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PubMed1999

Interaction of factors determining oxygen uptake at the onset of exercise.

Considerable debate surrounds the issue of whether the rate of adaptation of skeletal muscle O2 consumption (QO2) at the onset of exercise is limited by 1) the inertia of intrinsic cellular metabolic signals and enzyme activation or 2) the availability of O2 to the mitochondria, as determined by an extrinsic inertia of convective and diffusive O2 transport mechanisms. This review critically examines evidence for both hypotheses and clarifies important limitations in the experimental and theoretical approaches to this issue. A review of biochemical evidence suggests that a given respiratory rate is a function of the net drive of phosphorylation potential and redox potential and cellular mitochondrial PO2 (PmitoO2). Changes in both phosphorylation and redox potential are determined by intrinsic metabolic inertia. PmitoO2 is determined by the extrinsic inertia of both convective and diffusive O2 transport mechanisms during the adaptation to exercise and the rate of mitochondrial O2 utilization. In a number of exercise conditions, PmitoO2 appears to be within a range capable of modulating muscle metabolism. Within this context, adjustments in the phosphate energy state of the cell would serve as a cytosolic "transducer," linking ATP consumption with mitochondrial ATP production and, therefore, O2 consumption. The availability of reducing equivalents and O2 would modulate the rate of adaptation of QO2.

Animals↗
PubMedDate not supplied

Low glucocorticoid concentrations decrease oxidative phosphorylation of isolated rat brain mitochondria: an additional effect of dexamethasone.

The effects of hydrocortisone, triamcinolone, prednisolone and dexamethasone have been investigated in vitro using mitochondria isolated from rat brain. Respiratory control ratio (RCR), oxygen consumption, ATP synthesis, enzymatic activities of involved complexes and superoxide anion generation have been measured to assess the effects of these drugs. Our data showed that the decrease of RCR induced by glucocorticoids was due to a common inhibition of oxidative phosphorylation (State 3) and of complex V activity and a modification of the proton-fluxes through the mitochondrial inner membrane. These effects were quantitatively limited, since they occurred at concentrations lower than 2 nM. Dexamethasone was the only one able to induce a specific inhibition of complex I activity and to decrease the superoxide anion radical generation. Inhibition of complex V and partial reversion of uncoupling seem to be common properties of glucocorticoids. The theoretical consequence of these inhibitions could be the modulation of the mitochondrial function, oxygen consumption rate, ATP synthase activity and superoxide anion radical production, involved in many patho-physiological phenomena.

Animals↗
PubMed2003

Response of plant metabolism to too little oxygen.

Oxygen can fall to low concentrations within plant tissues, either because of environmental factors that decrease the external oxygen concentration or because the movement of oxygen through the plant tissues cannot keep pace with the rate of oxygen consumption. Recent studies document that plants can decrease their oxygen consumption in response to low oxygen concentrations to avoid internal anoxia. This adaptive response involves a restriction of respiration and a concomitant decrease in ATP consumption that results from the inhibition of a wide range of biosynthetic processes. The inhibition of respiration is rapid and occurs at oxygen concentrations well above the K(m)(oxygen) of cytochrome oxidase, indicating that an oxygen-sensing system triggers a coordinated inhibition of ATP formation and consumption. In addition to this, low oxygen concentrations lead to the induction of a plant-specific and energy-conserving pathway of sucrose degradation, which decreases oxygen consumption and improves plant performance. Low oxygen concentrations also lead to long-term morphological adaptations, which allow respiration per volume tissue to be decreased and oxygen entry to be increased. Recently, advances have been made in elucidating possible oxygen-sensing systems and regulatory components that are involved in these responses.

Adaptation, Physiological↗
PubMed1996

Role of adenosine 5'-triphosphate hydrolysis in the assembly of the bacteriophage T4 DNA replication holoenzyme complex.

Steady-state and pre-steady-state rates of ATP hydrolysis by the 44/62 accessory protein were determined to elucidate the role of ATP hydrolysis in bacteriophage T4 holoenzyme complex formation. Steady-state ATPase measurements of the 44/62 protein under various combinations of 45 protein, DNA substrate, and T4 exo- polymerase indicate that although the 44/62 protein synergistically hydrolyzes ATP in the presence of 45 protein and DNA substrate, the ATPase activity of 44/62 is diminished substantially upon the formation of the holoenzyme complex. The decrease in activity is primarily in kcat while the K(m) for ATP is changed unsubstantially by the various combinations. Data suggest that the decrease in the rate of ATP hydrolysis upon the addition of T4 exo- polymerase in the presence of 45 protein and DNA substrate is due to formation of a stable holoenzyme complex consisting of only the 45 protein and T4 exo- polymerase in a 1:1 ratio. The 44/62 protein acts catalytically to load 45 protein onto the DNA substrate and does not remain a component of the holoenzyme complex. Pre-steady-state kinetic analysis of the ATP hydrolysis reaction catalyzed by the 44/62 protein loading the 45 protein onto the DNA substrate in the absence or presence of polymerase is biphasic, in which a burst in ATP hydrolysis precedes the steady-state rate of ATP hydrolysis. An identical burst in ATP consumption is obtained under either condition, indicating that ATP hydrolysis is not required to load polymerase into the holoenzyme complex. The data suggest one turnover of ATP at each of the four ATPase active sites of the 44/62 protein per 45 protein loaded. ATP hydrolysis by the 44/62 protein under conditions of holoenzyme complex formation is the rate-limiting step in holoenzyme complex formation. The process of holoenzyme formation appears to be identical for leading and lagging strand synthesis.

Adenosine Triphosphatases↗
PubMed1986

The energy budget of Tetrahymena and the material fluxes into and out of the adenylate pool.

The material budget of the adenylate pool deals with all processes which physically establish and maintain this pool, while the energy budget is concerned with the intracompartmental ATP recycling. Both budgets were analysed in Tetrahymena thermophila exposed to various energy and material demands. Some of the general conclusions are: at a maximum growth rate the overall ATP consumption during one cell cycle is 10(-10) mol ATP; the contribution of osmoregulation and ciliary motion to the budget is about 1% each; at zero net growth, energy is consumed because of a continuous recycling of matter between the monomer and the polymer compartment. The rate of ATP production is about 1000-fold greater than the rate of adenylate monomer influx. The residence time of adenylate monomers within the pool is about 30 min, but for ATP molecules it is only 2 sec.

Adenosine Diphosphate↗
PubMed1999

Isoflurane alters proximal tubular cell susceptibility to toxic and hypoxic forms of attack.

BACKGROUND: Fluorinated anesthetics can profoundly alter plasma membrane structure and function, potentially impacting cell injury responses. Because major surgery often precipitates acute renal failure, this study assessed whether the most commonly used fluorinated anesthetic, isoflurane, alters tubular cell responses to toxic and hypoxic attack. METHODS: Mouse proximal tubule segments were incubated under control conditions or with a clinically relevant isoflurane dose. Cell viability (lactate dehydrogenase release), deacylation (fatty acid, such as C20:4 levels), and adenosine triphosphate (ATP) concentrations were assessed under one or more of the following conditions: (a) exogenous phospholipase A2 (PLA2) or C20:4 addition, (b) Ca2+ overload (A23187 ionophore), (c) increased metabolic work (Na ionophore), and (d) hypoxia- or antimycin A-induced attack. Isoflurane's effect on NBD phosphatidylserine uptake (an index of plasma membrane aminophospholipid translocase activity) was also assessed. RESULTS: Isoflurane alone caused trivial deacylation and no lactate dehydrogenase release. However, it strikingly sensitized to both PLA2- and A23187-induced deacylation and cell death. Isoflurane also exacerbated C20:4's direct membrane lytic effect. Under conditions of mild ATP depletion (Na ionophore-induced increased ATP consumption; PLA2-induced mitochondrial suppression), isoflurane provoked moderate/severe ATP reductions and cell death. Conversely, under conditions of maximal ATP depletion (hypoxia, antimycin), isoflurane conferred a modest cytoprotective effect. Isoflurane blocked aminophospholipid translocase activity, which normally maintains plasma membrane lipid asymmetry (that is, preventing its "flip flop"). CONCLUSIONS: Isoflurane profoundly and differentially affects tubular cell responses to toxic and hypoxic attack. Direct drug-induced alterations in lipid trafficking/plasma membrane orientation and in cell energy production are likely involved. Although the in vivo relevance of these findings remains unknown, they have potential implications for intraoperative renal tubular cell structure/function and how cells may respond to superimposed attack.

Adenosine Triphosphate↗
PubMedDate not supplied

Mitochondria and Ca(2+)in cell physiology and pathophysiology.

There is now a consensus that mitochondria take up and accumulate Ca(2+)during physiological [Ca(2+)](c)signalling. This contribution will consider some of the functional consequences of mitochondrial Ca(2+)uptake for cell physiology and pathophysiology. The ability to remove Ca(2+)from local cytosol enables mitochondria to regulate the [Ca(2+)] in microdomains close to IP3-sensitive Ca(2+)-release channels. The [Ca(2+)] sensitivity of these channels means that, by regulating local [Ca(2+)](c), mitochondrial Ca(2+)uptake modulates the rate and extent of propagation of [Ca(2+)](c)waves in a variety of cell types. The coincidence of mitochondrial Ca(2+)uptake with oxidative stress may open the mitochondrial permeability transition pore (mPTP). This is a catastrophic event for the cell that will initiate pathways to cell death either by necrotic or apoptotic pathways. A model is presented in which illumination of an intramitochondrial fluorophore is used to generate oxygen radical species within mitochondria. This causes mitochondrial Ca(2+)loading from SR and triggers mPTP opening. In cardiomyocytes, mPTP opening leads to ATP consumption by the mitochondrial ATPase and so results in ATP depletion, rigor and necrotic cell death. In central mammalian neurons exposed to glutamate, a cellular Ca(2+)overload coincident with NO production also causes loss of mitochondrial potential and cell death, but mPTP involvement has proven more difficult to demonstrate unequivocally.

Biological Transport↗
PubMed2001

Mitochondrial respiratory chain adjustment to cellular energy demand.

Because adaptation to physiological changes in cellular energy demand is a crucial imperative for life, mitochondrial oxidative phosphorylation is tightly controlled by ATP consumption. Nevertheless, the mechanisms permitting such large variations in ATP synthesis capacity, as well as the consequence on the overall efficiency of oxidative phosphorylation, are not known. By investigating several physiological models in vivo in rats (hyper- and hypothyroidism, polyunsaturated fatty acid deficiency, and chronic ethanol intoxication) we found that the increase in hepatocyte respiration (from 9.8 to 22.7 nmol of O(2)/min/mg dry cells) was tightly correlated with total mitochondrial cytochrome content, expressed both per mg dry cells or per mg mitochondrial protein. Moreover, this increase in total cytochrome content was accompanied by an increase in the respective proportion of cytochrome oxidase; while total cytochrome content increased 2-fold (from 0.341 +/- 0.021 to 0.821 +/- 0.024 nmol/mg protein), cytochrome oxidase increased 10-fold (from 0.020 +/- 0.002 to 0.224 +/- 0.006 nmol/mg protein). This modification was associated with a decrease in the overall efficiency of the respiratory chain. Since cytochrome oxidase is well recognized for slippage between redox reactions and proton pumping, we suggest that this dramatic increase in cytochrome oxidase is responsible for the decrease in the overall efficiency of respiratory chain and, in turn, of ATP synthesis yield, linked to the adaptive increase in oxidative phosphorylation capacity.

Adenosine Triphosphate↗
PubMed2004

Genetic manipulation of calcium-handling proteins in cardiac myocytes. II. Mathematical modeling studies.

We developed a mathematical model specific to rat ventricular myocytes that includes electrophysiological representation, ionic homeostasis, force production, and sarcomere movement. We used this model to interpret, analyze, and compare two genetic manipulations that have been shown to increase myocyte relaxation rates, parvalbumin (Parv) de novo expression, and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) overexpression. The model was used to seek mechanistic insights into 1) the relative contribution of two mechanisms by which SERCA2a overexpression modifies Ca2+ sequestration, i.e., more pumps and an increase in the SERCA2a-to-phospholamban ratio, 2) the mechanisms behind postrest potentiation and how Parv and SERCA2a influence this response, and 3) why Parv myocytes retain their fast kinetics when endogenous SERCA2a is partially impaired by thapsigargin (a condition used to mimic diastolic dysfunction). The model was also utilized to predict whether Parv metal-binding characteristics might be modified to improve diastolic and systolic functions and whether Parv or SERCA2a might affect diastolic Ca2+ levels and myocyte energetics. One outcome of the model was to demonstrate a higher peak and total ATP consumption in SERCA2a myocytes and more even distribution of ATP throughout the cardiac cycle in Parv myocytes. This may have implications for failing hearts that are energetically compromised.

Animals↗
PubMed1996

Membrane-metabolic coupling and ion homeostasis in anoxia-tolerant and anoxia-intolerant hepatocytes.

The relationship between membrane function and energy metabolism was studied in rainbow trout hepatocytes, an anoxia-intolerant cell system, and compared with the situation in hepatocytes from the goldfish, a typical anoxia-tolerant species. In trout hepatocytes, under normoxia and under chemical anoxia, inhibition of ATP consumption by the Na+ pump induced a decrease in ATP production of the same magnitude. In response to chemical anoxia, total ATP production was reduced to 15% and Na+ pump activity to 22% of the control rate under normoxia. Measurement of the cellular ATP content under these conditions revealed that, despite the reduction in Na+ pump activity, the cells became rapidly depleted of ATP, with the time course of this process resembling that observed in the anoxic rat hepatocyte. This is in contrast to the responses of goldfish hepatocytes, where, during chemical anoxia, 1) inhibition of the Na+ pump did not lead to a corresponding reduction in ATP production and 2) ATP levels, after a transient decrease, stabilized at a new steady state. To investigate the consequences of chemical anoxia on ion homeostasis, efflux and uptake rates of K+ were determined simultaneously. In the trout cells, chemical anoxia led to a decoupling of influx and efflux rates, the latter exceeding the former three- to eightfold. In contrast, goldfish hepatocytes were able to preserve ion homeostasis by a concerted decrease in Rb+ uptake and K+ efflux, so that the net flux of K+ was always close to zero. In neither species did chemical anoxia induce a change in pump density. Other potential control mechanisms are briefly discussed.

Adenosine Triphosphate↗
PubMed1996

[Cardiovascular effects of thyroid hormones].

The heart is a major target organ for thyroid hormone action, and marked changes occur in cardiac function in patients with hypothyroidism or hyperthyroidism. Triiodothyronine (T3)-induced changes in cardiac function can result from direct or indirect T3 effects. Direct T3 effects result from T3 action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T3 effects, which occur independently of nuclear T3 receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T3 effects are mediated by the binding of T3 to specific nuclear receptor proteins, which results in increased transcription of T3-responsive cardiac genes. The T3 receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. T3 increases the heart transcription of the myosin heavy chain (MHC) alpha gene and decreases the transcription of the MHC beta gene, leading to an increase of myosin V1 and a decrease in myosin V3 isoenzymes. Myosin V1, which is composed of two MHC alpha, has a higher myosin ATPase activity than myosin V3, which contains two MHC beta. The globular head of myosin V1, with its higher ATPase activity, leads to a more rapid movement of the globular head of myosin along the thin filament, resulting in an increased velocity of contraction. T3 also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum (SR). This T3 effect results from T3-induced increases in the level of the mRNA coding for the SR calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the SR. Overall, T3 leads to an increase in ATP consumption in the heart. In addition, less chemical energy of ATP is used for contractile purposes and more of it goes toward heat production, which causes a decreased efficiency of the contractile process in the hyperthyroid heart. The pathophysiologic basis for myxedema is the opposite of that discussed for the hyperthyroid heart. In addition to decreased direct effects of thyroid hormone in cardiac myocytes, indirect effects occur through decreases in peripheral oxygen consumption and changes in hemodynamic parameters. Myofibrillar swelling with loss of striation and interstitial fibrosis occurs on histologic examination of hypothyroid hearts. In addition, accumulation of mucopolysaccharide substances (Glycosaminoglycans) can be demonstrated. On electron microscopic examination, mitochondria show disruption and lipid inclusion. Cardiac papillary muscle obtained from animals with hypothyroidism shows a depression of the force velocity curve and reduced rate of tension development, indicating significant contractile abnormalities. In patients with hypothyroidism, a true enhanced incidence of hypertension (increased peripheral vascular resistance) has been found. In addition, hypercholesterolemia and impairment of fatty acid mobilization are associated with myxedema and present additional risk factors for the development of atherosclerotic cardiovascular disease.

Animals↗
PubMed1990

Biochemical basis of thyroid hormone action in the heart.

Thyroid hormone-induced changes in cardiac function have been recognized for over 150 years; however, the biochemical basis of triiodothyronine (T3) action in the heart has been intensely investigated only during the last two decades. T3-induced changes in cardiac function can result from direct or indirect T3 effects. Direct T3 effects result from T3 action in the heart itself and are mediated by nuclear or extranuclear mechanisms. Extranuclear T3 effects, which occur independent of nuclear T3 receptor binding and increases in protein synthesis, influence primarily the transport of amino acids, sugars, and calcium across the cell membrane. Nuclear T3 effects are mediated by the binding of T3 to specific nuclear receptor proteins, which results in increased transcription of T3-responsive cardiac genes. The T3 receptor is a member of the ligand-activated transcription factor family and is encoded by cellular erythroblastosis A (c-erb A) genes. The c-erb A protein is the cellular homologue of the viral erythroblastosis A (v-erb A) protein, which causes red cell leukemia in chickens. Currently, three T3-binding isoforms of the c-erb protein and two non-T3-binding nuclear proteins that exert positive and negative effects on T3-responsive cardiac genes have been identified. T3 increases the heart transcription of the myosin heavy chain (MHC) alpha gene and decreases the transcription of the MHC beta gene, leading to an increase of myosin V1 and a decrease in myosin V3 isoenzymes. Myosin V1, which is composed of two MHC alpha, has a higher myosin ATPase activity than myosin V3, which contains two MHC beta. The globular head of myosin V1, with its higher ATPase activity, leads to a more rapid movement of the globular head of myosin along the thin filament, resulting in an increased velocity of contraction. T3 also leads to an increase in the speed of diastolic relaxation, which is caused by the more efficient pumping of the calcium ATPase of the sarcoplasmic reticulum (SR). This T3 effect results from T3-induced increases in the level of the mRNA coding for the SR calcium ATPase protein, leading to an increased number of calcium ATPase pump units in the SR. Overall, thyroid hormone leads to an increase in ATP consumption in the heart. In addition, less chemical energy of ATP is used for contractile purposes and more of it goes toward heat production, which causes a decreased efficiency of the contractile process in the hyperthyroid heart.

Heart↗
PubMed1992

Mechanisms for anoxic survival in the vertebrate brain.

When energy supplies to the mammalian brain are significantly reduced by anoxia, for a very short time energy requirements are curtailed by such routes as the suppression of synaptic transmission, while energy supply is enhanced by an increase in cerebral blood flow and an increase in glycolysis. The reduction of ATP consumption is insufficient to match the greatly curtailed supply of ATP coming from anaerobic glycolysis and the hydrolysis of PCr, and within minutes ATP falls, there is a loss of ion homeostasis with depolarization, and cell death occurs. The anoxia-tolerant species, like the turtle, appear to employ similar mechanisms to reduce energy expenditure, but in addition to such means as increases in inhibitory neurotransmitters and the manipulation of ion channel activities, they are able to reduce the energy costs to a level that can be met by a greatly reduced supply from anaerobic glycolysis. In this way ATP levels are maintained for many hours, and anoxic depolarization, with its concomitant consequences such as an uncontrolled release of excitatory amino acids are avoided. The greater anoxic tolerance of the mammalian neonate brain is due in part to intrinsic lower metabolic requirements and, perhaps through mechanisms similar to those in the turtle, to suppress metabolic demand. Studies of the survival mechanisms of anoxia-tolerant brains of such species as the turtle and crucian carp are not only of value for investigating a remarkable neuronal adaptation, but they promise to provide a valuable model for the study of the etiology of hypoxic damage and survival strategies in the mammal.

Animals↗
PubMed1990

Quantification of aerobic energy turnover in epididymal bull spermatozoa.

Turnover rates of oxidative energy metabolism were measured as oxygen consumption in untreated and caffeine-stimulated epididymal bull spermatozoa respiring with lactate. Incubation of spermatozoa with 1 mM caffeine led to an increase in respiration of approx. 60%. The rate of uncoupled respiration and the vanadate-insensitive part of oxygen consumption were not affected by caffeine. The small effect of ouabain on respiration (-10%) indicated a minor contribution of Na+/K+-ATPase to the ATP consumption. The major part of ATP turnover was caused by motility shown by the strong linear correlation between respiration and motility in untreated and caffeine-treated spermatozoa. In comparison with ejaculated spermatozoa investigated in a previous study, epididymal cells exhibited the same rates of uncoupled and ouabain-sensitive respiration. The efficiency of transforming mitochondrially-produced ATP into cell motion was the same in epididymal and ejaculated spermatozoa. The ATP-producing capacity of sperm mitochondria was utilized in untreated epididymal, in caffeine-stimulated epididymal and in ejaculated spermatozoa, by 20-25%, 40-45% and 45-50%, respectively. The results showed that the capacity of mitochondrial. ATP formation remains unchanged after ejaculation and is utilized to a higher extent by stimulated motility. Treatment with caffeine affected epididymal spermatozoa in a similar manner.

Aerobiosis↗
PubMed1994

Hyperthyroidism stimulates mitochondrial proton leak and ATP turnover in rat hepatocytes but does not change the overall kinetics of substrate oxidation reactions.

Thyroid hormones have well-known effects on oxidative phosphorylation, but there is little quantitative information on their important sites of action. We have used top-down elasticity analysis, an extension of metabolic control analysis, to identify the sites of action of thyroid hormones on oxidative phosphorylation in rat hepatocytes. We divided the oxidative phosphorylation system into three blocks of reactions: the substrate oxidation subsystem, the phosphorylating subsystem, and the mitochondrial proton leak subsystem and have identified those blocks of reactions whose kinetics are significantly changed by hyperthyroidism. Our results show significant effects on the kinetics of the proton leak and the phosphorylating subsystems. Quantitative analyses revealed that 43% of the increase in resting respiration rate in hyperthyroid hepatocytes compared with euthyroid hepatocytes was due to differences in the proton leak and 59% was due to differences in the activity of the phosphorylating subsystem. There were no significant effects on the substrate oxidation subsystem. Changes in nonmitochondrial oxygen consumption accounted for -2% of the change in respiration rate. Top-down control analysis revealed that the distribution of control over the rates of mitochondrial oxygen consumption, ATP synthesis and consumption, and proton leak and over mitochondrial membrane potential (delta psi m) was similar in hepatocytes from hyperthyroid and littermate-paired euthyroid controls. The results of this study include the first complete top-down elasticity and control analyses of oxidative phosphorylation in hepatocytes from hyperthyroid rats.

Adenosine Triphosphate↗
PubMed1985

Endogenous metabolism by sperm in response to altered cellular ATP requirements.

A quantitative description of the relative importance of endogenous metabolism to overall ATP production has not been established for mammalian cells. We report herein results of experiments using sperm (selected because of their simple metabolic potential and absence of biosynthetic pathways) and calorimetry (chosen because it serves as a general monitor of metabolism) to assess the importance of endogenous metabolism to total ATP synthesis under several incubation conditions. In experiments in which sperm were incubated at different temperatures (20 degrees C, 25 degrees C, 30 degrees C or 35 degrees C) and with different substrates (glucose, fructose, lactate, or beta-hydroxybutyrate), endogenous metabolism occurred at a constant rate regardless of the rate of ATP turnover in the cells or the nature of the exogenous substrate available to them. Sperm incubated at 35 degrees C with glycolyzable substrates synthesized more ATP (9 mumol ATP . h-1/10(8) cells) than did sperm incubated with the nonglycolyzable substrate, lactate (6.2 mumol ATP . h-1/10(8) cells). To investigate this substrate-related difference in the rate of ATP synthesis, the motility of sperm incubated at 35 degrees C with glucose or with lactate was determined. The velocities of the sperm incubated with either substrate were identical, indicating that the rates of ATP consumption for support of motility were identical. Most of the additional ATP synthesized by cells with glycolyzable substrates was consumed in the process of substrate cycling of the metabolic intermediates of glucose.

3-Hydroxybutyric Acid↗
PubMed1982

Aging of the erythrocyte. XIV. ATP content does decrease.

ATP content shows a cell-age dependent decrease in bovine erythrocytes separated according to Murphy. An important point in the evaluation of results lies in considering differences in the ATP consumption between cells of various age during separation. The decrease in ATP occurs in spite of diminished ATPase activity in older cells.

Adenosine Triphosphate↗
PubMed1977

Adenine nucleotide metabolism of blood platelets. X. Formaldehyde stops centrifugation-induced secretion after A23187-stimulation and causes breakdown of metabolic ATP.

A23187 induced shape change, aggregation and secretion of platelets in plasma. When rapid cooling was used to stop secretion and centrifugation to separate the cells from the medium, maximal amounts of storage ATP plus ADP and preadsorbed [14C]serotonin were found in the supernatant immediately (less than 5 s) after A23187 addition. These results suggested that A23187 could cause shape change and aggregation through secreted ADP and not directly. When secretion was stopped with chilling and formaldehyde treatment before centrifugation, the secreted substances appeared after a lag of 60-120 s, i.e. after shape change was terminated and aggregation was well on its way. These two platelet responses thus seemed to be independent of secretion and induced directly by A23187. The absence of a lag period when secretion was stopped by chilling alone was thought to be due to centrifugation-induced secretion of platelets conditioned by A23187. Formaldehyde completely inhibited centrifugation-induced secretion. At 37 degrees C, formaldehyde caused rapid breakdown of metabolic ATP in platelets with a pattern dependent on the formaldehyde concentration: Below 50 mM, ATP was converted to inosine plus hypoxanthine via ADP, AMP and IMP and the adenylate energy charge was preserved. Above 100 mM, AMP was the end product with a drastic reduction in the adenylate energy charge. These changes were not due to lysis of the platelets, but were apparently caused by an formaldehyde-induced increase in cellular ATP consumption. Platelet secretion is usually associated with a conversion of metabolic ATP to hypoxanthine. Formaldehyde had to be used to stop secretion and since it caused breakdown of ATP, additional smaples were taken out for nucleotide determination during stirring of platelet-rich plasma with A23187. It was found that metabolic ATP was converted to inosine plus hypoxanthine only during the secretion step.

Adenosine Diphosphate↗