PLATELET SIZE DISTRIBUTION MEASUREMENTS AS INDICATORS OF SHEAR STRESS-INDUCED PLATELET-AGGREGATION

The mechanisms underlying shear stress-induced platelet aggregation (S IPA) were investigated by measuring changes in the platelet size distr ibutions resulting from the exposure of human platelet-rich plasma (PR P) to well-defined shear stresses in a modified viscometer. Exposure o f PRP to a shear stress of 100 dyne/cm(2) for 1 min at 37 degrees C re sulted in the loss of single platelets, an overall shift in the distri bution to larger particle sizes, and the generation of platelet fragme nts. Treatment of PRP prior to shearing with a monoclonal antibody dir ected against platelet glycoprotein (GP) IIb-IIIa (integrin alpha(IIb) beta(3)) at a concentration that completely inhibited ADP-induced plat elet aggregation also inhibited SIPA. Furthermore, incubation of PRP w ith a recombinant fragment of von Willebrand factor (vVWF) that abolis hes ristocetin-induced platelet agglutination significantly inhibited but did not eliminate SIPA. Pretreatment of PRP with the tetrapeptides RGDS or RGDV, which constitute the GP IIb-IIIa peptide recognition se quences on fibrinogen and VWF, almost completely blocked platelet aggr egation at 100 dyne/cm(2), whereas the negative control peptide RGES h ad no discernible effect. Finally, incubation of PRP with a monoclonal antibody directed against the platelet vitronectin receptor (integrin alpha(v) beta(3)) did not affect SIPA. These results indicate that bo th GP IIb-IIIa and GP Ib, the latter through its interaction with vWF, are required for SIPA at 100 dyne/cm(2); that the interaction of GP I Ib-IIIa with its adhesive ligands under shear stress can be inhibited by RGD-containing peptides; and that the vitronectin receptor on plate lets, which shares the same beta(3) subunit as GP IIb-IIIa, plays no r ole in SIPA. On the basis of these results, the assessment of platelet size distributions provides a sensitive and quantitative measurement for the study of SIPA.