PAR-stimulated factor IXa binding to a small platelet subpopulation requires a pronounced and sustained increase of cytoplasmic calcium

Calcium fluxes in the subpopulation of PAR-1 stimulated platelets are examined by measuring fluorescence changes in Fura Red/AM-loaded platelets following PAR-1 stimulation. Fluorescence changes in all platelets indicated calcium release from internal stores and influx of external calcium, a subpopulation of platelets displayed a pronounced increase in calcium transients and positive factor IXa binding, indicating that PAR-1 stimulated development factor IXa binding sites is dependent upon release of calcium from internal stores leading to sustained and pronounced calcium transients.