Mitochondria as Therapeutic Targets in Transplantation
Authors: Kourosh Saeb-Parsy, Jack L. Martin, Dominic M. Summers, Christopher J.E. Watson, Thomas Krieg, Michael P. Murphy
Journal: Trends in Molecular Medicine (2020)

The mitochondrial and metabolic changes that occur during transplantation have been neglected, compared with technical and immunological aspects. Knowledge of how these factors contribute to graft function may improve outcomes for patients.Of particular interest is how ischemic metabolism depletes adenine nucleotides and leads to an accumulation of NADH and succinate. These processes are greatly enhanced by temperature, emphasizing the importance of rapid cooling of organs during ischemia.The metabolic changes during ischemia prime the organ for damaging ischemia–reperfusion injury upon reperfusion. This leads to the release of damage-associated molecular patterns (DAMPs), such as mitochondrial DNA, that initiate an innate immune response, which can contribute to organ rejection. Advances in surgical procedures, technology, and immune suppression have transformed organ transplantation. However, the metabolic changes that occur during organ retrieval, storage, and implantation have been relatively neglected since the developments many decades ago of cold storage organ preservation solutions. In this review we discuss how the metabolic changes that occur within the organ during transplantation, particularly those associated with mitochondria, may contribute to the outcome. We show how a better understanding of these processes can lead to changes in surgical practice and the development of new drug classes to improve the function and longevity of transplanted grafts, while increasing the pool of organs available for transplantation. Advances in surgical procedures, technology, and immune suppression have transformed organ transplantation. However, the metabolic changes that occur during organ retrieval, storage, and implantation have been relatively neglected since the developments many decades ago of cold storage organ preservation solutions. In this review we discuss how the metabolic changes that occur within the organ during transplantation, particularly those associated with mitochondria, may contribute to the outcome. We show how a better understanding of these processes can lead to changes in surgical practice and the development of new drug classes to improve the function and longevity of transplanted grafts, while increasing the pool of organs available for transplantation. Development of organ transplantation has transformed lives [1.Linden P.K. History of solid organ transplantation and organ donation.Crit. Care Clin. 2009; 25: 165-184Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 2.Watson C.J. Dark J.H. Organ transplantation: historical perspective and current practice.Br. J. 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In situ flushing of donor organs with cold University of Wisconsin solution has become the most widely used procedure for organ preservation [1.Linden P.K. History of solid organ transplantation and organ donation.Crit. Care Clin. 2009; 25: 165-184Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar,2.Watson C.J. Dark J.H. Organ transplantation: historical perspective and current practice.Br. J. Anaesth. 2012; 108: i29-i42Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar]. This approach is thought to protect the organ by slowing metabolism through cooling, providing osmotic support that stops cell rupture following disruption of ionic gradients as the ATP:ADP ratio falls, and reducing the ischemia–reperfusion (IR) injury that occurs upon transplantation of the graft (see Glossary) into the recipient [11.Jahania M.S. et al.Heart preservation for transplantation: principles and strategies.Ann. Thorac. 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The precise duration depends on the quality of the organ, with organs from young, healthy donors being better able to tolerate longer periods of cold ischemia. Despite its widespread use and importance for transplantation, our understanding of why cold storage is protective is incomplete, and our understanding of the critical metabolic and mitochondrial changes that occur during organ retrieval, storage, and implantation in the recipient has lagged behind technical and immunological advances.Box 1Current Procedures in Deceased Donor Transplantation (See Figure I)RetrievalDonation after brainstem death (DBD): following the declaration of death by brainstem criteria, the donor continues to receive cardiorespiratory support until the time of donation when in situ cold perfusion and topical cooling is used to rapidly cool organs.Donation after circulatory death (DCD): following the withdrawal of life support death is declared by cardiocirculatory criteria. After a stand-off period, the donor is rapidly cannulated and the organs perfused with cold preservation solution before retrieval. More recently, normothermic regional perfusion (NRP, see later) has been used to minimize the effects of warm ischaemia during DCD.StorageFollowing retrieval, organs are stored for transport to the recipient. A range of techniques have been employed to minimize ischemia and ischemic injury during this period.Hypothermic static preservation: organs are stored in ice-cold preservation solutions that have been designed to provide an extracellular oncotic force to minimize cellular swelling, a buffer to counter intracellular acidosis and electrolytes to maintain ion gradients.Hypothermic machine perfusion: there has been renewed interest in the use of hypothermic machine perfusion, primarily in liver and kidney transplantation. It has been proposed that this has a beneficial effect by removing metabolites form the organs during storage and flushing capillaries. Some investigators have also oxygenated the preservation solution hypothermic oxygenated perfusion (HOPE). This has been shown to have promising outcomes in liver and kidney transplantation.NRP: following cardiocirculatory death, the donor is rapidly cannulated and the organs perfused in situ with oxygenated blood. During this period organs can be assessed, and therapeutic drugs or measures to improve organ function can be administered. The organs are then retrieved using a similar technique to DBD retrieval. Cardiothoracic and abdominal organs (with the exception of intestinal transplants) have been transplanted successfully using this technique.Ex situ normothermic oxygenated machine perfusion (NMP): this technique has also received widespread interest more recently with considerable investment into the development of technology for a range of organs to enable ex situ perfusion of the heart, lungs, liver, and kidneys. There is interest in the use of these technologies for a range of clinical situations including prolonging the period of time organs can be maintained ex situ to facilitate the transplant operation, assessment of organs prior to transplantation, reconditioning of organs, and as a potential platform for the delivery of agents. NMP can be used immediately following retrieval or following a period of hypothermic static preservation. Retrieval Donation after brainstem death (DBD): following the declaration of death by brainstem criteria, the donor continues to receive cardiorespiratory support until the time of donation when in situ cold perfusion and topical cooling is used to rapidly cool organs. Donation after circulatory death (DCD): following the withdrawal of life support death is declared by cardiocirculatory criteria. After a stand-off period, the donor is rapidly cannulated and the organs perfused with cold preservation solution before retrieval. More recently, normothermic regional perfusion (NRP, see later) has been used to minimize the effects of warm ischaemia during DCD. Storage Following retrieval, organs are stored for transport to the recipient. A range of techniques have been employed to minimize ischemia and ischemic injury during this period. Hypothermic static preservation: organs are stored in ice-cold preservation solutions that have been designed to provide an extracellular oncotic force to minimize cellular swelling, a buffer to counter intracellular acidosis and electrolytes to maintain ion gradients. Hypothermic machine perfusion: there has been renewed interest in the use of hypothermic machine perfusion, primarily in liver and kidney transplantation. It has been proposed that this has a beneficial effect by removing metabolites form the organs during storage and flushing capillaries. Some investigators have also oxygenated the preservation solution hypothermic oxygenated perfusion (HOPE). This has been shown to have promising outcomes in liver and kidney transplantation. NRP: following cardiocirculatory death, the donor is rapidly cannulated and the organs perfused in situ with oxygenated blood. During this period organs can be assessed, and therapeutic drugs or measures to improve organ function can be administered. The organs are then retrieved using a similar technique to DBD retrieval. Cardiothoracic and abdominal organs (with the exception of intestinal transplants) have been transplanted successfully using this technique. Ex situ normothermic oxygenated machine perfusion (NMP): this technique has also received widespread interest more recently with considerable investment into the development of technology for a range of organs to enable ex situ perfusion of the heart, lungs, liver, and kidneys. There is interest in the use of these technologies for a range of clinical situations including prolonging the period of time organs can be maintained ex situ to facilitate the transplant operation, assessment of organs prior to transplantation, reconditioning of organs, and as a potential platform for the delivery of agents. NMP can be used immediately following retrieval or following a period of hypothermic static preservation. Recently there has been greater understanding of how mitochondrial metabolism contributes to cell damage in general [15.Suomalainen A. Battersby B.J. Mitochondrial diseases: the contribution of organelle stress responses to pathology.Nat. Rev. Mol. Cell Biol. 2017; 19: 77-92Crossref PubMed Scopus (124) Google Scholar, 16.Nunnari J. Suomalainen A. Mitochondria: in sickness and in health.Cell. 2012; 148: 1145-1159Abstract Full Text Full Text PDF PubMed Scopus (1294) Google Scholar, 17.Wallace D.C. et al.Mitochondrial energetics and therapeutics.Annu. Rev. Pathol. 2010; 5: 297-348Crossref PubMed Scopus (432) Google Scholar, 18.Friedman J.R. Nunnari J. Mitochondrial form and function.Nature. 2014; 505: 335-343Crossref PubMed Scopus (651) Google Scholar, 19.Murphy M.P. Hartley R.C. Mitochondria as a therapeutic target for common pathologies.Nat. Rev. Drug Discov. 2018; 17: 865-886Crossref PubMed Scopus (137) Google Scholar], and to poor outcome in transplantation in particular [20.Dare A.J. et al.The mitochondria-targeted anti-oxidant MitoQ decreases ischemia-reperfusion injury in a murine syngeneic heart transplant model.J. Heart Lung Transplant. 2015; 34: 1471-1480Abstract Full Text Full Text PDF PubMed Scopus (50) Google Scholar, 21.Lin L. et al.Circulating mitochondria in organ donors promote allograft rejection.Am. J. Transplant. 2019; 19: 1917-1929Crossref PubMed Scopus (6) Google Scholar, 22.Martin J.L. et al.Succinate accumulation drives ischemia-reperfusion injury during organ transplantation.Nat. Metab. 2019; 1: 966-974Crossref PubMed Scopus (11) Google Scholar, 23.Martin J.L. et al.Mitochondrial mechanisms and therapeutics in ischaemia reperfusion injury.Ped. Nephrol. 2019; 34: 1167-1174Crossref PubMed Scopus (0) Google Scholar, 24.Kim K. et al.Clinical relevance of cell-free mitochondrial DNA during the early postoperative period in kidney transplant recipients.Sci. Reports. 2019; 9: 18607PubMed Google Scholar]. We discuss the mitochondrial and metabolic changes that contribute to tissue damage during transplantation and consider how to target these pathways therapeutically. The metabolic factors that impact on transplantation arise mainly because the organs to be transplanted are ischemic, often for long periods of time. In this review we focus on the general properties of solid organs; however, it is important to note that there are significant differences in ischemic metabolism between different organs. A clear exception is the lung, which is inflated with oxygen before storage, enabling aerobic metabolism in the alveolar epithelium to continue during storage [25.Yeung J.C. Keshavjee S. Overview of clinical lung transplantation.Cold Spring Harb. Perspec. Med. 2014; 4a015628Crossref PubMed Scopus (0) Google Scholar]. Unlike other organs, circulating blood removes oxygen from the lungs as opposed to supplying it to other organs, and the absence of circulation is not deleterious to an oxygenated lung. The heart is also a special case, where storage is preceded by the administration of cold cardioplegia, the hyperkalaemic composition of which ensures rapid cessation of contractile activity and thus energy consumption in addition to the metabolic effects of cooling [26.Conti V.R. et al.Cold cardioplegia versus hypothermia for myocardial protection. Randomised clinical study.J. Thorac. Cardiovasc. Surg. 1978; 76: 577-589Abstract Full Text PDF PubMed Google Scholar,27.Follette D. et al.Prolonged safe aortic clamping by combining membrane stabilization, multidose cardioplegia, and appropriate pH reperfusion.J. Thorac. Cardiovasc. Surg. 1977; 74: 682-694Abstract Full Text PDF PubMed Google Scholar]. Much of our knowledge of ischemic metabolism in the heart comes from studies on hearts held temporarily ischemic by cardioplegia during cardiac surgery. During ischemia the lack of ATP production by mitochondrial oxidative phosphorylation leads to a gradual decrease in the ATP:ADP ratio which eventually causes cell death [22.Martin J.L. et al.Succinate accumulation drives ischemia-reperfusion injury during organ transplantation.Nat. Metab. 2019; 1: 966-974Crossref PubMed Scopus (11) Google Scholar,28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar]. Then, upon implantation and reperfusion of the organ in the recipient, mitochondria contribute to IR injury to the organ similar to that which occurs during myocardial infarction and stroke [22.Martin J.L. et al.Succinate accumulation drives ischemia-reperfusion injury during organ transplantation.Nat. Metab. 2019; 1: 966-974Crossref PubMed Scopus (11) Google Scholar]. Mitochondria are central to IR injury, generating damaging reactive oxygen species (ROS) upon reperfusion, which cause tissue damage [29.Chouchani E.T. et al.A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury.Cell Metab. 2016; 23: 254-263Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar,30.Chouchani E.T. et al.Ischaemic accumulation of succinate controls reperfusion injury through mitochondrial ROS.Nature. 2014; 515: 431-435Crossref PubMed Scopus (951) Google Scholar]. Furthermore, the damage to mitochondria upon reperfusion releases damage-associated molecular patterns (DAMPs), which activate the innate immune response and enhance inflammatory damage to the tissue, and contribute to organ dysfunction and rejection [21.Lin L. et al.Circulating mitochondria in organ donors promote allograft rejection.Am. J. Transplant. 2019; 19: 1917-1929Crossref PubMed Scopus (6) Google Scholar,31.Nagakawa K. et al.Elevated plasma levels of mitochondria-derived Damage-Associated Molecular Patterns during liver transplantation: predictors for postoperative multi-organ dysfunction syndrome.Tohoku J. Exp. Med. 2020; 250: 87-93Crossref PubMed Scopus (1) Google Scholar,32.Hu Q. et al.Mitochondrial Damage-Associated Molecular Patterns (MTDs) are released during hepatic ischemia reperfusion and induce inflammatory responses.PLoS One. 2015; 10e0140105Crossref PubMed Scopus (32) Google Scholar]. Here, we consider the mitochondrial and metabolic changes during transplantation that contribute to graft damage, with a focus on the role of IR injury upon implantation. During conventional organ transplantation, from living donors or from donation after brainstem death (DBD) or donation after circulatory death (DCD) donors, periods of ischemia are inevitable (Box 1). The lack of oxygen during ischemia means there is no ATP production by oxidative phosphorylation, thus the organs cannot rely on the metabolism of fat, ketone bodies, or amino acids to maintain the supply of ATP. Instead, during storage anaerobic glycolysis is essential to maintain a sufficient ATP:ADP ratio to keep cells alive (Figure 1, bottom left, Key Figure). During ischemia there is still continual consumption of ATP for essential basic cell functions, such as maintaining ion gradients. In addition, in the absence of oxygen the mitochondrial proton motive force will no longer be sustained through proton pumping by the respiratory chain; instead the F0F1-ATP synthase will act in reverse, hydrolyzing ATP generated by glycolysis to pump protons across the mitochondrial inner membrane [28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar,33.Grover G.J. et al.Excessive ATP hydrolysis in ischemic myocardium by mitochondrial F1F0-ATPase: effect of selective pharmacological inhibition of mitochondrial ATPase hydrolase activity.Am. J. Physiol. Heart Circ. Physiol. 2004; 287: H1747-H1755Crossref PubMed Scopus (77) Google Scholar,34.Pham T. et al.Mitochondrial inefficiencies and anoxic ATP hydrolysis capacities in diabetic rat heart.Am. J. Physiol. Cell Physiol. 2014; 307: C499-C507Crossref PubMed Scopus (26) Google Scholar]. Although there is a protein, inhibitory factor-1, that can block this ATP hydrolysis in vitro and is activated at the decreased pH present during ischemia [35.Walker J.E. The ATP synthase: the understood, the uncertain and the unknown.Biochem. Soc. Trans. 2013; 41: 1-16Crossref PubMed Scopus (289) Google Scholar], its role in vivo is not well understood and mitochondrial ATP hydrolysis is still a significant drain on ATP during ischemia [28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar,33.Grover G.J. et al.Excessive ATP hydrolysis in ischemic myocardium by mitochondrial F1F0-ATPase: effect of selective pharmacological inhibition of mitochondrial ATPase hydrolase activity.Am. J. Physiol. Heart Circ. Physiol. 2004; 287: H1747-H1755Crossref PubMed Scopus (77) Google Scholar,34.Pham T. et al.Mitochondrial inefficiencies and anoxic ATP hydrolysis capacities in diabetic rat heart.Am. J. Physiol. Cell Physiol. 2014; 307: C499-C507Crossref PubMed Scopus (26) Google Scholar]. These processes together contribute to the gradual decrease in the ATP:ADP ratio during ischemia, causing a disruption in cell function and eventually cell death [28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar,36.Johnson T.A. et al.Shortage of cellular ATP as a cause of diseases and strategies to enhance ATP.Front. Pharmacol. 2019; 10: 98Crossref PubMed Scopus (19) Google Scholar]. To maintain glycolytic ATP generation, the NADH produced by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) must be recycled to NAD+ (Figure 1, bottom left). This is done by lactate dehydrogenase (LDH), which oxidizes NADH to NAD+, reducing pyruvate to lactate. The lack of blood flow to the organ during ischemia means that the glucose required to drive glycolysis cannot be provided from the circulation and so instead comes from breakdown of its glycogen stores. If the organs cannot continue to utilize glycogen, they can no longer sustain their ATP:ADP ratio. This could in principle occur because of the consumption of all the glycogen stores. However, it often seems to be due to the inability to utilize these stores because of the inhibition of glycolysis at GAPDH caused by the high NADH/NAD+ ratio and the low pH that arises during ischemia [22.Martin J.L. et al.Succinate accumulation drives ischemia-reperfusion injury during organ transplantation.Nat. Metab. 2019; 1: 966-974Crossref PubMed Scopus (11) Google Scholar,28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar]. The generation of lactate from pyruvate thus acts as a safety valve to prevent a build-up of NADH to maintain glycolytic ATP production. Under anaerobic glycolysis, such as in fast twitch muscle or red blood cells, lactate is exported from the cell by monocarboxylate transporters (MCTs) to the circulation which at the same time pump protons out of the cell to counteract cell acidification [37.Halestrap A.P. Wilson M.C. The monocarboxylate transporter family–role and regulation.IUBMB Life. 2012; 64: 109-119Crossref PubMed Scopus (333) Google Scholar]. The situation during ischemia is quite different, because in the absence of blood flow the concentration of lactate in the extracellular fluids rise and the pH drops, blocking efflux of lactate from the cell. Preservation solutions used for organ transplantation can counter the pH fall with phosphate or citrate buffers. However, the importance of lactate efflux from the cell is underappreciated. If lactate builds up in the cell then LDH will no longer be able to oxidize NADH to NAD+ and glycolysis will be blocked at GAPDH (Figure 1, bottom left). Another important metabolic change that occurs during ischemia is the degradation of purine nucleotides (Figure 1, bottom right). The lack of oxidative phosphorylation leads to a persistent lowering of the ATP:ADP ratio and a consequent accumulation of ADP [36.Johnson T.A. et al.Shortage of cellular ATP as a cause of diseases and strategies to enhance ATP.Front. Pharmacol. 2019; 10: 98Crossref PubMed Scopus (19) Google Scholar]. The action of adenylate kinase converts two ADP molecules to one ATP and an AMP, leading to the accumulation of AMP [28.Jennings R.B. Reimer K.A. The cell biology of acute myocardial ischemia.Annu. Rev. Med. 1991; 42: 225-246Crossref PubMed Google Scholar,36.Johnson T.A. et al.Shortage of cellular ATP as a cause of diseases and strategies to enhance ATP.Front. Pharmacol. 2019; 10: 98Crossref PubMed Scopus (19) Google Scholar]. The accumulated AMP is then deaminated by AMP deaminase to inosine monophosphate (IMP) which is in turn dephosphorylated to inosine by a 5′-nucleotidase [36.Johnson T.A. et al.Shortage of cellular ATP as a cause of diseases and strategies to enhance ATP.Front. Pharmacol. 2019; 10: 98Crossref PubMed Scopus (19) Google Scholar,38.Wu J. et al.HIF-1alpha in the heart: remodeling nucleotide metabolism.J. Mol. Cell. Cardiol. 2015; 82: 194-200Abstract Full Text Full Text PDF PubMed Google Scholar]. The inosine is then degraded to hypoxanthine by purine nucleoside phosphorylase. The hypoxanthine can then be further metabolized to xanthine and uric acid by xanthine oxidoreductase, although its variable tissue and species distribution and dependence on oxygen affects its contribution [36.Johnson T.A. et al.Shortage of cellular ATP as a cause of diseases and strategies to enhance ATP.Front. Pharmacol. 2019; 10: 98Crossref PubMed Scopus (19) Google Scholar]. AMP can also be dephosphorylated by a 5′-nucleotidase to adenosine, which can be further deaminated by adenosine deaminase to inosine. Thus, during ischemia the accumulation of AMP leads to its degradation and because of its dynamic equilibrium with ATP and ADP all adenine nucleotides are severely depleted by ischemia. This lack of ADP diminishes the ability of substrate level phosphorylation to generate ATP. An emerging metabolic marker of ischemia is the accumulation of succinate (Figure 1, top) [29.Chouchani E.T. et al.A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury.Cell Metab. 2016; 23: 254-263Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar,30.Chouchani E.T. et al.Ischaemic accumulation of succinate controls reperfusion injury through mitochondrial ROS.Nature. 2014; 515: 431-435Crossref PubMed Scopus (951) Google Scholar]. During aerobic metabolism electrons from many sources are passed on to the coenzyme Q (CoQ) pool within the mitochondrial inner membrane. In the absence of oxygen, the highly reduced CoQ pool can then pass electrons onto fumarate by reversal of succinate dehydrogenase (SDH), generating succinate [29.Chouchani E.T. et al.A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury.Cell Metab. 2016; 23: 254-263Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar,38.Wu J. et al.HIF-1alpha in the heart: remodeling nucleotide metabolism.J. Mol. Cell. Cardiol. 2015; 82: 194-200Abstract Full Text Full Text PDF PubMed Google Scholar]. In addition, glutamine and glutamate breakdown to 2-oxoglutarate and on to succinyl CoA may continue during ischemia leading to the build-up of succinate as the highly reduced CoQ pool will prevent its oxidation to fumarate [39.Zhang J. et al.Accumulation of succinate in cardiac ischemia primarily occurs via canonical Krebs cycle activity.Cell Reps. 2018; 23: 2617-2628Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. The fumarate for succinate generation likely enters mitochondria from the cytosol as malate in exchange for succinate via the dicarboxylate carrier, and within the matrix malate is converted to fumarate by fumarate hydratase [29.Chouchani E.T. et al.A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury.Cell Metab. 2016; 23: 254-263Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar,38.Wu J. et al.HIF-1alpha in the heart: remodeling nucleotide metabolism.J. Mol. Cell. Cardiol. 2015; 82: 194-200Abstract Full Text Full Text PDF PubMed Google Scholar]. The malate in the cytosol likely arises as a consequence of the highly reduced NADH/NAD+ pool that reduces oxaloacetate to malate by malate dehydrogenase. In addition, the accumulation of AMP and IMP during ischemia may activate the purine nucleotide cycle which generates fumarate which will be converted to malate by cytosolic fumarate hydratase [29.Chouchani E.T. et al.A unifying mechanism for mitochondrial superoxide production during ischemia-reperfusion injury.Cell Metab. 2016; 23: 254-263Abstract Full Text Full Text PDF PubMed Scopus (246) Google Scholar,38.Wu J. et al.HIF-1alpha in the heart: remodeling nucleotide metabolism.J. Mol. Cell. Cardiol. 2015; 82: 194-200Abstract Full Text Full Text PDF PubMed Google Scholar,40.Hagberg H. et al.Extracellular adenosine, inosine, hypoxanthine, and xanthine in relation to tissue nucleotides and purines in rat striatum during transient ischemia.J. 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