Creatine is synthesised in the liver and kidney. From here the creatine is transported to where it is required, mainly the skeletal muscle and brain. Creatine passes into the blood from the sites of synthesis and then into cells down its concentration gradient through sodium and chloride dependent transporters. The location and amount of these transporters matches the cellular expression of creatine kinase, the enzyme that is responsible for the formation of creatine phosphate (phosphocreatine) from creatine and ATP. The skeletal muscle and brain therefore have high amounts of these transporters as they have a high requirement for creatine phosphate. As type 2 skeletal fibres have a higher requirement for creatine phosphate compared to type 1 fibres, they have higher expressions of creatine kinase and creatine transporters to allow them to accumulate more creatine phosphate. Adrenaline, insulin-like growth factor 1 (IGF-1), insulin, and exercise can influence the uptake of creatine into skeletal muscle.
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