Ischemic-reperfusion injury (IRI)

• Ischemia-Reperfusion Injury (IRI)


  Restoring blood flow after transient ischemia may induce new or serious reversible or irreversible cell damage, especially after prolonged ischemia
• Swift reperfusion of ischemic tissue

  The most important therapeutic approach for reducing ischemia injury
• Oxygen free radicals mediate a reaction series

  1. Inducing inflammatory factors

  2. Impairing nitric oxide formation

  3. Inducing adhesion molecule expression

  4. Stimulating adherence between neutrophils and intact vessels
• Calcium overload


  Na+ and Ca2+ influx -> free radicals -> lipid peroxidation -> Membrane or mitochondria, lysosomal membrane and other fluidity ↓, permeability ↑
• Calcium homeostasis is critical for normal cell functioning
• Intracellular calcium concentration regulation

  1. Ca2+ flux across the plasma membrane (sarcolemma in myocytes)

  2. Endoplasmic reticulum Ca2+ release (sarcoplasmic reticulum in myocytes)

  3. Mitochondria
• Intracellular calcium is stored in sarcoplasmic reticulum (SR) and mitochondria in myocytes
• During ischemia-reperfusion or oxygen paradox

  Intracellular calcium increases markedly
• Calcium overload


  The phenomenon of impaired cellular structure and dysfunction
• Compared to extracellular calcium concentration (1.25 mmol/L), cytosolic free calcium level is maintained at extremely low concentrations (<0.1mmol/L)
• Most intracellular calcium is sequestered within mitochondria and endoplasmic reticulum in resting state of normal cells
• Ischemia


  Increases cytosolic calcium concentration, due to influx of Ca2+ across the plasma membrane and release of Ca2+ from mitochondria and endoplasmic reticulum
• Na+/Ca2+ exchanger


  The main electrogenic transporter of calcium, via cellular gradients for Na+ and Ca2+, as well as membrane potential
• Na+/Ca2+ exchanger in normal "forward mode"

  1. Supplements the activity of Ca2+ATPase in the sarcoplasmic reticulum to transport Ca2+ extracellularly to decrease myocyte cytosolic Ca2+ concentration

  2. For each Ca2+ extruded, three Na+ ions enter the myocyte

  3. The ratio of Na+/Ca2+ exchange is 3Na+:1Ca2+
• Excess intracellular Na+


  Actively extruded by Na+/K+-ATPase
• During times of very positive membrane potential or elevated intracellular Na+


  The Na+/Ca2+ exchanger can operate in "reverse mode", extruding Na+, and permitting increased Ca2+ influx
• During ischemia reperfusion


  • ATP production is decreased, resulting in Na+/K+-ATPase suppression
  • Intracellular Na+ overload is caused and the Na+/Ca2+ exchanger, which usually pumps out intracellular Ca2+ in exchange for Na+, is activated in "reverse mode", leading to intracellular Ca2+ overload
• Na+/H+ exchanger activation


  Intracellular H+ and protein kinase C (PKC)
• Acidosis during ischemia

  Leads to increased intracellular H+, which activates Na+/H+ exchanger, overloading intracellular Na+ concentration
• Increased intracellular Na+


  Activates the Na+/Ca2+ exchanger in "reverse mode"
• Protein kinase C (PKC) is another potential activator for Na+/Ca2+ exchange
• Signal transduction


  1. a-adrenergic receptor and G protein-phospholipase C (PLC) pathways

  2. Phosphatidylinositol (PI) is hydrolyzed by PLC to two second messengers, inositol 1,4,5-triphosphate (IP3) and 1,2-diacylglycerol (DAG)

  3. IP3 induces the release of Ca2+ from sarcoplasmic reticulum

  4. DAG activates PKC further to enhance Na+/H+ exchanger activity, increasing intracellular Ca2+ concentration
• Calcium can enter the cytosol


  Via damaged membranes
• Early loss of selective membrane permeability ultimately leads to membrane damage, a consistent feature of all cell injury types, affecting the mitochondria, the plasma membrane, and other cellular membranes
• The integrity and permeability of sarcoplasmic membrane is impaired during ischemia-reperfusion
• Damage also occurs in mitochondria, lysosomes, and other cellular-membraned organelles
• Ca2+ influx via damaged membranes per gradient


  May induce cellular dysfunction
• High intracellular Ca2+ concentration

  • Carries harmful effects
  • Impairs the respiration chain and energy production
  • Activates a number of Ca2+-dependent enzymes, including phospholipidase, protease, ATPase and endonuclease, to make different effects
• No-reflow phenomenon


  The phenomenon that the ischemic area is still not adequately perfused with blood after the restoration of blood perfusion
• Animal experiments demonstrate that certain ischemic regions cannot be reperfused sufficiently, even after removing occluded blood flow
• Neutrophilic activation and inflammatory factor release

  The pathophysiological basis for no-reflow phenomenon and microvascular hemorrhagic changes
• Microvascular dysfunction


  • Impaired endothelium-dependent arteriolar dilation
  • Enhanced fluid filtration
  • Leukocyte-plugging in capillaries
  • Plasma protein extravasation via postcapillary venules
• Activated neutrophils release several mediators, such as oxygen free radicals and proteolytic enzymes, which can directly induce tissue damage
• Neutrophils may also plug capillaries, mechanically blocking flow
• Activated neutrophils


  1. Adhere to endothelial cells or blood cells

  2. Release inflammatory factors (such as TNF-α) after margination

  3. Amplifying the local inflammatory reaction and increasing the permeability of the endothelial cell monolayer
• The inflammatory mediators released as a consequence of reperfusion also appear to activate endothelial cells in remote organs not exposed to the initial ischemic result
• Neutrophils, endothelial cell activation


  • Active substance release (e.g. FR, proteases, lysosomes, etc.)
• This distant response to ischemia-reperfusion can result in neutrophil-dependent microvascular injury that is characteristic of the multiple organ dysfunction syndromes (MODS)
• Activated endothelial cells may develop edema, narrowing blood flow and creating hemorrhagic abnormalities
• Activated neutrophils


  Promote expression of cellular adhesion molecules (CAMs), including the selectins (P-selectin, L-selectin, E-selectin), integrins (CD11/CD18), and immunoglobulin superfamily (ICAM-1,VCAM-1,etc.)