Pathobiology and clinical impact of reperfusion injury

Reperfusion injury refers to cellular death or dysfunction caused by restoration of blood flow to previously ischemic tissue. This should be differentiated from the normal reparative processes that follow an ischemic insult. Four types of reperfusion injury have been described in the literature: (1) lethal reperfusion injury, (2) nonlethal reperfusion injury (myocardial stunning), (3) reperfusion arrhythmias, and (4) vascular injury (including the no-reflow phenomenon). There is continued debate whether reperfusion itself is capable of killing viable myocytes, which otherwise would have survived the ischemic insult. However, there is firm evidence for the existence of myocardial stunning following various ischemic syndromes, including reperfusion therapy for acute myocardial infarction, unstable angina pectoris, vasospastic angina, effort-induced ischemia, coronary artery bypass surgery, and cardiac transplantation. Reperfusion arrhythmia is more common after short ischemic episodes than after long ischemic periods. Thus, while reperfusion arrhythmias in the setting of acute myocardial infarction are relatively rare, reperfusion arrhythmias may be an important cause of sudden death. The no-reflow phenomenon has been described following reperfusion in patients with acute myocardial infarction. Three major components have been proposed as mediators of reperfusion injury: (1) oxygen free radicals, (2) the complement system, and (3) neutrophils. Numerous experimental studies have shown short-term benefit by blocking various stages of the postischemic inflammatory response. Oxygen free radicals scavangers, complement inhiition, leukocyte depletion, and the use of antibodies against various adhesion molecules have shown a reduction of infarct size in many ischemic/reperfusion experimental models. However, many of these agents failed to show a benefit in the clinical setting. Moreover, the long-term benefit of such intervention is still unknown.     

Suppression of the degradation of adenine nucleotides during ischemia may not be a sufficient mechanism for infarct size limitation by preconditioning

Preconditioning is known to decelerate degradation of the tissue adenine nucleotides during ischemia and to delay ischemic myocardial necrosis. However, it is not known whether these two phenomena are related. To obtain an insight into this question, the present study examined whether adenosine and B2 receptor antagonists, which block the infarct size-limiting effect of preconditioning, modify the interstitial purine levels during preconditioning and subsequent sustained ischemia. In pentobarbital anesthetized open-chest rabbits, a microdialysis probe was placed in the territory of a branch of the left coronary artery, and perfused with Ringer solution. Preconditioning was performed with 5 min ischemia/5 min reperfusion. Dialysate adenosine and inosine were elevated from the baseline values of 0.064±0.011 and 0.329±0.044M to 0.189±0.069 and 4.106±1.451 M, respectively during preconditioning, but their elevation during a subsequent 20 min of ischemia was significantly lower compared with that in the non-preconditioned myocardium. This suppression of the purine accumulation during ischemia by preconditioning was not abolished by 2 g/kg of Hoe 140, a specific B2 receptor antagonist, or by 10 mg/kg of 8-phenyltheophylline, a non-selective adenosine receptor antagonist. Since the doses of Hoe 140 and 8-phenyltheophylline are sufficient to block the infarct size-limiting effect of preconditioning, the present results suggest that there is a dissociation between the suppression of adenine nucleotide degradation during ischemia by preconditioning and the enhancement of myocardial resistance against infarction. Thus, it is unlikely that a reduction of adenine nucleotide utilization by preconditioning is sufficient to protect the myocardium against ischemic necrosis.     

Preconditioning myocardium with ischemia

Summary Preconditioning and stunning are the chief adaptive changes induced in myocardium by a brief episode of reversible ischemia followed by arterial reperfusion. In the dog heart, both coexist for a period of at least 20 minutes of reperfusion, but after 120 minutes of reflow, preconditioning is much diminished, while stunning remains fully developed. Preconditioned, stunned, myocardium differs from control virgin myocardium in that adenine nucleotide content is reduced to about 50–70% of control, whereas creatine phosphate (CP) greatly exceeds normal-the so-called CP overshoot. When preconditioned myocardium is subjected to sustained ischemia, ATP utilization and anaerobic glycolysis occur at much slower rates than those observed in virgin myocardium. As a result of the early difference in metabolic rate, a longer period of ischemia is required for the ATP and lactate of the preconditioned tissue to reach the levels associated with irreversible injury. Associated with this change is a delay in myocyte death.The molecular events responsible for slower ischemic metabolism and associated tolerance of preconditioned, stunned tissue to a new ischemic episode are not known. Among the reactions that could cause a reduction in energy metabolism is reduced P expenditure by stunned myocardium attempting to contract during the initial phase of ischemia. However, results from in vivo and in vitro experiments suggest that although stunning may be necessary for preconditioning to develop, it alone is not sufficient to cause preconditioning. Alternatively, metabolic changes may be explained by depressed activity of the mitochondrial ATP ase during the epsiode of sustained ischemia. However, no direct experimental evidence supporting this hypothesis is available up to the present time.     

Normothermic ischemic cardiac arrest in the isolated working rabbit heart: Effects of dl-nebivolol and atenolol

Summary The effect of pretreatment with selective ₁-adrenoceptor blockers (dl-nebivolol or atenolol) on myocardial mechanical activity, mitochondrial function, morphology, and calcium cytochemistry was studied during normothermic ischemic arrest and reperfusion of isolated working rabbit hearts. The hearts subjected to 25 min of ischemia followed by 30 min of post-ischemic reperfusion showed typical signs of severe myocardial ischemic damage. The ultrastructural changes showed a good relation with the changes in mechanical activity and mitochondrial function. To determine whether these changes could be prevented or reduced by ₁-adrenoceptor blockade, dl-nebivolol or atenolol (0.62 mg/liter) was added to the perfusate 30 min before the induction of ischemia. The results showed that dl-nebivolol exerted a protective effect on recovery of mechanical activity, on mitochondrial function during reperfusion as well as on the ultrastructure as examined at the end of the reperfusion period. On the other hand, atenolol failed to protect the myocardium against ischemia-reperfusion damage in the isolated working rabbit heart.     

The relationship between regional blood flow and contractile function in normal,ischemic, and reperfused myocardium

Summary The prevailing paradigm of coronary physiology and pathophysiology is that a balance between blood flow (i.e., supply) and function (i.e., demand) exists under normal conditions and that an imbalance between supply and demand occurs during ischemia. However, this paradigm is derived largely from studies relating changes in total coronary inflow to global ventricular function. The present article examines the relationship between myocardial blood flow and function on a regional level and proposes that a change may be needed in the current paradigm of coronary pathophysiology. In normal myocardium, considerable heterogeneity of regional blood flow exists, indicating either similar heterogeneity of metabolic demand and function or questioning the precision of metabolic coupling between flow and function. After the onset of ischemia, a transient imbalance between the reduced blood flow and function may exist. However, myocardial function rapidly declines and during early steady-state ischemia regional myocardial blood flow and function are once again evenly matched. Such supply-demand balance may persist over prolonged periods of ischemia enabling the myocardium to remain viable through reduction of energy expenditure for contractile function, i.e., to hibernate. Whereas in hibernating ischemic myocardium, regional myocardial blood flow and function are both reduced but appropriately matched to one another, flow and function appear to be largely uncoupled in reperfused stunned myocardium. The clinical identification of viable but ischemic (hibernating) and postischemic (stunned) myocardium is of utmost importance in patients undergoing reperfusion procedures. A new paradigm of coronary and myocardial pathophysiology, encompassing a regional as well as a global view of perfusion and function, will have to include explanations for phenomena such as myocardial hibernation and myocardial stunning.     

Clinical relevance of myocardial “stunning”

Summary Experimental studies have demonstrated that myocardium reperfused after reversible ischemia exhibits prolonged depression of contractile function (stunning). Despite the multiplicity of clinical situations in which myocardial stunning would be expected to occur, investigation of this phenomenon in humans has been hindered by several major problems, including the limited accuracy of the methods available to measure regional left ventricular function, the inability to quantify regional myocardial blood flow during acute ischemia, the difficulty in establishing with certainty the beginning and end of an ischemic episode, and the uncontrolled influence of variables (such as preload, afterload, adrenergic tone, and inotropic therapy) that have a major impact on postischemic dysfunction. The main problem is to discern whether a reversible defect of contractility is caused by stunning, silent ischemia, or hibernation (i.e., chronic ischemia). This differential diagnosis requires the simultaneous measurement of regional myocardial function and flow, which thus far has not been generally possible. Despite these limitations, however, numerous clinical observations suggest that stunning does occur in various settings in which the myocardium is exposed to transient ischemia, including coronary angioplasty, exercise-induced angina, angina at rest (unstable or variant), acute myocardial infarction with early reperfusion, open-heart surgery, and cardiac transplantation. Recognition of this entity is important, amongst other reasons, because it is likely to cause significant morbidity and because it is potentially correctable with inotropic therapy or even preventable with antioxidant therapy. In addition, the appreciation of the phenomenon of myocardial stunning should allow the clinician to assess the efficacy of reperfusion therapy with greater accuracy and to recognize that patients should not be denied mechanical revascularization solely because of an abnormal left ventricular wall motion. Perhaps the most intriguing clinical implication of the concept of myocardial stunning is the possibility that in patients who exhibit frequent episodes of ischemia in the same territory, the myocardium may not be able to fully recover between episodes and thus may remain reversibly depressed for prolonged periods of time, or even chronically, which could account for some cases of ischemic cardiomyopathy. Our understanding of myocardial stunning in humans is still relatively crude and will not significantly improve until studies are performed that measure simultaneously regional myocardial perfusion and function (so that stunning can be differentiated from silent ischemia and hibernation). Future important areas of research should also include the elucidation of whether stunning can become chronic and the evaluation of therapies (such as antioxidant treatments) designed to prevent this contractile abnormality. Further knowledge regarding the clinical significance of myocardial stunning will be essential to improve our understanding of the pathophysiology of coronary artery disease and our management of the adverse manifestations associated with this disorder.     

Effects of dilevalol in the anesthetized pig with a partially occluded coronary artery

Summary The beta-adrenoceptor antagonist dilevalol in a total dose of 430 g/kg IV, potently suppressed isoprenaline-induced increases in heart rate and max LVdP/dt (dose ratios of 42±6 and 38±5, respectively, in anesthetized pigs), but a dose of 1430 g/kg did not appreciably modify phenylephrine-induced increases in arterial blood pressure (dose ratio< 4) in both anesthetized and conscious pigs. The actions of dilevalol on ischemic myocardium of anesthetized pigs were investigated following a reduction of left anterior descending artery flow by 85–90%. Dilevalol (300 g/kg), administered after 15 minutes of ischemia, did not affect the ischemia-induced changes in systemic hemodynamics (such as heart rate, max LVdP/dt and cardiac output), myocardial perfusion, and wall-thickening of the ischemic segment during the following 15 minutes of ischemia and 2 hours of reperfusion. The reasons for the lack of antiischemic actions are most likely the absence of negative chronotropy and an absence of afterload reduction by dilevalol.     

Blockade of the Na+-Ca2+ exchanger is more efficient than blockade of the Na+-H+ exchanger for protection of the myocardium from lethal reperfusion injury

Since the Na⁺-H⁺ exchanger (NHE) is not the only pathway of Na⁺ influx into cardiomyocytes during ischemia/reperfusion, we hypothesized that blockade of Na⁺-Ca²⁺ exchanger (NCX) may be a more efficient strategy than is NHE inhibition for protecting the myocardium from infarction. To test this hypothesis, we compared KB-R7943 (KBR), a novel selective NCX blocker, with cariporide, a selective NHE blocker, with regard to their protective effects against infarction. In isolated rabbit hearts, infarction was induced by 30-min global ischemia/2-h reperfusion, and infarct size was determined by tetrazolium staining and expressed as a percentage of area at risk (%IS/AR). Hearts received no drugs, or were infused with cariporide (1 M) for 10 min or KBR (0.3 or 10 M) for 5 min before ischemia or after the onset of reperfusion. Protein level of NCX was assessed by Western blotting. Cariporide infusion before ischemia significantly reduced %IS/AR from 63.9 ± 2.9% to 20.2 ± 3.0%, but its infusion upon reperfusion failed to achieve a significant protection (%IS/AR = 53.8 ± 4.6%). In contrast, KBR infusion similarly reduced infarct size both when infused before ischemia (%IS/AR = 33.3 ± 6.3% and 21.9 ± 4.7% by 0.3 and 10 M KBR, respectively) and when infused for only 5 min after reperfusion (%IS/AR = 35.3 ± 7.1% and 31.5 ± 2.1% by 0.3 and 10 M KBR, respectively). Protein levels of NCX after 30-min ischemia and 30-min ischemia/30-min reperfusion were similar to baseline values in both untreated controls and hearts treated with 0.3 M KBR upon reperfusion. These results suggest that lethal reperfusion injury is more efficiently suppressed by blockade of the NCX than by blockade of the NHE.     

Ischemic event characteristics determine the extent of myocardial stunning in conscious dogs

Both the severity and duration of postischemic myocardial dysfunction (stunned myocardium) are unpredictable and may vary considerably between subjects that underwent apparently similar ischemic insults. To explain this heterogeneous response of the heart to ischemia and reperfusion, we investigated the determinants of stunning in conscious dogs.Twenty-five dogs were chronically instrumented for measurement of global and regional myocardial performance (wall thickening) and myocardial perfusion (coloured microspheres). A hydraulic occluder was positioned around the LAD coronary artery. Conscious dogs were subjected to acute coronary artery occlusions of predetermined duration (2, 5 and 10 min), followed by complete reperfusion.Multiple regression analysis identified the following variables as determinants of postischemic contractile recovery: 1) the duration of ischemia (p<0.01), 2) the amount of collateral perfusion (p=0.01) and 3) left ventricular end-diastolic pressure during ischemia (p<0.01). Neither the severity of regional dyskinesia during ischemia nor indices of global systolic hemodynamic performance correlated with the rate of recovery.Our data confirm that myocardial stunning relates primarily to the intensity of preceding ischemia. Variations in the preexisting level of collateral perfusion may result in markedly different recovery profiles. Except for LV end-diastolic pressure during ischemia, indices of global and regional cardiac performance fail to predict the severity of postischemic contractile failure.Supported in part by the Nationaal Fonds voor Wetenschappelijk Onderzoek: grant #3.0071.90     

Role of a nonsteroidal anti-inflammatory agent,ibuprofen, in coronary revascularization after acute myocardial infarction

Summary The efficacy of using a nonsteroidal anti-inflammatory agent such as ibuprofen for the salvage of ischemic and reperfused myocardium was investigated by examining its ability to improve global and regional functions as well as to preserve high-energy phosphate compounds and inhibit creatine kinase release from an isolated in-situ pig heart subjected to 1 h of normothermic regional ischemia followed by 1 h of global hypothermic arrest and 1 h of normothermic reperfusion. Preperfusion of the heart for 15 min prior to ischemic insult with 50 M ibuprofen failed to mitigate the myocardial reperfusion injury. Ibuprofen, however, functioned as an anti-inflammatory agent, as judged by its ability to inhibit the influx of indium-111-labeled polymorphonuclear leukocytes and chromium-51 (⁵¹Cr)-labeled platelets into the ischemic and reperfused heart. It also blocked the cyclooxygenase pathway, as evidenced by the significant reduction of 6-keto-prostaglandin F₁ and thromboxane B₂ concentrations in the perfusate. Inhibition of cyclooxygenase resulted in increased accumulation of nonesterified fatty acids, particularly arachidonic acid, in the heart. These results suggest that although ibuprofen can inhibit polymorphonuclear leukocyte and platelet influx into the ischemic and reperfused heart, it causes further damage to the already ischemic heart by reducing prostacyclin concentration and increasing free fatty acids in the heart.     

Coronary vasomotion of the stunned myocardium

The term stunned moycardium describes a dysfunction of the myocardium which may persist for hours, days or even weeks after restoration of coronary blood flow following thrombolysis, percutaneous transluminal coronary angioplasty (PTCA) or aorto-coronary bypass grafting (7, 9). The stunned myocardium is characterized — despite marked dyskinesis — by near, normal levels of myocardial perfusion and normal or even enhanced oxygen consumption (2), probably due to higher oxygen requirements of the contractile elements. Thus, a mismatch between function and flow exists which has been recognized to be a characteristic finding of the stunned myocardium. However, coronary reperfusion may result in an attenuation of the endothelial response to coronary vasodilators such as acetylcholine, bradykinin, etc. (3). It is well established that prolonged myocardial ischemia is associated with irreversible myocardial damage and profound structural and functional derangements of the coronary microvasculature which may persist even if perfusion is restored (1). Local vasoactive substances such as the endotheliumdependent relaxing factor (EDRF) also play an important role in the regulation of myocardial perfusion. EDRF is a potent vasodilator which is released from the coronary endothelium but is reduced in patients with atherosclerotic plaques or hypercholesterolemia (=endothelial dysfunction). Since there are shear-stress dependent flow receptors, EDRF is released proportionally to coronary flow, i.e., the higher the flow the larger the artery and vice versa. However, flow regulation is dependent on several factors such as perfusion pressure, peripheral resistance, sympathetic activation, oxygen saturation, metabolic factors etc. EDRF is synthetized in the endothelial cells and is rapidly broken down during ischemia by superoxide radicals which inactivate EDRF and, thus, decrease flow induced vasodilation (6). Neutrophil aggregation may further contribute to this process by the release of proteolytic enzymes which can convert oxygen to superoxide anions, i.e., oxygen free radicals. These radicals may injure the endothelium when generated in large amounts such as during ischemia (4) and may sensitize the smooth vasculature to contraction by alpha-adrenergic mechanisms (8).     

Is preconditioning by nicotine responsible for the better prognosis in smokers with acute myocardial infarction?

Cigarette smoking is a well-known risk factor for acute myocardial infarction and sudden death. However, a history of smoking consistently has been associated with better hospital outcome in patients with acute myocardial infarction. The mechanism for the better outcome in smokers is not clear. It has been suggested that nicotine may have a preconditioning-like effect. This study assesses whether nicotine attenuates myocardial infarct size.Anesthetized rabbits were randomized to receive infusion of nicotine 80 g/kg (n=13) or saline (n=12) over 10 min. Twenty minutes after termination of infusion all rabbits underwent 30 min of coronary artery occlusion and 4 h of reperfusion. Risk zone was assessed by blue dye and infarct size by tetrazolium staining.Nicotine did not affect regional myocardial blood flow 15 min after treatment, during occlusion, or during reperfusion. Heart rate and mean systemic blood pressure were similar between the groups. Nicotine serum levels during occlusion were 9.5–22.0 ng/ml in the treated group, which are comparable to levels found in human smokers. No differences were found in the risk zone of nicotine compared to control rabbits (26±2% vs. 23±2% of the left ventricle, respectively), or infarct size (31±5% vs. 37±4% of risk zone). Since no effect on infarct size was found, a third group receiving higher dose of nicotine, 320 g/kg (n=6), was added. Infarct size was not different from the control group (39±6% of risk zone). Nicotine, given intravenously before ischemic insult, does not protect the myocardium. A preconditioning-like effect of nicotine is probably not the mechanism of reduced mortality in smoking patients with myocardial infarction.     

The p38 MAPK Inhibitor SB203580 Blocks Adenosine A<Subscript>1</Subscript> Receptor-Induced Attenuation of In Vivo Myocardial Stunning

Summary There is considerable evidence implicating a key role for p38 mitogen-activated protein kinase (MAPK) in ischemic and pharmacological preconditioning against myocardial infarction. However, there have been few, if any, studies examining the role of p38 MAPK in the protection of stunned myocardium. The purpose of this study was to determine whether p38 MAPK plays a role in the adenosine A₁ receptor anti-stunning effect in in vivo porcine myocardium. Regional myocardial stunning in anesthetized, open-chest pigs was induced by 15 min of left anterior descending coronary artery (LAD) occlusion and 3 h of reperfusion (RP). Animals were treated with either vehicle (n = 5), AMP579 (70 g/kg i.v.; 25 g/kg bolus + 1.5 g/kg/min for 30 min prior to ischemia, n = 5), the p38 MAPK inhibitor SB203580 (0.25 mg/kg i.v. bolus, n = 4) or a combination of SB203580 plus AMP579 (n = 5). Regional ventricular function was monitored by measurements of segment shortening and load insensitive parameters including preload recruitable stroke work (PRSW) and PRSW area (PRSWA). The ischemic area at risk was similar in all groups and there was no necrosis in any heart. Treatment with AMP579 significantly improved reperfusion regional PRSW and PRSWA compared to vehicle controls. The p38 inhibitor SB203580 alone did not alter the extent of myocardial stunning, but it abolished the beneficial effect of AMP579 pretreatment. These results provide the first evidence that p38 MAPK activation may play an important role in the mechanism by which adenosine agonists attenuate myocardial stunning.     

Heat stress increases cardiac HSP72i but fails to reduce myocardial infarct size in rabbits 24 hours later

Recent reports suggest that delayed myocardial protection (second window of preconditioning) occurs 24 hours after brief ischemic or thermal stress. In order to test this hypothesis, we subjected New Zealand White rabbits to a heating regimen (42°C for 15–20 minutes). Twenty four hours later, the effect of heat stress on infarct size was determined by conducting a 30 minute ischemia/3 hour reperfusion protocol. In a separate group of rabbits, Western blot analysis was used to verify that the heating regimen increased expression of HSP72i. The size of the region at risk was delineated by infusion of Unisperse blue and infarcted myocardium was identified by incubation of left ventricular slices in triphenyl tetrazolium chloride. In contrast to expectations, induction of HSP72i with thermal stress was not effective in limiting infarct size in rabbits 24 hours later, calling into question the concept that heat stress induces delayed or second window myocardial protection.     

Mast cell degranulation does not contribute to ischemic preconditioning in isolated rabbit hearts

Preconditioning the heart with a short period of ischemia makes it resistant to infarction from a subsequent ischemic insult. We have proposed that preconditioning is triggered by the release of endogenous substances including adenosine which activate protein kinase C through receptormediated cell signaling pathways. However, it has also been proposed that the initial brief ischemia may result in mast cell degranulation without significant myocardial damage, making it less likely that the toxic granule contents could be released to irreversibly damage vulnerable myocardial cells during the subsequent prolonged ischemia. To study the role of mast cells in ischemic preconditioning (PC) isolated rabbit hearts were subjected to 30 min of regional ischemia followed by 120 min of reperfusion. Infarct size was measured with triphenyltetrazolium chloride. In control hearts infarction was 31.9±2.6% of the risk zone. Preconditioning with 5 min of global ischemia and 10 min of reperfusion reduced infarct size to 5.6±6.1% (p<0.01). When disodium cromoglycate (DSCG) (10 M), a mast cell stabilizer, was infused shortly before the long ischemia it did protect the heart (12.8±2.9% infarction, p<0.01 vs control) which supports the mast cell theory. However, a mast cell degranulating agent, compound 48/80 (24 mg/L), added to the perfusate prior to the 30 min ischemic period could not mimic PC (39.7±5.6% infarction). Mast cell granules are rich in histamine, and the latter was assayed in myocardium by immunoassay as a marker of intact granules. In homogenized left ventricle from normal rabbit hearts and those following a standard PC protocol of 5-min global ischemia/10-min reperfusion, histamine contents were 9.3±1.4 and 8.9±1.4 ng/g wet tissue, respectively. Compound 48/80 reduced histamine levels to 2.9±0.6 ng/g (p<0.05 vs control). Although baseline histamine contents were 10-fold higher in rats, PC also had no effect, but compound 48/80 reduced content by 91%. Therefore, histamine tissue content and presumably mast cell granules were unaffected by a PC protocol which successfully protected ischemic myocardium, while pharmacological myocardial histamine depletion was not associated with protection. Hence, mast cells do not appear to be important in ischemic preconditioning. Although a mast cell stabilizer such as DSCG can protect ischemic myocardium, it may do so by one of its other properties, e.g., membrane stabilization.This study was supported in part by grants from the National Institutes of Health Heart, Lung, and Blood Institute, HL-20648 and HL-50688.     

Gallopamil Activity on Asynergic Viable Myocardium in Acute Myocardial Infarction: Insights on Stunned and Hibernating Myocardium

The influence of the calcium antagonist gallopamil on the contractility of asynergic viable myocardium after acute myocardial infarction treated with thrombolysis was investigated by two-dimensional echocardiography. Sixteen patients with 1 viable segment(s), identified during the low-dose phase (up to 10 g/kg/min) of a dobutamine echocardiographic test (up to 40 g/kg/min) performed 4–5 days after a first acute myocardial infarction, were given a gallopamil intravenous bolus (50 g/kg) 12–24 hours later. Two-dimensional echocardiography was done before and 15 minutes after the bolus. A score index of 1 (normokinesis) to 4 (dyskinesis) and a 16-segment model were used. A segment was considered viable when a resting asynergy (score 2) improvement of 1 grade was seen during low-dose dobutamine. Follow-up echocardiograms were done 3–5 months later. A total of 30 viable segments were found; of these, 10 showed sustained improvement in contractility (group A) during high-dose dobutamine, while 20 exhibited a biphasic response returning to their basal contractile state (group B). After the gallopamil bolus, 9 of 10 group A segments improved their contractility, in comparison with 0 of 20 group B segments (P < .001). Infarct-related vessel significant (75%) coronary stenosis was present in the tributary vessel of 0 of 10 group A and of 20 of 20 group B segments (P < .001). At follow-up, 9 of 10 group A segments showed a spontaneous contractile improvement; of the 20 group B segments, 8 of 10 that underwent revascularization (7 angioplasty, 3 bypass graft) showed contractile improvement, in comparison with 0 of 10 segments not revascularized (P = .001). We conclude that gallopamil may reverse the contractile dysfunction of postischemic stunned myocardium in patients with acute myocardial infarction, whereas no effects are apparent on ischemic/hibernating myocardium.     

Angina: Who Needs It? Cardioprotection in the Preconditioning Era

Over the past two decades, it has been demonstrated in various animal species that the myocardium possesses innate adaptive mechanisms that may render it more resistant to ischemic injury. Ischemic preconditioning, defined as the protection conferred to ischemic myocardium by prior episodes of brief sublethal ischemia, is one of the most potent of such adaptive phenomena. Extensive research over the past decade has alluded to the cellular mechanisms underlying this powerful means of reducing myocardial ischemia-reperfusion injury. Moreover, the possibility that such adaptive mechanisms might be inducible in the human heart has generated considerable excitement and enthusiastic research, which has significantly enhanced our understanding of the pathogenesis of ischemia-reperfusion injury. An insight into the mechanisms underlying the cardioprotective properties of ischemic preconditioning has, on the one hand, directed research aimed at identification of novel therapeutic agents for the treatment of ischemic heart disease, and on the other, questioned the use of potentially deleterious agents that may abolish the cardioprotective actions of ischemic preconditioning in patients with angina. Current studies are under way to evaluate the potential protection afforded by these preconditioning agents in patients with acute coronary syndromes, and some early reports provide some basis for optimism that a beneficial and clinically detectable improvement in myocardial protection may be possible. This article reviews our current knowledge of the cellular mechanisms responsible for mediation of ischemic preconditioning, the evidence for the existence of this phenomenon in humans, and its potential therapeutic applications.     

The dual effects of nitric oxide synthase inhibitors on ischemia-reperfusion injury in rat hearts

Objective Nitric oxide (NO) is known to act as a mediator of tissue injury as well as being a potent endogenous vasodilator. The functional and metabolic effects of NO on ischemia-reperfusion injury are still controversial. The aim of this study was to clarify the relationship between the degree of NO synthase (NOS) inhibition and the effects on ischemia-reperfusion injury.Methods and results Langendorff-perfused rat hearts were subjected to 30 minutes of global ischemia followed by 30 minutes of reperfusion. The recovery of left ventricular developed pressure (LVDP), creatine kinase (CK) release, and myocardial high energy phosphates were measured in hearts perfused with or without NOS inhibitors, L-N^G-monomethyl arginine (L-NMMA) or N^Gnitro-L-arginine methylester (L-NAME). NOS inhibitors exerted different effects on the recovery of LVDP and CK release depending on the concentration. The low dose of L-NMMA improved the recovery of LVDP, decreased the CK release during reperfusion, and preserved the myocardial adenosine triphosphate content after reperfusion. In contrast, the high dose of L-NMMA had adverse effects. L-NMMA reduced NO release in coronary efuent in a dose-dependent fashion. Both effects of L-NMMA were abolished by excessive co-administration of L-arginine and the same doses of D-N^G-monomethyl arginine (D-NMMA) showed no effect on ischemia-reperfusion injury. Therefore, both effects were due to NOS inhibition. In addition, L-NMMA suppressed the myocardial malondialdehyde accumulation, an indicator of oxidative stress, which might be attributed to the beneficial effects by partial NOS inhibition. On the other hand, the high dose L-NMMA signicantly decreased coronary ow during aerobic perfusion and reperfusion. Therefore, it is conceivable that the vasoactive NOS inhibition contributes to the harmful effects, which might exceed the benecial effects due to a decrease in oxidative stress.Conclusion The present results showed that NO inhibitors had dual effects on mechanical function and energy metabolism depending on the concentration. Non-vasoactive inhibition of NOS had benecial effects due to the suppression of oxidative injury. However, strong vasoactive inhibition of NOS exacerbated the ischemia-reperfusion injury.     

Reperfusion Injury: Does It Exist and Does It Have Clinical Relevance?

Although reperfusion is an absolute prerequisite for the survival of ischemic tissue, it is not necessarily without hazard. Many (but not all) cardiologists are of the opinion that some components of reperfusion may be detrimental and able to inflict injury over and above that attributable to the ischemia. In this article we define four sequelae of reperfusion that might be designated as reperfusion injury. We identify possible underlying mechanisms and consider whether any of these forms of reperfusion injury are of clinical relevance.     

Intracoronary hyperosmotic mannitol during reperfusion does not affect infarct size in ischemic,reperfused porcine hearts

Summary We investigated the effect of reperfusion with hyperosmotic mannitol on the infarct size in porcine hearts. The distal half of the left anterior descending coronary artery was occluded in each of 21 anesthetized pigs for 75 min and was reperfused for 2 h. During reperfusion mannitol (1075 mosmol/kg) was intracoronarily infused at a dose of 0.5 ml/min in 6 pigs (low mannitol group), at a dose of 1.5 ml/min in another 6 pigs (high mannitol group), and at a dose of 5 ml/min in 3 pigs for the first 8 min of reperfusion (very high mannitol group). 6 pigs served as controls. Although mannitol infusion increased plasma osmolality in the ischemic, reperfused myocardium in all experiments, the infarct size expressed as the ratio of the infarcted tissue over the area at risk of necrosis was not significantly influenced. Infarct size amounted to 72±25% in the control group, to 75±14% in the low mannitol group, to 78±18% in the high mannitol group, and to 93±8% in the very high mannitol group. These results clearly indicate that reperfusion with hyperosmotic mannitol after 75 min of ischemia does not exert any beneficial effect on the infarct size.