This document summarizes gastrointestinal hormones. It describes that GI hormones are secreted from endocrine cells in the stomach and small intestine. They are broadly divided into 3 categories: the gastrin family including gastrin and CCK; the secretin family including secretin, VIP, and GIP; and other polypeptides like motilin, neurotensin, substance P, somatostatin, ghrelin, guanylin, and PYY. For each hormone, it provides information on source, structure, functions, mechanisms, and factors that regulate their secretion.

1. GI
HORMONES

2.  GI Hormones – Hormones secreted from
the endocrine cells
 Especially in stomach & small intestine
 Endocrine cells that secrete
oEnteroendocrine - GI hormones
o Enterochromaffin – Serotonin
o APUD – amines & polypeptides
(neuroendocrine cells)
Carcinoid tumors originate from
neuroendocrine cells.

3. • Broadly divided into 3 categories
1.Gastrin family- Gastrin & CCK
2.Secretin family- Secretin, Vasoactive
intestinal peptide (VIP) & Gastric
inhibitory polypeptide (GIP)
3.Other polypeptides
CLASSIFICATION OF GI HORMONES

4. Gastrin
• Produced by G cells in the stomach
that are mainly in the antral region.
• Also present in hypothalamus,
ant pituitary, medulla & fetal pancreas.
• Gastrin as a neurotransmitter is also
secreted from vagus & sciatic nerve.
SOURCE

5. • A polypeptide hormone
• It has marked Heterogeneity
• Macroheterogeneity- Gastrin having diff polypeptide length
• Microheterogeneity-Gastrin having diff molecular structure
• G 34, G 17, G 14(depending on number of AA they possess)
• G 17-principal gastrin secreted from the stomach
• METABOLISM – Secreted from G cells of pyloric
antrum enters general circulation, activated in the
intestine & degraded in the kidney

6. Functions/ Actions
1. Stimulation of gastric acid & pepsin
secretion from stomach.
2. Most potent natural stimulator of HCl
secretion from parietal cells
3. Stimulates growth of gastric mucosa &
mucosa of intestine (Tropic action of Gastrin)
4. ↑ gastric motility
5. Contraction of muscles at the gastro-
esophageal junction, it prevents reflux
esophagitis

7. 6.Stimulates exocrine pancreatic
secretion
7. Stimulates insulin secretion
8. Stimulates mass movement of
large intestine
9.Colonic contraction that initiates
gastrocolic reflex after a meal

8. MECHANISM OF ACTION
• It binds with gastrin receptors on parietal
cells &↑ intracellular calcium concentration
via second messenger,IP3. Increase protein
kinases that stimulates acid secretion
Control of gastrin secretion-
• Stimuli that Increase - gastric distention, protein
products of digestion, increased vagal discharge &
calcium
• Decrease- acid in stomach, somatostatin, secretin, GIP,
VIP, calcitonin & glucagon

10. Cholecystokinin - Pancreozymin
(CCK-PZ or CCK)
• I cells in the upper intestine
• Nerves of distal ileum & colon
• Neurons in brain as neurotransmitter
( cerebral cortex) & in nerves in many
part of the body
• peptides & amino acids, & long chain
fatty acids

11. • polypeptide hormone
• Heterogeneity
• CCK 58, CCK 39, CCK 33, CCK 12, CCK 8, CCK 4
−Duodenum & jejunum CCK 12, CCK 8
−Enteric & pancreatic nerves CCK 4
−Brain CCK 58, CCK 8
• Half-life of CCK is about 5 minutes

12. FUNCTIONS OF CCK
1. Contraction of gall bladder- cck increases bile release into the
intestine following a meal
2. It stimulates pancreatic secretion rich in enzymes- cholecystokinin
– (CCK-PZ)
3. It inhibit the gastric acid secretion
4. It inhibit gastric motility, delays gastric emptying
5. Stimulates growth of pancreas
6. Enhances intestinal motility
7. Stimulates glucagon secretion
8. causes contraction of gall bladder
9. ↑ secretion of enterokinase
10. Enhance motility of small intestine
11. Stimulates colonic movements

13. MECHANISM OF ACTION- CCK
• 2 TYPES OF RECEPTORS
• Receptor - A – located in the gall bladder,
pancreas, GIT etc
• Receptor –B – Present in brain areas
• Cck act on both receptors activates
membrane phospholipase – C &
stimulates intracellular IP3 & DAG

14. REGULATION OF SECRETION -CCK
• Factors That Increases Cck Secretion
1. Contact of intestinal mucosa with
products of digest –FA, aa,& peptides
2. Bile & pancreatic juice - facilitates
digestion of protein & fat –they
provide positive feedback for cck
secretion

16. GI HORMONES OF SECRITIN FAMILY
• SECRETIN –first hormone to be
discovered
• Secreted from S cells – located in the
mucosa of upper part of small intestine
• STRUCTURE – polypeptide hormone
containing 27 aa

17. FUNCTIONS OF SECRETIN
1. Increases secretion of pancreatic juice rich
in bicarbonate
2. Increases alkaline bile secretion
3. Decreases gastric secretion & motility
4. Augments the action of cck to produce
pancreatic secretion rich in enzymes
5. Causes contraction of pyloric sphicnter

18. MECHANISM OF ACTION
• Secretin acts on adenylcyclase on the cell
membrane & increase cytosolic formation of
cAMP
REGULATION OF SECRETION
• Increase in secretin seen –products of protein
digestion entering the upper part of intestine,
stimulates watery and pancreatic secretion, which
neutrilizes the acid content in upper small
intestine.
• The increased Ph of duodenal & upper jejunal
content decreases secretin secretion by feedback
mechanism

19. GASTRIC INHIBITORY PEPTIDE-GIP
• Polypeptide hormone containing 42 aa
Sources - produced by K cells present in the mucosa of
duodenum & jejunum
FUNCTIONS
• In inhibits gastric secretion & motility
• It stimulates insulin secretion
REGULATION OF SECRETION
• Secretion of GIP is increased by glucose & fat in the
duodenum

20. Vasoactive intestinal peptide( VIP )
• A polypeptide containing 28 aa
SOURCE – secreted from mucosal cells of entire GIT
starting from stomach to colon -- more in the colon
• Found in nerves in the GIT
• also found in blood, brain & autonomic nerves
FUNCTION –
1 It markedly increases secretions of electrolytes & water
2 It causes vasodilatation --- decreases blood pressure
3 It decreases GI motility
4 It potentiates the action of acetylcholine on salivary
glands

21. OTHER GI HORMONES
Motilin
• polypeptide hormone containing 22 AA
• Enterochromaffin cells & Mo cells present
in the mucosa of GIT
• ↑ GI motility (interdigestive phase)
• a major regulator of MMC, that regulates
GI motility between meals.

22. Neurotensin
• A polypeptide hormone containing 13
AA
• Produced by neurons & mucosal cells
of ileum
• Inhibits gastric acid secretion but ↑
ileal blood flow

23. Substance P
• Secreted from Endocrine & nerve
cells in the GIT
• ↑ intestinal motility of the small
intestine

24. • Somatostatin
• a polypeptide containing either SS 14 or SS 28
• D cells in GIT, hypothalamus, D cells of
pancreas
• inhibits secretion of gastrin, VIP, GIP, secretin
& motilin
• inhibits pancreatic exocrine secretion; gastric
acid secretion & motility; gall bladder
contraction; & absorption of glucose, amino
acids & triglycerides

25. Ghrelin
• 28 amino-acid polypeptide
• Secreted by oxyntic cell in gastric
fundus
• Can be found in pancreas, intestine,
hypothalamus, pituitary gland

26. Factors influencing Ghrelin
secretion
• Food intake
• Most important
• Ghrelin level increases 1-2 hr prior to meal,
max just before eating and decreases
dramatically within 1 hr after meal
• Degree of ghrelin level decrease is in
proportion with calories and composition
of food (CHO can decrease ghrelin > fat)

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28. Guanylin
• polypeptide hormone containing 15 AA
• secreted from the paneth cells
• ↑ secretion of Cl-into the intestinal lumen
Peptide YY
• entire GIT (ileum & colon)
• inhibits the food intake
• inhibits gastric acid secretion & motility