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Department of Biology, York University, Toronto, Canada
Recent observations have suggested that creatine supplementation might have a beneficial effect on glucoregulation in skeletal muscle. However, conclusive studies on the direct effects of creatine on glucose uptake and metabolism are lacking. The objective of this study was to investigate the effects of creatine supplementation on basal and insulin-stimulated glucose transporter (GLUT4) translocation, glucose uptake, glycogen content, glycogen synthesis, lactate production, glucose oxidation and AMP-activated protein kinase (AMPK) phosphorylation in L6 rat skeletal muscle cells. Four treatment groups were studied: control, insulin (100 nM), creatine (0.5 mM) and creatine + insulin. After 48 h of creatine supplementation the creatine and phosphocreatine contents of L6 myoblasts increased by 
9.3- and 5.1-fold, respectively, but the ATP content of the cells was not affected. Insulin significantly increased 2-deoxyglucose uptake (1.9-fold), GLUT4 translocation (1.8-fold), the incorporation of D-[U-14C]glucose into glycogen (2.3-fold), lactate production (1.5-fold) and 14CO2 production (1.5-fold). Creatine neither altered the glycogen and GLUT4 contents of the cells nor the insulin-stimulated rates of 2-DG uptake, GLUT4 translocation, glycogen synthesis and glucose oxidation. However, creatine significantly reduced by 42% the basal rate of lactate production and increased by 40% the basal rate of 14CO2 production. This is in agreement with the 35% increase in citrate synthase activity and also with the 2-fold increase in the phosphorylation of both 
-1 and -2 isoforms of AMPK after creatine supplementation. We conclude that 48 h of creatine supplementation does not alter insulin-stimulated glucose uptake and glucose metabolism; however, it activates AMPK, shifts basal glucose metabolism towards oxidation and reduces lactate production in L6 rat skeletal muscle cells.
(Received 2 October 2003;
accepted after revision 24 December 2003;
first published online 9 January 2004)
Corresponding author G. Sweeney: Department of Biology, York University, Toronto, M3J 1P3, Ontario, Canada. Email: gsweeney{at}yorku.ca
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