Hormones of GIT
Humoral agents (biological active polypeptides)
secreted by cells in mucosa and
transported in the circulation to influence
functions of stomach, intestines, the
pancreas & gall bladder.
Act in a paracrine fashion.
Regulation of GIT
Gastrointestinal Hormones
• Biologically active polypeptides
• Secreted by;
i. nerve cells &
ii. gland cells in the mucosa of G.I.T
Enteroendocrine cells: (G cells or S cells)
> 15 types of cells identified in mucosa of stomach,
small intestine & colon
APUD cells (Amine Precursor Uptake & Decarboxylase cells) or
neuroendocrine cells found in lungs & GIT;
form carcinoid tumors.
• Regulate;
* Secretions &
* Motility of GIT
Gastrointestinal Hormones
• Classification:
1. On the basis of structural & functional similarity
a. Gastrin Family: Gastrin, CCK
b. Secretin Family: Secretin, Glucagon
Glycentin (GLI)
VIP & GIP
2. Other GI Hormones
Neurotensin Glucagon
Subs P Ghrelin
GRP Guanylin
Bombesin Urogastrone
Somatostatin
Gastrin
Produced by: G-cells (flask shaped) of antral mucosa of
stomach.
Respond to changes in gastric contents
Receptors of gastrin are present on microvilli
Function During fetal life Gastrin is found in pancreatic islets.
Not known
Also found in Ant & Int lobes of Pituitary, Hypothalamus,
Medulla, Vagus & Sciatic nerves.
Types: i. ‘Little’ Gastrin; Principal form, G17 i.e.17 A.Acids
Secreted by G-Cells of gastric antrum
in response to meal.
ii. ‘Big’ Gastrin; G34 i.e. 34 A. Acids
Secreted between meals
Secretion of Gastrin
Stimuli that increase Gastrin secretion
i. Luminal: Protein meal (Peptides & A. Acids)
Distention
ii. Neural: Increased vagal discharge via GRP
iii. Blood borne: ↑ Calcium
↑ Epinephrine
Stimuli that decrease Gastrin secretion
i. Luminal: Acid (low pH) in antrum
Somatostatin
ii. Blood-borne: Secretin, GIP, VIP, Glucagon, Calcitonin
GASTRIN
• Inactivated: In kidneys, S & L. intestines.
• Actions:
a. Secretion of Gastric acid, Pepsin
b. Stimulation of Gastric Motility
c. Contraction of Gastro-Esophageal sphincter
d. Trophic action: Growth of mucosa of
stomach, S & L intestines.
e. Insulin secretion
• Gastrin secreting tumor or Gastrinoma
(Zollinger - Ellison syndrome)
usually in non-beta-cell pancreas
Sign & symptoms attributed to gastrin i.e. duodenal
ulcers, steatorrhea.
Cholecystokinin-Pancreozymin (CCK- Pz)
• Now CCK
• Secreted by:
I-cells of duodenal & jejunal mucosa in response to
breakdown products of prot, fats in duodenum i.e.
a. monoglycerides & fatty acids (not TGs)
b. small peptides & a. acids.
Mostly
• Secreted as: prepro CCK
(i.e. CCK58,CCK39, CCK33, CCK12, CCK8)
Also found in nerve cells of distal ileum & colon.
Cholecystokinin
Functions:
• Contraction of gallbladder with relaxation of
sphincter of Oddi
• Secretion of pancreatic juice rich in enzymes
e.g. lipase, amylase, proteases
• CCK augments the effect of secretin on the secretion of
HCO3- from pancreas ( Alkaline)
• Inhibits gastric motility by contraction of pyloric sphincter
( gastric emptying time, ensuring adequate time for
digestion & absorption of fats)
• Motility of s.intestine & colon
• Secretion of enterokinase.
Secretin
Secreted by:
S-cells of mucosa of s.intestine
only one form exists.
Actions:
Secretion of HCO3- by ducts of pancreas & biliary tract to
neutralize H+ in small intestine
(essential for fat digestion; as the optimum activity of
pancreatic lipase is between pH 6-8, denatured at pH < 3)
Gastric secretion
Contraction of pyloric sphincter
Stimulated by:
Products of prot. digestion & acid (when pH < 4.5)
• GIP (Gastric Inhibitory Peptide):
Secreted by: K-cells in mucosa of duodenum/Jejunum
Stimulated by: Oral glucose ,
Fatty acids,
A. Acids in duodenum.
Actions: ↑ insulin secretion from pancreatic β-cells
• VIP (Vasoactive Intestinal Peptide):
Source: Neurons of mucosa & smooth muscle
Actions:
Stimulates intestinal secretion of electrolytes & H2O
↑ Pancreatic secretion
Inhibits Gastric secretions
Relaxation of Int. Smooth muscles& Bl.vessels.
• GRP (Gastrin Releasing Peptide) or Bombesin:
Secreted by: Neurons of Gastric mucosa
Action: ↑Gastric secretion
• Substance P:
Cosecreted with Ach
Actions:
Contraction of smooth muscle
↑ Salivary secretion
G-cells
GRP
Neurons
I-cells S-cells K-cells
Neurons
