The binding of epinephrine or glucagon to the corresponding membrane receptor has which of the following effects on glycogen metabolism?
**Core Concept**
The binding of epinephrine or glucagon to their respective membrane receptors triggers a cascade of intracellular signaling events that ultimately affect glycogen metabolism. This process involves the activation of **adenylate cyclase**, leading to an increase in **cyclic AMP (cAMP)** levels. cAMP serves as a second messenger, activating **protein kinase A (PKA)**.
**Why the Correct Answer is Right**
The activation of PKA by cAMP leads to the phosphorylation and activation of **phosphorylase kinase**, which in turn phosphorylates and activates **glycogen phosphorylase**. This enzyme catalyzes the breakdown of glycogen into glucose-1-phosphate, thereby promoting glycogenolysis. Concurrently, PKA also inhibits **glycogen synthase** by phosphorylating it, which reduces glycogen synthesis.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the binding of epinephrine or glucagon does not directly stimulate glycogen synthesis.
**Option B:** Incorrect as it does not accurately describe the effect on glycogen metabolism.
**Option C:** Incorrect because it inaccurately represents the outcome of epinephrine or glucagon binding on glycogen.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that both epinephrine and glucagon play crucial roles in the body's "fight or flight" response, with their effects on glycogen metabolism being critical for rapidly mobilizing glucose stores.
**Correct Answer:** D.