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1-isoform of 5'-AMP-activated protein kinase in rat skeletal muscle
1Laboratory of Nutrition Chemistry, Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University; 2Department of Medicine and Clinical Science, Graduate School of Medicine, Kyoto University; 3Laboratory of Food Science, Department of Food Sciences and Nutritional Health, Kyoto Prefectural University; and 4Laboratory of Sports and Exercise Medicine, Graduate School of Human and Environmental Studies, Kyoto University, Kyoto, Japan
Submitted 23 August 2005 ; accepted in final form 20 October 2005
Skeletal muscle expresses two catalytic subunits, 
1 and 2, of the 5'-AMP-activated protein kinase (AMPK), which has been implicated in contraction-stimulated glucose transport and fatty acid oxidation. Muscle contraction activates the 2-containing AMPK complex (AMPK2), but this activation may occur with or without activation of the 1-containing AMPK complex (AMPK1), suggesting that AMPK2 is the major isoform responsible for contraction-induced metabolic events in skeletal muscle. We report for the first time that AMPK1, but not AMPK2, can be activated in contracting skeletal muscle. Rat epitrochlearis muscles were isolated and incubated in Krebs-Ringer bicarbonate buffer containing pyruvate. In muscles stimulated to contract at a frequency of 1 and 2 Hz during the last 2 min of incubation, AMPK1 activity increased twofold and AMPK2 activity remained unchanged. Muscle stimulation did not change the muscle AMP concentration or the AMP-to-ATP ratio. AMPK activation was associated with increased phosphorylation of Thr172 of the -subunit, the primary activation site. Muscle stimulation increased the phosphorylation of acetyl-CoA carboxylase (ACC), a downstream target of AMPK, and the rate of 3-O-methyl-D-glucose transport. In contrast, increasing the frequency (
5 Hz) or duration (5 min) of contraction activated AMPK1 and AMPK2 and increased AMP concentration and the AMP/ATP ratio. These results suggest that 1) AMPK1 is the predominant isoform activated by AMP-independent phosphorylation in low-intensity contracting muscle, 2) AMPK2 is activated by an AMP-dependent mechanism in high-intensity contracting muscle, and 3) activation of each isoform enhances glucose transport and ACC phosphorylation in skeletal muscle.
exercise; twitch; glucose transport; acetyl-coenzyme A carboxylase; 
-oxidation
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