Maintaining Muscle Mass in Space
Absolute and Relative Muscle Weight in Response to Muscle Unloading in Rodents
This slide presents data on the response of rodent skeletal muscle to conditions of unloading, such as during space flight. In space, the muscles lose their ability to generate force, because there is no gravity or load for the muscles to work against. This creates a signal to the muscles that there is no need for them to maintain their normal size. The laboratory rat is an ideal subject for this type of experiment, because, as can be seen in the figure, rat muscles will atrophy dramatically over a period of a few days (the magenta line color). That is, the muscle loses protein, and the muscle size and weight decrease. This change can be seen in the upper figure, depicting the weight of the soleus muscle, one of the key muscle components in our calf region (located in the posterior or rear position of the lower leg). Sometimes, scientists normalize the change in muscle weight by correcting it to the body weight. Generally, the greater the total body weight of the animal, the greater the individual muscle weight. As can be seen in the lower figure, when the muscle weight is normalized to body weight, the relative muscle weight of the rat is still smaller than in the normal control animals that remain on Earth. This information suggests that the muscle tissues are a key target for atrophy, relative to the other tissues, when they are not allowed to generate force, as during exposure to the lack of gravity. Additional research suggests that the same deterioration that affects rodent muscle also affects human muscle, although it take more time to observe the changes, because the metabolism of human muscle is slower than that of rodents. Ongoing studies suggest that there is an imbalance in the capacity of the muscle to synthesize the proteins that make up muscle, relative to the rate that the proteins are broken down (called protein degradation). As a result of this imbalance, muscle weight becomes reduced due to the loss in protein.
Can you think of any way that one could try to prevent this muscle loss from occurring?
Caiozzo, V. J. ,Haddad, F., Baker, M. J., Herrick, R. E., Prietto, N., & Baldwin K. M. (1996). Microgravity induced transformations of myosin isoforms and contractile properties of skeletal muscle. J. Appl. Physiol. 81: 123-132.
ADDITIONAL NOTES FROM SPEAKER’S TRANSCRIPT (http://www.bioedonline.org/presentations/)
Why would someone want to study a rat in space? First, it is important to realize that rats express the same type of muscle fibers as humans. Rats are under the influence of gravity just as humans are, so when we flew our rats in space, we were able to track a process by weighing the muscle. There was a rapid drop in the weight of the muscle over a period of 16 days. Contrast the magenta line in the upper graph with the blue line, which shows how ground bearing animals on Earth were maintaining their weight. Muscle is much more sensitive to the lack of gravity—the state of unloading—than are other organ systems that contribute to one’s body weight.
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Funded by the following grant(s)
This work was supported by National Space Biomedical Research Institute through NASA cooperative agreement NCC 9-58.