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Abstract
Contributions of altered in vivo protein synthesis and degradation to unweighting
atrophy of the soleus muscle in tail-suspended young female rats were analyzed daily
for up to 6 days. Specific changes in myofibrillar and sarcoplasmic proteins were
also evaluated to assess their contributions to the loss of total protein. Synthesis
of myofibrillar and sarcoplasmic proteins was estimated by intramuscular (IM) injection
and total protein by intraperitoneal (IP) injection of flooding doses of 3H-phenylalanine. Total protein loss was greatest during the first 3 days following
suspension and was a consequence of the loss of myofibrillar rather than sarcoplasmic
proteins. However, synthesis of total myofibrillar and sarcoplasmic proteins diminished
in parallel beginning in the first 24 hours. Therefore sarcoplasmic proteins must
be spared due to a decrease in their degradation. In contrast, myofibrillar protein
degradation increased, thus explaining the elevated degradation of the total pool.
Following 72 hours of suspension, protein synthesis remained low, but the rate of
myofibrillar protein loss diminished, suggesting a slowing of degradation. These various
results show (1) acute loss of protein during unweighting atrophy is a consequence
of decreased synthesis and increased degradation of myofibrillar proteins, and (2)
sarcoplasmic proteins are spared due to slower degradation, likely explaining the
sparing of plasma membrane receptors. Based on other published data, we propose that
the slowing of atrophy after the initial response may be attributed to an increased
effect of insulin.
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Article info
Publication history
Accepted:
October 14,
1992
Received:
May 29,
1992
Footnotes
☆Supported in part by Grant No. NAG2-384 from the National Aeronautics and Space Administration (NASA).
Identification
Copyright
© 1993 Published by Elsevier Inc.
