In muscle physiology research, whether on Earth or in space, the rat is often used as a laboratory model because rat muscles come very close to approximating the structure, function, and biochemistry of human muscle. (Isn't that a sobering thought?!!) In laboratories on Earth, studies have shown that both humans and rats lose muscle mass when they do not use their muscles. This loss of muscle mass has been measured in experiments where humans have been confined to bed for some period of time. To simulate similar unloading (taking all weight off of them) of the muscles in rats, researchers use a special hind limb suspension apparatus that lifts the back legs of the rat off the ground, rendering the animals incapable of using their legs to bear weight (Figure 15). Of course, in space, there is no need to put astronauts into bed or to suspend the hind limbs of the rats because everyone and everything floats in space, unloading the muscles naturally.
In microgravity, since astronauts and rats can float instead of walk, leg muscles become weakened from lack of use. Past space flight studies have shown decreases in muscle mass for both the astronauts and the rats. Specific changes include loss of lower body mass and decreased muscle strength. Dr. Baldwin's experiment was designed to quantify the decrease in the mass of the rat's hind limb muscles, muscles that are usually used to oppose gravity on Earth. This was done in order to develop a better understanding of how human muscles might be affected by microgravity. Figure 16 illustrates the particular hind limb muscles of the rat that Dr. Baldwin focused on.
Before we continue with our discussion of how Dr. Baldwin measured the mass changes in rats, let's discuss for a moment the concepts of weight and mass. Here on Earth, any one of us can "weigh" ourselves on a scale. The weight measurement that we obtain is an indication of the gravitational attraction that the Earth has on the mass of our body. Therefore, since the magnitude of the pull of gravity doesn't change, any change in "weight" is actually a reflection of a change in the "mass" of our bodies. Of course, the mass of our bodies refers to the "amount" of total stuff that our bodies are made of. All of this makes sense because, as we lose weight, the size or "amount of stuff" that our bodies are made of becomes smaller. The opposite is true when we gain weight. In space, where gravity is virtually absent, the only physical indication that we can obtain about our bodies is a measurement of our nongravimetric (non = without, gravi = refers to gravity, and metric = measure) mass.
In preparation for the Skylab missions that occurred in the early 1970s, efforts were undertaken to design equipment for the measurement of a variety of human physiological changes that were known to occur in space. One of the first priorities in space medical research was to determine the cause and time course of the weight loss which always seemed to accompany space flight. At that time, the main problem that had to be solved in order to carry out such a study was the lack of an instrument that could be used in space for nongravimetric mass measurement. How could they develop a mass - measurement device that did not depend on weight? The only alternative to the determination of gravitational attraction, or weight, was some determination of the inertial property of mass. Here are four statements that you should try to understand individually and then try to blend together in order to understand what is meant by "the inertial property of mass":
Figure 17 illustrates the principle of nongravimetric mass measurement. A
sample mass (imagine this to be an astronaut in space) is placed between
two springs and constrained to move from right to left (linear motion)
along the longitudinal axes of the springs (Figure 17a). If the mass is
displaced from its rest position (Xo) to a new position (X) and the mass
assembly is released, it will undergo a round trip undamped oscillation
(undamped = uninterrupted by other forces or friction, oscillation =
back-and-forth motion) in a period (the time required to complete
one round trip of the motion, see Figure 17b).
The physical relationship between the period of oscillation
and mass is expressed in the following equation:
M = the mass of the object in kilograms, and
K = the spring constant (which is a measure of the spring's "stiffness") in newtons per meter.
p = pi = 3.14159
If this period of oscillation is accurately measured by an appropriate timer, an object's mass may be calculated by rearranging the equation to obtain the following relationship:
|Figure 18. The Body Mass Measurement Device is used in space to measure the astronaut's mass every day to determine any changes that may result from space flight.|
Figure 18 shows the current Body Mass Measurement Device (BMMD) that is used on the space shuttle. It allows the astronaut's mass to be measured based on the same principles that were developed and used in the Skylab program. After strapping the astronaut into the BMMD, the seat is unlocked and prepared by cocking the "displacement and release" device to allow the seat to begin oscillating. The timer is also turned on to enable a determination of the period of oscillation. As the astronaut takes a breath and holds it to avoid " jitter," the seat is released and begins to oscillate. The period of oscillation is recorded and a mass measurement is determined.
There is also a Small Mass Measuring Instrument (SMMI) that has been developed to determine inflight mass changes in small animals and other specimens. After all of this discussion about mass measurement in space, it is important for you to understand that, for very practical reasons, Dr. Baldwin did not use the SMMI to measure the muscle mass changes of rats. Rather than making inflight measurements, he waited until soon after the rats returned to Earth to begin handling the rats and determining their weights.
Rats are valuable models in muscle research, not only because their muscles are, in many ways, similar to human muscles, but also because rat muscles can be studied directly. That is, whole muscle groups can be removed from the rat to enable the researchers to study them more closely. Obviously, such a complete study is not possible with human muscles. Dr. Baldwin has performed muscle mass measurements by removing the muscles of rats that flew on a variety of missions. We will examine his results from both a nine-day mission and a six -day mission. Let's see how these measurements were carried out on these two missions.
Soon after the shuttle returned from nine days in space, the flight rats were unloaded from the RAHF, transported to a building near the shuttle landing site (where the control rats were located), and delivered to a waiting dissection team. Both the flight and control animals were weighed and then half of each group were immediately sacrificed by decapitation. The other half of the group were sacrificed nine days later (R+9) to see how rapidly the muscles regained their mass and strength. In either case, After sacrificing the animals, the muscles of interest were removed rapidly, along with other organs. The other organs were distributed to various scientists around the world who had other important investigations to carry out with these tissues. Then the muscles of interest were weighed. Of the particular muscle weights that are reported here, two come from the back of the thigh (Figure 16) and are known to be heavily involved in weight-bearing (antigravity) activities here on Earth. These include:
|Figure 19. Animal muscle weight comparisons between flight and control animals.|
The third muscle group that is reported on here is the tibialis anterior muscle, which is located in the ankle and is normally not heavily recruited in weight-bearing activity. The muscle weight results for each muscle group are shown in the graphs in Figure 19.
From the results in Figure 19, you can see that the total weights of the animals did not change very much. However, nine days of microgravity caused significant atrophy in both the fast twitch VL (15%) and slow twitch (22%). The slow-twitch VI experienced more atrophy than the VL, and interestingly, the tibialis anterior was not affected by microgravity. What does all of this mean?
First of all, since the body weights of the flight (FL) and control (CON) animals were not different, it is clear that the differences in muscle mass between FL and CON animals represent a true atrophy response. Secondly, these data indicate that the muscles that are primarily used as antigravity muscles on Earth (the slow twitch muscles) change more as a result of microgravity than the muscles that are used for bursts of activity (the fast twitch muscles). Third, while nine days of ground activity during the recovery period returned muscle mass back toward normal values, the recovery was incomplete, suggesting that a recovery period longer than the flight duration is needed to fully regain normal muscle mass, at least in the rat. Interestingly, the slow twitch Vl muscle, which is used more as an antigravity muscle, appeared to recover a greater portion of its mass than the VL After nine days back on Earth. This is consistent with the idea that weight-bearing is important in maintaining mass in the muscles that are composed chiefly of slow twitch muscle fibers.
|Table 5.||Control||Flight||% Change|
|233 ± 7.8||216 ± 4.3||-7.3%|
|Soleus Muscle weight
|102 ± 7.7||75 ± 7.2||-26.5%|
|Table 5. A comparison of animal and soleus muscle weight between flight and control animals.|
On a different space mission that lasted only six days, Dr. Baldwin examined changes in the rat's soleus muscle, a muscle that is located in the lower leg (Figure 16). This muscle consists primarily of slow twitch, Type I muscle fibers and is used extensively on Earth as an endurance muscle (similar to the vastus intermedius covered previously) to oppose the ever-present force of gravity. As you can see from Table 5, its muscle mass also decreased significantly as a result of space flight.
Let's look at how the Type I and Type II muscle fibers in the soleus seemed to change their distribution.
|Table 6.||Control||Flight||% Change|
|2300 ± 276||1749 ± 362||-24.0%|
|2098 ± 262||2041 ± 244||-2.7%|
|71.2 ± 7.1||60.6 ± 5.9||-14.9%|
|28.8 ± 7.1||39.4 ± 5.9||+36.8%|
|Table 6. A comparison of the cross-sectional area and percent area differences in Type I and Type II muscle fiber for flight and control rat soleus muscles.|
Look at Table 6, which shows that there was a decrease in the cross - sectional area of Type I (by 24%) and Type II (by 2.7%) muscle fibers in the soleus muscle. But remember, there was a decrease overall in the mass of the muscle so it would be expected that since the actual cross sectional area of the entire muscle would be decreased, then the Type I and Type II cross-sectional areas would naturally be decreased. The real indicator that Type I fibers might have actually been transformed into Type II muscle fibers can be seen by looking at the changes in percent area for each fiber type. This is a different kind of comparison to make because, by looking at the percent area that the Type I and Type II muscle fibers occupy in the total muscle, you can directly compare the flight and control muscles without regard to their size differences. That is, it evens out the total size differences that exist between the soleus muscles of the rats that flew in space and the soleus muscles of the rats that served as ground controls. Let's use an example to illustrate the difference between cross-sectional area and percent area.
Consider the following cake analogy. Let's say that one of your parents has just made two cakes, a large square cake that measures 40 cm X 40 cm and a small square cake that measures 20 cm X 20 cm (Figure 20). Let's also say that you are starving and your parent will allow you to eat a quarter of either cake. Of course, your hunger will be more satisfied if you eat a quarter of the large cake because you will have a larger cross-sectional area of the cake to eat. Since the other cake is smaller, the cross-sectional area of a quarter of that cake will obviously be smaller. The percent area of each piece, however, would be the same - 25%. Thus, using the percent area of the two cakes will even out their size differences when making a comparison, but not when making a decision on what cake to choose! Now, let's get back to the results of Dr. Baldwin's experiment.
Table 6 indicates that the percent area that the Type I fibers occupied in the muscle decreased almost 15% between the flight animals and the control animals, but the percent area that the Type II fibers occupied in the muscle increased almost 37% between the two groups of animals! This suggests that some of the Type I fibers were transformed into Type II fibers because they were not utilized during space flight to the extent that they are used and needed here on Earth.
These studies have shown that not only does the elimination of weight- bearing activity cause a decrease in muscle mass, but it also causes a transformation of Type I into Type II muscle fibers in the predominantly slow twitch antigravity muscles in the rat model. Efforts are now underway to translate these data into an understanding of how human muscle is affected by reduced loading here on Earth (due to long illnesses that confine humans to bed) and by the microgravity environment of space. This work is important because such an understanding will allow countermeasures to be developed that reverse the detrimental effects of muscle atrophy. Let's take a look now at some of the results from Dr. Baldwin's study of how space flight influences the endurance capacity of rat muscles and let's see what he has found out about some of the biochemical explanations for these effects.