In starvation:
**Core Concept**
Glucagon stimulates glycogenolysis and gluconeogenesis in the liver, playing a crucial role in maintaining blood glucose levels during fasting and starvation. This physiological response is mediated by the hormone glucagon, which binds to its receptor on the surface of hepatocytes and stimulates the activation of key enzymes involved in glycogen breakdown and glucose production.
**Why the Correct Answer is Right**
During starvation, the body's primary goal is to maintain blood glucose levels. Glucagon, secreted by the pancreas, triggers a cascade of events that ultimately lead to the breakdown of glycogen stores and the production of new glucose molecules through gluconeogenesis. This process is essential for providing the brain and other glucose-dependent tissues with the necessary energy. The key enzymes involved in this process include glucose-6-phosphatase, which catalyzes the final step of gluconeogenesis, and phosphorylase, which breaks down glycogen into glucose-1-phosphate.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the physiological response to starvation. While cortisol does play a role in gluconeogenesis, it is not the primary hormone involved in this process.
* **Option B:** This option is incorrect because it is a description of the role of insulin, which actually inhibits gluconeogenesis and glycogenolysis in the liver.
* **Option C:** This option is incorrect because it is a description of the role of adrenaline, which also inhibits glycogenolysis and stimulates glycogen synthesis in the liver.
**Clinical Pearl / High-Yield Fact**
In critically ill patients, the administration of glucagon-like peptide-1 (GLP-1) analogs has been shown to improve glycemic control and reduce the risk of hypoglycemia.
**Correct Answer: D. Glucagon stimulates glycogenolysis and gluconeogenesis in the liver during starvation.**