Glucagon stimulates
**Core Concept**
Glucagon is a peptide hormone secreted by the pancreas, primarily involved in glucose homeostasis. It plays a crucial role in raising blood glucose levels by stimulating glycogenolysis and gluconeogenesis in the liver.
**Why the Correct Answer is Right**
Glucagon acts by binding to its receptor on the surface of hepatocytes, which triggers a series of intracellular signaling events. The binding of glucagon to its receptor activates adenylyl cyclase, leading to an increase in intracellular cyclic AMP (cAMP) levels. Elevated cAMP levels, in turn, activate protein kinase A (PKA), which phosphorylates and activates key enzymes involved in glycogenolysis and gluconeogenesis, such as phosphoglucomutase and fructose-1,6-bisphosphatase. This results in the breakdown of glycogen to glucose and the production of glucose from non-carbohydrate sources, ultimately leading to an increase in blood glucose levels.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because glucagon does not stimulate insulin secretion. In fact, glucagon and insulin have opposing effects on glucose metabolism.
* **Option B:** This is incorrect because glucagon does not stimulate glycolysis. Glucagon's action is to increase blood glucose levels, whereas glycolysis is a pathway that breaks down glucose to produce energy.
* **Option C:** This is incorrect because glucagon does not stimulate lipolysis. While glucagon does stimulate the breakdown of glycogen, it does not directly stimulate the breakdown of triglycerides to fatty acids.
**Clinical Pearl / High-Yield Fact**
Glucagon's effects on glucose metabolism are mediated by its action on the liver, primarily through the stimulation of glycogenolysis and gluconeogenesis. This is in contrast to insulin, which acts to lower blood glucose levels by stimulating glucose uptake in peripheral tissues and inhibiting glycogenolysis and gluconeogenesis.
**Correct Answer: D**