Glycogen phosphorolysis can form a metabolic shuttle to support Ca2+ uptake by sarcoplasmic reticulum membranes in skeletal muscle.

In the presence of glycogen, ADP, phosphoglucomutase and hexokinase, the glycogen phosphorylase b activity associated to sarcoplasmic reticulum (SR) membranes stimulates Ca2+ uptake by SR membrane fragments in the absence of added ATP. Phosphoglucomutase and hexokinase lead to the formation of glucose 6-phosphate which in turn is used as an ATP regenerating system by the Ca2+ pump. It is proposed that a raise of cytosolic AMP and ADP concentrations after muscle contraction can activate an alternative metabolic route which would be used to ensure the maintenance of a low cytosolic Ca2+ concentration and avoid unnecessary metabolic energy depletion in muscle cells.