What are the Gastrointestinal Hormones?
Besides water and air, we need in order to live a healthy life is the food. Our body needs food as a source of fuel to power all of its processes. But the food doesn't enter in our body cells by any magic. Foods we take are needed to be digested. Our gastrointestinal system, which is a collection of cells, tissues, organs, and biochemical that allow for the digestion and absorption of food into the body.
The mucous membrane of stomach and intestine secretes some polypeptide hormones into the blood which regulate the activity of various parts of digestive system. These peptides are called gastrointestinal hormones or the G-I hormones.
These gastrointestinal hormones are considered as local hormones because they originate from the gastro-intestinal tract as well as act on it. Although the polypeptides secreted by the gastro-intestinal mucosa act on their target cells via the circulation, some of them may also act in a paracrine fashion.
Let us now have a brief discussion about different gastrointestinal hormones as follows:
It is called the Hormone of stomach. Gastrin is secreted by the mucosa of the stomach and also of small intestine. Gastrin is found in the pancreatic islets in fetal life but it is uncertain whether any gastrin is present in the pancreas of normal adults. Gastrin stimulates the secretion of gastric juice i.e., both pepsin and HCl and motility of stomach. The secretion of gastrin is stimulated when the food material comes in contact with the gastric mucosa, and thus, gastric digestion is facilitated. This gastrointestinal hormone also stimulates the growth of mucosa of stomach and intestine (both small and large) which may be considered as its trophic action. In addition, gastrin stimulates the secretion of insulin and glucagon from pancreas.
When the acid mixed, semi-digested food from stomach comes in contact with the mucosa of the small intestine, it secretes several gastrointestinal hormones. The intestinal hormones and their functions are as follows:
This intestinal polypeptide hormone is the first discovered hormone. Secretin has fallowing main functions—
i) It stimulates secretion of bile from liver.
ii) It increases secretion of bicarbonate and water in the pancreatic juice.
iii) Secretin decreases gastric acid secretion which causes contraction of pyloric sphincter.
Formerly it was believed that two separate polypeptide gastrointestinal hormones were liberated from intestine, one responsible for contraction of gall bladder and as a result, expulsion of bile from gall bladder to duodenum whereas the other responsible for secretion of enzymes in the pancreatic juice. These two hormones were named cholecystokinin (CCK) and pancreozymin (PZ) respectively. But recently it has been revealed that CCK and PZ are chemically indifferent, i.e., the same peptide hormone performs both the functions i.e., stimulation of gall bladder contraction and enzyme-rich pancreatic juice secretion. Therefore, the hormone is now called CCK-PZ. It may also inhibit gastric emptying and stimulate intestinal motility.
This is one of the gastrointestinal hormones stimulates secretion of intestinal juice.
This hormone stimulates the motility of villus and thereby facilitates absorption.
Gastric inhibitory peptide (GIP)
This polypeptide hormone is another of the gastrointestinal hormones which secreted from the mucosa of duodenum and jejunum. Its secretion is stimulated by presence of glucose and fat in duodenum. It inhibits gastric secretion and motility; hence it is so named. It also stimulates insulin secretion from pancreas. For this reason, it is sometimes called glucose dependent insulinotropic polypeptide.
Vasoactive intestinal polypeptide (VIP)
This polypeptide is originates from the nerves in the G-I tract. It may act as a neurotransmitter as well as a neuro-hormone. Its actions include as follow:
i) Stimulation of secretion of electrolytes and water in the intestinal juice.
ii) Relaxation of intestinal smooth muscles including sphincters.
iii) Dilatation of peripheral blood vessels.
iv) Inhibition of gastric acid secretion.
This is one of the gastrointestinal hormones which are duodenal polypeptide hormone causes contraction of intestinal smooth muscles.
It is a polypeptide secreted mainly from the mucosa of the ileum in response to presence of fatty acids and it inhibits gastrointestinal motility.
This polypeptide was originally isolated from hypothalamus, as the growth hormone inhibiting hormone (GIH). It is also secreted from the gastrointestinal mucosa and the D-cells of pancreas. It inhibits secretion of other G-I hormones like gastrin, secretin, CCK-PZ, VIP, GIP, motilin etc., and thereby inhibits secretion of exocrine pancreas and gastric HC1, gastric motility and contraction of gall bladder. So it may be considered as one of the gastrointestinal hormones.
Glucagon and Enteroglucagon
Glucagon is secreted from the gastric and intestinal mucosa in addition to the A cells of pancreatic islets. It appears to play a role in hyperglycemia of diabetes. The functions of glucagon have been described earlier under hormones of pancreas. A glucagon like hormone bailed enteroglucagon is secreted from the intestinal mucosa. It is immunologically similar but chemically different from glucagon (its polypeptide chain is larger than glucagon). Its functions are similar to glucagon but much less potent as another of the gastrointestinal hormones active in our body.