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E K Seppet

Publications and source records attributed to E K Seppet.

30 records · Page 2Linked to original sources
PubMed1987

[Energy-related disorders of myocardial contractility in calcium overload of the cardiomyocytes].

Langendorff perfusion of euthyroid rat heart with 10 mM phosphocreatine (PCr) in Krebs-Henseleit solution in conditions of Ca2+-paradox significantly potentiated the development of contracture and produced a 4-fold increase in perfusion pressure as compared to heart contractility in PCr-free, but otherwise similar conditions. Similarly, in case of hyperthyroid myocardium, addition of PCr into hypoxic perfusate (glucose omitted) in the presence of supranormal extracellular Ca2+ (5.0 mM) irreversibly inhibited the restitution of resting tension and developed tension during aerobic reperfusion. In the same conditions, PCr did not modulate contractility of euthyroid myocardium. These results are consistent with Ruigrok's hypothesis (1978) that intracellular Ca2+ redistribution is energy-dependent, provided PCr can penetrate into cardiomyocytes and stimulate ATP-dependent accumulation of Ca2+ in mitochondria, leading to an impairment of energy production. It is concluded that PCr exerts its cardioprotective effect only in case of a reversible (acute) phase of ischemia.

Animals↗
PubMed1985

Hormone regulation of cardiac energy metabolism. I. Creatine transport across cell membranes of euthyroid and hyperthyroid rat heart.

Hyperthyroid rat heart was studied with the purpose of identifying the mechanism for the significant decrease in total creatine (free creatine plus phosphocreatine) observed in this pathology and its consequences on heart function. Administration of L-thyroxine in doses of 50-100 micrograms/100 g of body weight during a week resulted in a reversible decrease of the total creatine by 40-50%. Simultaneously, remarkable changes in the creatine transport system across the cardiac cell membranes were observed: both the maximal rate of its active uptake and its passive movement along its concentration gradient were enhanced. In euthyroid hearts, the parameters of creatine uptake (Km approximately or equal to 0.05 mM, Vmax = 20 nmole/min/g dry weight) were similar to those for skeletal muscle and the passive movement of creatine was negligible. In hyperthyroid hearts the latter rate was enhanced to 0.4 mumole min/g dry weight, this showing reversible damages in the cell membrane structure induced by L-thyroxine. This conclusion is consistent with observed penetration of colloidal lanthanum into the cells of hyperthyroid hearts. Perfusion of hyperthyroid rat hearts with 50 mM creatine significantly restored creatine content in the cells, Hyperthyroid hearts with decreased creatine content were found to develop ischemic contracture more rapidly and in higher extent than the euthyroid hearts. Increased sensitivity to ischemic damage may be related to decreased efficiency of energy channeling via phosphocreatine pathway.

Animals↗
PubMed1980

Phosphocretine production coupled to the glycolytic reactions in the cytosol of cardiac cells.

Phosphocreatine production ctalyzed by a cytosolic fraction from cardiac muscle containing all glycolytic enzymes and creatine kinase in a soluble form has been studied in the presence of creatine, adenine nucleotides and different glycolytic intermedites as substrates. Glycolytic depletion of glucose, fructose 1,6bis(phosphate) and phosphoenolpyruvate to lactte was coupled to efficient phosphocreatine production. The molar ratio of phosphocreatine to lactate produced was close to 2.0 when fructose 1,6bis(phosphate) was used as substrate and 1.0 with phosphoenolpyruvate. In these processes the creatine kinase reaction was not the rate-limiting step: themass action ratio of the creatine kinase reaction was very close to its equilibrium value and the maximal rate of the forward creatine kinase reaction exceeded that of glycolytic flux by about 6-fold when fructose 1,6-bis(phosphate) was used as a substrate. Therefore, the creatine kinase raction was continuously in the state of quasi-equilibrium and the efficient syntheses of phosphocreatine observed is a result of constant removal of ADP by the glycolytic system at an almost unchanged level of ATP ([ATP]>>[ADP]), this leading to a continuous shift of the creatine kinase equilibrium position. When phosphocreatine was added initially at concentrations of 5---15 mM the rate of the coupled creatine kinase and glycolytic reactions was very significantly inhibited due to a sharp decrease in the steady-state concentration of ADP. Therefore, under conditions of effective phosphocreatine production in heart mitochondria, which maintain a high phosphocreatine: creatine ratio in the myoplasm in vivo, the glycolytic flux may be suppressed due to limited availability of ADP restricted by the creatine kinase system. The possible physiological role of the control of the glycolytic flux by the creatine kinase system is discussed.

Creatine Kinase↗
PubMed1979

[Steady state kinetics of pyruvate kinase from muscle].

The dependence of the forward pyruvate kinase reaction on the concentrations of ADP and Mg2+ was studied. It was shown that high total ADP concentrations (2-15 mM) exceeding total Mg2+ concentration in the medium lead to the inhibition of the pyruvate kinase reaction, whereas relatively high Mg2+ concentrations (up to 15 mM) do not cause any inhibition. The kinetics of the reaction can be described in the best way by a scheme incorporating the active PEP . E . Mg2+ . Mg . ADP complex and dead-end complexes containing free ADP. An analysis of the experimental data allows to determine all coefficients of the rate equation and to calculate the values of all kinetic parameters. The values of the constants obtained were used for mathematical simulation of the reaction on the basis of the kinetic scheme given. The mathematical model obtained describes satisfactorily the experimentally determined dependences, which is indicative of the correctness of the model.

Adenosine Diphosphate↗
PubMed1978

[Changes in the adrenal cortex during adaptation to different regimes of motor activity].

The hypertrophy of the adrenal glands was higher in the rats adapted to long-lasting exercises of moderate and mild intensities. Ultrastructural analysis showed that there were more mitochondria and vesicular cristae as well as the elements of cytoplasmatic reticulum and lipid droplets in the cells of adrenal cortex of the adapted rats. The data obtained suggest two types of adaptational changes in the adrenal cortex: 1) pronounced hypertrophy with low spontaneous functional activity and increased content of cytochrome a--a3; 2) less pronounced hypertrophy with higher content of corticosterone and lower content of cytochrome a--a3 in adrenals.

Adaptation, Physiological↗
PubMed1978

[Creatine phosphate synthesis coupled to glycolytic reactions in heart cell cytosol].

It is shown that a cytosolic fraction of proteins from rat heart, containing all glycolytic enzymes and creatine phosphokinase (CPK) in natural proportions, catalyses in the presence of creatine (Cr) the intensive creatine phosphate (CP) synthesis coupled to glycolytic depletion of glucose, fructose-1,6-diphosphate (FDP) and phosphoenolpyruvate (PEP). The ratio of CP to lactate produced was 1.0 in the case of PEP as glycolytic substrates. In the CP production process the CPK reaction was not the rate limiting step: the mass action ratio for CPK was equal to the apparent equilibrium constant for this reaction, and the maximal rate of the CPK reaction exceeded the observed rate of glycolysis by a factor 6. It is concluded that in such a system the components of the CPK reaction are in quasiequilibrium and the mechanism of CP synthesis can be considered to be a continuous decrease in ADP concentration due to its phosphorylation in the glycolytic reactions, ATP concentration being constant due to its continuous utilization and resynthesis, this leading to an increase in CP concentration. The possible role of the CPK system in regulation of glycolysis in muscle cells is discussed.

Adenine Nucleotides↗
PubMed1977

[A comparative study of the role of creatine phosphokinase isoenzymes in energy metabolism of skeletal and heart muscle].

It has been shown that the contents of mitochondria and mitochondrial isoenzyme of creatine phosphokinase are almost identical in skeletal and heart muscles. In mitochondria from both types of muscle creatine phosphokinase is functionally coupled to ATP-ADP translocase. This kind of coupling ensures complete conversion of mitochondrial ATP energy into the energy of creatine phosphate and effective control of oxidative phosphorylation by the creatine phosphokinase reaction. It has also been shown that all isoenzymes of creatine phosphokinase from heart and skeletal muscle have very similar kinetic properties. Significant differences have been found to exist between isoenzyme patterns of these muscles and also in distribution of different isoenzymes in the cells. In skeletal muscle cells creatine phosphokinase is present mainly as cytosolic MM isoenzyme; about 6% of total cellular activity is localised also in mitochondria. Due to high activity of cytosolic isoenzyme the total activity of creatine phosphokinase is about three times higher in skeletal muscle than in cardiac muscle. It has been also shown that phosphoenolpyruvate and glucose-6-phosphate do not have any inhibitory effect on creatine phosphokinase.

Animals↗
PubMedDate not supplied

Characteristics of Ca2(+)-stimulated ATPase in rat heart sarcolemma in the presence of dithiothreitol and alamethicin.

We have studied the activities of Ca2(+)-stimulated ATPase in rat heart sarcolemma upon modulating the redox state of membrane thiol groups with dithiothreitol (DTT). The suitability of alamethicin to unmask the latent activity of this enzyme was also investigated. The Ca2(+)-stimulated ATPase in sarcolemma exhibited two activation sites--one with low affinity (Km = 0.70 +/- 0.2 mM; Vmax = 10.0 +/- 2.2 mumol Pi/mg/h) and the other with high affinity (Km = 0.16 +/- 0.7 mM; Vmax = 4.6 +/- 0.8 mumol Pi/mg/h) for Mg2+ ATP. Alamethicin at a ratio of 1:1 with the sarcolemmal protein caused a 3-fold activation of Ca2(+)-stimulated ATPase without affecting its sensitivity to Ca2+ or Mg2+ ATP. Treatment of sarcolemma with deoxycholate or sodium dodecyl sulfate resulted in a total loss of the enzyme activity; high concentrations of alamethicin also showed a detergent-like action on the sarcolemmal vesicles. DTT at 5-10 mM concentrations caused a 4-5 fold activation of Ca2(+)-stimulated ATPase in sarcolemma and this effect was observed to be dependent on the concentration of Mg2+ ATP. DTT increased the affinity of the enzyme to Mg2+ ATP at the high affinit site and enhanced the Vmax at the low affinity site in addition to increasing the sensitivity of Ca2(+)-stimulated ATPase to Ca2+. DTT protected the Ca2(+)-stimulated ATPase against deterioration by detergents and restored the enzyme activity after treatment with N-ethylmaleimide. The mechanism of action of DTT on Ca2(+)-stimulated ATPase may involve the reduction of essential thiols at the active site of the enzyme or its interaction with specific DTT-dependent inhibitor protein. No changes in the sensitivity of sarcolemmal Ca2(+)-stimulated ATPase to orthovanadate was evident in the absence or presence of DTT and alamethicin. The results suggest the use of both DTT and alamethicin for the determination of Ca2(+)-stimulated ATPase activity in sarcolemmal preparations.

Alamethicin↗
PubMedDate not supplied

Thyroid hormones and the creatine kinase system in cardiac cells.

The paper reviews the current evidence on the role of thyroid hormones in regulating the creatine kinase energy transfer system at multiple structures in cardiac cells. 1) Thyroid hormones modulate the overall synthesis of phosphocreatine (PCr) by increasing the rate of mitochondrial oxidative phosphorylation. 2) Thyroid hormones regulate the total activity of creatine kinase and its isoenzyme distribution. In comparison with normal thyroid state (euthyroidism), hypothyroidism is characterized by decreased total creatine kinase activity owing to diminished fraction of creatine kinase. On the other hand, hyperthyroidism, while causing no change in total creatine kinase activity, leads to increased fractions of neonatal isoforms of creatine kinase, and, in case of prolonged hyperthyroidism, to decreased fraction of mitochondrial creatine kinase. The latter change is associated with partial uncoupling between mitochondrial creatine kinase and adenine nucleotide translocase reflected by decreased PCr/O ratio. 3) Hyperthyroidism leads to increased passive sarcolemmal permeability due to which the leakage of creatine along its concentration gradient occurs. As a result of (i) increased sarcolemmal permeability for creatine, (ii) uncoupling of mitochondrial PCr synthesis, and (iii) increased energy utilization rate the steady state intracellular PCr content decreases under hyperthyroidism which, in turn, increases the myocardial susceptibility to hypoxic damage. Thyroid state also modulates the protective effects of exogenous PCr on energetically depleted myocardium.

Animals↗
PubMedDate not supplied

Functional coupling as a basic mechanism of feedback regulation of cardiac energy metabolism.

In this review we analyze the concepts and the experimental data on the mechanisms of the regulation of energy metabolism in muscle cells. Muscular energetics is based on the force-length relationship, which in the whole heart is expressed as a Frank-Starling law, by which the alterations of left ventricle diastolic volume change linearly both the cardiac work and oxygen consumption. The second basic characteristics of the heart is the metabolic stability--almost constant levels of high energy phosphates, ATP and phosphocreatine, which are practically independent of the workload and the rate of oxygen consumption, in contrast to the fast-twitch skeletal muscle with no metabolic stability and rapid fatigue. Analysis of the literature shows that an increase in the rate of oxygen consumption by order of magnitude, due to Frank-Starling law, is observed without any significant changes in the intracellular calcium transients. Therefore, parallel activation of contraction and mitochondrial respiration by calcium ions may play only a minor role in regulation of respiration in the cells. The effective regulation of the respiration under the effect of Frank-Starling law and metabolic stability of the heart are explained by the mechanisms of functional coupling within supramolecular complexes in mitochondria, and at the subcellular level within the intracellular energetic units. Such a complex structural and functional organisation of heart energy metabolism can be described quantitatively by mathematical models.

Adenosine Triphosphate↗