The Phosphagen System (ATP-PCr) occurs in the cytosol (a gel-like substance) of the sarcoplasm of skeletal muscle, and in the myocyte's cytosolic compartment of the cytoplasm of cardiac and smooth muscle.
During muscle contraction: 
:HO + ATP → H + ADP + P (Mg assisted, utilization of ATP for Muscle contraction by ATPase)
:H + ADP + CP → ATP + Creatine (Mg assisted, catalyzed by creatine kinase, ATP is used again in the above reaction for continued muscle contraction)
:2 ADP → ATP + AMP (catalyzed by adenylate kinase/myokinase when CP is depleted, ATP is again used for muscle contraction)
Muscle at rest:
:ATP + Creatine → ADP + CP + H (Mg assisted, catalyzed by creatine kinase)
:ADP + P → ATP (during anaerobic glycolysis and oxidative phosphorylation)
When the Phosphagen System has been depleted of phosphocreatine (creatine phosphate), the resulting AMP produced from the adenylate kinase (myokinase) reaction is primarily regulated by the Purine Nucleotide Cycle.