Gastrointestinal Hormones Notes

Gastrointestinal Hormone Introduction

The gastrointestinal hormones are the hormones secreted in the Gl tract. These hormones belong to the family of local hormones and are polypeptides in nature. The major function of these hormones is to regulate the secretory activities and motility of the Gl tract.

Cells Secreting The Hormones

• Enteroendocrine Cells: The hormone-secreting cells in the Gl tract are called entero-endocrine cells. The enteroendocrine cells are the nerve cells and glandular cells which are present in the gastric mucosa, intestinal mucosa, and pancreatic cells.

Read And Learn More: Medical Physiology Notes

• APUD Cells or Neuroendocrine 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 Gl 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 cell is also present in other parts of the body particularly the brain, lungs J 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 the pyloric glands of the stomach. It is also secreted by TG cells in the stomach, duodenum, and jejunum. In the fetus, the islets of Langerhans also secrete this hormone.

Stimulant for Secretion: Gastrin is secreted from the stomach during the gastric (second) phase of gastric secretion and from the small intestine during the intestinal (third) phase of gastric secretion. The stimulants for secretion of gastrin:

• Presence of food in the stomach
• Stimulation of the local nervous plexus in the stomach and small intestine
• Vagovagal reflex during the gastric phase of gastric secretion: The 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:

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 the pancreas to release pancreatic hormones.

2. Secretin: Secretin is a peptide hormone with 27 amino acid residues. The historical importance of secretin is that this was the first ever hormone discovered. It was discovered in 1902 by Bayliss and Starling. It is secreted by the S cells of the duodenum, jejunum, and ileum.

Stimulant for Secretion: Secretin is first produced in an inactive form called prosecretin. The prosecretin is converted into secretin by the acidity of the chyme. The stimulant for the release and activation of prosecretin is the acid chyme entering the duodenum from the stomach. Products of protein digestion also stimulate the production of secretin.

Actions Major actions: Secretin stimulates exocrine pancreatic secretion. It acts on the cells of the pancreatic ductule via cyclic AMP and causes the secretion of large amounts of watery juice with high content of bicarbonate ions. The bicarbonate content of pancreatic juice (released by secretin) has functional significance.

Other actions: Secretin –

• Inhibits secretion of gastric juice
• Inhibits motility of the stomach
• Causes constriction of the pyloric sphincter
• 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 the pancreozymin stimulated the secretion of pancreatic juice with more amount of enzymes and the cholecystokinin stimulated the contraction of the 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 the mucosa of the duodenum and jejunum. A small quantity of the hormone is secreted in the ileum also.

Stimulant for Secretion: The 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 the small intestine.

Actions Major actions: Cholecystokinine:

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

Other actions: Cholecystokinin:

• Accelerates the activity of secretin to produce alkaline pancreatic juice with a large number of bicarbonate ions
• Increases the secretion of enterokinase
• Inhibits the gastric motility
• Increases the motility of the intestine
• Augments contraction of the pyloric sphincter
• Plays an important role in satiety by suppressing hunger
• 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 the duodenum and in the jejunum. It is also secreted in the antrum of the stomach.

Stimulant for Secretion: It is secreted when chyme containing glucose and fat enters the duodenum.

Actions:

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

Stimulant for Secretion: The presence of acid chyme in the stomach and intestine causes secretion of VIP.

Actions VIP:

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

6. Glucagon: Glucagon has 29 amino acid residues. It is secreted mainly by alpha cells of islets of Langerhans in the pancreas. It is also secreted by A cells in the stomach and L cells in the intestine. In the intestine, it is secreted as pre-proglucagon.

Stimulant for Secretion: The presence of food with more fat and protein in the stomach is the stimulant for glucagon secretion in the stomach and duodenum. Hypoglycemia is the stimulant for the secretion of pancreatic glucagon.

Action: Glucagon increases blood sugar levels.

7. Glicentin:

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

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 cells of pancreatic islets (see above). Structurally, it is similar to GLP-2 and glucagon. It is found in the brain also.
• Stimulant for Secretion: The presence of food with glucose in the small intestine stimulates the release of GLP-1.

Actions GLP-1:

• Stimulates the insulin secretion from (3 cells of islets in the pancreas
• Inhibits gastric motility.

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

Stimulant for Secretion: The presence of food with glucose in the small intestine stimulates the release of GLP-2 also.

Action: It is believed to suppress appetite.

10. Somatostatin: Somatostatin was first found in the hypothalamus and named as growth hormone inhibitory hormone. Now it is found in D cells of the stomach and upper part of the small intestine and D cells of pancreatic islets also. Somatostatin is secreted in two forms; one with 14 amino acids and the other one with 28 amino acids.

Stimulant For Secretion: The presence of chyme with glucose and proteins in the stomach and small intestine causes the release of somatostatin.

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 Gl hormones such as
  • Gastrin
  • CCK
  • VIP
  • GiP

11. Pancrea11c Polypeptide:

Source of Secretion: It is a polypeptide with 36 amino acid residues. It is secreted mainly by the PP cells of the islets of Langerhans in the pancreas. It is also found in the small intestine.

Stimulant for Secretion: Pancreatic polypeptide is secreted by the presence of chyme with proteins in the small intestine. It is also secreted in conditions like hypoglycemia, fasting, and exercise.

Actions Pancreatic polypeptide:

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

12. Peptide Yy: This polypeptide with 36 amino acid residues is structurally related to pancreatic polypeptide and neuropeptide Y. It is secreted in L cells of ileum and colon.

Stimulant for Secretion: The presence of fat-containing chyme stimulates the release of peptide YY.

Actions Peptide YY:

• Inhibits gastric secretion and motility
• Reduces secretion of pancreatic juice
• Inhibits intestinal motility and stops the passage of the bowel beyond the ileum (ileal brake)
• Suppresses appetite and food intake.

13. Neuropeptide Y: It contains 36 amino acid residues. It is structurally related to pancreatic polypeptide and peptide YY. It is secreted by enteric nerve endings, particularly in the ileum and colon. It is also secreted in the medulla, hypothalamus, and neurons of the autonomic nervous system (ANS).

Stimulant for Secretion: Secretion of neuropeptide Y is stimulated by fat-containing chyme.

Action: It increases the blood flow in enteric blood vessels and stimulates food intake.

14. Motilin: Motilin is built by 22 amino acid residues. It is secreted by Mo cells which are present in the stomach and intestine. It is also believed to be secreted by enterochromaffin cells of the intestine.

Stimulant for Secretion: Motilin is secreted when the chyme from the stomach enters the duodenum.

Actions Motilin:

• Accelerates gastric emptying
• Increases the mixing and propulsive movements of the small intestine
• Increases the peristalsis in the colon.

15. Substance P Source Of Secretion: Substance P is a neurotransmitter with 11 amino acid residues. It is secreted at the pain nerve endings in the brain and enteric nerve endings in the small intestine.

Stimulant for Secretion: Secretion of substance P in the intestine is caused by the presence of chyme.

Actions: In Gl tract, it increases the mixing and propulsive movements of the small intestine.

16. Ghrelin: Ghrelin is a recently discovered hormone. This 28 amino acid polypeptide is synthesized by epithelial cells in the fundus of the stomach. It is also produced in smaller amounts in the hypothalamus, pituitary, kidney, and placenta.

Stimulant for Secretion: Secretion of ghrelin increases during fasting and decreases when the stomach is full.

Actions: Ghrelin promotes the secretion of growth hormone (GH) by stimulating somatotropes (growth hormone synthesizing cells) in the anterior pituitary. The receptors for this hormone called growth hormone secretagogues receptors (GHS-R) were identified in the somatotropes before the discovery of the hormone itself.

The receptors are also found in adipose tissue, the heart, and the hypothalamus. Other actions of ghrelin are:

• Induces appetite and food intake by acting via the feeding center in the hypothalamus
• Stimulates gastric emptying.

Other Gastrointestinal Hormones:

The mucosa of the Gl tract secretes many other hormones such as:

• Enkephalins
• Dynorphin
• Neurotensin
• Serotonin
• Urogastrone
• Enterocrinin
• Villikinin
• Guanylin
• Bombesin.

However, the significant biological actions of these hormones on Gl tract are not clear.