Role of platelet surface negative charge in ristocetin-induced platelet agglutination

The effect of ristocetin on platelet electrophoretic mobility (EPM) was studied with the new automatic Laser Zee System 3000 with special reference to the effects of changing the conductance of the platelet suspension medium. Human platelets were extensively washed using several methods until they no longer agglutinated upon addition of ristocetin alone. Washed platelets in plasma-free Zeiller-Hannig's buffer which has a low specific conductance showed an EPM of-1.95+0.05pm/sec/V/cm (M+S. E. M., n=20) and showed high susceptibility towards EPM decrease by 0.075 mg/ml ristocetin alone without any change in medium conductance. Washed platelets in Tris-saline buffer showed an EPM of-0.98+0.03 pm/sec/V/cm and showed a significant decrease in EPM with 0.5mg/ml ristocetin, while they did not agglutinate on the further addition of 1/10 volume of normal plasma. Since the platelet EPM decreased by addition of ristocetin was restored to the control value after 2 washes with buffer, the change in platelet EPM in the presence of ristocetin was reversible, suggesting a weak attachment of ristocetin to platelets. Treatment of platelets with neuraminidase resulted in irreversible and dose-related parallel decreases in both EPM and sialic acid content. These tiresults suggest that platelet surface negative charge is decreased by ristocetin and this enhances binding of negatively charged von Willebrand factor to the platelet membrane.