Rapid and selective measurement of platelet-activating factor using a quantitative bioassay of platelet aggregation

A bioassay for the measurement of platelet-activating factor (PAF) based on the quantification of platelet aggregation was developed. The method used a platelet analyzer in conjunction with a multiwelled micromixing device and whole blood collected from male New Zealand White rabbits. This bioassay can be nonselective and used to quantitate platelet aggregation induced by any activator. The EC50 for platelet-activating factor, arachidonic acid, and adenosine diphosphate were 0.0232, 55, and 10 microM, and at these concentrations the half maximal aggregation response occurred at 7.5, 10.0, and 12.5 min, respectively. The bioassay was capable of the sensitive quantification of platelet aggregation in small volumes (50 microL) of citrated rabbit whole blood, using a short (i.e., 15-min) incubation period. This enabled multiple bioassays to be performed without the need for a large volume of whole blood to be collected from the rabbit. Selective measurement of platelet-activating factor was achieved by adding inhibitors of arachidonic acid- and adenosine diphosphate-dependent pathways of platelet activation, that is, acetylsalicyclic acid and phosphoenolpyruvate/pyruvate kinase, respectively, to the citrated rabbit whole blood immediately before bioassay. These inhibited arachidonic acid- and adenosine diphosphate-induced platelet aggregation, but had no effect on platelet aggregation induced by platelet-activating factor. Platelet-activating factor was selectively inhibited by its receptor antagonist BN 52021. This method for measuring platelet-activating factor was reproducible; at the EC50, the inter- and intrabioassay coefficients of variation were within acceptable limits at 13.17% and 9.75%, respectively.