Effects of calpeptin and DEVD-fmk on PS exposure (A and C) and microparticle release (B and D). Platelets were pretreated as indicated with no inhibitor or calpeptin (50 μg/mL) or calpeptin plus DEVD-fmk (25 μg/mL) and stimulated for the indicated times with thrombin plus collagen (1 U/mL; 20 μg/mL). (A) PS exposure induced by thrombin plus collagen was significantly increased by calpeptin at 5, 10, and 20 minutes (*P < .01, P < .001, andP < .002, respectively; n = 4). (C) PS exposure induced by agonist and enhanced by calpeptin was significantly inhibited by DEVD-fmk at 5, 10, and 20 minutes (P < .01, P < .007, and P < .001, respectively). (B) Thrombin plus collagen-induced microparticle release was significantly inhibited by calpeptin at 5, 10, and 20 minutes (P < .02, P < .005, and P < .0003, respectively) and (D) was not further inhibited by DEVD-fmk and calpeptin. Unstimulated values (dashed lines), with the exception of PS exposure after calpeptin pretreatment (shown in [A]), were not significantly different for platelets preincubated without and with inhibitors.