Phosphocreatine and oxygen consumption
Phosphocreatine (PCr) is degraded to form the ATP, which is required for contractile proteins (PCr + ADP à Cr + ATP) in muscles. Creatine is released intracellularly at the level of the contractile proteins and then diffusing into the mitochondria of the muscle cells which consume oxygen and produce ATP by a process called mitochondrial respiration. Creatine thus stimulates this production of ATP, thus allowing the reaction to occur in the opposite direction (Cr + ATP à PCr + ADP). This ATP, newly formed by the mitochondria, is immediately combined with creatine to reform phosphocreatine, which in turn will spread to the contractile proteins and allow the reconstitution of phosphocreatine stores. ATP production within the oxygen consuming mitochondria will allow regeneration of phosphocreatine at the level of the contractile proteins in proportion to the speed. Varying running speed makes it possible to take advantage of the of phosphocreatine-creatine shuttle (PCr/Cr) (Figure 30) between the mitochondria and the contractile proteins. Contrary to popular belief, the use of phosphocreatine is not restricted to a few seconds but occurs throughout exercise through its regeneration by aerobic metabolism.