Reperfusion injury overview

Editors-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editors-In-Chief: Shivam Singla, M.D [2]

Overview

Reperfusion injury, also known as ischemia-reperfusion injury (IRI) or re-oxygenation injury, is the tissue damage which results from the restoration of blood supply to the tissue after a period of ischemia, anoxia or hypoxia from different pathologies. During the period of absence of blood to the tissues a condition is created in which the resulting reperfusion will result in inflammation and oxidative damage through the involvement of various mechanisms mainly involving oxidation, free radical formation and complement activation which ultimately leads to cell death, rather than restoration of normal function.

Various intracellular or extracellular changes during ischemia leads to increased intracellular calcium and ATP depletion that will ultimately land up in the cell death if the ongoing process does not stopped. Reperfusion forms reactive oxygen species . This leads to Increased mitochondrial pore permeability, complement activation & cytochrome release, inflammation and edema formation, Neutrophil platelet adhesion and thrombosis leading to progressive tissue death. In Heart reperfusion injury is attributed to oxidative stress which in turn leads to arrhythmias, Infarction and Myocardial stunning. In case of trauma the resulting restoration of blood flow to the tissue after prolonged ischemia aggravates tissue damage by either directly causing additional injury or by unmasking the injury sustained during the ischemic period. Reperfusion injury can occur in any organ of body mainly seen in the heart, intestine, kidney, lung, and muscle, and is due to microvascular damage

Pathophysiology

Mainly divided into 2 phases

1) Ischemic phase

2) Reperfusion Phase

Ischemic Phase

Reperfusion injury ( Ischemic Phase)

During this phase mainly the dysregulation of metabolic pathways occurs and in the reperfusion phase there will be generation of free radicals.

• Ischemia when the blood supply to the tissues decreases with respect to the demand required to function properly. This results in deficiency in oxygen, glucose and various other substrates required for cellular metabolism. As previously dais the derangement or dysregulation of metabolic function begins in this phase. Due to less oxygen supply cellular metabolism shifts to anaerobic glycolysis causing the glycogen to breakdown resulting in the production of 2 ATP and a lactic acid. This decrease in tissue PH starts further inhibits the ATP generation by negative feed back mechanism. ATP gets broken down into ADP, AMP and IMP. This finally gets converted to adenosine, inosine, hypoxanthine and xanthine.

• Lack of ATP at the cellular level causes impairment in the function of ionic pumps - Na+/K+ and Ca²+ pumps. As a result cytosolic sodium rises which in turn withdraws water to maintain the osmotic equilibrium consequently resulting in the cellular swelling. To maintain ionic balance potassium ion escape from the cell. Calcium is released from the mitochondria to the cytoplasm and into extracellular spaces resulting in the activation of Mitochondrial calcium- dependent cytosolic proteases. These converts the enzyme xanthine dehydrogenase to xanthine oxidase. Phospholipases activated during ischemia promotes membrane degradation and increases level of free fatty acids

• Ischemia also induces expression of a large number of genes and transcription factors, which play a major role in the damage to the tissues.
  • Transcription factors
    • Activating protein-1 (AP-1)
    • Hypoxia-inducible factor-1 (HIF-1) which in turn activates transcription of VEGF, Erythropoietin and Glucose transporter-1
    • Nuclear factor-kappa b (NF-kb)
    • Activation of NF-kb occurs during both the ischemic and reperfusion phases

Reperfusion Phase

• Reperfusion damage occurs after myocardial reperfusion after a period of decreased oxygen supply. The damage from reperfusion injury is partially due to the affected tissue's inflammatory response. White blood cells transported to the region by fresh blood release a host of inflammatory factors such as interleukins and free radicals in response to tissue injury. Blood flow restored reintroduces oxygen inside cells that damages cellular proteins, DNA, and plasma membrane. Damage to the membrane of the cell will in effect cause further free radicals to be released. These reactive species can also act indirectly to turn on apoptosis through redox signaling. Also, leukocytes may build up in small capillaries, block them and cause more ischemia

• Mitochondrial dysfunction plays a significant role in reperfusion injury. Although the mitochondrial membrane is normally impermeable to ions and metabolites, ischemia changes permeability by elevating concentrations of intro-mitochondrial calcium, reducing concentrations of adenine nucleotides, and inducing oxidative stress. It gives primacy to the mitochondrial transfer pore permeability (MPTP), which opens when reperfusion occurs. This contributes to increased osmotic load in the mitochondrial body causing swelling and breakup, releasing proteins that induce apoptosis from mitochondria. Mitochondrial activity is impaired, and ATP is hydrolyzed, allowing degrading enzymes to activate. Finally, excessive activation of the Poly ADP ribose polymerase-1 (PARP-1) impairs the work of other organelles and speeds up the development of reactive oxygen species.

• Hypoxanthine is produced as breakdown product of the ATP metabolism in prolonged ischemia (60 minutes or more). As a consequence of higher oxygen availability the enzyme xanthine dehydrogenase is converted to xanthine oxidase. This oxidation contributes to the conversion of molecular oxygen into highly reactive superoxide and hydroxyl radicals. Xanthine oxidase also produces uric acid, which can act both as a pro-oxidant and as a reactive species scavenger such as per-oxinitrite. To produce the potent reactive species per-oxinitrite, too much nitric oxide formed during reperfusion reacts with superoxide. These radicals and reactive oxygen species attack lipids , proteins, and glycosaminoglycan from the cell membrane, causing further damage. Specific biological processes may also be initiated by redox signaling.

Risk Factors

Risk factors for reperfusion injury include

• Hypertension with left ventricular hypertrophy,
• Congestive heart failure,
• Increased age,
• Diabetes, and
• Hyperlipidemia

Natural History, Complications and Prognosis

Reperfusion injury may be responsible for about 50% of the total infarct size after an acute myocardial infarction as well as myocardial stunning, congestive heart failure and reperfusion arrhythmias such as ventricular arrhythmias.

Medical Therapy

Various proposed medical managements studied are:

• Therapeutic hypothermia

It has been shown in rats that neurons sometimes die completely 24 hours after the blood flow returns. Some claim that this delayed reaction is the result of the multiple inflammatory immune responses that occur during reperfusion. Such inflammatory reactions cause intracranial pressure, a pressure that leads to cell damage and cell death in some cases. Hypothermia has been shown to help reduce intracranial pressure and thus decrease the adverse effects of inflammatory immune responses during reperfusion. Besides that, reperfusion also increases free radical development. Hypothermia has also been shown to decrease the patient's development of deadly free radicals during reperfusion.  

• Hydrogen sulfide treatment

There are several preliminary studies in mice that seem to show that treatment with hydrogen sulfide ( H2S) could have a protective effect against reperfusion injury.

• Cyclosporine

In addition to its well-known immunosuppressive capabilities, the one-time administration of cyclosporine at the time of percutaneous coronary intervention (PCI) has been found to deliver a 40 percent reduction in infarct size in a small group proof of concept study of human patients with reperfusion injury published in The New England Journal of Medicine in 2008.

Cyclosporine has been confirmed in studies to inhibit the actions of cyclophilin D, a protein which is induced by excessive intracellular calcium flow to interact with other pore components and help open the MPT pore. Inhibiting cyclophilin D has been shown to prevent the opening of the MPT pore and protect the mitochondria and cellular energy production from excessive calcium inflows.

Reperfusion leads to biochemical imbalances within the cell that lead to cell death and increased infarct size. More specifically, calcium overload and excessive production of reactive oxygen species in the first few minutes after reperfusion set off a cascade of biochemical changes that result in the opening of the so-called mitochondrial permeability transition pore (MPT pore) in the mitochondrial membrane of cardiac cells.

The opening of the MPT pore leads to the inrush of water into the mitochondria, resulting in mitochondrial dysfunction and collapse. Upon collapse, the calcium is then released to overwhelm the next mitochondria in a cascading series of events that cause mitochondrial energy production supporting the cell to be reduced or stopped completely. The cessation of energy production results in cellular death. Protecting mitochondria is a viable cardio protective strategy.

Cyclosporine is currently in a phase II/III (adaptive) clinical study in Europe to determine its ability to ameliorate neuronal cellular damage in traumatic brain injury.

• TRO40303

TRO40303 is a new cardio protective compound that was shown to inhibit the MPT pore and reduce infarct size after ischemia-reperfusion. It was developed by Trophos company and currently is in Phase I clinical trial.

• Stem cell therapy

Recent investigations suggest a possible beneficial effect of mesenchymal stem cells on heart and kidney reperfusion injury.

• Superoxide dismutase

Superoxide dismutase is an important antioxidant enzyme that transforms superoxide anions to water and hydrogen peroxide. Recent work has demonstrated important therapeutic effects on pre-clinical models of reperfusion damage following an ischemic stroke .

• Metformin

A series of 2009 studies published in the Journal of Cardiovascular Pharmacology indicate that metformin may prevent injury to cardiac reperfusion by inhibiting Mitochondrial Complex I and opening up MPT pore and in rats.

• Cannabinoids

A research published in 2012 shows that the synthetic analog of phytocannabinoid tetrahydrocannabivarin (THCV), 8-Tetrahydrocannabivarin (THCV) and its 11-OH-8-THCV metabolite prevents hepatic ischemia / reperfusion injury by minimizing oxidative stress and inflammatory reactions through cannabinoid CB2 receptors, thereby lowering tissue damage and protective effects of inflammation. Pretreatment with a CB2 receptor antagonist, whereas a CB1 antagonist appeared to strengthen it, attenuated the defensive effects of somewhere else.

An earlier study published in 2011 found that cannabidiol (CBD) also protects against hepatic ischemia/reperfusion injury by attenuating inflammatory signals and oxidative and nitrative stress response, resulting in cell death and tissue damage, but is independent of classic CB1 and CB2 receptors.

References

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