Reperfusion injury

Postanoxic oxidative injury in rat hepatocytes: lactate-dependent protection against tert-butylhydroperoxide

Previous studies in this laboratory showed that hypoxia and anoxia enhance the susceptibility of hepatocytes to tert-butylhydroperoxide (TBH)-induced oxidative injury. To determine whether preceding exposure to anoxia affects postanoxic sensitivity to oxidative injury, viability was studied in hepatocytes incubated under anoxic conditions followed by reoxygenation without or with tert-butylhydroperoxide addition. Results showed that a preceding exposure to 60 min of anoxia substantially increased the vulnerability of cells to injury by the oxidant. Because substantial tissue lactate can accumulate during anoxia, the effect of increased lactate on postanoxic injury due to TBH was determined. Results showed that added lactate protected in a concentration-dependent manner. The TBH elimination rate was stimulated by lactate, and the pyruvate production rate approached the rate of TBH elimination. Thus, lactate protects against postanoxic oxidative injury by supplying reducing equivalents for peroxide reduction. This suggests that lactate accumulation during ischemia may be beneficial and that supplementation with lactate could be considered as a means to protect against postischemic injury.

Hydrogen peroxide-induced oxidative stress to the mammalian heart-muscle cell (cardiomyocyte): lethal peroxidative membrane injury

Oxidative stress induced by hydrogen peroxide (H2O2) may contribute to the pathogenesis of ischemic-reperfusion injury in the heart. For the purpose of investigating directly the injury potential of H2O2 on heart muscle, a cellular model of H2O2-induced myocardial oxidative stress was developed. This model employed primary monolayer cultures of intact, beating neonatal-rat cardiomyocytes and discrete concentrations of reagent H2O2 in defined, supplement-free culture medium. Cardiomyocytes challenged with H2O2 readily metabolized it such that the culture content of H2O2 diminished over time, but was not depleted. The consequent H2O2-induced oxidative stress caused lethal sarcolemmal disruption (as measured by lactate dehydrogenase release), and cardiomyocyte integrity could be preserved by catalase. During oxidative stress, a spectrum of cellular derangements developed, including membrane phospholipid peroxidation, thiol oxidation, consumption of the major chain-breaking membrane antiperoxidant (alpha-tocopherol), and ATP loss. No net change in the protein or phospholipid contents of cardiomyocyte membranes accompanied H2O2-induced oxidative stress, but an increased turnover of these membrane constituents occurred in response to H2O2. Development of lethal cardiomyocyte injury during H2O2-induced oxidative stress did not require the presence of H2O2 itself; a brief "pulse" exposure of the cardiomyocytes to H2O2 was sufficient to incite the pathogenic mechanism leading to cell disruption. Cardiomyocyte disruption was dependent upon an intracellular source of redox-active iron and the iron-dependent transformation of internalized H2O2 into products (e.g., the hydroxyl radical) capable of initiating lipid peroxidation, since iron chelators and hydroxyl-radical scavengers were cytoprotective. The accelerated turnover of cardiomyocyte-membrane protein and phospholipid was inhibited by antiperoxidants, suggesting that the turnover reflected molecular repair of oxidized membrane constitutents. Likewise, the consumption of alpha-tocopherol and the oxidation of cellular thiols appeared to be epiphenomena of peroxidation. Antiperoxidant interventions coordinately abolished both H2O2-induced lipid peroxidation and sarcolemmal disruption, demonstrating that an intimate pathogenic relationship exists between sarcolemmal peroxidation and lethal compromise of cardiomyocyte integrity in response to H2O2-induced oxidative stress. Although sarcolemmal peroxidation was causally related to cardiomyocyte disruption during H2O2-induced oxidative stress, a nonperoxidative route of H2O2 cytotoxicity was also identified, which was expressed in the complete absence of cardiomyocyte-membrane peroxidation. The latter mode of H2O2-induced cardiomyocyte injury involved ATP loss such that membrane peroxidation and cardiomyocyte disruption on the one hand and cellular de-energization on the other could be completely dissociated.(ABSTRACT TRUNCATED AT 400 WORDS)

Simultaneous determination of two antioxidants, uric and ascorbic acid, in animal tissue by high-performance liquid chromatography

A rapid and simple method for the simultaneous analysis of uric and ascorbic acid in extracts of animal tissue is described. The method uses reversed-phase ion-pair chromatography with ultraviolet detection. The technique allows efficient separation of both acids while showing high selectivity, recovery, reproducibility, and sample stability. Calculated levels of both substances in mouse liver tissue were 1.00 +/- 0.05 mumol ascorbic acid/g and 130 +/- 5 nmol uric acid/g.

Free radical scavenging is involved in the protective effect of L-propionyl-carnitine against ischemia-reperfusion injury of the heart

L-Propionyl-carnitine is known to improve the recovery of myocardial function and metabolic parameters reduced in the course of ischemia-reperfusion of the heart. The mechanism of this protective effect of L-propionyl-carnitine is not fully understood. The purpose of this study was to elucidate the effects of L-propionyl-carnitine in Langendorff perfused rat hearts subjected to 40 min of ischemia followed by 20 min of reperfusion. We tested the hypothesis that L-propionyl-carnitine suppresses generation of oxygen radicals and subsequent oxidative modification of myocardial proteins during reperfusion. Our data show that the protective effect of L-propionyl-carnitine in the course of ischemia-reperfusion is highly significant in terms both of mechanical properties of the heart (developed pressure) and of high-energy phosphates (ATP, creatine phosphate). Myocardial creatine phosphokinase (CPK) activity decreased in the course of the reperfusion period. The loss of CPK activity was partially prevented by L-propionyl-carnitine. Two other effects were observed when L-propionyl-carnitine was present in the perfusion solution: (i) the reperfusion-induced sharp increase in oxidative protein modification was completely prevented as detected by the formation of protein carbonyls, and (ii) generation of hydroxyl radicals was significantly inhibited as detected by the formation of the adducts with the spin trap 5,5-dimethyl-1-pyrroline-1-oxide. We conclude that the protective effect of L-propionyl-carnitine against ischemia-reperfusion injury of the heart is at least due in part to its ability to suppress the development of oxidative stress and free radical damage.

Leukocytic .O2- and cardiac dysfunctions in isolated perfused rat hearts

Superoxide radicals are supposed to contribute to myocardial reperfusion injury. Their origin, however, is still a matter of debate. Polymorphonuclear leukocytes (PMNL) have been discussed as a major postischaemic .O2- source [Lucchesi and Mullane (1986) Ann Rev Pharmacol Toxicol 26: 201-224]. We studied the role of .O2- derived from human polymorphonuclear leukocytes in reperfused and normoxic perfused isolated rat hearts. During reperfusion PMNL exerted deleterious effects on different parameters (e.g. contractility, coronary flow) of isolated rat hearts. Under normoxic perfusion conditions stimulation of PMNL with N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) caused bradycardia and a transient vasoconstriction. Superoxide dismutase (SOD) administration did not influence any of the PMNL effects mentioned, suggesting that leukocytic .O2- was not involved in PMNL-induced cardiac dysfunctions.

Simultaneous determination of inosine, hypoxanthine, xanthine, and uric acid and the effect of metal chelators

We describe a sensitive, reproducible method for the simultaneous determination of the ATP catabolites inosine, hypoxanthine, xanthine, and uric acid in biological samples and organ perfusate using reverse-phase chromatography and multiwavelength detection at 254, 270, and 292 nm. Sample preparation includes precipitating proteins with perchloric acid, neutralizing the sample, passing the supernatant over a polyethyleneimine column, and analyzing the collected fractions by high-performance liquid chromatography. Addition of metal chelators to the perchloric acid resulted in increased values for xanthine, hypoxanthine, and uric acid. The method was sensitive (limit of detection, 0.08 nmol on column; S/N = 4) and linear over the range 0.5-30 microM. Precision and accuracy of the method were evaluated for lung tissue and lung perfusate. Coefficients of variation ranged from 2.8 to 6.1% for perfusate and from 1.7 to 12.6% for tissue. Recoveries for all compounds exceeded 90%. We applied this method to rat lung tissue, lung perfusate, and rat and human blood. Advantages of this method are simultaneous quantitation with excellent sensitivity of all compounds, simplified peak identification by using multiwavelength detection, and improved accuracy by preventing loss of compounds with metal chelators.

Reactive oxygen metabolites and colitis: a disturbed balance between damage and protection. A selective review

Enhanced local production of reactive oxygen metabolites has been found in association with colitis, both experimentally and in humans. Cellular and biochemical systems involved have been identified, and 5-aminosalicylic acid-containing drugs but, more effectively, specific scavengers have been found to reduce the intestinal inflammatory process. The multitude of reactions in which oxygen metabolites participate provides a new area of research in intestinal inflammation. These basic studies might bring related clinical studies in an era of new anti-inflammatory drugs for inflammatory bowel disease specifically designed to scavenge toxic oxygen metabolites.

Therapeutic potential of vitamin E against myocardial ischemic-reperfusion injury

Myocardial ischemia is a disease process characterized by reduced coronary flow such that the supply of nutritive blood to heart muscle (myocardium) is insufficient for normal myocardial aerobic metabolism. Prompt reestablishment of coronary flow by invasive and noninvasive clinical procedures is the most direct and effective means of limiting myocardial damage in ischemic heart disease patients, although reperfusion carries with it an injury component which may reflect, at least to some degree, the toxic effects of partially reduced oxygen species and their participation in degenerative cellular processes such as membrane lipid peroxidation. Vitamin E, a lipophilic, chain-breaking antioxidant, is a prominent membrane constituent in heart muscle, where it modulates/regulates various aspects of heart muscle-cell metabolism and function. Vitamin E's beneficial effects against experimentally induced oxidative damage to the heart, along with inverse epidemiological correlations between plasma vitamin E level and either anginal pain or mortality due to ischemic heart disease, suggest that vitamin E might have protective and therapeutic roles against myocardial ischemic-reperfusion injury. Laboratory investigations aimed at addressing this possibility have demonstrated that vitamin E supplementation protects isolated hearts against ischemic-reperfusion injury, and relatively more inconsistent and limited data document cardioprotective effects of vitamin E in some animal models of myocardial ischemia-reperfusion, especially when administered prior to the ischemic period. Clinical attempts to establish whether vitamin E has therapeutic benefit in ischemic heart disease patients remain inconclusive, having relied upon a variety of nonuniformly controlled protocols and a single, rather subjective endpoint (anginal pain). Consequently, although laboratory data constitute a conceptual context for and indirect support of the idea that vitamin E could be a cardioprotectant against ischemic-reperfusion injury, compelling clinical evidence regarding vitamin E's therapeutic potential in the ischemic heart-disease patient is lacking. Elective coronary revascularization would appear to provide an attractive clinical setting for evaluating the therapeutic efficacy of vitamin E in the context of cardiac ischemia-reperfusion. Further biochemical work would still be required to define how vitamin E exerts any cardioprotective effect observed in these patients.

Effects of hypoxia and reoxygenation on the conversion of xanthine dehydrogenase to oxidase in Chinese hamster V79 cells

The effects of hypoxia and reoxygenation on the conversion of xanthine dehydrogenase to the free radical-producing xanthine oxidase in Chinese hamster V79 cells have been investigated using a newly developed fluorimetric enzyme assay. Hypoxia caused an increase in xanthine oxidase activity from 25% to 80% of the total activity of xanthine oxidase and dehydrogenase. The ratio returned to normal levels within 24 h of aerobic incubation. Hypoxia caused the release of xanthine oxidase in the medium of V79 cells and an increase in total protein concentration in the medium. There was an early change induced in lipid peroxidation markers and this was inhibited by allopurinol. The effects of glucose deprivation and calcium blockers were also investigated. Fura-2 AM was found to interact with V79 cells, making it impossible to determine intracellular calcium levels in V79 cells by this reagent.

Propranolol preserves ultrastructure in adult cardiocytes exposed to anoxia/reoxygenation: a morphometric analysis

The protective effect of d,l-propranolol was studied using freshly isolated canine ventricular cardiocytes (1.5 x 10(6)/mL) exposed to 30 min anoxia (95% N2/5% CO2) and 0, 3, 20, and 45 min of reoxygenation (95% O2/5% CO2). In addition to preventing lipid peroxide formation, propranolol maintained cellular viability, and minimized ultrastructural alterations. In the absence of propranolol, the outer mitochondria become swollen and rounded up within the first few minutes of reoxygenation. The perinuclear mitochondrial area increased only slightly. We observed that the cellular injury process proceeded differentially from the exterior to the interior, with a mitochondrial area increase and outer membrane rupture. Sarcolemmal damage was also observed with prevalent blebbing and membrane loss. The Z-lines became wider and more diffuse with reoxygenation. Injury to the nuclear double membrane was observed. Incubation with propranolol showed significant protection during postanoxia reoxygenation. In contrast, the more water soluble beta-blocker atenolol only exhibited slight protection. In addition, d-propranolol (the non beta-blocking isomer) and the antioxidant enzymes, SOD and catalase, showed significant protection. These data support previous findings concerning the antioxidant properties of propranolol which appear to be independent of beta-receptor blockade.

Reoxygenation injury and antioxidant protection: a tale of two paradoxes

Under certain circumstances, added antioxidants can protect tissues against reoxygenation injury after ischemia. Yet reperfusing blood carries many antioxidants with it. The implications of this "antioxidant paradox" are discussed.

Ascorbyl free radical as a reliable indicator of free-radical-mediated myocardial ischemic and post-ischemic injury. A real-time continuous-flow ESR study

The real-time kinetics of the release of ascorbyl free radicals in the coronary perfusate from isolated rat hearts submitted to an ischemia/reperfusion sequence has been achieved by continuous-flow ESR using high-speed acquisition techniques. Enhanced ESR detection of ascorbyl free radicals was obtained by addition of dimethyl sulfoxide (Me2SO), a strong cation chelator and oxidizing agent. A continuous-flow device allowed a direct monitoring of the ascorbyl free radical and/or ascorbate leakage in coronary perfusate by observation of the ascorbyl radical doublet (aH = 0.188 mT and g = 2.0054). 1. The results showed that ascorbyl free radical release occurred mainly during sequences of low-flow ischemia (90 min) coupled or not with 30 min of zero-flow ischemia followed by reperfusion (60 min). The kinetic profiles of ascorbyl-free-radical detection confirm in quantitative terms the expected correlation between the duration of the ischemic insult and the magnitude of ascorbate extracellular release upon reperfusion. There is indication that ascorbyl free radical depletion could be secondary to oxygen-derived-free-radical-induced cellular damage. 2. The amount of residual ascorbic acid was quantitated on myocardial tissue at the end of reperfusion using Me2SO as extracting solvent. Intense oxidation of ascorbate and chemical stabilization of the resulting free radical species provided by Me2SO allowed ESR measurement of a marked tissue ascorbate depletion related to the duration of ischemia. 3. Perfusion of superoxide dismutase during low-flow ischemia and the first 10 min of reperfusion greatly inhibited both extracellular release and endogenous ascorbate depletion. These results suggest that the ascorbate redox system constitutes a major protective mechanism against free-radical-induced myocardial injury. 4. The proposed direct ESR detection of ascorbyl free radicals in the coronary perfusates or in tissue extracts does not require extensive chemical preparation and conditioning of effluent or tissue samples. It provides an interesting straightforward alternative to the evaluation of detrimental free radical processes affecting the myocardium during ischemia and reperfusion.

Protective effects of sulfhydryl-containing angiotensin converting enzyme inhibitors against free radical injury in endothelial cells

The effects of SH-containing (captopril, epi-captopril, and the free-SH form of zofenopril) and non-SH-containing (enalaprilat and lisinopril) angiotension converting enzyme (ACE) inhibitors on free radical injury in cultured endothelial cells were studied. When cultured endothelial cells were exposed to a superoxide and hydroxyl radical generating system (dihydroxyfumarate + Fe3(+)-ADP) for 30 min, lipid peroxidation [malondialdehyde (MDA) formation] occurred, and cellular viability (trypan blue exclusion) decreased to 41%; concomitantly, plasma membrane blebbing, assessed by scanning electron microscopy, occurred in 65% of the cells. Preincubation of the cells with each of the SH-agents before free radical addition resulted in an equipotent concentration-dependent (10-200 microM) inhibition (15-60%) of MDA formation; both losses in cellular viability and percent blebbed cells were reduced significantly (P less than 0.05) by concentrations as low as 10 microM of each SH-agent. However, neither of the non-SH agents up to 200 microM produced any major effect. When the effects on hydroxyl radical formation in the system were assessed by ESR spin-trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), concentrations of 10 and 50 microM of the SH-agents reduced the intensity of the DMPO-OH adducts 20 and 50% respectively. Similar results were observed when the hydroxyl radical was generated from the Fenton-reagents (Fe2+ + H2O2), suggesting direct hydroxyl radical scavenging. Thus, these results demonstrate that the SH-containing ACE agents are capable of protecting the endothelial cells against free radical induced lipid peroxidation and cell injury; the mechanism may be due to direct hydroxyl radical scavenging.

A new cellular function for peroxisomes related to oxygen free radicals?

Although in cell biology peroxisomes are still 'young' organelles, it is becoming increasingly clear that they are involved in important cellular functions. Recent results have indicated the presence of the metalloenzyme superoxide dismutase in peroxisomes and the production of superoxide free radicals (O2-) in these oxidative organelles. These findings, together with other experimental evidence, point towards the existence of new roles for peroxisomes in cellular active oxygen metabolism, something that has a potential impact in multiple areas of cell biology, particularly in biochemistry and biomedicine.

Oxygen radical-mediated injury of myocytes-protection by propranolol

UIe effects of propranolol and atenolol on free radical mediated injury in myocytes were examined. Freshly isolated adult canine myocytes were incubated with a superoxide generating (from dihydroxyfumarate) and Fe-catalyzed free radical system. Exposure of the myocytes to free radicals for 20 min resulted in more than a 5-fold increase in thiobarbituric acid reactant (peroxide) formation and elevated levels of lactate dehydrogenase (LDH) activity released into the media compared to controls. Ultrastructurally, severe sarcolemmal damage, mitochondrial and myofibril derangements were evident. At 40 min, cellular viability (trypan blue exclusion) in the samples exposed to free radicals decreased to about one-third of controls; concomitantly, major losses in total cellular phospholipids occurred. When the cells were pretreated with 200 microM propranolol before the addition of free radicals, both peroxide formation and increased LDH release were inhibited; in agreement, complete ultrastructural preservation was observed. In addition, the subsequent losses in cellular viability and phospholipids were prevented. For comparison, the more water soluble beta-blocker, atenolol at 200 microM was shown ineffective in providing significant protection against the induced injury. The results suggest that propranolol may provide antiperoxidative protection to myocytes when elevated levels of free radicals are present.

Superoxide dismutase and catalase as therapeutic agents for human diseases. A critical review

The list of human and animal diseases for which oxygen radical scavenging therapy is being recommended continues to grow, based primarily on inferential evidence suggesting a potential role for oxygen-derived free radicals in various types of pathophysiology. Some distinct advances in pharmacologic manipulation of protein scavengers have been made which could ultimately greatly enhance the use of these reagents as drugs, as well as some innovative techniques for drug delivery (direct injection via endoscopy, iontophoresis). Unfortunately, most of the therapeutic reports in the literature, almost all of which are based on usage of standard (native) SOD and/or catalase, are still anecdotal and/or uncontrolled. A review of the human disease/treatment literature suggests that further tightening of the scientific design of such trials is still badly needed; hopefully better experimental design will be applied when products such as PEG conjugates or genetically engineered polymers are ready for testing.

Oxygen toxicity: unique cytoprotective properties of vitamin E succinate in hepatocytes

Freshly isolated rat hepatocytes suspensions were incubated under an atmosphere of 95% O2/5% CO2 or 95% air/5% CO2 for 10 h. Cell injury and death were observed between the 6th and 10th hour of incubation, only in 95% O2-treated hepatocytes. Oxygen-induced injury was preceded by marked lipid peroxidation and rapid depletion of cellular alpha tocopherol content. The exogenous administration of unesterified alpha tocopherol (T, 25 microM) resulted in a 20-fold increase in cellular T levels (4.2 nmol/10(6) cells) but failed to protect these hepatocytes from the toxic effects of oxygen. In contrast, hepatocytes incubated with 25 microM of the succinate ester of alpha tocopherol (TS) contained both TS (3.0 nmol/10(6) cells) and T (1.4 nmol/10(6) cells) and were completely protected from the toxic effects of oxygen, including the induction of lipid peroxidation. These findings suggest that TS cytoprotection results not from the cellular accumulation of T but rather, from cellular TS accumulation. The data also indicate that the depletion of cellular T is not the critical cellular event that is responsible for hyperoxia (reactive oxygen intermediate)-induced injury. Instead, it appears that TS possesses unique cytoprotective properties that intervene in the critical cellular events that lead to oxygen toxicity. Thus, vitamin E succinate and our hyperoxic hepatocyte preparation provide a promising new model system for the study and prevention of tissue damage resulting from the toxic effects of hyperoxia and reactive oxygen intermediates.

Superoxide dismutase as an inhibitor of postischemic microvascular permeability increase in the hamster

The purpose was to elucidate the involvement of superoxide radical (O2-.) in the postischemic increase in the vascular permeability in the hamster cheek pouch. Cheek pouches of anesthetized hamsters were everted, prepared for intravital microscopy, and superfused with a bicarbonate buffered saline solution. Local ischemia for 30 min was obtained using a cuff placed around the proximal part of the cheek pouch. The vascular permeability in the postcapillary venules was quantified as leakage of intravenously injected fluorescein labeled dextran (FITC-dextran, Mw 150,000), using intravital microscopy and fluorimetry. There was a significant and reversible permeability increase after the reperfusion started. In the first series of experiments, combined intravenous infusion and topical application of human recombinant extracellular superoxide dismutase C (EC-SOD C) reduced the postischemic permeability response by 80%. Bovine CuZn-SOD given in exactly the same way reduced the response by 60%. In the second series of experiments, inactivated EC-SOD C was given to the control animals and active EC-SOD C was given to the treated animals. The topical treatment was excluded. Only active EC-SOD C reduced significantly the postischemic permeability increase when present during the ischemic period. Treatment with mannitol (i.v.) did not alter the postischemic response. Since active EC-SOD C and CuZn-SOD but not inactivated EC-SOD C effectively inhibited the response, we suggest that the superoxide anion is involved in the mediation of the postischemic permeability increase in the hamster.

Real-time continuous-flow spin trapping of hydroxyl free radical in the ischemic and post-ischemic myocardium

Real-time monitoring of spin-trapped oxygen-derived free radicals released by the isolated ischemic and reperfused rat heart has been achieved by ESR analysis of the coronary effluents using continuous flow detection and high-speed acquisition techniques. Two nitrone spin traps 5,5-dimethyl pyrroline 1-oxide (Me2PnO) and 3,3,5,5-tetramethyl pyrroline 1-oxide (MePnO) have been separately perfused at a concentration of 40 mM during a sequence of 50 min of low-flow ischemia (1 ml/min) followed by 30 min of global ischemia and subsequent reperfusion at the control flow rate (14 ml/min). ESR spectra were sequentially obtained in 5-min or 30-s blocks during low-flow ischemia and reperfusion, respectively. 1. The results show the formation of OH. free radicals in the ischemic and reperfused heart, as demonstrated by the observation of Me2PnO-OH (aN = aH = 14.9 G; g = 2.0053) and Me4PnO-OH (aN = 15.2 G, aH = 16.8 G; g = 2.0055) spin adducts. There is no evidence of significant biological carbon-centered or peroxyl free radicals spin-adduct formation in the coronary effluents or in lipid extracts analyzed after reflow. 2. The OH. generation began 15-20 min after the onset of ischemia and was moderate, peaking at 30-40 min. During reperfusion, an intense formation of OH. spin adducts was observed, with a maximum at 30-60 s and a further gradual decrease over the following 2 min. 3. Cumulative integrated values of the amount of spin adducts released during the ischemic period show a Me2PnO-OH level fourfold greater than that of Me4PnO-OH. It was 2.5 times greater during reflow, reflecting slower kinetics with the more stable Me4PnO. 4. The original ESR detection technique developed in this study allows accurate real-time quantitative monitoring of the oxygen-derived free radicals generated during myocardial injury. It might provide a quick and reliable new means for assessing the efficacy of free-radical inhibitors.

Use of 3,3,5,5-tetramethyl-1-pyrroline-1-oxide spin trap for the continuous flow ESR monitoring of hydroxyl radical generation in the ischemic and reperfused myocardium

A new approach for in vivo spin trapping and quantitation of oxygen-derived free radicals has been developed using a continuous flow high speed ESR detection system. Spin adducts of OH. were detected as 1:1:1:1:1:1 sextets (aN=15.2 G, aH=16.8 G, g=2.0055) in the isolated rat heart when perfused with 3,3,5,5-tetramethyl-1-pyrroline-1-oxide (40 mM) during a 10-min control pretreatment (14 ml/min) followed by 50 min of low-flow ischemia (1 ml/min), 30 min of global ischemia and subsequent reperfusion at 14 ml/min. The ESR signals appeared within 15-20 min of low-flow ischemia and grew moderately during the remaining 30 min at a rate of 2-6 nmoles of spin adduct released per minute. Post-ischemic reperfusion was characterized by a burst of spin adduct formation at 30 s-1 min, corresponding to 51.8 nmoles of spin adduct released between 30 s and 1 min.

Deleterious effects of oxygen radicals in ischemia/reperfusion. Resolved and unresolved issues

Oxygen free radicals are known to be generated during periods of ischemia followed by reperfusion. There is still some controversy, however, concerning the use of electron paramagnetic resonance spectroscopy to accurately detect and identify the free radical species that are formed. There is no doubt that oxygen radicals are deleterious to the myocardium; free radicals cause left ventricular dysfunction and structural damage to myocytes and endothelial cells in both in vitro and in vivo preparations. Potential sources of these cytotoxic oxygen species include the xanthine oxidase pathway, activated neutrophils, mitochondria, and arachidonate metabolism, yet the crucial source of free radicals in the setting of ischemia and reperfusion is unresolved. There is little doubt that oxygen radicals play a role in the phenomenon of stunned myocardium induced by brief periods of ischemia followed by reperfusion; numerous studies have consistently observed that pretreatment with free radical scavengers and antioxidants enhances contractile function of stunned, postischemic tissue. Whether oxygen free radical scavengers administered only during reperfusion enhance recovery of stunned myocardium in models of brief ischemia remains to be determined. In models of prolonged ischemia (2 hours) followed by reperfusion, we have not observed a beneficial effect of scavengers on stunned myocardium. The issue of whether oxygen free radical scavengers are capable of reducing so-called irreversible or lethal reperfusion injury remains, in our opinion, unresolved. Although some studies have observed that agents such as superoxide dismutase and catalase reduce infarct size in ischemia and reperfusion models, many others have reported negative results. Additional studies will be needed to resolve this ongoing controversy. Oxygen free radicals may also contribute to reperfusion-induced arrhythmias in rodent heart preparations; however, less data are available in other animal models. The concept of reperfusion injury should not be considered a deterrent to reperfusion for the treatment of acute myocardial infarcts in the clinical setting. Thrombolytic therapy reduces myocardial infarct size, enhances recovery of left ventricular function, and improves survival. Whether incremental beneficial effects on these parameters will be obtained when oxygen radical-scavenging agents are used as adjuvant therapy to thrombolysis in patients remains to be determined.