Reversible MM-creatine kinase binding to cardiac myofibrils

Numb and Numblike regulate sarcomere assembly and maintenance

A sarcomere is the contractile unit of the myofibril in striated muscles such as cardiac and skeletal muscles. The assembly of sarcomeres depends on multiple molecules that serve as raw materials and participate in the assembly process. However, the mechanism of this critical assembly process remains largely unknown. Here, we found that the cell fate determinant Numb and its homolog Numblike regulated sarcomere assembly and maintenance in striated muscles. We discovered that Numb and Numblike are sarcomeric molecules that were gradually confined to the Z-disc during striated muscle development. Conditional knockout of Numb and Numblike severely compromised sarcomere assembly and its integrity and thus caused organelle dysfunction. Notably, we identified that Numb and Numblike served as sarcomeric α-Actin-binding proteins (ABPs) and shared a conserved domain that can bind to the barbed end of sarcomeric α-Actin. In vitro fluorometric α-Actin polymerization assay showed that Numb and Numblike also played a role in the sarcomeric α-Actin polymerization process. Last, we demonstrate that Numb and Numblike regulate sarcomeric α-Actinin-dependent (ACTN-dependent) Z-disc consolidation in the sarcomere assembly and maintenance. In summary, our studies show that Numb and its homolog Numblike regulate sarcomere assembly and maintenance in striated muscles, and demonstrate a molecular mechanism by which Numb/Numblike, sarcomeric α-Actin, and ACTN cooperate to control thin filament formation and Z-disc consolidation.

Creatine, Creatine Kinase, and Aging

With an ever aging population, identifying interventions that can alleviate age-related functional declines has become increasingly important. Dietary supplements have taken center stage based on various health claims and have become a multi-million dollar business. One such supplement is creatine, a major contributor to normal cellular physiology. Creatine, an energy source that can be endogenously synthesized or obtained through diet and supplement, is involved primarily in cellular metabolism via ATP replenishment. The goal of this chapter is to summarize how creatine and its associated enzyme, creatine kinase, act under normal physiological conditions, and how altered levels of either may lead to detrimental functional outcomes. Furthermore, we will focus on the effect of aging on the creatine system and how supplementation may affect the aging process and perhaps reverse it.

The decrease of NAD(P)H:quinone oxidoreductase 1 activity and increase of ROS production by NADPH oxidases are early biomarkers in doxorubicin cardiotoxicity

CONTEXT

Doxorubicin cardiotoxicity displays a complex and multifactorial progression.

OBJECTIVE

Identify early biochemical mechanisms leading to a sustained imbalance of cellular bioenergetics.

METHODS

Measurements of the temporal evolution of selected biochemical markers after treatment of rats with doxorubicin (20 mg/kg body weight).

RESULTS

Doxorubicin treatment increased lipid oxidation, catalase activity and production of H₂O₂ by Nox-NADPH oxidases, and down-regulated

NAD(P)H

quinone oxidoreductase-1 prior eliciting changes in reduced glutathione, protein carbonyls and protein nitrotyrosines. Alterations of mitochondrial and myofibrillar bioenergetics biomarkers were detected only after this oxidative imbalance was established.

CONCLUSIONS

NAD(P)H

quinone oxidoreductase-1 activity and increase of hydrogen peroxide production by NADPH oxidases are early biomarkers in doxorubicin cardiotoxicity.

Novel strategy for measuring creatine kinase reaction rate in the in vivo heart

In the heart, the creatine kinase (CK) system plays an important role in the cascade of ATP production, transportation, and utilization. The forward pseudo-first-order rate constant for the CK reaction can be measured noninvasively by the (31)P-magnetic resonance (MR) spectroscopy magnetization saturation transfer (MST) techniques. However, the measurement of MST in the in vivo heart is limited by the lengthy data acquisition time, especially for studies requiring spatial localization. This technical report presents a new method for measuring ATP production rate via CK that can reduce the MST data acquisition time by 82%. This method is validated using an in vivo pig model to evaluate the forward pseudo-first-order rate constant of myocardial CK reaction noninvasively.

Creatine kinase structural changes induced by substrates

Myofibrillar creatine kinase (CK) buffers the cellular ATP concentration during fluctuating ATP turnover in a muscle. In order to detect structural changes of the CK molecule due to bound substrates, the dynamics of free, ATP-bound, and ATP+creatine-bound CK were examined, using steady-state and time-resolved fluorescence spectroscopy. The intrinsic tryptophan fluorescence of non-labelled CK presented the smaller fluorescence lifetime 2.38 ns and rotation correlation time 27 ns for the CK-ATP (in comparison with the times 2.72 ns and 35 ns for the free CK), and their moderate return to the longer times 2.42 ns and 29 ns for the CK-ATP+creatine complex. Three conformations for the non-labelled CK were indicated also by different quenching of fluorescence by acrylamide. Data were confirmed by anisotropy experiments with CK-(FITC labelled), providing the same substrate dependence of the rotation times (34 ns, 27 ns and returning 30 ns). The results indicate the existence of three conformations arranged according to the "energy minimizing principle" by ligated substrates. In this way the data implicate another essential component of physiological control at the subcellular level in the transition of the nonreactive CK-ATP+creatine complex to the reactive enzyme molecule.

Creatine kinase binds more firmly to the M-band of rabbit skeletal muscle myofibrils in the presence of its substrates

Creatine kinase (CK) (E.C. 2.7.3.2) buffers cellular ATP concentration during fluctuating ATP turnover. Muscle cytosolic CK isoform interacts with various subcellular structures where it is functionally coupled with relevant ATPases. However, how this interaction affects its activity is not known. We have therefore studied the interaction of CK with myofibrils and the role of different conformational states of CK molecule induced by ATP, phosphocreatine, ADP and the ATP-creatine pair. Purified rabbit psoas myofibrils with CK specific activity of 0.4+/-0.02 IU/mg were used. The exchange rates between the myofibrillar M-band and its surroundings were measured with fluorofore conjugated CK (IAF) by the Fluorescence Lost in Photobleaching (FLIP) method within a very narrow pH range 7.1-7.15. For CK-IAF without docked substrates, the time derivative of the initial loss of the fluorescent signal within the M-band equalled -3.26 at the fifth second and the decrease reached 82% by the 67th second. For CK-IAF with added substrates, the derivatives fell into the range of -0.95 to -1.30, with respective decreases from 16 to 46% at the 67th second. The results show that the substrates slowed down the exchange rate. This indicates that the strength of the bond between CK and the M-band of myofibrils increased.

Dependence of Ca2+-induced contraction on ATP in alpha-toxin-permeabilized preparations of rat femoral artery

Effects of various concentrations of ATP on Ca(2+)-induced contraction were studied in alpha-toxin-permeabilized preparations obtained from the rat femoral artery. The contractile magnitude was highest in the presence of 1 mM ATP and decreased with both increasing and decreasing the concentration, suggesting the presence of an optimum ATP concentration in inducing contraction. The magnitude of the contractions in various concentrations of ATP correlated with the extent of the phosphorylated myosin light chain (MLC). The rate of contractions in the presence of 1 mM ATP under an inhibition of MLC phosphatase was faster than in the presence of 4 mM ATP, suggesting that the increased phosphorylation of MLC at 1 mM ATP results from an increased activity of MLC kinase. On the other hand, MLC phosphatase activity appeared unchanged, because the rates of relaxations under the inhibition of MLC kinase were not different in the presence of either 1 or 4 mM ATP. The high sensitivity to 1 mM ATP was absent in the preparations that were permeabilized with beta-escin or Triton X-100, suggesting the existence of an intracellular factor required for the increased activity of MLC kinase to ATP in the alpha-toxin-permeabilized preparations of the rat femoral artery.

Molecular biology of myocardial recovery

The use of LVADs that leads to a dramatic mechanical and hemodynamic unloading of the failing left ventricle offers a unique opportunity to investigate the mechanisms of remodeling and reverse remodeling. Although it is being increasingly realized that the LVAD unloading results in regression of hypertrophy and improvement of myocyte function and LV geometry, the cellular and molecular mechanisms responsible for these beneficial effects remain undefined. The favorable alterations in geometry that occur in parallel fashion at the organ, cellular, and molecular levels are most likely caused by the reduced LV wall stress/stretch as a consequence of the mechanical support provided by LVAD. If it is confirmed that LVAD unloading can contribute significantly to reverse remodeling, the role of LVADs may graduate from bridge-to-transplantation or destination therapy to bridge-to-recovery.

Effects of peroxynitrite on isolated cardiac trabeculae: selective impact on myofibrillar energetic controllers

Formation of peroxynitrite and cardiac protein nitration have been implicated in multiple cardiac disease states, but their contributions to disease initiation remain undefined. We have previously observed nitration of myofibrillar regions of cardiac myocytes in several experimental and clinical settings of cardiac myocyte dysfunction and postulated that oxidative insult to key components of the contractile apparatus may be initiating events. Here we tested the hypothesis that peroxynitrite alters myofibrillar contractile function, and investigated a mechanistic role for nitration in this process. Isolated rat ventricular trabeculae were exposed to physiologically relevant concentrations of peroxynitrite and ATP-dependent contractile responses were measured. Maximal trabecular force generation was significantly impaired following 300 nM peroxynitrite exposures. Several myofibrillar proteins demonstrated increased tyrosine nitration, the most significant increases occurred in the myosin heavy chain and the myofibrillar isoform of creatine kinase. Additional functional experiments were conducted using phosphocreatine (high energy phosphate substrate for myofibrillar creatine kinase) as the primary energy substrate. Myofibrillar creatine kinase-dependent force generation was impaired at peroxynitrite concentrations as low as 50 nM, suggesting potent inactivation of the enzyme. Extent of tyrosine nitration of myofibrillar creatine kinase was negatively correlated to myofibrillar creatine kinase-dependent force generation. These data demonstrate that the cardiac contractile apparatus is highly sensitive to peroxynitrite, and that MM-CK may be a uniquely vulnerable target.

Myocardial creatine kinase expression after left ventricular assist device support

OBJECTIVES

We examined whether unloading of the left ventricle with a ventricular assist device (LVAD) can result in normalization of the creatine kinase (CK) abnormalities in the failing human heart.

BACKGROUND

Left ventricular failure is associated with a decrease of myocardial total CK activity and a fetal shift in CK isoform expression that results in an increase in the cytosolic brain type homodimeric-creatine kinase (CK-B) subunit and decreases of the cytosolic muscle-creatine kinase (CK-M) and CK-mitochondrial (CK-Mt) isoforms. The mechanisms of this abnormality are not known.

METHODS

Total CK activity and CK protein isoform expression (Western blotting) were examined in 11 patients with end-stage cardiomyopathy. In 7 patients, myocardial tissue was also obtained after 4.1 +/- 1.1 months of left ventricular assist device (LVAD) support.

RESULTS

Left ventricular unloading produced by LVAD implantation resulted in a 270% +/- 114% increase in total CK activity (p < 0.01) that was associated with a 69% +/- 18% increase in CK-M protein expression (p < 0.01) and a 121% +/- 69% increase in CK-Mt protein expression (p < 0.01), but no significant change in CK-B expression.

CONCLUSIONS

Systolic and diastolic unloading provided by the LVAD resulted in increases of total CK activity as well as CK-Mt and CK-M protein expression. The failure of CK-B expression to decrease suggests that abnormalities other than increased loading are responsible for the increase in CK-B expression in the failing heart.

Intracellular distribution of peroxynitrite during doxorubicin cardiomyopathy: evidence for selective impairment of myofibrillar creatine kinase

Cardiac peroxynitrite and protein nitration are increased during doxorubicin cardiotoxicity, but the intracellular targets and functional consequences have not been defined. We investigated the intracellular distribution of protein nitration during doxorubicin cardiotoxicity in mice. Following in vivo cardiac function assessments by echocardiography, cardiac tissues were prepared for immunohistochemistry and electron microscopy 5 days after doxorubicin (20 mg kg(-1)) or vehicle control. Increased cardiac 3-nitrotyrosine was observed using light microscopy in doxorubicin treated animals. Immunogold electron microscopy (55,000x) revealed increased myofibrillar and mitochondrial 3-nitrotyrosine levels following doxorubicin, but cellular 3-nitrotyrosine density was 2 fold higher in myofibrils. We therefore investigated the actions of peroxynitrite on intact cardiac contractile apparatus. Skinned ventricular trabeculae were exposed to physiologically relevant peroxynitrite concentrations (50 or 300 nM) for 1 h, then Ca(2+) induced contractile responses were measured in the presence of ATP (4 mM) or phosphocreatine (12 mM) as high energy phosphate supplier. ATP maximal force generation was unaltered after 50 nM peroxynitrite, but phosphocreatine/ATP response was reduced (0.99+/-0.63 vs 1.59+/-0.11), suggesting selective inactivation of myofibrillar creatine kinase (MM-CK). Reduction of ATP maximal force was observed at 300 nM peroxynitrite and phosphocreatine/ATP response was further reduced (0.64+/-0.30). Western blotting showed concentration dependent nitration of MM-CK in treated trabeculae. Similarly, cardiac tissues from doxorubicin treated mice demonstrated increased nitration and inactivation of MM-CK compared to controls. These results demonstrate that peroxynitrite-related protein nitration are mechanistic events in doxorubicin cardiomyopathy and that the cardiac myofibril is an important oxidative target in this setting. Furthermore, MM-CK may be a uniquely vulnerable target to peroxynitrite in vivo.

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

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

Myocardial creatine kinase kinetics and isoform expression in hearts with severe LV hypertrophy

Left ventricular (LV) hypertrophy (LVH) results in a fetal shift in myocardial creatine kinase (CK) expression. Because CK plays an important role in intracellular energy production, transport, and utilization, this study was performed to characterize changes in CK expression and CK flux in severe pressure-overload LVH. Ascending aortic banding in 8-wk-old dogs resulted in LVH with a 92% increase in relative LV mass. In LVH hearts, CK-M isoform mRNA was decreased by 40% (P = 0.05) and protein was decreased by 50% (P < 0.01), whereas mitochondrial CK protein was decreased by 22% (P < 0.05). CK-B isoform mRNA was undetectable in normal hearts but was prominently expressed in LVH (P < 0.01); CK-B protein was increased by more than 10-fold in LVH (P < 0.01). Despite these changes, total CK activity was normal in LVH. Myocardial CK flux was examined using (31)P magnetic resonance spectroscopy magnetization transfer. The CK forward rate constant was similar in normal and LVH hearts at baseline and did not change in either group during dobutamine treatment. In hearts with LVH, the CK forward flux rate was reduced by approximately 60% (P < 0.05) and decreased further during dobutamine. Thus, although pressure-overload LVH caused alterations of expression of both CK mRNA and protein levels, LV performance and oxygen consumption in response to dobutamine were normal. However, myocardial free ADP was increased in LVH hearts. This finding suggests that the CK alterations result in a need for higher ADP levels to maintain ATP synthesis in the hypertrophied heart.

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.

Length-dependence of cross-bridge mediated activation of the cardiac thin filament

Length-dependent Ca(2+)activation of the thin filament plays a critical role in the steep force-length relationship of cardiac muscle (Frank-Starling relation). Recent evidence indicates that the increase in myofilament Ca(2+)sensitivity and Ca(2+)-troponin C affinity that occurs with increase in sarcomere length results from a cooperative activation of the thin filament by attached cross-bridges. At short sarcomere length the Ca(2+)sensitivity is lower because the access of cross-bridges for actin is reduced. The aim of this study was to determine the length-dependence of myosin-mediated thin filament activation in skinned bovine ventricular muscle, as assayed by the generation of force with progressive reduction of MgATP concentration in the absence of Ca(2+). If the interaction between myosin and actin is weaker at short sarcomere length there should be a lower MgATP concentration needed to maintain the relaxed state. Contrary to expectation, the force-pMgATP relationship was not significantly influenced by change in sarcomere length. However this relationship became length-sensitive in the presence of phosphate analogs which stabilize weak-binding cross-bridges. We suggest that sarcomere length modulates Ca(2+)sensitivity by controlling the size of the population of thin filament regulatory units in the weakly-bound state.

Brain tachykinins and the regulation of salt intake

The regulation of salt intake is achieved through the coordination of behavioral and physiological responses. Brain neuropeptides, such as the tachykinins, play an important role in orchestrating both of these responses. Intraventricular injections of NK3 receptor agonists, such as senktide, are potent in suppressing salt intake. Experimental results show that intraventricular injections of senktide that suppress salt intake have no effect on the ingestion of other tastes, such as sucrose. The means by which senktide suppresses salt intake was investigated in a series of experiments. Taste reactivity and lick rate analyses suggest that the activation of NK3 receptors reduces salt intake by modulating the oral-stimulating property of salt taste.

Myocardial creatine kinase kinetics in hearts with postinfarction left ventricular remodeling

This study examined whether alterations in myocardial creatine kinase (CK) kinetics and high-energy phosphate (HEP) levels occur in postinfarction left ventricular remodeling (LVR). Myocardial HEP and CK kinetics were examined in 19 pigs 6 wk after myocardial infarction was produced by left circumflex coronary artery ligation, and the results were compared with those from 9 normal pigs. Blood flow (microspheres), oxygen consumption (MVO2), HEP levels [31P magnetic resonance spectroscopy (MRS)], and CK kinetics (31P MRS) were measured in myocardium remote from the infarct under basal conditions and during dobutamine infusion (20 micrograms. kg-1. min-1 iv). Six of the pigs with LVR had overt congestive heart failure (CHF) at the time of study. Under basal conditions, creatine phosphate (CrP)-to-ATP ratios were lower in all transmural layers of hearts with CHF and in the subendocardium of LVR hearts than in normal hearts (P < 0.05). Myocardial ATP (biopsy) was significantly decreased in hearts with CHF. The CK forward rate constant was lower (P < 0.05) in the CHF group (0.21 +/- 0.03 s-1) than in LVR (0.38 +/- 0.04 s-1) or normal groups (0.41 +/- 0.03 s-1); CK forward flux rates in CHF, LVR, and normal groups were 6.4 +/- 2.3, 14.3 +/- 2.1, and 20.3 +/- 2.4 micromol. g-1. s-1, respectively (P < 0.05, CHF vs. LVR and LVR vs. normal). Dobutamine caused doubling of the rate-pressure product in the LVR and normal groups, whereas CHF hearts failed to respond to dobutamine. CK flux rates did not change during dobutamine in any group. The ratios of CK flux to ATP synthesis (from MVO2) under baseline conditions were 10.9 +/- 1.2, 8. 03 +/- 0.9, and 3.86 +/- 0.5 for normal, LVR, and CHF hearts, respectively (each P < 0.05); during dobutamine, this ratio decreased to 3.73 +/- 0.5, 2.58 +/- 0.4, and 2.78 +/- 0.5, respectively (P = not significant among groups). These data demonstrate that CK flux rates are decreased in hearts with postinfarction LVR, but this change does not limit the response to dobutamine. In hearts with end-stage CHF, the changes in HEP and CK flux are more marked. These changes could contribute to the decreased responsiveness of these hearts to dobutamine.

Compartmentalized energy transfer in cardiomyocytes: use of mathematical modeling for analysis of in vivo regulation of respiration

The mathematical model of the compartmentalized energy transfer system in cardiac myocytes presented includes mitochondrial synthesis of ATP by ATP synthase, phosphocreatine production in the coupled mitochondrial creatine kinase reaction, the myofibrillar and cytoplasmic creatine kinase reactions, ATP utilization by actomyosin ATPase during the contraction cycle, and diffusional exchange of metabolites between different compartments. The model was used to calculate the changes in metabolite profiles during the cardiac cycle, metabolite and energy fluxes in different cellular compartments at high workload (corresponding to the rate of oxygen consumption of 46 mu atoms of O.(g wet mass)-1.min-1) under varying conditions of restricted ADP diffusion across mitochondrial outer membrane and creatine kinase isoenzyme "switchoff." In the complete system, restricted diffusion of ADP across the outer mitochondrial membrane stabilizes phosphocreatine production in cardiac mitochondria and increases the role of the phosphocreatine shuttle in energy transport and respiration regulation. Selective inhibition of myoplasmic or mitochondrial creatine kinase (modeling the experiments with transgenic animals) results in "takeover" of their function by another, active creatine kinase isoenzyme. This mathematical modeling also shows that assumption of the creatine kinase equilibrium in the cell may only be a very rough approximation to the reality at increased workload. The mathematical model developed can be used as a basis for further quantitative analyses of energy fluxes in the cell and their regulation, particularly by adding modules for adenylate kinase, the glycolytic system, and other reactions of energy metabolism of the cell.

Use of gene targeting for compromising energy homeostasis in neuro-muscular tissues: the role of sarcomeric mitochondrial creatine kinase

We have introduced a single knock-out mutation in the mitochondrial creatine kinase gene (ScCKmit) in the mouse germ line via targeted mutagenesis in mouse embryonic stem (ES) cells. Surprisingly, ScCKmit -/- muscles, unlike muscles of mice with a deficiency of cytosolic M-type creatine kinase (M-CK -/-; Van Deursen et al. (1993) Cell 74, 621-631), display no altered morphology, performance or oxidative phosphorylation capacity. Also, the levels of high energy phosphate metabolites were essentially unaltered in ScCKmit mutants. Our results challenge some of the present concepts about the strict coupling between CKmit function and aerobic respiration.

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.

Coupling between myosin ATPase cycle and creatinine kinase cycle facilitates cardiac actomyosin sliding in vitro. A clue to mechanical dysfunction during myocardial ischemia

BACKGROUND

There is much evidence to support the favorable effects of the phosphocreatine shuttle on myocardial contraction and relaxation. However, experiments in which cardiac muscle fiber or myofibril was used have not elucidated its precise mechanism.

METHODS AND RESULTS

Active movements of fluorescently labeled actin filaments on a cardiac myosin layer coimmobilized with creatinine kinase (CK) onto a nitrocellulose-coated glass coverslip were studied under various concentrations of adenine nucleotides. At a constant phosphocreatine concentration (5 mmol/L, pH 7.1), the relation of sliding velocity to MgATP concentration followed Michaelis-Menten kinetics. The apparent Km was significantly smaller in the presence of CK (0.041 +/- 0.001 mmol/L) than in the absence of CK (0.080 +/- 0.001 mmol/L), indicating that coattached CK facilitated the propelling of actin filaments by the myosin ATPase. This phenomenon was also seen under acidic conditions (pH 6.7) as well as in the presence of inorganic phosphate (10 mmol/L. At a constant MgATP concentration (1 mmol/L), the inhibitory effect of MgADP on the actin-myosin interaction was weaker in the presence of CK than in the absence of CK. Another ATP-regenerating system, pyruvate kinase and phospho(enol)pyruvate, while maintaining a low ratio of [MgADP] to [MgATP], did not reduce the Km value (0.156 +/- 0.001 mmol/L), suggesting that the effect of coattached CK was not achieved only by prevention of MgADP accumulation.

CONCLUSIONS

Coupling between the ATPase cycle and the CK cycle may serve not only to maintain the ATP concentration within the myofibril but also to provide optimal conditions for cardiac actomyosin interaction. Consideration of this coupling will offer a clue to elucidating the systolic or diastolic dysfunction during myocardial ischemia or reperfusion.

Factors limiting adenosine triphosphatase function during high intensity exercise. Thermodynamic and regulatory considerations

It is widely accepted that a structural organisation favouring interaction between functionally-related enzymes is required for the economy and efficiency of metabolic reactions. Many functionally-related enzymes have been shown to be reversibly bound to cellular structures and to other enzymes at the sites where they are required. Resulting from this binding, close structural proximity and concentration of enzymes, a microenvironment is generated where the product of one enzyme is the substrate of the other. This reduces the diffusion distance for the substrate, saturates binding sites with maximal speed and, as a final outcome, increases the efficiency and economy of function behind these metabolic reactions. Available data indicate that the above-described association between adenosine triphosphatase (ATPase) and enzymes regenerating ATP has an important role in the regulation of ATPase function. A general consensus exists among published studies that the concentration of ATP ([ATP]) is not significantly decreased in fatigued muscle, even in those with severely diminished power output. However, in studies with isolated perfused hearts it has been possible to significantly reduce [ATP] in muscle cells without compromising mechanical activity. An explanation for this discrepancy is connected with local ATP regeneration in the vicinity of ATPase. Furthermore, when ATP regeneration is unable to balance ATP consumption a critical drop in the free energy of ATP hydrolysis is avoided by down-regulation of ATP consumption. The main function of local ATP regeneration is to maintain a low concentration of adenosine diphosphate ([ADP]), and the ADP/ATP ratio in the vicinity of the ATP-binding site of ATPase that is a prerequisite for high thermodynamic efficiency of ATP hydrolysis. Close proximity of creatine kinase and glycolytic enzymes to ATPase and high-affinity binding of substrates generate an ATPase microenvironment, where ADP and ATP are not in free equilibrium with those adenine nucleotides in the surrounding medium. In the physiological range of operation for important cellular ATPases (free energy change of 55 to 60 kJ/mol ATP) only a small fraction of energy, available in ATP, can be utilised, provided that no ATP regeneration takes place. However, ATP regeneration allows utilisation of most of the regenerating capacity, before ATP hydrolysis drops below the critical 55 kJ/mol. The importance of local ATP regeneration increases in parallel with an increase in the rate of ATPase turnover.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Creatine kinase activity associated with the contractile proteins of the guinea-pig carotid artery

Activity and role of creatine kinase associated with contractile proteins of vascular smooth muscle have been investigated using skinned guinea-pig carotid artery rings. Membrane solubilization was performed with the detergent Triton X-100. Creatine kinase activity, isoenzyme profile as well as mechanics were performed on the Triton skinned carotid artery rings. Total creatine kinase activity was 47.3 +/- 9.3 IU g-1 ww and electrophoresis showed BB, MB, and MM isoforms (BB-CK being the predominant isoenzyme). One hour incubation with Triton X-100, produced predominantly BB-CK remaining with the myofibrils with some MB, representing 23% of the preskinned creatine kinase activity. When relaxed carotid artery rings were exposed to pCa 9 in the presence of 250 microM ADP, 0 ATP, and 12 mM phosphocreatine, tension was not significantly different from resting tension, but changing to pCa 4.5 caused the carotid artery rings to generate 49.5 +/- 4.5% of maximal tension. When a high-tension rigor state was achieved (250 microM ADP, 0 ATP, 0 phosphocreatine, and pCa 9), the addition of 12 mM phosphocreatine effected significant relaxation. These observations implicate an endogenous form of creatine kinase, associated with the myofilaments, which is capable of producing enough ATP for submaximal tension generation and significant relaxation from rigor conditions. These results suggest co-localization of ATPase, MLCK, and creatine kinase on the contractile proteins of the carotid artery. Such an enzymic association may play a role in the energetic supply to the contractile apparatus of vascular smooth muscle.

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.

The creatine kinase system in smooth muscle

Despite the energetic flux being much lower in smooth muscle compared to striated muscles (such as the heart and skeletal muscle) creatine kinase (CK) has been found present and active in all smooth muscles studied to date. A complete CK circuit has been identified, with CK found in the mitochondria, contractile elements, membrane pumps and the cytoplasm. CK isoenzymes are coupled to many cellular energetic processes and appears to be involved in energy production and consumption by acting as an energy transducer. The CK system responds to pathological insults and development (e.g., hypertrophy and gestation respectively) by changes in sub-cellular distribution localization, isoenzymes, and specific activity. The conclusion from these observations is that creatine kinase is intimately involved in the energetic system of smooth muscle.

Approaching the multifaceted nature of energy metabolism: inactivation of the cytosolic creatine kinases via homologous recombination in mouse embryonic stem cells

To study the physiological role of the creatine kinase/phosphocreatine (CK/PCr) system in cells and tissues with a high and fluctuating energy demand we have concentrated on the site-directed inactivation of the B- and M-CK genes encoding the cytosolic CK protein subunits. In our approach we used homologous recombination in mouse embryonic stem (ES) cells from strain 129/Sv. Using targeting constructs based on strain 129/Sv isogenic DNA we managed to ablate the essential exons of the B-CK and M-CK genes at reasonably high frequencies. ES clones with fully disrupted B-CK and two types of M-CK gene mutations, a null (M-CK-) and leaky (M-CK1) mutation, were used to generate chimaeric mutant mice via injection in strain C57BL/6 derived blastocysts. Chimaeras with the B-CK null mutation have no overt abnormalities but failed to transmit the mutation to their offspring. For the M-CK- and M-CK1 mutations successful transmission was achieved and heterozygous and homozygous mutant mice were bred. Animals deficient in MM-CK are phenotypically normal but lack muscular burst activity. Fluxes through the CK reaction in skeletal muscle are highly impaired and fast fibres show adaptation in cellular architecture and storage of glycogen. Mice homozygous for the leaky M-CK allele, which have 3-fold reduced MM-CK activity, show normal fast fibres but CK fluxes and burst activity are still not restored to wildtype levels.

Myofibrillar creatine kinase and cardiac contraction

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

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

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

Creatine 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.

Force responses to rapid length changes in single intact cells from frog heart

1. Force transients in response to step perturbations in length were recorded in intact atrial cells from frog heart at various temperatures (6-15 degrees C). Length changes of various sizes and in either direction, complete in 0.5 ms, were applied to single myocytes near slack length (initial sarcomere length 2.1-2.2 microns) just before the peak of an isometric twitch. The frequency response of the force transducers used was 2-4 kHz in Ringer solution. 2. An early quick force recovery phase was clearly observed after the elastic force response to the length step and before the start of much slower recovery processes. The quick recovery phase became progressively faster with larger shortening steps and was almost as fast as that originally described in intact frog skeletal muscle fibres (rate constants above 1000 s-1 in large releases at 10 degrees C). 3. The force-extension relation determined at the end of the length change (T1 curve) indicates that an instantaneous shortening of 0.5-0.6% of the initial cell length (L0) brings the force to zero. The force--extension relation determined at the end of the quick recovery phase (T2 curve) showed that the early recovery leads to an almost complete restoration of the original force with small stretches and releases (up to 0.3% L0) and that it becomes negligible in shortening steps of about 1.4% L0. 4. The results suggest that the mechanical properties of attached cross-bridges and the rate of transitions between attached cross-bridge states are approximately the same in frog atrial cells and fast skeletal muscle fibres.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in creatine kinase M localization in acute ischemic myocardial cells. Immunoelectron microscopic studies

To clarify the changes in creatine kinase M localization along with the progress of myocardial ischemia, immunoelectron microscopic studies were performed using rabbit anti-canine creatine kinase M Fab'-horseradish peroxidase conjugate in 21 dogs. Myocardial ischemia was induced by occlusion of the left anterior descending coronary artery for 15 (n = 5), 30 (n = 5), 60 (n = 5), or 180 (n = 4) minutes. Two dogs were used as normal controls. As we have already demonstrated, most creatine kinase M in normal myocardial cells was localized over the entire A band in association with the thick filament, suggesting that creatine kinase in this region (A-band creatine kinase) was the enzyme coupled with myosin ATPase. After 15 minutes of ischemia, creatine kinase M showed only minimal changes in its location, indicating that A-band creatine kinase still has the ability to couple with myosin ATPase (reversible injury). However, after 30 minutes of ischemia, A-band creatine kinase diffused markedly to the I band (transitional phase), and after 60 minutes of ischemia, it leaked out to extracellular spaces (irreversible injury). After 180 minutes of ischemia, most A-band creatine kinase disappeared from the myocardial cells (coagulation necrosis). These features of creatine kinase M localization seemed to reflect each stage of ischemic cell injury. We conclude that myocardial ischemia results in a dissociation of creatine kinase molecules from the thick filament, which leads the energy transport system to destruction.

Functional characterization of mitochondrial oxidative phosphorylation in saponin-skinned human muscle fibers

The conditions of treatment of human skeletal muscle fibers from M. vastus lateralis with saponin were optimized to achieve complete permeabilization of cell membrane at intact mitochondrial oxidative phosphorylation. After 30 min of incubation with saponin all lactate dehydrogenase, 50% of creatine kinase, 30% of adenylate kinase and less than 20% of citrate synthase was released into the permeabilization medium. These skinned fibers behave similar to isolated mitochondria from human skeletal muscle: (i) the respiration with mitochondrial substrates can be stimulated by ADP, (ii) inhibited by carboxyatractyloside and (iii) it is possible to detect fluorescence changes of mitochondrial NAD(P)H on additions of substrates, uncoupler and cyanide. From a comparison of rates of respiration per cytochrome aa3 content of isolated human skeletal muscle mitochondria and saponin-skinned muscle fibers it was possible to calculate that almost 85% of mitochondria in those fibers are accessible for the investigation of oxidative phosphorylation. As shown by the investigation of biopsy samples of two patients with undefined myopathies these fibers are a suitable object for the replacement of isolated mitochondria in the diagnosis of mitochondrial myopathies and encephalomyopathies.

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

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

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.

Creatine kinase binding and possible role in chemically skinned guinea-pig taenia coli

The activity and role of creatine kinase (CK) associated with contractile proteins of smooth muscle have been investigated using skinned guinea-pig taenia coli fibers. Total CK activity was 163 +/- 22 IU/g (ww) and agarose electrophoresis showed BB, MB, and MM isoforms (BB-CK being the predominant isoenzyme). After skinning for 1 h with Triton X-100, BB-CK was specifically associated with the myofibrils, representing 22% of the preskinned CK activity. When relaxed fibers were exposed to pCa 9 in the presence of 250 microM ADP, 0 ATP and 12 mM PCr, tension was not significantly different from resting tension, but changing to pCa 4.5 caused the fibers to generate 59.1 +/- 5.2 percent of maximal tension. When a high-tension rigor state was achieved (250 microM ADP, 0 ATP, 0 PCr, and pCa 9), the addition of 12 mM PCr effected significant relaxation. These observations implicate an endogenous form of BB-CK, associated with the myofilaments and capable of producing enough ATP for submaximal tension generation and significant relaxation from rigor conditions. It was also shown that ADP is bound to the myofibrils and available for rephosphorylation by BB-CK. These results suggest co-localization of ATPase, MLCK and CK on the contractile proteins of the taenia coli. This enzymic association may play a role in the compartmentation of adenine nucleotides in smooth muscle.

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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Cardiac myofibrillar creatine kinase Km is not influenced by contractile protein binding

Subcellular microcompartmentation underlies the proposed phosphorylcreatine shuttle mechanism in mammalian cardiac tissue. In mitochondria, CK coupling to oxidative phosphorylation via adenine nucleotide translocase decreases the Km for ATP and suggests both a functional and physical integration. In the present studies, substrate Km of myofibrillar CK was unaltered when determined in the intact, native state or after removal from the myofibril. In contrast to mitochondria, close spatial proximity between cardiac myofibrillar CK and ATPase is sufficient to establish phosphorylcreatine shuttle microcompartmentation.

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.

An example of substrate channeling between co-immobilized enzymes. Coupled activity of myosin ATPase and creatine kinase bound to frog heart myofilaments

In myofilaments obtained by Triton X-100 lysis of frog heart cells in high ionic strength medium, the activity of bound creatine kinase cannot be detected by a coupled enzymatic assay. ATP is channelized toward myosin ATPase, through the unstirred layer near myofilaments and cannot diffuse into the bulk solution. Model systems based upon the coupled kinetics of enzymes co-immobilized on the same surface may explain this behaviour. This may also account for why myofilament-bound creatine kinase is more efficient than free enzyme in the cytosol for the physiological recycling of ADP into ATP.

Dependence upon high-energy phosphates of the effects of inorganic phosphate on contractile properties in chemically skinned rat cardiac fibres

The effects of inorganic phosphate (Pi) on mechanical properties of Triton X100 treated ventricular fibres have been studied in different substrate conditions. In the presence of both MgATP and phosphocreatine, increasing concentrations of Pi progressively decreased maximal active force, up to 50-60% at 20 mM Pi. The reduction in stiffness was slightly less. These effects appeared nearly independent of the diameter of the preparations. 20 mM Pi decreased Ca sensitivity of the myofilaments and increased the Hill coefficient of the tension/pCa relationship; furthermore, the time constant of tension recovery was decreased from 12.9 to 8.9 ms suggesting that the cycling rate of cross-bridges was increased in the presence of Pi. When MgATP was regenerated by the myofilament bound creatine kinase in the presence of phosphocreatine, Pi was less efficient in decreasing the maximal tension and it weakened the relaxing effect of MgATP upon rigor tension. These effects are related to the inhibition of creatine kinase by Pi. The effects of Pi on maximal force and kinetics of contraction were antagonized by the effects of a decrease in phosphocreatine. The results are discussed in terms of the antagonistic role of Pi increase and phosphocreatine decrease upon contractile properties of myofilaments during hypoxia in heart muscle.