Energy Requirements for Muscle Contraction
Muscle fibers are very energy-demanding cells whether at rest or during any form of exercise. This energy comes from either aerobic (with O2) or anaerobic (without O2) ATP production .
Generally, ATP is derived from four processes in skeletal muscle:
1. Aerobic production of ATP during most exercise and normal conditions
2. Anaerobic production of ATP during intensive short-term work
3. Conversion of a molecule called creatine (krē′a-tēn) phosphate to ATP
4. Conversion of two ADP to one ATP and one AMP (adenosine monophosphate) during heavy exercise
Aerobic respiration, which occurs mostly in mitochondria,requires O2 and breaks down glucose to produce ATP, CO2, and H2O. Aerobic respiration can also process lipids or amino acids to make ATP. Anaerobic respiration, which does not require O2, breaks down glucose to produce ATP and lactate.
In general, slow-twitch fibers work aerobically, whereas fast-twitch fibers are more suited for working anaerobically. Low-intensity, long-duration exercise is supported through mainly aerobic pathways. High-intensity, short-duration exercise, such as sprinting or carrying something very heavy, is supported through partially anaerobic pathways. There are very few, if any, activities that are supported through exclusively anaerobic pathways and those can only be sustained for a few seconds. Because exercise is not usually exclusively aerobic or anaerobic, we see both muscle fiber types contributing to most types of muscle function.
Historically, it was thought that ATP production in skeletal muscle was clearly delineated into either purely aerobic activities or purely anaerobic activities, and that the product of anaerobic respiration was principally lactic acid. Lactic acid was consid-ered to be a harmful waste product that must be removed from the body. However, it is now widely recognized that anaerobic respiration ultimately gives rise to lactic acid’s alternate chemical form, lactate. Moreover, it is now known that lactate is a critical metabolic intermediate that is formed and utilized continuously even under fully aerobic conditions. Lactate is produced by skeletal muscle cells at all times, but particularly during exercise, and is subsequently broken down (70–75%) or used to make new glucose (30–35%). Thus, the aerobic and anaerobic mechanisms of ATP production are linked through lactate.
Aerobic respiration is much more efficient than anaerobic respiration, but takes several minutes. With aerobic respiration pathways, the breakdown of a single glucose molecule produces approximately 18 times more ATP than that through anaerobic respiration pathways. Additionally, aerobic respiration is more flexible than anaerobic respiration because of the ability to break down lipids and amino acids to form ATP, as noted earlier.
Anaerobic respiration produces far less ATP than aerobic res-piration, but can produce ATP in a matter of a few seconds instead of a few minutes like aerobic respiration. However, ATP production rate by anaerobic respiration is too low to maintain activities for more than a few minutes.
Because muscle cells cannot store ATP, how do they generate enough ATP at a rate to keep pace with their high-energy demand? They store a different high-energy molecule called creatine phos-phate. Creatine phosphate provides a means of storing energy that can be rapidly used to help maintain adequate ATP in contracting muscle fibers. During periods of rest, as excess ATP is produced, the excess ATP is used to synthesize creatine phosphate. During exercise, especially at the onset of exercise, the small ATP reserve is quickly depleted. Creatine phosphate is then broken down to directly synthe-size ATP. Some of this ATP is immediately used, and some is used to restore ATP reserves. Figure 7.12 summarizes how aerobic and anaerobic respiration, lactic acid fermentation, and creatine phosphate production interact to produce a continuous supply of ATP.
When a muscle cell is working too strenuously for ATP stores and creatine phosphate to be able to provide enough ATP, anaerobic respiration predominates. Typically, the type II fibers are the primary anaerobic fibers. The type II fibers break down glucose into the intermediate, lactate, which can be shuttled to adjacent type I fibers to make ATP, or secreted into the blood for uptake by other tissues such as the liver to make new glucose. Thus, we see that in skeletal muscle, the type II fiber (anaerobic) pathways and the type I fiber (aerobic) pathways are not mutually exclusive. Rather, they work together, with lactate being the product of the type II fiber pathways that then serves as the starting point of the type I fiber pathways
Ultimately, if the use of ATP is greater than the production of ATP, the ATP:ADP ratio decreases, which interferes with the func-tioning of all of the major ATP-dependent enzymes in the muscle fibers. The ATP-dependent enzymes include the myosin head, the sarcoplasmic reticulum Ca2+ re-uptake pump, and the Na+/K+ pump for the resting membrane potential maintenance, all of which are required for proper muscle functioning. If the ATP:ADP ratio declines, an enzyme transfers one phosphate from one ADP to another ADP, generating one ATP and one AMP (adenosine mono-phosphate). The presence of AMP triggers a switch from anaerobic respiration to aerobic respiration of blood glucose and fatty acids. If this switch were not to occur, the muscles could not maintain their activity and could ultimately fail (see “Fatigue” in the next section). Figure 7.12 summarizes how aerobic and anaerobic respiration and creatine phosphate production interact to keep the muscles supplied with the ATP they need.
After intense exercise, the respiratory rate and volume remain elevated for a time, even though the muscles are no longer actively contracting. This increased respiratory activity provides the O2 to pay back the oxygen deficit. The recovery oxygen consumption is the amount of O2 needed in chemical reactions that occur to (1) convert lactate to glucose, (2) replenish the depleted ATP and creatine phosphate stores in muscle fibers, and (3) replenish O2 stores in the lungs, blood, and muscles. After the lactate produced by anaerobic respiration is converted to glucose and creatine phosphate levels are restored, respiration rate returns to normal.
The magnitude of the oxygen deficit depends on the intensity of the exercise, the length of time it was sustained, and the physical condition of the individual. The metabolic capacity of an individual in poor physical condition is much lower than that of a well-trained athlete. With exercise and training, a person’s ability to carry out both aerobic and anaerobic activities is enhanced.