Peptide Hormones in the Gastrointestinal Tract

Gastrointestinal hormones are peptides released from endocrine cells and neurons in the digestive tract. More than 30 hormone genes are currently known to be expressed in the gastrointestinal tract, which makes the gut the largest hormone-producing organ in the body. Modern biology makes it feasible to conceive the hormones under five headings. (1) The structural homology groups a majority of the hormones into nine families, each of which is assumed to originate from one ancestral gene. (2) The individual hormone gene often has multiple phenotypes due to alternative splicing, tandem organization, or differentiated maturation of the prohormone. By a combination of these mechanisms, more than 100 different hormonally active peptides are released from the gut. (3) Gut hormone genes are also widely expressed outside the gut, some only in extraintestinal endocrine cells and neurons but others also in other cell types. (4) The extraintestinal cells may express different bioactive fragments of the same prohormone due to cell-specific processing pathways. (5) Finally, endocrine cells, neurons, and, for instance, spermatozoa display differential release of gut peptides, so the same peptide may act as a blood-borne hormone, a neurotransmitter, a long-acting growth factor, or a fertility factor.