Glycogenin protein and mRNA expression in response to changing glycogen concentration in exercise and recovery

Glycogenin ( GN- 1) is essential for the formation of a glycogen granule; however, rarely has it been studied when glycogen concentration changes in exercise and recovery. It is unclear whether GN- 1 is degraded or is liberated and exists as apoprotein ( apo)- GN- 1 ( unglycosylated). To examine this, we measured GN- 1 protein and mRNA level at rest, at exhaustion ( EXH), and during 5 h of recovery in which the rate of glycogen restoration was influenced by carbohydrate ( CHO) provision. Ten males cycled ( 65% VO2max) to volitional EXH ( 117.8 +/- 4.2 min) on two separate occasions. Subjects were administered carbohydrate ( CHO; 1 g . kg(-1) . h(-1) Gatorlode) or water [ placebo ( PL)] during 5 h of recovery. Muscle biopsies were taken at rest, at EXH, and following 30, 60, 120, and 300 min of recovery. At EXH, total glycogen concentration was reduced ( P < 0.05). However, GN-1 protein and mRNA content did not change. By 5 h of recovery, glycogen was resynthesized to similar to 60% of rest in the CHO trial and remained unchanged in the PL trial. GN-1 protein and mRNA level did not increase during recovery in either trial. We observed modest amounts of apo-GN-1 at EXH, suggesting complete degradation of some granules. These data suggest that GN-1 is conserved, possibly as very small, or nascent, granules when glycogen concentration is low. This would provide the ability to rapidly restore glycogen during early recovery.