Key Anabolic Markers in Human Soleus Muscle after 21-Day Head-Down Tilt Bed Rest

Prolonged bed rest can have a significant negative effecton skeletal muscles, leading to muscle wasting and reduced strength.This process can take as little as 10 days in healthy individuals,with the loss of muscle mass and strength being particularly pronouncedduring the first week of immobilization. Head-down tilt bed rest(HDTBR) is a method used to simulate physiological changes thatoccur in weightlessness during spaceflight. This technique involveslying in bed with the head tilted downward. Here, we analyze thekey anabolic markers of the human m. soleus during21 days of HDTBR. Six healthy male volunteers, aged 25–35 years,were exposed to 21 days of strict bed rest at a tilt angle of –6°. Aneedle biopsy of the m. soleus wasperformed via the Bergström method before the onset of HDTBR andon day 21 after that. The biopsy material was immediately frozenin liquid nitrogen for further Western blot and PCR analysis. Astudy of mTORC1 substrates showed a significant decrease in p70and 4EBP1 phosphorylation after HDTBR. We also observed a significantdecrease in the phosphorylation of another ribosomal kinase, p90RSK,a significant increase in eEF2 phosphorylation, and an increasein eEF2k mRNA expression. In addition, the phosphorylation of AMPKand its substrate ACC decreased after HDTBR. The data obtained inthis work support the hypothesis that a decrease in protein synthesis, togetherwith an increase in proteolysis, contributes to the developmentof human m. soleus atrophy after21 days of HDTBR.