Extracellular creatine regulates creatine transport in rat and human muscle cells.

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Abstract
Muscle cells do not synthesize creatine; they take up exogenous creatine by specific Na+-dependent plasma membrane transporters. We found that extracellular creatine regulates the level of expression of these creatine transporters in L6 rat muscle cells. L6 myoblasts maintained for 24 hr in medium containing 1 mM creatine exhibited 1/3rd of the creatine transport activity of cells maintained for 24 hr in medium without creatine. Down-regulation of creatine transport was partially reversed when creatine-fed L6 cells were incubated for 24 hr in medium lacking creatine. Down-regulation of creatine transport occurred independently of amino acid and glucose transport. Furthermore, the down-regulation of creatine transporters by extracellular creatine was slowed by inhibitors of protein synthesis. These results suggest that creatine induces the expression of a protein that functionally inactivates the creatine transporters. Regulation of creatine transport by extracellular creatine also was observed in L6 myotubes and in cultures of human myoblasts and myotubes. Hence, the activity of creatine transport represents another site for the regulation of creatine homeostasis.