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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 12 Processes involved in thrombus formation. The hemostatic system involves<br />

interacting processes for the formation of a stable thrombus—platelet aggregation <strong>and</strong> blood<br />

clotting—but also a mechanism to dissolve the thrombus, i.e., fibrinolysis.<br />

which makes it very difficult to obtain a true reflection of the in vivo situation. In<br />

addition, measurements are usually made in venous fasting blood, while one is interested<br />

in thrombotic tendency in the arteries. Also, many different methods are<br />

used, which makes a comparison between studies difficult.<br />

A. Platelet Aggregation<br />

The activity of blood platelets is an important factor for thrombus formation. Aggregated<br />

platelets adhere to the injured blood vessel to form a hemostatic plug,<br />

excrete substances such as thrombin <strong>and</strong> calcium, <strong>and</strong> provide a phospholipid surface<br />

—all of which are important for blood coagulation.<br />

Several mechanisms are being proposed to explain the effects of fatty acids on<br />

platelet aggregation. Differences in fatty acid composition can change the arachidonic<br />

acid content of platelet <strong>and</strong> endothelial phospholipids. Arachidonic acid acts as a<br />

substrate for thromboxane A2 (TxA2) in platelets <strong>and</strong> prostacycline (PGI2) in endothelial<br />

cells, <strong>and</strong> the balance between these two eicosanoids affects platelet aggregation<br />

(Fig. 13). Fatty acids have also been reported to directly affect TxA2 receptors<br />

on platelet membranes. A third mechanism is that differences in fatty acid composition<br />

can affect the cholesterol content of membranes, <strong>and</strong> consequently affect the<br />

fluidity of platelet membranes <strong>and</strong> platelet activation.<br />

1. Measurement of Platelet Aggregation<br />

A broad scale of methods is available to measure platelet aggregation in vitro. First,<br />

the blood sample needs to be anticoagulated to avoid clotting of the blood in the<br />

test tube or in the aggregometer. Different anticoagulants are used, such as citrate,<br />

which depletes the sample from calcium, <strong>and</strong> heparin or hirudin, which cause an<br />

inhibition of the conversion of prothrombin to thrombin. In vitro platelet aggregation<br />

can then be measured in whole blood, in platelet-rich plasma, or—to remove the<br />

influence of possible interfering constituents from the plasma—in a washed platelet<br />

sample. Finally, the aggregation reaction in the test tube can be triggered with many<br />

different compounds, such as collagen, ADP, archidonic acid, <strong>and</strong> thrombin.<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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