Immediate Energy System

Immediate Energy System | Creatine Phosphate Energy Shuttle

Immediate Energy Sources

There are three sources of ATP available immediately to muscle:

	ATPase
ATP	———————>	ADP + Pi

	creatine kinase
ADP + CP	———————>	ATP + creatine

	adenylate kinase
ADP + ADP	———————>	ATP + AMP

Muscular stores of ATP are small and are sufficient for only about 1 s of maximal effort. Sarcoplasmic creatine phosphate (CP), also known as phosphocreatine (PCr), phosphorylate ADP. The creatine returns to the mitochondria to be rephosphorylated itself by mitochondrial creatine kinase and mitochondrial ATP. At high exercise intensities, adenylate kinase, also known as myokinase, is another important immediate source of ATP. However, these systems are limited as ATP production must come from other sources if maximal exercise lasts longer than 5-10 s.

Creatine Phosphate Energy Shuttle

As the adenine nucleotide charge regulates both glycolysis and the Kreb's cycle, ADP and ATP also stimulate and inhibit, respectively, the ETC. With the onset of exercise, the [ADP] is increased with ATP utilization. This initiates a series of biochemical events to replenish ATP and restore normal levels of ATP, ADP, and AMP. The creatine phosphate energy shuttle is suggested to regulate the ETC as Cr is known to stimulate mitochondrial respiration. Cr, formed with the phosphorylation of ADP and CP, moves to the mitochondria where it is itself rephosphorylated by a different creatine phosphokinase. The resultant CP then moves back into the myofibrils or other energy-utilizing sites. This process results in an increased mitochondrial [ADP] which stimulates the entire oxidative process.