Changes in oxygen tension affect cardiac mitochondrial respiration rate via changes in the rate of mitochondrial hydrogen peroxide production

Experimental Models of Ischemic Lung Damage for the Study of Therapeutic Reconditioning During Ex Vivo Lung Perfusion

Background.

Ex vivo lung perfusion (EVLP) may allow therapeutic reconditioning of damaged lung grafts before transplantation. This study aimed to develop relevant rat models of lung damage to study EVLP therapeutic reconditioning for possible translational applications.

Methods.

Lungs from 31 rats were exposed to cold ischemia (CI) or warm ischemia (WI), inflated at various oxygen fractions (FiO₂), followed by 3 h EVLP. Five groups were studied as follow: (1) C21 (control): 3 h CI (FiO₂ 0.21); (2) C50: 3 h CI (FiO₂ 0.5); (3) W21: 1 h WI, followed by 2 h CI (FiO₂ 0.21); (4) W50: 1 h WI, followed by 2 h CI (FiO₂ 0.5); and (5) W2h: 2 h WI, followed by 1 h CI (FiO₂ 0.21). Following 3 h EVLP, we measured static pulmonary compliance (SPC), pulmonary vascular resistance, lung weight gain (edema), oxygenation capacity (differential partial pressure of oxygen), and protein carbonyls in lung tissue (oxidative stress), as well as lactate dehydrogenase (LDH, lung injury), nitrotyrosine (nitro-oxidative stress), interleukin-6 (IL-6, inflammation), and proteins (permeability edema) in bronchoalveolar lavage (BAL). Perivascular edema was quantified by histology.

Results.

No significant alterations were noted in C21 and C50 groups. W21 and W50 groups had reduced SPC and disclosed increased weight gain, BAL proteins, nitrotyrosine, and LDH. These changes were more severe in the W50 group, which also displayed greater oxidative stress. In contrast, both W21 and W50 showed comparable perivascular edema and BAL IL-6. In comparison with the other WI groups, W2h showed major weight gain, perivascular edema, SPC reduction, drop of differential partial pressure of oxygen, and massive increases of BAL LDH and proteins but comparable increase of IL-6 and biomarkers of oxidative stress.

Conclusions.

These models of lung damage of increasing severity might be helpful to evaluate new strategies for EVLP therapeutic reconditioning. A model combining 1 h WI and inflation at FiO₂ of 0.5 seems best suited for this purpose by reproducing major alterations of clinical lung ischemia-reperfusion injury.

Hyperoxia during extracorporeal cardiopulmonary resuscitation for refractory cardiac arrest is associated with severe circulatory failure and increased mortality

Background

High levels of arterial oxygen pressures (PaO₂) have been associated with increased mortality in extracorporeal cardiopulmonary resuscitation (ECPR), but there is limited information regarding possible mechanisms linking hyperoxia and death in this setting, notably with respect to its hemodynamic consequences. We aimed therefore at evaluating a possible association between PaO₂, circulatory failure and death during ECPR.

Methods

We retrospectively analyzed 44 consecutive cardiac arrest (CA) patients treated with ECPR to determine the association between the mean PaO₂ over the first 24 h, arterial blood pressure, vasopressor and intravenous fluid therapies, mortality, and cause of deaths.

Results

Eleven patients (25%) survived to hospital discharge. The main causes of death were refractory circulatory shock (46%) and neurological damage (24%). Compared to survivors, non survivors had significantly higher mean 24 h PaO₂ (306 ± 121 mmHg vs 164 ± 53 mmHg, p < 0.001), lower mean blood pressure and higher requirements in vasopressors and fluids, but displayed similar pulse pressure during the first 24 h (an index of native cardiac recovery). The mean 24 h PaO₂ was significantly and positively correlated with the severity of hypotension and the intensity of vasoactive therapies. Patients dying from circulatory failure died after a median of 17 h, compared to a median of 58 h for patients dying from a neurological cause. Patients dying from neurological cause had better preserved blood pressure and lower vasopressor requirements.

Conclusion

In conclusion, hyperoxia is associated with increased mortality during ECPR, possibly by promoting circulatory collapse or delayed neurological damage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12872-021-02361-3.

Changes to both cardiac metabolism and performance accompany acute reductions in functional capillary supply

BACKGROUND

The relative importance of arteriole supply or ability to switch between substrates to preserve cardiac performance is currently unclear, but may be critically important in conditions such as diabetes.

METHODS

Metabolism of substrates was measured before and after infusion of polystyrene microspheres in the perfused working heart to mimic random capillary loss due to microvascular disease. The effect of acute loss of functional capillary supply on palmitate and glucose metabolism together with function was quantified, and theoretical tissue oxygen distribution calculated from histological samples and ventricular VO(2) estimated.

RESULTS

Microsphere infusion led to a dose-dependent decrease in rate-pressure product (RPP) and oxygen consumption (P<0.001). Microsphere infusion also increased work/unit oxygen consumption of hearts ('efficiency') by 25% (P<0.01). When corrected for cardiac work palmitate oxidation remained tightly coupled to very low workloads (RPP<2500 mmHg/min), illustrating a high degree of metabolic control. Arteriole occlusion by microspheres decreased the density of patent capillaries (P<0.001) and correspondingly increased the average capillary supply area by 40% (P<0.01). Calculated rates of oxygen consumption declined from 16.6±7.2 ml/100 ml/min to 12.4±9 ml/100 ml/min following arteriole occlusion, coupled with increases in size of regions of myocardial hypoxia (Control=22.0% vs. Microspheres=42.2%).

CONCLUSIONS

Cardiac mechanical performance is very sensitive to arteriolar blockade, but metabolite switching from fatty acid to glucose utilisation may also support cardiac function in regions of declining PO(2).

GENERAL SIGNIFICANCE

Preserving functional capillary supply may be critical for maintenance of cardiac function when metabolic flexibility is lost, as in diabetes.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

The difference in myocardial injuries and mitochondrial damages between asphyxial and ventricular fibrillation cardiac arrests

INTRODUCTION

Ventricular fibrillation (VF) and asphyxia account for most cardiac arrests but differ in cardiac arrest course, neurologic deficit, and myocardial damage. In VF resuscitation, cardiac mitochondria were known to be damaged via excess generation of reactive oxygen species. This study evaluated the difference of cardiac mitochondrial damages between VF and asphyxial cardiac arrests.

METHODS

In the VF + electrical shock (ES) group, VF was induced and untreated for 5 minutes, followed by 1 minute of cardiopulmonary resuscitation (CPR) and 1 ES of 5 J. Animals were killed immediately after ES. In the asphyxia group, cardiac arrest was induced by airway obstruction, and then pulselessness was maintained for 5 minutes, followed by 1 minute of CPR. The animals were killed immediately after CPR. The histology and ultrastructural changes of myocardium and complex activities and respiration of mitochondria were evaluated. The mitochondrial permeability transition pore opening was measured based on mitochondrial swelling rate.

RESULTS

The histopathologic examinations showed myocardial necrosis and mitochondrial damage in both cardiac arrests. Instead of regional damages of myocardium in the VF + ES group, the myocardial injury in the asphyxia group distributed diffusely. The asphyxia group demonstrated more severe mitochondrial damage than the VF + ES group, which had a faster mitochondrial swelling rate, more decreased cytochrome c oxidase activity, and more impaired respiration.

CONCLUSIONS

Both VF and asphyxial cardiac arrests caused myocardial injuries and mitochondrial damages. Asphyxial cardiac arrest presented more diffuse myocardial injuries and more severe mitochondrial damages than VF cardiac arrest.

Ascorbic acid mitigates the myocardial injury after cardiac arrest and electrical shock

PURPOSE

To examine the effects of ascorbic acid (AA) administrated during cardiopulmonary resuscitation (CPR) on the myocardial injury in a rat model of ventricular fibrillation (VF) and electrical shock (ES).

METHODS

VF was induced in male Wistar rats and left untreated for 5 min, followed by 1 min of CPR, and then one ES of 5 J. At the start of CPR, animals received either intravenous administration of AA (100 mg/kg) or Tempol (30 mg/kg), two antioxidants, or 0.9% saline (VF + ES group). After ES, animals were immediately killed. Myocardial lipoxidation was determined by malondialdehyde (MDA) assay. The histology and ultrastructural changes of myocardium were also evaluated. The mitochondrial permeability transition pore (mPTP) opening was measured based on the mitochondrial swelling rate. The complex activities and respiration of mitochondria were assessed, too.

RESULTS

Increased myocardial injury and mitochondrial damage in the VF + ES group were noted. AA and Tempol alleviated such damages. Both AA and Tempol improved accelerated mitochondrial swelling; decreased complex activities and respiratory dysfunction occurred in the VF + ES group. The animals receiving AA and Tempol during CPR had better successful resuscitation rates and 72-h survival than the VF + ES group.

CONCLUSIONS

Intravenous administration of AA and Tempol at the start of CPR may reduce lipid peroxidation and myocardial necrosis, diminish mitochondrial damage, facilitate resuscitation, and improve outcomes after VF + ES.