glucagon
Glucagon is insulin's opposite number — the “release the reserves” signal. When blood sugar falls, alpha cells secrete glucagon, which mainly acts on the liver to break down stored glycogen and to manufacture new glucose, pushing blood sugar back up. It is a frontline defense against hypoglycemia.
Glucagon is a peptide hormone that binds a G protein-coupled receptor on liver cells, raising cyclic AMP and activating protein kinase A. This stimulates glycogenolysis and gluconeogenesis while suppressing glycogen synthesis. During prolonged fasting it also promotes ketone production, providing an alternative fuel for the brain.
Therapeutically, injectable or nasal glucagon is the emergency rescue for severe hypoglycemia in people on insulin. Conversely, suppressing inappropriate glucagon is a goal of several diabetes drugs: GLP-1 based therapies lower glucagon in part by restoring the normal glucose dependence of its release.
Counterintuitively, amino acids stimulate both insulin and glucagon. The glucagon response prevents protein-rich, low-carb meals from triggering insulin-induced hypoglycemia — an example of how the islet balances opposing signals.