PLATELET-PLATELET CONTACT AND THROMBOXANE A(2) CONTRIBUTE TO ACTIN POLYMERIZATION IN PLATELETS STIMULATED WITH ADP

Adding adenosine diphosphate (ADP) to platelet-rich plasma (PRP) resul ts in a fall in the level of platelet monomeric globular (G)-actin ind icative of actin polymerization. There is an immediate fall in G-actin associated with shape change which is reversible, and a second phase or sustained response associated with second phase or irreversible agg regation. Previous studies suggested that platelet aggregation is a pr erequisite for second phase or sustained actin polymerization. Here we have examined further the relationship between platelet aggregation a nd actin polymerization in ADP-stimulated platelets by studying the ef fects of M148, a monoclonal antibody that inhibits aggregation by comb ining with the glycoprotein (Gp) IIb/IIIa complex, and the effects of dissociating GpIIb/IIIa by incubating platelets with EGTA at 37 degree s C. We also assessed the contribution of thromboxane A(2) (TXA(2)) by inhibiting its synthesis with aspirin. The results show that GpIIb/II Ia is involved in mediating the second phase or sustained actin polyme rization that occurs after activating platelets with ADP and confirm t he requirement for platelet aggregation. TXA(2) synthesis is not requi red for second phase or sustained actin polymerization, but TXA(2) con tributes to second phase or sustained actin polymerization, probably v ia promotion of further platelet-platelet contact.