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Intensified exercise training does not alter AMPK signaling in human skeletal muscle

¹Exercise Metabolism Group, School of Medical Sciences, RMIT University, Victoria 3083; ²St. Vincent's Institute of Medical Research, and Department of Medicine, University of Melbourne, Fitzroy, Victoria 3065; and ³Muscle, Ions and Exercise Group, School of Human Movement, Recreation and Performance, Centre for Rehabilitation, Exercise and Sports Science, Melbourne, Victoria University of Technology, Victoria 3011, Australia

Submitted 14 October 2003 ; accepted in final form 14 December 2003

The AMP-activated protein kinase (AMPK) cascade has been linked to many of the acute effects of exercise on skeletal muscle substrate metabolism, as well as to some of the chronic training-induced adaptations. We determined the effect of 3 wk of intensified training (HIT; 7 sessions of 8 x 5 min at 85% O_{2 peak}) in skeletal muscle from well-trained athletes on AMPK responsiveness to exercise. Rates of whole body substrate oxidation were determined during a 90-min steady-state ride (SS) pre- and post-HIT. Muscle metabolites and AMPK signaling were determined from biopsies taken at rest and immediately after exercise during the first and seventh HIT sessions, performed at the same (absolute) pre-HIT work rate. HIT decreased rates of whole body carbohydrate oxidation (P < 0.05) and increased rates of fat oxidation (P < 0.05) during SS. Resting muscle glycogen and its utilization during intense exercise were unaffected by HIT. However, HIT induced a twofold decrease in muscle [lactate] (P < 0.05) and resulted in tighter metabolic regulation, i.e., attenuation of the decrease in the PCr/(PCr + Cr) ratio and of the increase in [AMP_free]/ATP. Resting activities of AMPK1 and -2 were similar post-HIT, with the magnitude of the rise in response to exercise similar pre- and post-HIT. AMPK phosphorylation at Thr¹⁷² on both the 1 and 2 subunits increased in response to exercise, with the magnitude of this rise being similar post-HIT. Acetyl-coenzyme A carboxylase- phosphorylation was similar at rest and, despite HIT-induced increases in whole body rates of fat oxidation, did not increase post-HIT. Our results indicate that, in well-trained individuals, short-term HIT improves metabolic control but does not blunt AMPK signaling in response to intense exercise.

adenosine 5'-monophosphate-activated protein kinase; acetyl-coenzyme A carboxylase; glycogen; fat metabolism

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