Time-dependent changes in expression of troponin subunit isoforms in unloaded rat soleus muscle

This study focuses on the effects of mechanical unloading of rat soleus muscle on the isoform patterns of the three troponin (Tn) subunits: troponin T (TnT), troponin I (TnI), and troponin C (TnC). Mechanical unloading was achieved by hindlimb unloading (HU) for time periods of 7, 15, and 28 days. Relative concentrations of slow and fast TnT, TnI, and TnC isoforms were assessed by electrophoretic and immunoblot analyses. HU induced profound slow-to-fast isoform transitions of all Tn subunits, although to different extents and with different time courses. The effectiveness of the isoform transitions was higher for TnT than for TnI and TnC. Indeed, TnI and TnC encompassed minor partial exchanges of slow isoforms with their fast counterparts, whereas the expression pattern of fast TnT isoforms (TnTf) was largely increased after HU. Moreover, slow and fast isoforms of the different Tn were not affected in the same manner by HU. This suggests that the slow and fast counterparts of the Tn subunit isoforms are regulated independently in response to HU. The changes in TnTf composition occurred in parallel with previously demonstrated transitions within the pattern of the fast myosin heavy chains in the same muscles.