Gastrointestinal Hormones
The epithelium of the gastrointestinal
tract contains multiple cell types, including specialized cells termed enteroendocrine
cells that number less than 1% of the cell population and yet form the
largest endocrine system of the body. Enteroendocrine cells synthesize, store,
and release chemical transmitters that are involved in gastrointestinal
motility, secretion, and absorption and in regulation of appetite. These transmitters
are predominantly small polypeptides that are also found in the enteric nervous
system and the central nervous system. There are more than 30 gut peptide
hormone genes identified, which express more than 100 bioactive peptides. They
are grouped into “families” according to their primary structure. In this
section, the pancreatic polypeptide family will be discussed.
Peptide YY is one of the gut peptides that belongs to the pancreatic polypeptide family of peptides, which also includes pancreatic polypeptide and neuropeptide Y. Despite sharing structural similarities and the same 36 amino acid lengths, the gut peptides vary in their biologic functions and locations. Peptide YY, neuropeptide Y, and pancreatic polypeptide bind to a family of G-protein–linked receptors (called Y receptors). At present, five receptor subtypes have been identified.
Peptide
YY is secreted from L cells in the ileum and H cells in the colon in response
to an oral nutrient load.
Peptide
YY levels start to rise within 15 minutes of any caloric ingestion, long before
the nutrients themselves reach the distal gut, implying that other neural or
hormonal mechanisms are involved in its release. The actions of peptide YY are
largely inhibitory. It inhibits gastrointestinal motility, pancreatic and
gastric secretion, and chloride secretion, causing a delay in intestinal
transit, or the so-called ileal brake. This allows for a longer contact
time between nutrients and the small intestine. Peptide YY is also believed to
be involved in the regulation of food intake and
satiety, acting mainly via the Y2 receptors in the hypothalamus.
Pancreatic polypeptide is secreted by specialized pancreatic islet cells and inhibits gallbladder contraction and pancreatic exocrine secretion. It may influence food intake, energy metabolism, and the expression of gastric ghrelin and hypothalamic peptides. Neuropeptide Y is a neurotransmitter predominantly found in sympathetic neuron and is the most potent known stimulant of food intake.
