Thrombin-induced phosphoinositide hydrolysis in platelets. Receptor occupancy and desensitization

Area of Science:

• Biochemistry
• Cell Biology
• Hematology

Background:

• Platelets play a crucial role in hemostasis and thrombosis.
• Thrombin is a key enzyme in the coagulation cascade and a potent platelet activator.
• Phosphoinositide hydrolysis is a critical signaling pathway in platelet activation.

Purpose of the Study:

• To investigate the relationship between thrombin receptor occupancy and phosphoinositide hydrolysis in human platelets.
• To characterize the kinetics and regulatory mechanisms of thrombin-induced phosphoinositide hydrolysis.
• To determine the role of continuous thrombin presence and receptor desensitization in this signaling pathway.

Main Methods:

• Washed human platelets were labeled with [3H]inositol.
• Agonist-induced accumulation of labeled inositol phosphates was measured.
• Dose-response relationships, time courses, and effects of thrombin inhibitors (hirudin) and secondary agents were analyzed.

Main Results:

• Thrombin-induced inositol phosphate accumulation mirrored dense-granule secretion.
• Continuous presence of active thrombin was necessary for sustained signaling, with maximum accumulation between 5-10 minutes.
• Platelets exhibited desensitization to a second thrombin addition within 10 minutes, without precursor depletion.
• ADP-consuming enzymes reduced thrombin sensitivity, while cyclo-oxygenase inhibition had no effect.

Conclusions:

• Thrombin-induced phosphoinositide hydrolysis is mediated by a receptor similar to that for dense-granule secretion.
• This signaling requires continuous thrombin presence and exhibits receptor desensitization.
• Secondary agents only partially enhance thrombin-induced phosphoinositide hydrolysis.