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

Diastolic dysfunction in pulmonary artery hypertension: Creatine kinase and the potential therapeutic benefit of beta-blockers

Passive properties of the myocardium influence diastolic filling and cardiac output. In heart failure, changes in contributors to the passive properties of the ventricle, such as titin and collagen, and loss of the metabolic enzyme creatine kinase, increase resistance to filling resulting in diastolic dysfunction. Pulmonary artery hypertension (PAH) arises from interactions between the pulmonary vasculature and the right ventricle (RV) which ultimately leads to RV failure. Beta1-adrenergic receptor blockers (BB) act on the myocardium and are beneficial in left heart failure but are not used in PAH. We investigated whether BB improved survival and RV function in a rat model of PAH. Rats were injected with monocrotaline (60 mg/kg) to induce PAH and RV failure, or saline as controls (CON). When PAH was established, rats were treated with metoprolol (10 mg/kg per day) (MCT+BB) or vehicle (sucrose) (MCT); CON were treated with vehicle. In vivo measurement of RV compliance using pressure-volume catheter, indicated diastolic dysfunction in the RV of MCT rats was improved with BB treatment. Expression of creatine kinase protein and mRNA was lower in MCT rats compared to CON, with a trend for reversion by BB treatment. Isolated CON RV myocytes had a positive contraction response to faster pacing, whereas it was negative in MCT. MCT+BB cells had an intermediate response, indicating improved ability to respond to increased demand. BB improved diastolic function, partially restored metabolic enzymes and augmented contractility in PAH. These data support the hypothesis that BB may be beneficial in PAH by supporting RV function.

The Effect of Creatine Kinase Inhibition on Contractile Properties of Human Resistance Arteries

BACKGROUND

Creatine kinase (CK) is a main predictor of blood pressure, and this is thought to largely depend on high resistance artery contractility. We previously reported an association between vascular contractility and CK in normotensive pregnancy, but pregnancy is a strong CK inducer, and data on human hypertension are lacking. Therefore, we further explored CK-dependency of vascular contractility outside the context of pregnancy in normotensive and hypertensive women.

METHODS AND RESULTS

Nineteen consecutive women, mean age 42 years (SE 1.3), mean systolic/diastolic blood pressure respectively 142.6 (SE 5.9)/85.6 (3.4) mm Hg (9 hypertensive), donated an omental fat sample during abdominal surgery. We compared vasodilation after the specific CK inhibitor 2,4-dinitro-1-fluorobenzene (DNFB; 10(-6) mol/l) to sodium nitroprusside (10(-6) mol/l) in isolated resistance arteries using a wire myograph. Additionally, we assessed predictors of vasoconstrictive force. DNFB reduced vascular contractility to 24.3% (SE 4.4), P < 0.001, compared to baseline. Sodium nitroprusside reduced contractility to 89.8% (SE 2.3). Maximum contractile force correlated with DNFB effect as a measure of CK (r = 0.8), and with vessel diameter (r = 0.7). The increase in contractile force was 16.5 mN [9.1-23.9] per unit DNFB effect in univariable and 10.35 mN [2.10-18.60] in multivariable regression analysis.

CONCLUSION

This study extends on our previous findings in pregnant normotensive women of CK-dependent microvascular contractility, indicating that CK contributes significantly to resistance artery contractility across human normotension and primary hypertension outside the context of pregnancy. Further studies should explore the effect of CK inhibitors on clinical blood pressure.

Mitochondrial Bioenergetics and Fiber Type Assessments in Microbiopsy vs. Bergstrom Percutaneous Sampling of Human Skeletal Muscle

Microbiopsies of human skeletal muscle are increasingly adopted by physiologists for a variety of experimental assays given the reduced invasiveness of this procedure compared to the classic Bergstrom percutaneous biopsy technique. However, a recent report demonstrated lower mitochondrial respiration in saponin-permeabilized muscle fiber bundles (PmFB) prepared from microbiopsies vs. Bergstrom biopsies. We hypothesized that ADP-induced contraction (rigor) of smaller length microbiopsy PmFB causes a greater reduction in maximal respiration vs. Bergstrom, such that respiration could be increased by a myosin II ATPase-inhibitor (Blebbistatin; BLEB). Eleven males and females each received a 2 mm diameter percutaneous microbiopsy and a 5 mm diameter Bergstrom percutaneous biopsy in opposite legs. Glutamate/malate (5/0.5 mM)—supported respiration in microbiopsy PmFB was lower than Bergstrom at submaximal concentrations of ADP. 5 μM BLEB reduced this impairment such that there were no differences relative to Bergstrom ± BLEB. Surprisingly, pyruvate (5 mM)-supported respiration was not different between either biopsy technique ±BLEB, whereas BLEB increased succinate-supported respiration in Bergstrom only. H₂O₂ emission was lower in microbiopsy PmFB compared to Bergstrom PmFB in the presence of BLEB. Microbiopsies contained fewer type I fibers (37 vs. 47%) and more type IIX fibers (20 vs. 8%) compared to Bergstrom possibly due to sampling site depth and/or longitudinal location. These findings suggest that smaller diameter percutaneous biopsies yield lower glutamate-supported mitochondrial respiratory kinetics which is increased by preventing ADP-induced rigor with myosin inhibition. Microbiopsies of human skeletal muscle can be utilized for assessing mitochondrial respiratory kinetics in PmFB when assay conditions are supplemented with BLEB, but fiber type differences with this method should be considered.

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

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

Methods for assessing mitochondrial function in diabetes

A growing body of research is investigating the potential contribution of mitochondrial function to the etiology of type 2 diabetes. Numerous in vitro, in situ, and in vivo methodologies are available to examine various aspects of mitochondrial function, each requiring an understanding of their principles, advantages, and limitations. This review provides investigators with a critical overview of the strengths, limitations and critical experimental parameters to consider when selecting and conducting studies on mitochondrial function. In vitro (isolated mitochondria) and in situ (permeabilized cells/tissue) approaches provide direct access to the mitochondria, allowing for study of mitochondrial bioenergetics and redox function under defined substrate conditions. Several experimental parameters must be tightly controlled, including assay media, temperature, oxygen concentration, and in the case of permeabilized skeletal muscle, the contractile state of the fibers. Recently developed technology now offers the opportunity to measure oxygen consumption in intact cultured cells. Magnetic resonance spectroscopy provides the most direct way of assessing mitochondrial function in vivo with interpretations based on specific modeling approaches. The continuing rapid evolution of these technologies offers new and exciting opportunities for deciphering the potential role of mitochondrial function in the etiology and treatment of diabetes.

Mitochondrial creatine kinase activity and phosphate shuttling are acutely regulated by exercise in human skeletal muscle

Energy transfer between mitochondrial and cytosolic compartments is predominantly achieved by creatine-dependent phosphate shuttling (PCr/Cr) involving mitochondrial creatine kinase (miCK). However, ADP/ATP diffusion through adenine nucleotide translocase (ANT) and voltage-dependent anion carriers (VDACs) is also involved in this process. To determine if exercise alters the regulation of this system, ADP-stimulated mitochondrial respiratory kinetics were assessed in permeabilized muscle fibre bundles (PmFBs) taken from biopsies before and after 2 h of cycling exercise (60% ) in nine lean males. Concentrations of creatine (Cr) and phosphocreatine (PCr) as well as the contractile state of PmFBs were manipulated in situ. In the absence of contractile signals (relaxed PmFBs) and miCK activity (no Cr), post-exercise respiratory sensitivity to ADP was reduced in situ (up to 126% higher apparent K(m) to ADP) suggesting inhibition of ADP/ATP diffusion between matrix and cytosolic compartments (possibly ANT and VDACs). However this effect was masked in the presence of saturating Cr (no effect of exercise on ADP sensitivity). Given that the role of ANT is thought to be independent of Cr, these findings suggest ADP/ATP, but not PCr/Cr, cycling through the outer mitochondrial membrane (VDACs) may be attenuated in resting muscle after exercise. In contrast, in contracted PmFBs, post-exercise respiratory sensitivity to ADP increased with miCK activation (saturating Cr; 33% lower apparent K(m) to ADP), suggesting prior exercise increases miCK sensitivity in situ. These observations demonstrate that exercise increases miCK-dependent respiratory sensitivity to ADP, promoting mitochondrial-cytosolic energy exchange via PCr/Cr cycling, possibly through VDACs. This effect may mask an underlying inhibition of Cr-independent ADP/ATP diffusion. This enhanced regulation of miCK-dependent phosphate shuttling may improve energy homeostasis through more efficient coupling of oxidative phosphorylation to perturbations in cellular energy charge during subsequent bouts of contraction.

Dual cardiac contractile effects of the alpha2-AMPK deletion in low-flow ischemia and reperfusion

Because the question "is AMP-activated protein kinase (AMPK) alpha(2)-isoform a friend or a foe in the protection of the myocardium against ischemia-reperfusion injury?" is still in debate, we studied the functional consequence of its deletion on the contractility, the energetics, and the respiration of the isolated perfused heart and characterized the response to low-flow ischemia and reperfusion with glucose and pyruvate as substrates. alpha(2)-AMPK deletion did not affect basal contractility, respiration, and high-energy phosphate contents but induced a twofold reduction in glycogen content and a threefold reduction in glucose uptake. Low-flow ischemia increased AMPK phosphorylation and stimulated glucose uptake and phosphorylation in both alpha(2)-knockout (alpha(2)-KO) and wild-type (WT) groups. The high sensitivity of alpha(2)-KO to the development of ischemic contracture was attributed to the constitutive impairment in glucose transport and glycogen content and not to a perturbation of the energy transfer by creatine kinase (CK). The functional coupling of MM-CK to myofibrillar ATPase and the CK fluxes were indeed similar in alpha(2)-KO and WT. Low-flow ischemia impaired CK flux by 50% in both strains, showing that alpha(2)-AMPK does not control CK activity. Despite the higher sensitivity to contracture, the postischemic contractility recovered to similar levels in both alpha(2)-KO and WT in the absence of fatty acids. In their presence, alpha(2)-AMPK deletion also accelerated the contracture but delayed postischemic contractile recovery. In conclusion, alpha(2)-AMPK is required for a normal glucose uptake and glycogen content, which protects the heart from the development of the ischemic contracture, but not for contractile recovery in the absence of fatty acids.

Length dependence of cardiac myofilament Ca(2+) sensitivity in the presence of substitute nucleoside triphosphates

Although ATP is the immediate source of energy for muscle contraction other nucleoside triphosphates (NTP) can substitute for ATP as substrates for myosin and as sources of energy for contraction of skinned muscle fibers. However, experiments with skinned skeletal muscle fibers in the presence of substitute NTP indicate significant differences with respect to cross-bridge kinetics, force generation, and Ca(2+) regulation. In this study the length dependence of Ca(2+) sensitivity of skinned bovine cardiac muscle was analyzed in the presence of MgATP, MgCTP, MgUTP, and MgITP. Ca(2+) regulation in the presence of MgCTP and MgUTP was essentially the same as in the presence of MgATP, although the maximum force generated (at sarcomere length 2.4 microm) was about 25% less. However, the length dependence of Ca(2+) sensitivity was eliminated in the presence of MgUTP. With MgITP the maximum force generated (at sarcomere length 2.4 microm) was about the same as in the presence of MgATP, but there was an impairment of relaxation such that at pCa 8 the force developed was about 50-60% of that developed at pCa 5. Moreover, the Ca(2+)-dependent component showed no length-dependent sensitivity. Thus length modulation of Ca(2+) sensitivity is a function of the myosin substrate. Taken in conjunction with other data, the results are consistent with the hypothesis that length-dependence of Ca(2+) sensitivity is modulated at a step upstream from the force-generating reaction.

Direct evidence for the control of mitochondrial respiration by mitochondrial creatine kinase in oxidative muscle cells in situ

The efficiency of stimulation of mitochondrial respiration in permeabilized muscle cells by ADP produced at different intracellular sites, e.g. cytosolic or mitochondrial intermembrane space, was evaluated in wild-type and creatine kinase (CK)-deficient mice. To activate respiration by endogenous production of ADP in permeabilized cells, ATP was added either alone or together with creatine. In cardiac fibers, while ATP alone activated respiration to half of the maximal rate, creatine plus ATP increased the respiratory rate up to its maximum. To find out whether the stimulation by creatine is a consequence of extramitochondrial [ADP] increase, or whether it directly correlates with ADP generation by mitochondrial CK in the mitochondrial intermembrane space, an exogenous ADP-trap system was added to rephosphorylate all cytosolic ADP. Under these conditions, creatine plus ATP still increased the respiration rate by 2.5 times, compared with ATP alone, for the same extramitochondrial [ADP] of 14 microM. Moreover, this stimulatory effect of creatine, observed in wild-type cardiac fibers disappeared in mitochondrial CK deficient, but not in cytosolic CK-deficient muscle. It is concluded that respiration rates can be dissociated from cytosolic [ADP], and ADP generated by mitochondrial CK is an important regulator of oxidative phosphorylation.

Effect of ionic strength on length-dependent Ca(2+) activation in skinned cardiac muscle

The length-dependence of myofilament Ca(2+) sensitivity is considered to be an important component of the steep force-length relationship in cardiac muscle (Frank-Starling relation). Recent studies suggest that Ca(2+) sensitivity is a function of the number of strong-binding cross-bridge interactions formed at a given sarcomere length. However, the length-dependent step in the thin filament activation process is still unknown. This study was designed to test the hypothesis that sarcomere length influences the transition of the thin filament from the unattached (blocked) state to the weakly bound (closed) state. This hypothesis was tested by determining the length-dependence of Ca(2+) sensitivity as a function of ionic strength in skinned bovine ventricular muscle. Previous studies have shown that reduction in ionic strength below a critical level, in the absence of Ca(2+), shifts the thin filament to the closed state. In this study normal Ca(2+) regulation was maintained at low ionic strength but the length-dependence of Ca(2+) sensitivity and the length-dependence of Ca(2+) binding were eliminated. These results are consistent with the hypothesis that the transition from the blocked to the closed state is a function of filament geometry as well as Ca(2+) and ionic strength.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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.

Effects of creatine phosphate and inorganic phosphate on the sarcoplasmic reticulum of saponin-treated rat heart

1. Ventricular trabeculae from rat heart were permeabilized by treatment with saponin. In the presence of 150 nM Ca2+, application of 20 mM caffeine released Ca2+ from the sarcoplasmic reticulum (SR), resulting in a transient contracture. Ca2+ released from the SR was detected using fura-2 fluorescence. The amplitudes of the caffeine-induced Ca2+ transients were used to assess SR Ca2+ content. 2. In the absence of creatine phosphate (CP), introduction of 5-30 mM inorganic phosphate (Pi) caused a net release of Ca2+ from the SR. Subsequent caffeine-induced Ca2+ and tension transients were smaller in the presence of Pi. Under these conditions, 30 mM Pi decreased the caffeine-induced Ca2+ transients by 45 +/- 3.1% (mean +/- S.D., n = 14). On removal of Pi, the [Ca2+] transiently decreased and the caffeine-induced Ca2+ transients returned to control levels over 4-6 min. 3. In the presence of CP (5-15 mM), the Ca2+ transients were unaffected by the introduction of Pi (5-30 mM) or slightly increased in amplitude. Pi (30 mM) significantly increased the caffeine-induced Ca2+ transients by 7 +/- 8.8% (mean +/- S.D., n = 19, P < 0.05) in the presence of 15 mM CP. The release of Ca2+ on addition of Pi and decrease in [Ca2+] on Pi withdrawal was less pronounced or absent completely in the presence of CP. The inhibitory effects of Pi on caffeine-induced Ca2+ release became apparent as the [CP] was decreased from 5 to 0 mM. 4. In the presence of the creatine phosphokinase inhibitor dinitro-fluorobenzene (DNFB) the effects of Pi (in the presence of CP) were qualitatively similar to the results obtained in the absence of CP, although the decrease in caffeine-induced Ca2+ release was less pronounced. 5. These results suggest that the rise in [Pi]i during ischaemia or anoxia will have little effect on the regulation of Ca2+ by the SR while the [CP]i remains above 5 mM. However, as the [CP] decreases below 5 mM, the accumulation of Pi within the cytosol will progressively reduce the SR Ca2+ content. CP may act in conjunction with endogenous creatine phosphokinase to modify the response of the SR to Pi, and possible mechanisms are considered.

Myocardial ischemic contracture. Metabolites affect rigor tension development and stiffness

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

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

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

Myofibrillar creatine kinase and cardiac contraction

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

Compartmentation of creatine kinases during perinatal development of mammalian heart

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

Mitochondrial creatine phosphokinase deficiency in diabetic rat heart

The role of the mitochondrial end of the phosphocreatine energy shuttle was studied in the streptozotocin diabetic rat heart. Diabetic rats had 45 +/- 5% lower body weight and yielded 46 +/- 6% less mitochondria gm of protein than normals. Diabetic heart mitochondria had 32 +/- 7% lower creatine phosphokinase (CPK) activity and 59 +/- 10% lower oxygen consumption rate than normal heart mitochondria. Creatine (25 mM) did not stimulate oxygen uptake by diabetic heart although control (normal) heart mitochondria were stimulated. Inadequate mitochondrial energy production in the form of phosphocreatine could result in lower energy delivery to the myofibrillar contraction sites and might be an important factor in diabetic cardiomyopathy and weight loss.

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.

Contractile properties and creatine kinase activity of myofilaments following ischemia and reperfusion of the rat heart

After prolonged ischemia followed by reperfusion of the isolated rat heart, irreversible heart failure is associated with creatine kinase leakage from the cells. The possible implications of MM creatine kinase leakage from myofibrillar compartments on the contractile properties of ventricular muscle have been studied in control versus ischemic hearts. Total creatine kinase activity decreased in ischemic cells while creatine kinase and ATPase activities were not modified in isolated myofibrils. The efficiency of creatine kinase and phosphocreatine in the relaxation of rigor tension in skinned ventricular preparations was not changed after ischemia. Furthermore, neither the pCa/tension relationship nor the rate of tension development following length changes were modified by ischemia. These results show that the contractile properties of myofilaments as well as the functional coupling between myosin ATPase and creatine kinase are preserved in ischemic hearts suffering irreversible contractile failure.

Reversible MM-creatine kinase binding to cardiac myofibrils

Skinned rat papillary muscles and purified preparations of rat cardiac myofibrils were used to study the nature of the interaction of creatine kinase with cardiac myofibrils. High activity of creatine kinase (2 IU/mg protein in fibers and 0.9 IU/mg in purified myofibrils) was due mostly to reversibly bound enzyme. This activity could be removed and rebound. The process of creatine kinase rebinding was characterized by apparent Km value of 0.14 mg/ml (approximately equal to 2 X 10(6) M). Rebinding of creatine kinase to cardiac myofibrils restored the phenomenon of functional compartmentation of adenine nucleotides in myofibrillar space and restored the ability of phosphocreatine to decrease the rigor tension in the presence of MgADP. The physiological experiments with quick length changes showed that rebinding of creatine kinase to skinned papillary muscle also restored Ca sensitivity, increased maximal tension development, decreased stiffness, and restored the tension recovery after quick length changes in muscle under condition of inhibition of endogenous creatine kinase by 1-fluoro-2,4-dinitrobenzene. It is concluded that creatine kinase reversibly bound to cardiac myofibrils is involved in the energy supply for cardiac contraction.

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

The influence of phosphocreatine in the presence or absence of MgATP and MgADP was studied in Triton X-100-treated thin papillary muscles and ventricular strips of the rat heart. The pCa/tension relationships, the pMgATP/tension relationships, and the tension responses to quick length changes were analyzed. The results show three major consequences of the reduction of the phosphocreatine concentration in the presence of millimolar concentrations of the MgATP. (a) The resting tension and the maximal Ca2+-activated tension were increased, and the pCa/tension relationship was shifted toward higher pCa values and its steepness was decreased; these effects were enhanced by the inclusion of MgADP. (b) The time constant of tension recoveries after quick stretches applied during maximal activation was increased, while the extent of these recoveries was decreased. (c) The study of pMgATP/tension relationships in low Ca concentrations showed that the decrease in phosphocreatine induced a shift toward higher MgATP values with no changes in maximal rigor tension or the slope coefficient; these effects were increased by the increase in MgADP and were independent of the preparation diameter. Thus, modifications of the apparent Ca sensitivity and resting and maximal tension when phosphocreatine is decreased seem to be due to an increasing participation of rigor-like or slowly cycling cross-bridges spending more time in the attached state. These results suggest that endogenous creatine kinase is able to ensure maximal efficiency of myosin ATPase by producing a local high MgATP/MgADP ratio.