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Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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V. Neutrophil-Mediated Tissue Injury<br />

343<br />

Endotoxin<br />

CD14<br />

Macrophage<br />

TNF<br />

IL-1<br />

ICAM<br />

E-selectin<br />

Activated<br />

endothelial cell<br />

PAF<br />

IL-8<br />

C<br />

LBT4<br />

TF<br />

TF<br />

TM↓ ADP<br />

ase↓<br />

Activated<br />

endothelial cell<br />

PECAM<br />

VWF PGI2↓<br />

PAF<br />

Neutrophil<br />

adhesion<br />

Neutrophil<br />

activation<br />

Procoagulant<br />

Platelet<br />

adhesion<br />

Platelet<br />

activation<br />

FIGURE 11-5 The activation <strong>of</strong> endothelial cells at a site <strong>of</strong> inflammation. At sites <strong>of</strong> inflammation, activated macrophages release<br />

proinflammatory mediators including tumor necrosis factor- α (TNF- α ) and interleukin-1 (IL-1). These cytokines activate endothelial<br />

cells, converting their surface from antiadhesive and anticoagulant to proadhesive and procoagulant. Activated endothelium express<br />

intercellular adhesion molecules (ICAM) and selectin molecules that promote neutrophil adhesion. Activated endothelium also<br />

express platelet-activating factor (PAF), interleukin-8 (IL-8), and leukotriene-B4 (LTB4) and may bind complement , all <strong>of</strong> which will<br />

activate neutrophils while attached to endothelium. Decreased expression <strong>of</strong> adenosine diphosphatase and thrombomodulin as well<br />

as increased expression <strong>of</strong> platelet-endothelial cell adhesion molecules (PECAM) and von Willebrand’s factor promote platelet adhesion.<br />

Decreased production <strong>of</strong> prostaglandin I 2 (PGI2) and expression <strong>of</strong> PAF promotes platelet activation. Finally, tissue factor (TF),<br />

a potent activator <strong>of</strong> the extrinsic clotting pathway, may be expressed on activated endothelium.<br />

B . Ischemia/Reperfusion Injury<br />

Prolonged ischemia results in numerous metabolic and ultrastructural<br />

changes within cells. Ischemia decreases cellular<br />

oxidative phosphorylation resulting in failure to resynthesize<br />

high-energy phosphate compounds such as adenosine<br />

5 -triphosphate. Adenosine 5 -triphosphate catabolism during<br />

ischemia results in accumulation <strong>of</strong> hypoxanthine.<br />

Hypoxanthine is subsequently converted to oxygen radicals<br />

upon reperfusion <strong>of</strong> the tissue. Ischemia also promotes<br />

expression <strong>of</strong> leukocyte and platelet adhesion receptors on<br />

the cell surface; promotes expression <strong>of</strong> proinflammatory<br />

cytokines (e.g., IL-8) and bioactive agents (e.g., platelet-activating<br />

agents, endothelin, thromboxane A2); and suppresses<br />

certain gene products including nitric oxide, nitric oxide<br />

synthase, thrombomodulin, and prostacyclin.<br />

Restoration <strong>of</strong> blood flow after a period <strong>of</strong> ischemia<br />

places tissues at risk <strong>of</strong> further cellular necrosis ( Carden and<br />

Granger, 2000 ; Eltzschig and Collard, 2004). Reperfusion<br />

injury is primarily directed at the endothelial cells lining<br />

the microvasculature and is manifest as accelerated production<br />

<strong>of</strong> a variety <strong>of</strong> bioactive agents and expression <strong>of</strong><br />

adhesion molecules for neutrophils. Neutrophils appear to<br />

become activated while attached to endothelium-releasing<br />

oxygen radicals and granule contents directly onto the endothelial<br />

surface. In addition to production <strong>of</strong> superoxide by<br />

neutrophils, superoxide is generated from hypoxanthine<br />

within endothelial cells and is produced by mast cells and<br />

macrophages.<br />

Neutrophils may also contribute to tissue hypoperfusion<br />

after blood flow is reestablished. Activated neutrophils<br />

are rigid and tend to lodge in capillaries, obstructing<br />

blood flow. Neutrophil-neutrophil aggregates and plateletneutrophil<br />

aggregates also may contribute to capillary malperfusion<br />

by plugging capillaries resulting in prolonged<br />

maldistribution <strong>of</strong> blood flow. Platelet-neutrophil aggregates<br />

may also contribute to microthrombus formation.

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