The glucose stored in the liver as glyco-gen can be considered a reservoir for buffering blood glucose levels (A). Since glucose is the essential energy source of the CNS, blood glucose levels are regulated within a tight margin through the interplay of glucagon and insulin.
Muscle glycogen is not directly integrated into the blood glucose regulation system. Since muscles lack glucose-6-phosphatase, their cells are unable to convert glucose-6-phosphate formed during glycogen breakdown into glucose, so no glucose can be released into the blood. Rather, muscle glycogen serves as an energy reserve for the muscles. Since stored ATP and other high-energy phosphates last only seconds, while external energy supply through the blood takes some time, muscle gly-cogen is the sole intermediate energy source.
The storage and breakdown of muscle glycogen are not only influenced hor-monally, but also subject to allosteric regulation (B). During muscular activity, membrane depolarization and subsequent energy use create high levels of Ca2+ and AMP. Both substances activate glycogen phosphorylase, which catalyzes the first breakdown step towards glucose-1-phosphate. Glucose, glucose-6-phosphate, and ATP inhibit this enzyme. Glucose-6-phosphate, on the other hand, which is formed after glucose absorption, activates glycogen syn-thase.
blood is not sufficient to completely oxidize the glucose used by the muscle. Nevertheless, to ensure energy supply, glucose is converted to lactate during anaerobic glycolysis. Since the muscle has no further use for lactate, it is released into the bloodstream. In the liver, the lactate is used for gluconeo-genesis and the resulting glucose released into the blood. Continued activity, therefore, results in a net transfer of muscle glycogen to the liver. During the recuperation phase, the liver releases more glucose into the blood, which the muscles, in turn, use to rebuild glycogen stores.
During starvation or prolonged fasting when neither enough glycogen nor lac-tate is available to maintain adequate blood glucose levels, muscle protein is used for this purpose. Through transamination, specific, so-called gluco-genic amino acids are converted to alanine, which is released into the bloodstream. Upon reaching the liver, it is metabolized to glucose (gluconeo-genesis). Only through this cycle, called "Cori Cycle," is extended survival without food intake at all possible. In any case, it is bound to result in an unavoidable loss of muscle and fatty tissue.
In extreme situations, the muscle itself may also be used to supply glucose (C). In case of short-term, extreme muscular activity, the oxygen supplied by the
B. Glycogen Synthesis and Breakdown —
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