Karin Przyklenk

Regional ischemic 'preconditioning' protects remote virgin myocardium from subsequent sustained coronary occlusion.

BackgroundOne or more brief episodes of coronary artery occlusion protect or “precondition” the myocardium perfused by that artery from a subsequent episode of sustained ischemia. We sought to determine whether ischemic preconditioning protects only those myocytes subjected to brief coronary occlusion or whether brief occlusions in one vascular bed also limit infarct size andfor attenuate contractile dysfunction in remote virgin myocardium subjected to subsequent sustained coronary occlusion. Methods and ResultsIn the preliminary limb of the study, six anesthetized dogs underwent four episodes of 5-minute circumflex branch occlusion plus 5-minute reperfusion, followed by 1 hour of sustained left anterior descending coronary artery occlusion and 4.5 hours of reflow. Subendocardial blood flow during left anterior descending coronary artery occlusion (measured by injection of radiolabeled microspheres) was 0.07±0.03 mL. min-1. g tissue-1, similar to the value of 0.07±0.02 mL. min-1. g-1 observed in a group of eight concurrent control dogs. However, infarct size (assessed by triphenyltetrazolium staining) in the circumflex preconditioned group averaged 4±1% of the myocardium at risk, significantly less (p<0.05) than the value of 13+4% observed in the concurrent controls. An additional 18 dogs were then randomized to undergo either four episodes of circumflex branch occlusion (n=8) or no intervention (n=10) before 1 hour of left anterior descending coronary artery occlusion and 4.5 hours of reflow. Subendocardial blood flow averaged 0.08±0.02 versus 0.08±0.03 mL. min-1. g-1 in the control versus circumflex preconditioned groups, yet infarct size was significantly smaller in circumflex preconditioned dogs than in the controls (6±2% versus 16±5% of the risk region; p<0.05). At 4.5 hours following reperfusion, segment shortening in the left anterior descending coronary artery bed (assessed by sonomicrometry) averaged -21+19%o of baseline in control animals versus 13±12% of baseline in the preconditioned group (p=NS). Circumflex preconditioning did not, however, have an independent beneficial effect on contractile function: Regression analysis revealed that the trend toward improved function in circumflex preconditioned dogs reflected the smaller infarct sizes in this group. ConclusionsBrief episodes of ischemia in one vascular bed protect remote, virgin myocardium from subsequent sustained coronary artery occlusion in this canine model. These data imply that preconditioning may be mediated by factor(s) activated, produced, or transported throughout the heart during brief ischemia/reperfusion.

Previous Angina Alters In-Hospital Outcome in TIMI 4 A Clinical Correlate to Preconditioning?

BACKGROUND Ischemic preconditioning has been shown to reduce myocardial infarct size in experimental models, but its role in patients remains unclear. Angina before myocardial infarction reflects brief episodes of ischemia and may be a marker of preconditioning. As part of the Thrombolysis in Myocardial Infarction (TIMI) 4 study, we performed an analysis on the effect of a history of previous angina on in-hospital outcomes for patients with acute myocardial infarction. METHODS AND RESULTS Patients eligible for thrombolytic therapy were enrolled into the study. Data were collected from case report forms regarding previous history of angina, in-hospital outcome and 6-week follow-up. Two hundred eighteen patients had a history of previous angina at any time before acute myocardial infarction, and 198 patients did not have previous angina. Patients with any previous history of angina were less likely than with those without angina to experience in-hospital death (3% versus 8%) (P = .03), severe congestive heart failure (CHF) or shock (1% versus 7%, P = .006), or the combined end point of in-hospital death, severe CHF, or shock (4% versus 12%, P = .004). Moreover, patients with any history of angina were more likely to have a smaller creatine kinase (CK)-determined infarct size (119 versus 154 CK integrated units; P = .01) and were less likely to have Q waves on their ECG (57% versus 69%; P = .01). In the subset of patients who experienced angina within the 48 hours before infarction (compared with those who did not), there was a trend toward less likely in-hospital death (3% versus 6%; P = .09), a lower incidence of severe CHF or shock (1% versus 6% P = .008), a lower combined end point of death, CHF, or shock (3% versus 10%; P = .006), smaller infarct size assessed by CK (115 versus 151 CK units; P = .03), and a trend toward fewer Q-wave infarcts. However, patients with a history of previous angina did have a trend toward more recurrent ischemic pain. Of importance is that the beneficial in-hospital effects of previous angina were not dependent on angiographically visible coronary collaterals. CONCLUSIONS Previous angina confers a beneficial effect on in-hospital outcome after acute myocardial infarction. The reasons for this benefit are uncertain, but one potential mechanism for this observation may be ischemic preconditioning.

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.

Superoxide dismutase plus catalase improve contractile function in the canine model of the stunned myocardium

Fifteen minutes of coronary occlusion followed by reperfusion does not result in myocardial necrosis; however, the contractile function and high energy phosphate content of the previously ischemic myocardium remains depressed or ‘stunned’ for several hours to days after reperfusion. Oxygen-derived free radicals have been implicated in ischemia and reperfusion- induced injury in a variety of tissues. We wished to determine whether administration of free radical scavengers superoxide dismutase plus catalase before and during occlusion, and throughout reperfusion, could attenuate the ‘stunning’ produced by 15 minutes of left anterior descending coronary artery occlusion in anesthetized, open-chest dogs. Segment shortening in the previously ischemic zone recovered to within only ±10% of preinfusion values in the control group during 3 hours of reperfusion, while, in the treated dogs, segment shortening returned to a maximum of 56 ± 16% of preinfusion at 11/2 hours post-reperfusion (P < 0.0003 compared to controls). Similarly, superoxide dismutase + catalase-treated dogs exhibited improved wall thickening during reperfusion (+30% to +70% of preinfusion values), compared to controls (0% to +10%). However, this improvement in contractile function in the treated group was not accompanied by increased adenosine triphosphate stores in the previously ischemic zone (31.8 ± 0.8 vs. 28.2 ± 2.2 nmol/ mg protein for control vs. treated groups). Infusion of superoxide dismutase + catalase did not influence blood flow during occlusion or reperfusion. However, the treated group did exhibit a significant decrease in blood pressure during reperfusion. Hypotension during reperfusion did not appear to be the cause of the improved contractile function, as administration of sodium nitroprusside (an afterload-reducing agent with no free radical scavenging properties) to an additional group of dogs during reperfusion had no significant effect on segment shortening in the previously ischemic tissue. Thus, treatment with free radical scavengers significantly enhanced function, but did not improve high energy phosphate content, in the stunned myocardium.

Molecular Mechanisms of Ischemia–Reperfusion Injury in Brain: Pivotal Role of the Mitochondrial Membrane Potential in Reactive Oxygen Species Generation

Stroke and circulatory arrest cause interferences in blood flow to the brain that result in considerable tissue damage. The primary method to reduce or prevent neurologic damage to patients suffering from brain ischemia is prompt restoration of blood flow to the ischemic tissue. However, paradoxically, restoration of blood flow causes additional damage and exacerbates neurocognitive deficits among patients who suffer a brain ischemic event. Mitochondria play a critical role in reperfusion injury by producing excessive reactive oxygen species (ROS) thereby damaging cellular components, and initiating cell death. In this review, we summarize our current understanding of the mechanisms of mitochondrial ROS generation during reperfusion, and specifically, the role the mitochondrial membrane potential plays in the pathology of cerebral ischemia/reperfusion. Additionally, we propose a temporal model of ROS generation in which posttranslational modifications of key oxidative phosphorylation (OxPhos) proteins caused by ischemia induce a hyperactive state upon reintroduction of oxygen. Hyperactive OxPhos generates high mitochondrial membrane potentials, a condition known to generate excessive ROS. Such a state would lead to a “burst” of ROS upon reperfusion, thereby causing structural and functional damage to the mitochondria and inducing cell death signaling that eventually culminate in tissue damage. Finally, we propose that strategies aimed at modulating this maladaptive hyperpolarization of the mitochondrial membrane potential may be a novel therapeutic intervention and present specific studies demonstrating the cytoprotective effect of this treatment modality.

New Horizons in Cardioprotection Recommendations From the 2010 National Heart, Lung, and Blood Institute Workshop

Coronary heart disease is the largest major killer of American men and women and accounted for 1 of every 6 deaths in the United States in 2007.1 The annual incidence of myocardial infarction in the United States is estimated to be 935 000, with 610 000 new cases and 325 000 recurrent attacks. Survivors have a much higher chance of suffering from congestive heart failure, arrhythmias, and sudden cardiac death. Prognosis after an acute myocardial ischemic injury is primarily dependent on the amount of myocardium that undergoes irreversible injury.2–4 Large transmural infarcts yield a higher probability of cardiogenic shock, arrhythmias, adverse remodeling, and development of late chronic heart failure. Although it has been known since the early 1970s that the size of a myocardial infarction can be modified by various therapeutic interventions,5 early coronary artery reperfusion by fibrinolysis or percutaneous coronary intervention, including balloon angioplasty with or without stenting, remains the only established intervention capable of consistently reducing infarct size in humans. Although reperfusion has led to significant advances in patient care and reduction in hospital mortality, delays in seeking medical attention and inherent limitations in initiating fibrinolysis or percutaneous coronary intervention dictate that additional substantive improvements in morbidity and mortality can be achieved only with the development of new adjunctive therapies coupled with reperfusion. In addition, reperfusion therapy itself may induce reperfusion injury, a phenomenon that may encompass stunned myocardium, no-reflow phenomenon, and lethal myocardial cell death. If this injury could be prevented or minimized by administration of adjunctive therapy, then the net benefit of reperfusion could be enhanced. The problem of acute ischemic injury and myocardial infarction is not limited to patients with acute coronary artery syndrome. It remains a major problem in cardiac surgery as well. It is well documented that the incidence of myocardial necrosis after surgery, as determined by creatine kinase MB enzyme release and troponin levels, ranges somewhere between 40% and 60%, and, depending on its clinical definition, the incidence of myocardial infarction after coronary artery bypass graft surgery may be as high as 19%. The intermediate and long-term implications are considerable. In a recent retrospective analysis of 18 908 patients who underwent coronary artery bypass graft surgery and in whom long-term follow-up was available, it was shown that myocardial enzyme elevation within the first 24 hours of surgery was associated with increasing mortality over the course of months to years. This study confirms earlier reports that even small enzyme elevations after surgery are associated with worse long-term outcomes.4

Prospective Temporal Analysis of the Onset of Preinfarction Angina Versus Outcome: An Ancillary Study in TIMI-9B

BACKGROUND The timing of onset of angina before myocardial infarction in relation to outcome is unknown. METHODS AND RESULTS We prospectively determined the importance of the time of onset of preinfarction angina in relation to 30-day outcomes in the TIMI-9B study from standardized forms. Of the 3002 patients entered into the study, 425 reported angina before their myocardial infarction. Patients with angina onset within 24 hours of infarction had a lower 30-day cardiac event rate (mortality, recurrent myocardial infarction, heart failure, or shock) at 4% than those with onset of angina >24 hours (17%; P=.030). A history of any angina alone was not associated with reduced event rate. Peak creatine kinase levels tended to be lower in the group with angina within 24 hours. These benefits were not due to higher rates of use of antianginal medicines or aspirin and were not a consequence of differences in baseline characteristics or disease states (hypertension, hypercholesterolemia) among subgroups. CONCLUSIONS These temporal observations are consistent with the concept of preconditioning by preinfarction angina but do not rule out other mechanisms.

Preconditioning does not attenuate myocardial stunning.

BackgroundDespite numerous reports that one or more episodes of brief coronary artery occlusion preconditions the myocardium and dramatically reduces myocardial infarct size produced by a subsequent prolonged ischemia, we recently demonstrated that preconditioning does not attenuate contractile dysfunction in the peri-infarct tissue. However, the specific effects of preconditioning on myocardium in which wall motion has not been compromised by the preconditioning regimen per se and is further submitted to a short ischemic insult (that is, not confounded by necrosis) remain unknown. Methods and ResultsWe addressed these issues in the canine model of myocardial stunning. Eighteen anesthetized dogs underwent 15 minutes of coronary occlusion followed by 3 hours of reperfusion. Before the 15-minute coronary occlusion, each dog received one of three treatments: no intervention (control group, n=6), one episode of 5-minute coronary occlusion/S-minute reperfusion (PC5 group, n=6), or one episode of 2.5-minute coronary occlusion/S-minute reperfusion (PC25 group, n=6). Segment shortening (SS) in the ischemic/reperfused midmyocardium was monitored by sonomicrometry, and myocardial blood flow was assessed by injection of radiolabeled microspheres. All three groups were equally ischemic during the 15-minute coronary occlusion: Midmyocardial blood flow averaged 0.05±0.02, 0.07±0.04, and 0.08±0.03 ml/min/g in control, PC25, and PC5 groups, respectively. Before the 15-minute coronary occlusion, PC5 dogs exhibited significant stunning (SS=55% baseline; p<0.01 versus control), whereas PC25 dogs did not (SS=91% baseline; p=NS versus control). However, segment shortening during the subsequent 15-minute coronary occlusion was equally depressed at −25% to −42% of baseline values among the three groups. Furthermore, all three groups demonstrated a similar degree of stunning after reperfusion: SS at 3 hours after reflow averaged 24±12%, 34±16%, and 48±12% of baseline in control, PC25, and PC5 groups, respectively (p=NS). The degree of recovery of function after reperfusion correlated with the amount of midmyocardial blood flow during coronary artery occlusion. However, this relation was not different among the three groups: Specifically, for any given collateral flow during ischemia, preconditioning did not reduce the degree of stunning. ConclusionsPreconditioning neither preserves contractile function during a reversible ischemic insult nor prevents myocardial stunning during the initial hours of reflow.

Regulation of oxidative phosphorylation, the mitochondrial membrane potential, and their role in human disease

Thirty years after Peter Mitchell was awarded the Nobel Prize for the chemiosmotic hypothesis, which links the mitochondrial membrane potential generated by the proton pumps of the electron transport chain to ATP production by ATP synthase, the molecular players involved once again attract attention. This is so because medical research increasingly recognizes mitochondrial dysfunction as a major factor in the pathology of numerous human diseases, including diabetes, cancer, neurodegenerative diseases, and ischemia reperfusion injury. We propose a model linking mitochondrial oxidative phosphorylation (OxPhos) to human disease, through a lack of energy, excessive free radical production, or a combination of both. We discuss the regulation of OxPhos by cell signaling pathways as a main regulatory mechanism in higher organisms, which in turn determines the magnitude of the mitochondrial membrane potential: if too low, ATP production cannot meet demand, and if too high, free radicals are produced. This model is presented in light of the recently emerging understanding of mechanisms that regulate mammalian cytochrome c oxidase and its substrate cytochrome c as representative enzymes for the entire OxPhos system.

Postischemic Myocardial Stunning: A Clinically Relevant Phenomenon

Excerpt Coronary artery reperfusion by means of thrombolytic therapy is rapidly emerging as a fundamental strategy in the management of patients with acute myocardial infarction. Although successfu...