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1 Diabetes Biology, Novo Nordisk, DK-2880 Bagsværd; and 2 Copenhagen Muscle Research Centre, August Krogh Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
We examined whether the protein level and/or activity of glycogenin, the protein core upon which glycogen is synthesized, is limiting for maximal attainable glycogen levels in rat skeletal muscle. Glycogenin activity was 27.5 ± 1.4, 34.7 ± 1.7, and 39.7 ± 1.3 mU/mg protein in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. A similar fiber type dependency of glycogenin protein levels was seen. Neither glycogenin protein level nor the activity of glycogenin correlated with previously determined maximal attainable glycogen levels, which were 69.3 ± 5.8, 137.4 ± 10.1, and 80.0 ± 5.4 µmol/g wet wt in white gastrocnemius, red gastrocnemius, and soleus muscles, respectively. In additional experiments, rats were exercise trained by swimming, which resulted in a significant increase in the maximal attainable glycogen levels in soleus muscles (~25%). This increase in maximal glycogen levels was not accompanied by an increase in glycogenin protein level or activity. Furthermore, even in the presence of very high glycogen levels (~170 µmol/g wet wt), ~30% of the total glycogen pool continued to be present as unsaturated glycogen molecules (proglycogen). Therefore, it is concluded that glycogenin plays no limiting role for maximal attainable glycogen levels in rat skeletal muscle.
fiber type; training; supercompensation; proglycogen; macroglycogen
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