Glycogen metabolism hormonal regulation

Figure 21.17 Coordinate control of glycogen metabolism. Glycogen metabolism is regulated, in part, by hormone triggered cyclic AMP cascades. The sequence of reactions leading to the activation of protein kinase A ultimately activates glycogen degradalion. At the same time, protein kinase A also inactivates glycogen synthase, shutting down glycogen synthesis.

Glycogen Metabolism Is Regulated by Hormone-Induced Activation of Protein Kinase A... 

Fig. 7.18. Regulation of glycogen metabolism in muscle. Phosphorylase kinase stands at the center of regulation of glycogen metabolism. Phosphorylase kinase may exist in an active, phosphorylated form and an inactive, unphosphorylated form. Phosphorylation of phosphorylase kinase is triggered by hormonal signals (e.g. adrenahne) and takes place via an activation of protein kinase A in the cAMP pathway. In the absence of hormonal stimulation, phosphorylase kinase can also be activated by an increase in cytosolic Ca. The active phosphorylase kinase stimulates glycogen degradation and inhibits glycogen synthesis, in that, on the one side, it activates glycogen phosphorylase by phosphorylation, and on the other side, it inactivates glycogen synthase by phosphorylation.

Recently Exton and co-workers [93] have proposed that adrenergic responsiveness in skeletal muscle is regulated by thyroid hormones at two levels, i.e., 1) stimulation of /3-adrenergic receptors and adenylate cyclase activity and 2) increased activity of phosphoprotein phosphatases. Such results would explain the effect of thyroid hormones on glycogen metabolism in muscle although the primary mechanism of these actions remains unknown. 

Finally, no attempt will be made to review the extensive literature on the influence of various hormones on glycogen metabolism with regard to insulin, the view that its major action is to regulate the permeability of cells to D-glucose, rather than to control the activity of certain enzymes, should not be overlooked. 

Begins with hydrolysis of fat to glycerol and free fatty acids (is hormonally regulated - comparable to glycogen metabolism - Figure 18.11)... 

Explain the role of hormones in regulating glycogen metabolism. 

The role of the cAMP system can be exemplified by the consideration of the hormonal regulation of glycogen metabolism. The activity of glycogen synthase and phosphorylase are controlled by two hormones, adrenaline and the pancreatic polypep-... 

Storage and utilization of tissue glycogen, maintenance of blood glucose concentration, and other aspects of carbohydrate metabolism are meticulously regulated by hormones, including insulin, glucagon, epinephrine, and the glucocorticoids. 

---