Glucagonlike peptide 1: a newly discovered gastrointestinal hormone

Glucagonlike peptide (GLP) 1, a peptide of 30 amino acids with 50% sequence homology to glucagon, results from expression of the glucagon gene in the L cells of the distal intestinal mucosa. It is secreted early in response to mixed meals by mechanisms involving the presence of unabsorbed nutrients in the gut lumen or the absorptive process itself, but other mechanisms may also be involved. GLP-1 has two important actions. First, it stimulates insulin secretion and inhibits glucagon secretion and thereby inhibits hepatic glucose production and lowers blood glucose levels. It may have effects on glucose clearance independent of its pancreatic effects. It acts on recently cloned G protein-coupled specific receptors and seems to increase insulin secretion via cyclic adenosine monophosphate-dependent increases in intracellular calcium. It has been suggested that activation of the beta cells by GLP-1 is a prerequisite for glucose-induced insulin secretion. Second, it also potently inhibits gastrointestinal secretion and motility and is likely to act as an "ileal brake," possibly after activation of cerebral receptors. Therefore, GLP-1 physiologically seems to signal nutritional abundancy and enhance deposition of nutrients. Because of these effects, however, the peptide can completely normalize blood glucose levels in type 2 diabetics and is therefore of considerable pharmaceutical interest.