GLOSSARY

Glucagon is a 29-amino-acid peptide hormone secreted by the alpha cells of the islets of Langerhans and acts opposite to insulin. Its main job is to raise blood glucose during fasting, exercise and hypoglycaemia.

Main Effects

• Hepatic glycogenolysis: rapid breakdown of liver glycogen, releasing glucose into the blood.
• Gluconeogenesis: making glucose from amino acids, lactate and glycerol.
• Ketogenesis: increased production of ketone bodies in the liver, especially when insulin is low.
• Amino acids: increased hepatic uptake of amino acids; this is the liver–alpha cell axis.

Mechanism

Glucagon binds to a G-protein-coupled receptor on liver cells and, by raising cAMP and activating protein kinase A, switches on the enzymes that break down glycogen and inhibits those that build it.

Scientific Note