The independent effects of oxygen radical scavengers on canine infarct size. Reduction by superoxide dismutase but not catalase

Superoxide dismutase does not cause scar thinning after myocardial infarction

Previous studies demonstrated that treatment with superoxide dismutase, a scavenger of superoxide anions, limits the extent of myocardial injury in a canine preparation of regional myocardial ischemia and reperfusion. Little is known, however, about the effects of superoxide dismutase on the healing of a myocardial infarct. Therefore, this study was performed to determine whether treatment with superoxide dismutase during myocardial ischemia impairs formation of scar tissue after infarction. Dogs received 2 hour infusions of superoxide dismutase or albumin (controls) by way of the left atrium beginning 15 minutes before and ending 15 minutes after a 90 minute occlusion of the left circumflex coronary artery. Six weeks later the animals were killed. Two-dimensional echocardiography was performed before surgery and before induced death. Wall thickening in the central ischemic zone was decreased at 6 weeks compared with baseline studies (p less than 0.05), but the decrease was similar for both groups. The hydroxyproline concentrations (microgram/mg dry weight) of the scar tissue in the superoxide dismutase and control groups, respectively, were 35.3 +/- 3.8 and 28.7 +/- 5.0 (p less than 0.05). The ratios of the scar thickness to normal wall thickness were superoxide dismutase 0.91 +/- 0.03 and control 0.89 +/- 0.03 (p greater than 0.05). Thus, superoxide dismutase had no adverse effect on wall thickening or scar formation assessed 6 weeks after myocardial infarction, and may be useful to limit oxygen radical-mediated damage during reperfusion of the ischemic myocardium.

Effect of oxygen-derived free radical scavengers on infarct size following six hours of permanent coronary artery occlusion: salvage or delay of myocyte necrosis?

Oxygen-derived free radicals (O-2 and .OH) have been implicated in myocardial injury associated with coronary artery occlusion followed by reperfusion. While these cytotoxic oxygen species are predominantly produced upon reintroduction of molecular oxygen to previously ischemic tissue, they may also be generated throughout coronary occlusion in species (such as dog and man) in which native collateral vessels permit residual blood flow into the ischemic bed. To test this theory, 20 anesthetized, open-chest dogs underwent 6 h of permanent left anterior descending coronary artery occlusion: ten dogs were treated with the potent free radical scavenging enzymes superoxide dismutase (SOD: 5 mg/kg per hour) plus catalase (5 mg/kg per hour), while the remaining ten animals received saline. Infusion of drug or saline solution was begun 15 min prior to occlusion, and maintained throughout occlusion. Infusion of SOD + catalase did not significantly affect the extent of the area at risk of infarction (19.5 +/- 1.8% vs 24.0 +/- 1.4% of the left ventricle for the treated vs control group; P = NS), did not reduce myocardial oxygen demand (heart rate and arterial pressures were comparable for both groups), and did not alter collateral blood flow to the ischemic myocardium. However, mean infarct size in dogs treated with SOD + catalase (39.6 +/- 6.6% of the area at risk; 8.4 +/- 2.1% of the left ventricle) was significantly smaller than that observed in the saline controls (73.0 +/- 6.3% of the area at risk, P less than 0.01; 19.8 +/- 2.2% of the left ventricle, P less than 0.01). Thus, infusion of SOD + catalase prior to and during 6 h of coronary occlusion significantly reduced infarct size assessed at 6 h postocclusion. To determine whether this reduction in infarct size represented long-term salvage of ischemic myocardium, an additional 14 dogs (seven treated and seven controls) underwent the same procedure as described above; in this case the artery was reperfused after the 6-hour occlusion period, infusion of the SOD + catalase or saline solution stopped at 5-10 min postreperfusion, and the hearts examined at 30-48 h postocclusion. In contrast to the results at 6 h postocclusion, the necrosis at 30-48 h postocclusion was large, confluent and transmural in all dogs (infarct size = 21.2 +/- 2.5% vs 22.7 +/- 4.4% of the left ventricle for treated vs controls; P = NS). These results suggest that infusion of SOD + catalase in this model may delay, but not prevent, the development of ischemic necrosis.

Comparative anti-inflammatory activity of different superoxide dismutases and liposomal SOD in ischemia

Comparison of superoxide dismutases from different sources with respect to biological activity in the rat tourniquet poditis model shows that anti-ischemic activity is very variable although all the enzymes have the same specific enzymic activity. Both bovine Cu-SOD and E. coli Mn-SOD have excellent properties whereas yeast Cu-SOD and the homologous rat Cu-SOD show zero activity. The results confirm earlier demonstrations that (1) "All superoxide dismutases are equal but some are more equal than others", (2) at the dose levels used (compatible with possible clinical use) homologous enzyme is inefficient and hence human Cu-SOD may not be effective in humans, (3) liposomal encapsulation of bovine Cu-SOD greatly enhances biological efficacity, provides a slow release mechanism of the enzyme and provides a powerful drug for the treatment of ischemic injury.

Eicosanoids and myocardial ischaemia

The available data clearly suggest large alterations in myocardial eicosanoid generation during myocardial ischaemia and demonstrate important actions of eicosanoids on myocardial function during ischaemic conditions. These actions include direct effects on the injured myocardium as well as influences on other target cells, such as platelets and leukocytes. Selective modifications of eicosanoid generation, for example by providing exogenous PGI2 or by inhibiting oxygen toxicity are most challenging approaches for the design of new and potentially valuable cardioprotective agents. Antagonism of thromboxane formation and/or action might be of some value in ischaemia but appears to be less important for reperfusion injury. Leukotrienes and other noncyclic fatty acid peroxidation products are another group of potentially deleterious agents and there is a definite need for more selective inhibitors of leukotriene formation and/or action to establish their role in ischaemia.

Myocardial protection by antioxidant during permanent and temporary coronary occlusion in dogs

In an ischaemic heart model the lipid peroxidation, scavenger state and ultrastructure were studied, to determine the action of a new antioxidant of dihydroquinoline type (MTDQ-DA). In dog experiments, the left descending coronary artery (LAD) was ligated permanently (30 minutes, 1, 2 or 3 hours) or temporarily (30 minutes, 1 or 2 hours of ischaemia followed by 1 hour of recirculation). The experimental protocol involved two groups: control animals without antioxidant treatment and animals treated with antioxidant infusion during the ischaemic and reperfusion period. In both groups, the thiobarbituric acid reactive product, the malondialdehyde (MDA), reduced glutathione (GSH) and superoxide dismutase (SOD) were measured, to illustrate the injured or scavenged state of the membrane system. In nontreated animals the permanent and temporary LAD increased the MDA content, decreased GSH concentration (mainly during reperfusion) and reduced SOD activity. Treatment with MTDQ-DA diminishes the characteristic biochemical changes. According to ultrastructural investigations, irreversible alterations (Ca deposits in the mitochondria, disruption of intramitochondrial membranes, hypercontraction bands) occurred only in the control group. Anti-oxidant therapy is able to reduce the myocardial damages both quantitatively and qualitatively.

Evidence for a reversible oxygen radical-mediated component of reperfusion injury: reduction by recombinant human superoxide dismutase administered at the time of reflow

It has been suggested that the beneficial effects of reperfusing ischemic myocardium might be in part reversed by the occurrence of "reperfusion injury." One possible mechanism could be the generation of oxygen free radicals. Superoxide dismutase enzymatically scavenges superoxide radicals by dismutation to hydrogen peroxide. This study tested the hypothesis that administration of recombinant human superoxide dismutase (h-SOD) at the time of reflow after a period of prolonged global ischemia would result in improved recovery of myocardial metabolism and function by preventing or reducing a potentially harmful component of reperfusion. We also sought to determine whether catalase, an enzymatic scavenger of hydrogen peroxide, was a necessary addition for optimal benefit. Langendorff perfused rabbit hearts were subjected to 30 min of normothermic (37 degrees C) total global ischemia. At the moment of reperfusion, 12 control hearts received a 10 ml bolus of normal perfusate followed by 15 min of reperfusion with normal perfusate (group I), 12 hearts received 60,000 IU of h-SOD as a bolus followed by a continuous infusion of 100 IU/ml for 15 min (group II), and 12 hearts received 60,000 IU of h-SOD and 60,000 IU of catalase as a bolus followed by 100 IU/ml of both enzymes for 15 min (group III). Myocardial ATP and phosphocreatine (PCr) content and intracellular pH during ischemia and reperfusion were continuously monitored with 31P nuclear magnetic resonance (NMR) spectroscopy. During 30 min of normothermic global ischemia intracellular pH dropped from 7.11-7.18 to 5.58-5.80 in all three groups of hearts. Likewise myocardial PCr content fell rapidly to 7% to 8% and ATP fell more slowly to 29% to 36% of preischemic control content. After 45 min of reperfusion PCr recovered to 65 +/- 5% of control in untreated (group I) hearts compared with 89 +/- 8% in h-SOD-treated (group II) hearts (p less than .01 vs group I) and with 83 +/- 6% of control in h-SOD/catalase-treated (group III) hearts (p less than .05 vs group I). Recovery of isovolumic left ventricular developed pressure was 68 +/- 5% of control in h-SOD-treated (group II) hearts and 66 +/- 6% of control in h-SOD/catalase-treated (group III) hearts after 45 min of reflow, compared with 48 +/- 6% of control in untreated (group I) hearts (p less than .005 for groups II and III vs group I). The NMR data confirmed equal depletion of ATP and PCr content in all three groups of hearts.(ABSTRACT TRUNCATED AT 400 WORDS)

Reduction in experimental infarct size by recombinant human superoxide dismutase: insights into the pathophysiology of reperfusion injury

To determine the importance of reperfusion injury and the ability of the free-radical scavenger recombinant human superoxide dismutase (h-SOD) to prevent it, open-chest dogs underwent 90 min of proximal circumflex coronary artery occlusion, and only at the moment of reperfusion received either h-SOD (400,000 IU bolus into the left atrium followed by a 300,000 IU iv infusion over 1 hr) or saline. After 48 hr the surviving animals were killed and measurements were made of the risk region (by postmortem angiography) and infarct size (by gross pathology). All measurements were made by investigators blinded to treatment given, and the code was broken only at the end of the study. Hemodynamic variables and collateral flow during ischemia were similar in the two groups. Infarct size in control animals (n = 8) averaged 22.4 +/- 3.1% of the left ventricle and 52.2 +/- 7.1% of the risk region, compared with 13.3 +/- 0.8% of the left ventricle and 33.6 +/- 2.1% of the risk region in h-SOD-treated dogs (n = 8) (p less than .05). Infarcts in treated animals were not only smaller, but also exhibited a distinctive "patchiness," suggesting protection along vascular distributions. Furthermore, analysis of the relationship between infarct size and collateral flow measured during ischemia in the two groups indicated that protection by h-SOD was greatest in animals with the lowest collateral flows. This study supports the concept that reperfusion of ischemic myocardium results in a separate component of cell damage, presumably linked to the generation of oxygen free radicals on reflow. Since the h-SOD preventable reperfusion component of injury was most pronounced in hearts with the most severe ischemia, scavenging of oxygen radicals at the time of reflow may offer a novel and particularly promising therapeutic approach for the protection of ischemic myocardium.

Myocardial tissue iron delocalization and evidence for lipid peroxidation after two hours of ischemia

Ischemic tissue injury has been proposed to be in part due to oxygen-radical-mediated lipid peroxidation. In vitro studies of such reactions show that they are thermodynamically unfavorable unless catalyzed by transitional metals such as iron in low molecular weight species (LMWS iron), ie, the iron-ADP complex. This study tests for iron delocalization into a LMWS pool during myocardial ischemia and for increased tissue malondialdehyde (MDA), a product of lipid peroxidation. Anesthesia was induced in eight dogs (weighing 20 to 30 kg) with ketamine and maintained by ventilation with 1% halothane. The left anterior descending coronary artery was ligated in four animals, and the circumflex coronary artery was ligated in the other four. Two hours after ligation, the animals were sacrificed by a central venous injection of KCl. Tissue samples were immediately taken from the ischemic zone and from the corresponding nonischemic zone. MDA was determined by the thiobarbituric acid assay. LMWS iron was determined on a tissue ultrafiltrate by the o-phenanthroline assay. Statistical data analysis used the matched-pair two-tailed t test. LMWS iron was 18.3 nM/100 mg in ischemic tissue versus 13.1 nM/100 mg in nonischemic tissue (t = 4.14; P less than .01). MDA was 0.91 nM/100 mg in ischemic tissue versus 0.83 nM/100 mg in nonischemic tissue (t = 7.27; P less than .005). We conclude that there is a significant increase in tissue LMWS iron and in MDA after two hours of regional myocardial ischemia. This iron might be the catalyst for maturation of tissue injury during reperfusion as observed by other investigators.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial protection

Early studies of myocardial protection were designed to minimize ischemic injury. The next class and generation of investigations will most likely be designed to accelerate recovery following known myocardial injury. Such techniques will play an important role in allowing operations on acutely injured and ischemic myocardium and will be important in the treatment of postischemic injury when such injury occurs during the course of complex cardiac operations. Surgical aspects of myocardial metabolism are still rudimentary and many empiric observations require further exploration into the mechanisms by which such applications work.

Electrolysis-induced myocardial dysfunction. A novel method for the study of free radical mediated tissue injury

Oxygen-derived free radicals and other oxidizing species are thought to be involved in inflammation and ischemic tissue injuries. Recently, oxygen-derived free radicals also have been implicated in tissue injury of the myocardium subjected to ischemia/reperfusion. The purpose of this investigation was to determine if electrolysis of a physiological buffer would serve as a source of free radicals, and if these radicals would lead to alterations in myocardial function. Isolated Langendorff-perfused rabbit hearts perfused with buffer subjected to a 20 mA D.C. current for 2 min demonstrated significant increases in coronary perfusion pressure (37 +/- 6 mmHg), left ventricular end diastolic pressure (41 +/- 7 mmHg), and loss in left ventricular developed pressure (35 +/- 5%). The free radical scavengers, superoxide dismutase and a combination of tryptophan plus glycine, were effective in protecting the hearts from the effects of electrolysis. The presence of free radicals was semiquantitated with a radical-luminol chemiluminescent assay. In this assay a variety of radical scavengers and antioxidants were effective (i.e., dimethyl sulfoxide, nitro blue tetrazolium, ascorbate, superoxide dismutase, 1, 3-diphenylisobenzofuran, and glycine, catalase), whereas mannitol and tryptophan were not effective. The data indicate that electrolysis of a physiological buffer produces a milieu containing several reactive oxygen species or free radicals that have the potential to produce alterations in a biological system. This method has the advantage over existing protocols for the generation of radicals in that it is a blood-free and an enzyme-free system.

Failure of superoxide dismutase and catalase to alter size of infarction in conscious dogs after 3 hours of occlusion followed by reperfusion

Superoxide dismutase (SOD) and catalase (CAT), enzymes that degrade superoxide anion and hydrogen peroxide, respectively, reduce size of infarction in anesthetized, open-chest dogs subjected to coronary occlusion followed by reperfusion. To evaluate potential protective effects of these enzymes in conscious animals, three groups of dogs were instrumented at sterile surgery with a hydraulic occluder on the left circumflex (LCX) coronary artery, sonomicrometers to measure regional wall thickness, and catheters to monitor arterial and left ventricular pressures. Ten to 14 days after surgery, the animals were sedated with morphine sulfate (0.5 mg/kg). The LCX artery was occluded for 3 hr by inflation of the hydraulic cuff. Infusions of SOD (n = 7), CAT (n = 6), or saline (control group, n = 7) were begun 15 min before reperfusion and lasted for 45 min of reperfusion. The doses of SOD and CAT were 5 mg/kg, dissolved in 60 ml of saline, and infused at a rate of 1 ml/min. Myocardial blood flow was measured with tracer-labeled microspheres (15 micron diameter) before occlusion, after 5 to 10 min of occlusion, after 150 min of occlusion, and 5 to 10 min after reperfusion. Size of infarction was measured 24 hr later by dual-perfusion staining with Evans blue and triphenyl tetrazolium. Size of infarction (expressed as a percentage of area at risk) did not differ significantly among the three groups: control, 32 +/- 17% (mean +/- SD); SOD, 38 +/- 17%; CAT, 27 +/- 17%. Hemodynamic parameters and myocardial blood flows (measured before infusion of any agents) were not significantly different among the three groups. Serum SOD levels in SOD-treated dogs were 19 +/- 2 micrograms/ml at the onset of reperfusion and 29 +/- 3 micrograms/ml at the end of the infusion. Blood assays collected after infusion showed a monoexponential decay of SOD levels with a half-life of 22 +/- 6 min. We conclude that myocardial protection by SOD or CAT is model dependent. In conscious dogs subjected to 3 hr of coronary occlusion followed by reperfusion, SOD and CAT failed to alter size of infarction.

Reduction of the size of infarction by allopurinol in the ischemic-reperfused canine heart

This study was performed to assess the effect of allopurinol in a canine preparation of myocardial infarction. Dogs underwent occlusion of the left circumflex coronary artery for 90 min, followed by reperfusion for 6 hr. Three groups were studied: (1) control, (2) dogs receiving 25 mg/kg allopurinol 18 hr before occlusion and 50 mg/kg 5 min before occlusion, and (3) dogs receiving allopurinol as above plus 5 mg/kg superoxide dismutase over 1 hr beginning 15 min before reperfusion. Infarct size expressed as a percentage of the area at risk was 40 +/- 4 in the control group, 22 +/- 5 in the allopurinol group (p less than .05 vs control), and 17 +/- 4 in the allopurinol plus superoxide dismutase group (p less than .05 vs control). The differences in infarct size were not due to differences in myocardial oxygen supply or demand. Neutrophil superoxide anion production was not altered by allopurinol treatment. The results suggest that myocardial xanthine oxidase may generate oxygen radicals that play a role in myocardial injury due to ischemia and reperfusion.

Safety evaluation of free radical scavengers PEG-catalase and PEG-superoxide dismutase

Treatment with catalase and SOD (superoxide dismutase) could diminish the damage due to oxygen free radical formation, but these enzymes are rapidly removed from circulation. The covalent attachment of monomethoxypolyethylene glycol (PEG) to catalase and SOD extended their plasma half-lives. Toxicity of PEG-catalase and PEG-SOD was evaluated in mice and rats prior to their use as free radical scavengers. Rodents used in acute, subacute, and subchronic toxicologic studies could tolerate large doses of PEG-catalase and PEG-SOD without developing toxic signs. The conjugates did not affect survival rate, appearance, behavior, food intake, blood chemistry, hematology, or urinalysis. In general, body weight gains, organ weights, and histomorphology were also unaffected. Massive doses of PEG-catalase caused slight weight loss, splenic hypertrophy, and generalized splenic stimulation in mice. Massive doses of PEG-SOD resulted in vacuolation in splenic macrophages in rats. PEG-catalase and PEG-SOD circulated for 3 days and 8 days, respectively, in mice following i.v. or i.m. administration.

Free radicals in ischemic myocardial injury

Myocardial ischemia causes release of chemotactic factors, migration of neutrophils, peroxidation of lipids, and depletion of free radical scavengers. The invading neutrophils may injure the myocardial vasculature and sarcolemma by generating oxygen free radicals. Several agents that affect neutrophils or oxygen radicals were evaluated in a canine model of regional myocardial ischemia and reperfusion. Anesthetized dogs underwent occlusion and reperfusion of the left circumflex coronary artery. Infarct zone, area at risk of infarction, and total left ventricle were quantified by gravimetric and planimetric analysis. Limitation of infarct size by ibuprofen was associated with marked suppression of leukocyte accumulation within the ischemic myocardium. Neutrophil depletion by antiserum resulted in similar reductions of infarct size and was accompanied by a reduction in leukocyte infiltration. A combination of oxygen radical scavengers, superoxide dismutase plus catalase, decreased myocardial injury whether infusion began before occlusion or 75 min after occlusion. None of the treatments significantly altered hemodynamic indices of myocardial oxygen demand. Reduction of infarct size by ibuprofen, neutrophil antiserum, and free radical scavengers indicates that neutrophils and oxygen radicals participate in producing the irreversible damage to the myocardium during ischemia and reperfusion.