What is the role of glucagon in glucose metabolism?

• A. It promotes glucose uptake by muscle
• B. It inhibits glycogenolysis
• C. It lowers blood glucose by increasing insulin release
• D. It stimulates the liver to produce glucose when energy is needed.

Answer: (D)

The main concept here is how glucagon raises blood glucose during times of energy need. Glucagon signals the liver to release glucose when blood sugar is low or during fasting. It does this primarily by stimulating hepatic glycogenolysis (breaking down stored glycogen) and gluconeogenesis (making new glucose from substrates like lactate, glycerol, and amino acids). This hepatic glucose production increases the amount of glucose entering the bloodstream.

This action relies on glucagon receptors in liver cells that raise cyclic AMP and activate protein kinase A, which in turn activates glycogen phosphorylase and inhibits glycogen synthase, promoting glucose release. Glucagon’s effect is opposite to insulin in many respects: insulin promotes glucose uptake into muscle and fat and suppresses hepatic glucose production. The statement that glucagon stimulates the liver to produce glucose when energy is needed best captures its role in glucose metabolism.