Alkaline phosphatase prevents platelet stimulation by thromboxane-mimetics

Area of Science:

• Biochemistry
• Hematology
• Pharmacology

Background:

• Platelet activation is crucial for hemostasis and thrombosis.
• Thromboxane A2 (TxA2) and prostaglandin endoperoxides are key mediators of platelet aggregation.
• The role of alkaline phosphatase in platelet function is not fully understood.

Purpose of the Study:

• To investigate the effects of alkaline phosphatase on platelet aggregation, secretion, and thromboxane B2 generation.
• To determine the specific pathways of platelet activation inhibited by alkaline phosphatase.

Main Methods:

• Studied human platelet-rich plasma and washed platelets.
• Utilized various platelet agonists including arachidonate, U46619, EP171, PAF-acether, thrombin, ADP, and adrenaline.
• Measured platelet aggregation, adenosine 5'-triphosphate (ATP) secretion, and thromboxane B2 (TxB2) generation.
• Conducted in vivo experiments in guinea pigs to assess thrombocytopenia.

Main Results:

• Alkaline phosphatase abolished platelet aggregation and ATP secretion induced by arachidonate, TxA2, and prostaglandin endoperoxide mimetics.
• Inhibition occurred despite persistent TxB2 synthesis.
• Alkaline phosphatase did not affect aggregation induced by PAF-acether, thrombin, or the primary wave of ADP/adrenaline-induced aggregation.
• The secondary wave of ADP-induced aggregation was blocked.
• Alkaline phosphatase did not increase platelet cyclic AMP levels.
• Inorganic phosphate or ATP/creatine phosphokinase reversed the inhibitory effect.
• Alkaline phosphatase reduced arachidonate-induced thrombocytopenia in vivo.

Conclusions:

• Alkaline phosphatase specifically inhibits platelet activation at sites sensitive to TxA2 and prostaglandin endoperoxides.
• The phosphorylation/dephosphorylation state of alkaline phosphatase may modulate platelet activation.
• Results suggest the presence of ecto-protein kinases on platelet membranes.