All cells us ATP fo fuel their reactions and perform work. The concentration of ATP within most cells is generally around 5mM; it is kept at this steady state level because new ATP is synthesized as fast as it is utilized. Muscle cells present a special case because they are called upon for both sudden bursts and long, sustained periods of intense activity. During endurance exercise, a muscle may utilize a hundred to a thousand times as much ATP as it does during rest. Somehow the supply has to adjust and meet these enormous demands. ATP is supplied via three separate sources: creatine phosphate, the glycolysis-lactic acid system, and aerobic metabolism or oxidative phosphorylation.
THE HIGH-ENERGY PHOSPHATE SYSTEM; The amount of ATP present in muscle cells at any given moment is small. By itself, it is barely enough to sustain 5-6 seconds of intense activity, say a 50 meter dash. But as ATP is utilized, it is quickly replenished by the small reserve of energy stored as creatine phospate. Creatine phosphate very rapidly donates its high energy phosphate to ADP the moment ADP forms, converting it back to ATP. This extra source of ATP is easily mobilized and is very effective as long as it lasts. Unfortunately, this is limited because the store of creatine phosphate is small, only about four to five times larger than the original store of ATP. Normally, the supply of creatine phosphate is replenished by oxidative metabolism via the ATP produced by the Krebs cycle. But during sustained, intense exercise, there is not enough time for this to occur. Thus, after some 20-25 seconds of intense activity, we are back in the same place– NO ATP. We require additional sources.
THE GLYCOLYSIS-LACTID ACID SYSTEM
ATP can be supplied in a hurry through the anaerobic breakdown of glucose. Each time a glucose is chopped up by this anaerobic path, 2 ATP are formed. Its advantage is that it produces the ATP without O2, and it produces it fast. Though half as fast as the creatine phosphate system, it is two to three times faster than aerobic metabolism. It is limited, however, because on this path the hydrogens stripped off glucose that are normally bound for O2 to form water are taken up instead by pyruvate to form lactic acid. For each new ATP, this pathway is limited by this accumulation of lactic acid which produces fatigue. In addition, anaerobic glycolysis produces very small amounts of ATP, 2 per glucose consumed compared to oxidative phosphorylation, which yields 36 ATP per glucose.
AEROBIC METABOLISM– OXIDATIVE PHOSPHORYLATION
This system utilizes fats as well as glucose and glycogen. In contrast to creatine phosphate, or glycolysis aerobic metabolism to adjust to the increased demands of exercise. Thus, anaerobic processes are required not only for brief peak physical exertion, but also to supply energy of the beginning at the beginning of long-term muscular activity before aerobic metabolism becomes fully mobilized. Once this has occurred, an exhausted runner may experience a “second wind.”
Not all skeletal muscle cells are the same. The three types red/slow, red/fast, and white/fast; differ in their capacity to generate ATP, their speed of contraction, and their resistance to fatigue. In general, whole skeletal muscles in humans contain all three types, but in different proportions. Postural muscles of the back, for example, are continually active and have a high proportion of red/slow fibers. These fibers are specialized for aerobic metabolism. They contain the red respiratory pigment myoglobin, which stores O2 and facilitates the diffusion of O2 within the muscle to mitochondria. Further, the fibers are small, surrounded by many capillaries and they contract slowly so the blood supply of O2 can keep up with demand. Red/fast fibers are intermediate between red/slow and white/fast. White/fast fibers are abundant in muscles that have rapid, intense bursts of activity. Myoglobin is absent, mitochondria are sparse, and capillaries are less profuse. Glycolysis is well developed so that ATP is produced rapidly, but the muscle fatiques quickly when the limited glycogen stores are depleted. Muscles of the arms, which may be called upon to produce strong contractions over short periods of time (e.g., weight lifting), have a relatively large proportion of white/fast fibers.
Daryl Conant, M.Ed.