Gastrointestinal Hormone
Introduction :
Gastrointestinal Hormone- Certain parts of the digestive tract emit particular hormones. These are commonly referred to as gastrointestinal hormones. Of course, some hormones are released not only from the gastrointestinal region but also from other regions. These hormones regulate the motor and secretory activity of the digestive tract. These hormones are polypeptides in nature, and structural similarities between the chemical properties of these hormones themselves and other polypeptide hormones such as glucagon have been noted. Their secretion is determined primarily by the presence or absence of certain foods in the lumen of the gastrointestinal tract, and not by other glandular products in the bloodstream. The specific cells responsible for the synthesis of these hormones are not exactly known. These hormones are released immediately upon appropriate stimulation and are destroyed as soon as the corresponding stimulus is removed.
Gastrointestinal Hormones and Their Functions :-
1. Gastrin
2. Secretin
3. Cholecystokinin
4. Gastric inhibitory peptide (GIP)
5. Glucagon
6. Glucagon-like polypeptide-1 (GLP-1)
7. Glucagon-like polypeptide-2 (GLP-2)
8. Somatostatin
9. Pancreatic polypeptide
1. Gastrointestinal Hormone- Gastrin :-
Gastrin is a peptide consisting of 34 amino acid residues. The pyloric gland’s G cells release it mostly. Additionally, the stomach, duodenum, and jejunum’s TG cells emit it. In the fetus, the islets of Langerhans also secrete this hormone. Gastrin is secreted by the stomach during the gastric (2nd) phase of gastric secretion and by the small intestine during the intestinal (3rd) phase of gastric secretion.
Functions of Gastrin : – Stimulates gastric motility.
• It also stimulates chief cells and oxyntic cells to secrete pepsinogen and acid (HCl), stimulates the pancreas to secrete bicarbonate containing fluids and enzymes, stimulates the duodenal mucosa to release secretin.
• It also stimulates insulin secretion after protein ingestion.
• It also causes contraction of the gastroesophageal sphincter.
Regulation of Gastrin Secretion : –
1. Vagus nerve stimulation, acetylcholine, digested protein products, and bombesin stimulate gastrin secretion.
2. Somatostatin, gastric acid, secretin, glucagon, VIP, and GIP inhibit gastrin secretion.
2. Gastrointestinal Hormone- Secretin :-
Secretin is a peptide hormone consisting of 27 amino acid residues. The historical importance of secretin is that it was the first hormone ever discovered. It is secreted by S cells in the duodenum, jejunum, and ileum. Prosecretin, an inactive version of secretin, is first synthesized. It is converted to secretin by the acid in chyme.
Regulation of Secretin secretion :- The stimulus for the release and activation of prosecretin is the acidic chyme passing from the stomach into the duodenum. Protein digestion products also stimulate secretion.
Function of Secretin :- Secretin stimulates exocrine pancreatic secretion. It acts via cyclic AMP on the cells of the pancreatic duct to secrete large amounts of aqueous juice rich in bicarbonate ions. The bicarbonate content of pancreatic juice (released by secretin) has functional significance.
Other Effects:
i. Inhibits secretion of gastric juices
ii. Inhibits gastric motility
iii. Causes constriction of the pyloric sphincter
iv. Increases the effect of cholecystokinin on pancreatic secretions.
3. Gastrointestinal Hormone- Cholecystokinin :-
There are 39 residues of amino acids in cholecystokinin. Pancreozymin and cholecystokinin were once believed to be two distinct hormones. Large levels of pancreatic enzyme secretion are stimulated by pancreozymin, while gallbladder contractions are stimulated by cholecystokinin. It is now understood, meanwhile, that the pancreas and gallbladder are both affected by the same hormone. For this reason, It is known as cholecystokinin pancreozymin (CCK-PZ) or cholecystokinin (CCK) for this reason. I cells found in the mucosa of the duodenum and jejunum release cholecystokinin. Small amounts of the hormone are also secreted by the ileum.
Regulation of Cholecystokinin Secretion (CCK) :- The release of this hormone is stimulated by the presence of digestive products including fats and proteins in the upper part of the small intestine, i.e. chyme of fatty acids, peptides and amino acids.
Functions of Cholecystokinin (CCK) :-
i. Contraction of the gallbladder.
ii. Stimulates exocrine pancreatic secretion. uses inositol triphosphate as a second messenger to activate pancreatic acinar cells. Cholecystokinin causes secretion of pancreatic juice containing large amounts of enzymes.
iii. Stimulates secretin production, resulting in pancreatic juice that is alkaline and high in bicarbonate ions.
iv. Increases the secretion of enterokinase. BC
Inhibits gastric motility.
VI. Increases intestinal motility.
vii. It increases the contraction of the pyloric sphincter.
viii. It plays an important role in suppressing hunger and hence giving a feeling of fullness.
vii. Causes drug tolerance to opioids
4. Gastrointestinal Hormone- Gastric inhibitory peptide (GIP) :-
Gastric inhibitory peptide (GIP) is a peptide hormone consisting of 42 amino acid residues. It is secreted by K cells in the duodenum and jejunum. It is also secreted by the gastric antrum.
Regulation of Gastric Inhibitory Peptide (GIP) :– Secretion GIP is secreted when chyme containing glucose and fat enters the duodenum.
Functions of Gastric Inhibitory Peptide (GIP) :–
i. It stimulates the beta cells of the islets of Langerhans in the pancreas to secrete insulin. When glucose-containing chyme that is enters the small intestine, insulin is created. For this reason, it is referred to as glucose-dependent insulinotropic hormone.
ii. It stops the secretion of stomach fluid.
iii. Inhibits gastric motility.
5. Gastrointestinal Hormone- Glucagon :-
Glucagon consists of 29 amino acid residues. It is secreted primarily by the alpha cells of the islets of Langerhans in the pancreas. It is also secreted by the A cells of the stomach and the L cells of the intestine. It is secreted as preproglucagon in the gut.
Regulation of glucagon release :– The presence of food rich in fat and protein in the stomach stimulates glucagon release in the stomach and duodenum. Hypoglycemia stimulates pancreatic secretion of glucagon.
Functions of Glucagon :– Glucagon acts as an insulin antagonist, lowering blood glucose levels and ensuring a balance between energy stores and availability, which is important for overall metabolic health.
6. Gastrointestinal Hormone- Glucagon-like polypeptide-1 (GLP-1) :-
Glucagon-like peptide-1 (GLP-1): It is a neuropeptide and an incretin derived from the proglucagon gene. The main source of secretion of GLP-1 is the intestinal L-cells. GLP-1 is secreted by ileal L-cells and depends on the presence of dietary carbohydrates, proteins and lipids in the small intestine. It increases insulin secretion from the pancreas and is glucose dependent. It increases beta cell mass and insulin expression and secretion.
Regulation of Glucagon-like peptide-1 (GLP-1) secretion: – Presence of food containing glucose in the small intestine stimulates the release of GLP-1.
Functions of glucagon-like peptide-1 (GLP-1):
i. Promotes the release of insulin from the pancreatic islets’ beta cells
ii. Inhibits gastric motility
7. Gastrointestinal Hormone- Glucagon-like polypeptide-2 (GLP-2) :-
GLP-2, commonly referred to as glucagon-like polypeptide-2, is released by L-cells in the colon and ileum. It is structurally similar to GLP-1 and glucagon. Like GLP-1, it is also found in the brain.
Regulation of glucagon-like peptide-2 (GLP-2) secretion: – The presence of glucose-containing food in the small intestine also stimulates the release of GLP-2.
Function of glucagon-like peptide-2(GLP-2): – GLP-2 is thought to suppress appetite.
8. Gastrointestinal Hormone- Somatostatin :-
Somatostatin was first discovered in the hypothalamus and was called growth hormone inhibiting hormone. It is now present in the D cells of the islets of Langerhans as well as in the D cells of the stomach and upper part of the small intestine. Somatostatin is secreted in two forms, one containing 14 amino acids and the other containing 28 amino acids.
Regulation of somatostatin secretion: – The presence of chyme containing glucose and protein in the stomach and small intestine causes the release of somatostatin.
Functions of somatostatin: –
i. Inhibits secretion of growth hormone (GH) and thyroid stimulating hormone (TSH) from the pituitary gland
ii. Inhibits secretion and motility of gastric juices
iii. Inhibits secretion of pancreatic juices
9. Gastrointestinal Hormone- Pancreatic polypeptide :-
Pancreatic polypeptide is a polypeptide consisting of 36 amino acid residues. It is mainly secreted by PP cells in the islets of Langerhans in the pancreas. The small intestine contains it as well.
Regulation of pancreatic polypeptide secretion: Pancreatic polypeptide is secreted in the presence of chyme-containing proteins in the small intestine. It is also secreted under conditions such as hypoglycemia, fasting, and exercise.
Functions Pancreatic Polypeptide:-
i. Promotes the pancreatic islets of Langerhans type’ alpha cells to secrete excess glucagon.
ii. Reduces the amount of pancreatic juice secreted by the pancreatic exocrine part.
Conclusion :-
In summary, gastrointestinal hormones are important regulators of digestive health as they promote proper breakdown of food, absorption of nutrients, and maintenance of gastrointestinal homeostasis. Their dysfunction can lead to diseases such as ulcers, malabsorption, and obesity, highlighting their important role in gastrointestinal physiology. Each hormone has a specific function: gastrin stimulates secretion of gastric acid for protein digestion, secretin regulates bicarbonate release to neutralize gastric acid, and CCK stimulates secretion of bile and pancreatic enzymes. These hormones work together to regulate motility, enzyme secretion, and satiety, thus maintaining balance in the digestive system.