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Classification:
The GI hormones are grouped into following families
according to the structural and functional similarity-
A. Gastrin family:
1. Gastrin
Local hormones of alimentary tract 2. Cholecystokinin.
B. The secretin family:
1. Secretin
2. Glucagon
3. Gastric inhibitory polypeptide (GIP)
4. Vasoactive intestinal polypeptide (VIP).

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Glands of alimentary tract C. Other groups:

• Mucous glands: 1. Somatostatin
– They are single cell mucus secreting glands present in 2. Neurotensin
most part of GI tract on the surface epithelium. 3. Motilin
Sometimes they are called goblet cells. 4. Bombesin
• Tubular glands:
5. Substance P
– Present in stomach and upper duodenum. e.g.
oxyntic gland in stomach. 6. Gastrin releasing peptide (GRP)
• Pits: 7. Guanylin
– They represent invaginations of epithelium into 8. Serotonin
submucosa. e.g. Crypts of Lieberkühn are pits 9. Enkephalin
present in small intestine which contain specialized 10. Pancreatic polypeptide
secretory cells.
11. Ghrelin
• Complex glands: Salivary glands, pancreas and liver
12. Glucagon like peptide (GLP) etc.

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Enteroendocrine cells:
Local Hormones of GIT
 More than 15 types of hormone-secreting
 These are biologically active polypeptides that are enteroendocrine cells have been identified in the
secreted by nerve cells and gland cells in the mucosa, mucosa of the stomach, small intestine, and colon.
act in the paracrine fashion but also enter the Many of these secrete only one hormone and are
circulation. identified by letters (G cells, S cells, etc).
 These gastrointestinal hormones play roles in the  Others manufacture serotonin or histamine and are
regulation of gastrointestinal secretion and motility. called enterochromaffin or ECL cells, respectively.

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Cholecystokinin (CCK):
 Site of secretion: I cells of duodenal and upper jejunal
mucosa.
 Type of hormone: polypeptide containing 33 amino
acids.
 Stimulus for secretion:
Presence of mixtures of polypeptides & amino acids,
gastric HCl, fatty acids in small intestine.
 Inhibitor: CCK secretion is inhibited by somatostatin.

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Gastrin  Functions of cholecystokinin:

 Site of secretion: It is produced & stored by G cells or 1. Stimulates secretion of pancreatic juice rich in
Gastric cells of gastric antrum (mainly), proximal enzymes (ecbolic secretion).
duodenum & delta cells pancreatic islands. 2. Stimulates contraction of gall bladder.
 Type of hormone: large polypeptide 3. Relaxes sphincter of Oddi.
 Stimulus for secretion: presence of small peptides and 4. Stimulates gastric HCl & pepsinogen secretion.
amino acids in stomach and vagal stimulation. 5. Increases the motility of duodenum & small
 Inhibitory factors: H+ in the lumen of the stomach by intestine.
negative feedback control. 6. Inhibits gastric emptying. Thus meal containing fat
stimulates the secretion of CCK which slows gastric
emptying to allow more time for digestion.

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Secretin:
 Site of secretion: S cells of duodenal and jejunal
 Functions of gastrin: mucosa.
1. Stimulate secretion of gastric acid, pepsinogen,  Type of hormone: polypeptide containing 27 amino
intrinsic factor from parietal cell. acids.
2. Stimulates secretion of pancreatic juice  Stimulus for secretion: highly acidic food in small
containing HCO-3 & enzymes. intestine, gastric HCl, fatty acids, alcohol etc.
3. Stimulates secretion of hepatic bile.  Inhibitor: Secretin secretion is inhibited by
somatostatin.
4. Increases gastric & intestinal motility.
 Functions:
5. Stimulates the growth of gastric mucosa.
1. Stimulates pancreatic juice secretin rich in HCO3-
(hydralytic secretion) and helps in neutralizing the
acidic food from stomach.

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2. Stimulates HCO3- & water secretion from the

liver.
3. Augments the action of CCK to concentrate bile
in the gall bladder & increased hepatic bile flow.
4. Reduces gastric & duodenal motility.
5. It inhibits gastric acid secretion.
 By secreting HCO3- it provides an appropriate pH
for action of pancreatic enzymes which function
optimally in a slightly alkaline or neutral medium.
 Neutralization of acid chyme is a protective
mechanism against development of duodenal
ulcer

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Gastric inhibitory hormones:

They all inhibit gastric secretion to slow
passage of chyme from the stomach when the
small intestine is already filled.
– Gastric inhibitory peptide (GIP)
– Vasoactive intestinal peptide ( VIP)
– Somatostatin

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