Glucose is an Ineffective Substrate for Preservation of Machine Perfused Donor Hearts

Pushing the boundaries of innovation: the potential of ex vivo organ perfusion from an interdisciplinary point of view

Ex vivo machine perfusion (EVMP) is an emerging technique for preserving explanted solid organs with primary application in allogeneic organ transplantation. EVMP has been established as an alternative to the standard of care static-cold preservation, allowing for prolonged preservation and real-time monitoring of organ quality while reducing/preventing ischemia-reperfusion injury. Moreover, it has paved the way to involve expanded criteria donors, e.g., after circulatory death, thus expanding the donor organ pool. Ongoing improvements in EVMP protocols, especially expanding the duration of preservation, paved the way for its broader application, in particular for reconditioning and modification of diseased organs and tumor and infection therapies and regenerative approaches. Moreover, implementing EVMP for in vivo-like preclinical studies improving disease modeling raises significant interest, while providing an ideal interface for bioengineering and genetic manipulation. These approaches can be applied not only in an allogeneic and xenogeneic transplant setting but also in an autologous setting, where patients can be on temporary organ support while the diseased organs are treated ex vivo, followed by reimplantation of the cured organ. This review provides a comprehensive overview of the differences and similarities in abdominal (kidney and liver) and thoracic (lung and heart) EVMP, focusing on the organ-specific components and preservation techniques, specifically on the composition of perfusion solutions and their supplements and perfusion temperatures and flow conditions. Novel treatment opportunities beyond organ transplantation and limitations of abdominal and thoracic EVMP are delineated to identify complementary interdisciplinary approaches for the application and development of this technique.

Isolated murine skeletal muscles utilize pyruvate over glucose for oxidation

INTRODUCTION

Fuel sources for skeletal muscle tissue include carbohydrates and fatty acids, and utilization depends upon fiber type, workload, and substrate availability. The use of isotopically labeled substrate tracers combined with nuclear magnetic resonance (NMR) enables a deeper examination of not only utilization of substrates by a given tissue, but also their contribution to tricarboxylic acid (TCA) cycle intermediates.

OBJECTIVES

The goal of this study was to determine the differential utilization of substrates in isolated murine skeletal muscle, and to evaluate how isopotomer anlaysis provided insight into skeletal muscle metabolism.

METHODS

Isolated C57BL/6 mouse hind limb muscles were incubated in oxygenated solution containing uniformly labeled 13C6 glucose, 13C3 pyruvate, or 13C2 acetate at room temperature. Isotopomer analysis of 13C labeled glutamate was performed on pooled extracts of isolated soleus and extensor digitorum longus (EDL) muscles.

RESULTS

Pyruvate and acetate were more avidly consumed than glucose with resultant increases in glutamate labeling in both muscle groups. Glucose incubation resulted in glutamate labeling, but with high anaplerotic flux in contrast to the labeling by pyruvate. Muscle fiber type distinctions were evident by differences in lactate enrichment and extent of substrate oxidation.

CONCLUSION

Isotope tracing experiments in isolated muscles reveal that pyruvate and acetate are avidly oxidized by isolated soleus and EDL muscles, whereas glucose labeling of glutamate is accompanied by high anaplerotic flux. We believe our results may set the stage for future examination of metabolic signatures of skeletal muscles from pre-clinical models of aging, type-2 diabetes and neuromuscular disease.

The Position of the Heart During Normothermic Ex Situ Heart Perfusion is an Important Factor in Preservation and Recovery of Myocardial Function

Ex situ heart perfusion (ESHP) is being investigated as a method for the continuous preservation of the myocardium in a semiphysiologic state for subsequent transplantation. Most methods of ESHP position the isolated heart in a hanging (H) state, representing a considerable departure from the in vivo anatomical positioning of the heart and may negatively affect the functional preservation of the heart. In the current study, cardiac functional and metabolic parameters were assessed in healthy pig hearts, perfused for 12 hours, in either an H, or supported (S) position, either in nonworking mode (NWM) or working mode (WM). The cardiac function was best preserved in the S position hearts in WM (median 11 hour cardiac index (CI)/1 hour CI%: working mode perfusion in supported position = 94.77% versus nonworking mode perfusion in supported position = 62.80%, working mode perfusion in H position = 36.18%, nonworking mode perfusion in H position = 9.75%; p < 0.001). Delivery of pyruvate bolus significantly improved the function in S groups, however, only partially reversed myocardial dysfunction in the H heart groups. The hearts perfused ex situ in a semianatomical S position and in physiologic WM had better functional preservation and recovery than the H hearts in non-S position. Optimizing the positional support for the ex situ-perfused hearts may improve myocardial preservation during ESHP.

Prolonged (≥24 Hours) Normothermic (≥32 °C) Ex Vivo Organ Perfusion: Lessons From the Literature

For 2 centuries, researchers have studied ex vivo perfusion intending to preserve the physiologic function of isolated organs. If it were indeed possible to maintain ex vivo organ viability for days, transplantation could become an elective operation with clinicians methodically surveilling and reconditioning allografts before surgery. To this day, experimental reports of successfully prolonged (≥24 hours) organ perfusion are rare and have not translated into clinical practice. To identify the crucial factors necessary for successful perfusion, this review summarizes the history of prolonged normothermic ex vivo organ perfusion. By examining successful techniques and protocols used, this review outlines the essential elements of successful perfusion, limitations of current perfusion systems, and areas where further research in preservation science is required.

Sodium Pyruvate Ameliorates Influenza a Virus Infection In Vivo

Influenza A virus (IAV) causes seasonal epidemics annually and pandemics every few decades. Most antiviral treatments used for IAV are only effective if administered during the first 48 h of infection and antiviral resistance is possible. Therapies that can be initiated later during IAV infection and that are less likely to elicit resistance will significantly improve treatment options. Pyruvate, a key metabolite, and an end product of glycolysis, has been studied for many uses, including its anti-inflammatory capabilities. Sodium pyruvate was recently shown by us to decrease inflammasome activation during IAV infection. Here, we investigated sodium pyruvate’s effects on IAV in vivo. We found that nebulizing mice with sodium pyruvate decreased morbidity and weight loss during infection. Additionally, treated mice consumed more chow during infection, indicating improved symptoms. There were notable improvements in pro-inflammatory cytokine production (IL-1β) and lower virus titers on day 7 post-infection in mice treated with sodium pyruvate compared to control animals. As pyruvate acts on the host immune response and metabolic pathways and not directly on the virus, our data demonstrate that sodium pyruvate is a promising treatment option that is safe, effective, and unlikely to elicit antiviral resistance.

Myocardial Substrate Oxidation and Tricarboxylic Acid Cycle Intermediates During Hypothermic Machine Perfusion

BACKGROUND

The optimal substrate for hypothermic machine perfusion preservation of donor hearts is unknown. Fatty acids, acetate, and ketones are preferred substrates of the heart during normothermic perfusion, but cannot replete the tricarboxylic acid (TCA) cycle directly. Propionate, an anaplerotic substrate, can replenish TCA cycle intermediates and may affect cardiac metabolism. The purpose of this study was to determine myocardial substrate preferences during hypothermic machine perfusion and to assess if an anaplerotic substrate was required to maintain the TCA cycle intermediate pool in perfused hearts.

METHODS

Groups of rat hearts were perfused with carbon-13 (¹³C)-labeled substrates (acetate, β-hydroxybutyrate, octanoate, with and without propionate) at low and high concentrations. TCA cycle intermediate concentrations, substrate selection, and TCA cycle flux were determined by gas chromatography/mass spectroscopy and ¹³C magnetic resonance spectroscopy.

RESULTS

Acetate and octanoate were preferentially oxidized, whereas β-hydroxybutyrate was a minor substrate. TCA cycle intermediate concentrations except fumarate were higher in substrate-containing perfusion groups compared with either the no-substrate perfusion group or the no-ischemia control group.

CONCLUSIONS

The presence of an exogenous, oxidizable substrate is required to support metabolism in the cold perfused heart. An anaplerotic substrate is not essential to maintain the TCA cycle intermediate pool and support oxidative metabolism under these conditions.

The Role of Metabolomics in Current Concepts of Organ Preservation

In solid organ transplantation (Tx), both survival rates and quality of life have improved dramatically over the last few decades. Each year, the number of people on the wait list continues to increase, widening the gap between organ supply and demand. Therefore, the use of extended criteria donor grafts is growing, despite higher susceptibility to ischemia-reperfusion injury (IRI) and consecutive inferior Tx outcomes. Thus, tools to characterize organ quality prior to Tx are crucial components for Tx success. Innovative techniques of metabolic profiling revealed key pathways and mechanisms involved in IRI occurring during organ preservation. Although large-scale trials are needed, metabolomics appears to be a promising tool to characterize potential biomarkers, for the assessment of graft quality before Tx and evaluate graft-related outcomes. In this comprehensive review, we summarize the currently available literature on the use of metabolomics in solid organ Tx, with a special focus on metabolic profiling during graft preservation to assess organ quality prior to Tx.

Myocardial Functional Decline During Prolonged Ex Situ Heart Perfusion

BACKGROUND

Myocardial function declines in a time-dependent fashion during ex situ heart perfusion. Cell death and metabolic alterations may contribute to this phenomenon, limiting the safe perfusion period and the potential of ex situ heart perfusion to expand the donor pool. Our aim was to investigate the etiology of myocardial functional decline in ex situ perfused hearts.

METHODS

Cardiac function, apoptosis, effectors and markers of cell death, and metabolic function were assessed in healthy pig hearts perfused for 12 hours. These hearts were perfused in nonworking mode or working mode.

RESULTS

Cardiac function declined during ex situ heart perfusion regardless of perfusion mode but was significantly better preserved in the hearts perfused in working mode (11-hour cardiac index/1-hour cardiac index: working mode, 33%; nonworking mode, 10%; p = 0.025). The rate of apoptosis was higher in the ex situ perfused hearts compared with in vivo samples (apoptotic cells: in vivo, 0.13%; working mode, 0.54%; nonworking mode, 0.88%; p < 0.001), but the absolute values were low and out of proportion to the decline in function in either group. Myocardial dysfunction at the end of the perfusion interval was partially rescued by delivery of a pyruvate bolus.

CONCLUSIONS

A significant decline in myocardial function occurs over time in hearts preserved ex situ that is out of proportion to the magnitude of myocyte cell death present in dysfunctional hearts. Alterations in myocardial substrate utilization during prolonged ex situ heart perfusion may contribute to this phenomenon and represent an avenue to improve donor heart preservation.

Transplantation of Hearts Donated after Circulatory Death

Cardiac transplantation has become limited by a critical shortage of suitable organs from brain-dead donors. Reports describing the successful clinical transplantation of hearts donated after circulatory death (DCD) have recently emerged. Hearts from DCD donors suffer significant ischemic injury prior to organ procurement; therefore, the traditional approach to the transplantation of hearts from brain-dead donors is not applicable to the DCD context. Advances in our understanding of ischemic post-conditioning have facilitated the development of DCD heart resuscitation strategies that can be used to minimize ischemia-reperfusion injury at the time of organ procurement. The availability of a clinically approved ex situ heart perfusion device now allows DCD heart preservation in a normothermic beating state and minimizes exposure to incremental cold ischemia. This technology also facilitates assessments of organ viability to be undertaken prior to transplantation, thereby minimizing the risk of primary graft dysfunction. The application of a tailored approach to DCD heart transplantation that focuses on organ resuscitation at the time of procurement, ex situ preservation, and pre-transplant assessments of organ viability has facilitated the successful clinical application of DCD heart transplantation. The transplantation of hearts from DCD donors is now a clinical reality. Investigating ways to optimize the resuscitation, preservation, evaluation, and long-term outcomes is vital to ensure a broader application of DCD heart transplantation in the future.

Effect of Shen-Fu Injection Pretreatment to Myocardial Metabolism During Untreated Ventricular Fibrillation in a Porcine Model

Background:

Shen-Fu injection (SFI) can attenuate ischemia-reperfusion injury, protect cardiac function, and improve microcirculation during cardiopulmonary resuscitation. We hypothesized that SFI may also have an influence on myocardial metabolism during ventricular fibrillation (VF). In this study, we used SFI pretreatment prior to VF to discuss the changes of myocardial metabolism and catecholamine (CA) levels during untreated VF, trying to provide new evidence to the protection of SFI to myocardium.

Methods:

Twenty-four pigs were divided into three groups: Saline group (SA group), SFI group, and SHAM operation group (SHAM group). Thirty minutes prior to the induction of VF, the SFI group received 0.24 mg/ml SFI through an intravenous injection; the SA group received an equal amount of sodium chloride solution. The interstitial fluid from the left ventricle (LV) wall was collected through the microdialysis tubes during VF. Adenosine diphosphate (ADP), adenosine triphosphate (ATP), and Na⁺ -K⁺ -ATPase and Ca2⁺ -ATPase enzyme activities were measured after untreated VF. Peak-to-trough VF amplitude and median frequency were analyzed for each of these 5-s intervals.

Results:

The levels of glucose and glutamate were lower after VF in both the SA and SFI groups, compared with baseline, and the levels in the SFI group were higher than those in the SA group. Compared with baseline, the levels of lactate and the lactate/pyruvate ratio increased after VF in both SA and SFI groups, and the levels in the SFI group were lower than those in the SA group. In both the SA and SFI groups, the levels of dopamine, norepinephrine, and epinephrine increased significantly. There were no statistical differences between the two groups. The content of ATP, ADP, and phosphocreatine in the SFI group was higher than those in the SA group. The activity of LV Na⁺ -K⁺ -ATPase was significantly higher in the SFI group than in the SA group. Amplitude mean spectrum area (AMSA) was significantly lower in the SA and SFI groups at 8- and 12-min compared with 4-min. The AMSA in the SFI group was higher than that in the SA group at each time point during untreated VF.

Conclusions:

SFI pretreatment can improve myocardial metabolism and reduce energy exhaustion during VF, and it does not aggravate the excessive secretion of endogenous CAs.

Functional evaluation of rat hearts transplanted after preservation in a high-pressure gaseous mixture of carbon monoxide and oxygen

We recently succeeded in resuscitating an extracted rat heart following 24–48 hours of preservation in a high-pressure gaseous mixture of carbon monoxide (CO) and oxygen (O₂). This study aimed to examine the function of rat hearts transplanted after being preserved in the high-pressure CO and O₂ gas mixture. The hearts of donor rats were preserved in a chamber filled with CO and O₂ under high pressure for 24 h (CO24h) or 48 h at 4 °C. For the positive control (PC) group, hearts immediately extracted from donor rats were used for transplantation. The preserved hearts were transplanted into recipient rats by heterotopic cervical heart transplantation. CO toxicity does not affect the grafts or the recipients. Light microscopy and [¹⁸F]-fluorodeoxyglucose positron emission tomography revealed that there were no significant differences in the size of the myocardial infarction or apoptosis of myocardial cells in post-transplant hearts between the PC and CO24h groups. Furthermore, at 100 days after the transplantation, the heart rate, weight and histological staining of the post-transplanted hearts did not differ significantly between the PC and CO24h groups. These results indicate that the function of rat hearts is well preserved after 24 hours of high-pressure preservation in a CO and O₂ gas mixture. Therefore, high-pressure preservation in a gas mixture can be a useful method for organ preservation.

Protection of rat cardiac myocytes by fructose-1,6-bisphosphate and 2,3-butanedione

Earlier studies by our group showed that fructose-1,6-bisphosphate (FBP) enhances the hypothermic preservation of rat cardiac myocytes and the functional recovery of animal hearts after hypothermic storage. However, the mechanisms involved were not clear. We extended the cardiomyocyte studies by testing whether the FBP effects were due to chelation of extracellular calcium, leading to lower intracellular levels. We also tested effects of 2,3-butanedione monoxime (BDM), pyruvate, and adenine nucleotide precursors. Cardiomyocytes were incubated in ischemic suspension at 3°C, and aliquots examined over 48 to 72 hours for retention of rod-shaped morphology, a measure of viability. Cytosolic Ca²⁺ levels were measured in some experiments. FBP at 5 mM reduced the death rate even when added after one or two days of incubation. It caused cytosolic calcium levels that were 33% lower than controls in freshly-isolated cells and 70% lower after one day of incubation. EGTA protected against cell death similarly to FBP. These results indicated that one of the mechanisms by which FBP exerts protective effects is through chelation of extracellular calcium. BDM was strongly protective and reduced cytosolic calcium by 30% after one day of incubation. As with FBP, BDM was effective when added after one or two days of incubation. BDM may be useful in combination with FBP in preserving heart tissue. Pyruvate, adenine, and ribose provided little or no protection during hypothermia.