Persistence of hybrid fibers in rat soleus after spinal cord transection.
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.
Talmadge
R J
RJ
Department of Human Nutrition, Foods & Exercise, Virginia Polytechnic Institute and State University, Blacksburg 24061-0430, USA. bobt@vt.edu
Roy
R R
RR
Edgerton
V R
VR
D000818:Animals; D001835:Body Weight / Q000502:physiology; D002818:Cordotomy; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D007124:Immunoenzyme Techniques; D018485:Muscle Fibers, Skeletal / Q000166:cytology / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000294:innervation* / Q000378:metabolism; D018995:Myosin Heavy Chains / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000502:physiology*
D018995:Myosin Heavy Chains