The effect of aggregation and release on platelet prothrombin-converting activity.

Area of Science:

• Hematology
• Biochemistry
• Cell Biology

Background:

• Platelets play a vital role in hemostasis by providing procoagulant activity.
• Platelet prothrombin-converting activity (PPCA) is essential for converting prothrombin to thrombin.
• Understanding the mechanisms regulating PPCA is critical for hemostasis research.

Purpose of the Study:

• To investigate the relationship between platelet aggregation, dense body release, and PPCA.
• To determine if ADP-induced platelet aggregation leads to the exposure of procoagulant sites.
• To elucidate the role of dense body secretion in PPCA expression.

Main Methods:

• Utilized a two-stage assay measuring thrombin production with gel-filtered bovine platelets, factor Xa, and prothrombin.
• Monitored PPCA in response to ADP-induced platelet aggregation and shape change.
• Assessed the release of 14C-5-hydroxytryptamine from dense bodies.
• Measured the binding of 125I-factor Xa to platelets.
• Investigated the effect of theophylline and prostaglandin E1 on PPCA and serotonin release.

Main Results:

• PPCA expression was associated with platelet aggregation, not ADP-induced release or shape change.
• Dense body release was not required for PPCA expression during aggregation.
• 5-hydroxytryptamine release occurred after thrombin production onset and was inhibited by theophylline and prostaglandin E1 without significantly reducing PPCA.
• Increased binding of factor Xa to ADP-activated platelets (aspirin-treated) compared to unactivated platelets.
• ADP-activated platelet PPCA saturated at low factor Xa and Va concentrations, indicating specific site exposure.

Conclusions:

• ADP-induced platelet aggregation exposes specific procoagulant sites independently of dense body secretion.
• The mechanism of PPCA expression involves processes other than dense body release.
• These findings provide insights into the regulation of coagulation at the platelet level.