Satellite Cell Content Decreases and Muscle Physiology Is Altered during 2 Weeks of Physical Inactivity in Middle‐Aged Adults

Abstract

Atrophy of skeletal muscle during bed rest, an analog of spaceflight, occurs in older adults at a greater rate than young adults. Identifying the mechanism responsible for the age‐related difference is critical to promote the muscle health of the astronaut corps where the average age is approaching 50. Therefore, we sought to characterize changes in muscle physiology of middle‐aged adults after 2 weeks of bed rest. Muscle biopsies from the vastus lateralis were obtained from five (2 male, 3 female) healthy, middle‐aged adults (51±1 years) at 3 time points: prior to bed rest (pre‐BR), after 2 weeks of bed rest (post‐BR) and after one week of rehabilitation (post‐RE). Satellite cell (SC) content was determined via immunohistochemistry using an antibody against the unique satellite cell transcription factor Pax7. Muscle fiber type, cross‐sectional area and myonuclear content were also determined via immunohistochemistry and muscle loss was measured by DEXA. Subjects lost an average of 889±212 g of lean mass from their lower limbs during bed rest. SC content decreased post‐BR (0.09 SC/fiber) compared to pre‐BR (0.13 SC/fiber; 33% decrease), and partial recovery of satellite cell content occurred post‐RE (0.11 SC/fiber; 20% increase from post‐BR). The recovery in SC content occurred largely in SC associated with type 1 fibers, while content of SC associated with type 2 fibers remained relatively unchanged post‐RE. The loss of satellite cells can impair muscle regeneration following atrophy, warranting further work to determine if modulating SC activity during or following bed rest could improve skeletal muscle mass and health. This project was supported by NSBRI‐NNJ08ZSA002N, R01NR012973, and P30 AG024832.