II. Changes in the Endurance Level of Rat Muscle and Possible Biochemical Explanation for Such Changes

We have just seen that rat muscles lost mass, or atrophied, during space flight, particularly those muscles that are used extensively here on Earth to oppose the forces of gravity, i.e., the predominantly Type I antigravity muscles. We have also seen how Type I muscle fibers, used for endurance activities, seemed to transform themselves into Type II muscle fibers. This transformation probably occurs because of the animal's lack of need to stand up in space, but its different need to hop around using little bursts of activity to float from one side of the cage to the other. How do muscle atrophy and the Type l/Type II redistribution of muscle fibers affect the rat's overall endurance levels? You can probably guess the answer to that, but guesses aren't sufficient to answer questions scientifically. Dr. Baldwin carried out a study that examined how quickly the muscles of the rats that flew in space became fatigued and compared this result to the normal fatigue level for rats that did not fly in space. Fatigue levels offer an indication of endurance capability in the muscle.

In order to measure fatigue levels in the muscles of rats, it is not possible just to watch the rats exercise and try to figure out when they get tired. Not only are the rats not able to tell the researchers when their muscles have reached the point of fatigue, but the scientists require a technique that can provide a quantitative measurement to indicate to them when fatigue has occurred. Let's discuss the technique that Dr. Baldwin and his team used to measure fatigue levels. Remember, he performed this technique postflight on the rats that flew in space and on the control rats that remained on the ground.

The animals were anesthetized and, once unconscious, an incision was made in the skin of the left hind limb. The skin was separated from the animal's muscle groups in the leg and the soleus muscle was isolated. This means that the soleus muscle was identified and separated from the rest of the muscle groups while still remaining attached to the animal's leg. The hind limb of the animal was then placed into a device that provided rigid fixation of the hind limb so that it would remain stil l. The skin of the hind limb was pulled up, and the soleus muscle was bathed in a mineral oil so that the temperature of the muscle could be regulated (30 °C) and to prevent the muscle from drying out. Once the muscle was prepared in this manner, computers and other pieces of laboratory equipment were arranged so that they could be used to artificially stimulate the soleu s muscle to contract. A series of muscle contractions occurring over and over was used in order to produce fatigue in the muscle. After a certain point, the muscle is just not able to reach the level of tension, or tightness, that it coul d before it became fatigued. In order to induce these muscle contractions, the sciatic nerve in the leg was isolated and a wire electrode was placed around the nerve. The sciatic nerve is a motor nerve in the leg that ser ves as the pathway for signals that the brain sends for the muscles to contract and move. An electrical signal is passed through the electrodes, stimulating a sciatic nerve impulse (Figure 21). This elect rical stimulation of the sciatic nerve causes the muscle to contract.

Figure 21.

The electrical signals were delivered to the nerve at a frequency of 1 Hertz (1 Hz) which is equal to one electrical signal per second. Each electrical pulse lasted 300 milliseconds (300 ms = 0.30 seconds). For this experiment, the muscle was stimulated to produce isometric contractions (where the muscle contracts but does not shorten) and the muscle tension level was measured.

Figure 22.

The duration of this isometric fatigue test was 2 minutes. Figure 22 shows Dr. Baldwin's results comparing the percent of initial contraction that was achieved over time. As you can see, the flight rats were much less able to maintain contraction levels than the control rats that had remained on Earth. In fact, at the end of the fatigue test, the control group produced 64% of the initial isometric tension whereas the flight soleus muscles were capable of only producing 36% of their initial isometric tension. Certainly space flight had taken its toll.

We have just learned that space flight does affect the endurance level of rat muscles. The atrophied muscles are smaller and weaker and simply cannot perform as hard or as long as before. Prolonged and strong contraction of an atrophied muscle, then, leads more quickly to muscle fatigue. But it is not simply the size of the muscle that causes it to fatigue more rapidly. Studies performed on athletes here on Earth have shown that muscle fatigue increases in almost direct proportion to the rate of depletion of muscle glycogen.

Therefore, most fatigue probably results simply from our body's inability to supply energy to the muscle fibers for the muscle to perform its function. To explain the decrease in endurance levels for the rats that flew in space, Dr. Baldwin looked at some of the biochemical indicators of energy production in the muscles. Let's briefly discuss how the study of certain chemicals that come from the food we eat can help Dr. Baldwin understand how endurance levels are affected by space flight.

Metabolism includes all of the chemical reactions used by an organism to grow, feed, move, excrete waste products, and communicate. Metabolism has two major components: anabolism, or the synthesis (production) of biomolecules, and catabolism, or the oxidation (breakdown) of biomolecules for energy and the excretion of waste products. The manner in which any cell or organism, including humans, uses its foodstuffs (the stuff we eat) is organized into an orderly, very carefully regulated series of reaction steps and sequences known as metabolic pathways. Some of these metabolic pathways involve as many as a thousand different reactions before the final biological "event" takes place. For instance, a series of reactions occurs in the body after we eat certain foods. Foods are broken down into their basic chemical components and these chemicals continue to be broken down, or oxidized, and a chain of other chemicals are produced, or synthesized. Eventually, all of the reactions result in a final event. In our case, the biological event that we are talking about is movement, which is produced by our muscles.

We already know that ATP is the main ingredient that is necessary to fuel our muscle contractions, and, therefore, our movements. In order to produce ATP, our bodies must extract energy from the foods that we eat. As we digest our food, some of the chemicals from the food are broken into smaller chemicals (Figure 23). For instance:

carbohydrates (including glycogen) from the fruits, vegetables, pastas, potatoes, grains and other things we eat are broken down into simple sugars (including glucose);
fats from the ice cream, steaks, granola, chocolate and other things we eat are broken down into fatty acids and glycerol;
proteins from the fish, peanut butter, eggs, milk, cheese, beans, and other things we eat are broken down into amino acids.

Figure 23. Energy sources in the body include carbohydrates, proteins, and fats, an of which eventually break down to help the body produce ATP.

ATP can be formed from any one of these compounds. We have previously focused on our bodies' ability to produce glucose in the various metabolic energy systems. This is appropriate since our bodies prefer to use this carbohydrate pathway to produce ATP. However, we must realize that fats are also used to produce ATP. In fact, fatty acids are the major storage forms of energy in the body.

Now, there are many, many steps that the carbohydrates and fats must go through to be put into a form that can help the body produce ATP. Someday you may study all of these steps in a biochemistry class, but for the purposes of understanding this part of Dr. Baldwin's experiment, just remember that carbohydrates and fats are the primary foodstuffs that our bodies use to produce ATP. They are oxidized (broken down) by the body, and this occurs at a certain characteristic rate, or speed.

Dr. Baldwin was interested in seeing if space flight might influence the proportion of energy that was produced by the carbohydrates and by the fats that we eat. In order to do that, he had to focus on a particular step in the breakdown reaction of each of these compounds. As mentioned previously, there can be up to a thousand reactions that chemicals go through to reach their final destination and final purpose. Dr. Baldwin focused on the oxidation rate (or breakdown) of two of the most important chemicals that are produced along the energy production pathways in our bodies: palmitate and pyruvate.

Pyruvate is a chemical, or substrate, that is produced during the breakdown of carbohydrates.
Parmitate is a chemical, or substrate, that is produced during the breakdown of fats.

By looking at the rate at which these two substrates are oxidized, Dr. Baldwin could determine which of these two pathways dominated the body's production of energy in space. The questions were whether the body used up its carbohydrates or fats in space at the same rates that it does on Earth. In other words, by measuring these two substrates, he would be able to tell whether the carbohydrate oxidative capacity, the fat oxidative capacity, or both of these energy producing pathways were affected by space flight.

There is one more issue to discuss related to oxidative capacity. Different kinds of muscle fibers, Type I (slow twitch) and Type II (fast twitch), have different oxidative capacities. We know that slow twitch muscles, which are used for endurance activities, contain more blood vessels to continually supply needed fuel for such activities. In fact, slow twitch fibers are often called red muscle fibers. This would suggest that they have a greater oxidative capacity than the fast twitch muscle fibers, which are sometimes called white muscle fibers because they contain fewer blood vessels. Therefore, because of their greater blood supply, the slow twitch muscles would have more oxygen available to oxidize substrates. It would be expected, then, that the slow twitch muscles should be able to oxidize both pyruvate and palmitate at a higher rate than fast twitch muscles, either on Earth or in space. Dr. Baldwin measured and compared the pyruvate and palmitate oxidative capacity of the vastus intermedius (Vl) muscle, the red fibers of the vastus lateralis (RVL) muscle, and the white fibers of the vastus lateralis (WVL) muscle for both the flight animals and the control animals.

Let's see what Dr. Baldwin found.

Table 7. Pyruvate and palmitate oxidation in the vastus intermedius (VI) muscle, the red vastus lateralis (RVL) muscle, and the white vastus lateralis (WVL) muscle for both the flight and control rates.

Muscle Fiber R+O R+O R+9 R+9

Group Control Flight Control Flight

Pyruvate

VI 474 ± 31 468 ± 27 508 ± 27 485 ± 38

RVL 550 ± 29 493 ± 25 522 ± 25 458 ± 43

WVL 106 ± 9 105 ± 9 116 ± 5 122 ± 7

Palmitate

VI 73 ± 6 64 ± 5 75 ± 6 77 ± 6

RVL 58 ± 7 39 ± 6* 56 ± 4 51 ± 4

WVL 12 ± 1 8 ± 1* 10 ± 1 11 ± 2

From Table 7, you can see two kinds of comparisons: (1) a comparison between the flight animals and the control animals, and (2) a comparison among the three different muscle fiber groups. First, you can see that the capacity to oxidize pyruvate was not altered between the flight and control groups of animals. As expected, however, there were some major differences in the capacity to oxidize pyruvate between the slow twitch (RVL) and fast twitch (WVL) muscles in both experimental groups. In contrast, there were reductions in the capacity to oxidize palmitate in all three muscle types of the flight group compared to the control group. These reductions amounted to:

12% VI muscle groups
38% red VL muscle fibers
36% white VL muscle fibers

Thus, overall, there was a reduction in fatty acid oxidation capacity for the rats that flew in space. Surprisingly, however, this effect was completely reversed by nine days after their return from space, even though normal muscle mass had not been completely restored (refer back to Figure 19).

These results provide some explanation for why the endurance levels of the rats that flew in space were reduced. The rats that flew in space seemed to develop a greater dependence on using the carbohydrate pathway as an energy source. This greater dependence on carbohydrates could possibly reduce the endurance capacity of the animal because the animal would use up its glycogen stores more rapidly. In addition, the flight rats reduced their utilization of fats as an energy source. Since fatty acids are the major storage forms of energy in the body, any reduction in the use of fats could also cause a reduction in the animal's endurance capacity. It is clear that these metabolic pathways (in particular, the oxidation of fats in our bodies) are affected by space flight.

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