Inhibition by the caspase inhibitor DEVD-fmk of PS exposure in platelets activated by A23187 or thrombin plus collagen. Platelets were preincubated for 1 hour with no inhibitor (A), or FA-fmk, or DEVD-fmk (100 μmol/L in [B] and 25 μmol/L in [C]) and were treated for the indicated time with A23187 (1 μmol/L) or thrombin (1 U/mL) plus collagen (20 μg/mL). (A) Platelets without inhibitor treatment. Histograms of annexin V-FITC binding show time-dependent A23187-induced conversion of platelets from PS-negative to PS-positive. (B) Inhibition by DEVD-fmk of A23187-induced PS exposure. The data are the mean percentages ± SEM (n = 4) of annexin V-positive platelets as a function of time with A23187. Note that pretreatment with DEVD-fmk, but not with FA-fmk, significantly inhibited PS exposure (*P < .005 at 3 minutes andP < .009 at 5 minutes). (C) Inhibition by DEVD-fmk of thrombin plus collagen-induced PS exposure (n = 4). Pretreatment with DEVD-fmk significantly inhibited PS exposure (*P < .001 at 5 minutes, P < .008 at 10 minutes, and P < .01 at 20 minutes). The unstimulated baseline values in (B) and (C) (dashed lines) are not significantly different for platelets preincubated without (shown) or with inhibitors.