(A) Erk activity in PMA-treated CMK cells. CMK cells were serum-starved for 2 hours (•) or starved and then treated with 100 μmol/L of the Mek inhibitor, PD98059 for 1 hour (○). The cells were then stimulated with 50 ng/mL of PMA for various times as indicated. Activity was plotted as relative activation compared with baseline control. Significant increased Erk activity was seen in all stimulated time points (P < .001 all PMA treated time points, P < .01 all PMA+PD98059-treated time points, compared with baseline by analysis of variance [ANOVA], Student-Newman-Keuls Multiple Comparisons Test). (B) CMK cells were serum starved for 2 hours (▪) or starved and then treated with 100 μmol/L of the Mek inhibitor PD98059 for 1 hour (□). The cells were then stimulated with 100 ng/mL SCF for various times as indicated. Activity was plotted as relative activation compared with baseline control. Significant increased Erk activity was seen in samples stimulated for 5 or 15 minutes in the abscence of Mek inhibitor (* P < .05, ** P < .001 compared with baseline by ANOVA, Student-Newman-Keuls Multiple Comparisons Test). (C) Western blot analysis of above cell lysates using an anti-Phospho–MAPK antibody as described in Materials and Methods. Conditions are as indicated on the figure.