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.

Platelet function and ABO blood group.

An influence of the ABO blood group on von Willebrand and Factor VIII:C levels is known. Von Willebrand factor interacts with at least two platelet membrane receptors, but the effect of ABO group on platelet function is an unstudied area. The authors examined platelet function in 40 plateletpheresis donors using an impedance lumi-aggregometer. Aggregation responses to collagen, adenosine diphosphate (ADP), and ristocetin were measured and the adenosine triphosphate (ATP) release to thrombin. Twenty donors were Group O and 20 were Group A. Measurements of von Willebrand factor antigen (vWf:Ag), Factor VIII: C, and ristocetin co-factor (RiCoF) in the same group showed reduced levels of vWf:Ag and Factor VIII:C in Group O, as previously reported. The aggregation response to collagen and ADP and the release of ATP did not differ. The aggregation response to ristocetin, however, was better in Group O than in Group A despite the lower vWf:Ag levels. The explanation for this is unclear, but the data suggest an influence of blood group antigens on the interaction between von Willebrand's factor and platelets.

The effect of the platelet count on the aggregation response and adenosine triphosphate release in an impedance lumi-aggregometer.

Platelet aggregation studies are usually performed in either electrical impedance or optical systems and the release reaction assessed by quantitating adenosine triphosphate (ATP) from luminesence produced by the firefly luciferin-luciferase system. In evaluating the results of such studies, attention is paid to a variety of parameters such as the slope of the aggregation response and the maximum aggregation expressed as percent light transmission in optical and as Ohms in electrical impedance systems. Although threshold platelet counts are frequently cited below which the performance of these studies is technically difficult, the influence of the platelet count within the normal range on the results of such studies has not been prospectively addressed. This study examines the relationship between the aggregation response, ATP release and the platelet count in a lumi-impedance system. It is clear that the platelet count influences the results in this system and requires consideration in the interpretation if an erroneous conclusion is to be avoided.