Regulation of glycolytic flux in an energetically controlled cell-free system: the effects of adenine nucleotide ratios, inorganic phosphate, pH, and citrate

An integrative approach to the regulation of mitochondrial respiration during exercise: Focus on high-intensity exercise

During exercise, muscle ATP demand increases with intensity, and at the highest power output, ATP consumption may increase more than 100-fold above the resting level. The rate of mitochondrial ATP production during exercise depends on the availability of O₂, carbon substrates, reducing equivalents, ADP, P_i, free creatine, and Ca²⁺. It may also be modulated by acidosis, nitric oxide and reactive oxygen and nitrogen species (RONS). During fatiguing and repeated sprint exercise, RONS production may cause oxidative stress and damage to cellular structures and may reduce mitochondrial efficiency. Human studies indicate that the relatively low mitochondrial respiratory rates observed during sprint exercise are not due to lack of O₂, or insufficient provision of Ca²⁺, reduced equivalents or carbon substrates, being a suboptimal stimulation by ADP the most plausible explanation. Recent in vitro studies with isolated skeletal muscle mitochondria, studied in conditions mimicking different exercise intensities, indicate that ROS production during aerobic exercise amounts to 1-2 orders of magnitude lower than previously thought. In this review, we will focus on the mechanisms regulating mitochondrial respiration, particularly during high-intensity exercise. We will analyze the factors that limit mitochondrial respiration and those that determine mitochondrial efficiency during exercise. Lastly, the differences in mitochondrial respiration between men and women will be addressed.

Identification of alkaline pH optimum of human glucokinase because of ATP-mediated bias correction in outcomes of enzyme assays

Adenosine triphosphate (ATP) is a crucial substrate and energy source commonly used in enzyme reactions. However, we demonstrated that the addition of this acidic compound to enzyme assay buffers can serve as a source of unnoticed pH changes. Even relatively low concentrations of ATP (up to 5 mM) shifted pH of reaction mixtures to acidic values. For example, Tris buffer lost buffering capacity at pH 7.46 by adding ATP at a concentration higher than 2 mM. In addition to the buffering capacity, the pH shifts differed with respect to the buffer concentration. High ATP concentrations are commonly used in hexokinase assays. We demonstrated how the presence of ATP affects pH of widely used enzyme assay buffers and inversely affected K_M of human hexokinase 2 and S_0.5 of human glucokinase. The pH optimum of human glucokinase was never reported before. We found that previously reported optimum of mammalian glucokinase was incorrect, affected by the ATP-induced pH shifts. The pH optimum of human glucokinase is at pH 8.5-8.7. Suggested is the full disclosure of reaction conditions, including the measurement of pH of the whole reaction mixtures instead of measuring pH prior to the addition of all the components.

Sex differences in mitochondrial respiratory function in human skeletal muscle

Mitochondrial bioenergetic contributions to sex differences in human skeletal muscle metabolism remain poorly defined. The primary aim of this study was to determine whether mitochondrial respiratory kinetics differed between healthy young men and women in permeabilized skeletal muscle fibers. While men and women displayed similar ( P > 0.05) maximal respiration rates and abundance of mitochondrial/adenosine diphosphate (ADP) transport proteins, women had lower ( P < 0.05) mitochondrial ADP sensitivity (+30% apparent K_m) and absolute respiration rates at a physiologically relevant ADP concentration (100 μM). Moreover, although men and women exhibited similar carnitine palmitoyl transferase-I protein content- and palmitoyl-CoA-supported respiration, women displayed greater sensitivity to malonyl-CoA-mediated respiratory inhibition. These data establish baseline sex differences in mitochondrial bioenergetics and provide the foundation for studying mitochondrial function within the context of metabolic perturbations and diseases that affect men and women differently.

In the absence of phosphate shuttling, exercise reveals the in vivo importance of creatine-independent mitochondrial ADP transport

The transport of cytosolic adenosine diphosphate (ADP) into the mitochondria is a major control point in metabolic homeostasis, as ADP concentrations directly affect glycolytic flux and oxidative phosphorylation rates within mitochondria. A large contributor to the efficiency of this process is thought to involve phosphocreatine (PCr)/Creatine (Cr) shuttling through mitochondrial creatine kinase (Mi-CK), whereas the biological importance of alterations in Cr-independent ADP transport during exercise remains unknown. Therefore, we utilized an Mi-CK knockout (KO) model to determine whether in vivo Cr-independent mechanisms are biologically important for sustaining energy homeostasis during exercise. Ablating Mi-CK did not alter exercise tolerance, as the time to volitional fatigue was similar between wild-type (WT) and KO mice at various exercise intensities. In addition, skeletal muscle metabolic profiles after exercise, including glycogen, PCr/Cr ratios, free ADP/adenosine monophosphate (AMP), and lactate, were similar between genotypes. While these data suggest that the absence of PCr/Cr shuttling is not detrimental to maintaining energy homeostasis during exercise, KO mice displayed a dramatic increase in Cr-independent mitochondrial ADP sensitivity after exercise. Specifically, whereas mitochondrial ADP sensitivity decreased with exercise in WT mice, in stark contrast, exercise increased mitochondrial Cr-independent ADP sensitivity in KO mice. As a result, the apparent ADP Km was 50% lower in KO mice after exercise, suggesting that in vivo activation of voltage-dependent anion channel (VDAC)/adenine nucleotide translocase (ANT) can support mitochondrial ADP transport. Altogether, we provide insight that Cr-independent ADP transport mechanisms are biologically important for regulating ADP sensitivity during exercise, while highlighting complex regulation and the plasticity of the VDAC/ANT axis to support adenosine triphosphate demand.

Distinct effects of sorbic acid and acetic acid on the electrophysiology and metabolism of Bacillus subtilis

Sorbic acid and acetic acid are among the weak organic acid preservatives most commonly used to improve the microbiological stability of foods. They have similar pKa values, but sorbic acid is a far more potent preservative. Weak organic acids are most effective at low pH. Under these circumstances, they are assumed to diffuse across the membrane as neutral undissociated acids. We show here that the level of initial intracellular acidification depends on the concentration of undissociated acid and less on the nature of the acid. Recovery of the internal pH depends on the presence of an energy source, but acidification of the cytosol causes a decrease in glucose flux. Furthermore, sorbic acid is a more potent uncoupler of the membrane potential than acetic acid. Together these effects may also slow the rate of ATP synthesis significantly and may thus (partially) explain sorbic acid's effectiveness.

Who controls the ATP supply in cancer cells? Biochemistry lessons to understand cancer energy metabolism

Applying basic biochemical principles, this review analyzes data that contrasts with the Warburg hypothesis that glycolysis is the exclusive ATP provider in cancer cells. Although disregarded for many years, there is increasing experimental evidence demonstrating that oxidative phosphorylation (OxPhos) makes a significant contribution to ATP supply in many cancer cell types and under a variety of conditions. Substrates oxidized by normal mitochondria such as amino acids and fatty acids are also avidly consumed by cancer cells. In this regard, the proposal that cancer cells metabolize glutamine for anabolic purposes without the need for a functional respiratory chain and OxPhos is analyzed considering thermodynamic and kinetic aspects for the reductive carboxylation of 2-oxoglutarate catalyzed by isocitrate dehydrogenase. In addition, metabolic control analysis (MCA) studies applied to energy metabolism of cancer cells are reevaluated. Regardless of the experimental/environmental conditions and the rate of lactate production, the flux-control of cancer glycolysis is robust in the sense that it involves the same steps: glucose transport, hexokinase, hexosephosphate isomerase and glycogen degradation, all at the beginning of the pathway; these steps together with phosphofructokinase 1 also control glycolysis in normal cells. The respiratory chain complexes exert significantly higher flux-control on OxPhos in cancer cells than in normal cells. Thus, determination of the contribution of each pathway to ATP supply and/or the flux-control distribution of both pathways in cancer cells is necessary in order to identify differences from normal cells which may lead to the design of rational alternative therapies that selectively target cancer energy metabolism.

Effects of wollastonite on proliferation and differentiation of human bone marrow-derived stromal cells in PHBV/wollastonite composite scaffolds

In this study, the effects of wollastonite on proliferation and differentiation of human bone marrow-derived stromal cells (hBMSCs) have been investigated based on a polyhydroxybutyrate-co-hydroxyvalerate (PHBV)/ wollastonite (W) composite scaffolds system. Cell morphology, proliferation, and differentiation were measured. The results showed that the incorporation of wollastonite benefited hBMSCs adhesion, proliferation, and differentiation rate. In addition, an increase of proliferation and differentiation rate was observed when the wollastonite content in the PHBV/W composite scaffolds increased from 10 to 20 wt%. Based on our previous studies on PHBV/W composite discs, the differentiation measurements in this paper further proved that the wollastonite itself can stimulate the hBMSCs to differentiate toward osteoblasts without any osteogenic medium, and the ionic products (Ca and Si) released from wollastonite might contribute to this advantage. It is also suggested that the osteogenic differentiation of the hBMSCs can be affected by adjusting the wollastonite content in the composite scaffolds.

Protection of ischaemic-reperfused rat heart by dimethylamiloride is associated with inhibition of mitochondrial permeability transition

1. The aim of the present study was to assess whether protection afforded by the Na(+)/H(+) exchanger blocker dimethylamiloride (DMA) is associated with inhibition of mitochondrial permeability transition (MPT). The effects of DMA were compared with those of cyclosporine (Cs) A, an inhibitor of MPT. 2. Rat hearts were Langendorff perfused with Krebs'-bicarbonate medium containing 10 mmol/L glucose and were subjected to 25 min no-flow global ischaemia and 30 min reperfusion in the presence or absence of 10 micromol/L DMA or 0.2 micromol/L CsA. Cell viability was measured using tetrazolium stain. The MPT was determined by loading hearts with 2-deoxy-[(3)H]-glucose (2DG), which enters mitochondria only during MPT. Total heart 2DG content as an estimation of the extent of tissue damage was also measured. To assess whether DMA has any direct effect on glycolysis, a cell-free heart extract containing all the glycolytic enzymes was used. 3. Dimethylamiloride improved functional recovery (rate-pressure product) from 24 +/- 7 to 68 +/- 11% (P < 0.01) at reperfusion end, attenuated the increase in left ventricular end-diastolic pressure (from 29 +/- 7 to 6 +/- 3% 10 min after reperfusion onset; P < 0.01), improved cell viability (from 21.2 +/- 6.6 to 69.6 +/- 7.1% at reperfusion end; P < 0.05) and lessened lactate accumulation at the end of ischaemia (119 +/- 15 vs 163 +/- 14 micromol/g dry weight; P < 0.05). Dimethylamiloride limited MPT: 2DG mitochondrial entrapment, being 33.1 +/- 14.2 and 96.3 +/- 14.0 at reperfusion end in the treated and control hearts, respectively (P < 0.05), and concomitantly raised total 2DG content (51.3 +/- 4.4 vs 86.8 +/- 1.7 x 10(3) d.p.m./g wet weight in control and treated groups, respectively; P < 0.05). Cyclosporine A improved functional recovery and attenuated the amplitude of ventricular diastolic pressure in ischaemic-reperfused hearts. It also reduced mitochondrial entrapment (67.3 +/- 7.7%; P < 0.05 vs control) and increased total cell 2DG content (162.3 +/- 1.3 x 10(3) d.p.m./g wet weight; P < 0.01 vs control) at the end of reperfusion. Dimethylamiloride did not affect glucose consumption and lactate production in the cell-free heart extract. 4. In conclusion, DMA protects against the noxious effects of ischaemia-reperfusion and inhibits MPT, coinciding with present and previous findings concerning the effects of CsA. Dimethylamiloride also diminished lactate accumulation, although it did not exhibit any direct effect on glycolysis. These data suggest that blockade of Na(+)/H(+) exchange by DMA attenuates the extent of MPT in ischaemic-reperfused rat heart.

Multiple sprint work : physiological responses, mechanisms of fatigue and the influence of aerobic fitness

The activity patterns of many sports (e.g. badminton, basketball, soccer and squash) are intermittent in nature, consisting of repeated bouts of brief (<or=6-second) maximal/near-maximal work interspersed with relatively short (<or=60-second) moderate/low-intensity recovery periods. Although this is a general description of the complex activity patterns experienced in such events, it currently provides the best means of directly assessing the physiological response to this type of exercise. During a single short (5- to 6-second) sprint, adenosine triphosphate (ATP) is resynthesised predominantly from anaerobic sources (phosphocreatine [PCr] degradation and glycolysis), with a small (<10%) contribution from aerobic metabolism. During recovery, oxygen uptake (V-O2) remains elevated to restore homeostasis via processes such as the replenishment of tissue oxygen stores, the resynthesis of PCr, the metabolism of lactate, and the removal of accumulated intracellular inorganic phosphate (Pi). If recovery periods are relatively short, V-O2 remains elevated prior to subsequent sprints and the aerobic contribution to ATP resynthesis increases. However, if the duration of the recovery periods is insufficient to restore the metabolic environment to resting conditions, performance during successive work bouts may be compromised. Although the precise mechanisms of fatigue during multiple sprint work are difficult to elucidate, evidence points to a lack of available PCr and an accumulation of intracellular Pi as the most likely causes. Moreover, the fact that both PCr resynthesis and the removal of accumulated intracellular Pi are oxygen-dependent processes has led several authors to propose a link between aerobic fitness and fatigue during multiple sprint work. However, whilst the theoretical basis for such a relationship is compelling, corroborative research is far from substantive. Despite years of investigation, limitations in analytical techniques combined with methodological differences between studies have left many issues regarding the physiological response to multiple sprint work unresolved. As such, multiple sprint work provides a rich area for future applied sports science research.

Interactions of bioactive glasses with osteoblasts in vitro: effects of 45S5 Bioglass, and 58S and 77S bioactive glasses on metabolism, intracellular ion concentrations and cell viability

In a cell culture model of murine osteoblasts three particulate bioactive glasses were evaluated and compared to glass (either borosilicate or soda-lime-silica) particles with respect to their effect on metabolic activity, cell viability, changes in intracellular ion concentrations, proliferation and differentiation. 45S5 Bioglass caused extra- and intracellular alkalinization, a rise in [Ca2+]i and [K+]i, a small plasma membrane hyperpolarization, and an increase in lactate production. Glycolytic activity was also stimulated when cells were not in direct contact with 45S5 Bioglass particles but communicated with them only through the medium. Similarly, raising the pH of culture medium enhanced lactate synthesis. 45S5 Bioglass had no effect on osteoblast viability and, under most conditions, did not affect either proliferation or differentiation. Bioactive glasses 58S and 77S altered neither the ion levels nor enhanced metabolic activity. It is concluded that: (1) some bioactive glasses exhibit well-defined effects in osteoblasts in culture which are accessible to experimentation; (2) 45S5 Bioglass causes marked external and internal alkalinization which is, most likely, responsible for enhanced glycolysis and, hence, cellular ATP production; (3) changes in [H+] could contribute to alternations in concentrations of other intracellular ions; and (4) the rise in [Ca2+]i may influence activities of a number of intracellular enzymes and pathways. It is postulated that the beneficial effect of 45S5 on in vivo bone growth and repair may be due to some extent to alkalinization, which in turn increases collagen synthesis and crosslinking, and hydroxyapatite formation.

Comparison of caffeine and theophylline ingestion: exercise metabolism and endurance

This two-part investigation compared the ergogenic and metabolic effects of theophylline and caffeine. Initially (part A), the ergogenic potential of theophylline on endurance exercise was investigated. Eight men cycled at 80% maximum O(2) consumption to exhaustion 90 min after ingesting either placebo (dextrose), caffeine (6 mg/kg; Caff), or theophylline (4.5 mg/kg Theolair; Theo). There was a significant increase in time to exhaustion in both the Caff (41.2+/-4.8 min) and Theo (37.4+/-5.0 min) trials compared with placebo (32.6+/-3.4 min) (P<0.05). In part B, the effects of Theo on muscle metabolism were investigated and compared with Caff. Seven men cycled for 45 min at 70% maximum O(2) consumption (identical treatment protocol as in part A). Neither methylxanthines (MX) affected muscle glycogen utilization (P>0.05). Only Caff increased plasma epinephrine (P<0.05), but both MX increased blood glycerol levels (P<0.05). Muscle cAMP was increased (P<0.05) by both MX at 15 min and remained elevated at 45 min with Theo. This demonstrates that both MX are ergogenic and that this can be independent of muscle glycogen.

Acidosis and astrocyte amino acid metabolism

The relationship between acidosis and the metabolism of glutamine and glutamate was studied in cultured astrocytes. Acidification of the incubation medium was associated with an increased formation of aspartate from glutamate and glutamine. The rise of the intracellular content of aspartate was accompanied by a significant decline in the extracellular concentration of both lactate and citrate. Studies with either [2-(15)N]glutamine or [15N]glutamate indicated that there occurred in acidosis an increased transamination of glutamate to aspartate. Studies with L-[2,3,3,4,4-(2)H5]glutamine indicated that in acidosis glutamate carbon was more rapidly converted to aspartate via the tricarboxylic acid cycle. Acidosis appears to result in increased availability of oxaloacetate to the aspartate aminotransferase reaction and, consequently, increased transamination of glutamate. The expansion of the available pool of oxaloacetate probably reflects a combination of: (a) Restricted flux through glycolysis and less production from pyruvate of acetyl-CoA, which condenses with oxaloacetate in the citrate synthetase reaction; and (b) Increased oxidation of glutamate and glutamine through a portion of the tricarboxylic acid cycle and enhanced production of oxaloacetate from glutamate and glutamine carbon. The data point to the interplay of the metabolism of glucose and that of glutamate in these cells.

Effect of induced metabolic alkalosis on human skeletal muscle metabolism during exercise

The purpose of the study was to examine the roles of active pyruvate dehydrogenase (PDH(a)), glycogen phosphorylase (Phos), and their regulators in lactate (Lac(-)) metabolism during incremental exercise after ingestion of 0.3 g/kg of either NaHCO(3) [metabolic alkalosis (ALK)] or CaCO(3) [control (CON)]. Subjects (n = 8) were studied at rest, rest postingestion, and during constant rate cycling at three stages (15 min each): 30, 60, 75% of maximal O(2) uptake (VO(2 max)). Radial artery and femoral venous blood samples, leg blood flow, and biopsies of the vastus lateralis were obtained during each power output. ALK resulted in significantly (P < 0.05) higher intramuscular Lac(-) concentration ([Lac(-)]; ALK 72.8 vs. CON 65.2 mmol/kg dry wt), arterial whole blood [Lac(-)] (ALK 8.7 vs. CON 7.0 mmol/l), and leg Lac(-) efflux (ALK 10.0 vs. CON 4.2 mmol/min) at 75% VO(2 max). The increased intramuscular [Lac(-)] resulted from increased pyruvate production due to stimulation of glycogenolysis at the level of Phos a and phosphofructokinase due to allosteric regulation mediated by increased free ADP (ADP(f)), free AMP (AMP(f)), and free P(i) concentrations. PDH(a) increased with ALK at 60% VO(2 max) but was similar to CON at 75% VO(2 max). The increased PDH(a) may have resulted from alterations in the acetyl-CoA, ADP(f), pyruvate, NADH, and H(+) concentrations leading to a lower relative activity of PDH kinase, whereas the similar values at 75% VO(2 max) may have reflected maximal activation. The results demonstrate that imposed metabolic alkalosis in skeletal muscle results in acceleration of glycogenolysis at the level of Phos relative to maximal PDH activation, resulting in a mismatch between the rates of pyruvate production and oxidation resulting in an increase in Lac(-) production.

Creatine kinase, energy-rich phosphates and energy metabolism in heart muscle of different vertebrates

Maximal activities of creatine kinase, pyruvate kinase and cytochrome oxidase and total concentrations of creatine and phosphorylated adenylates were measured in cardiac muscle of hagfish, eight teleost species, frog, turtle, pigeon and rat. The ratio of creatine kinase to cytochrome oxidase with cytochrome oxidase as a rough estimate of aerobic capacity and cellular "energy turnover", was increased in myocardia of hagfish, turtle and crucian carp. These myocardia are likely to be frequently exposed to oxygen deficiency. In agreement with this, they possess a high relative glycolytic capacity as indicated by a high pyruvate kinase/cytochrome oxidase ratio. The creatine kinase/cytochrome oxidase ratio for the other myocardia varied within a factor of 2, except the value for cod myocardium which was below the others. Total creatine varied among species and was high in active species such as herring, pigeon and rat but also high in crucian carp. The variation in total concentration of phosphorylated adenylates was considerably less than the variation in total creatine. The high creatine kinase/cytochrome oxidase ratio in myocardia likely to be challenged by hypoxia may represent an enhanced efficiency for both "spatial" and "temporal" buffering of phosphorylated adenylates to attenuate the impact of a depressed energy liberation. As to the differences in total creatine, this factor influences not only the cellular energy distribution but possibly also contractility via an effect on the free phosphate level.

Relations between intracellular ions and energy metabolism under acidotic conditions: a study with nigericin in synaptosomes, neurons, and C6 glioma cells

Effects of nigericin were investigated in rat brain synaptosomes, cultured neurons, and C6 glioma cells to characterize the relations among ATP synthesis, [Na+]i, [K+]i, and [Ca2+]i, and pH under conditions when [H+]i is substantially increased and transmembrane electrical potential is decreased. Intracellular acidification and loss of K+ were accompanied by enhanced oxygen consumption and lactate production and a decrease in cellular energy level. Changes in the last three parameters were attenuated by addition of 1 mM ouabain. In synaptosomes treated with nigericin, neither respiration nor glycolysis was affected by 0.3 microM tetrodotoxin, whereas 1 mM amiloride reduced lactate production by 20% but did not influence respiration. In C6 cells, amiloride decreased the nigericin-stimulated rate of lactate generation by about 50%. The enhancement by nigericin of synaptosomal oxygen uptake and glycolytic rate decreased with time. However, there was only a small reduction in respiration and none in glycolysis in C6 cells. Measurements with ion-selective microelectrodes in neurons and C6 cells showed that nigericin also caused a rise in [Ca2+]i and [Na+]i. The increase in [Na+]i in C6 cells was partially reversed by 1 mM amiloride. It is concluded that nigericin-induced loss of K+ and subsequent depolarization lead to an increase in Na+ influx and stimulation of the Na+/K+ pump with a consequent rise in energy utilization; that acidosis inhibits mitochondrial ATP production; that a rise in [H+] does not decrease glycolytic rate when the energy state (a fall in [ATP] and rises in [ADP] and [AMP]) is simultaneously reduced; that a fall in [K+]i depresses both oxidative phosphorylation and glycolysis; and that the nigericin-induced alterations in ion levels and activities of energy-producing pathways can explain some of the deleterious effects of ischemia and hypoxia.

Phosphate-dependent glutaminase of rat skeletal muscle. Some properties and possible role in glutamine metabolism

A relatively high activity (26.7 nmol/min per mg mitochondrial protein) of phosphate-dependent glutaminase (EC 3.5.1.2; L-glutamine amidohydrolase) was found in rat skeletal muscle (mixed type from hindlegs) mitochondria incubated in 200 mM potassium phosphate (pH 8.2); the activity was lower in rat heart and diaphragm mitochondria. Phosphate-dependent glutaminase was also found in human skeletal muscle mitochondria, but the activity was about 3-5 times lower than in rat skeletal muscle. Multiplying the specific activity of mitochondrial glutaminase by the amount of mitochondrial protein present in 1 g of rat skeletal muscle the maximum glutaminase activity was found to be 0.352 mumol/min per g wet tissue. The rat skeletal muscle enzyme appears to be similar in many respects to phosphate-dependent glutaminase of the kidney (e.g., S0.5 for glutamine, K0.5 for phosphate, the pH activity profile, inhibition by glutamate). These properties make the skeletal muscle enzyme very similar to the 'kidney type' glutaminase isoenzyme of rat tissues. A significant difference between rat kidney and skeletal muscle enzymes is their adaptive response during acidosis. While the kidney enzyme increases during acidosis, the skeletal muscle glutaminase activity does not. A possible role of glutaminase in the glutamine metabolism in rat skeletal muscle is discussed.

A hysteretic cycle in glucose 6-phosphate metabolism observed in a cell-free yeast extract

The dynamics of a partial glycolytic reaction sequence which converts glucose 6-phosphate to triose phosphates is described. The study was performed with cell-free extracts from baker's yeast harvested in the logarithmic and stationary growth phases. The experiments are based on a flow-through reactor supplied with the desalted cell-free extract as well as glucose 6-phosphate, ATP and phosphoenolpyruvate. In the reaction system the quasi-irreversible reactions catalyzed by 6-phosphofructo-1-kinase, pyruvate kinase, and fructose-1,6-bisphosphatase are involved. When substrate is supplied continuously, only stable stationary states can be observed. With transient perturbations of the substrate supply, multiple stationary states appear. Cyclic transitions between unique stable stationary states were induced by appropriate changes of the rate of substrate supply. A hysteretic cycle could then be demonstrated when, during reverse transitions, a parameter region of multistability was passed. The presence (in resting yeast) or absence (in growing yeast) of fructose-1,6-bisphosphatase did not significantly influence the dynamic capabilities of the investigated reaction sequence. The kinetic properties of the cell-free extracts fit mathematical models developed for in vitro systems reconstituted from purified enzymes.

Possible mechanisms of the anaerobic threshold. A review

The anaerobic threshold consists of a lactate threshold and a ventilatory threshold. In some conditions there may actually be 2 ventilatory thresholds. Much of the work detailing the lactate threshold is strongly based on blood lactate concentration. Since, in most cases, blood lactate concentration does not reflect production in active skeletal muscle, inferences about the metabolic state of contracting muscle will not be valid based only on blood lactate concentration measurements. Numerous possible mechanisms may be postulated as generating a lactate threshold. However, it is very difficult to design a study to influence only one variable. One may ask, does reducing F1O2 cause an earlier occurrence of a lactate threshold during progressive exercise by reducing oxygen availability at the mitochondria? By stimulating catecholamine production? By shifting more blood flow away from tissues which remove lactate from the blood? Or by some other mechanism? Processes considered essential to the generation of a lactate threshold include: (a) substrate utilisation in which the ability of contracting muscle cells to oxidise fats reaches maximal power at lactate threshold; and (b) catecholaminergic stimulation, for without the presence of catecholamines it appears a lactate threshold cannot be generated. Other mechanisms discussed which probably enhance the lactate threshold, but are not considered essential initiators are: (a) oxygen limitation; (b) motor unit recruitment order; (c) lactate removal; (d) muscle temperature receptors; (e) metabolic stimulation; and (f) a threshold of lactate efflux. Some mechanisms reviewed which may induce or contribute to a ventilatory threshold are the effects of: (a) the carotid bodies; (b) respiratory mechanics; (c) temperature; and (d) skeletal muscle receptors. It is not yet possible to determine the hierarchy of effects essential for generating a ventilatory threshold. This may indicate that the central nervous system integrates a broad range of input signals in order to generate a non-linear increase in ventilation. Evidence indicates that the occurrence of the lactate threshold and the ventilatory threshold may be dissociated; sometimes the occurrence of the lactate threshold significantly precedes the ventilatory threshold and at other times the ventilatory threshold significantly precedes the lactate threshold. It is concluded that the 2 thresholds are not subserved by the same mechanism.

Effects of long-chain fatty-acyl esters of coenzyme A and carnitine on cell-free rat heart preparations

The purpose of this study was to investigate the effects of fatty acyl CoA and carnitine esters on the glycolytic system of the rat heart. Using a respiring incubation mixture containing a whole-heart homogenate it was observed that oleoyl-CoA slowed down the glucose disappearance whereas lactate accumulation did not change. Experiments were also performed by means of an incubation mixture prepared with a soluble heart extract, considered to contain all glycolytic enzymes present in heart fibres. Palmitoyl-CoA or oleoyl-CoA as well as palmitoyl carnitine, added separately or together, were unable to alter the glucose disappearance and lactate accumulation in this mixture. These data suggest that long chain acyl-esters have not direct inhibitory actions on the heart glycolytic activity. However, CoA esters seem to exert indirect inhibitory effects which may be relevant to the myocardium under oxygen restriction situations.

Phosphorus nuclear magnetic resonance of fast- and slow-twitch muscle

Phosphorus nuclear magnetic resonance (NMR) spectra were obtained at 109.3 MHz from isolated, arterially perfused cat biceps brachi (greater than 75% fast-twitch, glycolytic fibers) and soleus (greater than 92% slow-twitch, oxidative) muscles at 30 degrees C. The perfused muscles were stable with respect to O2 consumption, twitch characteristics, and ATP and phosphocreatine (PC) levels for up to 10 h. NMR spectra showed a higher PC/Pi ratio in the biceps (11) than in the soleus (1.7). Relatively higher Pi levels were observed in extracts of clamp-frozen muscles than in the intact muscles. This difference could be accounted for by artifactual hydrolysis of PC during muscle freezing. Based on the NMR and chemical data, the free cytosolic ADP level, calculated from the creatine kinase equilibrium, was 14 microM in the soleus and less than 1 microM in the biceps. Intracellular Pi concentration was 10 mM in the soleus and 3 mM in the biceps. Intracellular pH, estimated from the chemical shift of phosphate or 2-deoxyglucose 6-phosphate, was 7.0 in both muscles (perfusate pH 7.2). Both extracellular space and pH measurements were obtained from NMR spectra of muscles perfused with 10 mM sodium phenylphosphonate added to the perfusate. These results document larger differences in the phosphate metabolites in the two types of mammalian muscles than previously reported.

Adenine nucleotide metabolism in pigeon liver and heart: diurnal changes and correlations between indices

The adenine nucleotide and Pi content of pigeon liver and heart were determined, and the energy charge, phosphoryl potential and mass action ratio of adenylate kinase were calculated over the 24 hr period. All the indices of adenine nucleotide metabolism were shown to vary a 2- to 4-fold extent, both in liver and heart. The correlation coefficients for each of the adenine nucleotides and each of the calculated indices were computed and shown to be different for liver and heart. The difference between pigeon liver, pigeon heart and rat liver in the diurnal variation of adenine nucleotide metabolism, in the regulatory mechanism, and in metabolism on the whole is discussed.

Control of cellular redox potential as measured in a steady-state, cell-free system

A cell-free system consisting of rat liver mitochondria, liver cytosol, lactate, and the substrates intrinsic to the malate-aspartate shuttle was reconstituted for studies of steady-state substrate fluxes and, more specifically, to evaluate further the mechanism of control of the intra- and extramitochondrial steady states of the free NAD+/NADH ratios. Soluble (F1) ATPase or 2,4-dinitrophenol (DNP) were added in varying amounts to alter substrate fluxes and the constant energy state of this 'open' metabolizing system. The steady-state redox segregation (1.36 log NAD+/NADH ratio out vs NAD+/NADH in the mitochondrial matrix) was maximally about 3 kcal, and declined together with the membrane potential (delta psi) and log ATP/ADP, which obtain on imposing an increasing energy load on the system. It is concluded that transmembrane movement of reducing equivalents is coupled to electron transfer through delta psi, mediated by the electrogenic exchange of glutamate and aspartate. When delta psi was high (near State 4), delta G redox was approximately the same as that generated without flux of reducing equivalents [E. J. Davis, J. Bremer, and K. E. Akerman (1980) J. Biol. Chem. 255, 2277-2283], suggesting that delta Gredox is in near thermodynamic equilibrium with delta psi. If the steady-state ATP/ADP ratio was altered with an energy load (F1-ATPase), delta Gredox decreased more steeply than delta psi (tetraphenyl phosphonium-sensitive electrode used to measure delta psi). At comparable ranges of ATP/ADP, both delta Gredox and delta psi decreased more steeply with uncoupler than with an external ADP-regenerating system.

Reconstruction of steady state in cell-free systems. Interactions between glycolysis and mitochondrial metabolism: regulation of the redox and phosphorylation states

A reconstituted "open" system comprising respiring mitochondria and actively glycolyzing muscle extract was devised for studies of vectorially mediated interactions. Glycogen particles were the substrate for the glycolyzing enzymes. Purified soluble (F1) ATPase was added in varying quantities to establish a range of energetic steady states. The data generally confirm our recent conclusions (Wu and Davis, (1981) Arch. Biochem. Biophys. 208, 85-89) on the relative efficacy of the adenine nucleotides and their ratios, and of inorganic phosphate on flux through rate-controlling steps of glycolysis. When mitochondrial ATP synthesis was blocked, glycolytic flux was relatively rapid, and the lactate/pyruvate ratio increased with time to values up to greater than 300. If functional mitochondria were present, glycolytic flux was very strongly suppressed, provided the energy state (ATP/ADP) was high, and the phosphate concentration[Pi] was low. Adenine nucleotide control of glycolysis was to a large extent lost when the steady-state ATP/ADP was below about 10, or if [Pi] was elevated. In the two-phase system containing respiring mitochondria and components of the malate-aspartate shuttle, the ATP/ADP and both extra- and intramitochondrial NAD+/NADH ratios were maintained constant, and to various perturbable levels with varying energy load (ATPase). The gradient in reduction potentials attained values (delta Gredox) of up to about 2.5 kcal. The extramitochondrial redox state was not positively correlated with the external phosphorylation potential ([ATP]/[ADP] X [Pi]). The following conclusions are drawn on the basis of the present data, together with other reports (Davis, Bremer, and Akerman (1980) J. Biol. Chem. 255, 2277-2283) and (Klingenberg and Rottenberg (1977) Eur. J. Biochem. 73, 125-130): (a) the gradient in reduction potential is driven by the membrane potential (delta psi), mediated by the electrogenic glutamate-aspartate exchange, and the poise or set point of this gradient is a function of delta psi; and (b) the gradient of ATP/ADP ratios across the membrane is also driven principally by delta psi, mediated by the electrogenic ATP-ADP exchange. Hence, segregation of phosphorylation and reduction potentials is linked through a mutually shared electrical driving force.

Accumulation and disposal of tricarboxylic acid cycle intermediates during propionate oxidation in the isolated perfused rat heart

The role of the metabolite disposal mechanisms in the regulation of the tricarboxylic acid cycle pool size was studied in isolated perfused rat hearts oxidizing 2 mM propionate. Malate and succinate accumulated during the propionate metabolism. A further 118% increase in the malate concentration and 600% increase in the succinate concentration and a slight inhibition of the propionate uptake were observed during a subsequent KCl-induced arrest of the heart metabolizing propionate. When the mechanical activity of the heart was restored, the malate and succinate concentrations returned to the same levels as before the arrest of the heart, but the propionate uptake did not rise significantly. The mean disposal rates of the tricarboxylic acid cycle metabolites during the cardiac arrest and subsequent restoration of the activity were 1.4 and 2.4 mumol/min per g dry weight, respectively during cardiac arrest the malate carbon disposed was almost totally recovered as C3 compounds, whereas after the increase in the ATP-consumption most of it was oxidized. The result show that propionate is oxidized by heart muscle at an appreciable rate but the disposal rate of the tricarboxylic acid cycle intermediates is not tightly regulated by the cellular energy state. Although the metabolite pool size of the tricarboxylic acid cycle responds to change in the ATP consumption, the energy state appears to have a greater effect on the fate of the C3 compounds formed than on the actual rate of C4 compound disposition.

Compartmentation of citrate in relation to the regulation of glycolysis and the mitochondrial transmembrane proton electrochemical potential gradient in isolated perfused rat heart

Subcellular fractionation of tissue in nonaqueous media was employed to study metabolite compartmentation in isolated perfused rat hearts. The mitochondrial and cytosolic concentrations of citrate and 2-oxoglutarate, total concentrations of the glycolytic intermediates and rate of glycolysis were measured in connection with changes in the rate of cellular respiration upon modulation of the ATP consumption by changes of the mechanical work load of the heart. The concentrations of citrate and 2-oxoglutarate in the mitochondria were 16- and 14-fold, respectively, greater than those in the cytosol of beating hearts. The cytosolic citrate concentration was low compared with concentrations which have been employed in demonstrations of the citrate inhibition of glycolysis. In spite of the low activities reported for the tricarboxylate carrier in heart mitochondria, the cytosolic citrate concentration reacted to perturbations of the mitochondrial citrate concentration, and inhibition of glycolysis at the phosphofructokinase step could be observed concomitantly with an increase in the cytosolic citrate concentration. The delta pH across the inner mitochondrial membrane calculated from the 2-oxoglutarate concentration gradient and the mitochondrial membrane potential calculated from the adenylate distribution gave an electrochemical potential difference of protons compatible with chemiosmotic coupling in the intact myocardium.