Tail suspension induces activation of p38 and ERK1/2 in WT mice, which is attenuated in Capns1-knockout mice, and inhibition of calpain attenuates phosphorylation of ERK1/2 and p38 in cardiomyocytes under microgravity.A–C, mice with cardiomyocyte-specific deletion of Capns1 (KO) and their WT littermates (WT) were subjected to tail suspension (TS) for 14 or 28 days. A, a representative Western blot from three different hearts in each group for total and phosphorylated p38 and ERK1/2. B, top, a representative Western blot from two of six different hearts in each group for total and phosphorylated ERK1/2; bottom, quantitation for phosphorylated ERK1/2 (p-ERK1/2) relative to total ERK1/2. C, top, a representative Western blot from two of six different hearts in each group for total and phosphorylated p38; bottom, quantitation for phosphorylated p38 (p-p38) relative to total p38. Data are mean ± S.D. (error bars), n = 6 in each group. Two-way ANOVA followed by Newman–Keuls test was performed for statistical analysis. B, interaction, F = 39.53, p < 0.0001; row factor, F = 0.1185, p = 0.7366; column factor, F = 19.48, p = 0.0008. C, interaction, F = 82.32, p < 0.0001; row factor, F = 14.01, p = 0.0028; column factor, F = 101.8, p < 0.0001. *, p < 0.05 versus sham + WT; †, p < 0.05 versus KO + vehicle. D–F, neonatal mouse cardiomyocytes were subjected to SMG in the presence of calpain inhibitor-III or vehicle for 6 and 24 h. D, a representative Western blot from three different cell cultures for total and phosphorylated p38 and ERK1/2. E, top, a representative Western blot from three different cell cultures with each in duplicate for total and phosphorylated ERK1/2; bottom, quantitation for phosphorylated ERK1/2 (p-ERK1/2) relative to total ERK1/2. F, top, a representative Western blot from three different cell cultures with each in duplicate for total and phosphorylated p38; bottom, quantitation for phosphorylated p38 (p-p38) relative to total p38. Data are mean ± S.D., n = 3 in each group. Two-way ANOVA followed by Newman–Keuls test was performed for statistical analysis. E, interaction, F = 46.37, p < 0.0001; row factor, F = 5.254, p = 0.0329; column factor, F = 42.27, p < 0.0001. F, interaction, F = 7.9, p = 0.0108; row factor, F = 155.2, p < 0.0001; column factor, F = 5.059, p = 0.0359. *, p < 0.05 versus sham + vehicle; †, p < 0.05 versus SMG + vehicle.