The Function of Glucagon
In the 1920s, Kimball and Murlin studied pancreatic extracts, and found an additional substance with hyperglycemic properties. They described glucagon in 1923. The amino acid sequence of glucagon was described in the late 1950s. A more complete understanding of its role in physiology and disease was not established until the 1970s, when a specific radioimmunoassay was developed.
Glucagon generally elevates the amount of glucose in the blood by promoting gluconeogenesis and glycogenolysis.
Glucose is stored in the liver in the form of glycogen, which is a starch-like polymer chain made up of glucose molecules. Liver cells (hepatocytes) have glucagon receptors. When glucagon binds to the glucagon receptors, the liver cells convert the glycogen polymer into individual glucose molecules, and release them into the bloodstream, in a process known as glycogenolysis. As these stores become depleted, glucagon then encourages the liver and kidney to synthesize additional glucose by gluconeogenesis. Glucagon turns off glycolysis in the liver, causing glycolytic intermediates to be shuttled to gluconeogenesis.
Glucagon also regulates the rate of glucose production through lipolysis.
Glucagon production appears to be dependent on the central nervous system through pathways yet to be defined. In invertebrate animals, eyestalk removal has been reported to affect glucagon production. Excising the eyestalk in young crayfish produces glucagon-induced hyperglycemia.