The adaptive responses in several mediators linked with hypertrophy and atrophy of skeletal muscle after lower limb unloading in humans.
OBJECTIVE
To determine the adaptive changes in several molecules regulating muscle hypertrophy and atrophy after unloading, we examined whether unilateral lower limb suspension changes the mRNA and protein levels of SRF-linked (RhoA, RhoGDI, STARS and SRF), myostatin-linked (myostatin, Smad2, Smad3 and FLRG) and Foxo-linked (P-Akt, Foxo1, Foxo3a and Atrogin-1) mediators.


METHODS
A single lower limb of each of eight healthy men was suspended for 20 days. Biopsy specimens were obtained from the vastus lateralis muscle pre- and post-suspension.


RESULTS
The volume of the vastus lateralis muscle was significantly decreased after unloading. The amount of RhoA, RhoGDI or SRF protein in the muscle was not significantly changed post-suspension. An RT-PCR semiquantitative analysis showed increased levels of myostatin mRNA but not Smad2, Smad3 or FLRG mRNA. Unloading did not elicit significant changes in the amount of p-Smad3 or myostatin protein in the muscle. The amount of p-Akt protein was markedly reduced in the unloaded muscle. Lower limb SUSPENSION DID NOT INFLUENCE THE EXPRESSION PATTERN OF FOXO1, FOXO3A OR ATROGIN-1.


CONCLUSIONS
Unloading inducing a mild degree of muscle atrophy may decrease p-Akt and increase myostatin but not SRF-linked mediators.
Research Center for Physical Fitness, Sports and Health, Toyohashi University of Technology, Toyohashi, Japan. ksakuma@hse.tut.ac.jp
K
K
Sakuma
K
K
Watanabe
N
N
Hotta
T
T
Koike
K
K
Ishida
K
K
Katayama
H
H
Akima
D000222:Adaptation, Physiological; D051858:Forkhead Transcription Factors / Q000235:genetics / Q000378:metabolism*; D007839:Functional Laterality; D006801:Humans; D006984:Hypertrophy / Q000378:metabolism; D007103:Immobilization; D047908:Intracellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism; D007866:Leg; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism / Q000473:pathology*; D009133:Muscular Atrophy / Q000378:metabolism* / Q000473:pathology; D055435:Myostatin / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000032:analysis; D012016:Reference Values; D015290:Second Messenger Systems / Q000502:physiology; D026362:Serum Response Factor / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology; D055815:Young Adult
D051858:Forkhead Transcription Factors; D047908:Intracellular Signaling Peptides and Proteins; D055435:Myostatin; D012333:RNA, Messenger; D026362:Serum Response Factor