar CK (PCr and MgADP as substrates) but not when MgATP was added directly. These results suggest that myofibrillar CK was inhibited by ROS. Active stiffness and the time constant of tension changes after quick stretches applied to the fibers were dose-dependently increased by H2O2 only in the presence of PCr. In addition, myofibrillar CK but not myosin ATPase enzymatic activity was depressed after incubation with either ROS. These results suggest that ROS mainly alters CK in myofibrils, probably by the oxidation of its essential sulfhydryl groups. Such CK inactivation results in a decrease in the intramyofibrillar ATP-to-ADP ratio. The effects of ROS on cytosolic and bound CKs may take part in the overall process of myocardial stunning after cardiac ischemia and reperfusion.

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

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

Potentiation of the twitch responses by inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase in frog atrial fibres

In frog atrial fibres, cyclopiazonic acid as well as thapsigargin, which are inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, induced a significant increase in the twitch amplitude without detectable changes in its kinetics. The measurements performed on chemically skinned fibres show that cyclopiazonic acid has no effect on the properties of contractile proteins. In the presence of a T-type Ca2+ channel blocker or L-type Ca2+ channel blocker, cyclopiazonic acid still induced a potentiation of the twitch while no effect was found in the presence of a Na(+)-Ca2+ exchange blocker. The effect of cyclopiazonic acid was not related to any modification in myofibrillar Ca2+ sensitivity or in Ca2+ influx through Ca2+ channels. It is proposed that the inhibition of the sarcoplasmic reticulum Ca(2+)-ATPase resulted in a potentiation of the effect of the Ca2+ influx and that the major role of the sarcoplasmic reticulum was to limit the intracellular Ca2+ concentration.

Creatine kinase, energy-rich phosphates and energy metabolism in heart muscle of different vertebrates

Maximal activities of creatine kinase, pyruvate kinase and cytochrome oxidase and total concentrations of creatine and phosphorylated adenylates were measured in cardiac muscle of hagfish, eight teleost species, frog, turtle, pigeon and rat. The ratio of creatine kinase to cytochrome oxidase with cytochrome oxidase as a rough estimate of aerobic capacity and cellular "energy turnover", was increased in myocardia of hagfish, turtle and crucian carp. These myocardia are likely to be frequently exposed to oxygen deficiency. In agreement with this, they possess a high relative glycolytic capacity as indicated by a high pyruvate kinase/cytochrome oxidase ratio. The creatine kinase/cytochrome oxidase ratio for the other myocardia varied within a factor of 2, except the value for cod myocardium which was below the others. Total creatine varied among species and was high in active species such as herring, pigeon and rat but also high in crucian carp. The variation in total concentration of phosphorylated adenylates was considerably less than the variation in total creatine. The high creatine kinase/cytochrome oxidase ratio in myocardia likely to be challenged by hypoxia may represent an enhanced efficiency for both "spatial" and "temporal" buffering of phosphorylated adenylates to attenuate the impact of a depressed energy liberation. As to the differences in total creatine, this factor influences not only the cellular energy distribution but possibly also contractility via an effect on the free phosphate level.

Myocardial ischemic contracture. Metabolites affect rigor tension development and stiffness

Myocardial ischemia is characterized by a decrease in phosphocreatine (PCr) and Mg(2+)-ATP contents as well as an accumulation of myosin ATPase reaction products (inorganic phosphate [P(i)], protons, and Mg(2+)-ADP). The possibility that these metabolites play a role in rigor tension development was checked in rat ventricular Triton X-100-skinned fibers. Rigor tension was induced by stepwise decreasing [Mg(2+)-ATP] in the presence or in the absence of 12 mmol/L PCr. To mimic the diastolic ionic environment of the myofibrils, [free Ca2+] was set at 100 nmol/L (pCa 7); [free Mg2+], at 1 mmol/L; and ionic strength, at 160 mmol/L. In control conditions (pH 7.1, with no added P(i) or Mg(2+)-ADP), the pMg(2+)-ATP for half-maximal rigor tension (pMg(2+)-ATP50) was 5.07 +/- 0.03 in the presence of PCr. After withdrawal of PCr, the pMg2+)-ATP50 value was shifted toward higher Mg(2+)-ATP values (3.57 +/- 0.03). Addition of 20 mmol/L P(i) shifted the pMg(2+)-ATP50 to 3.71 +/- 0.04 (P < .05) in the absence of PCr and in the opposite direction to 4.98 +/- 0.02 (P < .01) in the presence of PCr. Acidic pH (6.6) strongly increased pMg(2+)-ATP50 in both the absence (3.90 +/- 0.03, P < .001) and presence (5.44 +/- 0.02, P < .001) of PCr. Conversely, Mg(2+)-ADP (250 mumol/L) decreased pMg(2+)-ATP50 to 3.26 +/- 0.06 (P < .001) in the absence of PCr; at pMg(2+)-ATP 4, no rigor tension was observed until PCr concentration was decreased to < 2 mmol/L. At acidic pH, maximal rigor tension was lower by 29% compared with control conditions, whereas in the presence of Mg(2+)-ADP, maximal rigor tension developed to 143% of the control value; P(i) had no effect. The tension-to-stiffness (measured by the quick length-change technique) ratio was lower in rigor (no PCr and pMg(2+)-ATP 6) than during Ca2+ activation in the presence of both PCr and ATP. Compared with control rigor conditions, this parameter was unchanged by Mg(2+)-ADP and decreased by acidic pH, suggesting a proton-induced decrease in the amount of force per crossbridge. In addition to their known effects on active tension, Mg(2+)-ADP and protons affect rigor tension and influence ischemic contracture development. It is concluded that ischemic contracture and increased myocardial stiffness may be mediated by a decreased PCr and local Mg(2+)-ADP accumulation. This emphasizes the importance of myofibrillar creatine kinase activity in preventing ischemic contracture.

In situ study of myofibrils, mitochondria and bound creatine kinases in experimental cardiomyopathies

Human cardiomyopathy has been extensively studied in the last decade, and knowledge of the functional and structural alterations of the heart has grown. However, understanding of the pathogenesis has come mostly from experimental studies. A number of work have been designed to elucidate if alterations of the contractile apparatus of cardiac cells contribute to the impairment of heart mechanics in cardiomyopathies. As well, an important question is to be solved: whether energy supply of the contraction-relaxation cycle is sufficient in the myopathic heart. Use of cardiac fibers skinned by different techniques allows to evaluate functional ability of myofibrils, mitochondria and bound creatine kinase which plays an important role in cardiomyocyte energy metabolism. The data presented in this chapter show that experimental cardiomyopathies of various types have some common features. These are an increase in calcium sensitivity of myofibrils and a depression of functional activity of mitochondrial creatine kinase. Possible mechanisms and physiological significance of these changes are discussed.

Myofibrillar creatine kinase and cardiac contraction

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

Compartmentation of creatine kinases during perinatal development of mammalian heart

Maturation of the cardiac cell is characterized by increasing diversity of isozymic expression of creatine kinases. Expression of the M-CK isozyme always precedes that of mitochondrial isozyme (mi-CK), however the expression of an isoform does not inform about its localization or cellular function. The functional role of isozymes binding to sites of energy utilization and production characteristic of the adult myocardium can be evidenced by the functional coupling of M-CK to myofibrillar ATPase and mito-CK to translocase in Triton X-100 and saponin skinned fibers. Functional activity of M-CK and mito-CK were investigated during perinatal development. Both functional activities appear during late fetal life in species mature at birth like guinea pig, and in the first postnatal weeks in immature species like rat or rabbit. Thus, the functional activity of bound CK isozymes is not associated with birth per se but with the general process of cell maturation. Localization of CK in the cytosol appears optimal for the transfer of glycolytic production of ATP to sites of utilization in an immature heart. During cell maturation, the increasing contribution of oxidative phosphorylation to ATP production, the apparition and binding of mi-CK to mitochondria, the binding of M-CK to myofibrils, turn the cell in a compartmentalized system of energy production. This provides the cellular basis for energy transfer by the PCr-Cr-CK system between sites of ATP production and utilization. Compartmentation of both Ca handling and energy turnover leads to a highly structured cell organization and could be essential for the efficiency of heart function.

Creatine kinase in non-muscle tissues and cells

Over the past years, a concept for creatine kinase function, the 'PCr-circuit' model, has evolved. Based on this concept, multiple functions for the CK/PCr-system have been proposed, such as an energy buffering function, regulatory functions, as well as an energy transport function, mostly based on studies with muscle. While the temporal energy buffering and metabolic regulatory roles of CK are widely accepted, the spatial buffering or energy transport function, that is, the shuttling of PCr and Cr between sites of energy utilization and energy demand, is still being debated. There is, however, much circumstantial evidence, that supports the latter role of CK including the distinct, isoenzyme-specific subcellular localization of CK isoenzymes, the isolation and characterization of functionally coupled in vitro microcompartments of CK with a variety of cellular ATPases, and the observed functional coupling of mitochondrial oxidative phosphorylation with mitochondrial CK. New insight concerning the functions of the CK/PCr-system has been gained from recent M-CK null-mutant transgenic mice and by the investigation of CK localization and function in certain highly specialized non-muscle tissues and cells, such as electrocytes, retina photoreceptor cells, brain cells, kidney, salt glands, myometrium, placenta, pancreas, thymus, thyroid, intestinal brush-border epithelial cells, endothelial cells, cartilage and bone cells, macrophages, blood platelets, tumor and cancer cells. Studies with electric organ, including in vivo 31P-NMR, clearly reveal the buffer function of the CK/PCr-system in electrocytes and additionally corroborate a direct functional coupling of membrane-bound CK to the Na+/K(+)-ATPase. On the other hand, experiments with live sperm and recent in vivo 31P-NMR measurements on brain provide convincing evidence for the transport function of the CK/PCr-system. We report on new findings concerning the isoenzyme-specific cellular localization and subcellular compartmentation of CK isoenzymes in photoreceptor cells, in glial and neuronal cells of the cerebellum and in spermatozoa. Finally, the regulation of CK expression by hormones is discussed, and new developments concerning a connection of CK with malignancy and cancer are illuminated. Most interesting in this respect is the observed upregulation of CK expression by adenoviral oncogenes.

Bioenergetics. Dissecting the role of creatine kinase

The phenotype of "gene knockout" mice deficient in a creatine kinase isoform sheds new light on the physiological function of the "phosphocreatine circuit."

Contractile properties of skinned preparations from ischaemic canine myocardium and coronary arteries

The influence of prolonged ischaemia on the regulation of contraction in the myocardium and in the smooth muscle of coronary arteries was investigated. Chemically skinned preparations were used which enabled the contraction to be studied with the environment of the contractile filaments controlled. Myocardial ischaemia was produced in anesthetized adult beagle dogs by occlusion of the left anterior descending artery for 3 h and followed by 30 min reperfusion. Myocardial tissue and segments from coronary arteries were obtained from the ischaemic infarcted wall region ("in vivo ischaemic") and compared with control preparations from perfused coronary arteries and from the free wall of the left ventricle. Coronary and myocardial preparations were also obtained from the heart after a 3 h period in vitro under anoxic conditions at 37 degrees C ("in vitro ischaemic") simulating a state of extreme ischaemia. Control myocardial fibres were fully relaxed at pCa (-log-[Ca2+]) 9 and developed 24 +/- 5% (n = 7) of maximum force at intermediate calcium concentration (pCa 5.5). In contrast, the in vivo and in vitro ischaemic preparations produced force at pCa 9 (28 +/- 13 and 39 +/- 8%, respectively, n = 5 and 7) and showed an increased force development at pCa 5.5 (53 +/- 11 and 75 +/- 5%). The in vivo and in vitro ischaemic coronary arteries relaxed more slowly following calcium removal than control vessels. The in vitro ischaemic vascular preparations developed active force at pCa 9 and showed increased levels of myosin light chain phosphorylation and reduced phosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A model system of coupled activity of co-immobilized creatine kinase and myosin

Myosin and creatine kinase were co-immobilized onto Immunodyne films to mimic the behaviour of creatine kinase bound to the M-line of myofilaments. The Mg-ATPase activity of bound myosin was studied by a coupled enzymatic assay, which detects Mg-ADP in the bulk solution by means of pyruvate kinase and lactate dehydrogenase. The competition for Mg-ADP between pyruvate kinase and creatine kinase either free in solution or co-immobilized with myosin was studied at various creatine phosphate concentrations. Bound creatine kinase competed efficiently when present in very low amounts, corresponding to an activity ratio higher than 1:20,000 between creatine kinase and pyruvate kinase and a molar ratio higher than 1:1000 between creatine kinase and myosin. The Mg-ADP produced by myosin ATPase in the vicinity of the film did not diffuse into the bulk solution but, in the presence of creatine phosphate, was recycled into Mg-ATP by the neighbouring creatine kinase. The existence of an unstirred layer near the surface of the film is sufficient to explain the channeling of ADP (or ATP) between co-immobilized myosin and creatine kinase, without direct interaction or 'intimate coupling' between the enzymes. The problem now is to determine the importance of this kind of facilitated diffusion in the myofilaments in vivo.

In situ compartmentation of creatine kinase in intact sarcomeric muscle: the acto-myosin overlap zone as a molecular sieve

Creatine kinase isoenzymes (CK = ATP: creatine N-phosphoryl transferase, EC 2.7.3.2) were localized in situ in cryosections of intact sarcomeric muscle by immunocytochemical staining. Similar to cardiac muscle, spermatozoa and photoreceptor cells, mitochondrial-type CK (Mi-CK) localization in skeletal muscle was also restricted to mitochondria. Besides the well-documented localization of muscle-type (M-CK) at the M-line and at the sarcoplasmic reticulum, surprisingly, most of the sarcoplasmic M-CK was also highly compartmentalized and was mainly confined to the I-band. The localization of M-CK at the I-band coincided with that of adenylate kinase and aldolase. In intact muscle, the diffusion equilibrium decisively favours occupancy by all three enzymes of the I-band, with the acto-myosin overlap region of the A-band acting as a molecular sieve, excluding to a large extent all three enzymes from the acto-myosin overlap region. This indicates that in intact muscle, this region of the A-band may be less accessible in vivo to soluble, sarcoplasmic enzymes than thought before. If muscle were permeabilized by chemical skinning before fixation, I-band CK, as well as aldolase and adenylate kinase, were solubilized and disappeared from the myofibrils, but the fraction of M-CK which was specifically associated with the M-line remained bound to the myofibrils. Implications of these findings are discussed with respect to the functional coupling of I-band-CK with glycolysis, to the formation of large multienzyme complexes of glycolytic enzymes with CK and to the supply of energy for muscle contraction in general.

Protein kinase C enhances myosin light-chain kinase effects on force development and ATPase activity in rat single skinned cardiac cells

Many neurohormones alter the force of cardiac contraction by variations in the intracellular Ca2+ concentration. alpha 1-Adrenergic and muscarinic stimulations, rather, modify the sensitivity of contractile proteins to Ca(2+)-calmodulin-myosin light-chain kinase (MLCK) complex induces a large increase in Ca2+ sensitivity (0.14 pCa unit) of these easily accessible myofilaments. This increase is further enhanced by up to 0.19 pCa unit when protein kinase C (PKC) is added together with MLCK. Similarly, the Ca2+ ATPase activity of skinned cells in suspension is increased in the presence of MLCK and further in the presence of both kinases. 32P-labelling and SDS/PAGE show that these changes are associated with light-chain 2 (LC2) phosphorylation together with phosphorylation of troponin I and troponin T when PKC is added. Although to a smaller extent than in smooth muscle, phosphorylation of cardiac myosin LC2 may be involved in the modulation of heart contractility.

Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis

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Functional development of the creatine kinase system in perinatal rabbit heart

The functional development of the creatine kinase system has been studied in rabbit heart during perinatal growth. Fiber bundles were obtained from left ventricles of fetal rabbits at the 30th day of gestation, newborn rabbits aged 1, 3, 8, and 17 days, and adult rabbits. Total creatine kinase activity was constant during perinatal development, whereas myofibrillar bound creatine kinase activity increased 15-fold during the first postnatal week. Functional activity of myofibrillar creatine kinase was assayed in Triton X-100-skinned fibers by its ability to induce active tension in the absence of ATP or to relax rigor tension. It was very low in 1-day-old newborns and increased during the first 2 weeks to reach adult levels 17 days after birth. Functional activity of mitochondrial creatine kinase was determined in saponin-skinned fibers. Creatine-stimulated respiration appeared only after birth and increased gradually between 1 and 17 days after birth. The results show that, although the two creatine kinase isoforms (mitochondrial and myofibrillar) are expressed at different stages during development, their functional activities appear in parallel in mitochondria and myofibrils. Early postnatal development is characterized by binding of creatine kinase isoenzymes to intracellular organelles. Such compartmentation participates in the postnatal cardiac cellular maturation.

In vivo regulation of mitochondrial respiration in cardiomyocytes: specific restrictions for intracellular diffusion of ADP

Relative diffusivities of ADP and creatine in cardiomyocytes were studied. The isolated rat cardiomyocytes were lysed with saponin (40 micrograms/ml) to perforate or completely disrupt sarcolemma that was evidenced by leakage of 80-100% lactate dehydrogenase. In these cardiomyocytes mitochondria were used as 'enzymatic probes' to determine the average local concentration of substrates exerting acceptor control of respiration--ADP or creatine (the latter activates respiration via mitochondrial creatine kinase reaction)--when their concentrations in the surrounding medium were changed. The kinetic parameters for ADP and creatine in control of respiration of saponin-treated cardiomyocytes were compared with those determined in isolated mitochondria and skinned cardiac fibers. The apparent Km for creatine (at 0.2 mM ATP) was very close and in a range of 6.0-6.9 mM in all systems studied, showing the absence of diffusion difficulties for this substrate. On the contrary, the apparent Km for ADP increased from 18 +/- 1 microM for isolated mitochondria to 250 +/- 59 microM for cardiomyocytes with the lysed sarcolemma and to 264 +/- 57 microM for skinned fibers. This elevation of Km was not eliminated by inhibition of myokinase with diadenosine pentaphosphate. When 25 mM creatine was present, the apparent Km for ADP decreased to 36 +/- 6 microM. These data are taken to indicate specific restrictions of diffusion of ADP most probably due to its interaction with intermediate binding sites in cardiomyocytes. The important role of phosphocreatine-creatine kinase system of energy transport is to overcome the restrictions in regulation of energy fluxes due to decreased diffusivity of ADP.