What is Secretina
Secretin is a peptide hormone of 27 amino acids, with a particular role in the history of physiology: released by the S cells of the duodenal crypts, secretin is the first hormone discovered by man, thanks to the studies of Bayliss and Starling in 1902.
The release of secretin is stimulated by the decrease in gastric pH, therefore by the "increase in the acidity of its content." Not surprisingly, secretin acts primarily on the pancreas, stimulating it to secrete a diluted pancreatic juice rich in bicarbonate, very important for buffering the acidity of the gastric chyme. This mechanism, in addition to ensuring optimal conditions for the activity of digestive enzymes (which work at best at slightly basic pH), it protects the duodenal mucosa from the insult of the acidity of the chyme.
Functions
The action of secretin is comparable to that of a firefighter; it is in fact released in response to the "increase in the acidity of the digested material coming from the stomach (gastric chyme), which would risk" burning "the mucosa of the proximal tract of the small intestine (called the duodenum).
To extinguish this fire, secretin stimulates the liver and pancreas to release secretions rich in liquids and bicarbonates.
The release of secretin is stimulated - as well as by the acidity of the kilo coming from the pylorus - also by the presence of free fatty acids and bile salts. When the pH of the duodenum approaches neutrality (> 4.5) the release of secretin is inhibited , since the organism no longer needs its biological action.
In addition to the pancreatic level, secretin also acts on the liver, stimulating biliary production. With a paracrine effect, this hormone also promotes glandular secretion of the duodenum (Brunner's glands), again with the aim of buffering the acidity of the chyme. At the same time it acts on the G cells responsible for the secretion of gastrin, reducing their activity and consequently increasing the gastric pH.
Secretin also enhances the action of cholecystokinin, a hormone that stimulates the synthesis of pancreatic hormones and promotes the secretion of bile in the intestine.
Recently it has been discovered that secretin is also released from the posterior pituitary in response to an increase in plasma osmolarity, and acts at the hypothalamic level favoring the synthesis and release of vasopressin (ADH). This last hormone is also called antidiuretic because it opposes the production of urine, conserving water in the organism; its action is therefore important when the osmolarity of the plasma increases (ie when it becomes more concentrated, therefore poorer in water).
Secretin stimulus test
The intravenous or endoluminal injection (into the duodenum via a tube) of secreatin is practiced in the diagnostic field to study the responses of the organs sensitive to its action, first of all the pancreas. In case of suspected pancreatitis, cystic fibrosis, pancreatic insufficiency, tumors of the pancreas and gastrinoma, the secretin stimulus test can provide important information about organ health. For example, it is possible to aspirate from the duodenum the pancreatic juices, released there thanks to the stimulus of secretin administered into a vein, and analyze them in the laboratory for the diagnosis of pancreatic insufficiency (although for this purpose the dosage of chymotrypsin in the faeces is often preferred). In the presence of gastrinoma (a pancreatic hormone secreting gastrin), intravenous injection of secretin significantly increases gastrin levels (gastrin); this does not happen in the healthy patient, since in normal conditions gastrin is produced mainly in the stomach, and its release is not significantly influenced by secretin.
