Phosphorylation and activity of p38 MAPK are increased during in vitro storage of platelets. Mouse platelets were isolated from fresh PRP or PRP stored in the presence of SB203580 (40μM) or DMSO (vehicle), washed in Tyrode-N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid buffer, and lysed immediately. (A) Immunoprecipitated phospho-p38 MAPK was detected by Western blot analysis using a specific antibody for p38 MAPK and quantified (mean ± SEM); n = 4. (B) The activity of phosphorylated p38 MAPK was assessed in lysates from 2 different preparations of freshly isolated or stored platelets by its ability to phosphorylate the substrate fusion protein ATF-2. (C) Phosphorylation of p38 and ATF-2 in platelets that were treated with 100μM CCCP for the indicated times. Total p38 MAPK was assessed in platelet lysates by Western blot analysis using a specific antibody for p38 MAPK.