1H nuclear magnetic resonance studies of sarcoplasmic oxygenation in the red cell-perfused rat heart

Myoglobin: Oxygen Depot or Oxygen Transporter to Mitochondria? A Novel Mechanism of Myoglobin Deoxygenation in Cells (review)

In this review, we shortly summarize the data of our studies (and also corresponding studies of other authors) on the new mechanism of myoglobin (Mb) deoxygenation in a cell, according to which Mb acts as an oxygen transporter, and its affinity for the ligand, like in other transporting proteins, is regulated by the interaction with the target, in our case, mitochondria (Mch). We firstly found that contrary to previously formulated and commonly accepted concepts, oxymyoglobin (MbO₂) deoxygenation occurs only via interaction of the protein with respiring mitochondria (low p_O2 values are necessary but not sufficient for this process to proceed). Detailed studies of the mechanism of Mb-Mch interaction by various physicochemical methods using natural and artificial bilayer phospholipid membranes showed that: (i) the rate of MbO₂ deoxygenation in the presence of respiring Mch fully coincides with the rate of O2 uptake by mitochondria from a solution irrespectively of their state (native coupled, freshly frozen, or FCCP-uncoupled), i.e. it is determined by the respiratory activity of Mch; (ii) Mb nonspecifically binds to membrane phospholipids of the outer mitochondrial membrane, while any Mb-specific protein or phospholipid sites on it are lacking; (iii) oxygen uptake by Mch from a solution and the uptake of Mb-bound oxygen are two different processes, as their rates are differently affected by proteins (e.g. lysozyme) that compete with MbO₂ for binding to the mitochondrial membrane; (iv) electrostatic forces significantly contribute to the Mb-membrane interactions; the dependence of these interactions on ionic strength is provided by the local electrostatic interactions between anionic groups of phospholipids (the heads) and invariant Lys and Arg residues near the Mb heme pocket; (v) interactions of Mb with phospholipid membranes promote conformational changes in the protein, primarily in its heme pocket, without significant alterations in the protein secondary and tertiary structures; and (vi) Mb-membrane interactions lead to decrease in the affinity of myoglobin for O2, which could be monitored by the increase in the MbO₂ autooxidation rate under aerobic conditions and under anaerobic ones, by the shift in the MbO₂/Mb(2) equilibrium towards the ligand-free protein. The decrease in the affinity of Mb for the ligand should facilitate O2 dissociation from MbO₂ at physiological p_O2 values in cells.

Myoglobin's old and new clothes: from molecular structure to function in living cells

Myoglobin, a mobile carrier of oxygen, is without a doubt an important player central to the physiological function of heart and skeletal muscle. Recently, researchers have surmounted technical challenges to measure Mb diffusion in the living cell. Their observations have stimulated a discussion about the relative contribution made by Mb-facilitated diffusion to the total oxygen flux. The calculation of the relative contribution, however, depends upon assumptions, the cell model and cell architecture, cell bioenergetics, oxygen supply and demand. The analysis suggests that important differences can be observed whether steady-state or transient conditions are considered. This article reviews the current evidence underlying the evaluation of the biophysical parameters of myoglobin-facilitated oxygen diffusion in cells, specifically the intracellular concentration of myoglobin, the intracellular diffusion coefficient of myoglobin and the intracellular myoglobin oxygen saturation. The review considers the role of myoglobin in oxygen transport in vertebrate heart and skeletal muscle, in the diving seal during apnea as well as the role of the analogous leghemoglobin of plants. The possible role of myoglobin in intracellular fatty acid transport is addressed. Finally, the recent measurements of myoglobin diffusion inside muscle cells are discussed in terms of their implications for cytoarchitecture and microviscosity in these cells and the identification of intracellular impediments to the diffusion of proteins inside cells. The recent experimental data then help to refine our understanding of Mb function and establish a basis for future investigation.

Measuring oxygenation in vivo with MRS/MRI--from gas exchange to the cell

Oxygen plays a major role as a substrate in metabolic processes in numerous signaling pathways, in redox metabolism, and in free radical metabolism. To study the role of oxygen in normal and pathophysiological states, methods that can be used noninvasively are required. This review examines the potential of nuclear magnetic resonance techniques to study tissue oxygenation. It is written from a systems perspective, looking at detection methods with respect to the path that oxygen takes in the mammalian system-from the lungs, through the vascular system, into the interstitial space, and finally into the cell. Methods discussed range from those that are quantifiable, such as the assessment of spin lattice relaxation time in fluorocarbon solutions, to those that are more correlative, such as assessment of lactate and high energy phosphates. Since the methods vary in their site of application, sensitivity, and specificity to the quantification of oxygen, this review provides examples of how each method has been applied. This may facilitate the reader's understanding of how to optimally apply different methods to study specific biomedical problems.

Implication of CO inactivation on myoglobin function

Myoglobin (Mb) has a purported role in facilitating O2diffusion in tissue, especially as cellular Po2drops or the respiration demand increases. Inhibiting Mb with CO under conditions that accentuate the facilitated diffusion role should then elicit a significant physiological response. In one set of experiments, the perfused myocardium received buffer with decreasing Po2(225, 129, and 64 mmHg). Intracellular Po2declined, as reflected in the1H NMR Val E11 signal of MbO2(67%, 32%, and 18%). The addition of 6% CO further reduced the available MbO2(11%, 9%, and 7%), as evidenced by the decline of the MbO2Val E11 signal intensity at −2.76 ppm. In a second set of experiments, electrical stimulation increased the heart rate (300, 450, and 540 beats/min) and correspondingly the O2consumption rate (MV̇o2). Intracellular Po2also declined, as reflected in the slight drop in the MbO2signal (100%, 96%, and 82%). MV̇o2increased (100%, 114%, 165%). The addition of 3% CO in the stimulated hearts further decreased the available MbO2(46%, 44%, and 29%). In all cases, CO inactivation of Mb does not induce any change in the respiration rate, contractile function, and high-energy phosphate levels. Moreover, the MbCO/MbO2partition coefficient shifts dramatically from its in vitro value during hypoxia and increased work. The observation suggests a modulation of an intracellular O2gradient. Overall, the experimental observations provide no evidence of a facilitated diffusion role for Mb in perfused myocardium and implicate a physiologically responsive intracellular O2gradient.

Role of myoglobin as a scavenger of cellular NO in myocardium

Recent studies have detected a (1)H nuclear magnetic resonance (NMR) reporter signal of metmyoglobin (metMb) during bradykinin stimulation of an isolated mouse heart. The observation has led to the hypothesis that Mb reacts with cellular nitric oxide (NO). However, the hypothesis depends on an unequivocal detection of metMb signals in vivo. In solution, nitrite oxidization of Mb produces a characteristic set of paramagnetically shifted (1)H NMR signals. In the upfield spectral region, MbO(2) and MbCO exhibit the gammaCH(3) Val E11 signals at -2.8 and -2.4 ppm, respectively. In the same spectral region, nitrite oxidation of Mb produces a set of signals at -3.7 and -4.7 ppm at 35 degrees C. Previous studies have confirmed the visibility of metMb signals in perfused rat myocardium. With bradykinin infusion, perfusion pressure and rate-pressure product decrease, consistent with endogenous NO formation. However, neither myocardial O(2) consumption nor high-energy phosphate levels, as reflected in the (31)P NMR signals, show any significant change. Bradykinin still triggers a similar physiological response even in the presence of CO that is sufficient to inhibit 86% Mb. In all cases, the (1)H NMR spectra from perfused rat myocardium reveal no metMb signals. The results suggest that bradykinin-induced NO does not interact significantly with cellular Mb to produce an NMR-detectable quantity of metMb in the perfused rat myocardium. As a consequence, the experiments cannot confirm the intriguing proposal that Mb acts as a cellular NO scavenger.

Biological plausibility for carbon monoxide as a copollutant in PM epidemiologic studies

Several recent epidemiologic studies investigating the short-term effects of particulate matter (PM) concentrations have shown carbon monoxide (CO) to have the strongest and most consistent statistical relationship with hospital admissions for cardiac diseases. This article suggests a potential hypothesis for these epidemiologic observations. Oxygen (O2) is transported, in reversible combination with hemoglobin, from the lungs to the tissues, where it diffuses into cardiac myocytes. Within the myocyte a portion of the O2 diffuses directly to the mitochondria, while the remaining O2 is transported by facilitated diffusion bound to myoglobin, a heme protein found in muscle. Within the mitochondria, O2 reacts to produce adenosine triphosphate (ATP), a high-energy phosphate compound that provides energy for all cell functions. Accordingly, the sustained production of ATP depends on the continuous delivery of O2 to the mitochondria, and failure at any point in the O2 transport system will compromise ATP production and myocardial function. Myoglobin, a fundamental constituent of cardiac muscle is essential for delivering O2 to the mitochondria. Myoglobin concentrations in cardiac tissue were 50% lower in patients with heart failure than in patients dying from noncardiac causes. Myoglobin concentrations are also severely depressed in animal models of congestive heart failure. Consequently, the role of myoglobin as a cellular transporter of O2 is seriously impaired by heart disease. Carbon monoxide reduces O2 transport to the tissues and, within the tissues, binds with myoglobin to form carboxymyoglobin (COMb). Thus, in cardiac patients CO further exacerbates the disease-related loss of myoglobin function. This further disrupts O2 transport and promotes adverse consequences for the compromised heart. Moreover, during hypoxia CO has the propensity of leaving the blood and binding with myoglobin in the intracellular compartment. Elderly persons with preexisting cardiopulmonary disorders appear to be at maximum risk of harmful health effects due to ambient air pollution exposure. Many of these disorders result in generalized or regional hypoxia. It is reasonable to hypothesize that CO also moves out of the blood of these patients and into the heart tissue whenever they are under hypoxic stress, such as exercise. Accordingly, CO binds with the marginal myoglobin concentrations present in the hearts of cardiac patients and further compromises cardiac function, resulting in poor tolerance of activity. Therefore, reduced cardiac myoglobin in people with heart disease, further exacerbated by CO moving into the cardiac tissue during episodes of hypoxia, may account for the positive association between ambient CO concentrations and hospitalization for heart disease.

Intracellular convection, homeostasis and metabolic regulation

Two views currently dominate experimental approaches to metabolic regulation. The first, let us call it Model 1, assumes that cells behave like a watery bag of enzymes. The alternative Model 2, however, assumes that 3-dimensional order and structure constrain metabolite behavior. A major problem in cell metabolism is determining why essentially all metabolite concentrations are remarkably stable (homeostatic) over large changes in pathway fluxes-for convenience, this is termed the [s] stability paradox. During large-scale transitions from maintenance metabolic rates to maximally activated work, contrasting demands of intracellular homeostasis versus metabolic regulation obviously arise. Data accumulated over the last 3-4 decades now make it clear that the demands of homeostasis prevail: during rest-work transitions, metabolites such as ATP and O(2) are notably and rigorously homeostatic; other intermediates usually do not vary by more than 0.5- to threefold over the resting condition. This impressive homeostasis is maintained despite changes in pathway fluxes that can exceed two orders of magnitude. Classical or Model 1 approaches to this problem can explain metabolite homeostasis, but the mechanisms for each metabolite, each enzyme locus, are necessarily specific. Thus Model 1 approaches basically do not provide a global explanation for the [s] stability paradox. Model 2 takes a different tack and assumes that an intracellular convection system acts as an over-riding 'assist' mechanism for facilitating enzyme-substrate encounter. Model 2 postulates that intracellular movement and convection are powered by macromolecular motors (unconventional myosins, dyneins, kinesin) running on actin or tubulin tracks. For fast and slow muscle fibers, microfilaments are concentrated near the periphery (where convection may be most important), but also extend throughout the actomyosin contractile apparatus both in horizontal and vertical dimensions. To this point in the development of the field, Model 1 and Model 2 approaches have operated as 'two solitudes', each considering the other incompatible with its own experimental modus operandi. In order to finally assemble a model that can sensibly explain a realistic working range of metabolic systems, opening of channels of communication between the above two very differing views of metabolic regulation would seem to be the requirement for the future.

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.

Mitochondrial respiratory control can compensate for intracellular O(2) gradients in cardiomyocytes at low PO(2)

In isolated single cardiomyocytes with moderately elevated mitochondrial respiration, direct evidence for intracellular radial gradients of oxygen concentration was obtained by subcellular spectrophotometry of myoglobin (Mb). When oxygen consumption was increased by carbonyl cyanide m-chlorophenylhydrazone (CCCP) during superfusion of cells with 4% oxygen, PO(2) at the cell core dropped to 2.3 mmHg, whereas Mb near the plasma membrane was almost fully saturated with oxygen. Subcellular NADH fluorometry demonstrated corresponding intracellular heterogeneities of NADH, indicating suppression of mitochondrial oxidative metabolism due to relatively slow intracellular oxygen diffusion. When oxygen consumption was increased by electrical pacing in 2% oxygen, radial oxygen gradients of similar magnitude were demonstrated (cell core PO(2) = 2.6 mmHg). However, an increase in NADH fluorescence at the cell core was not detected. Because CCCP abolished mitochondrial respiratory control while it was intact in electrically paced cardiomyocytes, we conclude that mitochondria with intact respiratory control can sustain electron transfer with reduced oxygen supply. Thus mitochondrial intrinsic regulation can compensate for relatively slow oxygen diffusion within cardiomyocytes.

Radial and longitudinal diffusion of myoglobin in single living heart and skeletal muscle cells

We have used a fluorescence recovery after photobleaching (FRAP) technique to measure radial diffusion of myoglobin and other proteins in single skeletal and cardiac muscle cells. We compare the radial diffusivities, D(r) (i.e., diffusion perpendicular to the long fiber axis), with longitudinal ones, D(l) (i.e., parallel to the long fiber axis), both measured by the same technique, for myoglobin (17 kDa), lactalbumin (14 kDa), and ovalbumin (45 kDa). At 22 degrees C, D(l) for myoglobin is 1.2 x 10(-7) cm(2)/s in soleus fibers and 1.1 x 10(-7) cm(2)/s in cardiomyocytes. D(l) for lactalbumin is similar in both cell types. D(r) for myoglobin is 1.2 x 10(-7) cm(2)/s in soleus fibers and 1.1 x 10(-7) cm(2)/s in cardiomyocytes and, again, similar for lactalbumin. D(l) and D(r) for ovalbumin are 0.5 x 10(-7) cm(2)/s. In the case of myoglobin, both D(l) and D(r) at 37 degrees C are about 80% higher than at 22 degrees C. We conclude that intracellular diffusivity of myoglobin and other proteins (i) is very low in striated muscle cells, approximately 1/10 of the value in dilute protein solution, (ii) is not markedly different in longitudinal and radial direction, and (iii) is identical in heart and skeletal muscle. A Krogh cylinder model calculation holding for steady-state tissue oxygenation predicts that, based on these myoglobin diffusivities, myoglobin-facilitated oxygen diffusion contributes 4% to the overall intracellular oxygen transport of maximally exercising skeletal muscle and less than 2% to that of heart under conditions of high work load.

Oxygen supply and oxidative phosphorylation limitation in rat myocardium in situ

The 1H-NMR signal of the proximal histidyl-N(delta)H of deoxymyoglobin is detectable in the in situ rat myocardium and can reflect the intracellular PO2. Under basal normoxic conditions, the cellular PO2 is sufficient to saturate myoglobin (Mb). No proximal histidyl signal of Mb is detectable. On ligation of the left anterior descending coronary artery, the Mb signal at 78 parts/million (ppm) appears, along with a peak shoulder assigned to the corresponding signal of Hb. During dopamine infusion up to 80 microg. kg(-1) x min(-1), both the heart rate-pressure product (RPP) and myocardial oxygen consumption (MVO2) increase by about a factor of 2. Coronary flow increases by 84%, and O2 extraction (arteriovenous O2 difference) rises by 31%. Despite the increased respiration and work, no deoxymyoglobin signal is detected, implying that the intracellular O2 level still saturates MbO2, well above the PO2 at 50% saturation of Mb. The phosphocreatine (PCr) level decreases, however, during dopamine stimulation, and the ratio of the change in P(i) over PCr (DeltaP(i)/PCr) increases by 0.19. Infusion of either pyruvate, as the primary substrate, or dichloroacetate, a pyruvate dehydrogenase activator, abolishes the change in DeltaP(i)/PCr. Intracellular O2 supply does not limit MVO2, and the role of ADP in regulating respiration in rat myocardium in vivo remains an open question.

Oxygen, homeostasis, and metabolic regulation

Even a cursory review of the literature today indicates that two views dominate experimental approaches to metabolic regulation. Model I assumes that cell behavior is quite similar to that expected for a bag of enzymes. Model II assumes that 3-D order and structure constrain metabolite behavior and that metabolic regulation theory has to incorporate structure to ever come close to describing reality. The phosphagen system may be used to illustrate that both approaches lead to very productive experimentation and significant advances are being made within both theoretical frameworks. However, communication between the two approaches or the two 'groups' is essentially nonexistent and in many cases (our own for example) some experiments are done in one framework and some in the other (implying some potential schizophrenia in the field). In our view, the primary paradox and problem which no one has solved so far is that essentially all metabolite concentrations are remarkably stable (are homeostatic) over large changes in pathway fluxes. For muscle cells O2 is one of the most perfectly homeostatic of all even though O2 delivery and metabolic rate usually correlate in a 1:1 fashion. Four explanations for this behavior are given by traditional metabolic regulation models. Additionally, there is some evidence for universal O2 sensors which could help to get us out of the paradox. In contrast, proponents of an ultrastructurally dominated view of the cell assume intracellular perfusion or convection as the main means for accelerating enzyme-substrate encounter and as a way to account for the data which have been most perplexing so far: the striking lack of correlation between changes in pathway reaction rates and changes in concentrations of pathway substrates and intermediates, including oxygen. The polarization illustrated by these two views of living cells extends throughout the metabolic regulation field (and has caused the field to progress along two surprisingly independent paths with minimal communication between them). The time may have come when cross talk between the two fields may be useful.

Myoglobin: Just an Oxygen Store or Also an Oxygen Transporter?

Besides acting as an oxygen store during times of reduced blood oxygen supply, myoglobin can also facilitate intracellular oxygen transport by diffusion of oxymyoglobin along a PO(2) gradient. We reassess the importance of myoglobin-facilitated oxygen diffusion by applying new findings on the intracellular diffusivity of myoglobin in a model calculation.

Cross-species studies of glycolytic function

Researchers probing the functional properties of glycogen (glucose) fermentation to lactate typically work within either one of two theoretical frameworks or models. The first assumes that the cell is analogous to a watery bag of enzymes, while the second assumes that three dimensional order and structure constrain the behaviors of glycolytic intermediates, of glycolytic enzymes, and of integrated glycolytic pathway functions per se. The former approach has been quite successful in accounting for many glycolytic functions but has not been able to satisfactorily explain a hallmark property of the pathway: namely, that large scale change in pathway flux is reflected in only modest changes in concentrations of pathway intermediates. Despite being composed of very different kinds of enzymes, the pathway is remarkably homeostatic by criterion of stability of concentrations of its intermediates in different metabolic states. The view of the cell as a system in which enzyme, substrate, and modulator mobilities are constrained by intracellular structures, the second framework above, posits intracellular perfusion or convection as a means for increasing rates of enzyme-substrate encounter and as an explanation for how high glycolytic pathway fluxes and homeostasis of pathway intermediates can be sustained simultaneously.

The metabolic implications of intracellular circulation

Two views currently dominate research into cell function and regulation. Model I assumes that cell behavior is quite similar to that expected for a watery bag of enzymes and ligands. Model II assumes that three-dimensional order and structure constrain and determine metabolite behavior. A major problem in cell metabolism is determining why essentially all metabolite concentrations are remarkably stable (are homeostatic) over large changes in pathway fluxes-for convenience, this is termed the [s] stability paradox. For muscle cells, ATP and O(2) are the most perfectly homeostatic, even though O(2) delivery and metabolic rate correlate in a 1:1 fashion. In total, more than 60 metabolites are known to be remarkably homeostatic in differing metabolic states. Several explanations of [s] stability are usually given by traditional model I studies-none of which apply to all enzymes in a pathway, and all of which require diffusion as the means for changing enzyme-substrate encounter rates. In contrast, recent developments in our understanding of intracellular myosin, kinesin, and dyenin motors running on actin and tubulin tracks or cables supply a mechanistic basis for regulated intracellular circulation systems with cytoplasmic streaming rates varying over an approximately 80-fold range (from 1 to >80 micrometer x sec(-1)). These new studies raise a model II hypothesis of intracellular perfusion or convection as a primary means for bringing enzymes and substrates together under variable metabolic conditions. In this view, change in intracellular perfusion rates cause change in enzyme-substrate encounter rates and thus change in pathway fluxes with no requirement for large simultaneous changes in substrate concentrations. The ease with which this hypothesis explains the [s] stability paradox is one of its most compelling features.

Comparative analysis of NMR and NIRS measurements of intracellular PO2 in human skeletal muscle

1H NMR has detected both the deoxygenated proximal histidyl NdeltaH signals of myoglobin (deoxyMb) and deoxygenated Hb (deoxyHb) from human gastrocnemius muscle. Exercising the muscle or pressure cuffing the leg to reduce blood flow elicits the appearance of the deoxyMb signal, which increases in intensity as cellular PO2 decreases. The deoxyMb signal is detected with a 45-s time resolution and reaches a steady-state level within 5 min of pressure cuffing. Its desaturation kinetics match those observed in the near-infrared spectroscopy (NIRS) experiments, implying that the NIRS signals are actually monitoring Mb desaturation. That interpretation is consistent with the signal intensity and desaturation of the deoxyHb proximal histidyl NdeltaH signal from the beta-subunit at 73 parts per million. The experimental results establish the feasibility and methodology to observe the deoxyMb and Hb signals in skeletal muscle, help clarify the origin of the NIRS signal, and set a stage for continuing study of O2 regulation in skeletal muscle.

Two research paths for probing the roles of oxygen in metabolic regulation

Tissues such as skeletal and cardiac muscles must sustain very large-scale changes in ATP turnover rate during equally large changes in work. In many skeletal muscles these changes can exceed 100-fold. Examination of a number of cell and whole-organism level systems identifies ATP concentration as a key parameter of the interior milieu that is nearly universally 'homeostatic'; it is common to observe no change in ATP concentration even while change in its turnover rate can increase or decrease by two orders of magnitude or more. A large number of other intermediates of cellular metabolism are also regulated within narrow concentration ranges, but none seemingly as precisely as is [ATP]. In fact, the only other metabolite in aerobic energy metabolism that is seemingly as 'homeostatic' is oxygen--at least in working muscles where myoglobin serves to buffer oxygen concentrations at stable and constant values at work rates up to the aerobic maximum. In contrast to intracellular oxygen concentration, a 1:1 relationship between oxygen delivery and metabolic rate is observed over biologically realistic and large-magnitude changes in work. The central regulatory question is how the oxygen delivery signal is transmitted to the intracellular metabolic machinery. Traditional explanations assume diffusion as the dominant mechanism, while proponents of an ultrastructurally dominated view of the cell assume an intracellular perfusion system to account for the data which have been most perplexing to metabolic biochemistry so far: the striking lack of correlation between changes in pathway reaction rates and changes in concentrations of pathway substrates, including oxygen and pathway intermediates.

Intracellular gradients of O2 supply to mitochondria in actively respiring single cardiomyocyte of rats

We demonstrated in a previous study [Takahashi, E., K. Sato, H. Endoh, Z.-L. Xu, and K. Doi. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H225-H233, 1998] that significant radial gradients of intracellular PO2 may be produced in an uncoupled actively respiring, single isolated cardiomyocyte of the rat. The present study was designed to further determine whether such intracellular PO2 gradients can be a limiting factor of oxidative metabolism in uncoupled cardiomyocytes. The NAD(P)H fluorescence of a single cardiomyocyte was captured by a digital charge-coupled device camera and quantitated with a subcellular spatial resolution by a ratio-imaging technique. In the conditions that we demonstrated significant radial PO2 gradients (cells treated with 1 microM carbonyl cyanide m-chlorophenylhydrazone and superfused with 2.09% or 3.14% O2 gas at 27 degreesC), we demonstrated significant augmentation of NAD(P)H fluorescence near the core of an individual cell. The heterogeneous fluorescence pattern was not found in the control cell, whereas fluorescence intensity averaged over the cell was increased by hypoxia. These results suggest the possibility that oxidative phosphorylation near the core of actively respiring, uncoupled cardiomyocytes may be severely suppressed due to insufficient diffusional oxygen supply (hypoxic core) even if regions near the sarcolemma are adequately oxygenated.

Impact of diffusional oxygen transport on oxidative metabolism in the heart

The resistance for the oxygen molecule to diffuse from the capillary blood to the cell surface produces remarkably large gradients of oxygen partial pressure (PO2) in the extracellular space. In addition, the intracellular radial gradients of PO2 may not be ignored particularly when the cellular oxygen consumption is increased. These PO2 gradients together result in a quite low intracellular PO2 in the cardiomyocyte in vivo. Thus, the cellular oxidative metabolism may be limited by diffusional transport of oxygen from the capillary blood to mitochondria. In this review, quantitative aspects and physiological relevances of the PO2 gradient in the myocardium are discussed.

Direct observation of radial intracellular PO2 gradients in a single cardiomyocyte of the rat

The purpose of the present study was to directly visualize radial gradients of intracellular PO2 in a single individual cardiomyocyte isolated from the rat ventricle. Microspectrophotometry with the use of cytosolic myoglobin as an oxygen probe was conducted at 410 nm. When the quiescent cell was incubated with 1 microM carbonyl cyanide m-chlorophenylhydrazone to increase oxygen consumption approximately eightfold, gradual decreases in myoglobin oxygen saturation (SMb) were demonstrated toward the core of the cell, whereas these decreases disappeared when the cell was treated with 2 mM NaCN. These results highlighted the importance of diffusional oxygen transport in determining intracellular oxygenation in cardiac cells. From the measured SMb, we assessed the profile of radial changes in intracellular PO2 at the mean SMb comparable to that in vivo ( approximately 0.5). Quite steep PO2 gradients were demonstrated in the vicinity of the sarcolemma that were rapidly attenuated toward the cell core. These radial profiles of intracellular PO2 demonstrate the significance of myoglobin-facilitated diffusion of oxygen. Furthermore, the shallow gradients of PO2 near the center of the cell might arise from partial depression of oxygen consumption near the cell core.

A simplified sequence for observing deoxymyoglobin signals in vivo: myoglobin excitation with dynamic unexcitation and saturation of water and fat (MEDUSA)

This paper describes a new, simplified pulse sequence for observing NMR signals from deoxymyoglobin in vivo. Paramagnetically shifted resonances from deoxymyoglobin can be exploited to noninvasively calculate intracellular oxygen tension in striated muscle. However, special sequences are required to observe these weak signals against the larger water and fat signals encountered in vivo. The pulse sequence described here, which is based on inversion recovery sequences, efficiently suppresses both water and fat resonances and can be implemented with short repetition rates. Moreover, it is perfectly suited for studies with surface coils, where RF inhomogeneities render other popular suppression sequences ineffective.

Determination of deoxymyoglobin changes during graded myocardial ischemia: an in vivo 1H NMR spectroscopy study

1H NMR spectroscopy was used to detect the proximal histidyl Ndelta proton signal of deoxymyoglobin from canine hearts in vivo during graded myocardial ischemia. The NMR signal intensity provided an indicator of intracellular oxygenation in myocardium. The relationship between the myocardial blood flow and the deoxymyoglobin concentration was successfully measured during resting, partial, and complete coronary artery occlusion conditions. The results demonstrate the feasibility to detect deoxymyoglobin using a 1H NMR spectroscopy technique in living hearts for the first time and the possibility to use this technique for investigating myocardial oxidative metabolism nondestructively and repetitively.

Metabolic response in Arenicola marina to limiting oxygen as reflected in the 1H-NMR oxymyoglobin signal

Many intertidal animals can endure prolonged periods of environmental stress and have developed strategies to preserve a functioning energy state in the cell. Recent 1H/31P-NMR techniques have allowed investigators to monitor directly mammalian tissue metabolism in vivo. In particular, the signals of myoglobin (Mb) offer a unique opportunity to explore the intracellular oxygen-partial-pressure [p(O2)] interaction in Arenicola marina, a standard model to study hypoxia tolerance in invertebrates. The present study reveals that the 1H-NMR MbO2 signal at -2.9 ppm is detectable in tissue and reflects directly the oxygenated state. As the p(O2) declines, MbO2 saturation and oxygen consumption decrease. However, phosphotaurocyamine concentration remains unaltered until the MbO2 saturation falls below 33%. The extracellular to intracellular p(O2) gradient appears substantial. The study establishes the 1H-NMR technique as an approach to measure the intracellular p(O2) with an oxygenated state marker and presents the interrelationship between oxygen and the metabolic adaptation during hypoxic stress.