Glucagon is a single-chain polypeptide of 29 amino acids. It has significant homology with several other polypeptide hormones, including secretin, vasoactive intestinal peptide (VIP), and GIP. Glucagon is synthesized from preproglucagon, a 180-amino-acid precursor. An amino-terminal signal peptide is followed by glicentin-related pancreatic peptide, glucagon, GLP-1, and glucagon-like peptide-2. Processing of the protein occurs in a tissue-specific fashion; this results in different secretory peptides in pancreatic a cells and intestinal a-like cells (L cells).
In the pancreatic a cell, the granule consists of a central core of glucagon surrounded by a halo of glicentin. Intestinal L cells contain only glicentin and presumably lack the enzyme required to process this precursor to glucagon. Enteroglucagon binds to hepatic glucagon receptors and stimulates adenylyl cyclase with 10—20% of the potency of glucagon. GLP-1 is an extremely potent potentiator of insulin secretion (see above), although it apparently lacks significant hepatic actions.
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