Platelets, von Willebrand factor, and prostaglandin I2

Area of Science:

• Biochemistry
• Hematology
• Cell Biology

Background:

• Platelet agglutination is crucial for hemostasis.
• Factor VIII-related von Willebrand factor activity (FVIIIvWF) mediates platelet aggregation.
• Prostaglandin I2 (PGI2) is a potent vasodilator and inhibitor of platelet activation.

Purpose of the Study:

• To investigate the mechanism by which PGI2 modulates FVIIIvWF-mediated platelet agglutination.
• To determine if PGI2 affects FVIIIvWF binding to platelets or platelet-platelet interactions.

Main Methods:

• Platelet agglutination assays using human FVIIIvWF and bovine FVIIIvWF with ristocetin.
• Measurement of 125I-FVIIIvWF binding to platelets.
• Treatment of platelets with PGI2, theophylline, and 2-aminoethylisothiouronium bromide (AET).

Main Results:

• PGI2 inhibited or reversed platelet agglutination mediated by FVIIIvWF and ristocetin, or bovine FVIIIvWF alone.
• PGI2 did not inhibit the binding of FVIIIvWF to platelets, even when platelets were pre-treated with PGI2.
• Platelets treated with AET also bound FVIIIvWF without agglutinating, suggesting altered cohesion sites.

Conclusions:

• FVIIIvWF-mediated agglutination requires both functional platelet FVIIIvWF binding sites and platelet-platelet cohesion sites.
• PGI2 and AET alter platelet surface cohesion sites, rather than FVIIIvWF binding sites.
• Endothelial PGI2 may prevent excessive platelet accumulation on subendothelium without impairing platelet binding to subendothelial FVIIIvWF.