Which of the following is NOT an action of glucagon?
**Core Concept**
Glucagon is a peptide hormone secreted by the pancreas that plays a crucial role in glucose metabolism. It stimulates the breakdown of glycogen to glucose and promotes gluconeogenesis, thereby increasing blood glucose levels.
**Why the Correct Answer is Right**
Glucagon acts on the liver to stimulate glycogenolysis, the breakdown of glycogen to glucose, through the activation of the enzyme glycogen phosphorylase. This process is mediated by glucagon receptors on the surface of hepatocytes, which trigger a signaling cascade that ultimately leads to the activation of glycogen phosphorylase. Additionally, glucagon promotes gluconeogenesis, the generation of new glucose molecules from non-carbohydrate precursors, by stimulating the expression of key enzymes involved in this pathway.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucagon does stimulate glycogenolysis, but it does not promote glycogenesis, the synthesis of glycogen from glucose.
**Option B:** This option is incorrect because glucagon does inhibit glycolysis, the breakdown of glucose to pyruvate, by reducing the activity of key enzymes involved in this pathway.
**Option C:** This option is incorrect because glucagon does promote lipolysis, the breakdown of triglycerides to fatty acids and glycerol, which can then be used as energy sources.
**Clinical Pearl / High-Yield Fact**
Glucagon is often used clinically to manage hypoglycemia, particularly in patients with diabetes. However, it is essential to note that glucagon is not effective in patients with pancreatic alpha-cell dysfunction, such as those with pancreatic cancer or pancreatitis.
**Correct Answer: D. Glucagon does not promote glycogenesis.**