Epinephrine glycogen degradation

Starvation ( ] glucagon i insulin) Glycogen exhaustion Diabetes ( f glucagon [ insulin) Glycogen degradation Excitement ( ] epinephrine). Glycogen degradation... 

Correct answer = B. Epinephrine and glucagon both cause increased glycogen degradation in the liver. Therefore, glycogen phosphorylase activity is increased, whereas glycogen synthase activity is decreased. Both cAMP-dependent protein kinase and its substrate, phosphorylase kinase, are also activated. 

The action of glucagon and epinephrine in the stimulation of glycogen degradation in their target tissues is mediated by a process involving cyclic AMP (Chap. 6). 

The signal-transduction processes in the liver are more complex than those in muscle. Epinephrine can also elicit glycogen degradation in the liver. However, in addition to binding to the p-adrenergic receptor, it binds to the 7TM a-adrenergic receptor, which then initiates the phosphoinositide... 

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). 

The target proteins affected by cAMP depend on the cell type. In addition, several hormones may activate the same G protein. Therefore different hormones may elicit the same effect. For example, glycogen degradation in liver cells is initiated by both epinephrine and glucagon. 

Epinephrine promotes the conversion of glycogen to glucose by activating adenylate cyclase, an enzyme whose product, cAMP, initiates a reaction cascade that activates the glycogen degrading enzyme glycogen phosphorylase. 

In this section, we first discuss how GPCR signals are transduced to an effector protein, a process that is similar for all receptors of this type. Then we focus on pathways in which cAMP is the second messenger, using the epinephrine-stimulated degradation of glycogen as an example. 

Exercise and stress Glucose t Tissue cAMP 4-Glucose t Blood Epinephrine t Glycogen degradation t... 

Epinephrine, acting at the (3-receptors, transmits a signal through G proteins to adenylate cyclase, which increases cAMP and activates protein kinase A. Hence, regulation of glycogen degradation and synthesis in liver by epinephrine and glucagon are similar (see Fig. 28.8). 

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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