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Skeletal Muscle Adaptation to Exercise Training

AMP-Activated Protein Kinase Mediates Muscle Fiber Type Shift

¹ Research Division, Joslin Diabetes Center and Department of Medicine, Brigham and Women's Hospital, and Harvard Medical School, Boston, Massachusetts
² Dartmouth Medical School, Hanover, New Hampshire

Address correspondence and reprint requests to Laurie J. Goodyear, PhD, Senior Investigator and Head, Section on Metabolism, Joslin Diabetes Center, One Joslin Place, Boston, MA 02215. E-mail: laurie.goodyear{at}joslin.harvard.edu

Abbreviations: AICAR, 5-aminoimidazole-4-carboxamide-1-ß-D-ribofuranoside; AMPK, AMP-activated protein kinase; MHC, myosin heavy chain; PGC, peroxisome proliferator–activated receptor- coactivator

Regular endurance exercise has profound benefits on overall health, including the prevention of obesity, cardiovascular disease, and diabetes. The objective of this study was to determine whether AMP-activated protein kinase (AMPK) mediates commonly observed adaptive responses to exercise training in skeletal muscle. Six weeks of voluntary wheel running induced a significant (P < 0.05) fiber type IIb to IIa/x shift in triceps muscle of wild-type mice. Despite similar wheel running capacities, this training-induced shift was reduced by 40% in transgenic mice expressing a muscle-specific AMPK2 inactive subunit. Sedentary mice carrying an AMPK-activating mutation (1TG) showed a 2.6-fold increase in type IIa/x fibers but no further increase with training. To determine whether AMPK is involved in concomitant metabolic adaptations to training, we measured markers of mitochondria (citrate synthase and succinate dehydrogenase) and glucose uptake capacity (GLUT4 and hexokinase II). Mitochondrial markers increased similarly in wild-type and AMPK2-inactive mice. Sedentary 1TG mice showed a 25% increase in citrate synthase activity but no further increase with training. GLUT4 protein expression was not different in either line of transgenic mice compared with wild-type mice and tended to increase with training, although this increase was not statistically significant. Training induced a 65% increase in hexokinase II protein in wild-type mice but not in AMPK2-inactive mice. Hexokinase II was significantly elevated in sedentary 1TG mice, without an additional increase with training. AMPK is not necessary for exercise training-induced increases in mitochondrial markers, but it is essential for fiber type IIb to IIa/x transformation and increases in hexokinase II protein.

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