Expression and functional implications of troponin T isoforms in soleus muscle fibers of rat after unloading

The expression pattern of troponin T (TnT) isoforms was studied in rat soleus muscle fibers in control and after hindlimb unloading (HU) conditions. To determine the functional consequence of TnT expression, the fibers were also examined for their calcium activation characteristics. With regard to TnT expression, four populations of fibers were distinguished in control muscle. Slow fibers expressing only slow isoforms of TnT (TnT1s, 2s, 3s) were predominant (54%). Hybrid slow fibers (16%) differed from slow fibers by the additional expression of two TnTf isoforms. Hybrid fast fibers (22%) expressed slow and fast isoforms of TnT while fast fibers (8%) expressed only fast TnT isoforms. The expression of the other regulatory protein isoforms was checked for each population. The contractile experiments revealed steeper slopes of the tension/pCa relationship from hybrid slow fibers expressing fast TnT in a completely slow molecular environment. The expression of TnTs in hybrid fast fibers did not modulate the intrinsic co-operativity. After HU, the fast population was increased and reached 55%. The slow population decreased to 41% and a very small amount of hybrid slow fibers remained (4%). These data demonstrated the implication of TnT isoforms in the calcium activation properties and, more particularly, in the modulation of co-operativity within the myofibrillar lattice. Regulation of TnT expression appeared as a very fast and complete process compared to moderate changes of TnC and TnI.