Gastrointestinal Tract Hormones

The digestive tract is a complex process involving various enzymes and hormone secretion. In this blog, we will read about the three major types of GI hormones in detail. The three major GI hormones are:

• Gastrin- Gastrin is produced by G cells located in the stomach's antrum.
• Cholecystokinin (CCK)- CCK is released from I cells, which are present in the duodenum and jejunum, part of the small intestine.
• Secretin- Secretin is released from S cells, which are also present in the duodenum and jejunum, which are parts of the small intestine. 

Gastrin

The Main Stimuli For Gastrin Release Are Listed Below:

• Peptides and various amino acids (AA) are major stimulants for gastrin. Foods containing protein that are partially digested in the stomach are the most potent stimulus for gastrin.
• Carbohydrates and fats do not act as stimuli for the release of gastrin.
• Distension of the stomach at the antrum causes the release of gastrin.
• If calcium particles are present in large amounts in food, they can also stimulate the release of gastrin.
• Parasympathetic stimuli can also increase gastrin secretion. However, in this case, acetylcholine is not the neurotransmitter. 
• Gastrin-releasing peptides (GRP) are the neurotransmitters used to release gastrin in the parasympathetic system.

All the stimulants mentioned above will stimulate G cells, either by the presence of peptides or by stomach distension. Thus, gastrin is produced.

• The most dominant effect of gastrin is to stimulate parietal or oxyntic which are present in the fundus and body region of the stomach. They can produce H+ ions and intrinsic factor (IF) 
• The major action of gastrin is the stimulation of the parietal cell. 
• Gastrin can also stimulate the nearby gastrin chief cells, also known as peptic cells, that produce pepsinogen.
• H+ ions are inhibitors of G cells. G cells stimulate and produce gastrin, which in turn stimulates the release of H+ ions that come back to the G cell. Thus, negative feedback can be seen in this process. 

We have read about the stimulants and how they trigger different cells in the GI tract. Now let us read about the various functions of the Gastrin hormone:

1. Stimulation of parietal or oxyntic cells for acid secretion and stimulation of chief cells for production of pepsinogen. 
2. They stimulate the growth of GI mucosa, which is called the trophic effect of gastrin.
3. It enhances the motility of the pyloric region of the stomach, which in turn enhances the emptying of the stomach.
4. Antibodies are produced against the parietal cells in pernicious anaemia. It is an autoimmune disease with antibodies against parietal cells.
5. When parietal cells are destroyed, H+ ions secretion decreases, and there are fewer inhibitors of G cells. Due to this, gastrin production increases.

• When gastrin is released, it passes through the blood to enter parietal cells for further stimulation. As G cells are located in the antrum and parietal cells are present in the fundus or body region of the stomach, gastrin cannot diffuse directly into parietal cells; it has to enter blood circulation and then move into parietal cells.
• A patient with pernicious anaemia has an antibody against parietal cells. Thus, gastrin production and the serum level of gastrin will be enhanced.
  • Anti-parietal cell antibodies destroy parietal cells, decreasing intrinsic factor production. As intrinsic factors help in the absorption of vitamin B12, the patient will also suffer from vitamin B12 deficiency, which is known as megaloblastic anaemia. 
  • Therefore, a pernicious anaemia patient will have megaloblastic anaemia and the serum level of gastrin will be enhanced due to the loss of negative feedback.

Also Read: Pancreas Hormones: Location and Function

Cholecystokinin (CCK)

The main Stimulus for CCK hormone to be released are:

1. Peptides are the dominant stimulus for CCK. 
2. Fats are the next major stimulus for CCK. The intact triglyceride (TG) molecule is not a stimulus for the release of CCK. It has to be digested into free fatty acids (FFA) and monoglycerides (MG).
3. Carbohydrate is not a stimulus for CCK. 

Functions of CCK

• The major action of CCK occurs at the duodenum, where the pancreatic duct is released. The acinar cells of the exocrine pancreas produce enzymes such as amylase, protease, and lipase, while the duct cells produce bicarbonate ions.
• The bile duct is also present in the same region in the duodenum. 
• When peptides or fatty acids are present around I cells, CCK is produced. 
• The major action of CCK is to stimulate the acinar cells of the exocrine pancreas, which enhances the release of enzymes responsible for the digestion of protein and fats.
• Under the influence of CCK, the pancreas produces huge amounts of enzymes, which are released in the second part of the duodenum. Here, more peptides and free fatty acids are generated, which in turn stimulate the I cells. 

We can conclude that some amount of peptides and free fatty acids can stimulate I cells. I cells produce CCK, and CCK stimulates the production of enzymes from the pancreas, which again causes the release of peptides and free fatty acids, which again stimulates I cells. This is a typical positive feedback system. Thus, we can say that the major action of CCK is to produce enzymes from the pancreas.

• Along with enzymes, bile acid is also essential for the proper digestion of fats at the duodenum. Contraction of the gallbladder causes a release of bile acids. The next major action of gastrin is the contraction of the gallbladder. Any substance that causes the contraction of the gallbladder is known as cholagogue. Thus, CCK can be termed as a potent cholagogue.
• For easy production of bile, the sphincter of Oddi needs to be relaxed as well. CCK is responsible for the relaxation of the sphincter of Oddi as well. 
• As food particles in the stomach are acidic, they should not reach the duodenum because the H+ ions present in them will disturb the pH of the duodenum. Therefore, CCK causes contraction of the pyloric sphincter so that the food content of the stomach doesn’t reach the duodenum during digestion.
• Once digestion is over, food has to pass through the distal part of the intestine, which needs to be relaxed to receive food. Thus, one of the major functions of CCK is to increase intestinal motility. Though it decreases stomach motility, CCK can help to enhance intestinal motility.
• CCK also helps in the growth of the pancreas. Exocrine pancreatic growth, in particular, is enhanced under the influence of CCK. 
• CCK and secretin augment one another's actions. Thus, CCK also helps regulate secretin hormones.

Also Read: Hemostasis: Stages, Steps and Clinical Applications

Secretin

• Secretin is the first GI hormone to be discovered by two scientists named Bayliss and Starling. 

Stimulus for secretin is received by the action of:

• H+ ions are the most potent stimulus for secretin.
• Peptides can stimulate secretin.
• Fat has a very minor role in the production of secretin.
• Carbohydrates cannot stimulate the release of secretin.

Secretin is produced by S cells located at the second part of the duodenum. When food particles from the stomach containing large amounts of H+ Ions reach the duodenum, they stimulate the S cells, and secretin is produced. The main Functions of Secretin are as follows:

• Stimulant of the duct cell of the exocrine pancreas. 
• The pancreas's duct cells produce bicarbonate. When bicarbonates reach the duodenum, they neutralise H+ ions. As soon as H+ ions are neutralised, they no longer stimulate S cells, and the production of secretin stops. This is a typical negative feedback system.  
• Thus, secretin mainly helps in increasing the production of bicarbonate from the exocrine pancreatic duct cells. It also increases bicarbonate production from the biliary tree and Brunner’s glands. As Brunner’s glands are located in the upper part of the duodenum, therefore all of its production is alkaline in nature. 
• This secreting action helps decrease the function of stomach acids. Secretin also maintains the duodenum's alkaline pH. 
• Secretin augments the actions of CCK. 

There are some other GASTROINTESTINAL HORMONES also which aid in digestion and absorption. They are:

• Somatostatin – It helps in the inhibition of gastric acid secretion. It also helps in the inhibition of bicarbonate produced from cholangiocytes. 
• Pancreatic polypeptide – It is secreted from pancreatic PP cells, which are part of islets of Langerhans portion of the pancreas. It helps in the inhibition of pancreatic bicarbonate secretion, gall bladder contraction and gut motility.
• Peptide YY - They are produced from L cells, which are present in the distal ileum. Fat is the stimulus responsible for the release of peptides YY.
  • Peptide YY is also known as the ileal brake hormone. It decreases stomach and intestine motility. It breaks the connection between the ileum and colon and fatty food particles cannot pass through it by decreasing stomach and intestine motility. 
• Neuropeptide (NPY) – It is a neurotransmitter in the central nervous system. It is a stimulus for increased food intake. Although it is not a GI hormone, it belongs to the same family of peptides.
• Glucagon-like peptide 1 (GLP 1) – It is produced from L cells. It is known as incretin. GI Peptides, which stimulate the release of insulin from pancreatic beta cells, are known as incretin. GLP 1 is the most potent incretin produced in the small intestine. 
• Oxyntomodulin -It is also produced from L cells of the small intestine. It helps in the inhibition of food intake. It also inhibits gastric motility. 
• Glucose-dependent insulinotropic polypeptide (GIP) – Old name for GIP is a gastric inhibiting peptide. Earlier, it was known that GIP is the inhibitor of the release of acid from the stomach. Later on, it was discovered that the most important physiological function of GIP is the incretin effect. Therefore, the name has been changed from gastric inhibiting peptide to glucose-dependent insulinotropic polypeptide. 
• Guanylin – This peptide is an agonist guanylin receptor. It increases fluid absorption in the small intestine. It is secreted from the goblet cells, which are located predominantly in the ileum and colon of the small intestine. 
• Motilin – It is produced from Mo cells present in the stomach and duodenum. It helps to initiate and regulate migrating motor complexes (MMC). It also enhances intestinal motility.
• Neurotensin – It is secreted from various endocrine cells. It helps in the stimulation of histamine release. 

Also Read: Synapse: Components, Types and Function

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