Regarding the regulation and physiological effects of glucagon, all of the following statements are correct EXCEPT:

Explanation

## Glucagon: Regulation and Physiological Effects ### Correct Statements (Options 0, 1, 2) **Option 0: Glucagon secretion and neural control** - Glucagon is secreted by pancreatic alpha (α) cells in the islets of Langerhans - Primary stimulus: **hypoglycaemia** (blood glucose < 70 mg/dL) - Secondary stimuli: amino acids (especially arginine), sympathetic activation - Sympathetic nervous system enhances glucagon secretion during stress and hypoglycaemia - **TRUE** **Option 1: cAMP-mediated signalling** - Glucagon binds to a **G-protein coupled receptor** (7-transmembrane domain) - Activates Gs protein → stimulates adenylyl cyclase - Adenylyl cyclase converts ATP → cAMP - cAMP activates protein kinase A (PKA) - PKA phosphorylates target enzymes (phosphorylase kinase, glycogen synthase, etc.) - **TRUE** — this is the canonical second-messenger mechanism for glucagon **Option 2: Hepatic glucose output** - Glucagon stimulates **glycogenolysis** (breakdown of glycogen to glucose) - PKA phosphorylates phosphorylase kinase → activates glycogen phosphorylase - Glucagon stimulates **gluconeogenesis** (synthesis of new glucose from lactate, amino acids, glycerol) - PKA phosphorylates PFK-2, shifting the balance toward gluconeogenesis - Increases availability of substrate (amino acids, lactate) - Both pathways increase hepatic glucose output - **TRUE** ### Incorrect Statement (Option 3) — THE ANSWER **Option 3: Glucagon and glucose uptake in muscle and fat** **Key Point:** Glucagon does **NOT** increase glucose uptake in skeletal muscle or adipose tissue. In fact, glucagon has **minimal or no effect** on glucose uptake in these tissues. - Glucagon receptors are present primarily on **hepatocytes** and **adipocytes** (for lipolysis) - Glucagon has **very few receptors** on skeletal muscle - Glucagon does **not** promote GLUT4 translocation in muscle or fat - Instead, glucagon promotes **glucose output** from the liver and **lipolysis** in adipose tissue - The effect of glucagon is to **raise** blood glucose, not to promote its uptake by peripheral tissues This is the opposite of insulin: insulin lowers blood glucose by promoting uptake; glucagon raises blood glucose by promoting hepatic output. ### Comparative Table: Insulin vs. Glucagon on Glucose Metabolism | Parameter | Insulin | Glucagon | | --- | --- | --- | | Blood glucose effect | ↓ Decreases | ↑ Increases | | Hepatic glycogenolysis | Inhibits | Stimulates | | Hepatic gluconeogenesis | Inhibits | Stimulates | | Muscle glucose uptake | Stimulates (GLUT4) | No effect | | Adipose glucose uptake | Stimulates (GLUT4) | No effect | | Lipolysis | Inhibits | Stimulates | | Mechanism | Tyrosine kinase | G-protein/cAMP/PKA | **Clinical Pearl:** During fasting or hypoglycaemia, glucagon rises and insulin falls. Glucagon mobilizes hepatic glucose; it does not pull glucose into muscle or fat. This is why patients with glucagon deficiency (e.g., in advanced liver disease) are prone to severe hypoglycaemia — they lose the ability to raise blood glucose. **High-Yield:** Glucagon is a **hyperglycaemic hormone**. It raises blood glucose by promoting hepatic output, not by promoting peripheral glucose uptake. Any option claiming glucagon increases glucose uptake in muscle or fat is **wrong**.