PD98059, SB203580, and SB202190 function as specific kinase inhibitors in HUVEC. (A) Cells were left unstimulated or were exposed to PMA (for 30 minutes) or arsenite (for 60 minutes) in the presence or absence of 20 μmol/L PD98059 after 30 minutes of pretreatment as indicated. After cell lysis, ERK or p38 were immunoprecipitated and assayed for activity as described. (B) HUVEC were left unstimulated or were stimulated with arsenite or PMA as described above either without or with 30 minutes of preincubation with 20 μmol/L SB203580. After cell lysis, kinases MAPKAP kinases 2 and 3 or ERK were immunoprecipitated and assayed for activity as described in Materials and Methods. (C) Experiments were performed as described in (B), but with the use of 20 μmol/L SB202190 instead of SB203580. Western blots show equal protein loading of the respective kinases.