a pancreatic hormone produced by cells in the islets of Langerhans. Glucagon, a protein of low molecular weight, strongly opposes the action of insulin; it raises the concentration of sugar (glucose) in the blood by promoting the breakdown of glycogen (the form in which glucose is stored in the liver). It also reduces the rate of glycogen synthesis, promotes the breakdown of protein, and promotes the metabolism of fat. Gastrointestinal glucagon, another form, is secreted into the blood when glucose is ingested; its only action appears to be to stimulate the secretion of insulin.