KINETICS OF FREE PLATELET DECREASE AFTER ADP - EFFECT OF FIBRINOGEN BINDING INHIBITORS

Evaluation of platelet aggregation in vitro has generally been perform ed by the turbidometric method, This technique is largely insensitive to early events of aggregation which, are best evaluated by following the decrease in number of free-platelets after agonist addition, In th is study the kinetics of free-platelet decrease after adenosine-5-diph osphate (ADP) addition was analyzed in platelet rich plasma and in who le blood samples obtained from 29 normal donors and 5 thrombocytopenic subjects, Analysis of experimental data using a single exponential de cay equation allowed us to compute the maximal velocity and the rate c onstant of the reaction as well as the number of un-reactive platelets , The velocity of aggregation was positively correlated with platelet number and was markedly increased by the presence of red blood cells, The process was unaffected by platelet cyclooxygenase inhibition and w as also independent of thrombospondin or von Willebrand factor binding to GpIIb-IIIa, Fibrinogen receptor blockade by either anti-GpIIb-IIIa monoclonal antibody or by the RGDS analogue, MK-852, reduced, in a do se dependent manner, the aggregation velocity, Simple decay estimation s based on the difference between counts made before and 15 s after AD P addition, were used to evaluate the anti-aggregating effect of MK-85 2, Compared to turbidometry, the method provided a more accurate dose- response curve and gave, in addition, a significantly higher IC50 valu e (0.21 +/- 0.05 vs 0.11 +/- 0.04 mu mol/l p < 0.01, means +/- SD), We conclude that the kinetics of free-platelet decay allows characteriza tion of early events of the platelet response to ADP by quantitative p arameters, In addition, this technique may represent a significant imp rovement over turbidometry in the laboratory monitoring of the anti-ag gregating effect of fibrinogen binding inhibitors.