BINDING OF FIBRINOGEN TO PLATELET GLYCOPROTEIN (GP) IIb/IIIa IS CRUCIAL FOR SHEAR-INDUCED PLATELET AGGREGATION
It is well known that human platelets can aggregate in vitro under certain shear stress without adding aggregating inducers. However, the mechanism of this shear-induced platelet aggregation has not been clarified yet. In this paper, we have investigated the role of fibrinogen and GP IIb/IIIa in shear-induced platelet aggregation. Citrated human platelet-rich plasma (PRP) was subjected to controlled shear stress levels in a polycarbonate cone and plate viscometer at 37°C for 2 minutes. After shearing the particle count was measured by an electronic particle counter. Particles with sizes from 3 to 20 μ cum were considered as single platelets. In unsheared PRP most of the particles were single platelets, but platelet doublets and platelet fragments larger than 3 μ cum were also counted. After exposure to shear rate of 3,600 - 9,000 sec−1 , the particle counts were decreased in a shear rate dependent manner, while LDH leakage from platelets was not significantly increased and 3H-serotonin release was 2-7%. Scanning electronmicroscopy clearly showed the presence of large platelet aggregates when the particle counts were decreased. Platelets from two patients with thrombasthenia and one patient with afibrinogenemia, however, failed to aggregate at a shear rate of 9,000 sec−1. Shear-induced aggregation was inhibited by monoclonal antibody to GPIIb/IIIa (1 μg/ml) and synthetic peptide, Arg-Gly-Asp-Ser, (1 mM). When fibrinogen was added to PRP from a patient with afibrinogenemia, shear-induced aggregation became evident as seen in normal platelets. Apyrase and hirudin showed no effect on shear-induced aggregation. Indomethacin (100 μM) and TXA2 synthetase inhibitor, OKY-046 (100 μM) markedly inhibited aggregation, while TXA2 competitive inhibitor, ONO-3708 (100 μM) exhibited only partial inhibition.Our results indicate that binding of fibrinogen to GPIIb/lIIa is also crucial for shear-induced platelet aggregation and that the exposure of fibrinogen receptor on GPIIb/IIIa may partially depend upon TXA2 synthesis in platelets.