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

Abstract

Skeletal muscle expresses two catalytic subunits, α1 and α2, 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 α2-containing AMPK complex (AMPKα2), but this activation may occur with or without activation of the α1-containing AMPK complex (AMPKα1), suggesting that AMPKα2 is the major isoform responsible for contraction-induced metabolic events in skeletal muscle. We report for the first time that AMPKα1, but not AMPKα2, 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, AMPKα1 activity increased twofold and AMPKα2 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 Thr172of the α-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 AMPKα1 and AMPKα2 and increased AMP concentration and the AMP/ATP ratio. These results suggest that 1) AMPKα1 is the predominant isoform activated by AMP-independent phosphorylation in low-intensity contracting muscle, 2) AMPKα2 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.