Control glycogen degradation

Glycogen degradation and glycogen synthesis are controlled both by allosteric regulation and by hormonal control. 

Fig. 4. Dual control of glycogen degradation and glycogen synthesis by protein kinase A.

Skeletal muscle cells lack glucagon receptors. Hormonal control of glycogen degradation is achieved by epinephrine via P-adrenergic activation of adenylate cyclase, resulting in enhanced cytoplasmic cyclic AMP levels. Neural activation of skeletal muscle cells considerably increases the cytoplasmic Ca level. Cyclic AMP and Ca " act in a synergistic fashion to fully express the activity of glycogen phosphorylase in the process described above (Devlin, 1992). 

This elegant system of hormonal control ensures that the reactions involved in glycogen degradation and synthesis do not compete with one another. In this way they provide glucose when the blood level is too low, and they cause the storage of glucose in times of excess. 

Epinephrine is a hormone synthesized in the adrenal glands from tyrosine (see p. 352). Its release is subject to neuronal control. This emergency hormone mainly acts on the blood vessels, heart, and metabolism. It constricts the blood vessels and thereby increases blood pressure (via ai and a2 receptors) it increases cardiac function (via P2 receptors) it promotes the degradation of glycogen into glucose in the liver and muscles (via P2 receptors) and it dilates the bronchia (also via P2 receptors). 

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