N-ethylmaleimide induces disaggregation of platelets preaggregated by ADP and thrombin and decreases cytoplasmic calcium level

The effect of N-ethylmaleimide (NEM, 1-200 mu M) on ADP- and thrombin-induc ed platelet aggregation and thrombin-induced increase in intracellular Ca2 concentration was studied. Addition of NEM to platelets preaggregated with ADP or thrombin induces platelet disaggregation. The anti-aggregant activi ty of NEM was different for ADP- and thrombin-induced aggregations. At 200 mu M concentration, NEM completely disaggregated ADP-induced aggregates and only partially disaggregated thrombin-aggregated platelets. NEM did not in fluence the thrombin-induced increase in cytoplasmic Ca2+ and had no effect on the basal level of Ca2+ in the cytosol of non-activated platelets. Howe ver, NEM decreased the level of thrombin-mobilized Ca2+ in the cytosol of a ctivated platelets. Thus, NEM fan induce disaggregation of ADP- and thrombi n-preaggregated platelets by activating a system which removes Ca2+ from th e platelet cytosol.