Editing Creatine (section)

===Phosphocreatine system===
[[File:Creatine kinase and phosphocreatine energy shuttle.png|thumb|464x464px|class=skin-invert-image|Proposed creatine kinase/phosphocreatine (CK/PCr) energy shuttle. CRT = creatine transporter; ANT = adenine nucleotide translocator; ATP = adenine triphosphate; ADP = adenine diphosphate; OP = oxidative phosphorylation; mtCK = mitochondrial creatine kinase; G = glycolysis; CK-g = creatine kinase associated with glycolytic enzymes; CK-c = cytosolic creatine kinase; CK-a = creatine kinase associated with subcellular sites of ATP utilization; 1 – 4 sites of CK/ATP interaction.]]
Creatine is transported through the blood and taken up by tissues with high energy demands, such as the brain and skeletal muscle, through an active transport system. The concentration of [[adenosine triphosphate|ATP]] in skeletal muscle is usually 2–5&nbsp;mM, which would result in a muscle contraction of only a few seconds.<ref name="ncbi.nlm.nih.gov">{{cite journal | vauthors = Wallimann T, Wyss M, Brdiczka D, Nicolay K, Eppenberger HM | title = Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the 'phosphocreatine circuit' for cellular energy homeostasis | journal = The Biochemical Journal | volume = 281 ( Pt 1) | issue = Pt 1 | pages = 21–40 | date = January 1992 | pmid = 1731757 | pmc = 1130636 | doi = 10.1042/bj2810021 }}</ref> During times of increased energy demands, the [[phosphagen]] (or ATP/PCr) system rapidly resynthesizes ATP from [[adenosine diphosphate|ADP]] with the use of [[phosphocreatine]] (PCr) through a reversible reaction catalysed by the enzyme [[creatine kinase]] (CK). The phosphate group is attached to an NH center of the creatine. In skeletal muscle, PCr concentrations may reach 20–35&nbsp;mM or more. Additionally, in most muscles, the ATP regeneration capacity of CK is very high and is therefore not a limiting factor. Although the cellular concentrations of ATP are small, changes are difficult to detect because ATP is continuously and efficiently replenished from the large pools of PCr and CK.<ref name="ncbi.nlm.nih.gov" /> A proposed representation has been illustrated by Krieder et al.<ref name=":3" /> Creatine has the ability to increase muscle stores of PCr, potentially increasing the muscle's ability to resynthesize ATP from ADP to meet increased energy demands.<ref>{{cite journal | vauthors = Spillane M, Schoch R, Cooke M, Harvey T, Greenwood M, Kreider R, Willoughby DS | title = The effects of creatine ethyl ester supplementation combined with heavy resistance training on body composition, muscle performance, and serum and muscle creatine levels | journal = Journal of the International Society of Sports Nutrition | volume = 6 | issue = 1 | pages = 6 | date = February 2009 | pmid = 19228401 | pmc = 2649889 | doi = 10.1186/1550-2783-6-6 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Wallimann T, Tokarska-Schlattner M, Schlattner U | title = The creatine kinase system and pleiotropic effects of creatine | journal = Amino Acids | volume = 40 | issue = 5 | pages = 1271–96 | date = May 2011 | pmid = 21448658 | pmc = 3080659 | doi = 10.1007/s00726-011-0877-3 }}.</ref><ref>T. Wallimann, M. Tokarska-Schlattner, D. Neumann u.&nbsp;a.: ''The Phosphocreatine Circuit: Molecular and Cellular Physiology of Creatine Kinases, Sensitivity to Free Radicals, and Enhancement by Creatine Supplementation.'' In: ''Molecular System Bioenergetics: Energy for Life.'' 22. November 2007. {{doi|10.1002/9783527621095.ch7}}C</ref>

Creatine supplementation appears to increase the number of [[myonuclei]] that satellite cells will 'donate' to damaged [[muscle fiber]]s, which increases the potential for growth of those fibers. This increase in myonuclei probably stems from creatine's ability to increase levels of the myogenic transcription factor MRF4.<ref>{{cite journal | vauthors = Hespel P, Eijnde BO, Derave W, Richter EA | title = Creatine supplementation: exploring the role of the creatine kinase/phosphocreatine system in human muscle | journal = Canadian Journal of Applied Physiology | volume = 26 Suppl | pages = S79-102 | year = 2001 | pmid = 11897886 | doi = 10.1139/h2001-045 }}</ref>