Low-intensity contraction activates the α1-isoform of 5′-AMP-activated protein kinase in rat skeletal muscle

Skeletal muscle expresses two catalytic subunits, alpha 1 and alpha 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 alpha 2-containing AMPK complex (AMPK alpha 2), but this activation may occur with or without activation of the alpha 1-containing AMPK complex (AMPK alpha 1), suggesting that AMPK alpha 2 is the major isoform responsible for contraction-induced metabolic events in skeletal muscle. We report for the first time that AMPK alpha 1, but not AMPK alpha 2, 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, AMPK alpha 1 activity increased twofold and AMPK alpha 2 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 Thr(172) of the alpha-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 AMPK alpha 1 and AMPK alpha 2 and increased AMP concentration and the AMP/ ATP ratio. These results suggest that 1) AMPK alpha 1 is the predominant isoform activated by AMP-independent phosphorylation in low- intensity contracting muscle, 2) AMPK alpha 2 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.