Glucagon is a peptide hormoneproduced by alpha cells of the pancreas. It raises the concentration of glucose and fatty acids in the bloodstream and is considered to be the main catabolic hormone of the body. Its effect is opposite to that of insulinVikt och hälsa lowers extracellular glucose.
The pancreas releases glucagon when the amount of glucose in the bloodstream is too low. Glucagon causes the liver to engage in glycogenolysis : converting stored glycogen into glucosewhich is released into the bloodstream. Insulin allows glucose to be taken up and used by insulin-dependent tissues.
Thus, glucagon and insulin are part of a feedback system that keeps blood glucose levels stable. Glucagon increases energy expenditure and is elevated under conditions of stress. Glucagon is a amino acid polypeptide. The polypeptide has a molecular mass of Daltons.
The hormone is synthesized and secreted from alpha cells α-cells of the islets of Langerhanswhich are located in the endocrine portion of the pancreas. Glucagon is produced from the preproglucagon gene Gcg. Preproglucagon first has its signal peptide removed by signal peptidaseforming the amino acid protein proglucagon.
In intestinal L cellsproglucagon is cleaved to the alternate products glicentin 1—69glicentin-related pancreatic polypeptide 1—30oxyntomodulin 33—69glucagon-like peptide 1 72— orand glucagon-like peptide 2 — In rodents, the alpha cells are located in the outer rim of the islet.
Human islet structure is much less segregated, and alpha cells are distributed throughout the islet in close proximity to beta cells. Glucagon is also produced by alpha cells in the stomach. Recent research has demonstrated that glucagon production may also take place outside the pancreas, with the gut being the most likely site of extrapancreatic glucagon synthesis.
Glucagon generally elevates the concentration of glucose in the blood by promoting gluconeogenesis and glycogenolysis. Glucose is stored in the liver in the form of the polysaccharide glycogen, which is a glucan a polymer made up of glucose molecules.
Liver cells hepatocytes have glucagon receptors. When glucagon binds to the glucagon receptors, the liver cells convert the glycogen 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.