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

Prevention of ischemia-reperfusion injury by the allergy drug lodoxamide tromethamine

Lodoxamide tromethamine, an orphan antiallergy drug, inhibits degranulation of mast cells that reside in the myocardium and inhibits xanthine oxidase located in myocytes and predominantly in the vascular endothelium. The hypothesis evaluated was that lodoxamide tromethamine would attenuate oxygen free radical damage. Isolated working rat hearts were perfused with Krebs-Henseleit buffer containing 0, 1, 10, 100, or 1,000 mumol/L lodoxamide tromethamine at 37 degrees and 24 degrees C with ischemic times of 22 and 93 minutes, respectively. These ischemic intervals yielded 50% survival and 50% return of function in untreated hearts. Lodoxamide treatment alone at the onset of reperfusion was also studied. Performance end points were aortic flow, pressure, and coronary flow. Biochemical analyses included serotonin collected from coronary effluent as a marker of mast cell degranulation, uric acid for xanthine oxidase inhibition, myocardial adenosine triphosphate, and carbonyl group concentrations. Performance data demonstrated that lodoxamide was beneficial in a log-linear dose response when given continuously at both temperatures. Percent of preischemic values for untreated and maximal responses at 1,000 mumol/L of lodoxamide were as follows: a mortality of 50% in nontreated hearts versus 0%; aortic flow, 47% to 94% (37 degrees C), 46% to 86% (24 degrees C); cardiac output, 60% to 98% (37 degrees C), 58% to 97% (24 degrees C); adenosine triphosphate, 59% to 90% (37 degrees C), 48% to 65% (24 degrees C). Serotonin was undetectable from any hearts. Uric acid concentrations and carbonyl group content did not change with increasing dose. Lodoxamide demonstrated no benefit when given only during reperfusion, suggesting injury occurred at times other than reperfusion.

Oxygen free radicals and myocardial damage: protective role of thiol-containing agents

It has been suggested that the sudden presence of oxygen during reperfusion after a period of ischemia may be toxic for the myocardial cell. The oxygen molecule is capable of producing reactions in the cell, forming highly reactive free radicals, and inducing lipid peroxidation of membranes, altering their integrity and increasing their fluidity and permeability. The ischemic and reperfused cardiac cell is the prime candidate for this reaction sequence and may explain the molecular mechanism underlying the pathologic events related to membrane dysfunction and calcium homeostasis. However, the myocardium has a series of defense mechanisms including the enzymes superoxide dismutase (SOD), catalase, and glutathione peroxidase plus other endogenous antioxidants such as vitamin E, ascorbic acid, and cysteine to protect the cell against the cytotoxic oxygen metabolites. The prerequisite for oxygen free radical involvement in ischemia and reperfusion damage is that ischemia alters the defense mechanisms against oxygen toxicity. It is known that ischemia may impair mitochondrial SOD and, with reperfusion, oxidative stress may occur as shown by tissue accumulation and release of oxidized glutathione. This tripeptide molecule in the cofactor of glutathione peroxidase, the enzyme that removes hydrogen and lipid peroxides. Its formation and subsequent release is a reliable index of oxidative damage. In our study, we investigated the effects of N-acetylcysteine on oxidative damage in the isolated rabbit heart. N-acetylcysteine increases, in a dose-dependent manner (from 10(-7) to 10(-5) M), the myocardial glutathione content and provides an important degree of protection against ischemia and reperfusion. Oxidative stress does not occur, mitochondrial function is maintained, enzyme release is reduced, and contractile recovery is increased. Similarly, we administered N-acetylcysteine in the pulmonary artery of coronary artery disease patients undergoing coronary bypass grafting (150 mg/kg in 1 hour followed by 150 mg/kg in 4 hours). The degree of oxidative stress on reperfusion was reduced and recovery of cardiac function improved. In this article, we review the cardioprotective role of thiol-containing agents.

Modulation of catecholamine cardiomyopathy by allopurinol

It has been suggested that cardiac injury by catecholamines may be the result of coronary constriction leading to ischemic damage. Allopurinol (ALLO) has been shown to reduce the extent of myocardial necrosis in various systems. Hence the possibility that ALLO might limit norepinephrine (NE) injury was tested. Rabbit hearts were infused with NE (3 micrograms/min/kg) for 90 minutes, with or without ALLO (50 micrograms/min/kg). Control specimens infused with saline solution plus ALLO were also prepared. Hearts were excised 48 hours later and studied as isovolumic isolated heart preparations. Peak systolic pressure, coronary flow, and myocardial oxygen consumption were significantly reduced in the hearts infused with NE but not in the NE + ALLO hearts. Myocardial adenosine triphosphate and glycogen concentrations were 29% and 26% lower in the NE hearts compared with control hearts. These reductions were absent in the NE + ALLO group. Moreover, rates of creatine phosphokinase and lactic dehydrogenase release were sharply elevated in the NE hearts but not in those also given ALLO. These findings are consistent with the changes observed histologically. The amount of myocardial damage was less in the ALLO + NE group compared with the NE group (p less than 0.02). This appears to be the first report to demonstrate that ALLO reduces myocyte damage by NE. Possible mechanisms include decreased free radical production, scavenging of free radicals, and preservation of the adenine nucleotide pool. Because xanthine oxidase activity is absent in the rabbit, the latter two mechanisms are more likely explanations for the findings.

Reduction of canine myocardial infarct size by a diffusible reactive oxygen metabolite scavenger. Efficacy of dimethylthiourea given at the onset of reperfusion

A number of scavengers of reactive oxygen metabolites reduce myocardial injury when given before ischemia and reperfusion, but few, if any, have proven to be effective when given near the onset of reperfusion. This is particularly true when infarct size is measured after at least 48 hours of reperfusion, when the full extent of myocardial damage has become apparent. Dimethylthiourea (DMTU) is an extremely diffusible, potent scavenger of hydroxyl radical, hydrogen peroxide, and hypochlorous acid, with a long half-life of 43 hours. Sixteen chloralose-anesthetized dogs underwent 90 minutes of left anterior descending coronary artery (LAD) occlusion followed by 48 hours of reperfusion. Collateral flow was measured by radioactive microspheres. Infarct size and risk area were measured by a postmortem dual-perfusion technique using triphenyl tetrazolium chloride and Evan's blue dye. In eight dogs, therapy with DMTU (500 mg/kg i.v.) was given during the last 15 minutes of ischemia and the first 15 minutes of reperfusion. In eight control dogs, the same volume of 0.9% saline was given during the last 15 minutes of ischemia through the first 15 minutes of reperfusion. Infarct size as a percent of risk area was reduced in the DMTU-treated group compared with the saline-treated controls (DMTU = 42 +/- 4% versus saline = 59 +/- 4%, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Allopurinol pretreatment improves evoked response recovery following global cerebral ischemia in dogs

The reperfusion of previously ischemic tissue may lead to the formation of highly reactive free radicals that promote tissue injury. Xanthine oxidase has been implicated as one source of these free radicals. We examined the role of xanthine oxidase in brain injury using a cerebrospinal fluid compression model of global cerebral ischemia with 15 minutes of ischemia and 4 hours of reperfusion. Seven dogs were pretreated with the xanthine oxidase inhibitor allopurinol (50 mg/kg for 5 days). Neurophysiological recovery was monitored with cortical somatosensory evoked potentials. As an attempt to correlate brain recovery with the mechanism of protection, free brain malondialdehyde was measured at the end of reperfusion by high-performance liquid chromatography. Brain water content was measured by wet-dry weights. Compared with seven untreated control dogs, allopurinol pretreatment significantly improved recovery of somatosensory evoked potentials after 4 hours of reperfusion. However, the amount of free malondialdehyde in the allopurinol-treated dogs was 32% greater than that in the controls. Brain water content was similar in the two groups. These results suggest that xanthine oxidase contributes to brain injury after ischemia and reperfusion. However, tissue damage caused by xanthine oxidase may be mediated through mechanisms other than free radical production.

Chronic administration of allopurinol fails to exert any cardioprotective effect in rats submitted to permanent coronary artery ligation

It has been shown that allopurinol, an inhibitor of xanthine oxidase, may limit the extent of myocardial infarction in dogs. In the present work, we studied the effect of a chronic administration of allopurinol on myocardial infarct size measured histochemically 48 h after in situ left coronary artery ligation in the rat. Our results indicate that allopurinol pretreatment does not produce any limitation of the extent of necrosis, but induces a significant increase in the volume of the non-ischemic portion of the myocardium, accompanied by an increase in protein content. This phenomenon, which could be due to the development of an edema in the non-ischemic portion of the myocardium, may well explain some discrepancies reported in previous experimental studies in which the infarct size was conventionally expressed as a percentage of the total volume of ventricular tissue. We have also shown that allopurinol pretreatment failed to improve the residual cardiac function in rats after left coronary artery ligation. We conclude that the enzyme xanthine oxidase is probably not involved in the pathophysiology of myocardial infarction in the rat because of the absence of collateral vasculature in this species which prevents any oxygen supply to the ischemic zone. In most other mammals such as the dog, the existence of a collateral system maintains a residual blood flow and oxygen supply to the ischemic portion of ligated hearts, allowing the xanthine oxidase-induced production of superoxide anions to be activated, thereby initiating peroxidative lesions in membrane lipids.

Failure of allopurinol to protect against cerebral injury when given after the start of hypoxia

One cause of ischemic brain injury is free radical formation during recirculation. Allopurinol inhibits xanthine oxidase, an important source of free oxygen radicals. It is known that allopurinol pre-treatment has a protective action during cerebral ischemia. In the present study we exposed slices from the rat hippocampus to 9 minutes of hypoxia to test whether it is sufficient that allopurinol is present in the tissue at the time of reoxygenation. Forty-six slices loaded with allopurinol (10(-5) M) prior to reoxygenation (during hypoxia) were compared to 34 control slices. The response of the pyramidal cell population to orthodromic stimulation was reduced in both groups and there was not a significant difference between the two groups.

Effects of allopurinol on ischemic experimental pancreatitis

The effect of allopurinol, a xanthine oxidase inhibitor, on canine experimental ischemic pancreatitis was studied. The animals were divided into nine groups: 1. Group 1. Control with pancreatic ischemia; 2. Group 2. Received allopurinol once, previous to ischemia; 3. Group 3. Received allopurinol once, immediately after ischemia; 4. Group 4. Received allopurinol immediately after ischemia and then daily; and 5. Groups 5, 6, and 7 were controls for the operation, allopurinol, and its vehicle, respectively; 6. Group 8 (pancreatic ischemia) and Group 9 (that received allopurinol after ischemia and daily) were also studied histologically. Serum amylase was determined in all animals. In Groups 1 and 5, following the ischemic period, hyperamylasemia developed and a peak was reached 24 h after ischemia. In Group 2, a significant decrease of amylase levels was found, compared to matched controls immediately after ischemia and then rose, reaching on the fifth day a peak that was less than the controls at 24 h. In Group 3, the serum amylase level increased immediately to values similar to controls; later, there was a drop to levels lower than those found in controls, followed by a peak on the fifth day. In Group 4, there was no significant elevation in the amylase values. Groups 6 and 7 showed no changes of amylasemia. In this experimental model, allopurinol blocked or ameliorated significantly cellular injury, as shown by a decrease of amylase levels in blood, and of histopathological changes, depending on dose and time of administration. These results offer the possibility of a prophylactic therapy for chronic relapsing and idiopathic pancreatitis.

Oxygen-derived free radicals and myocardial reperfusion injury: an overview

Reperfusion of acutely ischemic myocardium is associated with various distinctive pathophysiologic derangements, which are collectively referred to as reperfusion injury. Among these, three have been attributed to oxygen radicals: a) arrhythmias, b) transient mechanical dysfunction ("stunning"), and c) cell death. a) Reperfusion-induced arrhythmias. Although the precise mechanism for reperfusion arrhythmias remains to be determined, considerable evidence suggests that oxygen radicals play an important pathogenetic role in these rhythm disturbances. b) Transient mechanical dysfunction ("myocardial stunning"). Studies suggest that this abnormality is caused by events occurring in the initial seconds of reperfusion, and therefore represents a manifestation of sublethal, reversible reperfusion injury. Although our understanding of the mechanism of myocardial stunning is still fragmentary, there is overwhelming evidence for a pathogenetic role of oxygen radicals. c) Cell death. The evidence that reperfusion causes extension of the infarct produced by the antecedent ischemia is highly controversial. Although several studies have reported reduction of infarct size with antioxidants applied at the time of reperfusion, numerous other investigations have failed to reproduce these results. At present, there is no obvious explanation for this discrepancy. What is clear is that short-term administration of antioxidants at the time of reperfusion will not produce sustained limitation of infarct size. However, the possibility that long-term administration of antioxidants will produce sustained limitation of infarct size merits further consideration. In conclusion, there is strong evidence that the generation of oxygen radicals upon reperfusion plays an important pathogenetic role in two manifestations of reperfusion injury, namely, arrhythmias and stunning. Intense controversy persists regarding whether oxygen radicals contribute to extending cell death upon reperfusion and whether reperfusion in itself causes cell death. On the basis of the evidence available at this time, oxygen radicals appear to be important in the genesis of relatively mild, sublethal forms of myocellular damage, but their role in the genesis of lethal myocellular injury remains to be established.

Xanthine oxidase inhibition does not limit canine infarct size

BACKGROUND

Evidence supporting the role of xanthine oxidase in myocardial reperfusion injury is based on studies with pharmacological interventions used to inhibit enzyme function. Controversy exists, however, regarding the true role of xanthine oxidase in reperfusion injury. This study was performed to determine whether xanthine oxidase inhibition limits myocardial injury due to coronary artery occlusion and reperfusion.

METHODS AND RESULTS

Anesthetized dogs underwent coronary artery occlusion (90 minutes) and reperfusion (6 hours). Oxypurinol (28 mg/kg) or amflutizole (30 mg/kg), chemically unrelated inhibitors of xanthine oxidase, or vehicle was infused intravenously 15 minutes before and 3 hours after reperfusion. Regional myocardial blood flow was determined with radiolabeled microspheres. Infarct size was determined with the tetrazolium method. Myocardial infarct size (percent of risk region) was less in oxypurinol-treated dogs, 32 +/- 16%, compared with that of the control group, 46 +/- 15%. Infarct size for the amflutizole-treated dogs, 40 +/- 21%, was not significantly different from that of the control group. There were no differences in rate-pressure product or collateral blood flow to account for differences in infarct size. Uric acid concentration in the coronary venous plasma increased after reperfusion in the dogs treated with vehicle but not in the drug-treated dogs. Xanthine oxidase inhibition was demonstrated in each of the drug treatment groups, but only oxypurinol limited the extent of myocardial injury.

CONCLUSIONS

Previously reported cardioprotective effects of allopurinol, noted to occur only when the drug was administered chronically, may be related to a property of oxypurinol, a major metabolite of allopurinol. The beneficial effect of oxypurinol is unrelated to inhibition of superoxide formation during xanthine oxidase-catalyzed oxidation of xanthine and hypoxanthine.

The occurrence of oxidative stress during reperfusion in experimental animals and men

Reperfusion is the prerequisite for the ischemic myocardium to recover its metabolic and mechanical function. However, reperfusion after a prolonged period of ischemia in the experimental animal may exacerbate, or at least accelerate, the occurrence of ischemic injury, whilst in humans at the least it is not beneficial. This entity has been called reperfusion damage, since much of the damage is believed to be caused by events occurring at the moment of reperfusion rather than by changes occurring during ischemia. The existence of reperfusion damage, however, has been questioned, and evidence in favour of the concept is sparse. At the moment the molecular events occurring at the time of reperfusion are not completely understood, and the relative importance of several proposed deleterious mechanisms is not yet established. One of the most fashionable ideas for the cause of reperfusion damage is that the function of cell membrane is modified by oxygen radicals generated at the moment of reperfusion. Evidence in favour of and against this hypothesis is described in detail in the present article.

Treatment with deferoxamine during ischemia improves functional and metabolic recovery and reduces reperfusion-induced oxygen radical generation in rabbit hearts

BACKGROUND

Iron may play a central role in oxygen radical generation during myocardial ischemia and after reperfusion. Because conditions during ischemia may also liberate iron, we hypothesized that administration of the iron chelator deferoxamine during ischemia would result in improved functional and metabolic recovery after postischemic reperfusion.

METHODS AND RESULTS

Isolated, perfused rabbit hearts were studied by phosphorus-31 nuclear magnetic resonance spectroscopy. The hearts received one of three treatments: deferoxamine at the onset of 30 minutes of global ischemia (n = 9), deferoxamine as a bolus followed by a continuous 15-minute infusion begun at reflow (n = 9), or standard perfusate (n = 7). Hearts treated with deferoxamine during ischemia showed better recovery of developed pressure than did control hearts (63.2 +/- 7.5% versus 41.2 +/- 2.9% of baseline) (p = 0.02) and better recovery of myocardial phosphocreatine content (92.4 +/- 10.3% versus 68.2 +/- 4.5% of baseline, p less than 0.05). These functional and metabolic benefits were comparable to those obtained with deferoxamine treatment during early reperfusion. In 15 additional hearts, intraischemic treatment with deferoxamine resulted in no reduction in oxygen radical concentrations as measured on frozen tissue by electron paramagnetic resonance spectroscopy at end ischemia, but the treatment eliminated the reperfusion-induced increase of free radical generation observed in control hearts (2.9 +/- 0.01 versus 7.0 +/- 0.07 microM, p less than 0.001). The magnitude of reduction was similar to that when deferoxamine was given at the onset of reflow (2.4 +/- 0.02 microM, p less than 0.001 versus control).

CONCLUSIONS

These results demonstrate improved functional and metabolic recovery of myocardium treated with deferoxamine during ischemia, accompanied by a reduction in reperfusion-induced oxygen free-radical generation to the same degree as reflow treatment, confirming the importance of iron in the pathogenesis of myocardial reperfusion injury.

Recombinant tissue-type plasminogen activator: current concepts and guidelines for clinical use in acute myocardial infarction. Part II

The extraordinarily high prevalence of coronary heart disease, coupled with the alarming incidence of MI in Western society, has encouraged the investigation and development of pharmacologic agents that can be employed widely, quickly, effectively, and safely. Recombinant t-PA has played a vital role in the treatment of MI, restoring coronary arterial patency, limiting infarct size, preserving ventricular function, and improving patient survival. It has been shown to be safe when given to carefully selected patients and, although indications for clinical use have been relatively restricted, they appear to be expanding considerably. Future investigations must continue to focus on patient selection to allow treatment for all patients who would derive benefit and to establish dosing regimens and adjuvant therapies that will maximize coronary reperfusion while concomitantly limiting reocclusion and hemorrhagic complications.

Are leucocyte-derived free radicals involved in ischaemia in human legs?

Leucocyte-derived free radicals were monitored in 30 stage II peripheral vascular disease (PVD) patients in an open placebo-controlled study. Linked to a transcutaneous oxygen pressure (TcPO2) monitor, they performed two consecutive standard treadmill tests (5 min, 2 km h-1, 12% slope) before and after 15-days treatment with placebo or a leucocyte-derived free radical scavenger (Piroxicam, 20 mg day-1), the second test being carried out at the TcPO2 half-recovery time. Blood samples were collected at baseline, at the maximum walking times and the TcPO2 half recovery times. The total and differential leucocyte counts, the percentage of cells with pseudopodia or cytoplasmatic irregularities, the filterability rates (using a positive pressure Nuclepore filter system) of the main leucocyte subfractions and plasma oxidant activity were monitored. Compared with values before treatment and with the placebo-treated group Piroxicam therapy significantly (P less than 0.001) reduced the final half-recovery time, the percentage of cells with pseudopodia and the level of plasma oxidant activity (P less than 0.01) and kept the granulocyte filterability rate stable, showing leucocyte-derived free radicals are involved in peripheral ischaemia.

High-performance liquid chromatographic detection of hydroxylated benzoic acids as an indirect measure of hydroxyl radical in heart: its possible link with the myocardial reperfusion injury

The present report describes a method suitable for the indirect assay of hydroxyl radical (OH.), which is likely to be produced during reperfusion of ischemic myocardium. Isolated rat heart perfused by the Langendorff technique was subjected to 30 min of ischemia, followed by 30 min of reperfusion. Salicylic acid (2 mM) was added to the perfusion circuit to trap any OH. radical generated during the experiment. 2,5- and 2,3-dihydroxybenzoic acids (hydroxylated products of salicylic acid) were identified by authentic standards as well as by pure OH.-generating system using high-performance liquid chromatography with electrochemical detection. In addition to serving as a chemical trap for the detection of OH., salicylate attenuated myocardial reperfusion injury as evidenced by reduced formation of creatine kinase, decreased lipid peroxidation, and improved myocardial contractile functions during reperfusion. These results thus provide direct evidence for the presence of OH. in heart and link it to the myocardial reperfusion injury.

Cardioprotective effects of amlodipine on ischemia and reperfusion in two experimental models

The cardioprotective effect of amlodipine, a long-acting dihydropyridine derivative, was studied in 2 experimental models of ischemia and reperfusion. Isolated and blood-perfused feline hearts were made globally ischemic for 60 minutes and then reperfused for 60 minutes. Alterations of left ventricular developed pressure and compliance were monitored in both amlodipine-treated hearts and saline-treated control animals. Changes in perfusion pressure indicated that amlodipine significantly reduced myocardial oxygen consumption and coronary vascular resistance. Furthermore, a progressive increase in resting left ventricular diastolic pressure indicated that amlodipine, administered before the onset of global ischemia, attenuated the development of ischemic contracture. Return of contractile function 60 minutes after reperfusion and maintenance of tissue concentrations of electrolytes were significantly better in the amlodipine-treated group than in the control animals. In intact canine hearts, regional myocardial ischemia was induced for 90 minutes, followed by 6 hours of reperfusion. Although the hemodynamic variables and the size of the region of risk did not differ significantly between treated animals and control animals, the infarct size was significantly smaller in the amlodipine-treated group than in the control animals, and a gradual reduction in coronary blood flow was observed in the control group that was prevented in the amlodipine group. A comparison of these findings with those observed with oxygen radical scavengers also is discussed. A detailed report of these studies was published in The American Journal of Cardiology (1989;64:101I-116I). This review is included here to maintain continuity of the symposium for the convenience of the reader.

Intracoronary infusion of superoxide dismutase and reperfusion injury in the pig heart

The effects of an intracoronary infusion of superoxide dismutase on infarct size were studied in 16 pigs submitted to a 48-min coronary occlusion of the mid left anterior descending coronary artery followed by reperfusion for 24 h. Areas at risk marked with fluorescein and infarct sizes calculated with triphenyl tetrazolium chloride staining 24 h after the occlusion were similar in the five control animals with coronary reperfusion alone, in the five animals with an intracoronary infusion of lactate Ringer initiated 3 min before reperfusion and maintained for 33 min and in the six animals with superoxide dismutase added to the solution of lactate Ringer and infused at a rate of 2500 units/min. The ratios infarct size/area at risk were respectively 0.50 +/- 0.10, 0.65 +/- 0.04 in the three study groups (NS). The extent of intramyocardial hemorrhage, evaluated by morphometric analysis was also similar 0.90 +/- 0.29 x 10(6), 0.70 +/- 0.14 and 1.62 +/- 0.42 red blood cells/mm3 of tissue (NS). The superoxide dismutase infusion, however, resulted in significantly fewer early reperfusion arrhythmias which involved 23 +/- 15 s of each minute electrocardiographic recording in the superoxide dismutase group, compared to 37 +/- 13 s in the lactate Ringer group and 45 +/- 14 s in the control group (p = 0.004). The lack of an effect of intracoronary infusion of superoxide dismutase on infarct size suggests that in this experimental model, extracellular superoxide radicals generated during early reperfusion have no major role on myocardial cell necrosis and microvascular damage. Reperfusion arrhythmias were, however, reduced.

Myocardial reperfusion injury: an assessment by the spasm of resistance vessel concept of ischemic heart disease

Myocardial reperfusion injury will be discussed in context to the spasm of resistance vessel concept of ischemic heart disease. This hypothesis attributes symptoms in this disorder directly to primary spasm of resistance vessels, and is based in part on a study of no-reflow which provided evidence that no-reflow is due to ischemia-induced injury-spasm of resistance vessels. Studies of no-reflow and reperfusion injury are rather similar, and the concept asserts that ischemia-induced injury-spasm causing no-reflow is involved in reperfusion injury. It is recognized that oxygen free radicals cause both myocardial and vascular injury during reperfusion injury, and the concept suggests that vascular injury contributes significantly to reperfusion injury by inducing the sequence of injury-spasm, no-reflow, fresh ischemia, and fresh ischemic reperfusion injury. In keeping with this, the possible involvement of spasm and no-reflow in reperfusion injury occasionally is mentioned. However, it seems to be generally accepted that reperfusion injury is due essentially solely to direct myocardial injury by free radicals, and possible reasons will be explored for a relative disinterest in spasm and no-reflow in reperfusion injury.

Does reperfusion extend necrosis? A study in a single territory of myocardial ischemia--half reperfused and half not reperfused

The purpose of this study was to confirm or disprove the existence of reperfusion-induced extension of necrosis. To avoid the effect of the variability of collateral circulation when groups of dogs are compared, we compared the effect of reperfusion and nonreperfusion on myocardial necrosis in a single ischemic territory, half of which was reperfused and half of which was not. The left anterior descending coronary artery (LAD) territory between its last diagonal branch and the apex was studied because it was found to have uniform collateral blood flow. In 20 dogs, the LAD was occluded for 90-240 minutes to produce necrosis of different degrees of transmurality. Before release of this occlusion, the LAD was occluded distally halfway to the apex to keep the distal half nonreperfused. After 5 minutes of proximal reperfusion. Monastral blue dye was injected into the left atrium for demarcation of the reperfused region, and the heart was arrested, excised, cut parallel to the LAD, and placed into triphenyl tetrazolium chloride (TTC) solution for delineation of the region of necrosis. The validity of TTC staining under the conditions of this study was confirmed by light and electron microscopy. The transmurality of necrosis, measured within 1 or 0.5 cm on either side of the boundary, ranged from 30% to 88% of wall thickness and was not different in the reperfused compared with the nonreperfused region (paired t test). Reperfusion did not advance the epicardial edge of necrosis compared with the nonreperfused region. In conclusion, at 5 minutes after reperfusion, comparison of necrosis in the reperfused and nonreperfused halves of a single ischemic territory could not demonstrate an extension of necrosis by reperfusion.

Combined adenosine and lidocaine administration limits myocardial reperfusion injury

The endogenous compound adenosine may play a role in limiting myocardial ischemia-reperfusion injury through its ability to cause vasodilation, modulate cardiac adrenergic responses, inhibit neutrophil function, or modulate energy supply and demand of the myocardium. The local anesthetic lidocaine has been shown to be protective against myocardial ischemia-reperfusion injury, although its mechanism of action remains unresolved. We hypothesized that administration of exogenous adenosine during reperfusion would limit the size of the infarct that results from a period of ischemia and reperfusion only when the animals are treated with lidocaine. Male, mongrel dogs (13.0-20.0 kg) were anesthetized (30 mg/kg i.v. sodium pentobarbital), and a left thoracotomy was performed. The left circumflex coronary artery (LCx) was isolated and instrumented with an electromagnetic flow probe, a 25-gauge nonobstructing intracoronary catheter, and a critical stenosis. The dogs were allocated randomly to one of four groups: 1) control, n = 13, (saline), 2) adenosine, n = 13, (0.15 mg/kg/ml/min i.c. for the first hour of reperfusion), 3) lidocaine, n = 9, (2.0 mg/kg i.v. given immediately before coronary artery occlusion and just before reperfusion), or 4) adenosine plus lidocaine, n = 11. The LCx was occluded for 90 minutes and reperfused for 6 hours. Regional myocardial blood flow (RMBF) was determined (n = 6 per group) at 80 minutes of occlusion and at 45 minutes of reperfusion with radiolabeled microspheres. RMBF determinations revealed an increase in blood flow to the inner two thirds of the myocardium at 45 minutes of reperfusion only in the presence of the combined treatment. Adenosine treatment alone or lidocaine treatment alone did not affect RMBF. Quantification of infarct size (triphenyltetrazolium method) expressed as a percent of the area at risk revealed a significant limitation of infarct size only in the group treated with both adenosine and lidocaine: control, 47.8 +/- 6.6%; adenosine, 45.0 +/- 3.2%; lidocaine, 46.9 +/- 6.0%; and adenosine and lidocaine, 20.8 +/- 5.6%. Statistical analyses were performed with two-way analysis of variance to account for the two individual drug treatments. The findings show that intracoronary administration of exogenous adenosine, at the dose used, is only effective at limiting myocardial infarct size when administered to lidocaine-treated animals.

Myocardial ischemia, reperfusion and free radical injury

Reperfusion of coronary arteries to limit myocardial ischemic injury and extent of myocardial necrosis is possible by either the use of fibrinolytic therapy, coronary angioplasty or coronary artery bypass surgery. The concept that early reperfusion may salvage jeopardized myocardium is derived from basic experimental studies which purported to demonstrate that the ultimate extent of irreversible myocardial injury could be reduced by reperfusion of the ischemic myocardium within 3 hours from the onset of regional myocardial ischemia. It is firmly established that salvage of ischemic myocardium is dependent on early restoration of blood flow to the myocardium at risk. Despite dependency on reoxygenation for ultimate survival, myocardial tissue that is reperfused and reoxygenated may be subjected to additional injurious insult due to reactive metabolites of oxygen. The cytotoxic species of oxygen are referred to as "oxygen free radicals." Coincident with the influx of inflammatory cells into the reperfused region is an additional loss of otherwise viable myocardial cells. There is strong support for the concept that the polymorphonuclear leukocyte is a contributor to the phenomenon of "reperfusion" or "reoxygenation" injury in the blood perfused heart. This discussion focuses on the role of the neutrophil as a potential contributor to the extension of tissue injury and reviews those interventions, which although in the experimental stage, offer promise of becoming therapeutically important in the future and may help elucidate the mechanisms underlying the potentially deleterious role of the neutrophil in situations involving whole blood reperfusion of the ischemic myocardium.

Cytochrome c oxidase and cardiolipin alterations in response to skeletal muscle ischaemia and reperfusion

The effect of 2 and 4 h of tourniquet ischaemia followed by 1 h of reperfusion on the major mitochondrial phospholipids and on the cytochrome c oxidase kinetic parameters has been investigated in rat skeletal muscle. There was no change either in the mitochondrial phospholipid content or in the Vmax and the Km of the enzyme after 2 h of ischaemia with and without subsequent reperfusion. Four hours of ischaemia had no effect on the lecithin and the cephalin content, while the cardiolipin content decreased as well as the Vmax of the enzyme (P less than 0.05). Tissue reperfusion caused a dramatic decrease in both cardiolipin (55% of the control, P less than 0.001) and Vmax (38% of the control, P less than 0.001). The corresponding reduction in lecithin and cephalin contents was 12% and 14% respectively (P less than 0.05). The Km remained unchanged at all conditions. These findings suggest that mitochondrial dysfunction in response to ischaemia and reperfusion could be a consequence of the reperfusion itself following severe ischaemia. The results are discussed in terms of cardiolipin peroxidation and cytochrome oxidase as a functional parameter.

Improved survival in intestinal ischemia by allopurinol not related to xanthine-oxidase inhibition

Allopurinol, a xanthine-oxidase (XO) inhibitor, has been used to improve the resistance to ischemia with disappointing results that have been attributed to administration regimen of the drug. Our aim was to investigate the effect of different administration schedules of allopurinol on the survival in rats undergoing intestinal ischemia testing the blockade of XO. Intestinal ischemia was achieved by 90 min of clamping the superior mesenteric artery (SMA) close to its origin from the aorta. Three groups of animals were evaluated: A-group: only the allopurinol solvent was given; B-group: the full dose of allopurinol (100 mg/k b.w.) was given iv and C-group: the 75% dose was administered orally 24 hr before and the remaining 25% was administered 30 min before. Survival was evaluated at 48 hr and the blockade of XO was assayed by High Efficacy Liquid Chromatography (HELC) in homogenate of intestinal wall. Survival was only improved in the C-group (P = 0.02). Levels of hypoxanthine were significantly increased both in B-group and C-group (P = 0.003) when compared with the A-group. Levels of uric acid in B-group (P = 0.0003) and C-group (P = 0.0009) were significantly decreased with respect to A-group. That means that an effective blockade of XO is achieved whichever the regimen of administration. Allopurinol and oxypurinol levels were significantly increased (P = 0.05 and P = 0.008) in C-group when compared with B-group. We conclude that the protective effect of allopurinol on survival in intestinal ischemia in rats is not related to the blockade of XO but rather to the allopurinol and oxypurinol levels in intestinal wall.

Lipid peroxidation during myocardial ischaemia induced by pacing

Oxygen derived free radical generation can be shown in experimental models of myocardial ischaemia and reperfusion and may cause cellular damage by peroxidizing polyunsaturated membrane phospholipids. An attempt was made to quantify human intracardiac lipid peroxidation during transient myocardial ischaemia by measuring the aortic and coronary sinus concentrations of malondialdehyde (a marker of lipid peroxidation) before, during, and after incremental pacing. Twenty six patients were paced until they had severe chest pain or 2 mm ST segment depression or they reached a paced rate of 180 beats/min. They were divided into two groups according to whether or not lactate was produced during pacing. Twelve patients (group 1), all with coronary artery disease, produced myocardial lactate during pacing. None of the other 14 patients (group 2), half of whom had coronary disease, produced lactate during pacing. Concentrations of malondialdehyde in the aorta and coronary sinus were significantly higher in group 1 than in group 2. Five minutes after the end of pacing coronary sinus malondialdehyde concentrations in group 1 had increased significantly from baseline values. There were no changes with time in the coronary sinus concentration of malondialdehyde in group 2 or in the aorta in either group. The negative malondialdehyde extraction ratio in group 1 suggests that intracardiac lipid peroxidation occurs during transient human myocardial ischaemia.

Oxygen-derived free radicals and postischemic myocardial reperfusion: therapeutic implications

Oxygen-derived free radicals have been implicated in the pathogenesis of various disease states, including myocardial ischemia and reperfusion. In this article, we review 1) the evidence linking free radical production and myocardial injury during myocardial ischemia and reperfusion and 2) results of studies of the effects of the pharmacological therapies available potentially to prevent free radical-mediated injury. Free radicals can be produced during ischemia and reperfusion by several different biochemical pathways. Of these, the xanthine oxidase reaction and the output of free radicals by neutrophils that have accumulated in damaged tissue have been studied extensively. When produced, free radicals can potentially damage myocytes or endothelial cells through peroxidation of membrane lipids or damage to proteins or nucleic acids. Using electron spin resonance spectroscopy, several studies have shown a 'burst' of oxygen free radicals immediately after reperfusion. Moreover, exogenous generation of intravascular free radicals has been shown to produce marked vascular and myocyte damage, as well as contractile dysfunction. 'Anti-free radical' interventions, such as xanthine oxidase inhibitors and free radical scavengers have been reported to prevent contractile dysfunction and reperfusion-induced arrhythmias after an episode of reversible ischemic injury. However, after more severe episodes of ischemia, such interventions have had conflicting effects on myocardial infarct size. 'Anti-free radical' interventions could be of potential use in situations where reversible ischemic injury occurs. In situations where reperfusion is achieved after irreversible ischemic injury has occurred, the potential beneficial effect of these treatments on infarct size is more doubtful.

A method for monitoring the effectiveness of allopurinol pretreatment in the prevention of ischemic/reperfusion injury

The protective actions of allopurinol in ischemic/reperfusion injury seem critically determined by the drug pretreatment regimen and may involve generalized alterations in tissue antioxidant status. In the present study, 12 male swine to be used as donors and recipients in a heart-lung transplantation study were treated with allopurinol given orally at a dose of 50 mg/kg for 4 days prior to surgery. Red cells from allopurinol-treated animals showed a progressive decrease in susceptibility to in vitro peroxidative challenge. Although the degree and time-course of protection showed some degree of interanimal variation, maximal effects were obtained in most animals after 2-3 days. The extent of red cell protection in both donor and recipient animals correlated significantly with the functional viability of the transplanted lung, as assessed by tissue water content. It is suggested that the susceptibility of red cells to in vitro oxidative damage may provide a useful functional assessment of generalized alterations in antioxidant status produced by pharmacological interventions.

Oxygen free radical-mediated heart injury in animal models and during bypass surgery in humans. Effects of alpha-tocopherol

There is evidence that oxygen free radicals play a role in myocardial ischemic and reperfusion injury. We investigated the effect of ischemia and reperfusion on glutathione status. Reperfusion after prolonged ischemia (60 min) induced an important release of reduced (GSH) and oxidized (GSSG) glutathione, concomitant with an increase of tissue GSSG and no recovery of mechanical function, indicating that reperfusion results in oxidative stress. These alterations are associated with tissue and mitochondrial calcium accumulation, loss of mitochondrial function, and membrane damage. We also determined the arteriocoronary sinus difference for GSH and GSSG of 16 CAD patients undergoing coronary artery bypass. Patients were divided in two groups according to the length of clamping period: 25 +/- 2 min (group 1), and 55 +/- 6 min (group 2). In group 1, reperfusion resulted in a transient release of GSH, GSSG, CPK, and lactate, with return to preclamping values in 10 minutes. In group 2, reperfusion determined a sustained and pronounced release of GSH, GSSG, CPK, and lactate during declamping, suggesting the occurrence of an oxidative stress. Using an in vitro model, administration of alpha-tocopherol bound with albumin showed protection of mitochondrial function, improved recovery of contraction, and reduced oxidative stress during reperfusion.

Neutrophil depletion limited to reperfusion reduces myocardial infarct size after 90 minutes of ischemia. Evidence for neutrophil-mediated reperfusion injury

Reperfusion of ischemic myocardium may accelerate necrosis of injured myocytes. To determine the role of neutrophil leukocytes in this process, we examined whether neutrophil depletion during reperfusion could modify infarct size in anesthetized dogs. The proximal circumflex coronary artery was occluded for 90 minutes and then reperfused for 2 hours via an extracorporeal circuit with either whole blood (n = 11) or with blood depleted of neutrophils by leukocyte filters (n = 11). The leukocyte filters caused near-total neutropenia in blood reperfusing the ischemic myocardium (7 +/- 7 neutrophils/microliters compared with 2,551 +/- 317/microliters in controls, mean +/- SEM; p less than 0.001. Infarct size was measured by planimetry of myocardial slices stained with triphenyltetrazolium chloride (TTC), and the accuracy of TTC for identifying necrotic myocardium was verified by electron microscopy. The size of the ischemic risk region was the same in the control (41.6 +/- 1.0%) and neutropenic (41.8 +/- 2.1%) groups. Collateral blood flow to the risk region was the same in control (0.15 +/- 0.03 ml/min/g) and neutropenic (0.13 +/- 0.03 ml/min/g) groups. Among dogs with collateral flow less than 0.2 ml/min/g, infarct size was reduced in the neutropenic group (27.7 +/- 6.7% of risk region, n = 8), compared with control dogs (52.5 +/- 5.7%; n = 7; p = 0.02). Multiple linear regression described the relation between infarct size, risk region size, and collateral flow in the control group, and the same regression relation was used to predict infarct size for the neutropenic group. Mean predicted infarct size in the neutropenic group (n = 11) was 16.8 +/- 3.4% of left ventricle, whereas mean observed infarct size was 9.6 +/- 3.1% (p less than 0.01). The extent of the no-reflow zone (absence of thioflavin-S-fluorescence) was also less in the neutropenic than the control group (2.2 +/- 0.8% vs. 8.1 +/- 2.7% of the risk region, p less than 0.05). Neutropenia limited to the reperfusion period is associated with significant reductions in the extent of the infarct and no-reflow zones after 90 minutes of ischemia. These findings support the hypothesis that reperfusion necrosis occurs after prolonged myocardial ischemia and indicate that neutrophil leukocytes are important mediators of such reperfusion injury.

Cardioprotective effects of amlodipine in the ischemic-reperfused heart

Amlodipine is a dihydropyridine derivative belonging to the group of pharmacologic calcium entry blocking agents and is characterized as having a slow onset and relatively long duration of action with minimal effects on cardiac electrophysiology and myocardial contractility. The protective effect of amlodipine was studied in isolated blood-perfused feline hearts made globally ischemic for 60 minutes followed by reperfusion for 60 minutes. Ischemic-induced alterations of left ventricular developed pressure and complicance were monitored. In 11 control and 7 drug-treated hearts, amlodipine produced significant decreases in myocardial oxygen consumption (6.2 +/- 0.4 to 4.4 +/- 0.4 ml oxygen/min/100 g) and coronary vascular resistance, as assessed by changes in perfusion pressure (120 +/- 1 to 100 +/- 4 mm Hg). Amlodipine administered before the onset of global ischemia decreased the development of ischemic contracture as reflected by a progressive increase in resting left ventricular diastolic pressure. The return of contractile function, 60 minutes afer reperfusion, improved significantly in the amlodipine-treated group compared with controls, and there was better maintenance of the tissue concentration of Na+, Ca2+ and K+. A canine model of regional myocardial ischemia (90 minutes) followed by 6 hours of reperfusion was used to assess the cardioprotective effects of amlodipine, 150 micrograms/kg, administered 15 minutes before reperfusion. Infarct size, expressed as a percentage of the area at risk, was smaller in the amlodipine-treated group (n = 10) than in the control group (n = 10) (34.5 +/- 3.8% vs 45.9 +/- 2.8%, p = 0.027). Risk region size did not differ between groups and both groups were comparable with respect to the hemodynamic parameters of heart rate, blood pressure and rate-pressure product. Amlodipine prevented the gradual reduction in coronary blood flow observed in the control group. It is concluded that amlodipine reduces myocardial ischemic injury by mechanism(s) that may involve a reduction in myocardial oxygen demand as well as by positively influencing transmembrane Ca2+ fluxes during ischemia and reperfusion.

Intracoronary adenosine administered after reperfusion limits vascular injury after prolonged ischemia in the canine model

Myocardial salvage after reperfusion may be limited by deleterious vascular changes in the previously ischemic microcirculatory bed. This could result in a progressive decrease in blood flow in the capillary bed to potentially viable myocytes (no-reflow phenomenon). The effect of intracoronary adenosine on these changes was assessed in 15 closed-chest dogs subjected to 2 hours of proximal left anterior descending artery (LAD) occlusion followed by 3 hours of reperfusion. Animals randomly received adenosine (n = 8) 3.75 mg/min into the proximal LAD or an equivalent volume of saline (control) (n = 7) for 1 hour after reperfusion. Endothelial-dependent and independent coronary vasodilator reserve was determined using a chronically implanted volume-flowmeter on the mid-LAD at baseline and 1 and 3 hours after reperfusion with acetylcholine and papaverine infusions, respectively, into the proximal vessel. Regional myocardial blood flow was measured serially with radioactive microspheres and regional contractile function with contrast ventriculography. Both agonists produced a significant increase in LAD flow before occlusion. Endothelial-dependent and independent vasodilatory reserve was significantly reduced (p less than 0.05) at 1 and 3 hours after reperfusion in control animals compared with adenosine treatment. A progressive decrease in mid-LAD flow and increase in coronary vascular resistance after reperfusion was observed in control animals (p less than 0.05). The treated group manifested improved regional myocardial blood flow in endocardial regions from the central (0.73 +/- 0.15 versus 0.24 +/- 0.11 ml/g/min; p less than 0.02) and lateral ischemic zones (0.80 +/- 0.15 versus 0.34 +/- 0.12 ml/g/min; p less than 0.05) 3 hours after reperfusion. A significant reduction (p less than 0.05) in endocardial and midmyocardial flow compared with baseline was seen in control animals at 3 hours. Intravascular and interstitial neutrophil infiltration was reduced in adenosine animals and this was associated with relative ultrastructural preservation of endothelial cells. Regional ventricular function in the ischemic zone was improved in the adenosine group 3 hours after reperfusion (13.4 +/- 3.9% versus -5.3 +/- 1.6%; p less than 0.001). This study demonstrates that selective administration of adenosine after reperfusion significantly attenuates functional and structural abnormalities in the microvasculature after prolonged (2 hours) regional ischemia in the canine model. Prevention of microvascular injury and the non-reflow phenomenon by adenosine may preserve reversibly injured myocytes following restoration of blood flow to previously ischemic myocardium.

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.

Glutathione disulfide formation and lipid peroxidation during cardiac ischemia and reflow in the dog in vivo

The content of glutathione disulfide (GSSG) in tissue, coronary sinus blood plasma, and in cardiac lymph was measured in a well-characterized model of regional cardiac ischemia and reflow in dogs in vivo in order to assess the magnitude of the oxidant stress produced. No increase in GSSG content was observed during 60 min of occlusion of the circumflex or left anterior descending arteries, or during up to 70 min of reflow. The contents of 11-, 12-, and 15-hydroxyeicosatetraenoates (HETEs) in total lipids also were not increased following 60 min of regional ischemia and up to 60 min of reflow. In addition, global ischemia produced by aortic crossclamping and cardiopulmonary bypass did not increase HETE content. In contrast, infusion of tertiary butyl hydroperoxide (tBHP) into the left atrium produced readily measurable increases in GSSG content with or without prior induction of myocardial ischemia. Infusion of tBHP also increased tissue contents of the HETEs. These findings indicate that the canine myocardium subjected to ischemia-reflow conditions does not generate large amounts of reactive oxygen and does not form significant amounts lipid peroxidation products.

Reperfusion injury and its pharmacologic modification

Reperfusion injury includes a spectrum of events, such as reperfusion arrhythmias, vascular damage and no-reflow, and myocardial functional stunning. The concept of reperfusion injury remains controversial with many proposed mechanisms when applied to humans, whereas in animal models, there are two main proposed mechanisms: calcium over-load and formation of oxygen free radicals. To prove that reperfusion injury is specifically caused by reperfusion would require evidence that an intervention given at the time of reperfusion can diminish or abolish the injury as in the case of arrhythmias, which are thought to be mediated by excess recycling of cytosolic calcium with delayed afterdepolarizations and ventricular automaticity. In the case of myocardial stunning, the phenomenon may be mediated, at least in part, by a burst of free radicals formed within the first minute of reperfusion and improved by free radical scavengers given at the time of reperfusion. The alternate hypothesis is that cytosolic calcium overload damages mechanisms for normal intracellular calcium regulation so that the stunned myocardium responds to agents that are thought to increase intracellular cytosolic calcium, such as beta-receptor agonists. A further component of reperfusion injury, under active investigation, is microvascular damage with alterations at the level of platelets, leukocytes, and endothelial integrity. From the therapeutic point of view, the divergent results of experimental interventions and the possibility that the abrupt onset of reperfusion in animals differs from the situation in humans with thrombolysis means that the best way currently available to limit reperfusion injury is by minimizing the ischemic period by early reperfusion and by optimizing the metabolic status of the ischemic myocardium at the end of the ischemic period.

Distribution of xanthine dehydrogenase and oxidase activities in human and rabbit tissues

The activity of xanthine dehydrogenase in human postmortem tissues is surprisingly high in brain and heart; activity was found in most tissue samples, whereas many samples contained little or no oxidase activity. We have confirmed the high level of oxidase activity in liver in which tissue conversion of dehydrogenase to oxidase appears complete. We have also confirmed the virtual absence of either activity in fresh human placenta. Fresh rabbit tissues similarly show considerable dehydrogenase activity in brain and heart. In view of the stability and generalised distribution of dehydrogenase activity, our results suggest that some modification of existing ideas on the physiological and pathological roles of the enzyme may be needed.

Allopurinol and dimethylthiourea reduce brain infarction following middle cerebral artery occlusion in rats

Free radicals have been shown to play an important role in ischemia-reperfusion injury in several organ systems; however, the role of free radicals in central nervous system ischemia has been less well studied. Many potential free radical-generating systems exist. The primary products of these reactions, superoxide and hydrogen peroxide, may combine to produce hydroxyl radicals. Of the many potential sources of free radical generation, the enzyme xanthine oxidase has been shown to be important in ischemia in noncerebral tissue. We investigated the effect of the hydroxyl radical scavenger dimethylthiourea and the xanthine oxidase inhibitor allopurinol on infarct volume in a model of continuous partial ischemia. Male Sprague-Dawley rats were treated with dimethylthiourea or allopurinol before middle cerebral artery occlusion. Infarct volume was measured by triphenyltetrazolium chloride staining of brains removed 3 or 24 hours after occlusion. Stroke volume was reduced by 30% after dimethylthiourea treatment and by 32-35% after allopurinol treatment. At 24 hours after stroke, cortical tissue was more effectively protected than caudate tissue with both agents. Pretreatment with dimethylthiourea and allopurinol also significantly reduced cerebral edema formation and improved blood-brain barrier function as measured by fluorescein uptake. Our results imply that hydroxyl radicals are important in tissue injury secondary to partial cerebral ischemia and that xanthine oxidase may be the primary source of these radicals.

Effect of superoxide dismutase on myocardial infarct size in the canine heart after 6 hours of regional ischemia and reperfusion: a demonstration of myocardial salvage

Available data demonstrate that oxygen free radicals and derived reactive species of oxygen are produced during myocardial ischemia as well as upon reperfusion of the ischemic tissue. The present study was designed to determine if polyethylene glycol-conjugated superoxide dismutase (PEG-SOD), with its extended plasma half-life in excess of 30 hours in contrast to the native form of the enzyme (Native-SOD), could provide protection to the ischemic myocardium subjected to a 6-hour regional ischemia followed by reperfusion for 24 hours. We hypothesized that myocardial injury due to an ischemic interval is a dynamic process involving the sustained production of cytotoxic oxygen radicals that may continue beyond the ischemic interval. The ability to demonstrate a protective effect of the free radical scavenger enzyme superoxide dismutase would require the continued presence of the antioxidant during the ischemic interval and especially during reperfusion. To test this hypothesis, 22 anesthetized, open-chest dogs underwent 6 hours of circumflex coronary artery occlusion followed by reperfusion for 24 hours. Rapid administration of either Native-SOD (1,000 U/kg), PEG-SOD (1,000 U/kg), PEG-albumin (PEG-ALB), or 0.9% sodium chloride solution for injection (saline) was administered via the left atrium 15 minutes before occlusion of the vessel. A continuous infusion of an additional 1,000 U/kg of the respective enzyme interventions or an equivalent volume of PEG-ALB or saline was given during the 6-hour coronary artery occlusion and terminated 15 minutes after reperfusion. The animals were euthanized 24 hours after reperfusion, and the myocardial region at risk and the infarct region were quantitated by the tetrazolium method. The area of myocardium at risk of infarction, expressed as a percent of the left ventricle, did not differ among the groups: Native-SOD (n = 8), 46.2 +/- 1.8%; PEG-SOD (n = 6), 45.7 +/- 2.1%; PEG-ALB, 38.4 +/- 2.3% (n = 4); and saline 46.0 +/- 2.1% (n = 4). Hemodynamic parameters, the calculated rate-pressure-product, as well as regional myocardial blood flow (radiolabeled microsphere method) in the endocardial, midmyocardial, and epicardial segments of the risk and the nonrisk regions were comparable for all groups. Mean infarct size, determined 24 hours after reperfusion, in the group treated with PEG-SOD was 47.1 +/- 2.9% of the area at risk (n = 6), significantly smaller than that observed in each of the other treatment groups: Native-SOD, 63.5 +/- 2.2% (n = 8); PEG-ALB, 64.6 +/- 2.4% (n = 4); saline, 70.8 +/- 2.2% (n = 4).(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular cardiology: new avenues for the diagnosis and treatment of cardiovascular disease

This review summarizes some of the major advances in the investigation of molecular mechanisms underlying both normal and abnormal cardiovascular function. Four major areas are highlighted including cardiac muscle, the blood vessel, atherosclerosis and thrombosis/thrombolysis. The remarkable strides in understanding multifactorial diseases such as atherosclerosis, and the development of innovative new therapies such as the use of thrombolytic agents produced by recombinant deoxyribonucleic acid (DNA) technology, are noted. Moreover, it is concluded that the past decade of basic research has provided a solid framework for improvements in the diagnosis and therapy of other forms of cardiovascular disease as well. An evaluation of current trends in basic cardiovascular research suggests that diagnostic and therapeutic approaches to disease will increasingly target specific molecular processes underlying the pathophysiologic state.

Endothelial and myocardial injury during ischemia and reperfusion: pathogenesis and therapeutic implications

Early reperfusion remains the most effective way of limiting myocardial necrosis and improving ventricular function in experimental models and human patients. However, the introduction of oxygen and cellular elements, especially the neutrophil, into the ischemic zone may initiate a deleterious cascade of events that limits myocardial salvage after reperfusion. Although the pathogenesis of reperfusion injury remains controversial, recent studies have suggested that the endothelium may play a critical role. Endothelial cells maintain flow in the microcirculation by secreting a number of vasodilatory compounds and substances that prevent plugging of capillaries by inhibiting neutrophil adherence and platelet aggregation. Reperfusion of ischemic myocardium accelerates structural and functional changes in endothelial cells, resulting in a progressive decrease in microcirculatory flow ("no reflow" phenomenon). Numerous studies suggest that activated neutrophils mediate vascular damage by releasing reactive oxygen species and potent proteolytic enzymes. The administration of therapeutic agents that limit endothelial disruption and neutrophil plugging has shown promising results in limiting myocardial reperfusion injury in experimental models.

Myocardial salvage after regional beta-adrenergic blockade

The aim of the study was to determine whether regional beta-adrenergic blockade via the coronary sinus limited myocardial damage after coronary artery occlusion in the canine model. Accordingly, open-chest anesthetized dogs were randomly allocated to one of three groups: a control group and groups treated with propranolol (in doses of 0.02, 0.2, and 2.0 mg/kg) given either intravenously or via the coronary sinus. The hypoperfused zone (i.e., risk area) and the extent of myocardial damage were assessed by autoradiography and triphenyltetrazolium chloride staining, respectively. Myocardial damage expressed as a percent of the hypoperfused zone was 84 +/- 5% in the control group (n = 9) and 78 +/- 7% (0.02 mg/kg, n = 7, NS), 63 +/- 6% (0.2 mg/kg, n = 7, p less than 0.05), and 62 +/- 7% (2.0 mg/kg, n = 9, p less than 0.02) in the groups receiving intravenous propranolol and 73 +/- 6% (0.02 mg/kg, n = 7, NS), 58 +/- 7% (0.2 mg/kg, n = 7, p less than 0.01), and 44 +/- 9% (2.0 mg/kg, n = 9, p less than 0.001) in groups receiving propranolol via the cardiac veins. There was a significant enhancement of myocardial salvage with increasing doses of propranolol delivered via the cardiac veins (linear regression trend, p less than 0.05). In contrast, myocardial damage expressed as a percent of the hypoperfused zone remained comparable with propranolol doses of 0.2 and 2.0 mg/kg administered intravenously (linear regression trend, NS).

IN CONCLUSION

(1) regional beta-adrenergic blockade via the cardiac veins afforded significant myocardial salvage and (2) the regional administration of propranolol resulted in significant reduction of myocardial damage in a dose-dependent fashion.

Spin trapping of free radicals produced in vivo in heart and liver during endotoxemia

Studies using free radical scavengers and measurements of lipid peroxidation have suggested that free radicals are generated during endotoxemia. Conclusions from these studies have implied that free radicals may participate in the sequence of pathologic events following endotoxin challenge in the experimental animal. Current inferences of free radical generation and involvement have been derived from indirect evidence and are therefore inconclusive. To quantitate the generation of free radicals in vivo during endotoxemia this study employed the use of electron paramagnetic resonance spectroscopy (EPR) combined with spin trapping techniques. Five minutes before intraperitoneal endotoxin administration, trimethoxy-a-phenyl-t-butyl-nitrone [(MeO)3 PBN] was administered intraperitoneally. Experimental animals were always matched with control animals receiving no endotoxin. At either five minutes or twenty-five minutes following endotoxin administration animals were decapitated and hearts and livers were rapidly taken for lipid extraction and EPR evaluation. Analysis of the EPR spectra revealed hyperfine splitting constants that indicated the presence of carbon-centered radical spin adducts in both organ tissues from animals exposed to endotoxin for twenty-five minutes. No signals were present in hearts and livers taken five minutes after endotoxin administration. EPR evaluation did not indicate spin adduct formation in control tissue. These data directly demonstrate that activation of processes in vivo involving free radical generation occur early during endotoxemia, but are not detectable immediately after the endotoxin challenge.