Submitted on August 24, 2007
Accepted on January 2, 2008
AMPK Activation Attenuates S6K1, 4E-BP1, and eEF2 Signaling Responses to High-frequency Electrically Stimulated Skeletal Muscle Contractions
David M. Thomson1, Christopher A. Fick1, and Scott E. Gordon2*
1 Human Performance Laboratory, Department of Exercise and Sport Science, and Department of Physiology, East Carolina University, Greenville, North Carolina, United States
2 Human Performance Laboratory, East Carolina University, 363 Ward Sports Medicine Building, Greenville, North Carolina, 27858, United States; Human Performance Laboratory, Department of Exercise and Sport Science, and Department of Physiology, East Carolina University, Greenville, North Carolina, United States
* To whom correspondence should be addressed. E-mail: gordonsc{at}ecu.edu.
Regulation of protein translation through Akt and the downstream mammalian target of rapamycin (mTOR) pathway is an important component of the cellular response to hypertrophic stimuli. It has been proposed that 5'-AMP-activated protein kinase (AMPK) activation during muscle contraction may limit the hypertrophic response to resistance-type exercise by inhibiting translational signaling. However, experimental manipulation of AMPK activity during such a stimulus has not been attempted. Therefore, we investigated whether AMPK activation can attenuate the downstream signaling response of the Akt/mTOR pathway to electrically stimulated lengthening muscle contractions. Extensor digitorum longus (EDL) muscles (n=8/group) were subjected to a 22-min bout of lengthening contractions by high-frequency sciatic nerve electrical stimulation (STIM) in young adult (8 mo.) Fischer344 x Brown Norway male rats. 40 min prior to electrical stimulation, rats were subcutaneously injected with saline (SAL) or 5-aminoimidazole-4-carboxamide-1-4-ribofuranoside (AICAR; 1 mg/g BW), an AMPK activator. Stimulated and contralateral resting muscles were removed at 0, 20 and 40 min post-STIM, and AMPK, Acetyl CoA carboxylase (ACC), Akt, eukaryotic initiation factor 4E-binding protein (4E-BP1), 70-kDa ribosomal protein S6 kinase (S6k), and eukaryotic elongation factor 2 (eEF2) phosphorylations were assessed by western blot. AICAR treatment increased (p
0.05) post-STIM AMPK (Thr172) and ACC phosphorylation (Ser79/221), inhibited post-STIM S6k (Thr389) and 4E-BP1 (gel shift) phosphorylation, and elevated post-STIM eEF2 phosphorylation (Thr56). These findings suggest that translational signaling downstream of Akt/mTOR can be inhibited after lengthening contractions when preceded by AMPK activation, and that energetic stress may be antagonistic to the hypertrophic translational signaling response to loaded muscle contractions.
