Brief overview: glucagon history and physiology.

Glucagon is a peptide hormone that is produced primarily by the alpha cells in the islet of Langerhans in the pancreas, but also in intestinal enteroendocrine cells and in some neurons. Approximately 100 years ago, several research groups discovered that pancreatic extracts would cause a brief rise in blood glucose before they observed the decrease in glucose attributed to insulin. An overall description of the regulation of glucagon secretion requires the inclusion of its sibling insulin because they both are made primarily by the islet and they both regulate each other in different ways. For example, glucagon stimulates insulin secretion, whereas insulin suppresses glucagon secretion. The mechanism of action of glucagon on insulin secretion has been identified as a trimeric guanine nucleotide-binding protein (G-protein)-mediated event. The manner in which insulin suppresses glucagon release from the alpha cell is thought to be highly dependent on the peri-portal circulation of the islet through which blood flows downstream from beta cells to alpha cells. In this scenario, it is via the circulation that insulin is thought to suppress the release of glucagon. However, high levels of glucose also have been shown to suppress glucagon secretion. Consequently, the glucose-lowering effect of insulin may be additive to the direct effects of insulin to suppress alpha cell function, so that in vivo both the discontinuation of the insulin signal and the condition of low glucose jointly are responsible for induction of glucagon secretion.