Abstract
BackgroundGlycogen-depleting exercise can lead to supercompensation of muscle glycogen stores, but the biochemical mechanisms of this phenomenon are still not completely understood.MethodsUsing chronic low-frequency stimulation (CLFS) as an exercise model, the tibialis anterior muscle of rabbits was stimulated for either 1 or 24 hours, inducing a reduction in glycogen of 90% and 50% respectively. Glycogen recovery was subsequently monitored during 24 hours of rest.ResultsIn muscles stimulated for 1 hour, glycogen recovered basal levels during the rest period. However, in those stimulated for 24 hours, glycogen was supercompensated and its levels remained 50% higher than basal levels after 6 hours of rest, although the newly synthesized glycogen had fewer branches. This increase in glycogen correlated with an increase in hexokinase-2 expression and activity, a reduction in the glycogen phosphorylase activity ratio and an increase in the glycogen synthase activity ratio, due to dephosphorylation of site 3a, even in the presence of elevated glycogen stores. During supercompensation there was also an increase in 5′-AMP-activated protein kinase phosphorylation, correlating with a stable reduction in ATP and total purine nucleotide levels.ConclusionsGlycogen supercompensation requires a coordinated chain of events at two levels in the context of decreased cell energy balance: First, an increase in the glucose phosphorylation capacity of the muscle and secondly, control of the enzymes directly involved in the synthesis and degradation of the glycogen molecule. However, supercompensated glycogen has fewer branches.
Highlights
Glycogen, the branched polymer of glucose, is the main energy reserve in muscle and is important for glucose homeostasis
Muscle glycogen levels are regulated by glycogenin, muscle glycogen synthase and glycogen phosphorylase. mGS is the key enzyme for glycogen synthesis
AMPK play a central role in muscle adaptation to contraction [8] and regulating glycogen synthesis through acute effects such as glucose availability and chronic effects involving the control of gene expression [9]
Summary
The branched polymer of glucose, is the main energy reserve in muscle and is important for glucose homeostasis. MGS is the key enzyme for glycogen synthesis. Muscle glucose uptake and phosphorylation control glycogen synthesis through substrate availability [3,4,5]. The 59-AMP-activated protein kinase (AMPK) acts as a sensor of cellular energy charge [7]. It is activated by phosphorylation under conditions of energetic stress, such as muscle contraction. The enhanced activity of AMPK during contraction leads to increased muscle contraction-dependent glucose transport via an increase in GLUT4 recruitment to the cell membrane [3]. AMPK play a central role in muscle adaptation to contraction [8] and regulating glycogen synthesis through acute effects such as glucose availability and chronic effects involving the control of gene expression [9]. Glycogen-depleting exercise can lead to supercompensation of muscle glycogen stores, but the biochemical mechanisms of this phenomenon are still not completely understood
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