Induction and Assessment of Ischemia-reperfusion Injury in Langendorff-perfused Rat Hearts

The biochemical events surrounding ischemia reperfusion injury in the acute setting are of great importance to furthering novel treatment options for ...

Medical University of South Carolina

Reperfusion of the ischemic myocardium leads to a burst of reactive O(2) species (ROS), which is a primary determinant of postischemic myocardial dysfunction. We tested the hypothesis that early O(2) delivery and the cellular redox state modulate the initial myocardial ROS production at reperfusion. Isolated buffer-perfused rat hearts were loaded with the fluorophores dihydrofluorescein or Amplex red to detect intracellular and extracellular ROS formation using surface fluorometry at the left ventricular wall. Hearts were made globally ischemic for 20 min and then reperfused with either 95% or 20% O(2)-saturated perfusate. The same protocol was repeated in hearts loaded with dihydrofluorescein and perfused with either 20 or 5 mM glucose-buffered solution to determine relative changes in NADH and FAD. Myocardial O(2) delivery during the first 5 min of reperfusion was 84.7 +/- 4.2 ml O(2)/min with 20% O(2)-saturated buffer and 354.4 +/- 22.8 ml O(2)/min with 95% O(2) (n = 8/group, P < 0.001). The fluorescein signal (intracellular ROS) was significantly increased in hearts reperfused with 95% O(2) compared with 20% O(2). However, the resorufin signal (extracellular ROS) was significantly increased with 20% O(2) compared with 95% O(2) during reperfusion. Perfusion of hearts with 20 mM glucose reduced the (.)NADH during ischemia (P < 0.001) and the (.)ROS at reperfusion (P < 0.001) compared with 5.5 mM-perfused glucose hearts. In conclusion, initial O(2) delivery to the ischemic myocardium modulates a compartment-specific ROS response at reperfusion such that high O(2) delivery promotes intracellular ROS and low O(2) delivery promotes extracellular ROS. The redox state that develops during ischemia appears to be an important precursor for reperfusion ROS production.

O2 delivery and redox state are determinants of compartment-specific reactive O2 species in myocardial reperfusion.

Previous reports note an increase in both reactive oxygen species (ROS) and nitric oxide (*NO) at the onset of myocardial reperfusion. We tested the hypothesis that inhibition of *NO or ROS production at the time of reperfusion improves recovery of post-ischemic myocardial function.

Inhibition of peroxynitrite precursors, NO and O2, at the onset of reperfusion improves myocardial recovery.

Epinephrine (adrenaline) is widely used as a primary adjuvant for improving perfusion pressure and resuscitation rates during cardiopulmonary resuscitation (CPR). Epinephrine is also associated with significant myocardial dysfunction in the post-resuscitation period. We tested the hypothesis that the cardiac effects of epinephrine vary according to the duration of cardiac arrest.

Cardiovascular response to epinephrine varies with increasing duration of cardiac arrest.

Recent scientific evidence has demonstrated the importance of good quality chest compressions without interruption to improve cardiac arrest resuscitation rates, and suggested that a de-emphasis on minute ventilation is needed. However, independent of ventilation, the role of oxygen and the optimal oxygen concentration during CPR is not known. Previous studies have shown that ventilation with high oxygen concentration after CPR is associated with worse neurologic outcome. We tested the hypothesis that initial ventilation during CPR without oxygen improves resuscitation success.

Oxygen requirement during cardiopulmonary resuscitation (CPR) to effect return of spontaneous circulation.

During cardiac arrest (CA), myocardial perfusion is solely dependent on cardiopulmonary resuscitation (CPR) although closed-chest compressions only provide about 10-20% of normal myocardial perfusion. The study was conducted in a whole animal CPR model to determine whether CPR-generated oxygen delivery preserves or worsens mitochondrial function. Male Sprague-Dawley rats (400-450 g) were randomly divided into four groups: (1) BL (instrumentation only, no cardiac arrest), (2) CA(15) (15 min cardiac arrest without CPR), (3) CA(25) (25 min cardiac arrest without CPR) and (4) CPR (15 min cardiac arrest, followed by 10 min CPR). The differences between groups were evaluated by measuring mitochondrial respiration, electron transport chain (ETC) complex activities and mitochondrial ultrastructure by transmission electron microscopy (TEM). The CA(25) group had the greatest impairment of mitochondrial respiration and ETC complex activities (I-III). In contrast, the CPR group was not different from the CA(15) group regarding all measures of mitochondrial function. Complex I was more susceptible to ischemic injury than the other complexes and was the major determinant of mitochondrial dysfunction. Observations of mitochondrial ultrastructure by TEM were compatible with the biochemical results. The findings suggest that, despite low blood flow and oxygen delivery, CPR is able to preserve heart mitochondrial function and viability during ongoing global ischemia. Preservation of complex I activity and mitochondrial function during cardiac arrest may be an important mechanism underlying the beneficial effects of CPR which have been shown in clinical studies.

Preservation of mitochondrial function with cardiopulmonary resuscitation in prolonged cardiac arrest in rats.

Isolated hearts used in the study of ischemia-reperfusion induced myocardial reactive oxygen species (ROS) have typically been perfused with crystalloid buffer. Limitations of crystalloid buffer which may exaggerate the production of ROS, include a requirement for higher oxygen tension and the absence of the intrinsic erythrocyte antioxidant defenses. Using a novel recirculating blood-perfused rat heart model, we measured H(2)O(2) concentration in the blood (as an indicator of ROS formation) and tissue glutathione concentration (an overall measure of oxidant stress) following ischemia and reperfusion.

Measurement of hydrogen peroxide and oxidant stress in a recirculating whole blood-perfused rat heart model.

Rapid post-ischemic re-oxygenation is necessary to minimize ischemic injury, but itself can induce further reperfusion injury through the induction of reactive oxygen species. Utilization of oxygen within the cell primarily occurs in the mitochondria. The objective of this study was to determine heart mitochondrial function after 1 h of controlled arterial oxygenation following cardiac arrest and restoration of spontaneous circulation (ROSC). We hypothesized that arterial hyper-oxygenation following ROSC would result in greater impairment of heart mitochondrial function.

Post-cardiac arrest hyperoxia and mitochondrial function.

The effect of hyperoxygenation at reperfusion, particularly in the setting of cardiac arrest, remains unclear. This issue was studied in a prolonged cardiac arrest model consisting of 25 min cardiac arrest in a rat resuscitated with cardiopulmonary bypass (CPB). The objective of this study was to determine the effect of hyperoxygenation following prolonged cardiac arrest resuscitation on mitochondrial and cardiac function.

Hyperoxemic reperfusion after prolonged cardiac arrest in a rat cardiopulmonary bypass resuscitation model.

The neutrophil elastase inhibitor sivelestat (ONO-5046) possesses unknown mechanisms of cardioprotection when infused following global ischemia, even in the absence of neutrophils. Since myocardial ischemia-reperfusion injury is strongly associated with endothelial dysfunction and reactive oxygen species (ROS) generation during reperfusion, we have tested the hypothesis that infusion of sivelestat during postischemic low flow would preserve endothelial and contractile function and reduce infarct size through an ROS-mediated mechanism. Isolated male rat hearts, subjected to global ischemia of 25 minutes, were reperfused with low flow with or without sivelestat followed by a full flow reperfusion. Hearts treated with sivelestat showed a significant improvement of LV contractile function and a reduction in infarct size. Infusion of L-NAME (nonspecific blocker of endothelial nitric oxide synthase (eNOS)) along with sivelestat during reperfusion reversed the preservation of contractile function and infarct size. In vitro EPR spin trapping experiments showed that sivelestat treatment decreased superoxide adduct formation in bovine aortic endothelial cells (BAECs) subjected to hypoxia-reoxygenation. Similarly, dihydroethidine (DHE) staining showed decreased superoxide production in LV sections from sivelestat-treated hearts. Taken together, these results indicate that sivelestat infusion during postischemic low flow reduces infarct size and preserves vasoreactivity in association with decreased ROS formation and the preservation of nitric oxide.

Sivelestat attenuates myocardial reperfusion injury during brief low flow postischemic infusion.

While inhibition of class I/IIb histone deacetylases (HDACs) protects the mammalian heart from ischemia reperfusion (IR) injury, class selective effects remain unexamined. We hypothesized that selective inhibition of class I HDACs would preserve left ventricular contractile function following IR in isolated hearts. Male Sprague Dawley rats (n=6 per group) were injected with vehicle (dimethylsulfoxide, 0.63mg/kg), the class I/IIb HDAC inhibitor trichostatin A (1mg/kg), the class I HDAC inhibitor entinostat (MS-275, 10mg/kg), or the HDAC6 (class IIb) inhibitor tubastatin A (10mg/kg). After 24h, hearts were isolated and perfused in Langendorff mode for 30min (Sham) or subjected to 30min global ischemia and 120min global reperfusion (IR). A saline filled balloon attached to a pressure transducer was placed in the LV to monitor contractile function. After perfusion, LV tissue was collected for measurements of antioxidant protein levels and infarct area. At the conclusion of IR, MS-275 pretreatment was associated with significant preservation of developed pressure, rate of pressure generation, rate of pressure relaxation and rate pressure product, as compared to vehicle treated hearts. There was significant reduction of infarct area with MS-275 pretreatment. Contractile function was not significantly restored in hearts treated with trichostatin A or tubastatin A. Mitochondrial superoxide dismutase (SOD2) and catalase protein and mRNA in hearts from animals pretreated with MS-275 were increased following IR, as compared to Sham. This was associated with a dramatic enrichment of nuclear FOXO3a transcription factor, which mediates the expression of SOD2 and catalase. Tubastatin A treatment was associated with significantly decreased catalase levels after IR. Class I HDAC inhibition elicits protection of contractile function following IR, which is associated with increased expression of endogenous antioxidant enzymes. Class I/IIb HDAC inhibition with trichostatin A or selective inhibition of HDAC6 with tubastatin A was not protective. This study highlights the need for the development of new strategies that target specific HDAC isoforms in cardiac ischemia reperfusion.

Selective inhibition of class I but not class IIb histone deacetylases exerts cardiac protection from ischemia reperfusion.

Ischemia-reperfusion (IR) injury comprises a significant portion of morbidity and mortality from heart and brain diseases worldwide. This enduring clinical problem has inspired myriad reports in the scientific literature of experimental interventions seeking to elucidate the pathology of IR injury. Elective cardiac surgery presents perhaps the most viable scenario for protecting the heart and brain from IR injury due to the opportunity to condition the organs prior to insult. The physiological parameters for the preconditioning of vital organs prior to insult through mechanical and pharmacological maneuvers have been heavily examined. These investigations have revealed new insights into how preconditioning alters cellular responses to IR injury. However, the promise of preconditioning remains unfulfilled at the clinical level, and research seeking to implicate cell signals essential to this protection continues. Recent discoveries in molecular biology have revealed that gene expression can be controlled through posttranslational modifications, without altering the chemical structure of the genetic code. In this scenario, gene expression is repressed by enzymes that cause chromatin compaction through catalytic removal of acetyl moieties from lysine residues on histones. These enzymes, called histone deacetylases (HDACs), can be inhibited pharmacologically, leading to the de-repression of protective genes. The discovery that HDACs can also alter the function of non-histone proteins through posttranslational deacetylation has expanded the potential impact of HDAC inhibitors for the treatment of human disease. HDAC inhibitors have been applied in a very small number of experimental models of IR. However, the scientific literature contains an increasing number of reports demonstrating that HDACs converge on preconditioning signals in the cell. This review will describe the influence of HDACs on major preconditioning signaling pathways in the heart and brain.

Sverre E. Aune

Department of Medicine/Cardiology,
Gazes Cardiac Research Institute,
Department of Medicine/Cardiology, Gazes Cardiac Research Institute

Medical University of South Carolina

Induction and Assessment of Ischemia-reperfusion Injury in Langendorff-perfused Rat Hearts

Sverre E. Aune

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Medical University of South Carolina

Induction and Assessment of Ischemia-reperfusion Injury in Langendorff-perfused Rat Hearts

Department of Medicine/Cardiology,
Gazes Cardiac Research Institute,
Department of Medicine/Cardiology, Gazes Cardiac Research Institute