Gastrointestinal (GI) 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. Major function of these hormones is to regulate the secretory activities and motility of the GI tract.

CELLS SECRETING THE HORMONES

Enteroendocrine Cells

Enteroendocrine cells are the hormone-secreting cells in GI tract. These are the nerve cells and glandular cells which are present in the gastric mucosa, intestinal mucosa and the pancreatic cells.

Neuroendocrine Cells or APUD Cells

Enteroendocrine cells which secrete hormones from amines are known as amine precursor uptake and

decarboxylation cells (APUD cells) or neuroendocrine cells. For the synthesis of a GI hormone, first a precursor substance of an amine is taken up by these cells. Later, this precursor substance is decarboxylated to form the amine. From this amine, the hormone is synthesized. Because of the uptake of the amine precursor and decarboxylation of this precursor substance, these cells are called APUD cells. This type of cells is also present in other parts of the body, particularly the brain, lungs

and the endocrine glands.

Enterochromaffin Cells

Enteroendocrine cells which secrete serotonin are called enterochromaffin cells.

DESCRIPTION OF GASTROINTESTINAL HORMONES

1. 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 Gastrin is secreted from stomach during the gastric (second) phase of gastric secretion and from small intestine during the intestinal (third) phase of gastric secretion.

Stimulant for Secretion

Stimulants for secretion of gastrin are:

i. Presence of food in the stomach.

ii. Stimulation of local nervous plexus in stomach and small intestine.

iii. Vagovagal reflex during the gastric phase of gastric secretion: Gastrin-releasing polypeptide

is released at the vagal nerve ending. It causes the secretion of gastrin by stimulating the G cells

or TG cells.

 Actions

Gastrin:

i. Stimulates gastric glands to secrete gastric juice with more pepsin and hydrochloric acid.

ii. Accelerates gastric motility.

iii. Promotes growth of gastric mucosa.

iv. Stimulates secretion of pancreatic juice, which

is rich in enzymes.

v. Stimulates islets of Langerhans in pancreas to release pancreatic hormones.

2. SECRETIN

Secretin is a peptide hormone with 27 amino acid residues. 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. Secretin is first produced in an inactive form called prosecretin. It is converted into secretin by the acidity of chyme.

Stimulant for Secretion

Stimulant for the release and activation of prosecretin is the acid chyme entering the duodenum from stomach. Products of protein digestion also stimulate secretin secretion.

Actions

Major 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 actions

Secretin:

i. Inhibits secretion of gastric juice

ii. Inhibits motility of stomach

iii. Causes constriction of pyloric sphincter

iv. Increases the potency of action of cholecystokinin on pancreatic secretion.

3. CHOLECYSTOKININ

Cholecystokinin is made up of 39 amino acid residues. Previously it was thought that there were two separate hormones, namely pancreozymin and cholecystokinin. It was thought that pancreozymin stimulated the secretion of pancreatic juice with large amount of enzymes and the cholecystokinin stimulated the contraction of gallbladder. But now 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.

Stimulant for Secretion

Stimulant for the release of this hormone is the presence of chyme-containing digestive products of fats and proteins, viz. fatty acids, peptides and amino acids in the upper part of small intestine.

Actions

Major actions

Cholecystokinin:

i. Contracts gallbladder.

ii. 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

Cholecystokinin:

i. Accelerates the activity of secretin to produce alkaline pancreatic juice, with large amount of

bicarbonate ions.

ii. Increases the secretion of enterokinase.

iii. Inhibits the gastric motility.

iv. Increases the motility of intestine.

v. Augments contraction of pyloric sphincter.

vi. Plays an important role in satiety by suppressing hunger.

vii. Induces drug tolerance to opioids.

4. GLUCOSE-DEPENDENT INSULINOTROPIC HORMONE

Earlier it was called gastric inhibitory peptide (GIP). It is a peptide hormone, formed by 42 amino acid residues. It is secreted by K cells in duodenum and in jejunum. It is also secreted in antrum of stomach.

Stimulant for Secretion

GIP is secreted when chyme containing glucose and fat enters the duodenum.

Actions

Gastric inhibitory peptide (GIP):

i. Stimulates the beta cells in the islets of

Langerhans in pancreas to release insulin.

It causes insulin secretion, whenever chime with glucose enters the small intestine. Hence

it is called glucose-dependent insulinotropic hormone.

ii. Inhibits the secretion of gastric juice.

iii. Inhibits gastric motility.

Recent studies reveal that GIP does not show significant action on gastric secretion.

5. VASOACTIVE INTESTINAL POLYPEPTIDE

Vasoactive intestinal polypeptide (VIP) contains 28 amino acid residues. This polypeptide is secreted in the stomach and small intestine. A small amount of this hormone is also secreted in large intestine.

Stimulant for Secretion

Presence of acid chyme in the stomach and intestine causes secretion of VIP.

Actions

Vasoactive intestinal polypeptide (VIP):

i. Dilates splanchnic (peripheral) blood vessels.

ii. Inhibits hydrochloric acid secretion in gastric juice.

iii. Stimulates secretion of succus entericus with large amounts of electrolytes and water.

iv. Relaxes smooth muscles of intestine.

v. Augments action of acetylcholine on salivaryglands.

vi. Stimulates insulin secretion.

6. GLUCAGON

Glucagon has 29 amino acid residues. 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.

Stimulant for Secretion

Presence of food with more fat and protein in the stomach is the stimulant for glucagon secretion in

stomach and duodenum. Hypoglycemia is the stimulant for secretion of pancreatic glucagon.

Action

Glucagon increases blood sugar level.

7. GLICENTIN

Glicentin polypeptide is secreted by L cells in duodenum and jejunum and α-cells of pancreatic islets. It is also secreted in brain. Precursor of this hormone is the preproglucagon. In intestine, the preproglucagon is converted into glicentin and glucagon-like polypeptide-2 (GLP-2). In pancreas, it is converted into glucagon, glucagon-like polypeptide-1 (GLP-1) and major proglucagon fragment.

Stimulant for Secretion

Glicentin is secreted when chyme with fat and protein enters the intestine.

Action

Like glucagon, glicentin also increases the blood sugar level.

8. GLUCAGON-LIKE POLYPEPTIDE-1

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.

Stimulant for Secretion

Presence of food with glucose in the small intestine stimulates the release of GLP-1.

Actions

Glucagon-like polypeptide-1 (GLP-1):

i. Stimulates the insulin secretion from β-cells of islets in pancreas

ii. Inhibits gastric motility.

9. GLUCAGON-LIKE POLYPEPTIDE-2

Glucagon-like polypeptide-2 (GLP-2) is secreted by L cells in ileum and colon (see above). Structurally, it is similar to GLP-1 and glucagons. Like GLP-1, it is also found in brain.

Stimulant for Secretion

Presence of food with glucose in the small intestine stimulates the release of GLP-2 also.

Action

GLP-2 is believed to suppress appetite.

10. SOMATOSTATIN

Somatostatin was first found in hypothalamus and named as growth hormone-inhibiting hormone. Now it is found in D cells of stomach and upper part of small intestine and D cells of pancreatic islets also. Somatostatin is secreted

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