In muscle, phosphorylase b is inactivated by:
**Core Concept**
Phosphorylase b is a key enzyme involved in glycogenolysis in muscle cells. It catalyzes the breakdown of glycogen to glucose-1-phosphate, which is then converted to glucose-6-phosphate and enters glycolysis. The activity of phosphorylase b is tightly regulated by phosphorylation and dephosphorylation reactions.
**Why the Correct Answer is Right**
The correct answer is **Glucagon**. When glucagon binds to its receptor on the surface of muscle cells, it activates a signaling cascade that ultimately leads to the phosphorylation of phosphorylase kinase. This enzyme then phosphorylates and activates phosphorylase b, allowing it to break down glycogen. However, when glucagon is removed, the activity of phosphorylase b is inhibited by dephosphorylation. This is the correct answer because glucagon is a hormone that stimulates glycogenolysis in muscle cells.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because insulin, not adrenaline, inhibits glycogenolysis in muscle cells. Insulin stimulates glycogenesis and inhibits glycogenolysis by activating protein phosphatase 1, which dephosphorylates and inactivates phosphorylase b.
**Option B:** This option is incorrect because adrenaline (epinephrine) actually activates glycogenolysis in muscle cells by stimulating the breakdown of glycogen to glucose-1-phosphate.
**Option C:** This option is incorrect because this is not a correct description of the regulation of phosphorylase b. The correct regulation involves phosphorylation and dephosphorylation reactions, not just inhibition by a hormone.
**Clinical Pearl / High-Yield Fact**
Glucagon and adrenaline are both hormones that stimulate glycogenolysis in muscle cells, but they have different effects on the body. Glucagon is released when blood glucose levels are low, while adrenaline is released in response to stress or excitement.
**Correct Answer:** B. Adrenaline.