Insulin and glucagon are antagonistic hormones. Glucagon is produced by the alpha cells of the islets of Langerhans in response to a low blood sugar concentration. The only cells that have glucagon receptors are liver cells and fat cells. Glucagon raises the blood glucose concentration by breaking down glycogen in the liver (glycogenolysis), reducing the amount of glucose absorbed by liver cells and increasing gluconeogenesis.Gluconeogenesis is the formation of glucose from non-carbohydrate sources e.g. fatty acids and amino-acids.