Index
It is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach.
It also regulates secretions in the pancreas and liver.
It is a peptide hormone produced in the S cells of the duodenum, which are located in the intestinal glands. In humans, the secretin peptide is encoded by the SCT gene.
Secretin helps regulate the pH of the duodenum by inhibiting the secretion of gastric acid from the parietal cells of the stomach and stimulating the production of bicarbonate from the ductal cells of the pancreas.
It also stimulates bile production by the liver; bile emulsifies fats from the diet in the duodenum so that the pancreatic lipase can act on them.
Prostaglandin is a precursor of secretin, which is present in digestion. This is stored in this unusable form and is activated with gastric acid in the small intestine to neutralize the pH and ensure that the small intestine is not damaged by the acid mentioned above.
In 2007, it was discovered that secretin plays a role in osmoregulation by acting on the hypothalamus, the pituitary gland, and the kidney.
Structure
Secretin is initially synthesized as a 120-amino acid precursor protein known as prosecretin.
This precursor contains an N-terminal signal peptide, spacer, secretin (residues 28-54), and a C-terminal peptide of 72 amino acids.
Secretin also has an amidated carboxy-terminal amino acid that is valine.
Physiology
Production and secretion:
Secretin is synthesized in cytoplasmic secretory granules of S cells, which are found mainly in the mucosa of the duodenum and smaller amounts in the jejunum of the small intestine.
This is released into the circulation and intestinal lumen in response to a low duodenal pH that varies between 2 and 4.5 depending on the species; the acidity is due to the hydrochloric acid in the chyme that enters the duodenum from the stomach through the pyloric sphincter.
In addition, the secretion of secretin is increased by the products of the digestion of proteins that bathe the mucosa of the upper small intestine.
The release of secretin is inhibited by H2 antagonists, which reduce gastric acid secretion. As a result, if the pH in the duodenum increases above 4.5, secretin can not be released.
Functions
PH regulation:
Secretin works mainly to neutralize the pH in the duodenum, allowing the digestive enzymes of the pancreas (for example, pancreatic amylase and pancreatic lipase) to function optimally.
Secretin is directed to the pancreas; pancreatic centroacuatic cells have secretin receptors in their plasma membrane. Since secretin binds to these receptors, it stimulates the activity of adenylate cyclase and converts ATP into cyclic AMP.
Secretin also increases the secretion of water and bicarbonate from the Brunner duodenal glands to buffer the incoming protons of the acid chyme and also reduces the secretion of acid by the parietal cells of the stomach. It does this through at least three mechanisms:
- By stimulating the release of somatostatin.
- By inhibiting the release of gastrin in the pyloric antrum.
- By directly decreasing the regulation of the secretory mechanism of parietal acid.
Counterattacks the peaks of blood glucose concentration triggering a more significant release of insulin from the pancreas after consumption of oral glucose
Osmoregulation:
Secretin modulates the transport of water and electrolytes in the pancreatic duct cells, liver cholangiocytes, and epithelial cells of the epididymis.
Secretin is found in the magnocellular neurons of the paraventricular and supraoptic nuclei of the hypothalamus and along the neurohypophysial tract to the neurohypophysis.
During the increase in osmolality, it is released from the posterior pituitary. It activates the release of vasopressin in the hypothalamus.
It is also necessary to carry out the significant effects of angiotensin II. In the absence of secretin or its receptor in genetically inactivated genes, the central injection of angiotensin II failed to stimulate water intake and vasopressin release.
It has been suggested that abnormalities in such release of secretin could explain the underlying abnormalities of type D syndrome of inappropriate hyperhidrosis of antidiuretic hormone (SIADH).
In these individuals, vasopressin release and response are standard, although abnormal renal expression, translocation of aquaporin 2, or both are found.
It has been suggested that “secretin as a neurosecretory hormone of the posterior pituitary gland, therefore, could be the vasopressin-independent mechanism long sought to solve the puzzle that has baffled doctors and physiologists for decades.”
Food intake:
Secretin and its receptor are found in discrete nuclei of the hypothalamus, including the paraventricular nucleus and the arcuate nucleus, which are the primary sites in the brain for regulating the homeostasis of the body’s energy.
It was found that both the central and the peripheral injection of Sct reduced the food intake in the mouse, which indicates an anorexigenic role of the peptide. The central melanocortin system mediates this function of the peptide.
Applications
Secretin is used in diagnostic tests for pancreatic function.
It is injected, and the pancreatic result can be obtained with magnetic resonance imaging, a non-invasive procedure. The secretions generated can be gathered through an endoscope or tubes inserted through the mouth into the duodenum.
