COMP expression and secretion in platelets. (A) Western blot of suspensions of resting platelets without centrifugation from humans, rats, and mice. (B) Representative immunofluorescence images of human resting platelets (scale bar, 5 μm). Mouse washed platelets were treated with thrombin (0.1 U/mL) at different time points (C) or stimulated with thrombin for 60 minutes at different doses (D). Representative western blot analysis for supernatant and pellet of platelets isolated from platelet suspensions by centrifugation. Protein band density was normalized to the corresponding β-actin and then to the mean of the corresponding control group (0 minutes or 0 U/mL). Bar graphs show the band densitometry with statistics. n = 3, *P < .05 vs 0 minutes (C) or 0 U/mL (D). (E) Coimmunoprecipitation (IP) of COMP and thrombin in the supernatant of thrombin-activated mouse platelets (0.1 U/mL, 60 minutes). Rabbit IgG was as negative control for IP. Input fractions isolated prior to precipitation were detected for loading controls.