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1 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada
Muscle contraction increases glucose uptake and fatty acid (FA) metabolism in isolated rat skeletal muscle, due at least in part to an increase in AMP-activated kinase activity (AMPK). However, the extent to which AMPK plays a role in the regulation of substrate utilization during contraction is not fully understood. We examined the acute effects of 5-aminoimidazole-4-carboxamide riboside (AICAR; 2 mM), a pharmacological activator of AMPK, on FA metabolism and glucose oxidation during high intensity tetanic contraction in isolated rat soleus muscle strips. Muscle strips were exposed to two different FA concentrations (low fatty acid, LFA, 0.2 mM; high fatty acid, HFA, 1 mM) to examine the role that FA availability may play in both exogenous and endogenous FA metabolism with contraction and AICAR. Synergistic increases in AMPK 
2 activity (+45%; P < 0.05) were observed after 30 min of contraction with AICAR, which further increased exogenous FA oxidation (LFA: +71%, P < 0.05; HFA: +46%, P < 0.05) regardless of FA availability. While there were no changes in triacylglycerol (TAG) esterification, AICAR did increase the ratio of FA partitioned to oxidation relative to TAG esterification (LFA: +65%, P < 0.05). AICAR significantly blunted endogenous TAG hydrolysis (LFA: –294%, P < 0.001; HFA: –117%, P < 0.05), but had no effect on endogenous oxidation rates, suggesting a better matching between TAG hydrolysis and subsequent oxidative needs of the muscle. There was no effect of AICAR on the already elevated rates of glucose oxidation during contraction. These results suggest that FA metabolism is very sensitive to AMPK 2 stimulation during contraction.
(Received 20 December 2004;
accepted after revision 14 March 2005;
first published online 17 March 2005)
Corresponding author A. C. Smith: Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada, N1G 2W1. Email: asmith09{at}uoguelph.ca
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