Gastrointestinal Hormones

Hormone Source of secretion  Actions
Gastrin G cells in stomach TG cells in GI tract Islets in fetal pancreas Anterior pituitary Brain
 		 Stimulates gastric secretion and motility Promotes growth of gastric mucosa Stimulates release of pancreatic hormones Stimulates secretion of pancreatic juice Stimulates secretion of pancreatic hormones
 
Secretin S cells of small intestine
 		 Stimulates secretion of watery and alkaline pancreatic secretion Inhibits gastric secretion and motility Constricts pyloric sphincter Increases potency of cholecystokinin action
 
Cholecystokinin  I cells of small intestine
 		 Contracts gallbladder Stimulates pancreatic secretion with enzymes Accelerates secretin activity Increases enterokinase secretion Inhibits gastric motility Increases intestinal motility Augments contraction of pyloric sphincter Suppresses hunger Induces drug tolerance to opioids
 
Gastric inhibitory peptide (GIP)
 		 K cells in duodenum and jejunum Antrum of stomach
 

Stimulates insulin secretion

Inhibits gastric secretion and motility
Vasoactive intestinal polypeptide (VIP)
 		 Stomach Small and large intestines
 		 Dilates splanchnic (peripheral) blood vessels Inhibits Hcl secretion in gastric juice Stimulates secretion of succus entericus Relaxes smooth muscles of intestine Augments acetylcholine action on salivary glands Stimulates insulin secretion
 
Glucagon α-cells in pancreas A cells in stomach L cells in intestine
 		 Increase blood sugar level
Glicentin  L cells in duodenum and jejunum  Increases blood sugar level
Glucagon-like polypeptide-1 (GLP-1)
 		 α-cells in  pancreas Brai

Stimulates insulin secreation

Inhibits gastric motility
GLP-2 L cells in ileum and colon  Suppresses appetite
 
Ghrelin Stomach Hypothalamus Pituitary Kidney Placenta
 		 Promotes growth hormone (GH) release Induces appetite and food intake Stimulates gastric emptying
Substance P BrainSmall intestine  Increases movements of small intestine
 
Motilin Mo cells in stomach and  intestine Enterochromoffin cells in intestine
 		 Accelerates gastric emptying Increases movements of small intestine Increases peristalsis in colon 
Peptide YY  L cells of ileum and colon
 		 Inhibits gastric secretion and motility Reduces secretion of pancreatic juice Inhibits intestinal motility and bowel passage Suppresses appetite and food intake
 
Pancreatic polypeptide  PP cells in  pancreas Small intestine
 

Increase secreation of glucagons

Decrease pancreatic secretion
Somatostatin Hypothalamus D cells in pancreas D cells in stomach and  small intestine Inhibits secretion of growth hormone Inhibits gastric secretion and motility Inhibits secretion of pancreatic juice Inhibits secretion of GI hormones
Neuropeptide Y
 		 Ileum and colon Brain and autonomic nervous system (ANS)
 		 Increases blood flow in enteric blood vessels

Gastrointestinal Hormones-

Gastrointestinal (GI) hormones are the hormones secreted in GI tract. These hormones are polypeptides in nature and belong to the family of local hormones.

Description of Gastrointestinal Hormones-

  • Gastrin
  • Secretin
  • Cholecystokinin
  • Gastric inhibitory peptide (GIP)
  • Vasoactive intestinal polypeptide (VIP)
  • Somatostatin
  • Pancreatic polypeptide
  • Peptide YY
  • Neuropeptide Y
  • Motilin
  • Substance P
  • Ghrelin
  • Glucagon
  • Glicentin
  • Glucagon-like polypeptide-1 (GLP-1)
  • GLP-2

Gastrin-

 Gastrin is a peptide with 34 amino acid residues. It is secreted mainly by the G cells of pyloric glands of stomach. It is also secreted by TG cells in stomach, duodenum and jejunum. In fetus, the islets of Langerhans also secrete this hormone.

Actions Gastrin-

  • Stimulates gastric glands to secrete gastric juice with more pepsin and hydrochloric acid.
  • Accelerates gastric motility.
  • Promotes growth of gastric mucosa.
  • Stimulates secretion of pancreatic juice, which is rich in enzymes.
  • Stimulates islets of Langerhans in pancreas to release pancreatic hormones.

Secretin-

Historical importance of secretin is that, it was the first ever hormone discovered. It was discovered in 1902 by Bayliss and Starling. It is secreted by the S cells of duodenum, jejunum and ileum.

Actions-

Secretin stimulates exocrine pancreatic secretion. It acts on the cells of pancreatic ductule via cyclic AMP and causes secretion of large amount of watery juice with high content of bicarbonate ion. Bicarbonate content of pancreatic juice (released by secretin) has functional significance.

Other Action-

  • Inhibits secretion of gastric juice
  • Inhibits motility of stomach
  • Causes constriction of pyloric sphincter
  • Increases the potency of action of cholecystokinin on pancreatic secretion

Cholecystokinin-

it is established that the same hormone has actions on both pancreas and gallbladder. So, it is named as cholecystokinin-pancreozymin (CCK-PZ) or cholecystokinin (CCK). Cholecystokinin is secreted by I cells in mucosa of duodenum and jejunum. A small quantity of the hormone is secreted in the ileum also.

Actions-

Major actions

  • Contracts gallbladder.
  • Stimulates exocrine pancreatic secretion- It activates the pancreatic acinar cells via the second messenger inositol triphosphate. Cholecystokinin causes secretion of pancreatic juice with large amount of enzymes.

 Other actions-

  • Accelerates the activity of secretin to produce alkaline pancreatic juice, with large amount of bicarbonate ions.
  • Increases the secretion of enterokinase.
  • Inhibits the gastric motility.
  • Increases the motility of intestine.
  • Augments contraction of pyloric sphincter.
  • Plays an important role in satiety by suppressing hunger.
  • Induces drug tolerance to opioids.

Gastric inhibitory peptide-

It is secreted by K cells in duodenum and in jejunum. It is also secreted in antrum of stomach.

Actions-

  • Stimulates the beta cells in the islets of Langerhans in pancreas to release insulin. It causes insulin secretion, whenever chyme with glucose enters the small intestine. Hence it is called glucose-dependent insulinotropic hormone.
  • Inhibits the secretion of gastric juice.
  • Inhibits gastric motility.

Vasoactive intestinal polypeptide-

This polypeptide is secreted in the stomach and small intestine. A small amount of this hormone is also secreted in large intestine.

Actions-

  • Dilates splanchnic (peripheral) blood vessels. 
  • Inhibits hydrochloric acid secretion in gastric juice. 
  • Stimulates secretion of succus entericus with large amounts of electrolytes and water.
  • Relaxes smooth muscles of intestine.
  • Augments action of acetylcholine on salivary glands.
  • Stimulates insulin secretion.

Somatostatin-

it is found in D cells of stomach and upper part of small intestine and D cells of Pancreatic islets also.

Actions Somatostatin-

  • Inhibits the secretion of growth hormone (GH) and thyroid-stimulating hormone (TSH) from anterior pituitary
  • Inhibits gastric secretion and motility
  • Inhibits secretion of pancreatic juice
  • Inhibits secretion of GI hormones such as-
  1. Gastrin
  2. Cholecystokinin (CCK)
  3. Vasoactive intestinal polypeptide (VIP)
  4. Gastric inhibitory peptide (GIP).

Pancreatic polypeptide-

It is secreted mainly by the PP cells of the islets of Langerhans in pancreas. It is also found in small intestine.

Actions-

  • Increases the secretion of glucagon from α-cells of islets of Langerhans in pancreas.
  • Decreases the secretion of pancreatic juice from exocrine part of pancreas.

Polypeptide YY -

It is secreted in L cells of ileum and colon.

Actions-

 In GI tract, substance P increases the mixing and propulsive movements of small.

Glucagon-

It is secreted mainly by alpha cells of islets of Langerhans in pancreas. It is also secreted by A cells in the stomach and L cells in the intestine. In intestine, it is secreted as preproglucagon.

Action-

 Glucagon increases blood sugar level.

Glucagon-like polypeptide-1 (GLP-1)

 Is secreted in α-cells of pancreatic islets (see above). Structurally, it is similar to GLP-2 and glucagon. It is found in brain also.

Actions-

  • Stimulates the insulin secretion from β-cells of islets in pancreas
  • Inhibits gastric motility.

Glucagon-like polypeptide-2 (GLP-2)

 Is secreted by L cells in ileum and colon.Structurally, it is similar to GLP-1 and glucagons. Like GLP-1, it is also found in brain.

Action-

GLP-2 is believed to suppress appetite.

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