Glycogen in muscle

Notes Assuming a moderately active 70 kg man with 40% of body weight as muscle (28 kg), a liver weight of 1.8 kg, a plasma volume of 3 L, and 9 kg of adipose tissue. Endurance trained individuals store -125-150 mmol/kg wet muscle of glycogen in muscle and 400-700 mmol/kg wet tissue of glycogen in liver,... 

McArdle s disease is associated with excessive deposits of glycogen in muscle, and Hers disease with its deposition in liver. In both cases phosphorylase levels in the affected tissues are very low. In spite of this, glycogen synthesis is unimpaired, which is incompatible with glyco-genesis occurring through the action of phosphorylase. 

Figure 11-2 Roles of phosphofructose kinase and fructose 1,6-bisphosphatase in the control of the breakdown and storage (—+) of glycogen in muscle. The uptake of glucose from blood and its release from tissues is also illustrated. The allosteric effector fructose 2,6-bisphosphate (Fru-2,6-P2) regulates both phosphofructokinase and fructose 2,6-bisphosphatase. These enzymes are also regulated by AMP if it accumulates. The activity of phosphofructokinase-2 (which synthesizes Fru-2,6-P2) is controlled by a cyclic AMP-dependent kinase and by dephosphorylation by a phosphatase.

Glycogen is stored mainly by the liver and skeletal muscle as an energy reserve. The role of stored glycogen in muscle is to provide a source of energy upon prolonged muscle contraction. In contrast, glycogen stored in the liver is used to maintain blood glucose levels. 

Phosphofructokinase deficiency (Tarui s disease) is an inborn error of glycogen metabolism characterised by a phosphofructokinase deficiency in the muscles, and associated with abnormal deposition of glycogen in muscle tissues, occasionally with myoglobinuria. The symptoms are similar to those seen in McArdle s disease. Classic Tarui s disease typically presents in childhood with exercise intolerance and anaemia. The fatal infantile variant presents in the first year of life. All reported patients have died by age four years. A late-onset variant manifests itself during later adulthood with progressive limb weakness without myoglobinuria or cramps. It is an autosomal recessive inheritance. Males are slightly more often affected than females. 

R. G. Shulman and D.L. Rothman. 2001. The "glycogen shunt" in exercising muscle A role for glycogen in muscle energetics and fatigue Proc. Natl. Acad. Sci. USA 98 457-461. (PubMed) (Full Text in PMC)... 

C) have lower than normal amounts of glycogen in muscle tissue... 

Patients with this rare type have deposits of abnormal glycogen in muscle. Exercise intolerance, unresponsiveness to glucose administration, and hemolysis (caused by decreased glycolysis in erythrocytes) are noted clinically, producing hyperbilirubinemia, pigmenturia, and reticulocytosis. The specific enzyme defect can be demonstrated. 

It is certainly possible to increase the amount of glycogen in muscle, but a question remains about how long it will last during vigorous exercise. Ultimately, all diet considerations for athletes are very individual, and what works for one may not work for another. 

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