AGONIST-INDUCED ACTIN POLYMERIZATION IS REQUIRED FOR THE IRREVERSIBILITY OF PLATELET-AGGREGATION

Cytochalasin D was used to investigate the role of intracellular cyto- skeleton in the stabilization of platelet aggregation induced by stron g platelet agonists, Incubation of gel-filtered platelets with increas ing concentrations of cytochalasin D resulted in a dose-dependent inhi bition of actin polymerization and association of actin-binding protei ns with the Triton X-100-insoluble material induced by the thromboxane analogue. U46619, and the thrombin receptor activating peptide, TRAP. The same concentrations of cylochalasin D did not significantly inhib it platelet aggregation promoted by the two agonists. The addition of the chelating agent EDTA to fully aggregated platelets, that had been treated With cytochalasin D. resulted in the rapid and almost complete disaggregation. EDTA did not cause disaggregation of control, solvent -treated. aggregated platelets. The degree of platelet disaggregation induced by EDTA was dependent on the dose of cylochalasin D used, and was correlated with the inhibition of the cytoskeletal reorganization. Aggregation of cytochalasin D-treated platelets stimulated with U4661 9 or TRAP was also reverted by the addition of the tetrapeptide RGDS o r the fibrinogen gamma-chain dodecapeptide, which competitively interf ere with fibrinogen binding to the glycoprotein IIb-IIIa complex. Thes e results indicate that the intracellular cytoskeleton plays an essent ial role, in the stabilization of the fibrinogen platelet interaction, and is necessary for the irreversibility of platelet aggregation indu ced by strong agonists.