Reaction of oxygen with the respiratory chain in cells and tissues

Response of mitochondrial reactive oxygen species generation to steady-state oxygen tension: implications for hypoxic cell signaling

Mitochondria are proposed to play an important role in hypoxic cell signaling. One currently accepted signaling paradigm is that the mitochondrial generation of reactive oxygen species (ROS) increases in hypoxia. This is paradoxical, because oxygen is a substrate for ROS generation. Although the response of isolated mitochondrial ROS generation to [O(2)] has been examined previously, such investigations did not apply rigorous control over [O(2)] within the hypoxic signaling range. With the use of open-flow respirometry and fluorimetry, the current study determined the response of isolated rat liver mitochondrial ROS generation to defined steady-state [O(2)] as low as 0.1 microM. In mitochondria respiring under state 4 (quiescent) or state 3 (ATP turnover) conditions, decreased ROS generation was always observed at low [O(2)]. It is concluded that the biochemical mechanism to facilitate increased ROS generation in response to hypoxia in cells is not intrinsic to the mitochondrial respiratory chain alone but may involve other factors. The implications for hypoxic cell signaling are discussed.

Mitochondrial function in vivo evaluated by NADH fluorescence: from animal models to human studies

Normal mitochondrial function is a critical factor in maintaining cellular homeostasis in various organs of the body. Due to the involvement of mitochondrial dysfunction in many pathological states, the real-time in vivo monitoring of the mitochondrial metabolic state is crucially important. This type of monitoring in animal models as well as in patients provides real-time data that can help interpret experimental results or optimize patient treatment. The goals of the present review are the following: 1) to provide an historical overview of NADH fluorescence monitoring and its physiological significance; 2) to present the solid scientific ground underlying NADH fluorescence measurements based on published materials; 3) to provide the reader with basic information on the methodologies used in the past and the current state of the art fluorometers; and 4) to clarify the various factors affecting monitored signals, including artifacts. The large numbers of publications by different groups testify to the valuable information gathered in various experimental conditions. The monitoring of NADH levels in the tissue provides the most important information on the metabolic state of the mitochondria in terms of energy production and intracellular oxygen levels. Although NADH signals are not calibrated in absolute units, their trend monitoring is important for the interpretation of physiological or pathological situations. To understand tissue function better, the multiparametric approach has been developed where NADH serves as the key parameter. The development of new light sources in UV and visible spectra has led to the development of small compact units applicable in clinical conditions for better diagnosis of patients.

Modeling of oxygen transport and cellular energetics explains observations on in vivo cardiac energy metabolism

Observations on the relationship between cardiac work rate and the levels of energy metabolites adenosine triphosphate (ATP), adenosine diphosphate (ADP), and phosphocreatine (CrP) have not been satisfactorily explained by theoretical models of cardiac energy metabolism. Specifically, the in vivo stability of ATP, ADP, and CrP levels in response to changes in work and respiratory rate has eluded explanation. Here a previously developed model of mitochondrial oxidative phosphorylation, which was developed based on data obtained from isolated cardiac mitochondria, is integrated with a spatially distributed model of oxygen transport in the myocardium to analyze data obtained from several laboratories over the past two decades. The model includes the components of the respiratory chain, the F₀F₁-ATPase, adenine nucleotide translocase, and the mitochondrial phosphate transporter at the mitochondrial level; adenylate kinase, creatine kinase, and ATP consumption in the cytoplasm; and oxygen transport between capillaries, interstitial fluid, and cardiomyocytes. The integrated model is able to reproduce experimental observations on ATP, ADP, CrP, and inorganic phosphate levels in canine hearts over a range of workload and during coronary hypoperfusion and predicts that cytoplasmic inorganic phosphate level is a key regulator of the rate of mitochondrial respiration at workloads for which the rate of cardiac oxygen consumption is less than or equal to approximately 12 μmol per minute per gram of tissue. At work rates corresponding to oxygen consumption higher than 12 μmol min⁻¹ g⁻¹, model predictions deviate from the experimental data, indicating that at high work rates, additional regulatory mechanisms that are not currently incorporated into the model may be important. Nevertheless, the integrated model explains metabolite levels observed at low to moderate workloads and the changes in metabolite levels and tissue oxygenation observed during graded hypoperfusion. These findings suggest that the observed stability of energy metabolites emerges as a property of a properly constructed model of cardiac substrate transport and mitochondrial metabolism. In addition, the validated model provides quantitative predictions of changes in phosphate metabolites during cardiac ischemia.

To function properly over a range of work rates, the heart must maintain its metabolic energy level within a range that is narrow relative to changes in the rate of energy utilization. Decades of observations have revealed that in cardiac muscle cells, the supply of adenosine triphosphate (ATP)—the primary currency of intracellular energy transfer—is controlled to maintain intracellular concentrations of ATP and related compounds within narrow ranges. Yet the development of a mechanistic understanding of this tight control has lagged behind experimental observation. This paper introduces a computational model that links ATP synthesis in a subcellular body called the mitochondrion with ATP utilization in the cytoplasm, and reveals that the primary control mechanism operating in the system is feedback of substrate concentrations for ATP synthesis. In other words, changes in the concentrations of the products generated by the utilization of ATP in the cell (adenosine diphosphate and inorganic phosphate) effect changes in the rate at which mitochondria utilize those products to resynthesize ATP.

Two Sites of Interaction of Anions with Cytochrome a in Oxidized Bovine Cytochrome c Oxidase

An interaction between cytochrome a in oxidized cytochrome c oxidase (CcO) and anions has been characterized by EPR spectroscopy. Those anions that affect the EPR g = 3 signal of cytochrome a can be divided into two groups. One group consists of halides (Cl-, Br-, and I-) and induces an upfield shift of the g = 3 signal. Nitrogen-containing anions (CN-, NO2-, N3-, NO3-) are in the second group and shift the g = 3 signal downfield. The shifts in the EPR spectrum of CcO are unrelated to ligand binding to the binuclear center. The binding properties of one representative from each group, azide and chloride, were characterized in detail. The dependence of the shift on chloride concentration is consistent with a single binding site in the isolated oxidized enzyme with a Kd of approximately 3 mm. In mitochondria, the apparent Kd was found to be about four times larger than that of the isolated enzyme. The data indicate it is the chloride anion that is bound to CcO, and there is a hydrophilic size-selective access channel to this site from the cytosolic side of the mitochondrial membrane. An observed competition between azide and chloride is interpreted by azide binding to three sites: two that are apparent in the x-ray structure plus the chloride-binding site. It is suggested that either Mg2+ or Arg-438/Arg-439 is the chloride-binding site, and a mechanism for the ligand-induced shift of the g = 3 signal is proposed.

15O Radioactivity clearance is faster after intracarotid bolus injection of 15O-labeled oxyhemoglobin than after 15O-water injection

The authors tested the hypothesis that the oxygen content of brain tissue is negligible by injecting an intracarotid bolus of 15O-labeled tracer into rats. Under the hypothesis, the clearance rates of 15O radioactivity from the brain after injections of both 15O-labeled water (H(2)15O) and 15O-labeled oxyhemoglobin (HbO15O) should be identical. However, the logarithmic slope of the 15O radioactivity curve after HbO15O injection (0.494 +/- 0.071 min-1) was steeper than that after H(2)15O injection (0.406 +/- 0.038 min-1) (P<0.001, n = 13), where the time range used in the comparison was between 60 and 120 seconds after the injection. A possible interpretation of this result is that nonmetabolized O15O may dwell in the brain tissue for a finite period of time before it is eventually metabolized or returned to the blood stream unaltered. These findings contradict assumptions made by models currently used to measure cerebral oxygen metabolism.

Myoglobin function reassessed

The heart and those striated muscles that contract for long periods, having available almost limitless oxygen, operate in sustained steady states of low sarcoplasmic oxygen pressure that resist change in response to changing muscle work or oxygen supply. Most of the oxygen pressure drop from the erythrocyte to the mitochondrion occurs across the capillary wall. Within the sarcoplasm, myoglobin, a mobile carrier of oxygen, is developed in response to mitochondrial demand and augments the flow of oxygen to the mitochondria. Myoglobin-facilitated oxygen diffusion, perhaps by virtue of reduction of dimensionality of diffusion from three dimensions towards two dimensions in the narrow spaces available between mitochondria, is rapid relative to other parameters of cell respiration. Consequently, intracellular gradients of oxygen pressure are shallow, and sarcoplasmic oxygen pressure is nearly the same everywhere. Sarcoplasmic oxygen pressure, buffered near 0.33 kPa (2.5 torr; equivalent to approximately 4 micro mol l(-1) oxygen) by equilibrium with myoglobin, falls close to the operational K(m) of cytochrome oxidase for oxygen, and any small increment in sarcoplasmic oxygen pressure will be countered by increased oxygen utilization. The concentration of nitric oxide within the myocyte results from a balance of endogenous synthesis and removal by oxymyoglobin-catalyzed dioxygenation to the innocuous nitrate. Oxymyoglobin, by controlling sarcoplasmic nitric oxide concentration, helps assure the steady state in which inflow of oxygen into the myocyte equals the rate of oxygen consumption.

Characterization and function of mitochondrial nitric-oxide synthase

The mitochondrial production of nitric oxide is catalyzed by a nitric-oxide synthase. This enzyme has the same cofactor and substrate requirements as other constitutive nitric-oxide synthases. Its occurrence was demonstrated in various mitochondrial preparations (intact, purified mitochondria, permeabilized mitochondria, mitoplasts, submitochondrial particles) from different organs (liver, heart) and species (rat, pig). Endogenous nitric oxide reversibly inhibits oxygen consumption and ATP synthesis by competitive inhibition of cytochrome oxidase. The increased K(m) of cytochrome oxidase for oxygen and the steady-state reduction of the electron chain carriers provided experimental evidence for the direct interaction of this oxidase with endogenous nitric oxide. The increase in hydrogen peroxide production by nitric oxide-producing mitochondria not accompanied by the full reduction of the respiratory chain components indicated that cytochrome c oxidase utilizes nitric oxide as an alternative substrate. Finally, effectors or modulators of cytochrome oxidase (the irreversible step in oxidative phosphorylation) had been proposed during the last 40 years. Nitric oxide is the first molecule that fulfills this role (it is a competitive inhibitor, produced at a fair rate near the target site) extending the oxygen gradient to tissues.

Improved renal preservation with PB-3 flush solution during 72 h of cold storage

Previously, PB-2 flush solution has been found to be superior to Collins 2 solution (C-2) in extending renal viability in the dog. To further characterize preservation mechanisms, we studied mitochondrial oxidative function during 72 h of cold storage comparing PB-3, UW-1, and C-2 flush storage solutions. Complex 1 dependent mitochondrial oxidative phosphorylation (MOP) was found to be significantly less (P < 0.001) at 5 h and 72 h of cold storage (P < 0.001) for the C-2 compared to the other flush groups. Complex 2 dependent MOP had parallel results, having significantly less function (P < 0.002) at 5 h and 72 h (P < 0.02) of cold storage in the C-2 group compared to the other groups. PB-3 and UW-1 solutions were noted to be comparable, suggesting possible equivalent preservation efficacy. Nevertheless the components of PB-3 at the level of MOP contributed significantly to better preservation compared to C-2 solution.

Effect of hyperoxia, hypercapnia, and hypoxia on cerebral interstitial oxygen tension and cerebral blood flow

The assessment of cerebral interstitial oxygen tension (piO(2)) can provide valuable information regarding cerebrovascular physiology and brain function. Compartment-specific cerebral piO(2) was measured by (19)F NMR following the infusion of an oxygen-sensitive perfluorocarbon directly into the interstitial and ventricular space of the in vivo rat brain. (19)F T(1) measurements were made and cerebral piO(2) were obtained through in vitro calibrations. The effects of graded hyperoxia, hypercapnia, and hypoxia on piO(2) and cerebral blood flow (CBF) were investigated. Under normoxia (arterial pO(2) approximately 120 mm Hg), piO(2) was approximately 30 mm Hg and jugular venous pO(2) was approximately 50 mm Hg. During hyperoxia (arterial pO(2) = 90-300 mm Hg), piO(2) increased linearly with the arterial pO(2). Following hypercapnia (arterial pCO(2) = 20-60 mm Hg), the piO(2) increased sigmoidally with increasing CBF. With hypoxia (arterial pO(2) = 30-40 mm Hg), CBF increased approximately 56% and piO(2) decreased to approximately 15 mm Hg. The hypoxia-induced CBF increase was effective to some extent in compensating for the reduced piO(2). This methodology may prove useful for investigating cerebral piO(2) under pathologically or functionally altered conditions. Magn Reson Med 45:61-70, 2001.

A metabolic control analysis of kinetic controls in ATP free energy metabolism in contracting skeletal muscle

A system analysis of ATP free energy metabolism in skeletal muscle was made using the principles of metabolic control theory. We developed a network model of ATP free energy metabolism in muscle consisting of actomyosin ATPase, sarcoplasmic reticulum (SR) Ca(2+)-ATPase, and mitochondria. These components were sufficient to capture the major aspects of the regulation of the cytosolic ATP-to-ADP concentration ratio (ATP/ADP) in muscle contraction and had inherent homeostatic properties regulating this free energy potential. As input for the analysis, we used ATP metabolic flux and the cytosolic ATP/ADP at steady state at six contraction frequencies between 0 and 2 Hz measured in human forearm flexor muscle by (31)P-NMR spectroscopy. We used the mathematical formalism of metabolic control theory to analyze the distribution of fractional kinetic control of ATPase flux and the ATP/ADP in the network at steady state among the components over this experimental range and an extrapolated range of stimulation frequencies (up to 10 Hz). The control analysis showed that the contractile actomyosin ATPase has dominant kinetic control of ATP flux in forearm flexor muscle over the 0- to 1.6-Hz range of contraction frequencies that resulted in steady states, as determined by (31)P-NMR. However, flux control begins to shift toward mitochondria at >1 Hz. This inversion of flux control from ATP demand to ATP supply control hierarchy progressed as the contraction frequency increased past 2 Hz and was nearly complete at 10 Hz. The functional significance of this result is that, at steady state, ATP free energy consumption cannot outstrip the ATP free energy supply. Therefore, this reduced, three-component muscle ATPase system is inherently homeostatic.

The dynamic regulation of myocardial oxidative phosphorylation: analysis of the response time of oxygen consumption

Although usually steady-state fluxes and metabolite levels are assessed for the study of metabolic regulation, much can be learned from studying the transient response during quick changes of an input to the system. To this end we study the transient response of O2 consumption in the heart during steps in heart rate. The time course is characterized by the mean response time of O2 consumption which is the first statistical moment of the impulse response function of the system (for mono-exponential responses equal to the time constant). The time course of O2 uptake during quick changes is measured with O2 electrodes in the arterial perfusate and venous effluent of the heart, but the venous signal is delayed with respect to O2 consumption in the mitochondria due to O2 diffusion and vascular transport. We correct for this transport delay by using the mass balance of O2, with all terms (e.g. O2 consumption and vascular O2 transport) taken as function of time. Integration of this mass balance over the duration of the response yields a relation between the mean transit time for O2 and changes in cardiac O2 content. Experimental data on the response times of venous [O2] during step changes in arterial [O2] or in perfusion flow are used to calculate the transport time between mitochondria and the venous O2 electrode. By subtracting the transport time from the response time measured in the venous outflow the mean response time of mitochondrial O2 consumption (tmito) to the step in heart rate is obtained. In isolated rabbit heart we found that tmito to heart rate steps is 4-12 s at 37 degrees C. This means that oxidative phosphorylation responds to changing ATP hydrolysis with some delay, so that the phosphocreatine levels in the heart must be decreased, at least in the early stages after an increase in cardiac ATP hydrolysis. Changes in ADP and inorganic phosphate (Pi) thus play a role in regulating the dynamic adaptation of oxidative phosphorylation, although most steady state NMR measurements in the heart had suggested that ADP and Pi do not change. Indeed, we found with 31P-NMR spectroscopy that phosphocreatine (PCr) and Pi change in the first seconds after a quick change in ATP hydrolysis, but remarkably they do this significantly faster (time constant approximately 2.5 s) than mitochondrial O2 consumption (time constant 12 s). Although it is quite likely that other factors besides ADP and Pi regulate cardiac oxidative phosphorylation, a fascinating alternative explanation is that the first changes in PCr measured with NMR spectroscopy took exclusively place in or near the myofibrils, and that a metabolic wave must then travel with some delay to the mitochondria to stimulate oxidative phosphorylation. The tmito slows with falling temperature, intracellular acidosis, and sometimes also during reperfusion following ischemia and with decreased mitochondrial aerobic capacity. In conclusion, the study of the dynamic adaptation of cardiac oxidative phosphorylation to demand using the mean response time of cardiac mitochondrial O2 consumption is a very valuable tool to investigate the regulation of cardiac mitochondrial energy metabolism in health and disease.

Mitochondrial respiration in the low oxygen environment of the cell. Effect of ADP on oxygen kinetics

Oxygen levels in the intracellular microenvironment of tissues such as heart are extremely low, at 1-2% of standard atmospheric oxygen pressure. Kinetic studies with isolated mitochondria suggest a regulatory role of oxygen under these conditions, particularly in active states at high ADP concentration, when oxygen affinity was lower than in the resting state at ADP limitation. The oxygen pressure at 50% of maximum flux, p50, was 0.035 and 0.057 kPa in heart and liver mitochondria, respiring in state 3 on substrates for complex I or II and II, respectively. p50 in the resting state 4 was 0.02 kPa. The apparent kinetic efficiency, Jmax/p50, increased from the resting to the active state, despite the decrease of oxygen affinity, 1/p50. Consequently, the relative increase of respiratory flux by ADP activation, expressed as the adenylate control ratio, declined under hypoxia, but not to the extreme of a complete loss of the scope for activation, which would occur at constant Jmax/p50. High oxygen affinity is achieved by an excess capacity of cytochrome c oxidase relative to the respiratory chain and a correspondingly low turnover rate of this enzyme, consistent with the concept of kinetic trapping of oxygen [1].

The signal transduction function for oxidative phosphorylation is at least second order in ADP

To maintain ATP constant in the cell, mitochondria must sense cellular ATP utilization and transduce this demand to F0-F1-ATPase. In spite of a considerable research effort over the past three decades, no combination of signal(s) and kinetic function has emerged with the power to explain ATP homeostasis in all mammalian cells. We studied this signal transduction problem in intact human muscle using 31P NMR spectroscopy. We find that the apparent kinetic order of the transduction function of the signal cytosolic ADP concentration ([ADP]) is at least second order and not first order as has been assumed. We show that amplified mitochondrial sensitivity to cytosolic [ADP] harmonizes with in vitro kinetics of [ADP] stimulation of respiration and explains ATP homeostasis also in mouse liver and canine heart. This result may well be generalizable to all mammalian cells.

Kinetic trapping of oxygen in cell respiration

Cell respiration in eukaryotes is catalysed by mitochondrial enzyme cytochrome c oxidase. In bacteria there are many variants of this enzyme, all of which have a binuclear haem iron-copper centre at which O2 reduction occurs, and a low-spin haem, which serves as the immediate electron donor to this centre. It is essential that the components of the cell respiratory system have a high affinity for oxygen because of the low concentration of dissolved O2 in the tissues; however, the binding of O2 to the respiratory haem-copper oxidases is very weak. This paradox has been attributed to kinetic trapping during fast reaction of O2 bound within the enzyme's binuclear haem iron-copper centre. Our earlier work indicated that electron transfer from the low-spin haem to the oxygen-bound nuclear centre may be necessary for such kinetic oxygen trapping. Here we show that specific decrease of the haem-haem electron transfer rate in the respiratory haem-copper oxidase from Escherichia coli leads to a corresponding decrease in the enzyme's operational steady-state affinity for O2. This demonstrates directly that fast electron transfer between the haem groups is a key process in achieving the high affinity for oxygen in cell respiration.

Pulmonary lactate release in patients with sepsis and the adult respiratory distress syndrome

PURPOSE

Elevated arterial lactate concentrations in patients with sepsis have been interpreted as evidence of peripheral, nonpulmonary tissue hypoxia. These patients often develop pulmonary failure manifested by the acute respiratory distress syndrome (ARDS). As the result of tissue hypoxia or inflammation, the lungs of patients with sepsis and ARDS may become a source of lactate release into the circulation.

MATERIALS AND METHODS

Pulmonary lactate release was measured in 19 patients with sepsis, arterial lactate > or = 2.2 mm, and gastric mucosal pH > 7.30. A normal gastric mucosal pH served as a marker of adequate splanchnic oxygenation. Pulmonary lactate release was computed as the product of the cardiac index and the difference in plasma L-lactate concentration in simultaneously obtained arterial and mixed venous blood samples. Lung injury was graded with the Lung Injury Score using radiographic and physiologic data.

RESULTS

The lungs of patients with minimal or no lung injury (lung injury score <1) produced significantly less lactate than those with moderate or severe lung injury (lung injury score > or = 1) (P < .005). The Lung Injury Score correlated with pulmonary lactate release (r2 = .73; P < .0001). This relationship resulted primarily from increases in mixed venous-arterial lactate differences (r2 = .59). The Lung Injury Score correlated weakly with the cardiac index (r2 = .32). Arterial lactate concentration did not correlate with pulmonary lactate release, systemic oxygen transport, or systemic oxygen consumption.

CONCLUSIONS

The lungs of patients with sepsis and ARDS may produce lactate. Pulmonary lactate release correlates with the severity of lung injury. The contribution of pulmonary lactate release should be considered when interpreting arterial lactate concentration as an index of systemic hypoxia.

Control of mitochondrial and cellular respiration by oxygen

Control and regulation of mitochondrial and cellular respiration by oxygen is discussed with three aims: (1) A review of intracellular oxygen levels and gradients, particularly in heart, emphasizes the dominance of extracellular oxygen gradients. Intracellular oxygen pressure, pO2, is low, typically one to two orders of magnitude below incubation conditions used routinely for the study of respiratory control in isolated mitochondria. The pO2 range of respiratory control by oxygen overlaps with cellular oxygen profiles, indicating the significance of pO2 in actual metabolic regulation. (2) A methodologically detailed discussion of high-resolution respirometry is necessary for the controversial topic of respiratory control by oxygen, since the risk of methodological artefact is closely connected with far-reaching theoretical implications. Instrumental and analytical errors may mask effects of energetic state and partially explain the divergent views on the regulatory role of intracellular pO2. Oxygen pressure for half-maximum respiration, p50, in isolated mitochondria at state 4 was 0.025 kPa (0.2 Torr; 0.3 microM O2), whereas p50 in endothelial cells was 0.06-0.08 kPa (0.5 Torr). (3) A model derived from the thermodynamics of irreversible processes was developed which quantitatively accounts for near-hyperbolic flux/pO2 relations in isolated mitochondria. The apparent p50 is a function of redox potential and protonmotive force. The protonmotive force collapses after uncoupling and consequently causes a decrease in p50. Whereas it is becoming accepted that flux control is shared by several enzymes, insufficient attention is paid to the notion of complementary kinetic and thermodynamic flux control mechanisms.

Electromyogram spectrum changes during sustained contraction related to proton and diprotonated inorganic phosphate accumulation: a 31P nuclear magnetic resonance study on human calf muscles

The calf muscles of five clinically healthy men were submitted to isometric exercise and examined by 31P nuclear magnetic resonance (NMR) spectroscopy and electromyography (EMG) to evaluate the influence of proton (H+) and diprotonated forms of inorganic phosphate (H2PO4-) accumulation on EMG spectrum changes. The experiments were performed in a supra-conducting magnet (2.35 Tesla, 35-cm effective diameter) using a surface coil (7-cm diameter) positioned against the calf muscles. The EMG surface electrodes were applied on the gastrocnemius medialis muscle and acquisition of both NMR and EMG signals was synchronized. The exercise consisted of a sustained isometric contraction at 70% of the maximal voluntary contraction until exhaustion. A continuous decrease in phosphocreatine content and a large concomitant increase in H2PO4- was observed in the calf muscles of each subject. A significant increase in H+ concentration was also found when considering the whole population but intracellular acidosis was low for two subjects. Moreover, a quasilinear decrease in mean power frequency (MPF) was found during the test. Changes in MPF were correlated with variations in H+ and H2PO4- concentration but a more significant relationship was found when MPF changes were correlated with H2PO4- concentration. An interpretation of EMG spectrum changes in terms of an accumulation of by-products of anaerobic metabolism and an increase in the relative number of activated slow fibres is proposed.

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.

Oxygen transport in newborns at different gestational ages

Oxygen (O2) transport was assessed through the affinity between O2 and hemoglobin (Hb) in 123 newborns of 28 to 40 week gestational ge, with a minimum of 9 newborns for each gestational age group (see table). In order to assess the O2-Hb affinity, we studied the correlation between the pO2 and the Hb saturation for each gestational age, obtaining estimates of the oxy-hemoglobin dissociation curves corresponding to each gestational age (see fig. 3). The pO2 levels corresponding to the 50% saturation (P50) for each gestational age were estimated from there. All newborns were from single vaginal deliveries with no fetal distress before birth and with an adequate weight for gestational age. The latter was calculated according to the date of the last menstrual period (78% of the cases), echography (10.6% of the cases) or neonatal physical exam (11.4% of the cases). A P50 vs. gestational age linear regression showed a high determination rate (r2 = 0.957, p less than 0.00001) (see fig. 2) which supports the hypothesis of the P50 linear growth; decrease in the Hb-O2 affinity with increasive gestational age (Hb-O2 affinity is different in newborns of different gestational ages). With these results one may conclude that the Hb-O2 uptake varies according to gestational age (P50 changes linearly as gestational age increases) and that a single measurement of pO2 in a newborns, blood does not accurately evaluate the amount of O2 that is transported to the tissues, because the transport capacity depends, among other factors, upon gestational age. The Hb saturation better represents the amount of O2 that can get to the cell level.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathobiochemistry of CO poisoning

A hypothesis for the pathobiochemical mechanism of CO poisoning, amenable to in vivo testing with optical reflectance spectrophotometry, is presented. It differs from the classical formulation in which loss of cytochrome c oxidase function is attributed entirely to O2 depletion, by including ligation and oxygenation of CO as essential components of the inhibitory process.

The polyphasic nature of the respiratory process at the mitochondrial level

The kinetics of oxygen consumption by rat liver mitochondria, respiring under a variety of metabolic conditions, have been studied. Respiration was initiated by injecting oxygen into anaerobic suspensions of mitochondria. It was found that, irrespective of the metabolic state of the mitochondria and the nature of the respiratory substrate, the rates of electron flow and oxygen consumption follow the pattern of a polyphasic reaction. The rates of oxygen uptake during the first phase are extremely fast and depend on oxygen concentration. The second phase represents a transition in which net oxidation of cytochrome-c oxidase stops and the rates of oxygen consumption suddenly decrease. The third phase is characterized by its changeability. Depending on initial conditions the rates may increase, decrease, or remain constant, although the reaction is not one of zero order. During the last phase, the rates decrease and the oxidase becomes increasingly reduced. It is postulated that the mitochondrial respiratory process is basically a cyclic event in which the redox state of the membrane and the rates of oxygen consumption oscillate with amplitudes and frequencies conditioned by the energy demand and energy-yielding capacity of the cell.

O2 dependence of in vivo brain cytochrome redox responses and energy metabolism in bloodless rats

Oxygen-dependent changes in brain cytochrome redox state and cerebrocortical energy metabolism were evaluated in fluorocarbon-circulated rats at hematocrits of less than 1%. Redox levels of three respiratory chain cytochrome complexes, b, c, and a,a3 (cytochrome c oxidase), were continuously measured directly through the intact skulls of animals using reflectance spectrophotometry. The in vivo redox status of cytochromes at different FiO2 was directly compared with in vitro measured changes in cortical metabolites known to reflect energy production, i.e., glucose, pyruvate, lactate, phosphocreatine (PCr), ADP, and ATP. Lowering the FiO2 to less than 1.0 caused the cytochromes to become increasingly more reduced. This was associated with increased tissue accumulation of pyruvate and lactate and a concomitant increase in the lactate/pyruvate (L/P) ratio. At FiO2 = 0.6, cytochromes b, c, and a,a3 were 57, 53, and 46% reduced, respectively. There was no apparent cerebral energy deficit since changes in cortical PCr, ADP, and ATP concentrations were not statistically significant. Bloodless animals did not survive below FiO2 = 0.5. At this FiO2, the inability of the animals to sustain arterial pressure correlated (r = 0.87) with depletion of PCr and further increases in the L/P ratio (r = 0.66). Yet, the cortical ATP content was reduced by only 9% of control value. These data provide direct evidence that fluorocarbon emulsion (FC-43) sustains brain oxygenation and energy metabolism at high partial pressures of molecular O2. At lower FiO2, however, mitochondrial O2 uptake becomes limited as a function of decreasing perfusion pressure.

Potassium ion homeostasis and mitochondrial redox activity in brain: relative changes as indicators of hypoxia

This study was directed at relating ion transport and mitochondrial redox activity during hypoxia, as a step toward definition of brain oxygen sufficiency. To accomplish this, extracellular potassium ion activity (K+o) was recorded by ion-selective microelectrodes while reduction/oxidation (redox) ratios of cytochrome oxidase (cytochrome a,a3) were monitored by reflection spectrophotometry in cerebral cortex of rats anesthetized with pentobarbital. In normoxia, neuronal activation by direct cortical stimulation produced transient oxidation of cytochrome a,a3 and elevation of K+o. Moderate hypoxia (PaO2 above 50 mm Hg) resulted in reduction of cytochrome a,a3 but only slight elevation of K+o. At this level of hypoxia, cytochrome a,a3 continued to respond to neuronal activation with transient shifts toward oxidation and rates of K+o reaccumulation were unchanged from control. When PaO2 was further decreased below a critical threshold, stimulus-provoked oxidative responses of mitochondrial reactants were replaced by shifts toward reduction, but rates of reaccumulation of K+, spilled into the extracellular space by neuronal activation, remained unchanged. Only during severe hypoxia (PaO2 less than 20 mm Hg) was it possible in some animals to record a slowing in the reaccumulation of K+o without provocation of spreading cortical depression. These data indicate that ion transport activity in cerebral cortex is more refractory to hypoxia than is mitochondrial redox functioning. They suggest an in vivo parallel to the "cushioning" effect of mitochondria in vitro, in which oxygen consumption remains constant despite fluctuations in oxygenation and redox ratios, and also that there may be a greater anaerobic capacity to provide energy for ion transport in mammalian brain than has previously been appreciated.

Effect of decreased oxygen availability on NADH and lactate contents in human skeletal muscle during exercise

Eight men cycled for 5 min at 120 +/- 6 W (mean +/- SE) at which O2 uptake was 50% of its maximal normoxic value, breathing room air (21% O2; normoxia) on one occasion and 11% O2 in N2 (respiratory hypoxia/hypoxic--Resp. Hx.) on the other. Biopsies were taken from the quadriceps femoris muscle. Oxygen uptake during exercise was not significantly different between Resp. Hx (1.59 +/- 0.08 1 min-1) and normoxia (1.55 +/- 0.08 1 min-1). At rest, muscle lactate was the same under both conditions but was four times higher after Resp. Hx (33.2 +/- 5.2 mmol kg-1 dry wt) than normoxic cycling (8.6 +/- 1.0 mmol kg-1 dry wt; P less than 0.01). The muscle lactate/pyruvate (which is proportional to cytosolic NADH/NAD) was significantly higher after Resp. Hx.(76 +/- 19) than after normoxic cycling (26 +/- 2; P less than 0.05). At rest, analytically determined NADH averaged 0.14 +/- 0.02 mmol kg-1 dry wt under both conditions. However, exercise during Resp. Hx. resulted in a significantly higher NADH content (0.17 +/- 0.01) than exercise during normoxia (0.12 +/- 0.01; P less than 0.01). Indirect evidence indicates that the difference in muscle NADH reflects a difference in the mitochondrial redox state (Sahlin & Katz 1986). The increased muscle NADH during Resp. Hx. therefore indicates a relative lack of O2 at the cellular level (muscle hypoxia). It is suggested that the increased lactate production during Resp. Hx. is a consequence of the cellular adaptation to muscle hypoxia (i.e. increases in cytosolic ADP, AMP, Pi and NADH).

Redox state and lactate accumulation in human skeletal muscle during dynamic exercise

The relationship between the redox state and lactate accumulation in contracting human skeletal muscle was investigated. Ten men performed bicycle exercise for 10 min at 40 and 75% of maximal oxygen uptake [VO2(max.)], and to fatigue (4.8 +/- 0.6 min; mean +/- S.E.M.) at 100% VO2(max.). Biopsies from the quadriceps femoris muscle were analysed for NADH, high-energy phosphates and glycolytic intermediates. Muscle NADH was 0.20 +/- 0.02 mmol/kg dry wt. of muscle at rest, and decreased to 0.12 +/- 0.01 (P less than 0.01) after exercise at 40% VO2(max.), but no change occurred in the [lactate]/[pyruvate] ratio. These data, together with previous results on isolated cyanide-poisoned soleus muscle, where NADH increased while [lactate]/[pyruvate] ratio was unchanged [Sahlin & Katz (1986) Biochem. J. 239, 245-248], suggest that the observed changes in muscle NADH occurred within the mitochondria. After exercise at 75 and 100% VO2(max.), muscle NADH increased above the value at rest to 0.27 +/- 0.03 (P less than 0.05) and 0.32 +/- 0.04 (P less than 0.001) mmol/kg respectively. Muscle lactate was unchanged after exercise at 40% VO2(max.), but increased substantially at the higher work loads. At 40% VO2(max.), phosphocreatine decreased by 11% compared with the values at rest, and decreased further at the higher work loads. The decrease in phosphocreatine reflects increased ADP and Pi. It is concluded that muscle NADH decreases during low-intensity exercise, but increases above the value at rest during high-intensity exercise. The increase in muscle NADH is consistent with the hypothesis that the accelerated lactate production during submaximal exercise is due to a limited availability of O2 in the contracting muscle. It is suggested that the increases in NADH, ADP and Pi are metabolic adaptations, which primarily serve to activate the aerobic ATP production, and that the increased anaerobic energy production (phosphocreatine breakdown and lactate formation) is a consequence of these changes.

Redox state changes in human skeletal muscle after isometric contraction

Subjects maintained an isometric contraction of the quadriceps femoris muscle at two-thirds maximal voluntary contraction (m.v.c.) force for 5 s (5.0 +/- 0.3 s; mean +/- S.E. of mean; n = 6) or until fatigue (52 +/- 4 s; n = 13). Muscle biopsies were obtained at rest, immediately after the contractions and also at 1 and 4 min of recovery after contraction to fatigue. In all subjects 5 s isometric contraction resulted in an increase of muscle NADH (0.084 +/- 0.012 at rest to 0.203 +/- 0.041 mmol/kg dry wt.) and a decrease of phosphocreatine (PC; change in concentration = -17.3 +/- 3.8 mmol/kg dry wt.). Glucose-6-phosphate concentration was more than doubled whereas lactate increased in only four of the six subjects. The two subjects who did not show any increase in lactate also had the lowest increase in NADH. At fatigue NADH increased to 0.226 +/- 0.032 mmol/kg dry wt. which was not significantly different from the value after 5 s contraction. Muscle PC was nearly depleted and lactate increased 12-fold above resting levels. The major part (65%) of the NADH increase at fatigue had reverted after 1 min recovery but only a slight further decrease occurred between 1 and 4 min of recovery. In relative terms the time course of the changes in muscle NADH during the first minute of recovery was similar to that of PC resynthesis, suggesting a common regulator such as O2 availability. In contrast to the delayed return of NADH concentration, PC resynthesis continued during the later part of the recovery period and PC concentration was almost fully restored after 4 min of recovery. It is concluded that muscle NADH is already maximally increased in the first seconds of muscle contraction at two-thirds m.v.c. Indirect evidence indicates that this increase reflects a reduction of the mitochondrial NAD-NADH redox couple. The rapid establishment of a reduced mitochondrial redox state at the start of muscle contraction will probably lead to a reduction of the redox state in the cytoplasm also and therefore be important for enhancing lactate formation.

Control of oxidative metabolism and oxygen delivery in human skeletal muscle: a steady-state analysis of the work/energy cost transfer function

The concept of transfer function for organ performance (work output vs. biochemical input) is developed for skeletal and cardiac muscle under steady-state exercise conditions. For metabolic control by the ADP concentration, the transfer function approximates a Michaelis-Menten hyperbola. Variation of the work identifies metabolic operating points on the transfer function corresponding to ADP concentrations or to a ratio of inorganic phosphate to phosphocreatine that can be determined by phosphorus nuclear magnetic resonance. This operating point is characterized by the fraction (V/Vmax) of maximal activity of oxidative metabolism in the steady state. This quantity appears to be useful in predicting the degree to which metabolic homeostasis is effective; poorly controlled metabolic states can readily be identified and are used in the diagnosis and therapy of metabolic disease in the organs of neonates and adults.

Impact of profound reductions of PaO2 on O2 transport and utilization in congenital heart disease

A group of eight adult patients with congenital cyanotic heart disease (CCHD) with PaO2 values of less than 32 mm Hg at rest and/or exercise were studied. Four of the patients were re-studied after heart-lung transplantation and restoration of PaO2 to normal values. All eight patients showed increased red cell counts (polycythemia), whereas frankly elevated hemoglobin concentrations were found in only one patient. There was no impressive change in the affinity of hemoglobin for O2 as compared to normal subjects. Blood lactate concentrations were normal at rest before transplantation, rose very modestly during moderate exercise, and were normal following transplantation indicating that the concept of anaerobic threshold is not valid in the present group of patients. Cardiac index was not elevated in the pretransplantation period, indicating that an elevated cardiac output is not an essential adaptive mechanism for dealing with hypoxia. In the pre-transplant period, O2 consumption was elevated as compared to normal values and almost doubled during exercise despite a further decline in PaO2 and SaO2; this establishes that the rate of mitochondrial O2 utilization is maintained despite profound reduction in PaO2. These patients are capable of moderate exercise and normal brain function despite severe hypoxia and the absence or attenuation of various adaptive mechanisms for dealing with hypoxia. Further study of the specifics of O2 transport and utilization in similar patients should prove rewarding.

Mitochondrial respiration following acute hypoxia in the perfused rat heart

Mitochondria isolated from tissues of hypoxic animals have increased respiratory capacity (State 3 respiration) when assayed in vitro at ambient oxygen tensions. The present study utilized the isolated perfused rat heart to determine whether or not this change could be produced in the absence of the neural and hormonal changes that accompany hypoxia in vivo. Following 10-min single pass retrograde perfusion with normoxic Krebs-Henseleit buffer (PO2 greater than or equal to 600 mmHg), perfusion was continued for up to 15 min with either normoxic or hypoxic buffer (PO2 less than or equal to 150 mmHg). After 10 min of hypoxic perfusion State 3 respiration of the mitochondria from the hypoxic hearts was 13 to 15% higher (P less than or equal to 0.05) than that of normoxic hearts when assayed with either glutamate/malate or succinate as substrate but was unchanged when TMPD and ascorbate was the substrate. Succinate-supported State 4 respiration of the hypoxic mitochondria also showed a small (10%) but significant (P less than or equal to 0.05) increase. These changes were not abolished by preperfusing the heart with propranolol (10(-7), 10(-6), or 10(-5) M) indicating that the response was not attributable to release of local stores of catecholamines. Respiratory control and ADP/O ratios as well as contents of cytochrome c and aa3 of the mitochondria from the hypoxic hearts were similar to those of normoxic hearts indicating that the mitochondria remained intact and tightly coupled. We concluded that the hypoxia-induced increase in mitochondrial State 3 respiration, while independent of neural and hormonal influences from the body requires an intracellular event, since they cannot be reproduced by subjecting isolated mitochondria to hypoxia in vitro.

Intermediary metabolism of the lung

The lung is a metabolically active organ that is engaged in secretion, clearance and other maintenance functions that require reducing potential, energy and substrates for biosynthesis. These metabolic requirements are met in part through uptake and catabolism of glucose which represents the major fuel utilized by lung tissues. Gluconeogenesis does not occur, and glycogen stores are limited so that the lung depends on the circulation for its glucose requirement. Other substrates can be metabolized by lung and contribute to the metabolic pool although their role has been less thoroughly studied. Glucose is catabolized in the lung by cytoplasmic and mitochondrial pathways that are responsive to regulatory mechanisms as in other tissues. Activity of the pentose cycle pathway of glucose catabolism is relatively high and generates the NADPH required for biosynthesis of lipid, detoxification reactions, and protection against oxidant stress. The ATP content of the lung is maintained by oxidative metabolism at levels comparable to other metabolically active organs. Alterations in lung intermediary metabolism may depress amine clearance, alter lung permeability, and influence the lung response to oxidant stress.

The effect of intracellular oxygen concentration on lactate release, pyridine nucleotide reduction, and respiration rate in the rat cardiac tissue

By measuring the absorbance change due to myoglobin oxygenation in hemoglobin-free isolated perfused rat hearts, we analyzed effects of perfusion pressure and heart rate upon the intracellular oxygen concentration. With Langendorff perfusion, the cardiac tissue was kept normoxic (above 50 microM O2) at aortic pressure above 50 cm H2O, but became hypoxic (8 microM O2) at 30 cm H2O. The increase in cardiac work, expressed as the product of peak systolic pressure and heart rate, increased oxygen consumption at aortic pressure of 50-200 cm H2O. The heart was kept normoxic under these conditions. Lactate release, oxygen consumption, and the oxidation-reduction state of pyridine nucleotide were measured as a function of myoglobin oxygenation under various normoxic and anoxic conditions. Pyridine nucleotide fluorescence and lactate release started to increase as the intracellular oxygen concentration decreased to 6 and 10 microM, respectively. Oxygen consumption was kept constant until the oxygen concentration decreased to 10 microM and slowed down below it. A close relationship between oxygen consumption and lactate release was observed. Infusions of epinephrine and norepinephrine under normoxic perfusion conditions increased cardiac work, oxygen consumption, and lactate release. More than 50% of myoglobin was then deoxygenated even under normoxic perfusion conditions. The increase in lactate release was ascribable to the increase in glycolytic flux caused by hypoxia. The change of pyridine nucleotide fluorescence by epinephrine was also explained by hypoxia in cardiac tissue.

Oxygen affinity of the respiratory chain of Acanthamoeba castellanii

Apparent Km values for O2 for the soil amoeba Acanthamoeba castellanii determined polarographically and by bioluminescence gave similar values (0.37 and 0.41 microM respectively). Mitochondria oxidizing succinate or NADH in the presence or absence of ADP gave values in the range 0.21-0.36 microM-O2. Oxidation of respiratory-chain components to 50% of the aerobic steady states in intact cells was observed at the following O2 concentrations: cytochrome aa3, 0.1-0.25 microM; cytochrome c, 0.3-0.6 microM; cytochrome b, 0.35-0.45 microM; flavoprotein, 2 microM. In isolated mitochondria corresponding values for a-, c- and b-type cytochromes were 0.007, 0.035-0.05 and 0.06-0.09 microM-O2. It is concluded that an O2 gradient exists between plasma membrane and mitochondria in A. castellanii.

Periportal and pericentral pyridine nucleotide fluorescence from the surface of the perfused liver: evaluation of the hypothesis that chronic treatment with ethanol produces pericentral hypoxia

Pyridine nucleotide fluorescence made from the surface of the hemoglobin-free perfused rat liver was measured continuously by using a "micro-light guide" placed on selected periportal and pericentral regions of the liver lobule. From the portal oxygen tension at which pyridine nucleotide reduction first occurred in pericentral regions, the oxygen gradient across the liver lobule was estimated in livers from rats treated chronically with ethanol or sucrose. Chronic treatment with ethanol increased the average lobular oxygen gradient from 275 to 400 torr (1 torr = 133 Pa), primarily due to the increase in the oxygen gradient in pericentral regions. Ethanol treatment also increased hepatic oxygen uptake significantly, from 110 to 144 (mumol/g)/hr. Treatment with the antithyroid drug 6-propyl-2-thiouracil reversed the effect of ethanol on O2 uptake and on the lobular oxygen gradient. The oxygen gradients measured with the micro-light guide were confirmed by direct measurement of tissue oxygen tensions in periportal and pericentral areas by using an oxygen electrode. These data are consistent with the hypothesis that chronic treatment with ethanol causes the pericentral region of the liver lobule to become susceptible to hypoxic cellular injury. This may be responsible, at least in part, for the localized hepatotoxic effects of ethanol.

Mechanism for selective perivenular hepatotoxicity of ethanol

Chronic alcohol administration to baboons results in perivenular lesions in the liver. To study possible mechanisms, the effect of ethanol on splanchnic oxygen consumption was measured. Acute ethanol administration increased splanchnic oxygen consumption in the control baboons, but the consequences of this effect on oxygenation of the perivenular zones were offset by a concomitant rise in blood flow, resulting in unchanged hepatic venous oxygen tensions. In alcohol-fed baboons, splanchnic oxygen consumption was not increased, either in the withdrawal state or after ethanol infusion. To study the magnitude of the shift in redox state induced by ethanol in the perivenular zones, we compared the effects of ethanol on the lactate/pyruvate ratio in hepatic venous blood (an approximation of that in perivenular hepatocytes) with the ratio in total liver. Prior to ethanol infusion, the lactate and pyruvate were the same in liver and in hepatic venous blood. By contrast, in all baboons, ethanol produced a much greater rise in the lactate/pyruvate ratio and decreased pyruvate more in hepatic venous blood than in total liver. Moreover, in isolated rat hepatocytes, the ethanol-induced redox shift was markedly exaggerated by oxygen tensions similar to those found in centrolobular zones. This suggests that the normally low oxygen tensions existing in perivenular zones exaggerate the ethanol-induced redox shift, a change which may contribute to the exacerbation of the damage in the perivenular area of the hepatic lobule.

Response of cyt a,a3 in the situ canine heart to transient ischemic episodes

Experiments were performed to examine the response of cyt a,a3 to transient ischemic and hypoxic episodes in the empty, fibrillating canine heart in situ. Using a dual wavelength, differential spectrophotometer, reaction spectra show an absorption peak at approximately 605 nm consistent with that obtained from purified cyt a,a3. The characteristics of the averaged reaction spectrum in the interval 590 nm to 610 nm indicate that hemoglobin/myoglobin contribute no more than 23% to the signal measured at 605 nm. A regimen of one 30 sec global ischemia (GI) repeated once every 3 minutes over a 90 min period showed no appreciable signal deterioration. Therefore, five such interventions were subsequently used as the test perturbation. Studies of the effects of ischemic episodes of 30 and 60 min show that the response of cyt a,a3 to this test intervention was smaller (90 +/- 6% and 89 +/- 7%) than that observed prior to the ischemic episode. Changes in coronary perfusion pressure (+/- 10 Torr) produced an immediate oxidation/reduction of cyt a,a3. In the working heart, just prior to fibrillation, 6 sec to interrupted ventilation resulted in a continuous reduction of cyt a,a3. The data from these studies show: 1) The redox state of cyt a,a3 may be continuously monitored in the canine heart in situ. 2) Following ischemias of 30 and 60 min duration, respiratory chain function may be impaired; and 3) The well-perfused epicardium is extremely sensitive to small changes in oxygen delivery.

Oxygen transport and the function of myoglobin. Theoretical model and experiments in chicken gizzard smooth muscle

We studied the steady-state oxygen transfer across thin layers of respiring chicken gizzard smooth muscle and compared three models for oxygen consumption with respect to their influence on the facilitation of oxygen diffusion by myoglobin. These models assumed zero-order, Michaelis-Menten or exponential kinetics. The transport equation was solved for these models with simultaneous oxygen facilitation assuming chemical equilibrium between oxygen and myoglobin. Experimental flux data were obtained in two situations: a) high oxygen pressure throughout the layer of tissue providing maximum oxygen consumption and oxygen permeability, and b) anoxic conditions in part of the layer and with submaximal oxygen consumption and desaturation of myoglobin. Measurements in the presence of functional myoglobin were compared with data obtained after abolishing the transport function of myoglobin by application of 1 kPa carbon monoxide. It was found that oxygen consumption interferes with the facilitation effect. The oxygen pressure at half maximum oxygen consumption in the Michaelis-Menten model was 0.3 +/- 0.1 (S.E) kPa. The facilitation of the oxygen transport by myoglobin was 50 to 100% of the maximum value to be expected on the basis of the prevailing myoglobin concentration.

A common denominator in the etiology of adult respiratory distress syndrome

The initiating factor in ARDS is a matter of controversy. Some investigators relate ARDS development to diffuse pulmonary microemboli after stress ranging from sepsis to non-thoracic and thoracic trauma. Others indicate hyperoxic exposure as the causative agent. This investigation looked for a common factor in ischemia and hyperoxic exposure in lung which could cause the genesis of ARDS. Studies of oxidative phosphorylation, succinate dehydrogenase activity and ATP level were performed on ischemic and 100% O2 exposed lung. Results in both showed decreased respiration rate below the basal rate, decreased SDH activity, followed by marked decrease in ATP levels in pulmonary tissue. Decrease in respiration (ATP production) capacity and ATP levels in ischemic lung were such that normal cell functions could not be sustained if returned to normal circulation. Hyperbaric O2 therapy would subsequently decrease energy metabolism in regions of normal circulation and in previously ischemic regions.