Skeletal muscle adapts to loading; atrophying when exposed to unloading on Earth or
in spaceflight. Significant atrophy (decreases in muscle fiber cross-section of 11
- 24 %) in humans has been noted after only 5 days in space. Since muscle strength
is determined both by muscle cross-section and synchronization of motor unit recruitment,
a loss in muscle size weakens astronauts, which would increase risks to their safety
if an emergency required maximal muscle force. Numerous countermeasures have been
tested to prevent atrophy. Resistant exercise together with growth hormone and IGF-I
are effective countermeasures to unloading as most atrophy is prevented in animal
models. The loss of muscle protein is due to an early decrease in protein synthesis
rate and a later increase in protein degradation. The initial decrease in protein
synthesis is a result of decreased protein translation, caused by a prolongation in
the elongation rate. A decrease in HSP70 by a sight increase in ATP may be the factors
prolonging elongation rate. Increases in the activities of proteolytic enzymes and
in ubiquitin contribute to the increased protein degradation rate in unloaded muscle.
Numerous mRNA concentrations have been shown to be altered in unloaded muscles. Decreases
in mRNAs for contractile proteins usually occur after the initial fall in protein
synthesis rates. Much additional research is needed to determine the mechanism by
which muscle senses the absence of gravity with an adaptive atrophy. The development
of effective countermeasures to unloading atrophy will require more research.
Microgravity - exercise - atrophy - protein metabolism - countermeasure - molecular
- muscle - human
