Energetic basis for reduced contractile reserve in isolated rat hearts

Energetic communication between mitochondria and nucleus directed by catalyzed phosphotransfer

Exchange of information between the nucleus and cytosol depends on the metabolic state of the cell, yet the energy-supply pathways to the nuclear compartment are unknown. Here, the energetics of nucleocytoplasmic communication was determined by imaging import of a constitutive nuclear protein histone H1. Translocation of H1 through nuclear pores in cardiac cells relied on ATP supplied by mitochondrial oxidative phosphorylation, but not by glycolysis. Although mitochondria clustered around the nucleus, reducing the distance for energy transfer, simple nucleotide diffusion was insufficient to meet the energetic demands of nuclear transport. Rather, the integrated phosphotransfer network was required for delivery of high-energy phosphoryls from mitochondria to the nucleus. In neonatal cardiomyocytes with low creatine kinase activity, inhibition of adenylate kinase-catalyzed phosphotransfer abolished nuclear import. With deficient adenylate kinase, nucleoside diphosphate kinase, which secures phosphoryl exchange between ATP and GTP, was unable to sustain nuclear import. Up-regulation of creatine kinase phosphotransfer, to mimic metabolic conditions of adult cardiac cells, rescued H1 import, suggesting a developmental plasticity of the cellular energetic system. Thus, mitochondrial oxidative phosphorylation coupled with phosphotransfer relays provides an efficient energetic unit in support of nuclear transport.

Activation time of myocardial oxidative phosphorylation in creatine kinase and adenylate kinase knockout mice

Our goal was to determine whether mice genetically altered to lack either creatine kinase (M/MtCK(-/-)) or adenylate kinase (AK(-/-)) show altered properties in the dynamic regulation of myocardial oxygen consumption (MVO(2)). We measured contractile function, oxygen consumption, and the mean response time of oxygen consumption to a step increase in heart rate [i.e., mitochondrial response time (t(mito))] in isolated Langendorff-perfused hearts from wild-type (n = 6), M/MtCK(-/-) (n = 6), and AK(-/-) (n = 4) mice. Left ventricular developed pressure was higher in M/MtCK(-/-) hearts (88.2 +/- 6.8 mmHg) and lower in AK(-/-) hearts (46.7 +/- 9.4 mmHg) compared with wild-type hearts (60.7 +/- 10.1 mmHg) at the basal pacing rate. Developed pressure fell slightly when heart rate was increased in all three groups. Basal MVO(2) at 300 beats/min was 19.1 +/- 2.4, 19.4 +/- 1.5, and 16.3 +/- 1.9 micromol x min(-1) x g dry wt(-1) for M/MtCK(-/-), AK(-/-), and wild type, respectively, which increased to 25.5 +/- 3.7, 25.4 +/- 2.6, and 22.0 +/- 2.6 micromol. min(-1) x g(-1), when heart rate was increased to 400 beats/min. The t(mito) was significantly faster in M/MtCK(-/-) hearts: 3.0 +/- 0.3 versus 7.3 +/- 0.6 and 8.0 +/- 0.4 s for M/MtCK(-/-), AK(-/-), and wild-type hearts, respectively. Our results demonstrate that MVO(2) of M/MtCK(-/-) hearts adapts more quickly to an increase in heart rate and thereby support the hypothesis that creatine kinase acts as an energy buffer in the cytosol, which delays the energy-related signal between sites of ATP hydrolysis and mitochondria.

Myocardial energetics in cardiac hypertrophy

1. This review is presented with the intent of illustrating the representative studies of functional and myocardial energetic consequences of hearts with postinfarction left ventricular (LV) remodelling or with concentric hypertrophy and diastolic LV dysfunction in porcine models. 2. Both eccentric and concentric cardiac hypertrophy are associated with the abnormal myocardial energetics that are most severe in hearts with congestive heart failure (CHF). Presently, these abnormalities cannot be satisfactorily explained to be the cause(s) of the dysfunction of failing hearts or cause the progress from compensated cardiac hypertrophy to CHF. 3. Mechanisms governing abnormal myocardial high-energy phosphate (HEP) metabolism in hearts with cardiac hypertrophy and CHF are unclear. Myocardial energy metabolism studies use both kinetic and thermodynamic models. The thermodynamic studies examine the myocardial steady state levels of high- and low-energy phosphate, which indicate myocardial energy state or phosphorylation potential that is defined by the ratio of [ATP]/([ADP][Pi]). The kinetics studies examine the reaction velocity that is regulated by: (i) quantity and activity of the key enzymes; (ii) the concentrations of all the substrates and products; and (iii) the Michaelis-Menten constants of each substrate of the reaction. 4. Significant alterations in myocardial concentrations of phosphocreatine (PCr), ATP and ADP, myocardial oxidative phosphorylation (OXPHOS) protein expression and substrate preference are found in hearts with postinfarction LV remodelling and CHF. However, to define a causal relationship is a different matter. 5. Future studies of animal models of LV hypertrophy or heart failure using gene manipulation may provide additional insights to answer the persisting question of whether limitations of ATP synthetic or transport capacities contribute to the pathogenesis of LV remodelling or failure.

Mitochondrial ATPase and high-energy phosphates in failing hearts

This study examined high-energy phosphates (HEP) and mitochondrial ATPase protein expression in hearts in which myocardial infarction resulted in either compensated left ventricular remodeling (LVR) or congestive heart failure (CHF). The response of HEP (measured via (31)P magnetic resonance spectroscopy) to a modest increase in the cardiac work state produced by dobutamine-dopamine infusion and pacing (if needed) was examined in 17 pigs after left circumflex coronary artery ligation (9 with LVR and 8 with CHF) and compared with 7 normal pigs. In hearts with LVR, the baseline phosphocreatine (PCr)-to-ATP ratio decreased, and calculated ADP increased; these changes were most severe in hearts with CHF. HEP levels did not change in normal or LVR hearts during dobutamine-dopamine infusion. However, in hearts with CHF, the PCr-to-ATP ratio decreased further, and free ADP increased. The mitochondrial protein levels of the F(0)F(1)-ATPase subunits were normal in hearts with compensated LVR. However, in failing hearts, the alpha-subunit decreased by 36%, the beta-subunit decreased by 16%, the oligomycin sensitivity-conferring protein subunit decreased by 40%, and the initiation factor 1 subunit decreased by 41%. Thus in failing hearts, reductions in mitochondrial F(0)F(1)-ATPase protein expression are associated with increased myocardial free ADP.

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.

High-energy phosphate metabolism and creatine kinase in failing hearts: a new porcine model

BACKGROUND

This study aimed to create a pig model of heart failure secondary to severe aortic stenosis and to examine the relationship between the alterations in myocardial high-energy phosphate (HEP) metabolism and protein expression of creatine kinase (CK) isoforms.

METHODS AND RESULTS

Sixteen pigs with left ventricular hypertrophy (LVH) secondary to ascending aortic banding and 10 normal pigs (N) were studied. Myocardial protein levels of CK isoforms (Western blot), HEP levels, and CK kinetics ((31)P MR spectroscopy) were measured under basal conditions. Nine of the 16 animals with LVH developed congestive heart failure (CHF), as evidenced by ascites (100 to 2000 mL). LV weight/body weight ratio (g/kg) was 2.18+/-0.15 in N hearts, 3.04+/-0.14 in hearts with LVH (P<0.01), and 4.23+/-0.36 in hearts with CHF (P<0.01 versus LVH). Right ventricle weight/body weight ratio and LV end-diastolic pressure were significantly higher in hearts with CHF (each P<0.01 versus N or LVH). Myocardial phosphocreatine/ATP ratios and the CK forward flux rates were decreased in LVH hearts, most severely in hearts with CHF. CK-M/beta-actin ratios were 2.21+/-12 (N), 1.69+/-0.15 (LVH), and 1.39+/-0.27 (CHF, P<0.05 versus N). CK-mitochondria (CK-Mt)/beta-actin ratios were 1.40+/-0.09 (N), 1.24+/-0.09 (LVH), and 1.02+/-0.08 (CHF, P<0.05 versus N or LVH). The severity of the reduction of CK flux rate was linearly related to the severity of the decrease of CK-Mt/beta-actin (r=0.68, P<0.01).

CONCLUSIONS

In this new model of heart failure/hypertrophy, the abnormal myocardial HEP metabolism is related to the decreased CK-Mt protein level, which in turn is related to the severity of the hypertrophy.

Metabolically derived potential on the outer membrane of mitochondria: a computational model

The outer mitochondrial membrane (OMM) is permeable to various small substances because of the presence of a voltage-dependent anion channel (VDAC). The voltage dependence of VDAC's permeability is puzzling, because the existence of membrane potential on the OMM has never been shown. We propose that steady-state metabolically derived potential (MDP) may be generated on the OMM as the result of the difference in its permeability restriction for various charged metabolites. To demonstrate the possibility of MDP generation, two models were considered: a liposomal model and a simplified cell model with a creatine kinase energy channeling system. Quantitative computational analysis of the simplified cell model shows that a MDP of up to -5 mV, in addition to the Donnan potential, may be generated at high workloads, even if the OMM is highly permeable to small inorganic ions, including potassium. Calculations show that MDP and DeltapH, generated on the OMM, depend on the cytoplasmic pH and energy demand rate. Computational modeling suggests that MDP may be important for cell energy metabolism regulation in multiple ways, including VDAC's permeability modulation and the effect of electrodynamic compartmentation. The osmotic pressure difference between the mitochondrial intermembrane space and the cytoplasm, as related to the electrodynamic compartmentation effects, might explain the morphological changes in mitochondria under intense workloads.

Effects of free radicals on cytosolic creatine kinase activities and protection by antioxidant enzymes and sulfhydryl compounds

The main purpose of this study was to investigate the effect of free radicals and experimental diabetes on cytosolic creatine kinase activity in rat heart, muscle and brain. Hydrogen peroxide decreased creatine kinase activity in a dose dependent manner which was reversed by catalase. Xanthine/xanthine oxidase, which produces superoxide anion, lowered the creatine kinase activity in the same manner whose effect was protected by superoxide dismutase. N-acetylcysteine and dithiothreitol also significantly ameliorated the effect of Xanthine/xanthine oxidase and hydrogen peroxide. Experimental diabetes of twenty-one days (induced by alloxan), also caused a similar decrease in the activity of creatine kinase. This led us to the conclusion that the decrease in creatine kinase activity during diabetes could be due to the production of reactive oxygen species. The free radical effect could be on the sulfhydryl groups of the enzyme at the active sites, since addition of sulfhydryl groups like N-acetylcysteine and dithiothreitol showed a significant reversal effect.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Failing energetics in failing hearts

The perpetual and vigorous nature of heart muscle work requires efficient myocardial energetics. This depends not only on adequate ATP production, but also on efficient delivery of ATP to muscle ATPases and rapid removal of ADP and other by-products of ATP hydrolysis. Indeed, recent evidence indicates that defects in communication between ATP-producing and ATP-consuming cellular sites are a major factor contributing to energetic deficiency in heart failure. In particular, the failing myocardium is characterized by reduced catalytic activity of creatine kinase, adenylate kinase, carbonic anhydrase, and glycolytic enzymes, which collectively facilitate ATP delivery and promote removal of ADP, Pi, and H+ from cellular ATPases. Although energy transfer through adenylate kinase and glycolytic enzymes has been recognized as an adaptive mechanism supporting compromised muscle energetics, in the failing myocardium the total compensatory potential of these systems is diminished. A gradual accumulation of defects at various steps in myocardial energetic signaling, along with compromised compensatory mechanisms, precipitates failure of the whole cardiac energetic system, ultimately contributing to myocardial dysfunction. These advances in our understanding of the molecular bioenergetics in heart failure provide a new perspective toward improving the energetic balance of the failing myocardium.

Age peculiarities of changes in the contractile function of isolated rat hearts during prolonged perfusion

The experiments on the isolated hearts from adult and old rats have shown that during first hours of prolonged perfusion the contractile function decreased to a greater degree in adult rats compared to the old. Following a 3-h perfusion, there occurred a marked fall of the contractile function in old animals. The creatine phosphokinase activity in a perfusate grew sharply in the old rats and changed insignificantly in the adult. The Na,K-ATPase got activated in adult rats, which was prevented by the preliminary injection of actinomycin D, and declined in the old. It appeared that adult hearts had a better endurance of prolonged perfusion and kept sustaining a definite level of performance almost twice as long as the old. Under prolonged perfusion conditions, there has been synthesized a regulatory factor (invertor) in the heart of adult animals, which reduced the myocardial contractile function. The latter was evidenced from a fact that perfusate from adult hearts 'donors' promoted the decrease of contractility of the adult hearts 'recipients'. The preliminary injection of actinomycin D to adult rats prevented a developing decrease of the contractile function but did not induce any changes in the old. This permits to conclude that synthesis of the regulatory peptide factor promotes an economization and adaptation of the adult animal heart.

Role of MgADP in the development of diastolic dysfunction in the intact beating rat heart

Sarcomere relaxation depends on dissociation of actin and myosin, which is regulated by a number of factors, including intracellular [MgATP] as well as MgATP hydrolysis products [MgADP] and inorganic phosphate [Pi], pHi, and cytosolic calcium concentration ([Ca2+]c). To distinguish the contribution of MgADP from the other regulators in the development of diastolic dysfunction, we used a strategy to increase free [MgADP] without changing [MgATP], [Pi], or pHi. This was achieved by applying a low dose of iodoacetamide to selectively inhibit the creatine kinase activity in isolated perfused rat hearts. [MgATP], [MgADP], [Pi], and [H+] were determined using 31P NMR spectroscopy. The [Ca2+]c and the glycolytic rate were also measured. We observed an approximately threefold increase in left ventricular end diastolic pressure (LVEDP) and 38% increase in the time constant of pressure decay (P < 0.05) in these hearts, indicating a significant impairment of diastolic function. The increase in LVEDP was closely related to the increase in free [MgADP]. Rate of glycolysis was not changed, and [Ca2+]c increased by 16%, which cannot explain the severity of diastolic dysfunction. Thus, our data indicate that MgADP contributes significantly to diastolic dysfunction, possibly by slowing the rate of cross-bridge cycling.