SINGLE-FIBER AND WHOLE MUSCLE ANALYSES OF MHC ISOFORM PLASTICITY - INTERACTION BETWEEN T-3 AND UNLOADING

Previous data suggest that separate interventions of hyperthyroidism ( T-3) and hindlimb suspension (HS) act on some but not all slow type I fibers in the soleus muscle. This may be due to the presence of ''refr actory'' fibers that are unresponsive to either of these interventions . Alternatively, T-3 and HS might act on different populations of slow type I fibers in the soleus muscle. Adult female Sprague-Dawley rats were assigned to ii control, 2) T-3, 3) HS, or 4) T-3+HS. Nine animals were assigned to each group. Single-fiber electrophoretic analyses (n = 40 per muscle) of the soleus muscle demonstrated that the HS reduce d the percentage of slow type I fibers from similar to 80% (control) t o similar to 40% (HS) of the fiber population. Although hyperthyroidis m affected a greater percentage of Blow type I fibers than I-IS, a sma ll population (similar to 10% of the slow type I fibers) were unaffect ed by T-3. The combined intervention, in contrast, transformed all slo w type I fibers into fibers expressing various combinations of fast my osin heavy chain (MHC) isoforms. These findings demonstrate that the s oleus muscle does not contain so-called refractory fibers. They furthe r suggest that the soleus muscle contains different populations of slo w type I fibers that vary in their sensitivity to altered physiologica l conditions.