ATP during exercise

Figure 14.7. Sources of ATP During Exercise. In the initial seconds, exercise is powered by existing high phosphoryl transfer compounds (ATP and creatine phosphate). Subsequently, the ATP must be regenerated by metabolic pathways.

EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

As is often the case, tissue-specific control mechanisms operate to optimise adaptation to particular conditions. For example, muscle contraction requires an increase in cytosolic calcium ion concentration (see Section 7.2.1, Figure 7.4). During exercise when energy generation needs to be increased, or from a more accurate metabolic point of view, when the ATP-to-ADP ratio falls rapidly, and the accompanying rise in [Ca2 + ] activate (i) glycogen phosphorylase which initates catabolism of... 

P NMR spectra of human forearm muscle showing the effect of exercise. A Before exercise B and C during 19 minutes of exercise D 5-6 minutes after C. Peak assignments 1, /3-phosphorus of ATP 2, -phosphorus of ATP 3, y-phosphorus of ATP 4, phosphocreatine 5, Pi. Phosphocreatine is used as a major source of energy during exercise. It is hydrolyzed to creatine and Pi. Note that the level of ATP remains relatively constant during exercise because it is produced and used at about the same rate. After G. Radda, Science 233, 641 (1986). Reprinted with permission from the American Association for the Advancement of Science. 

Mitochondrial oxidative ADP phosphorylation is the primary source of ATP in skeletal muscle during aerobic exercise. Thus, to maintain the tree-energy state of the cytoplasmic phosphoenergetic compounds ATP, ADP, and inorganic phosphate (PI), oxidative phosphorylation is modulated to match the rate of ATP utilization during exercise. The mitochondrial components of Wu et a/. s model are illustrated in Figure 7.13. Additional components include ATP hydrolysis, adenylate kinase,... 

Muscle lactate, ATP, and CP levels during exercise after physical training in man. J. Appl. Physiol. 33 199-203. 

During exercise, [ATP] falls and [AMP] rises. Recall that AMP is an allosteric activator of glycogen phosphorylase b. Thus, even in the absence of covalent modification by phosphorylase kinase, glycogen is degraded. 

Figure 16.17 Regulation of glycolysis in muscle. At rest (left), glycolysis is not very active (thin arrows). The high concentration of ATP inhibits phosphofructokinase (PFK), pyruvate kinase, and hexokinase. Glucose 6-phosphate is converted into glycogen (Chapter 21). During exercise (right), the decrease in the ATP/AMP ratio resulting from muscle contraction activates phosphofructokinase and hence glycolysis. The flux down the pathway is increased, as represented by the thick arrows.

F. 22.13. Changes in ATP, ADP, and AMP concentrations in skeletal muscle during exercise. The concentration of ATP decreases by only approximately 20% during exercise, and the concentration of ADP rises. The concentration of AMP, produced by the adenylate kinase reaction, increases manyfold and serves as a sensitive indicator of decreasing ATP levels. 

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