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1Department of Physiology and Biophysics, University of California at Irvine, Irvine 92697; and 2Brain Research Institute and 3Department of Physiological Sciences, University of California at Los Angeles, Los Angeles, California 90095
Submitted 28 March 2003 ; accepted in final form 23 April 2003
The goal of this study was to use the model of spinal cord isolation (SI),
which blocks nearly all neuromuscular activity while leaving the motoneuron
muscle-fiber connections intact, to characterize the cellular processes linked
to marked muscle atrophy. Rats randomly assigned to normal control and SI
groups were studied at 0, 2, 4, 8, and 15 days after SI surgery. The slow
soleus muscle atrophied by 
50%, with the greatest degree of loss
occurring during the first 8 days. Throughout the SI duration, muscle protein
concentration was maintained at the control level, whereas myofibrillar
protein concentration steadily decreased between 4 and 15 days of SI, and this
was associated with a 50% decrease in myosin heavy chain (MHC) normalized to
total protein. Actin relative to the total protein was maintained at the
control level. Marked reductions occurred in total RNA and DNA content and in
total MHC and actin mRNA expressed relative to 18S ribosomal RNA. These
findings suggest that two key factors contributing to the muscle atrophy in
the SI model are 1) a reduction in ribosomal RNA that is consistent
with a reduction in protein translational capacity, and 2)
insufficient mRNA substrate for translating key sarcomeric proteins comprising
the myofibril fraction, such as MHC and actin. In addition, the marked
selective depletion of MHC protein in the muscles of SI rats suggests that
this protein is more vulnerable to inactivity than actin protein. This
selective MHC loss could be a major contributor for the previously reported
loss in the functional integrity of SI muscles. Collectively, these data are
consistent with the involvement of pretranslational and translational
processes in muscle atrophy due to SI.
myosin heavy chain messenger ribonucleic acid; actin messenger ribonucleic acid; poly(A) messenger ribonucleic acid; ribonucleic acid; deoxyribonucleic acid; protein translation
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