| Glycogen
plays a major role in supporting the energy demands of skeletal muscles
during high intensity exercise. Despite its importance, the amount
of glycogen stored in skeletal muscles is so small that a large fraction
of it can be depleted in response to a single bout of high intensity
exercise. For this reason, it is generally recommended to ingest food
after exercise to replenish rapidly muscle glycogen stores, otherwise
one's ability to engage in high intensity activity might be compromised.
But what if food is not available? It is now well established that,
even in the absence of food intake, skeletal muscles have the capacity
to replenish some of their glycogen at the expense of endogenous carbon
sources such as lactate. This is facilitated, in part, by the transient
dephosphorylation-mediated activation of glycogen synthase and inhibition
of glycogen phosphorylase. There is also evidence that muscle glycogen
synthesis occurs even under conditions conducive to an increased oxidation
of lactate post-exercise, such as during active recovery from high
intensity exercise. Indeed, although during active recovery glycogen
resynthesis is impaired in skeletal muscle as a whole because of increased
lactate oxidation, muscle glycogen stores are replenished in Type
IIa and IIb fibers while being broken down in Type I fibers of active
muscles. This unique ability of Type II fibers to replenish their
glycogen stores during exercise should not come as a surprise given
the advantages in maintaining adequate muscle glycogen stores in those
fibers that play a major role in fight or flight responses.
KEY
WORDS: Glycogen synthase, glycogen phosphorylase, Cori cycle,
glyconeogenesis.
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