Expression of thin-filament regulatory protein isoforms in mouse soleus muscle after unloading and during reloading. A: Western blots using mAbs CH1 recognizing α and β tropomyosins, TnI-1 recognizing all TnI isoforms, T12 recognizing fast skeletal muscle TnT, and CT3 recognizing cardiac and slow skeletal muscle TnT showed that 4 wk of unloading did not produce significant change in the expression of tropomyosin, TnI, and slow TnT in mouse soleus muscle, whereas the expression of fast TnT switched to more low-molecular-weight splice forms, which returned to the control pattern after 7 days of reloading. During the later phase of reloading, the expressions of slow TnT and slow TnI were increased, most clearly at 45 and 60 days. B: densitometry quantification further demonstrated an increase in the ratio of slow vs. fast TnI (ssTnI and fsTnI), which became higher than that in normal soleus muscle after 45 and 60 days of reloading, indicating an increase in slow fiber contents as a secondary adaptation possibly to the decreased fatigue resistance. Values are presented as means ± SE; n = 3 in CON, n = 3 in HU, n = 3 in 3DR, n = 3 in 7DR, n = 3 in 15DR, n = 4 in 30DR, n = 7 in 45DR, and n = 4 in 60DR. *P < 0.05 in 1-way ANOVA with mean comparison in Tukey's test.