Mitochondrial respiratory control in the myocardium

Oxidation of NADH and ROS production by respiratory complex I

Kinetic characteristics of the proton-pumping NADH:quinone reductases (respiratory complexes I) are reviewed. Unsolved problems of the redox-linked proton translocation activities are outlined. The parameters of complex I-mediated superoxide/hydrogen peroxide generation are summarized, and the physiological significance of mitochondrial ROS production is discussed. This article is part of a Special Issue entitled Respiratory complex I, edited by Volker Zickermann and Ulrich Brandt.

Mitochondrial production of reactive oxygen species

Numerous biochemical studies are aimed at elucidating the sources and mechanisms of formation of reactive oxygen species (ROS) because they are involved in cellular, organ-, and tissue-specific physiology. Mitochondria along with other cellular organelles of eukaryotes contribute significantly to ROS formation and utilization. This review is a critical account of the mitochondrial ROS production and methods for their registration. The physiological and pathophysiological significance of the mitochondrially produced ROS are discussed.

Role of NADH/NAD+ transport activity and glycogen store on skeletal muscle energy metabolism during exercise: in silico studies

Skeletal muscle can maintain ATP concentration constant during the transition from rest to exercise, whereas metabolic reaction rates may increase substantially. Among the key regulatory factors of skeletal muscle energy metabolism during exercise, the dynamics of cytosolic and mitochondrial NADH and NAD+ have not been characterized. To quantify these regulatory factors, we have developed a physiologically based computational model of skeletal muscle energy metabolism. This model integrates transport and reaction fluxes in distinct capillary, cytosolic, and mitochondrial domains and investigates the roles of mitochondrial NADH/NAD+ transport (shuttling) activity and muscle glycogen concentration (stores) during moderate intensity exercise (60% maximal O2 consumption). The underlying hypothesis is that the cytosolic redox state (NADH/NAD+) is much more sensitive to a metabolic disturbance in contracting skeletal muscle than the mitochondrial redox state. This hypothesis was tested by simulating the dynamic metabolic responses of skeletal muscle to exercise while altering the transport rate of reducing equivalents (NADH and NAD+) between cytosol and mitochondria and muscle glycogen stores. Simulations with optimal parameter estimates showed good agreement with the available experimental data from muscle biopsies in human subjects. Compared with these simulations, a 20% increase (or approximately 20% decrease) in mitochondrial NADH/NAD+ shuttling activity led to an approximately 70% decrease (or approximately 3-fold increase) in cytosolic redox state and an approximately 35% decrease (or approximately 25% increase) in muscle lactate level. Doubling (or halving) muscle glycogen concentration resulted in an approximately 50% increase (or approximately 35% decrease) in cytosolic redox state and an approximately 30% increase (or approximately 25% decrease) in muscle lactate concentration. In both cases, changes in mitochondrial redox state were minimal. In conclusion, the model simulations of exercise response are consistent with the hypothesis that mitochondrial NADH/NAD+ shuttling activity and muscle glycogen stores affect primarily the cytosolic redox state. Furthermore, muscle lactate production is regulated primarily by the cytosolic redox state.

Molecular system bioenergetics: regulation of substrate supply in response to heart energy demands

This review re-evaluates regulatory aspects of substrate supply in heart. In aerobic heart, the preferred substrates are always free fatty acids, and workload-induced increase in their oxidation is observed at unchanged global levels of ATP, phosphocreatine and AMP. Here, we evaluate the mechanisms of regulation of substrate supply for mitochondrial respiration in muscle cells, and show that a system approach is useful also for revealing mechanisms of feedback signalling within the network of substrate oxidation and particularly for explaining the role of malonyl-CoA in regulation of fatty acid oxidation in cardiac muscle. This approach shows that a key regulator of fatty acid oxidation is the energy demand. Alterations in malonyl-CoA would not be the reason for, but rather the consequence of, the increased fatty acid oxidation at elevated workloads, when the level of acetyl-CoA decreases due to shifts in the kinetics of the Krebs cycle. This would make malonyl-CoA a feedback regulator that allows acyl-CoA entry into mitochondrial matrix space only when it is needed. Regulation of malonyl-CoA levels by AMPK does not seem to work as a master on-off switch, but rather as a modulator of fatty acid import.

Influence of calcium-induced workload transitions and fatty acid supply on myocardial substrate selection

Because of differences in energy yield and oxygen demand, the selection of oxidative fuels is important in the hypoxic or ischemic heart muscle. The aim of the present study was to clarify the contradictions observed in the effects of workload and fatty acid supply on myocardial fuel preference in isolated perfused rat hearts. Nuclear magnetic resonance spectroscopy combined with the administration of substrates labeled with the stable isotope carbon 13 and isotopomer analysis of glutamate labeling offers an opportunity to simultaneously measure metabolic fluxes in pathways feeding into the tricarboxylic acid (TCA) cycle. The work output was modulated by changes in extracellular calcium. In the presence of 5 mmol/L glucose, 0.5 mmol/L octanoate in the perfusate dominated the oxidative metabolism, and workload had little effect on the ratio of glucose to fatty acid utilization. This was the case even when the octanoate concentration was lowered to 50 micromol/L. The relative rate of replenishment of the TCA cycle intermediates was higher at a low workload. The redox state of flavoproteins in the intact heart was monitored fluorometrically to obtain an estimate of the mitochondrial reduced/oxidized nicotinamide-adenine dinucleotide ratio (NADH/NAD ratio) for assessment of the dominant level of regulation of cell respiration, and the myoglobin spectrum was simultaneously monitored to evaluate the oxygenation status of the myocardium. Commencement of octanoate infusion (50 micromol/L or 0.5 mmol/L) caused a large but transient reduction of mitochondrial NAD and, conversely, its cessation elicited NADH oxidation and rebound reduction. During glucose oxidation, an increase in workload led to oxidation of the mitochondrial NADH, but this effect was much smaller in the presence of 50 micromol/L octanoate and absent in the presence of 0.5 mmol/L. This indicates that strong control of oxygen consumption during glucose oxidation is exerted in the mitochondrial respiratory chain, whereas equal control during fatty acid oxidation is exerted within the metabolic pathway upstream from the respiratory chain. It is concluded that when a medium-chain fatty acid is available, myocardial workload and energy consumption have little influence on fuel preference and glucose oxidation remains suppressed.

Cardiac energy metabolism homeostasis: role of cytosolic calcium

The heart is capable of dramatically altering its overall energy flux with minimal changes in the concentrations of metabolites that are associated with energy metabolism. This cardiac energy metabolism homeostasis is discussed with regard to the potential cytosolic control network responsible for controlling the major energy conversion pathway, oxidative phosphorylation in mitochondria. Several models for this cytosolic control network have been proposed, but a cytosolic Ca(2+) dependent parallel activation scheme for metabolism and work is consistent with most of the experimental results. That model proposes that cytosolic Ca(2+) regulates both the utilization of ATP by the work producing ATPases as well as the mitochondrial production of ATP. Recent studies have provided evidence supporting this role of cytosolic Ca(2+). These data include the demonstration that mitochondrial [Ca(2+)] can track cytosolic [Ca(2+)] and that the compartmentation of cytosolic [Ca(2+)] can facilitate this process. On the metabolic side, Ca(2+) has been shown to rapidly activate several steps in oxidative phosphorylation, including F(1)F(0)-ATPase ATP production as well as several dehydrogenases, which results in a homeostasis of mitochondrial metabolites similar to that observed in the cytosol. Numerous problems with the Ca(2+) parallel activation hypothesis remain including the lack of specific mechanisms of mitochondrial Ca(2+) transport and regulation of F(1)F(0)-ATPase, the time dependence of Ca(2+) activation of cytosolic ATPases as well as oxidative phosphorylation, and the role of cytosolic compartmentation. In addition, the lack of cytosolic or mitochondrial [Ca(2+)] measurements under in vivo conditions is problematic. Several lines of investigation to address these issues are suggested. A model of the cardiac energy metabolism control network is proposed that includes a Ca(2+) parallel activation component together with more classical elements including metabolite feedback and cytosolic compartmentation.

Theoretical studies on the regulation of oxidative phosphorylation in intact tissues

The theoretical studies on the regulation of oxidative phosphorylation that were performed with the aid of kinetic models of this process are overviewed. A definition of the regulation of the flux through a metabolic pathway is proposed and opposed to the control exerted by particular enzymes over this flux. Different kinetic models of oxidative phosphorylation proposed in the literature are presented, of which only the model proposed by myself and co-workers was extensively used in theoretical studies on the regulation and compensation in the oxidative phosphorylation system. These theoretical studies have led to the following conclusions: (1) in isolated mitochondria, an increase in the activity of an artificial ATP-using system stimulates mitochondria mainly via changes in [ADP], while changes in [ATP] and [P(i)] play only a minor role; (2) in non-excitable tissues (e.g. liver), hormones (acting via some cytosolic factor(s)) activate directly both ATP usage and at least some enzymes of the ATP-producing block; (3) in excitable tissues (e.g. skeletal muscle), neural signals stimulate (via some cytosolic factor(s)) in parallel all the steps of oxidative phosphorylation together with ATP usage and substrate dehydrogenation; (4) the decrease in the flux through cytochrome oxidase caused by a decrease in oxygen concentration is, at least partially, compensated by a decrease in Delta p and increase in the reduction level of cytochrome c. A theoretical prediction is formulated that there should exist and be observable a universal cytosolic factor/regulatory mechanism which directly activates (at least in excitable tissues) all complexes of oxidative phosphorylation during an increased energy demand.

Metabolic control of contractile performance in isolated perfused rat heart. Analysis of experimental data by reaction:diffusion mathematical model

The intracellular mechanisms of regulation of energy fluxes and respiration in contracting heart cells were studied. For this, we investigated the workload dependencies of the rate of oxygen consumption and metabolic parameters in Langendorff-perfused isolated rat hearts.(31)P NMR spectroscopy was used to study the metabolic changes during transition from perfusion with glucose to that with pyruvate with and without active creatine kinase system. The experimental results showed that transition from perfusion with glucose to that with pyruvate increased the phosphocreatine content and stability of its level at increased workloads. Inhibition of creatine kinase reaction by 15-min infusion of iodoacetamide decreased the maximal developed tension and respiration rates by a factor of two.(31)P NMR data were analyzed by a mathematical model of compartmentalized energy transfer, which is independent from the restrictions of the classical concept of creatine kinase equilibrium. The analysis of experimental data by this model shows that metabolic stability-constant levels of phosphocreatine, ATP and inorganic phosphate-at increased energy fluxes is an inherent property of the compartmentalized system. This explains the observed substrate specificity by changes in mitochondrial membrane potential. The decreased maximal respiration rate and maximal work output of the heart with inhibited creatine kinase is well explained by the rise in myoplasmic ADP concentration. This activates the adenylate kinase reaction in the myofibrillar space and in the mitochondria to fulfil the energy transfer and signal transmission functions, usually performed by creatine kinase. The activity of this system, however, is not sufficient to maintain high enough energy fluxes. Therefore, there is a kinetic explanation for the decreased maximal respiration rate of the heart with inhibited creatine kinase: i.e. a kinetically induced switch from an efficient energy transfer pathway (PCr-CK system) to a non-efficient one (myokinase pathway) within the energy transfer network of the cell under conditions of low apparent affinity of mitochondria to ADP in vivo. This may result in a significant decrease in the thermodynamic affinity of compartmentalized ATPase systems and finally in heart failure.

Left ventricular myocardial adenosine triphosphate changes during reperfusion of ventricular fibrillation: the influence of flow and epinephrine

OBJECTIVE

To determine whether epinephrine in combination with high flow worsens left ventricular (LV) myocardial high-energy phosphate stores during reperfusion of ischemic ventricular fibrillation (VF).

DESIGN

Blinded, prospective block randomized, placebo controlled study.

SETTING

University medical center research laboratory.

SUBJECTS

A total of 22 mixed breed swine weighing 22.0+/-3.3 kg (SD).

INTERVENTIONS

Open-chest swine, anesthetized with alpha-chloralose, underwent 10 mins of nonperfused VF followed by reperfusion with cardiopulmonary bypass for 90 mins and then defibrillation. Animals were block randomized to four groups for reperfusion: Group 1 (n = 5), high flow (100 mL/kg/min) and epinephrine (2.5 microg/kg/min); Group 2 (n = 5), high flow and placebo; Group 3 (n = 6), low flow (30 mL/kg/min) and epinephrine; and Group 4 (n = 6), low flow and placebo.

MEASUREMENTS AND MAIN RESULTS

In vivo LV creatine phosphate (CP) and adenosine triphosphate (ATP) were determined using whole wall and spatially localized 31P NMR spectroscopy at 4.7 Tesla. During perfusion of the fibrillating myocardium, epinephrine significantly increased aortic pressure (p < .05) and improved defibrillation rates (p < .01). ATP levels during reperfusion were significantly decreased within all groups compared with baseline. There were no differences in ATP levels between groups. High flow, independent of epinephrine, was associated with increased preservation of ATP (p < .05), increased CP/ATP ratios (p < .02) in all layers of the LV wall, and decreased aortic and cardiac vein lactates (p < .001).

CONCLUSIONS

Epinephrine, in combination with flow higher than standard cardiopulmonary resuscitation flows, increased perfusion pressure and defibrillation rates, but did not significantly alter myocardial ATP during VF reperfusion in the in vivo heart Reperfusion flow, independent of epinephrine, is a critical determinant of myocardial ATP preservation.

Quantification of the relative contribution of parallel pathways to signal transfer: application to cellular energy transduction

A simple mathematical formalism designed to quantify the relative contribution of parallel pathways to signal transduction is presented and applied to the regulation of the respiration rate by ATP, ADP and Pi concentrations in response to an increase of energy demand in isolated mitochondria. Theoretical studies were performed by means of the computer model of oxidative phosphorylation developed previously. Many earlier experimental studies have shown that externally-manipulated concentrations of all three metabolites can influence the respiration rate significantly. However, the effect of changes in [ATP], [ADP] and [Pi] that actually take place during an increased energy demand have not been determined in a quantitative way. It was shown in the present paper that [ADP] is the main regulatory factor which stimulates respiration during transition from state 4 to state 3 imposed by an addition of increasing amounts of an artificial ADP-regenerating system. Changes in [ATP] and [Pi] contribute to the respiration rate increase very weakly, and only in the nearest neighbourhood of state 3. Generally, changes in [ADP] are responsible for approx. 90% of the respiration rate increase during the state 4-->state 3 transition, while the remaining approx. 10% is due to changes in [Pi] and [ATP].

Regulation of ATP supply during muscle contraction: theoretical studies

The dynamic computer model of oxidative phosphorylation developed previously and successfully tested for large-scale changes in fluxes and metabolite concentrations was used to study the question of how the rate of ATP production by oxidative phosphorylation is adjusted to meet the energy demand during muscle contraction, which causes a great increase in ATP consumption in relation to the resting state. The changes in the respiration rate and ATP/ADP ratio after the onset of maximal work measured experimentally were compared with simulated changes in the respiration rate and ATP/ADP in several different cases, assuming direct activation of different steps by an external effector. On the basis of the computer simulations performed, it was possible to conclude which enzymes/metabolic blocks should be directly activated to cause the experimentally observable changes in fluxes and metabolite concentrations. The theoretical results obtained suggest that the parallel direct activation of actinomyosin-ATP-ase and oxidative phosphorylation by an external effector (for example calcium ions) is the main mechanism responsible for fitting of ATP production to ATP consumption, while the negative feedback via an increase in ADP concentration (decrease in ATP/ADP), which indirectly activates the ATP supply, plays only a minor role. Additionally, the conclusion is drawn that most of the oxidative phosphorylation steps should be directly activated in order to explain the observed changes in the respiration rate and ATP/ADP ratio (and also in other parameters) during muscle contraction. It is suggested that there should exist a universal external activator/regulatory mechanism which causes a parallel stimulation of different enzymes/processes. A possible nature of such an activator is shortly discussed.

Compartmentalized energy transfer in cardiomyocytes: use of mathematical modeling for analysis of in vivo regulation of respiration

The mathematical model of the compartmentalized energy transfer system in cardiac myocytes presented includes mitochondrial synthesis of ATP by ATP synthase, phosphocreatine production in the coupled mitochondrial creatine kinase reaction, the myofibrillar and cytoplasmic creatine kinase reactions, ATP utilization by actomyosin ATPase during the contraction cycle, and diffusional exchange of metabolites between different compartments. The model was used to calculate the changes in metabolite profiles during the cardiac cycle, metabolite and energy fluxes in different cellular compartments at high workload (corresponding to the rate of oxygen consumption of 46 mu atoms of O.(g wet mass)-1.min-1) under varying conditions of restricted ADP diffusion across mitochondrial outer membrane and creatine kinase isoenzyme "switchoff." In the complete system, restricted diffusion of ADP across the outer mitochondrial membrane stabilizes phosphocreatine production in cardiac mitochondria and increases the role of the phosphocreatine shuttle in energy transport and respiration regulation. Selective inhibition of myoplasmic or mitochondrial creatine kinase (modeling the experiments with transgenic animals) results in "takeover" of their function by another, active creatine kinase isoenzyme. This mathematical modeling also shows that assumption of the creatine kinase equilibrium in the cell may only be a very rough approximation to the reality at increased workload. The mathematical model developed can be used as a basis for further quantitative analyses of energy fluxes in the cell and their regulation, particularly by adding modules for adenylate kinase, the glycolytic system, and other reactions of energy metabolism of the cell.

Simulation of state 4 → state 3 transition in isolated mitochondria

The mathematical dynamic model of oxidative phosphorylation developed previously and in the accompanying paper was modified to involve isolated mitochondria conditions; it was also used to simulate state 4 --> state 3 transition in rat liver mitochondria incubated with succinate as respiratory substrate and glucose-hexokinase as an ADP-regenerating system. Changes in the respiration rate, protonmotive force and reduction level of ubiquinone and cytochrome c as well as the internal (i) and external (e) ATP/ADP ratio between state 4 and state 3 were calculated and compared with the experimental data. Flux control coefficients with respect to oxygen consumption flux for different reactions and processes of oxidative phosphorylation were simulated for different values of the respiration rate (state 4, state 3 and intermediate states). Flux control coefficients for the oxidation, phosphorylation and proton leak subsystems with respect to the oxidation, phosphorylation and proton leak fluxes for different values of the respiration rate were also calculated. These theoretical data were compared with the experimental results obtained in the frame of metabolic control analysis and the 'top-down' approach to this analysis. A good agreement was obtained. Simulated time courses of the respiration rate, the protonmotive force (Deltap) and other parameters after addition of a small amount of ADP to mitochondria in state 4 mimicked at least semiquantitatively the experimentally measured time courses of these parameters. It was concluded, therefore, that in the present stage, the model is able to reflect different properties of the oxidative phosphorylation system in a broad range of conditions fairly well, allows deeper insight into the mechanisms responsible for control and regulation of this process, and can be used for simulation of new experiments, thus inspiring experimental verification of the theoretical predictions.

Mechanisms of ischemic preconditioning in rat myocardium. Roles of adenosine, cellular energy state, and mitochondrial F1F0-ATPase

BACKGROUND

Adenosine has been proposed as one mediator for the preconditioning effect in the myocardium of some animals, but recent investigations have shown that this may not be the mechanism in the rat heart, although the effect itself is clearly demonstrable. The cellular energy state has been shown to be better in preconditioned hearts, and the role of ATP consumption has been discussed. The role of inhibition of mitochondrial F1F0-ATPase as a mechanism for the preservation of ATP in preconditioned hearts remains controversial.

METHODS AND RESULTS

Three-minute global ischemia followed by 9 minutes of reperfusion was used to precondition Langendorff-perfused rat hearts, and control hearts were perfused under normoxic conditions for the same time. The duration of sustained ischemia in both groups of hearts was 21 minutes, after which the hearts were reperfused for 15 minutes to evaluate their mechanical and metabolic recovery. Separate experiments were performed for tissue metabolite determinations, mitochondrial ATPase activity measurements, and 31P nuclear magnetic resonance studies. The recovery of the rate-pressure product was better in the preconditioned group. Three-minute preconditioning ischemia caused inhibition of the mitochondrial ATPase that persisted throughout the 9-minute intervening reperfusion so that at the early stages of sustained ischemia the enzyme activity was still more inhibited in preconditioned hearts. ATP was better preserved in preconditioned hearts than in control hearts during sustained ischemia. The accumulation of adenosine and its degradation products during sustained ischemia was greater in the control group. More lactate and H+ ions accumulated in this group, indicating higher anaerobic glycolysis. Also, inhibition of mitochondrial ATPase by oligomycin slowed ATP depletion during ischemia.

CONCLUSIONS

The results indicate that preconditioning causes inhibition of rat heart mitochondrial ATPase that persists during reperfusion so that the enzyme is inhibited from the very beginning of the sustained ischemia. This inhibition leads to sparing of high-energy phosphates and improves the time-averaged energy state during ischemia. Although a causal relationship is difficult to prove, this reversible inhibition may contribute to postischemic recovery of the heart.

Calcium and 2-oxoglutarate-mediated control of aspartate formation by rat heart mitochondria

Studies of the influence of calcium on the metabolism of cardiac mitochondria have indicated that calcium activates key enzymes involved in the citric acid cycle. Calcium-mediated activation of one of these enzymes, 2-oxoglutarate dehydrogenase, has been shown to cause a marked decrease in the steady-state concentration of 2-oxoglutarate in both heart and liver mitochondria. In liver, 2-oxoglutarate is a potent inhibitor of oxalacetate transamination to aspartate and activation of this enzyme by calcium-mobilizing hormones leads to a stimulation of aspartate formation and gluconeogenesis. Since mitochondrial aspartate formation is a key step in the malate/aspartate shuttle, we investigated the control of aspartate formation by cardiac mitochondria. In mitochondria incubated with glutamate and malate, activation of 2-oxoglutarate dehydrogenase by calcium led to an inhibition of aspartate formation. However, calcium caused a stimulation of aspartate production when incubations were supplemented with pyruvate as an additional substrate. Estimates of the mitochondrial redox potential (NADH/NAD+) indicated that both calcium and pyruvate increased the redox potential. The observed influence of calcium on aspartate formation was found to be due to a balance between is inhibitory effect, caused by an increased redox potential, and its stimulatory effect, caused by a decreased 2-oxoglutarate concentration. Under conditions in which the redox component was held constant, a kinetic analysis indicated that the apparent Ki for 2-oxoglutarate inhibition of aspartate formation is 0.2 mM. The data suggest that activation of cardiac 2-oxoglutarate dehydrogenase by calcium could lead to stimulation of the mitochondrial oxidation of cytosolic NADH via the malate/aspartate cycle.

Changes in mitochondrial function induced in isolated guinea-pig ventricular myocytes by calcium overload

1. Changes in [Ca2+]i and pHi, mitochondrial membrane potential (psi m) and mitochondrial [NADH] have been measured independently using fluorescent techniques in single isolated guinea-pig ventricular myocytes subjected to Ca2+ overload. 2. The changes in NADH autofluorescence on the inhibition or uncoupling of respiration are consistent with the signal emanating from the mitochondrial NADH. 3. Removal of Ca2+ and Mg2+ from the bathing Tyrode solution induced a modest fall in both [Ca2+]i and pHi, a small slowly developing depolarization of psi m and an initial fall followed by a rise in mitochondrial [NADH]. 4. In myocytes that maintained an intact sarcolemma, return to Ca(2+)-containing fluid elicited a strong but brief intracellular acidification, a rise in [Ca2+]i which generally recovered more slowly to stabilize above the initial level in Tyrode solution, a steep fall in mitochondrial [NADH] and a brief transient recovery followed by a large sustained depolarization of psi m. NADH autofluorescence and mitochondrial depolarization often reached values that were not further increased by uncoupling respiration although recovery of NADH was elicited by inhibitors of respiration. 5. These changes were reduced when the Ca2+ overload was less severe as evidenced by a reduced hypercontracture upon Ca2+ repletion. A similar reduction could be routinely achieved by elevation of [Mg2+]o during the period of Ca2+ depletion. 6. These results suggest that the well-established depletion of energy-rich phosphates that occurs on Ca2+ overload is due to the combined effects of the failure of the citric acid cycle to provide sufficient mitochondrial NADH for the respiratory chain and an uncoupling of respiration from ATP production due to depolarization of psi m. The former effect could result from the depletion of sarcoplasmic amino acids and the latter from increased Ca2+ cycling across the mitochondrial wall provoked by the elevated [Na+]i and [Ca2+]i.

Metabolic compartmentation and substrate channelling in muscle cells. Role of coupled creatine kinases in in vivo regulation of cellular respiration--a synthesis

The published experimental data and existing concepts of cellular regulation of respiration are analyzed. Conventional, simplified considerations of regulatory mechanism by cytoplasmic ADP according to Michaelis-Menten kinetics or by derived parameters such as phosphate potential etc. do not explain relationships between oxygen consumption, workload and metabolic state of the cell. On the other hand, there are abundant data in literature showing microheterogeneity of cytoplasmic space in muscle cells, in particular with respect to ATP (and ADP) due to the structural organization of cell interior, existence of multienzyme complexes and structured water phase. Also very recent experimental data show that the intracellular diffusion of ADP is retarded in cardiomyocytes because of very low permeability of the mitochondrial outer membrane for adenine nucleotides in vivo. Most probably, permeability of the outer mitochondrial membrane porin channels is controlled in the cells in vivo by some intracellular factors which may be connected to cytoskeleton and lost during mitochondrial isolation. All these numerous data show convincingly that cellular metabolism cannot be understood if cell interior is considered as homogenous solution, and it is necessary to use the theories of organized metabolic systems and substrate-product channelling in multienzyme systems to understand metabolic regulation of respiration. One of these systems is the creatine kinase system, which channels high energy phosphates from mitochondria to sites of energy utilization. It is proposed that in muscle cells feed-back signal between contraction and mitochondrial respiration may be conducted by metabolic wave (propagation of oscillations of local concentration of ADP and creatine) through cytoplasmic equilibrium creatine and adenylate kinases and is amplified by coupled creatine kinase reaction in mitochondria. Mitochondrial creatine kinase has experimentally been shown to be a powerful amplifier of regulatory action of weak ADP fluxes due to its coupling to adenine nucleotide translocase. This phenomenon is also carefully analyzed.

Ferrylmyoglobin formation induced by acute magnesium deficiency in perfused rat heart causes cardiac failure

The oxidation states of intracellular myoglobin and cytochrome oxidase aa3 were monitored by reflectance spectrophotometry in isolated perfused rat hearts subjected to an acutely magnesium deficient environment. After exposure to low extracellular [Mg2+]o (i.e., 0.3 mM) for 30 min, more than 80% of the oxymyoglobin converted to its deoxygenated form. The level of reduced cytochrome oxidase aa3 also increased about 80% in low [Mg2+]o. The deoxymyoglobin was converted further to a species identified as ferrylmyoglobin by its reaction with Na2S to form ferrous sulfmyoglobin which was optically visible. This process, set into motion by acute Mg deficiency, resulted from a direct accessibility of the exogenous peroxide to the cytosolic protein. The results suggest that a pathway leading to cardiac tissue damage, induced by magnesium deficiency, is probably involved in the generation of a ferrylmyoglobin radical which could be prevented by addition of ascorbate, which is known to be a one-electron reductant of this hypervalent form of myoglobin. In further studies, we also investigated whether addition of different concentrations of ascorbic acid (AA) to the perfusate could enhance myocardial function after exposure to low [Mg2+]o perfusion. Four concentrations of AA (0.5, 1, 5, 10 mM) were tested, and the results indicate that they exert their effects in a concentration-dependent manner; 1 mM AA was the most effective dose in improving aortic output in a Mg-deficient heart. Ferrylmyoglobin formation was found to be formed considerably before intracellular release of either creatine phosphokinase or lactic dehydrogenase. These studies may have wide implications as a new mechanism by which low extracellular Mg2+ can induce myocardial injury and subsequent cardiac failure.

Retarded diffusion of ADP in cardiomyocytes: possible role of mitochondrial outer membrane and creatine kinase in cellular regulation of oxidative phosphorylation

Possible reasons for retarded intracellular diffusion of ADP were investigated. The isolated skinned cardiac fibers were used to study apparent kinetic parameters for externally added ADP in control of mitochondrial respiration. Participation of myosin-ATPase in binding of ADP within cells as it was supposed earlier (Saks, V.A., Belikova, Yu.O. and Kuznetsov, A.V. (1991) Biochim. Biophys. Acta 1074, 302-311) was completely excluded, since myosin-deprived skinned cardiac fibers ('ghosts') displayed the same kinetic parameters as intact ones (Kmapp for ADP about 300 microM). Significantly lower apparent Km values were obtained for fibers with osmotically disrupted outer mitochondrial membrane (25-35 microM), which was close to that observed for isolated heart mitochondria. The data obtained are in favor of limitation of ADP movement via anion-selective low-conductance porine channels in the outer membrane of mitochondria. It is proposed that the permeability of this membrane is controlled by some unknown intracellular factor(s). In the presence of saturating concentrations of creatine (25 mM) the apparent Km for ADP significantly decreases due to coupling of creatine kinase and oxidative phosphorylation reactions in mitochondria. This coupling is not observed in KCl medium in which mitochondrial creatine kinase is detached from the membrane. It is concluded that in the cells in-vivo ADP movement between cytoplasm and intramitochondrial space is controlled by low-conductivity anion channels in the outer membrane. Thus, the mitochondrial creatine kinase reaction coupled to the adenine nucleotide translocase is an important mechanism in control of oxidative phosphorylation in vivo due to its ability to manifold amplify these very weak ADP signals from cytoplasm.

Evaluation of myocardial energy status in vivo by NMR spectroscopy

NMR spectroscopy is a powerful and non-invasive technique with which to study cardiac energy metabolism in vivo. This method makes use of the "spin" properties of certain atomic nuclei. The naturally occurring phosphorus nucleus (P-31) is visible by NMR and phosphorus-31 NMR spectra contain signals from the major components of energy metabolism. In vivo, the phosphocreatine to ATP ratio (PCr/ATP) is used as an index of the energy status and viability of the myocardium. However, it is the response of this metabolic index to differing physiological and pharmacological stresses that has helped to elucidate the mechanisms that regulate cellular respiration and to highlight abnormalities in heart failure. As there are many technical difficulties involved with cardiac NMR, 31-phosphorus studies of skeletal muscle have provided an indirect way of studying abnormalities in myocardial metabolism in vivo. One of the unique features of NMR is that it permits in vivo measurements of fluxes through key enzymes in energy metabolism using magnetization transfer. Determination of the rates of energy transfer through the creatine kinase reaction and energy turnover in vivo will provide new insights into the control of energy metabolism in health and disease. Alternatively, carbon-13 NMR can be used to measure fluxes through the different metabolic pathways of synthesis and catabolism following administration of selectively labelled carbon-13 substrates. In conclusion, the non-invasive and versatile nature of NMR spectroscopy makes it an ideal method to assess and evaluate energy metabolism in vivo.

Effect of macromolecules on the regulation of the mitochondrial outer membrane pore and the activity of adenylate kinase in the inter-membrane space

Macromolecules as components of the physiological mitochondrial environment were substituted by dextrans of different molecular weight. The addition of 10% dextran (molecular weights varying between 20 and 500 kDa) affected neither basic mitochondrial parameters (state 4 and state 3 respiration) nor kinetic properties of soluble kinases. A significant increase by 10% dextran was however observed of the voltage sensitivity of isolated porin when reconstituted in planar bilayers. The pores adapted the low conducting state already at a voltage of 10 mV. This effect of the macromolecules may explain the higher diffusion resistance of adenine nucleotides across the outer membrane as observed in different experiments: (i) the Michaelis constant of adenylate kinase in the inter-membrane space increased, in contrast to the soluble enzyme, from 118 +/- 10 microM to 193 +/- 20 microM ADP, (ii) in the presence of competing external pyruvate kinase, the mitochondrial utilization of ADP, produced by adenylate kinase in the inter-membrane space, was improved 3-fold suggesting a reduced ADP diffusion out of the outer mitochondrial compartment. The influence of the various dextrans correlated with the increase in molecular weight of the dextrans. The effect on the kinetic constants was dependent on the dextran concentration in terms of weight and not of molarity. The oncotic pressure and viscosity of dextran solutions with different molecular weight showed a comparable dependence. In general, the data indicate that the outer membrane pore responds to an increased oncotic pressure by reducing adenine nucleotide permeability. This suggests the physiological existence of a third adenine nucleotide compartment between the two envelope membranes which may be important especially at high metabolic fluxes.

Effects of hydrogen peroxide on mitochondrial enzyme function studied in situ in rat heart myocytes

Our previous work indicated that energy transduction, as measured by myocyte respiration, was inhibited by hydrogen peroxide, but the mitochondrial membrane potential was relatively unaffected. Therefore, we determined in the present study the critical steps in mitochondrial energy transduction by measuring the sensitivity to hydrogen peroxide of NADH-CoQ reductase, ATP synthase, and adenine nucleotide translocase in situ in myocytes. Adult rat heart cells were isolated using collagenase and incubated in the presence of 0.1-10 mM hydrogen peroxide for 30 min. Activities of NADH-CoQ reductase and oligomycin-sensitive ATP synthase were assayed enzymatically with sonicated myocytes, and adenine nucleotide translocase activities were determined by atractyloside-inhibitable [14C]ADP uptake of myocytes, permeabilized by saponin. The NADH-CoQ reductase and ATP synthase activities were inhibited to 77% and 67% of control, respectively, following an exposure to 10 mM hydrogen peroxide for 30 min. The adenine nucleotide translocase activities were inhibited in a concentration- and time-dependent manner and by 10 mM hydrogen peroxide to 44% of control. The dose-response relationship indicated that the translocase was the most susceptible to hydrogen peroxide among the three enzymes studied. Combined treatment of myocytes with 3-amino-1,2,4-triazole, 1,3-bis(2-chloroethyl)-1-nitrosourea and diethyl maleate (to inactivate catalase, to inhibit glutathione reductase activity, and to deplete glutathione, respectively) enhanced the sensitivity of translocase to hydrogen peroxide, supporting the view that the cellular defense mechanism is a significant factor in determining the toxicity of hydrogen peroxide. The results indicate that hydrogen peroxide can cause dysfunction in mitochondrial energy transduction, principally as the result of inhibition of adenine nucleotide translocase.

Respiratory control and substrate effects in the working rat heart

31P n.m.r. spectroscopy was used to measure the concentration of phosphates commonly proposed to control oxidative phosphorylation. The effect of loading conditions, beta-adrenergic stimulation and different substrates (acetate, pyruvate or glucose) was examined under steady-state conditions in the isolated working rat heart. Oxygen consumption and haemodynamic variables were monitored continuously. In response to a 2-fold increase in afterload, there were no significant changes in [ADP], [ATP]/[ADP], or [ATP]/[ADP][Pi]. In the presence of isoprenaline, these variables also tended not to change from afterload. However, isoprenaline, at identical perfusion pressures, consistently decreased the phosphorylation potential and [ATP]/[ADP], but had little effect on [ADP]. Substrates altered the phosphate metabolites in a manner independent of oxygen consumption, and had only minor effects on the relationship between phosphates and work, in contrast with other studies. Thus, metabolites of ATP synthesis are not normally involved in respiratory control. The 31P n.m.r. spectrum can vary greatly, but does not predict oxygen consumption in this preparation. Substrates have no effect on the mechanism of respiratory control. Thus the normal control of respiration in the heart at steady state cannot occur at the level of its substrates. Rather, there must be concerted regulation of the numerous pathways, involving allostery and covalent modification. The attention of future research should be shifted away from the metabolites of ATP and towards identifying the effectors of such regulation.

Vascular oxidative metabolism under different metabolic conditions

Control of respiration in vascular smooth muscle was examined while the metabolic state of the tissue was manipulated. During KCl-induced contractures in the presence of 5 mM glucose, oxygen consumption increased by 10 nmol/per min g without any decrease in phosphocreatine (PCr) or ATP as determined by 31P-NMR indicating a control of respiration which does not involve changes in high-energy phosphates (e.g., ADP, phosphorylation potential). However, when aortae with resting tone in the absence of substrate were then provided with 5 mM 2-deoxyglucose as the sole substrate, oxygen consumption increased 7.4 nmol/min per g while PCr decreased by more than 50% (resulting in a 2-fold increase in the calculated free ADP) with no change in tension from resting tone. During a subsequent KCl induced contracture in the presence of 2-deoxyglucose, oxygen consumption increased an additional 7.2 nmol/min per g while PCr continued to decline. Therefore, at least two mechanisms of respiratory control may exist in sheep aorta, one dependent and the other independent of changes in high-energy phosphates.

Control of mitochondrial ATP synthesis in the heart

Images

An extended dynamic model of oxidative phosphorylation

The presented model based on an earlier one (Korzeniewski, B. and Froncisz, W. (1989) Studia Biophys. 132, 173-187) simulates concentration changes in time of chemical compounds and thermodynamic forces during respiration of cell suspension in a closed chamber. A set of differential equations solved numerically describes the utilization of oxygen up to anaerobiosis and the behaviour of the system after a sudden pulse of oxygen. Flux control coefficients for most important reactions (enzymes) of oxidative phosphorylation were calculated. A good qualitative and (when a direct comparison is possible) quantitative agreement with experimental results can be observed. The following conclusions can be drawn from the simulation: (1) Wilson's steady state model is not in contradiction with sharing of the control over the respiration between some steps and displacement of the ATP/ADP carrier from equilibrium. (2) The overshoot characteristics of the delta microH+ time-course after reoxygenation can be explained without using the lag-phase kinetics of ATP-synthetase. (3) A 'hot region' (sharp changes of many parameters) can be distinguished when the oxygen concentration approaches zero; only cytochrome oxidase is clearly sensitive on oxygen concentration in all its range. (4) Control over oxidative phosphorylation is shared mainly between inputs of the system (ATP utilization and substrate dehydrogenation) and the proton leak.

Cardiac contractile function, oxygen consumption rate and cytosolic phosphates during inhibition of electron flux by amytal--a 31P-NMR study

In order to investigate the potential role of cytosolic phosphates ([ATP], [ADP] and [Pi]) in the integration of mitochondrial respiration and mechanical function in the perfused heart, inhibition of the substrate end of the respiratory chain by amytal has been employed. A stepwise increase in amytal concentration (from 0.2 to 1.2 mM) resulted in the progressive abolition of the cardiac oxygen consumption, rate (VO2) in hearts oxidizing pyruvate (5 mM). The inhibition curve for VO2 was S-shaped, with K0.5 = 1.1 mM, and independent of the initial VO2 values varied by coronary flow and isoproterenol (Iso) addition. ADP-stimulated respiration of isolated mitochondria (malate + pyruvate) was twice as sensitive to amytal inhibition, whereas state 2 respiration (before ADP addition) had the same sensitivity as cardiac VO2. Decrease in VO2 was followed by a decline in phosphocreatine (PCr) content and augmentation of Pi at nearly constant ATP level and intracellular pH as assessed by the 31P-NMR method. These changes were associated with an elevation of cytosolic free [ADP] and a reduction of the [ATP]/[ADP] ratio and ATP affinity calculated from creatine kinase equilibrium. Concomitantly, pressure-rate product (PRP), maximal rates of contraction and relaxation fell down and the end diastolic pressure (EDP) rose at all initial loads. Amytal-inhibited hearts retained the capability to respond to Iso stimulation (0.1 microM, about 50% enhancement of PRP) even at 1 mM amytal, but their response to elevation of coronary flow was greatly diminished. Alterations in the PRP value induced by the inhibitor at a fixed coronary flow correlated negatively with cytosolic [ADP] and [Pi], and positively with [ATP]/[ADP] and A(ATP). In contrast, EDP correlated with all these parameters in the opposite manner. However, when PRP was varied by coronary flow in the absence of the inhibitor or at its fixed concentrations, such correlations were absent. These data imply that cytosolic phosphates can serve as a feedback between energy production and utilization when the control point(s) is (are) at the mitochondria. In contrast, other regulatory mechanisms should be involved when control is distributed among different steps located both in energy producing and utilizing systems.

The contribution of the leak of protons across the mitochondrial inner membrane to standard metabolic rate

This paper presents and assesses the hypothesis that the proton leak across the mitochondrial inner membrane is an important contributor to standard metabolic rate, and that increases in the amount of mitochondrial inner membrane may be important in causing changes in proton leak and in the standard metabolic rate. The standard metabolic rate of an animal is known to be a function of body mass, phylogeny and thyroid status, and is largely attributed to the metabolically active internal organs. The total area of mitochondrial inner membrane in these organs correlates well with standard metabolic rate over a wide range of body masses in both ectotherms and endotherms. In hepatocytes isolated from rats, proton leak across the mitochondrial inner membrane accounts for about 30% of the resting oxygen consumption, and the distribution of control over respiration suggests that changes in mitochondrial inner membrane surface area will be accompanied by significant changes in the proton leak. This change in the leak will result in significant changes in resting oxygen consumption, but changes in ATP demand may also have a role to play in determining resting respiration rate. Extrapolation of these results to other tissues and other animals suggests that the hypothesis has the potential to explain a substantial proportion of the variation in standard metabolic rate with body mass, phylogeny and thyroid status. However, in most cases the quantitative contribution of proton leak compared to cellular ATP turnover has yet to be experimentally determined.

Regulation of oxidative phosphorylation in the mammalian cell

The cell is capable of maintaining a steady-state flux of energy from mitochondrial oxidative phosphorylation, producing ATP, to the cytosolic adenosinetriphosphatases (ATPases), performing work. Considerable effort has been devoted to investigating the individual mechanisms involved in these two processes. However, less effort has been directed toward learning how these reactions of energy metabolism interact through the cytosol to maintain the observed steady state in the intact cell. The "classical" model for the cytosolic interaction of these two processes involves the feedback of ATP hydrolysis products, ADP and Pi, from the ATPases to oxidative phosphorylation. This model is based on data from isolated mitochondria in which the rate of oxidative phosphorylation is controlled by the concentration of ADP and Pi. Yet, recent data from intact tissues with high oxidative phosphorylation capacities (i.e., heart, brain, and kidney) indicate that the cytosolic concentration of ADP and Pi do not change significantly with work. These data imply that this simple feedback model is not adequate to explain the regulation of energy metabolism in these tissues. Other sites within the oxidative phosphorylation process must be playing a regulatory role or the kinetics of ATP synthesis must be very different than currently believed to establish the steady state. This review covers the potential sites within oxidative phosphorylation which may be regulated through cytosolic transducers to result in the necessary feedback network regulating the steady-state flow of energy in the cell. These sites will include substrate delivery to the cytochrome chain, the processes involved in the phosphorylation of ADP to ATP, and the delivery of oxygen.

Adenosine triphosphate turnover in humans. Decreased degradation during relative hyperphosphatemia

The regulation of ATP metabolism by inorganic phosphate (Pi) was examined in five normal volunteers through measurements of ATP degradation during relative Pi depletion and repletion states. Relative Pi depletion was achieved through dietary restriction and phosphate binders, whereas a Pi-repleted state was produced by oral Pi supplementation. ATP was radioactively labeled by the infusion of [8(14)C]adenine. Fructose infusion was used to produce rapid ATP degradation during Pi depletion and repletion states. Baseline measurements indicated a significant decrease of Pi levels during phosphate depletion and no change in serum or urinary purines. Serum values of Pi declined 20 to 26% within 15 min after fructose infusion in all states. Urine measurements of ATP degradation products showed an eightfold increase within 15 min after fructose infusion in both Pi-depleted and -supplemented states. Urinary radioactive ATP degradation products were fourfold higher and urinary purine specific activity was more than threefold higher during Pi depletion as compared with Pi repletion. Our data indicate that there is decreased ATP degradation to purine end products during a relative phosphate repletion state as compared to a relative phosphate depletion state. These data show that ATP metabolism can be altered through manipulation of the relative Pi state in humans.

Substrate dependence of energy metabolism in isolated guinea-pig cardiac muscle: a microcalorimetric study

1. The effects of glucose, pyruvate and lactate on basal metabolism and on contraction-related energy expenditure of thin trabeculae isolated from guinea-pig heart were studied using a microcalorimetric technique. 2. Resting heat rates of cardiac ventricular muscle measured in the presence of substrate-free solution (56 +/- 20 mW (g dry weight)-1), 10 mM-lactate (54 +/- 12 mW (g dry weight)-1) and 10 mM-glucose (63 +/- 24 mW (g dry weight)-1) did not differ significantly. Increasing the external glucose concentration (up to 100 mM) and/or adding insulin (up to 80 units l-1) had virtually no effect on the measured resting heat rate. 3. With 10 mM-pyruvate as substrate resting heat rate was substantially larger (106 +/- 40 mW (g dry weight)-1) than with glucose, lactate or substrate-free solution. The concentrations of pyruvate producing a half-maximal increase in resting heat rate as compared to substrate-free solution ranged between 0.4 and 1.2 mM. 4. In order to test whether the development of an anoxic core contributed to the substrate dependence of resting heat production the critical PO2 (i.e. the PO2 that produced a just-noticeable decrease in heat rate) was determined in cylindrical preparations of various diameters. It was found that none of the preparations had an anoxic core at rest in a solution equilibrated with 100% oxygen. 5. From the dependence of the critical PO2 on the diameter of the preparation the diffusion coefficient of oxygen through cardiac muscle was calculated using a modification of Hill's equation (Hill, 1928). The O2 diffusion coefficient was found to be 1.09 X 10(-5) cm2 s-1. 6. Contraction-related heat production was also found to be dependent on the substrate used. In the presence of 10 mM-pyruvate it was about 60% larger than in the presence of 10 mM-glucose, 10 mM-lactate or with substrate-free solution. 7. Isometric force of contraction showed the same substrate dependence as contraction-related heat production and increased with a similar time course during repetitive stimulation. 8. The possible mechanisms underlying the substrate dependence of myocardial energy metabolism are discussed. It is suggested that the increased energy expenditure observed in the presence of pyruvate may be related to a decrease in intracellular phosphate and/or to an increase in intracellular pH.

Developmental changes in the relation between phosphate metabolites and oxygen consumption in the sheep heart in vivo

This study examines the role of phosphate metabolites in the regulation of mitochondrial oxygen consumption of the heart in vivo as a function of development. We used an open chest lamb/sheep preparation in which myocardial oxygen consumption (MVO2) was monitored via an extracorporeal shunt from the coronary sinus. Phosphate metabolites were monitored simultaneously using 31P nuclear magnetic resonance with a surface coil overlying the left ventricle. Graded infusions of epinephrine were used to increase MVO2 in both neonatal lambs (age 5-12 d, n = 8), and mature sheep (26-86 d, n = 6). The maximal increase in MVO2 achieved was 220 +/- 38% in the newborns and 350 +/- 66% in the mature animals. Associated with these increases in MVO2 in the newborn lambs are significant (P less than 0.001) decreases in PCr/ATP, and increases in calculated ADP and intracellular Pi. This was in contrast to the mature sheep, in which there were no significant changes in PCr/ATP, ADP, or Pi. In conclusion, we find that (a) there are changes in PCr/ATP, Pi, and ADP in newborn animals with moderate increases in work that are not apparent in mature animals of the same species and (b) that these changes suggest that cytosolic ATP hydrolysis products may be more important in regulation of myocardial energy metabolism in the newborn than in the adult.

Hypothyroidism in rats does not lower mitochondrial ADP/O and H+/O ratios

We investigated reports that mitochondria isolated from hypothyroid rats have decreased ADP/O and H+/O ratios. We observed no decrease in the H+/O ratio in mitochondria from hypothyroid rats, in the presence of either 2% (w/v) fatty-acid-free bovine serum albumin or 100 nM free Ca2+. The ADP/O ratio in mitochondria isolated from hypothyroid rats in the presence of 2% fatty-acid-free bovine serum albumin was measured. Under normal experimental conditions we found no decrease in the ADP/O ratio, relative to that measured for littermate controls. At the low concentrations of mitochondrial protein used in the previously reported studies, the ADP/O ratio of mitochondria from hypothyroid rats was decreased, whereas that for control rats was only slightly decreased. The difference between the ADP/O ratios measured for mitochondria form hypothyroid rats and from control rats under these conditions was eliminated by inhibition of endogenous adenylate kinase. We suggest that the lowering of the apparent ADP/O ratio in mitochondria from hypothyroid rats at low concentrations of mitochondrial protein is an experimental artefact resulting from the breakdown of ADP to AMP.

Oxygen and substrate dependence of hepatic cellular respiration: sinusoidal oxygen gradient and effects of ethanol in isolated perfused liver and hepatocytes

The oxygen dependence of hepatic cellular respiration was studied by employing simultaneous organ spectrophotometry of cytochromes and hemoglobin, the latter used as an intrasinusoidal optical oxygen probe. The Km of cytochrome aa3 for oxygen was found to be 6.8 microM in the isolated perfused liver and 0.3 microM in suspensions of isolated hepatocytes. The results indicate that the sinusoid-to-cell pO2 gradient is about 5 torr. Optical determination of the average effective pO2 indicates that the axial sinusoidal O2 profile does not conform to zero-order O2 uptake in the liver. Because of extensive NAD+ reduction, ethanol increases the thermodynamic driving force of oxidative phosphorylation, and it also increased the oxygen consumption in both the perfused liver and the hepatocyte suspension, but had no effect on the grade of steady-state cytochrome aa3 reduction, the cellular energy state [ATP]/[ADP].[Pi], or the Km of cytochrome aa3 for oxygen. The results indicate that hepatic energy metabolism is oxygen independent at very low O2 concentrations, but that the sinusoidal axial O2 concentration is anomalous, probably due to the spatial arrangement of the metabolizing systems.