Glucagon is secreted by the a cells of the islets of Langerhans in the pancreas. A major stimulus for its secretion is hypoglycemia (low blood glucose levels). Glucagon acts on the hepatic cells of the liver to cause glycogenolysis, the breakdown of glycogen, by activating the enzyme phosphorylase. It also enhances gluconeogenesis (formation of glucose) from amino acids and lactate. Thus, the major actions of glucagon oppose those of insulin. The a and b cells in the islets have a close functional relationship with one another; there is intraislet regulation of glucagon by insulin and of insulin by glucagon ( 3.7). Because of this, it is claimed that it is difficult to separate the direct effects of changes in plasma glucose levels on glucagon secretion from the a cells from control of glucagon secretion by insulin. When plasma glucose levels increase about twofold, glucagon secretion is inhibited by concurrent changes in b cell activity rather than by the direct effects of glucose or insulin on the a cells (38).
Glucagon must attach to its specific receptor in the plasma membrane to activate cellular responses. This specific glucagon receptor is a member of a superfamily of guanine nucleotide-binding protein-coupled receptors, and it is also a member of a smaller subfamily of homologous receptors for peptides structurally related to glucagon GLP-1 (glucagon-like peptide-1), GIP (gastric inhibitory peptide), VIP (vasoactive intestinal peptide), secretin, GRF (growth hormone-releasing factor), and PACAP (pituitary adenylate cyclase-activating polypeptide). Using specific glucagon receptor mRNA expression in rat tissues, glucagon receptor mRNA was observed to be relatively abundant in liver, adipose tissue, and pancreatic islets, as expected, but also in heart, kidney, spleen, thymus, and stomach. Low levels were found in adrenal glands, small intestine, thyroid, and skeletal muscle. No expression was observed in testes, lung, large intestine, or brain ( 39).
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