Hydrolysis of ATP is the immediate energy source for almost all energy-requiring processes in the cell:
The store of ATP is limited and must therefore be continuously replenished. Regeneration of ATP occurs through aerobic and anaerobic processes by which energy-rich chemical substances (carbohydrates, fat and phosphocreatine) are transformed into compounds with less stored energy (lactate, H2O, CO2 and creatine). This is achieved by sequences of chemical reactions by which part of the change in free energy is used for the synthesis of ATP through a reversal of reaction (1). The ATP-ADP cycle constitutes a basic feature of energy metabolism in all cells and is an intermediate between energy-utilizing and energy-consuming processes.
Skeletal muscle is a unique tissue in terms of the large variation in energy turnover. Transition from rest to exercise involves a drastic increase in energy demand and the rate of ATP utilization can increase more than 100 times. This corresponds to a utilization of the whole muscle store of ATP in about 2-3 s. To maintain a constant muscle ATP concentration, which is necessary for cellular homeostasis, the rate of ATP regeneration must equal the rate of ATP utilization. To meet the energy requirements skeletal muscle is faced with intricate problems related to supply of fuels and oxygen as well as control of the energetic processes. Adjustment of the rate of ATP regeneration to energy requirements is very precise and involves both feed-forward and feedback control mechanisms. A detailed discussion of the control of energy metabolism is outside the scope of this chapter and the reader is referred to other reviews or textbooks.
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