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J Appl Physiol (April 28, 2005). doi:10.1152/japplphysiol.00275.2005
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Submitted on March 8, 2005
Accepted on April 28, 2005

Glycogenin Activity and mRNA Expression in Response to Volitional Exhaustion in Human Skeletal Muscle

Jane Shearer1*, Terry E. Graham1, Danielle S. Battram1, Deborah L. Robinson1, Erik A. Richter2, Rhonda J. Wilson1, and Marica Bakovic1

1 Department of Human Biology and Nutritional Science, University of Guelph, Guelph, ON, Canada
2 Copenhagen Muscle Research Centre, University of Copenhagen, Copenhagen, Denmark

* To whom correspondence should be addressed. E-mail: jshearer{at}ucalgary.ca.

Glycogenolysis results in the selective catabolism of individual glycogen granules by glycogen phosphorylase. However, once the carbohydrate portion of the granule is metabolized, the fate of glycogenin, the protein primer of granule formation is not known. To examine this, male subjects (n=6) exercised to volitional exhaustion (Exh) on a cycle ergometer at 75% VO2 max. Muscle biopsies were obtained at rest, 30 min and Exh (99 ±10 min). At rest, total glycogen concentration (Gt) was 497 ± 41 and declined to 378 ± 51 mmol glucosyl units/kg dw following 30 min of exercise (p<0.05). There were no significant changes in proglycogen (PG), macroglycogen (MG), glycogenin activity or mRNA in this period (p≥0.05). Exh resulted in decreases in Gt, PG and MG as well as glycogenin activity (p<0.05). These decrements were associated with a 1.9 ±0.4 fold increase in glycogenin mRNA over resting values (p<0.05). Glycogenolysis in the initial exercise period (0-30min) was not adequate to induce changes in glycogenin, however, later in exercise when concentration and granule number decreased further, decrements in glycogenin activity and increases in glycogenin mRNA were demonstrated. Results show that glycogenin becomes inactivated with glycogen catabolism and that this event coincides with an increase in glycogenin gene expression as exercise and glycogenolysis progress.




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