| Abstract: | The effects of a chronic (up to 360 days) reduction in neuromuscular activity (defined as electrical activation and loading) on myosin heavy chain (MHC) isoform expression in the rat soleus muscle were studied. A complete mid‐thoracic (T7‐T8) spinal cord transection (ST) was used to induce a reduction in soleus muscle neuromuscular activity. Electrophoretic analyses revealed that MHC‐I was progressively decreased after ST, accounting for approx. 90% of the total soleus MHC in controls and only approx. 12% 1 year after ST. The reductions in the proportion of MHC‐I were countered by increases in MHC‐IIa and MHC‐IIx with the increase in MHC‐IIx preceding the increase in MHC‐IIa. Curiously, MHC‐IIb was expressed only at very low levels. Thus, a complete transformation from predominantly MHC‐I to MHC‐IIb did not occur. Many fibers (up to approx. 80%) contained multiple MHCs (hybrid fibers) after ST. The proportion of hybrid fibers was maintained at a high level (approx. 50%) 1 year after ST. These data suggest that: 1) a prolonged reduction in neuromuscular activity was not sufficient to induce high level MHC‐IIb expression by the soleus muscle; and 2) hybrid fibers were not simply transitional fibers. Thus, it appears that under appropriate conditions hybrid fibers may represent a “stable” fiber phenotype. Anat Rec 255:188–201, 1999. © 1999 Wiley‐Liss, Inc. |
