Ristocetin-induced platelet aggregate formation and adherence to the probe of an impedance aggregometer

The examination of the interaction between ristocetin and platelets generally is performed in an optical aggregometer where aggregate formation is detected by a change in turbidity. Ristocetin also will induce conductivity changes in an impedance aggregometer with either fresh whole blood or fresh platelet-rich plasma due to aggregate attachment to the electrical probe. Formaldehyde-fixed platelets, however, do not attach, and although fresh platelets agglutinate at extremes of pH, probe adhesion does not occur unless the pH is within a certain range. Incubation of platelet preparations with neuraminidase, colchicine, or cytochalasin-b affects neither aggregate formation nor probe adherence. Incubation of platelet preparations with a fibronectin tetrapeptide inhibitor eliminates aggregate formation in response to collagen but is without effect on ristocetin-induced attachment of aggregates to the impedance probe. It is suggested that the ability of platelet aggregates to attach to an impedance probe in response to ristocetin is dependent on additional aspects of the functional integrity of platelet preparations and that these changes required for probe adherence may have in vivo significance.