Myoglobin-dependent oxidative metabolism in the hypoxic rat heart

Computational Analysis Reveals Unique Binding Patterns of Oxygenated and Deoxygenated Myoglobin to the Outer Mitochondrial Membrane

Myoglobin (Mb) interaction with the outer mitochondrial membrane (OMM) promotes oxygen (O₂) release. However, comprehensive molecular details on specific contact regions of the OMM with oxygenated (oxy-) and deoxygenated (deoxy-)Mb are missing. We used molecular dynamics (MD) simulations to explore the interaction of oxy- and deoxy-Mb with the membrane lipids of the OMM in two lipid compositions: (a) a typical whole membrane on average, and (b) specifically the cardiolipin-enriched cristae region (contact site). Unrestrained relaxations showed that on average, both the oxy- and deoxy-Mb established more stable contacts with the lipids typical of the cristae contact site, then with those of the average OMM. However, in steered detachment simulations, deoxy-Mb clung more tightly to the average OMM, and oxy-Mb strongly preferred the contact sites of the OMM. The MD simulation analysis further indicated that a non-specific binding, mediated by local electrostatic interactions, existed between charged or polar groups of Mb and the membrane, for stable interaction. To the best of our knowledge, this is the first computational study providing the molecular details of the direct Mb-mitochondria interaction that assisted in distinguishing the preferred localization of oxy- and deoxy-Mb on the OMM. Our findings support the existing experimental evidence on Mb-mitochondrial association and shed more insights on Mb-mediated O₂ transport for cellular bioenergetics.

Myoglobin Interaction with Lactate Rapidly Releases Oxygen: Studies on Binding Thermodynamics, Spectroscopy, and Oxygen Kinetics

Myoglobin (Mb)-mediated oxygen (O₂) delivery and dissolved O₂ in the cytosol are two major sources that support oxidative phosphorylation. During intense exercise, lactate (LAC) production is elevated in skeletal muscles as a consequence of insufficient intracellular O₂ supply. The latter results in diminished mitochondrial oxidative metabolism and an increased reliance on nonoxidative pathways to generate ATP. Whether or not metabolites from these pathways impact Mb-O₂ associations remains to be established. In the present study, we employed isothermal titration calorimetry, O₂ kinetic studies, and UV-Vis spectroscopy to evaluate the LAC affinity toward Mb (oxy- and deoxy-Mb) and the effect of LAC on O₂ release from oxy-Mb in varying pH conditions (pH 6.0–7.0). Our results show that LAC avidly binds to both oxy- and deoxy-Mb (only at acidic pH for the latter). Similarly, in the presence of LAC, increased release of O₂ from oxy-Mb was detected. This suggests that with LAC binding to Mb, the structural conformation of the protein (near the heme center) might be altered, which concomitantly triggers the release of O₂. Taken together, these novel findings support a mechanism where LAC acts as a regulator of O₂ management in Mb-rich tissues and/or influences the putative signaling roles for oxy- and deoxy-Mb, especially under conditions of LAC accumulation and lactic acidosis.

Nitrite as regulator of hypoxic signaling in mammalian physiology

In this review we consider the effects of endogenous and pharmacological levels of nitrite under conditions of hypoxia. In humans, the nitrite anion has long been considered as metastable intermediate in the oxidation of nitric oxide radicals to the stable metabolite nitrate. This oxidation cascade was thought to be irreversible under physiological conditions. However, a growing body of experimental observations attests that the presence of endogenous nitrite regulates a number of signaling events along the physiological and pathophysiological oxygen gradient. Hypoxic signaling events include vasodilation, modulation of mitochondrial respiration, and cytoprotection following ischemic insult. These phenomena are attributed to the reduction of nitrite anions to nitric oxide if local oxygen levels in tissues decrease. Recent research identified a growing list of enzymatic and nonenzymatic pathways for this endogenous reduction of nitrite. Additional direct signaling events not involving free nitric oxide are proposed. We here discuss the mechanisms and properties of these various pathways and the role played by the local concentration of free oxygen in the affected tissue.

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.

Hypoxia-induced left ventricular dysfunction in myoglobin-deficient mice

Myoglobin-deficient mice are viable and have preserved cardiac function due to their ability to mount a complex compensatory response involving increased vascularization and the induction of the hypoxia gene program (hypoxia-inducible factor-1alpha, endothelial PAS, heat shock protein27, etc.). To further define and explore functional roles for myoglobin, we challenged age- and gender-matched wild-type and myoglobin-null mice to chronic hypoxia (10% oxygen for 1 day to 3 wk). We observed a 30% reduction in cardiac systolic function in the myoglobin mutant mice exposed to chronic hypoxia with no changes observed in the wild-type control hearts. The cardiac dysfunction observed in the hypoxic myoglobin-null mice was reversible with reexposure to normoxic conditions and could be prevented with treatment of an inhibitor of nitric oxide (NO) synthases. These results support the conclusion that hypoxia-induced cardiac dysfunction in myoglobin-null mice occurs via a NO-mediated mechanism. Utilizing enzymatic assays for NO synthases and immunohistochemical analyses, we observed a marked induction of inducible NO synthase in the hypoxic myoglobin mutant ventricle compared with the wild-type hypoxic control ventricle. These new data establish that myoglobin is an important cytoplasmic cardiac hemoprotein that functions in regulating NO homeostasis within cardiomyocytes.

Role of myoglobin in regulating respiration

The 1H NMR Val E11 signal provides a unique opportunity to observe carbon monoxide (CO) inhibition of Mb in the in vivo myocardium and to assess the functional role of Mb in regulating respiration. Upon carbon monoxide infusion, the MbO2 Val E11 signal at -2.76 ppm gradually disappears, and a new signal at -2.26 ppm, corresponding to MbCO, emerges. These signals yield the intracellular partial pressure of both O2 and CO and the extent of Mb inactivation, since CO binds more tightly to Mb than O2. Although contractile function decreases slightly to a steady state level, it shows no dose dependence on pCO. Up to 80% MbCO saturation, the contractile function remains at the steady state level. Neither the PCr concentration nor the oxygen consumption rate is significantly perturbed. Above 80% MbCO saturation, the oxygen consumption rate starts to decline. The experimental observations raise provocative questions about the functional role of Mb in the cell.

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.

Analysis of the myoglobin gene in heart disease

Analysis of the myoglobin gene from a large number of patients with cardiac disease disclosed a single substantive mutation. However, no evidence of biochemical or physiological dysfunction due to this mutation was detected. We conclude that, within the limits of presently available techniques, myoglobin mutations are unlikely to contribute substantially to the genotypic background of cardiac disease in the general population.

Disruption of myoglobin in mice induces multiple compensatory mechanisms

Myoglobin may serve a variety of functions in muscular oxygen supply, such as O(2) storage, facilitated O(2) diffusion, and myoglobin-mediated oxidative phosphorylation. We studied the functional consequences of a myoglobin deficiency on cardiac function by producing myoglobin-knockout (myo(-/-)) mice. To genetically inactivate the myoglobin gene, exon 2 encoding the heme binding site was deleted in embryonic stem cells via homologous recombination. Myo(-/-) mice are viable, fertile, and without any obvious signs of functional limitations. Hemoglobin concentrations were significantly elevated in myo(-/-) mice. Cardiac function and energetics were analyzed in isolated perfused hearts under resting conditions and during beta-adrenergic stimulation with dobutamine. Myo(-/-) hearts showed no alteration in contractile parameters either under basal conditions or after maximal beta-adrenergic stimulation (200 nM dobutamine). Tissue levels of ATP, phosphocreatine ((31)P-NMR), and myocardial O(2) consumption were not altered. However, coronary flow [6.4 +/- 1.3 ml.min(-1).g(-1) [wild-type (WT)] vs. 8.5 +/- 2.4 ml.min(-1).g(-1) [myo(-/-)] [and coronary reserve [17.1 +/- 2.1 (WT) vs. 20.8 +/- 1.1 (myo(-/-) ml. min(-1).g(-1) were significantly elevated in myo(-/-) hearts. Histological examination revealed that capillary density also was increased in myo(-/-) hearts [3,111 +/- 400 mm(-2) (WT) vs. 4,140 +/- 140 mm(-2) (Myo(-/-)]. These data demonstrate that disruption of myoglobin results in the activation of multiple compensatory mechanisms that steepen the pO(2) gradient and reduce the diffusion path length for O(2) between capillary and the mitochondria; this suggests that myoglobin normally is important for the delivery of oxygen.

Dynamics of tissue oxygenation in isolated rabbit heart as measured with near-infrared spectroscopy

We investigated the role of myoglobin (Mb) in supplying O2 to mitochondria during transitions in cardiac workload. Isovolumic rabbit hearts (n = 7) were perfused retrogradely with hemoglobin-free Tyrode solution at 37 degrees C. Coronary venous O2 tension was measured polarographically, and tissue oxygenation was measured with two-wavelength near-infrared spectroscopy (NIRS), both at a time resolution of approximately 2 s. During transitions to anoxia, 68 +/- 2% (SE) of the NIRS signal was due to Mb and the rest to cytochrome oxidase. For heart rate steps from 120 to 190 or 220 beats/min, the NIRS signal decreased significantly by 6.9 +/- 1.3 or 11.1 +/- 2.1% of the full scale, respectively, with response times of 11.0 +/- 0.8 or 9.1 +/- 0.5 s, respectively. The response time of end-capillary O2 concentration ([O2]), estimated from the venous [O2], was 8.6 +/- 0.8 s for 190 beats/min (P < 0.05 vs. NIRS time) or 8.5 +/- 0.9 s for 220 beats/min (P > 0.05). The mean response times of mitochondrial O2 consumption (VO2) were 3.7 +/- 0.7 and 3.6 +/- 0.6 s, respectively. The deoxygenation of oxymyoglobin (MbO2) accounted for only 12-13% of the total decrease in tissue O2, with the rest being physically dissolved O2. During 11% reductions in perfusion flow at 220 beats/min, Mb was 1.5 +/- 0.4% deoxygenated (P < 0.05), despite the high venous PO2 of 377 +/- 17 mmHg, indicating metabolism-perfusion mismatch. We conclude that the contribution of MbO2 to the increase of VO2 during heart rate steps in saline-perfused hearts was small and slow compared with that of physically dissolved O2.

Carbon monoxide inhibition of regulatory pathways in myocardium

The 1H nuclear magnetic resonance (NMR) myoglobin (Mb) Val E11 signal provides a unique opportunity to assess the functional role of Mb in the cell. On CO infusion in perfused myocardium, the MbO2 signal at -2.76 parts per million (ppm) gradually disappears, whereas the corresponding MbCO signal emerges at -2.26 ppm, reflecting the state of Mb inhibition. Up to 76.8% MbCO saturation, myocardial O2 consumption (MVO2) remains constant, whereas the rate-pressure product (RPP) has already dropped to 92% of the control level. At 87.6% MbCO saturation, the lactate formation rate has increased by a factor of two, and MVO2 begins to decline. However, the ratio CO/O2 is still 1/10, well below the inhibition threshold for cytochrome oxidase activity. The MVO2 decline in the face of an adequate O2 supply and an unperturbed high-energy phosphate level implies that Mb may play a role in directly regulating respiration, mediated potentially by a shift in NADH/NAD. Although nitrite inhibits Mb, nitrite also directly affects the myocardial function.

Diffusion of myoglobin in skeletal muscle cells--dependence on fibre type, contraction and temperature

We measured the diffusion coefficient of myoglobin (DMb) inside mammalian skeletal muscle cells with a microinjection technique. A small bolus of horse Mb was injected into a single muscle fibre and the subsequent time-dependent changes of the Mb profiles along the fibre axis were measured with a microscope-photometer. For fibres of the rat soleus muscle at 22 degrees C, a DMb of 1.3.10(-7) cm2/s was found, confirming a result obtained previously by us for rat diaphragm muscle with a photo-oxidation technique. In the extensor digitorum longus muscle of the rat, a higher value of 1.9.10(-7) cm2/s was measured. Auxotonic muscle contractions did not change the apparent DMb. For the temperature range between 22 degrees C and 37 degrees C, a temperature coefficient. Q10, of 1.5 was calculated. The implication of this result for the role of Mb in the facilitation of oxygen transport was examined. Model calculations show that with this relatively low DMb value, the intracellular oxygen supply can be improved only slightly.

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

The proximal histidine N delta H proton of deoxymyoglobin experiences a large hyperfine shift resulting in its 1H nuclear magnetic resonance (NMR) signal appearing at approximately 76 ppm (at 35 degrees C), downfield of the diamagnetic spectral region. 1H NMR of this proton is used to monitor sarcoplasmic oxygen pressure in isolated perfused rat heart. This method monitors intracellular oxygenation in the whole heart and does not reflect oxygenation in a limited region. The deoxymyoglobin resonance intensity is reduced upon conversion of myoglobin to the ferric form by sodium nitrite. 1H resonances of the N delta H protons of the alpha and beta subunits of bovine deoxyhemoglobin do not interfere with the measurement of myoglobin deoxygenation in blood-perfused rat heart. We find that steady-state myoglobin deoxygenation is increased progressively (and reversibly) as oxygenation of the perfusing medium is decreased in both saline and red blood cell-perfused hearts at constant work output. An eightfold increase in the heart rate of the blood-perfused heart resulted in no change in the deoxymyoglobin signal intensity. Intracellular PO2 of myoglobin-containing cells is maintained remarkably constant in changing work states.

Relationship between myoglobin contents and increases in cyclic GMP produced by glyceryl trinitrate and nitric oxide in rabbit aorta, right atrium and papillary muscle

Effects of glyceryl trinitrate (GTN) and nitric oxide (NO) on the cardiac functions and myocardial cyclic GMP (cGMP) contents were examined in comparison with those in the aorta and correlated with myoglobin (an inhibitor of soluble guanylate cyclase) contents using the preparations isolated from the reserpinized rabbit. GTN (10(-10)-10(-4) mol/l) produced a dose-dependent relaxation in the aorta. However, this compound exerted no effect on the rate of the spontaneous beat of the right atrium and the contraction of the papillary muscle. A transient and significant increase in cGMP was observed in the aorta with GTN (3 x 10(-6) mol/l). Although the increase was also observed in the right atrium, it was much smaller. No definite change was observed in papillary muscle. Increases in cGMP produced by NO (3 x 10(-6) mol/l) were larger and significant in all tissues; (AUCcGMP(GTN)/AUCcGMP(NO)) ratio was 30.1 for the aorta, 65.0 for the right atrium and 16.3% for the papillary muscle. Although higher concentrations of NO were necessary in the right atrium and papillary muscle to induce increases in cGMP, no differences were noted in the three tissues as regards the maximum accumulation of this substance. Furthermore, kinetic analysis of NO-induced increases in tissue cGMP indicated no marked difference in the production rate among the three tissues, while the rate of elimination of cGMP was lower in the aorta than in the atrium or the papillary muscle. The increases in cGMP observed in these three tissues were inversely related to the contents of myoglobin in respective tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced free radical generation during reperfusion of hypothermically arrested hearts

Several studies indicate the presence of hydroxyl radical (OH.) as well as its involvement in the myocardial reperfusion injury. A transition metal-like iron is necessary for the conversion of superoxide anion (O2-) to a highly reactive and cytotoxic hydroxyl radical (OH.). In the present study, we have examined the generation of OH. and free iron in reperfused hearts following either normothermic (37 degrees C) or hypothermic ischemia (5 degrees C). Employing the Langendorff technique, isolated rat hearts were subjected to global ischemia for 30 min at 37 degrees C or 5 degrees C and were then reperfused for 15 min at 37 degrees C. The results of the study suggest that both the OH. generation in myocardium and free iron release into perfusate were significantly lower in hearts made ischemic at 5 degrees C as compared to 37 degrees C. Release of myoglobin and lactic acid dehydrogenase into perfusate also followed a similar pattern. Furthermore, in in vitro studies, chemically generated O2- at 5 degrees C caused a significantly lower rate of oxidation of oxymyoglobin as well as generation of OH. and free iron as compared to 37 degrees C. These results suggest that (1) reperfusion of hypothermic ischemic heart is associated with a reduction in the generation of OH. and cellular damage compared to that of normothermic ischemic heart, and (2) myoglobin, an intracellular protein, is a source of free iron and plays a role in the reperfusion injury mediated by free radicals.

Effects of oxyradicals on oxymyoglobin. Deoxygenation, haem removal and iron release

We have examined the effects of O2-derived free radicals on oxymyoglobin, the myocardial intracellular protein involved in the storage and transport of O2. The oxyradicals generated by the xanthine/xanthine oxidase system decreased the concentration of oxymyoglobin. Based on the decreases in absorbance peaks at 581 nm and 415 nm it is estimated that out of a 10 nmol decrease in oxymyoglobin, 5 nmol appears to be oxidized to ferrimyoglobin (deoxygenation), while haem was removed from the other 5 nmol of haem protein. These processes were inhibited by both catalase alone and superoxide dismutase in combination with catalase, but not by either superoxide dismutase alone or deferoxamine. These results suggest that among H2O2, OH. and O2.-, only H2O2 causes the removal of haem and the oxidation of oxymyoglobin. Furthermore, the oxyradicals also released 3 microM free iron from oxymyoglobin, which is at least 5-fold less than the 15 nmol loss of oxymyoglobin. The loss of oxymyoglobin also preceded the release of free iron. These results indicate that oxymyoglobin oxidation and haem removal occur before the removal of free iron. Thus myoglobin appears to be highly susceptible to free radical attack, and this may represent yet another mechanism of free radical-mediated cellular injury.

Oxidation of cardiac myoglobin in vivo by sodium nitrite or hydroxylamine

A non-vascularized fish heart model was used to assess the oxidation of cardiac myoglobin in vivo by compounds known to cause methemoglobinemia. Buffalo sculpin (Enophrys bison) were cannulated from the afferent branchial artery to permit repeated blood sampling and injected intraperitoneally with sodium nitrite, hydroxylamine or aniline. Methemoglobin was formed by sublethal levels of sodium nitrite or hydroxylamine. For hydroxylamine, the time to peak effect was less than 1 h. For sodium nitrite, the onset was less rapid and the effect more prolonged. Aniline had no effect on hemoglobin at any concentration tested. Cardiac myoglobin, assayed at the time of peak effect on hemoglobin, was oxidized in a dose-dependent manner by sodium nitrite or hydroxylamine. At high doses of sodium nitrite (50 and 100 mg/kg), the oxidation of myoglobin exceeded that of hemoglobin. The reverse was true of hydroxylamine at all concentrations tested. This study suggests that possibility that cardiac myoglobin is oxidized in occupational or other exposures to sodium nitrite, hydroxylamine and related compounds.

Comparative oxygen affinity of fish and mammalian myoglobins

Myoglobins from rat, coho salmon (Oncorhynchus kisutch), buffalo sculpin (Enophrys bison) hearts, and yellowfin tuna (Thunnus albacares) red skeletal muscle were partially purified and their O2 binding affinities determined. Commercially prepared sperm whale myoglobin was employed as an internal standard. Tested at 20 degrees C, myoglobins from salmon and sculpin bound O2 with lower affinity than myoglobins from the rat or sperm whale. Oxygen binding studies at 12 degrees C and 37 degrees C suggest that this difference is adaptive, permitting myoglobins from cold-adapted fish to function at physiologically relevant temperatures. Taken together, purification and O2 binding data obtained in this study reveal a previously unrecognized diversity of myoglobin structure and function.

Myoglobin-mediated oxygen delivery to mitochondria of isolated cardiac myocytes

Myoglobin-mediated oxygen delivery to intracellular mitochondria is demonstrated in cardiac myocytes isolated from the hearts of mature rats. Myocytes are held at high ambient oxygen pressure, 40-340 torr (5-45 kPa); sarcoplasmic myoglobin is fully oxygenated. In this condition oxygen availability does not limit respiratory rate; myoglobin-facilitated diffusion contributes no additional oxygen flux and, since oxygen consumption is measured in steady states, the storage function of myoglobin vanishes. Carbon monoxide, introduced stepwise, displaces oxygen from intracellular oxymyoglobin without altering the optical spectrum of the largely oxidized intracellular mitochondria. A large part, about one-third, of the steady-state oxygen uptake is abolished by carbon monoxide blockade of myoglobin oxygenation. The myoglobin-dependent component of the oxygen uptake decreases linearly with decreasing fraction of intracellular oxymyoglobin, with a slope near unity. Studies using inhibitors of mitochondrial electron transport indicate that myoglobin-delivered oxygen uptake depends on electron flow through the mitochondrial electron transport chain. We conclude that cardiac mitochondria accept two additive simultaneous flows of oxygen: a flow of dissolved oxygen to cytochrome oxidase and a flow of myoglobin-bound oxygen to a mitochondrial terminus. Myoglobin-mediated oxygen delivery supports ATP generation by heart cells at physiological ambient oxygen pressure.

The effect of myoglobin-facilitated oxygen transport on the basal metabolism of papillary muscle

A mathematical model of oxygen diffusion into cylindrical papillary muscles is presented. The model partitions total oxygen flux into its simple and myoglobin-facilitated components. The model includes variable sigmoidal, exponential, or hyperbolic functions relating oxygen partial pressure to both fractional myoglobin saturation and rate of oxygen consumption. The behavior of the model was explored for a variety of saturation- and consumption-concentration relations. Facilitation of oxygen transport by myoglobin was considerable as indexed both by the elevation of oxygen partial pressure on the longitudinal axis of the muscle and by the fraction of total oxygen flux at the muscle center contributed by oxymyoglobin. Despite its facilitation of oxygen flux at the muscle center, myoglobin made only a negligible contribution to the total oxygen consumption averaged over the muscle cross-section. Hence the presence of myoglobin fails to explain either the experimentally determined basal metabolism-muscle radius relation or the stretch effect observed in isolated papillary muscle.