Hormones of the Gut
Over two dozen hormones have been identified in various parts of the gastrointestinal system.
- All of them are peptides.
- Many of them are also found in other tissues, especially the brain.
- Many act in a paracrine manner as well as being carried in the blood as true hormones.
- Their importance to health is uncertain as no known deficiency disorders have been found for any of them.
We shall look at only five of them:
- Gastrin is a mixture of several peptides, of which the most active contains 14 amino acids.
- It is secreted by cells in the stomach and duodenum
- It stimulates the exocrine cells of the stomach to secrete gastric juice, a mixture of
- hydrochloric acid and
- the proteolytic enzyme pepsin.
- It is a polypeptide of 27 amino acids.
- It is secreted by cells in the duodenum when they are exposed to the acidic contents of the emptying stomach.
- It stimulates the exocrine portion of the pancreas to secrete bicarbonate into the pancreatic fluid (thus neutralizing the acidity of the intestinal contents).
- A mixture of peptides, of which an octapeptide (8 amino acids) is the most active.
- Like secretin, it is secreted by cells in the duodenum when they are exposed to the acidic contents of the emptying stomach.
- It acts
- on the gall bladder stimulating it to contract and force its contents of bile into the intestine
- on the pancreas stimulating the release of pancreatic digestive enzymes into the pancreatic fluid.
- There is some evidence that CCK acts on the brain as a satiety signal (i.e., "that´s enough food for now").
This mixture of peptides acts on
- the stomach where it inhibits the release of gastrin
- the duodenum where it inhibits the release of secretin and cholecystokinin
- the pancreas where it inhibits the release of glucagon.
Taken together, all of these actions lead to a reduction in the rate at which nutrients are absorbed from the contents of the intestine.
Somatostatin is also secreted by the hypothalamus and the pancreas.
Neuropeptide Y contains 36 amino acids. It is a potent feeding stimulant and causes increased storage of ingested food as fat.
Neuropeptide Y is also secreted by cells in the hypothalamus.
6 June 1999