Glucagon stimulates –
**Core Concept**
Glucagon is a hormone produced by the pancreas that plays a crucial role in glucose homeostasis. It acts as a counter-regulatory hormone to insulin, promoting the breakdown of glycogen and the release of glucose into the bloodstream.
**Why the Correct Answer is Right**
Glucagon acts on the liver to stimulate glycogenolysis, the breakdown of glycogen to glucose. This process is mediated by the enzyme glycogen phosphorylase, which is activated by glucagon binding to its receptor. The glucagon receptor is a G protein-coupled receptor that activates adenylate cyclase, leading to an increase in cyclic AMP (cAMP) levels. cAMP then activates protein kinase A (PKA), which phosphorylates and activates glycogen phosphorylase. This results in the breakdown of glycogen to glucose, which is then released into the bloodstream.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucagon does not stimulate glycolysis, which is the breakdown of glucose to pyruvate. Glucagon actually inhibits glycolysis to conserve glucose and promote its release into the bloodstream.
**Option B:** This option is incorrect because glucagon does not stimulate lipolysis, which is the breakdown of triglycerides to fatty acids. Although glucagon can stimulate lipolysis indirectly by promoting the breakdown of glycogen and releasing glucose into the bloodstream, it is not a direct effect.
**Option C:** This option is incorrect because glucagon does not inhibit gluconeogenesis, which is the synthesis of glucose from non-carbohydrate sources. Glucagon actually stimulates gluconeogenesis to increase glucose levels in the bloodstream.
**Clinical Pearl / High-Yield Fact**
Glucagon is often used clinically to stimulate glycogen breakdown and glucose release in cases of hypoglycemia. It is particularly useful in emergencies when rapid glucose administration is necessary.
**Correct Answer:** C.