Energy transport from mitochondria to myofibril by a creatine phosphate shuttle in cardiac cells

High-Energy Phosphates and Ischemic Heart Disease: From Bench to Bedside

The purpose of this review is to bridge the gap between clinical and basic research through providing a comprehensive and concise description of the cellular and molecular aspects of cardioprotective mechanisms and a critical evaluation of the clinical evidence of high-energy phosphates (HEPs) in ischemic heart disease (IHD). According to the well-documented physiological, pathophysiological and pharmacological properties of HEPs, exogenous creatine phosphate (CrP) may be considered as an ideal metabolic regulator. It plays cardioprotection roles from upstream to downstream of myocardial ischemia through multiple complex mechanisms, including but not limited to replenishment of cellular energy. Although exogenous CrP administration has not been shown to improve long-term survival, the beneficial effects on multiple secondary but important outcomes and short-term survival are concordant with its pathophysiological and pharmacological effects. There is urgent need for high-quality multicentre RCTs to confirm long-term survival improvement in the future.

Compartmentation of membrane processes and nucleotide dynamics in diffusion-restricted cardiac cell microenvironment

Orchestrated excitation-contraction coupling in heart muscle requires adequate spatial arrangement of systems responsible for ion movement and metabolite turnover. Co-localization of regulatory and transporting proteins into macromolecular complexes within an environment of microanatomical cell components raises intracellular diffusion barriers that hamper the mobility of metabolites and signaling molecules. Compared to substrate diffusion in the cytosol, diffusional restrictions underneath the sarcolemma are much larger and could impede ion and nucleotide movement by a factor of 10(3)-10(5). Diffusion barriers thus seclude metabolites within the submembrane space enabling rapid and vectorial effector targeting, yet hinder energy supply from the bulk cytosolic space implicating the necessity for a shunting transfer mechanism. Here, we address principles of membrane protein compartmentation, phosphotransfer enzyme-facilitated interdomain energy transfer, and nucleotide signal dynamics at the subsarcolemma-cytosol interface. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

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

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

Modeling of spatial metabolite distributions in the cardiac sarcomere

Although a high ATP diffusion rate implies homogeneous distribution of the principal energetic currency in the cytosol, local diffusion barriers represented by macromolecular structures can render ATP concentrations to be inhomogeneous. A method is presented here that provides apparent diffusion coefficient values in local intracellular regions and allows the estimation of spatial metabolite distribution. The apparent local diffusion coefficient for ATP in cardiac myofibrils was determined from the analysis of diffusion-dependent rightward shift of the substrate dependence for actomyosin ATPase activity using the reaction-diffusion model, which accounted for the properties of phosphotransfer reactions. This functional analysis, which took into account the local diffusional ATP delivery to the active sites, provided an apparent value that was three orders of magnitude lower than that defined by direct methods for the cytosol. The low value of the diffusion coefficient was shown to define unusual properties of the intracellular space in working heart, where small reductions in ATP levels in the surrounding cytosol result in a large drop in [ATP] inside myofibrils. This drop is critical for vital cellular functions, and the analysis presented here defines its physical basis. The diffusion barriers thus defined explain the coexistence of pathological energy deficit with almost normal average ATP levels.

Changes in cardiac contractility related to calcium-mediated changes in phosphorylation of myosin-binding protein C

Ca ions can influence the contraction of cardiac muscle by activating kinases that specifically phosphorylate the myofibrillar proteins myosin-binding protein C (MyBP-C) and the regulatory light chain of myosin (RLC). To investigate the possible role of Ca-regulated phosphorylation of MyBP-C on contraction, isolated quiescent and rhythmically contracting cardiac trabeculae were exposed to different concentrations of extracellular Ca and then chemically skinned to clamp the contractile system. Maximum Ca-activated force (F(max)) was measured in quiescent cells soaking in 1) 2.5 mM Ca for 120 min, 2) 1.25 mM for 120 min, or 3) 1.25 mM for 120 min followed by 10 min in 7.5 mM, and 4) cells rhythmically contracting in 2.5 mM for 20 min. F(max) was, respectively, 21.5, 10.5, 24.7, and 32.6 mN/mm(2). Changes in F(max) were closely associated with changes in the degree of phosphorylation of MyBP-C and occurred at intracellular concentrations of Ca below levels associated with phosphorylation of RLC. Monophosphorylation of MyBP-C by a Ca-regulated kinase is necessary before beta-adrenergic stimulation can produce additional phosphorylation. These results suggest that Ca-dependent phosphorylation of MyBP-C modulates contractility by changing thick filament structure.

AMP deamination and purine exchange in human skeletal muscle during and after intense exercise

1. The present study examined the regulation of human skeletal muscle AMP deamination during intense exercise and quantified muscle accumulation and release of purines during and after intense exercise. 2. Seven healthy males performed knee extensor exercise at 64.3 W (range: 50-70 W) to exhaustion (234 s; 191-259 s). In addition, on two separate days the subjects performed exercise at the same intensity for 30 s and 80 % of exhaustion time (mean, 186 s; range, 153-207 s), respectively. Muscle biopsies were obtained from m.v. lateralis before and after each of the exercise bouts. For the exhaustive bout femoral arterio-venous concentration differences and blood flow were also determined. 3. During the first 30 s of exercise there was no change in muscle adenosine triphosphate (ATP), inosine monophosphate (IMP) and ammonia (NH3), although estimated free ADP and AMP increased 5- and 45-fold, respectively, during this period. After 186 s and at exhaustion muscle ATP had decreased (P < 0.05) by 15 and 19 %, respectively, muscle IMP was elevated (P < 0. 05) from 0.20 to 3.65 and 5.67 mmol (kg dry weight)-1, respectively, and muscle NH3 had increased (P < 0.05) from 0.47 to 2.55 and 2.33 mmol (kg d.w.)-1, respectively. The concentration of H+ did not change during the first 30 s of exercise, but increased (P < 0.05) to 245.9 nmol l-1 (pH 6.61) after 186 s and to 374.5 nmol l-1 (pH 6. 43) at exhaustion. 4. Muscle inosine and hypoxanthine did not change during exercise. In the first 10 min after exercise the muscle IMP concentration decreased (P < 0.05) by 2.96 mmol (kg d.w.)-1 of which inosine and hypoxanthine formation could account for 30 %. The total release of inosine and hypoxanthine during exercise and 90 min of recovery amounted to 1.07 mmol corresponding to 46 % of the net ATP decrease during exercise or 9 % of ATP at rest. 5. The present data suggest that AMP deamination is inhibited during the initial phase of intense exercise, probably due to accumulation of orthophosphate, and that lowered pH is an important positive modulator of AMP deaminase in contracting human skeletal muscle in vivo. Furthermore, formation and release of purines occurs mainly after intense exercise and leads to a considerable loss of nucleotides.

Mitochondrial response to heart rate steps in isolated rabbit heart is slowed after myocardial stunning

The oxidative capacity of mitochondria isolated from myocardium is undiminished after myocardial stunning, which is remarkable because stunning affects many other cellular functions. The aim of the present study was to assess the mitochondrial oxidative response in intact rather than isolated myocardium. The mean response time of mitochondrial O2 consumption to heart rate steps (tmito) was measured before and after 15-minute ischemia or high-flow hypoxia in isolated rabbit hearts. The tmito was calculated from the time course of venous O2 tension to steps in heart rate, with corrections made for diffusion and vascular transport delay. Isovolumic hearts were perfused with Tyrode's solution at 37 degrees C. Developed left ventricular pressure at 35 minutes of reperfusion was decreased significantly to 67 +/- 3% after ischemia (mean +/- SEM, n = 8) and to 79 +/- 6% after hypoxia (n = 8) relative to the control condition (n = 8), without increased cellular creatine kinase release. Before ischemia or hypoxia, tmito was 4.3 +/- 0.3 seconds. During reperfusion after ischemia or hypoxia, the increase in tmito (by 62 +/- 10% and 64 +/- 18%, respectively) was significantly larger than that in time controls (24 +/- 12% increase). The major determinant of decreased contractility and slower mitochondrial response appeared to be O2 deprivation and/or reintroduction rather than other consequences of stopped flow. O2 consumption at a given rate-pressure product was not increased after ischemia or hypoxia, indicating undiminished cardiac contractile economy. Brief ischemia or hypoxia, resulting in stunning, was associated with a slowing of the in vivo mitochondrial oxidative response, indicating that energy transfer and/or signaling between energy-consuming sites and mitochondria is affected in stunned myocardium.

Evidence for rapid consumption of millimolar concentrations of cytoplasmic ATP during rigor-contracture of metabolically compromised single cardiomyocytes

Cytoplasmic ATP can be measured continuously in single cardiac myocytes by monitoring the luminescence from microinjected firefly luciferase. We show here that the signals are markedly influenced by changes in cytoplasmic pH, and the calibration of the signals as ATP concentration is markedly affected by cytoplasmic protein. Measurements with a pH-sensitive fluorescent dye show that intracellular pH (pHi) can be clamped at pH 7.08 by perfusing cells with a modified bicarbonate-buffered Krebs saline containing 92 mM NaHCO3 and equilibrated with 20% CO2. Calibration of the firefly luciferase signal in vitro in the presence of high concentrations of BSA (180 mg/ml), to simulate the cytoplasmic protein concentration, revealed a shift in Km (ATP) to 2 mM, from approx. 400 microM in the absence of albumin in an identical ionic milieu. Luciferase measurements in pH-clamped cells show that metabolically poisoned isolated rat ventricle cardiomyocytes enter rigor at a cytoplasmic ATP concentration of between 1 and 2 mM. As the cells shorten in rigor, a process that is complete in 30-40 s, the cytoplasmic ATP concentration falls simultaneously to a level of typically 20 microM. When cyanide is removed 10 min later, to simulate reoxygenation, the signal recovers over a period of 2-3 min to a level approx. 70% of the original in the healthy cell. These studies indicate that rigor-mediated depletion of cytoplasmic ATP in metabolically poisoned cardiomyocytes is considerably more extreme than hitherto indicated.

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.

Differences in nucleotide compartmentation and energy state in isolated and in situ rat heart: assessment by 31P-NMR spectroscopy

Free cytosolic concentrations of ATP, PCr, ADP and 5'-AMP, and the cytosolic [ATP]/[ADP].[Pi] ratio, were determined in isolated and in situ rat hearts using 31P-NMR spectroscopy. Total tissue metabolite concentrations were determined by HPLC analysis of freeze-clamped, perchloric acid-extracted tissue. In in situ myocardium the PCr/ATP ratio was 2.7 +/- 0.2 determined from 31P-NMR data (using either PCr/beta-NTP or PCr/gamma-NTP), and 1.9 +/- 0.1 (P < 0.01) determined from total tissue concentrations. 31P-NMR-determined and total tissue [PCr] were in excellent agreement (49.6 +/- 8.4 and 49.5 +/- 1.0 mumol.g-1 dry wt, respectively), whereas 31P-NMR-determined [ATP] (18.6 +/- 3.2 mumol.g-1 dry wt) was only 71% of the total tissue concentration (26.1 +/- 1.7 mumol.g-1 dry wt, P < 0.01). Isolation and Langendorff perfusion of rat hearts with glucose as substrate reduced total tissue [ATP] and [PCr] and the 31P-NMR-determined PCr/ATP ratio fell to 1.5 +/- 0.1. This value agreed well with the total tissue ratio of 1.4 +/- 0.1, and there was excellent agreement between 31P-NMR-determined and total tissue [PCr] and [ATP] values in the perfused heart. Addition of pyruvate to perfusate increased the 31P-NMR-determined PCr/ATP ratio to 1.7 +/- 0.1 due to elevated [PCr], and there remained excellent agreement between NMR-determined and total tissue [PCr] and [ATP] values. Free cytosolic [ADP] (from the creatine kinase equilibrium) was 5% of total tissue ADP, and free cytosolic [5'-AMP] (from the adenylate kinase equilibrium) ranged from 0.2-0.3% of total tissue 5'-AMP. Bioenergetic state, indexed by [ATP]/[ADP].[Pi], was much lower in isolated perfused hearts (30 mM-1) vs. in situ myocardium (approximately 150 mM-1). In summary, we observe a substantial disproportionality between total tissue PCr/ATP and 31P-NMR-determined PCr/ATP in highly energised in situ myocardium but not in isolated perfused hearts. This appears due to an NMR invisible ATP compartment approximating 29% of total tissue ATP in situ. Additionally, more than 95% of ADP and more than 99% of 5'-AMP exist in bound forms in perfused and in situ myocardium. The physiological significance of these observations is unclear. However, substantial differences between 31P-NMR visible and total tissue [ATP] introduces significant errors in conventional estimation of free cytosolic [ADP], [5'-AMP] and [ATP]/[ADP].[Pi] from in vivo 31P-NMR data.

Does gastric tonometry work? No

Gastrointestinal tonometry is supposed to diagnose gut mucosal hypoxia using gastric luminal PCO2 and arterial bicarbonatemia, which are substituted in a modified Henderson-Hasselbach equation. This article reviews some of the problems inherent to the multiple assumptions underlying this technique. Tonometry is influenced by several local factors and by systemic acid-base imbalances that are unrelated to oxygenation. Tonometry is a rather crude and cumbersome method of gut capnometry, a technology that may provide valuable information regarding visceral perfusion, but not necessarily oxygenation.

Bound inorganic phosphate and early contractile failure in global ischaemia

Inorganic phosphate (Pi) accumulates extremely rapidly in ischaemic heart muscle and intracellular binding of this metabolite may account for the precipitous loss of function seen at the onset of severe ischaemia. We have used 31P-NMR spectroscopy to measure the free cytosolic [Pi] and chemical assay techniques to measure total tissue Pi at 0, 1, 2, 3, 4, 5, and 12 min of complete global ischaemia in the isolated isovolumic rat heart. At zero time, the Pi assayed chemically was 30.77 +/- 5.52 mumol/g dry wt (mean +/- SD, n = 7) whilst Pi assayed by NMR was 3.39 +/- 1.21 mumol/g dry wt (n = 15). Thus, 27.38 mumol/g dry wt of Pi was bound at a cytosolic [Pi] of 0.82 mM. After 12 min of ischaemia, 49.88 mumol/g dry wt of Pi was bound at a cytosolic [Pi] of 4.11 mM. When all data were fitted, using a non-linear, least squares fit (p < 0.05), to the binding isotherm: Bound Pi = Bmax'. [Pi]/Kd'+[Pi], the apparent binding parameters Kd' and Bmax' were estimated to be 1.1 +/- 0.6 mM and 64.0 +/- 10.2 mumol/g dry wt respectively. During the first minute of global ischaemia when the rate-pressure product had decreased by 79% of its pre-ischaemic value, bound Pi had increased by 58% and free cytosolic [Pi] by 162%. When functional and metabolite changes were expressed as a fraction of the total change which occurred during the 12-min ischaemic period, bound Pi had the profile most similar to the rate-pressure product. Both the amount of bound Pi and free cytosolic [Pi] correlated with loss of contractile function as the ischaemic period progressed. The results show that during ischaemia, Pi is bound progressively as free cytosolic [Pi] is increased as the result of high energy phosphate hydrolysis. While these results are consistent with the possibility that Pi binding may contribute to ischaemic contractile failure, no molecular explanation for the possible effect of bound Pi on contraction has been proposed.

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.

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.

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.

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

Images

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.

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.

Comparison of myosin and creatine kinase isoforms in left ventricles of young and senescent Fischer 344 rats after treatment with triiodothyronine

The effects of perturbation of thyroid hormone levels on expression of myosin and creatine kinase isoforms were examined in maturing and senescent rats. Whereas LV of maturing rats contain only alpha-MHC, LV of senescent rats contain nearly equal amounts of both alpha-MHC and beta-MHC. When maturing rats were made hypothyroid by treatment for 14 days with the antithyroid agent, propylthiouracil (PTU), beta-MHC and beta-MHC mRNA levels increased significantly. Administration of T3 to senescent rats, or maturing rats made hypothyroid by PTU treatment produced similar decreases in levels of both beta-MHC and beta-MHC mRNA. In contrast, treatment with T3 produced little change in creatine kinase isoform distribution. Thus, thyroid hormone appears to play a critical role in regulating expression of the isoforms of myosin but not of creatine kinase.

Effects of age on myosin and creatine kinase isoforms in left ventricles of Fischer 344 rats

Left ventricles of hearts from male Fischer 344 rats of 2, 8 and 23 months of age were analyzed to determine if aging results in significant alterations in the isoform distribution of myosin and creatine kinase protein and mRNAs. Left ventricles of maturing (2-month) rats contained almost exclusively alpha-myosin heavy chain (MHC) mRNA and protein. In adults (8 months) there was a 5-fold increase in beta-MHC (fetal isoform) and an approximate 10% decrease in alpha-MHC mRNA levels, relative to 2 months. By 23 months (senescence), beta-MHC mRNA levels had increased by 11-fold and alpha-MHC mRNA levels had decreased by about 30%. These changes corresponded to an increase in the relative proportion of beta-MHC protein, from undetectable levels at 2 months, to about 40% by 8 months and to about 60% by 23 months. Increased levels of beta-MHC and its mRNA in older rats correlated with decreased serum thyroid hormone levels. The specific activity of creatine kinase in crude homogenates decreased with age, as has been reported previously. Relative to 2-month controls, the specific activity of creatine kinase had decreased by 21% at 8 months and by 37% at 23 months. Analysis of creatine kinase activity showed no large increase in levels of the fetal (B) isoform with age, as was found for myosin. Levels of mRNAs encoding the B and M isoforms of creatine kinase were significantly reduced in senescent rats. Thus, the decreased levels of creatine kinase in aging rats is correlated with decreased levels of mRNA encoding the BCK and MCK isoforms but not an isoform shift.

Changes in myosin and creatine kinase mRNA levels with cardiac hypertrophy and hypothyroidism

Rats were treated with three methods which produce alterations in the expression of myosin isozymes: coarctation of the abdominal aorta, treatment with low doses of isoproterenol, and administration of propylthiouracil. The steady-state levels of the left ventricle mRNAs for alpha myosin heavy chain (alpha-MHC), beta myosin heavy chain (beta-MHC), M creatine kinase (MCK), and B creatine kinase (BCK) were then determined using Northern and slot blot hybridizations. Cardiac hypertrophy was induced by an acute systolic pressure overload, or beta adrenergic stimulation. At 7 days following systolic pressure overload, the induced cardiac hypertrophy was accompanied by alterations in the levels of MHC mRNAs, as has been previously reported. In RNA from left ventricles of treated animals alpha-MHC mRNA levels decreased by 15% by day 3 and 20% by day 7. In contrast, beta-MHC mRNA levels increased to 250% of control levels by day 3 and then declined to a value 150% of controls by day 7. Levels of MCK and BCK mRNAs showed little or no changes by day 3; at day 7 both MCK and BCK mRNAs showed decreases of 20% relative to controls. Cardiac hypertrophy induced by low doses of isoproterenol produced decreases of alpha-MHC mRNA levels to 70% of control values at day 3 and 50% at day 7. Over the same time periods there was an increase in the levels of the fetal mRNA isoform (beta-MHC) to 190%, then 130% of control values, respectively. At 3 days, both BCK and MCK mRNA levels had declined by approximately 20-25%. By 7 days, MCK mRNA levels had decreased by approximately 50% and BCK mRNA levels by 30%. Hypothyroidism induced by PTU treatment led to a 50% decrease in alpha-MHC mRNA levels by day 3, which then further decreased to 10% of control levels at day 7. beta-MHC mRNA levels increased to 350% of control levels at day 3 and then decreased to 275% of control levels at day 7. For creatine kinase mRNAs the level of the M isoform was increased by 30% at day 3, whereas there appeared to be no significant change in levels of B isoform mRNA at this time. At day 7 neither BCK nor MCK mRNA levels were significantly different from controls. These results show three treatments which produce an alteration in myosin mRNA isoforms produce little or no change in creatine kinase isoform mRNAs. Thus, the MHC and CK genes respond differently to either cardiac hypertrophy or a reduction in thyroid hormone levels.

Free creatine available to the creatine phosphate energy shuttle in isolated rat atria

To measure the actual percentage of intracellular free creatine participating in the process of energy transport, the incorporation of [1-14C]creatine into the "free" creatine and phosphocreatine (PCr) pools in spontaneously beating isolated rat atria, under various conditions, was examined. The atria were subjected to three consecutive periods, control, anoxia, and postanoxic recovery, in medium containing tracers of [1-14C]creatine. The tissue content and specific activity of creatine and PCr were determined at the end of each period. The higher specific activity found for tissue PCr (1.87 times) than creatine, independent of the percentage of total intracellular creatine that was present as free creatine, provides evidence for the existence of two separate pools of free creatine. Analysis of the data shows that in the normal oxygenated state approximately equal to 9% of the total intracellular creatine is actually free to participate in the process of energy transport (shuttle pool). About 36% of the total creatine is bound to unknown intracellular components and the rest exists as PCr. The creatine that was taken up and the creatine that was released from the breakdown of PCr have much greater access to the site of phosphorylation than the rest of the intracellular creatine. A sharp increase in the specific activity of residual PCr on prolongation of anoxic time was also observed. This provides evidence for a nonhomogeneous pool of PCr, for the most recently formed (radioactive) PCr appeared to be hydrolyzed last.

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.

Pathophysiology of cardiomyocytes

Isolated cardiomyocytes lend themselves very well to the quantification of pathological damage and to the determination of reversible versus irreversible changes. These single cells were used to study the cellular response to a variety of pathologic stimuli that impair structure and function. Degenerative alterations are accompanied by hyperactivation and irreversible rounding up of otherwise quiescent rod-shaped cells. Stereotypic degenerative changes and loss of sarcolemma-bound Ca2+ were seen during prolonged severe hypoxia, exposure to either depolarizing concentrations of potassium, veratrine, acylcarnitines, cationic amphiphiles, free-radical-generating systems, cardiac glycosides, or uncouplers of oxidative phosphorylation. Since the presence of extracellular Ca2+ is a prerequisite to obtain cell degeneration in most of these aggressive insults and since cellular Ca2+ overload parallels the damage, we studied the influence of representative compounds of the various subclasses of Ca2+ antagonists: verapamil, nifedipine, nicardipine, and diltiazem (Ca2+ blockers with high affinity for cardiac slow Ca2+ channels), cinnarizine, flunarizine, lidoflazine, and mioflazine (Ca2+ blockers with no affinity for cardiac slow Ca2+ channels). The non-slow-channel-blocking drugs were generally superior in protection against the imposed insults suggesting that prevention of Ca2+ overload is not correlated with slow channel blockade. For the latter group of drugs, other (hitherto not elucidated) mechanisms of membrane-drug interactions seem to be responsible for the preservation of Ca2+ homeostasis during the induction of pathological Ca2+ influx.

Phosphorylcreatine shuttle enzymes during perinatal heart development

Mammalian heart development, from the time of weaning until adulthood, is characterized by progressive and significant enhancement in functional performance. Aerobic metabolism and contractile protein ATPase activity increase in parallel with augmented cardiac function. The present studies examined the potential contribution of phosphorylcreatine shuttle enzymes to the developmentally linked alterations in heart performance. Mitochondrial ATPase specific activity was not altered between weanling and adult heart; however, creatine kinase activity was enhanced approximately threefold. Myofibrillar ATPase activity doubled over the developmental time course, while creatine kinase activity increased to an even greater extent. Enhanced myofibrillar ATPase activity was not due to alterations in either calcium sensitivity or ATPase activity measured in purified myosin. Both the mitochondrial and myofibrillar creatine kinase enzyme activities are enhanced during normal heart growth; however, relatively greater enhancement of the myofibrillar component occurs. Thus, enzymatic reactions comprising the phosphorylcreatine shuttle system are dramatically increased during normal heart development. This mechanism deserves consideration as a potentially powerful contributor to enhanced cardiac function during the perinatal period.

The creatine phosphate energy shuttle--the molecular asymmetry of a "pool"

The creatine phosphate shuttle energy transfer mechanism was postulated on the basis of the hexokinase acceptor theory of insulin action. It proposes that the movement of chemical energy from the mitochondrion to the myofibril is in the form of creatine phosphate. This occurs because there are isozymes of creatine phosphokinase bound to the inner membrane of the sarcosome and to the A band of the myofibril. These isozymes have been shown to act as transducers of energy from ATP to creatine phosphate at the translocase site and from creatine phosphate back to ATP at the myofibrillar compartment. Calculations show that there is no significant amount of transformation of creatine phosphate to ATP in the intervening space between the mitochondrion and the myofibril so that, essentially, transport between the oxidative sites and the contractile apparatus is through the creatine phosphate shuttle. There is also evidence that another terminus for this shuttle is the microsome so that muscle activity tends to increase energy supply for protein synthesis.

Disruption of myofibrillar energy use: dual mechanisms that may contribute to postischemic dysfunction in stunned myocardium

The abnormalities in regional function produced by myocardial ischemia persist after the ischemic episode resolves. Since a close functional coupling exists between myofibrillar creatine kinase and myosin ATPase, a disruption of this coupling could adversely influence myocardial function and might provide a mechanism for the myocardial dysfunction observed. The purpose of the present study was to determine if an alteration in the activity of creatine kinase associated with the myofibril occurs in the postischemic period. Anesthetized open-chest dogs (n = 6) underwent coronary occlusion for 15 minutes, followed by reperfusion for 15 minutes. In reperfused myocardium, adenine nucleotide content was decreased (72 +/- 10% of nonischemic myocardium, p less than 0.05), documenting the presence of previous ischemia. The creatine phosphate content of reperfused myocardium returned to normal, indicating resumption of myocardial energy production. The creatine kinase activity of purified myofibrils isolated from reperfused myocardium was decreased by 17 +/- 7% compared to that of nonischemic myofibrils (p less than 0.03). In addition, the free adenosine diphosphate concentration in reperfused myocardium was calculated to be 96 microM and was less than the Km of adenosine diphosphate determined for myofibrillar creatine kinase (105 microM). The results suggest two putative mechanisms for disruption of energy use in postischemic myocardium: decreased creatine kinase activity associated with the myofibril, and limitation of substrate necessary for maximal creatine kinase activity.

Myokinase and contractile function of glycerinated muscle fibers

Glycerinated rabbit psoas muscle fibers containing native CPK, ATPase, and myokinase activities were used and isometric contraction and relaxation responses to either ADP or ATP + CP or to ATP alone in the presence and absence of P1, P5-di(adenosine-5'-pentaphosphate), a myokinase inhibitor, were compared. In previous (14) work it was shown that CP generated more efficient and faster contraction and relaxation of glycerinated muscle fibers than ATP. The present work deals with the role of myokinase in the differential response of fibers to CP and ATP. Inhibition of the myokinase activity of these fibers caused slight diminution of the rate of contraction at physiological concentrations of ATP. Uninhibited fibers were not able to reach maximum contraction, because the tension began to drop gradually even in the presence of Ca2+. Addition of Ap5A permitted maximum contraction and the ability to stay at the contracted state. In the case of CP + adenosine nucleotides (ATP or ADP), myokinase activity decreased the rate of tension development which was statistically significant after 5-7 sec of contraction. Thus, a higher tension was obtainable when myokinase was inhibited. At high concentration of adenine nucleotides (greater than 2 mM) and in the absence of Ap5A, not only the maximum tension never was reached, but a spontaneous drop in tension was observed before addition of EGTA, as was seen with ATP alone. Relaxation was faster and more complete in the presence of uninhibited myokinase activity except that the ADP was low (125 mM). These observations provide further evidence for a close functional interaction of these three enzymes in the mechanism of contraction and relaxation, giving further support to the notion of the creatine-phosphocreatine energy shuttle.

Role of myofibrillar creatine kinase in the relaxation of rigor tension in skinned cardiac muscle

In the absence of creatine phosphate, MgATP produced relaxation of rigor tension in chemically-skinned right papillary muscles of the rat, the half maximal effect being obtained at 1.8 mM MgATP. In the presence of 12 mM creatine phosphate and 250 microM ADP, a decrease in MgATP concentration even to 10(-9) M never induced rigor tension. At a very low MgATP concentration (10(-6) M), the half maximal relaxing effect was obtained with 2 mM creatine phosphate, a value close to the Km of isolated MM-creatine kinase for this substrate, or with 14 microM MgADP, a value 5 times lower than the reported Km. An exogenous MgATP regenerating system (phosphoenol pyruvate + pyruvate kinase) was not able to fully relax the fibres. When MM-creatine kinase was inhibited by fluorodinitrobenzene, the dependency of rigor tension on MgATP became the same as it was without creatine phosphate. After washing out the fluorodinitrobenzene the addition of exogenous MM-creatine kinase for half an hour fully relaxed rigor tension; moreover, this effect persisted even after prolonged washout. These results show that endogenous MM-creatine kinase is able to ensure maximal efficiency of myosin ATPase by producing a localized high MgATP/MgADP ratio; they also suggest the existence of rapidly exchangeable binding sites for MM-creatine kinase in cardiac myofibrils.

Aequorin measurements of free calcium in single heart cells

The performance of the heart depends on the concentrations of free calcium ions in the cytoplasm of the myocytes. However, direct evidence for changes in free Ca concentration in physiological events during response to drugs and in pathogenesis has been difficult to obtain because of technical problems in measuring free Ca at 10(-7) M in cells with a volume of only a few picolitres. Here we describe measurements made with the Ca-sensitive photoprotein aequorin in single ventricular myocytes isolated from rat heart. We have detected signals from resting and contracting cells, and from cells exposed to media of altered ionic composition (raised K, lowered Na), ouabain and metabolic inhibitors. We report that free Ca in metabolically-poisoned myocytes is remarkably stable and that severe injury to the cell occurs before the free Ca concentration rises above 1-3 X 10(-7) M, hence cell damage seems to be a cause, not a consequence, of a rise in free Ca. The technique used here should help to resolve many uncertainties regarding free Ca in heart function, and should be particularly valuable for investigating the role of free Ca in ischaemic pathogenesis.