This video will address an important concept related to the energetic needs of muscles during exercise. Adenosine Triphosphate or ATP is a extremely important compound required for muscular work. ATP is the chemical currency used to supply the energy needed for muscle contraction. When this high energy phosphate is broken down, it releases a tremendous amount of energy to the cell, initiating muscle contraction. Of all the high energy compounds in the body, only ATP can be used directly to supply the energy needed for the mechanical work of exercise. This is an important concept when it comes to regulating the energetic needs of the body when going from rest to exercise. Shown here is the actually site in muscle where ATP is broken down or hydrolyzed. The energy released allows for the cross bridge formation between the myosin and actin filaments leading to the initiation of power stroke and eventually muscle contraction. When we zoom in the region of the myosin filament or the cross bridge formation the actin filament occurs we see the enzyme ATPase. This enzyme is responsible for the breakdown of ATP and its energy release. It is very important to realize that no other enzymes are present in this region. Thus, no other high energy compound can be used to activate muscle contraction, it must be ATP, and ATP alone. Therefore, it is of the utmost importance to ensure that ATP levels in muscle are maintained if you wish to exercise for any significant duration. The issue of maintaining ATP during exercise is further complicated by the fact that the concentration of ATP in muscle at rest is extremely low. As expected, when we go from rest to submaximal exercise, the rate of ATP utilization by the muscle increases dramatically. Since muscle ATP levels are low at rest, it is imperative that the rate of ATP production increases to an equal extent of that of utilization if we wish to continue to exercise for more than several seconds. With very intense exercise, it is possible for the rate of ATP utilization to increase beyond the capacity for ATP production to keep pace. In this case, the individual will either fatigue very rapidly and be forced to stop exercising or reduce their exercise intensity thereby lowering the rate of ATP utilization allowing production to keep pace. Thus it is essential for ATP producing pathways to be turned on or activated during exercise to match the rate of utilization. This figure shows that our stored macronutrients, carbohydrates, fats and proteins are broken down to supply the fuel for ATP production. The metabolic breakdown of these macronutrients supplies the energy for ATP synthesis from ADP. The ATP now produced can be directly used for the mechanical work of exercise. How the body activates the ATP producing pathways during exercise is controlled enzymatically. Key enzymes in those ATP producing pathways mentioned on the previous slide get turned on as the muscle senses the need for greater energy production. This sensing is primarily controlled by the energy charges of the cell shown here. Basically, when the concentration of ATP in the muscle cell drops below normal resting levels, the energy charge drops and ATP producing pathways are activated. At rest, the energy charge in muscle is approximately 0.85. The energy charge will drop during exercise dependent upon the exercise intensity and rate of ATP utilization. Notice that as the energy charge decreases, ATP generating pathways are turned on while ATP utilizing pathways, such as biosynthetic reactions are turned off to save the precious ATP from muscle contraction. Shown here are the three main ATP producing pathways related to fat, carbohydrate and protein metabolism. The complete oxidation of these macronutrients will lead to the eventual production of ATP in the mitochondria of muscle cells. Please notice that this process results in the utilization of oxygen by the mitochondria. This is why our rate of oxygen consumption increases during exercise to sustain the increase requirement for ATP production. This is also why we can measure oxygen consumption, via indirect calorimetry to estimate ones metabolic rate. Mitochondria are organelles in our cells, that are responsible for the majority of ATP production, needed both at rest and during exercise. Again, I emphasize that these ATP producing pathways must be activated when we go from rest to exercise to match ATP production with that of utilization. This is what an isolated mitochondrion looks like in a muscle cell. If the bulk of the energy needed for a specific athletic event comes from the mitochondrial ATP production, it is considered to be an aerobic activity, because oxygen is both required and consumed. Endurance activities, lasting ten minutes and longer fall into this category. Short term high intensity exercise lasting seconds must get the majority of ATP from pathways that do not require oxygen as they need ATP immediately and do not have the time to completely oxidize our macronutrients in the mitochondria. Activities that rely on these immediate energy sources that do not require oxygen for ATP production are known as anaerobic activities. Those immediate energy sources for high intensity explosive anaerobic activities are shown here. First there is that ATP already present in the muscle cell at rest, but remember its concentration is very low and will not last long. Second, we have creatine phosphate which can directly produce ATP enzymatically. Notice that oxygen is not involved or consumed in this process. At rest, there is about three to four times more creatine phosphate in muscle than ATP. Carbohydrate is the only macronutrient that can be broken down both aerobically and anaerobically to supply ATP. The anaerobic pathway involves the breakdown of glucose in glycolysis, and results in the production of two ATPs, and two lactic acids or lactates. Again, this is anaerobic, as no oxygen is required for this pathway. The major aerobic sources for ATP production during exercise come from the oxidation of carbohydrates and fats. Notice that one, oxygen is required. And two, the oxidation of glucose yields 30 ATP as opposed to only 2 ATP if we break down glucose anaerobically. Thus, for the very same glucose molecule, we get 15 times more ATP via the aerobic versus the anaerobic pathway. This is extremely important as we'll see in subsequent videos as there is a limited amount of carbohydrates stored in the body. So if we were able to use our glucose aerobically, we will not only get more bang for the back but we will also spare our precious carbohydrate stores. Finally, notice that the energy dense fats such as the 16 carbon palmitate, yields a tremendous amount of ATP upon oxidation. We will discus this in more detail later in module one. In summary, only ATP can be used directly for mechanical work required during exercise. At rest there is only a very small amount of ATP in muscle. Thus, ATP generating pathways must be activated to maintain ATP levels if exercise is to continue for more than several seconds. The drop in energy charge during the exercise activates ATP generating pathways. Mitochondria are the source of ATP produced for the oxidation of the carbohydrates, fats and proteins.
