Energy dependence of enzyme release from hypoxic isolated perfused rat heart tissue

Substrate dependence of the postischemic cardiomyocyte recovery: Dissociation between functional, metabolic and injury markers

Defining the substrate that influences the most favourably the myocardial post-ischemic recovery is subject of debates, due to dissociation between functional and biochemical benefits. Hence, we studied the effects of either glucose or different fatty acids on the functional and metabolic recovery of post-ischemic cardiomyocytes in a substrate-free hypoxia model of simulated ischemia-reperfusion. Rat cardiomyocytes were submitted to a 2.5 h simulated ischemia followed by a 2 h reoxygenation without substrate (control), or with either glucose, octanoic acid, oleic acid, or elaidic acid. During simulated ischemia, electromechanical function gradually disappeared while the cellular viability and mitochondrial function declined. During control simulated reperfusion, cardiomyocytes recovered near normal function but a significant reduction in the action potential amplitude and rate persisted. The addition of glucose or oleic acid during simulated reperfusion promoted a faster, better and sustain functional recovery. Amongst the fatty acids, the functional recovery was slower with elaidic and octanoic acids as compared with oleic acid. The mitochondrial function was better improved during simulated reperfusion with glucose than with the tested fatty acids, among which elaidic acid was the less unfavourable. Paradoxically, the addition of whichever substrate during simulated reperfusion tended to worsen the cellular viability. Thus, cardiomyocytes recovery strongly relies on the characteristics of the substrate supplied at the onset of simulated reperfusion: glucidic or lipidic nature, chain-length, insaturation degree. Moreover, these data suggest that defining the appropriateness of a given substrate for the post-ischemic cardiomyocyte recovery is closely related to the functional and the biological endpoints in consideration.

Enhanced gene expression of Na(+)/Ca(2+) exchanger attenuates ischemic and hypoxic contractile dysfunction

Enhanced gene expression of the Na(+)/Ca(2+) exchanger in failing hearts may be a compensatory mechanism to promote influx and efflux of Ca(2+), despite impairment of the sarcoplasmic reticulum (SR). To explore this, we monitored intracellular calcium (Ca(i)(2+)) and cardiac function in mouse hearts engineered to overexpress the Na(+)/Ca(2+) exchanger and subjected to ischemia and hypoxia, conditions known to impair SR Ca(i)(2+) transport and contractility. Although baseline Ca(i)(2+) and function were similar between transgenic and wild-type hearts, significant differences were observed during ischemia and hypoxia. During early ischemia, Ca(i)(2+) was preserved in transgenic hearts but significantly altered in wild-type hearts. Transgenic hearts maintained 40% of pressure-generating capacity during early ischemia, whereas wild-type hearts maintained only 25% (P < 0.01). During hypoxia, neither peak nor diastolic Ca(i)(2+) decreased in transgenic hearts. In contrast, both peak and diastolic Ca(i)(2+) decreased significantly in wild-type hearts. The decline of Ca(i)(2+) was abbreviated in hypoxic transgenic hearts but prolonged in wild-type hearts. Peak systolic pressure decreased by nearly 10% in hypoxic transgenic hearts and >25% in wild-type hearts (P < 0.001). These data demonstrate that enhanced gene expression of the Na(+)/Ca(2+) exchanger preserves Ca(i)(2+) homeostasis during ischemia and hypoxia, thereby preserving cardiac function in the acutely failing heart.

The role of mitochondria in the salvage and the injury of the ischemic myocardium

The relationships between mitochondrial derangements and cell necrosis are exemplified by the changes in the function and metabolism of mitochondria that occur in the ischemic heart. From a mitochondrial point of view, the evolution of ischemic damage can be divided into three phases. The first is associated with the onset of ischemia, and changes mitochondria from ATP producers into powerful ATP utilizers. During this phase, the inverse operation of F0F1 ATPase maintains the mitochondrial membrane potential by using the ATP made available by glycolysis. The second phase can be identified from the functional and structural alterations of mitochondria caused by prolongation of ischemia, such as decreased utilization of NAD-linked substrates, release of cytochrome c and involvement of mitochondrial channels. These events indicate that the relationship between ischemic damage and mitochondria is not limited to the failure in ATP production. Finally, the third phase links mitochondria to the destiny of the myocytes upon post-ischemic reperfusion. Indeed, depending on the duration and the severity of ischemia, not only is mitochondrial function necessary for cell recovery, but it can also exacerbate cell injury.

Mechanisms of hippocampal reoxygenation injury

Mechanisms of 12 min of hypoxia and subsequent reoxygenation were studied in rat hippocampal slices. General cell injury in reoxygenation was indicated by increased lactate dehydrogenase (LDH). Increase in conjugated dienes (CD) showed that oxygen radical burst induced lipid peroxidation (LPO). ATP increase was also involved in reoxygenation injury, since cyanide, an inhibitor of ATP synthesis, decreased this damage. The results obtained on using inhibitors of oxygen radicals generation, i.e., allopurinol, indomethacin, rotenone, and antimycin A, strongly suggest that the sources of oxygen radicals were the xanthine/xanthine oxidase system, prostaglandin synthesis, and mitochondrial respiratory chain. The involvement of oxygen radicals in oxidative stress was confirmed also by using chain-breaking antioxidants, trolox alpha-tocopherol and stobadine, [(-)-cis-2,8-dimethyl-2,3,4,4a,5,9b-hexahydro-1H-pyrido (4,3b)indole]. Stobadine added at the onset of reoxygenation was most effective, acting in a dose-dependent manner and found to be without effect when applied in hypoxia. Cytochrome-c oxidase was decreased in reoxygenated hippocampal slices treated with stobadine.

Mechanisms of hippocampal reoxygenation injury. Treatment with antioxidants

The effects of hypoxia of different durations (8, 12 or 15 min) and of subsequent reoxygenation were studied in rat hippocampal slices by measuring enzyme activities related to oxidative stress: superoxide dismutase (SOD), cytochrome c oxidase and lactate dehydrogenase (LDH). Simultaneously the degree of lipid peroxidation was estimated by measuring conjugated dienes (CD). Reoxygenation after 8-min of hypoxia induced general cell injury indicated by increased LDH activity. Reoxygenation after 12-min of hypoxia started lipid peroxidation assessed by an increase in CD, and after 15-min of hypoxia followed by reoxygenation CD were found to be significantly decreased, suggesting lipid degradation. The injury induced by a hypoxia of 12 min and reoxygenation was reduced by SOD and catalase, indicating that oxygen radicals were involved in this process. The oxygen radicals originating from the xanthine/xanthine oxidase system, from the synthesis of prostaglandins, as well as from the mitochondrial respiratory chain, since allopurinol, indomethacin and rotenone decreased while antimycin increased reoxygenation injury. An increase in ATP may also have been involved as cyanide, an inhibitor of ATP synthesis, decreased the reoxygenation injury. The chain-breaking antioxidants trolox, alpha tocopherol and the pyridoindole stobadine were effective in preventing reoxygenation injury, indicating the involvement of lipid peroxidation in this process.

S-nitrosoglutathione induces formation of nitrosylmyoglobin in isolated hearts during cardioplegic ischemia--an electron spin resonance study

Previously, it has been shown that *NO donor S-nitrosoglutathione (GSNO) improves the postischemic functional recovery in crystalloid buffer-perfused isolated rat hearts subjected to cardioplegic ischemia. Supplementation of cardioplegic solution with nitronyl nitroxide, a scavenger of *NO, antagonized this protective effect. Using low temperature ESR, we have detected nitrosylmyoglobin (MbNO) in rat hearts subjected to cardioplegic ischemia in the presence of GSNO (20-200 mumol/l). During aerobic reperfusion MbNO signal intensity gradually decreased, but persisted for up to 30 min of aerobic reperfusion. We conclude that MbNO is an endogenous marker of *NO release in myocardial tissues. Implications of MbNO formation are discussed with respect to cardioprotection during ischemia- and reperfusion-induced myocardial injury.

Bioenergetics, ischemic contracture and reperfusion injury

The mammalian heart is normally well oxygenated and anaerobic glycolysis is extremely rare except for the production of extra ATP during extreme exercise like a marathon race. Anaerobic glycolysis plays a role when there is a serious impairment in coronary blood flow such as during heart attack and open heart surgery. The control of glycolysis in ischemic myocardial tissue appears to be extremely complex. During aerobic glycolysis, phosphofructokinase is the most important regulatory enzyme that controls the energy requirements of the cell. Under anaerobic conditions, however, glyceraldehyde-3-phosphate dehydrogenase becomes the key enzyme because it responds promptly to any changes in the essential supply of co-factors for oxidation. The conversion of pyruvate to acetyl CoA (aerobic metabolism) involves a series of chain reactions primarily catalyzed by pyruvate dehydrogenase complex which is situated at the cross roads between both aerobic and anaerobic glycolysis. It is important to remember that substrate utilization is carefully controlled by substrate availability. During aerobic metabolism, control mechanisms using fatty acids, lactate and glucose as energy substrates regulate the rate of ATP production according to energy demand. This precise mechanism is upset during ischemia and post-ischemic reperfusion for reasons discussed in this review. The demand for ATP can no longer be met by its supply because of severely reduced anaerobic glycolysis and significantly inhibited beta-oxidation of fatty acids. The impairment of bioenergetics is discussed in the context of several diseases such as cardiomyopathy, heart failure, diabetes, arrhythmias, cardiac surgery, heart-lung transplantation, and also in aging and oxidative stress. The regulation of energy metabolism in preconditioned heart is also discussed. Finally, methods used to preserve energy in ischemic myocardium are summarized and quantitation of the high-energy phosphates is discussed. This review challenges scientists to discover drugs which will stimulate energy supply during myocardial ischemia.

Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): nonperoxidative purine and pyrimidine nucleotide depletion

Hydrogen peroxide (H2O2) overload may contribute to cardiac ischemia-reperfusion injury. We report utilization of a previously described cardiomyocyte model (J. Cell. Physiol., 149:347, 1991) to assess the effect of H2O2-induced oxidative stress on heart-muscle purine and pyrimidine nucleotides and high-energy phosphates (ATP, phosphocreatine). Oxidative stress induced by bolus H2O2 elicited the loss of cardiomyocyte purine and pyrimidine nucleotides, leading to eventual de-energization upon total ATP and phosphocreatine depletion. The rate and extent of ATP and phosphocreatine loss were dependent on the degree of oxidative stress within the range of 50 microM to 1.0 mM H2O2. At the highest H2O2 concentration, 5 min was sufficient to elicit appreciable cardiomyocyte high-energy phosphate loss, the extent of which could be limited by prompt elimination of H2O2 from the culture medium. Only H2O2 dismutation completely prevented ATP loss during H2O2-induced oxidative stress, whereas various free-radical scavengers and metal chelators afforded no significant ATP preservation. Exogenously-supplied catabolic substrates and glycolytic or tricarboxylic acid-cycle intermediates did not ameliorate the observed ATP and phosphocreatine depletion, suggesting that cardiomyocyte de-energization during H2O2-induced oxidative stress reflected defects in substrate utilization/energy conservation. Compromise of cardiomyocyte nucleotide and phosphocreatine pools during H2O2-induced oxidative stress was completely dissociated from membrane peroxidative damage and maintenance of cell integrity. Cardiomyocyte de-energization in response to H2O2 overload may constitute a distinct nonperoxidative mode of injury by which cardiomyocyte energy balance could be chronically compromised in the post-ischemic heart.

Isolated cells in the study of the molecular mechanisms of reperfusion injury

Isolated cells make it possible to study mechanisms of cell and tissue injury under well-defined conditions, including the interaction of different cells in coculture experiments. Isolated cells, either in suspension or in monolayer cultures, have also been used to study the mechanism of reperfusion injury--in this case better termed as reoxygenation injury in view of the experimental approach taken. In hepatocytes, Kupffer, and endothelial cells, reoxygenation injury resulted in necrosis primarily mediated by reactive oxygen species released by various sources such as mitochondria (hepatocytes) and NADPH oxidase (Kupffer cells). In contrast, contracture was a characteristic feature of reoxygenation injury occurring in cardiomyocytes without loss of cytosolic enzymes. Beside reactive oxygen species, Kupffer cells were activated to release prostanoids and a decrease in endothelial cell-mediated fibrinolysis occurred upon reoxygenation. Reoxygenation injury in endothelial cells was significantly increased when neutrophils were added at the time of reoxygenation, presumably due to additional generation of reactive oxygen species and the release of proteases. As exemplified for the liver, these experiments suggest a mechanism of reperfusion injury in which the various cell types of a given tissue differ significantly in their response to hypoxia-reoxygenation but in which they interact with each other in a complex pathobiochemical network via various mediators such as cytokines, and tissue damaging effector molecules such as reactive oxygen species. Future experiments with isolated cells will allow detailed analysis of the underlying molecular mechanisms.

Comparison of the effect of a mitochondrial uncoupler, 2,4-dinitrophenol and adrenaline on oxygen radical production in the isolated perfused rat liver

Using the isolated perfused rat liver, we examined the effect of stimulation of mitochondrial respiration by 2,4-dinitrophenol (2,4-DNP) and adrenaline on reactive oxygen species (ROS) production, liver damage and lipid peroxidation. ROS production was monitored by luminol- and lucigenin-enhanced chemiluminescence and oxygen uptake was measured simultaneously. Liver damage and lipid peroxidation were evaluated by measuring hepatic lactate dehydrogenase (LDH) and thiobarbituric acid reacting substances (TBARS) release. Tissue ROS level decreased and oxygen uptake increased soon after 2,4-DNP infusion. On termination of 2,4-DNP infusion, there was a sharp increase in lucigenin-enhanced chemiluminescence, which declined slowly, but luminol-enhanced chemiluminescence did not change prominently. Hepatic LDH and TBARS release increased gradually during 2,4-DNP infusion and were manifested by termination of the infusion. Allopurinol did not affect ROS production and TBARS release, but delayed increases in LDH release after termination of 2,4-DNP infusion. Adrenaline, which stimulates mitochondrial respiration without uncoupling caused similar but smaller ROS changes observed in 2,4-DNP. LDH and TBARS release were not affected significantly by adrenaline infusion. These results indicate that uncoupling of oxidative phosphorylation decreases ROS production and restoration of oxidative phosphorylation enhances ROS production and liver damage. Xanthine oxidase is unlikely to contribute to enhanced ROS production after termination of 2,4-DNP but has some protective effect during uncoupling.

Respective roles of free radicals and energy supply in hypoxic rat liver injury after reoxygenation

Livers from fasted rats subjected to 60 min of hypoxia followed by 25 min of reflow exhibited a significant release of lactate dehydrogenase (LDH) and protein into the perfusate together with high rates of oxygen consumption, depletion of hepatic glutathione (GSH) and accumulation of thiobarbituric acid reactants (TBAR) in the liver. These changes were observed in the presence and absence of added xanthine (25 microM) and were significantly diminished when experiments were carried out in the presence of either 1 mM allopurinol or 100 microM Trolox. Allopurinol inhibited by 79% the production of uric acid by the liver, which was not altered by Trolox. Hypoxia-reflow studies performed in the presence of 25 microM 2,4-dinitrophenol (DNP) showed a drastic enhancement in LDH (244%) and protein (104%) efflux from the liver, compared with the effects found in its absence, with a moderate increase (35%) in tissue TBAR levels. Liver perfusion in the presence of both allopurinol and DNP exhibited a normalization of the tissue content of GSH and TBAR, while the net increase in LDH and protein release elicited by DNP alone was inhibited by only 20 and 25%, respectively. Similar results were obtained in experiments in which allopurinol was replaced by Trolox. These studies indicate that production of oxygen free-radicals are involved in hypoxic liver injury upon reflow, but its relative importance is significantly diminished when energy stores are severely diminished.

Oxidative changes in hypoxic-reoxygenated rabbit heart: a consequence of hypoxia rather than reoxygenation

Tissue changes consistent with oxidative damage in hypoxic/reoxygenated heart tissue have not been well documented. We recently reported that oxidative perturbations were evident in isolated-perfused rat heart tissue subjected to as little as 10 min hypoxia and that these changes were not exacerbated by reoxygenation. The mechanism and species specificity of this finding is not known. Rabbit hearts, which lack measurable xanthine oxidase activity, were examined for evidence of hypoxia-induced injury. The release of lactate dehydrogenase into the coronary effluent gradually increased during the retrograde perfusion of isolated rabbit hearts with hypoxic medium (containing 10 mM glucose and 2.5 mM calcium), and was slightly enhanced upon reoxygenation after 60 min hypoxia. Cardiac glutathione content decreased significantly while glutathione disulfide, protein-glutathione mixed disulfides, thiobaribturic acid reactive substances (TBARS), and protein carbonyl contents increased significantly after 60 min of hypoxia, compared to oxygenated controls. These values were unaltered after 4 min of reoxygenation except for a loss of TBARS. The oxidative changes observed in hypoxic rabbit hearts may be caused by energy deficiency impairing normal reductive processes or by the generation of reactive oxygen species as a result of abnormal cell functions, but cannot be related to xanthine oxidase activity.

Oxidative stress during hypoxia in isolated-perfused rat heart

These data suggest that oxidative stress occurs at the low oxygen tensions which exist during perfusion of rat heart tissue with hypoxic medium. Importantly, no evidence was found for additional oxidative injury after 4 min reoxygenation when enzyme release is maximal in this system suggesting the oxygen paradox is unrelated to oxidative stress. However, the oxidative changes evident after 10-15 min of hypoxia do support the occurrence of free radical mediated injury at low oxygen tensions, and it is possible this injury is involved in the changes which lead to cell lysis at reoxygenation. The source of this oxidative stress is not known, but appears to be greater in mitochondria and may arise from an increased production of reactive oxygen species by this organelle. Whether the observed oxidative changes are directly injurious to a cell is not yet clear.

Mitochondria, oxygen and reperfusion damage

Reperfusion of the ischaemic or hypoxic heart elicits a number of oxygen dependent processes such as cell lysis and Ca2+ uptake. It is known that the energisation of mitochondria, which requires oxygen, plays a key role in these processes and that the organelle actively sequesters Ca2+ under these circumstances. In this brief review we discuss how oxidants derived from mitochondrial electron transport may perturb mitochondrial calcium handling on reoxygenation of the hypoxic myocardium. In addition we show that the immunosuppressive agent cyclosporin has little or no effect on the oxygen dependent increase in total cell Ca2+ which occurs when hypoxic myocytes are reoxygenated. This result suggests that the Ca2+ dependent mitochondrial pore, which is known to function under conditions of oxidative stress, does not play a major role in the perturbation of Ca2+ homeostasis which occurs on reoxygenation of hypoxic hearts.

Concepts related to the study of reactive oxygen and cardiac reperfusion injury

The phenomenon of reperfusion injury remains poorly defined. Questions remain about whether injury occurs in addition to that produced by hypoxia or ischemia, or whether the observed changes simply reflect the unmasking of an underlying injury. Various pathological processes which occur upon the return of oxygen to hypoxic and ischemic heart tissue have been quantitated to assess the extent of reperfusion injury, yet it is not known if they reflect identical or different processes. In addition, the mechanism(s) responsible for these diverse changes may not be the same in the various model systems used to study reperfusion injury. Although reactive oxygen species clearly are formed at reperfusion, conclusive evidence that they are producing injury, particularly during the first seconds, is not available. Several sources of these reactive oxygen species have been proposed but none have been clearly linked with injury in several species or model systems. As research in the field of reperfusion injury continues, it is imperative for scientists to clearly define the system they are using so that studies examining mechanisms of cell lysis at reperfusion are not confused with those assessing the occurrence and mechanisms of damage in addition to that produced by oxygen deprivation.