Fig. 6. Stretch activation of p38 requires TNF- receptor-mediated signaling. Primary myoblast cultures were prepared from wild-type (WT) or p55^–/– p75^–/– mice (KO). Myoblasts were seeded in six-well Bioflex® plates and allowed to proliferate to 65% confluence. Stretch was conducted in fresh growth medium for 6 hours. Time-matched, non-stretched myoblasts were used as controls. Whole cell extracts of myoblasts were prepared and analyzed by western blotting for levels of the phosphorylated form and the total levels of the kinases tested. The optical density of the phosphorylated form of each kinase detected western blotting was normalized to the respective total level and analyzed by paired t-test to compare control and stretched myoblasts for each type of myoblasts (n=3). ^*P<0.05; ^**P<0.01.