Long duration spaceflight missions out of lower earth orbit, back to the lunar surface, or possibly to Mars highlight the importance of preserving muscle mass and function. Muscle atrophy occurs within days of exposure to microgravity and prevailing thought is that a primary mechanism for muscle atrophy is a reduction in skeletal muscle protein synthesis. This dissertation examines the ability of skeletal muscle to recover muscle protein synthesis with slight perturbation, such as ambulatory reloading during disuse as well as partial loading, similar to body mass seen on the moon or Mars. We use traditional precursor-product labeling to measure protein synthesis, but use a relatively novel tracer, deuterium oxide, in order to make cumulative measures of protein synthesis over 24 h. The overarching goal of this dissertation is to define the response of skeletal muscle protein synthesis to different loading parameters in order to better understand the contribution of protein synthesis to skeletal muscle mass during disuse.
In the first study, we demonstrate that muscle atrophy during 5 days of hindlimb unloading is in part due to a decrease in protein synthesis. We also highlight the ability of skeletal muscle to adapt by allowing two 1 h ambulatory reloading sessions on days 2 and 4. Although this countermeasure is able to rescue protein synthesis in soleus and gastrocnemius, it is unable attenuate any losses in muscle mass.
In the second study, we compare partial weight loading to traditional hindlimb unloading. Weight bearing of 1/3 or 1/6 body weight is able to attenuate losses in muscle mass seen with unloading. Protein synthesis is maintained after 21 days of the experimental protocol, suggesting that protein synthesis is responsive to load and is likely not the only mechanism for determining muscle mass.
In the final study, the effects of < 1 Gy x-ray exposure and partial weight suspension are measured to better understand the complex space environment, which includes a wide variety of radiation. Surprisingly, we found no effects of radiation on muscle protein synthesis in 1 G or partial loading.
Targeting only protein synthesis may not be enough of a stimulus as evidenced by the data in this dissertation. Future plans should use a multiple-systems approach to counteract atrophy by increasing protein synthesis to maintain/elevate muscle mass during periods when it is otherwise compromised.
- Fluckey, James Professor