14.1.9—Glucagon cell signalling

When blood glucose falls, pancreatic alpha cells release glucagon. It acts mainly on liver cells to mobilise stored glycogen and support glucose production, increasing glucose release into the blood.

Glucagon control chain:

1. Alpha cells in the pancreatic islets detect a fall in blood glucose and release glucagon; beta-cell insulin secretion decreases.
2. Glucagon travels in the blood and binds to receptors on liver-cell membranes.
3. Liver-cell signalling activates enzymes that break glycogen down to glucose (glycogenolysis).
4. The liver releases more glucose into the blood, opposing the fall. If the fall persists, the liver can also support glucose production from non-carbohydrate substrates.

When blood glucose is high, insulin promotes glucose uptake and storage, whereas glucagon is the opposing low-glucose response. Together, these hormone pathways form negative feedback that returns blood glucose towards its functional range.

The glucagon enzyme cascade described here is a liver response; muscle glycogen is not directly released into the blood. This card covers hormonal control, not glucose test strips or biosensor readouts.