Secretin is basically a hormone that works to enhance digestion and, at the same time, protects the stomach as well as the gut. However, when the levels of secretin are abnormal in the stomach, the condition can be diagnosed as a pancreatic disorder. It has been established that secretin influences the fluid equilibrium as well as the amount of various other digestive hormones. Moreover, it is useful for the health of the heart, lungs and brain.

Secretin is made in our duodenum, which has a vital part in the digestive process. In addition, this hormone also seems to be engaged in osmoregulation - the procedure used by our body to maintain a balance of salt as well as keep the fluids stable internally. Aside from being produced in the duodenum, it is also possible to introduce secretin into our body. Usually, the hormone is introduced in the body during a medical examination undertaken to ascertain the functioning of the pancreas.

It is interesting to note that secretin is one of the first hormones identified by scientists. In 1902, English physiologists William Bayliss and Ernest Starling undertook a study to find the way our nervous system regulates the digestive process. At that time, scientists were aware of the fact that the pancreas released various digestive juices when the ingested food (also called chime) passes into the duodenum through the pyloric sphincter. During the course of their study, these two physiologists severed the all the nerves leading to the pancreas of the experimental animals and discovered that nervous system was in no way involved with digestive process. They came to the conclusion that the inner lining of the intestines secrete a substance which is transported to the pancreas through the bloodstream. This substance actually stimulates the pancreas. Bayliss and Starling named this substance secreted by the intestinal walls as secretin, which was, incidentally, the first of its kind of "chemical messenger" identified by researchers. In present times, such substances are referred to as a hormone. In fact, it was Starling who also coined the term hormone in 1905.

A series of reactions occur in succession when we start digesting the ingested food. Along with others, secretin plays a vital role during the digestive process. When the ingested food passes from the stomach downwards, it undergoes a changeover and the partially digested food is called chime, which is extremely acidic owing to the action of the gastric juices on it. When the food passes on into the small intestine, secretin starts working on it.

Among various other things, secretin stimulates bicarbonate production in the digestive tract. The bicarbonate works to counteract the acid content in chyme, thereby protecting the intestines from any damage caused by the acids. The intestines are different from the stomach, which has the ability to bear the harsh acidic action of the gastric juices that are secreted in the initial state of the digestive process. In fact, as the intestinal walls start secreting secretin, it sends a message to the stomach to stop producing gastric juices. This also protects the intestines from any damages that may be caused by the acids in the gastric juices. The message also says that since the digestive process has begun, the production of gastric juices in the stomach must stop allowing the stomach to take some rest.

In addition, secretin stimulates pancreatic juice production, besides triggering bile production in the liver. These are required for lubricating the chyme while it passes through the gastrointestinal (GI) track. At the same time, if the chyme contains high levels of glucose, secretin also triggers insulin production to deal with the excessive glucose.

During the test undertaken for secretion stimulation, the patient is administered secretin injection to ascertain the manner in which the pancreas responds. If the pancreas of a patient does not produce sufficient pancreatic juices even after the stimulation, it is an indication that the patient is enduring some medical condition that needs to be diagnosed and treated. This test is usually an uncomfortable one because in this case secretin is usually introduced via a stomach tube. Many people usually gag when the tube is introduced into or removed from the stomach.

Secretin possesses another characteristic - it was the first hormone that was identified by researchers. In the beginning of the 1900s, researchers who were studying the various aspects of the digestive tract observed that this hormone seemed to have a vital role in the digestive process as it sent signals to make the process somewhat moderate. These findings were contrary to the earlier conception that our digestive process was governed by the nervous system. The discovery not only changed the perception of the scientists about how our body functioned, but also led to the discovery of many other similar "chemical messengers", which were later known as hormones.

Functions of secretin hormone

Although secretin's primary function in our body is to motivate the pancreas to release digestive juices that contain high amounts of bicarbonate that counteract the acids contained in the gastric juices secreted in the stomach as the partially digested food (chime) moves from the stomach into the small intestine, this hormone has numerous other functions. Secretin also triggers the stomach to make and release another enzyme known as pepsin. The primary work of pepsin is to break down the proteins in ingested foods into small particles and simpler substances making it easy for the body to absorb them as nutrients. It is believed that secretin may also have a role in several activities of the brain such as triggering and using the neurotransmitter serotonin. Several places in our central nervous system (CNS) have secretin receptors. Such receptors are also present in our eyes. However, scientists are yet to ascertain whether this hormone possesses the ability to improve symptoms related to autism owing to its affects on the pancreas or its action on the brain. However, some scientists believe that there is some kind of association between the functioning of the gastrointestinal system and the brain. In the case of children with autism, their stomach is unable to digest proteins from ingested foods properly. Hence, the protein usually passes into their bloodstream and sooner or later reaches the brain.

Primarily, secretin works to counteract the pH in our duodenum, thereby enabling the digestive juices produced in the pancreas such as pancreatic lipase and pancreatic amylase to function in an optimal way.

Pancreas is the main target of secretion. In fact, secretin receptors are present in the plasma membrane of the pancreatic centroacinar cells. When secretin attaches itself to these receptors, the enzyme activates adenylate cyclase activity and transforms ATP into cyclic AMP in pancreatic islets. In the signal transduction within the cells (intracellular), cyclic AMP works as the subsequent messenger and triggers the organs to release a fluid that is rich in bicarbonate content. This fluid streams into the small intestine. A base (alkaline substance) bicarbonate works to counteract the acids in the gastric juices, thereby creating a favourable pH in which the digestive enzymes are able to work effectively in the small intestine.

Among other things, secretin also helps to promote bicarbonate and water secretion from the Brunner’s gland in the duodenum to serve as a buffer to the inward bound protons of chyme, which has high acidic content. At the same time, the water and bicarbonate also decreases the secretion of acids by the stomach's parietal cells. This is done by no less than three different mechanisms. Firstly, this is done by triggering somatostatin release and, secondly, by slowing down gastrin secretion in the pyloric antrum. Another mechanism by which this is achieved is by directly down regulating the secretory mechanics of the parietal cell acid.

Secretin also works against spikes in blood glucose concentration by triggering release of insulin from the pancreas. The function of secretin takes place after one intakes glucose. Secretin also modulates water as well as promotes transport of electrolyte within the pancreatic duct cells, epididymis epithelial cells and cholangiocytes. In addition, it has been found that this enzyme also has a role in regulating the absorption of renal water, which is independent of vasopressin (an anti-diuretic hormone).

In the hypothalamus, secretin is present in the magnocellular neurons found in the supraoptic and paraventricular nuclei. It is also present in the neurohypophysial tract extending up to neurohypophysis. When the osmolality is increased (higher concentration of solute in a solution), the posterior pituitary releases secretin. Secretin also triggers the release of vasopressin in the hypothalamus. Moreover, secretin is necessary for undertaking the central effects of the hormone angiotensin II. When secretin or secretin receptors are absent in gene knockout animals (genetically modified animals), it was found that injecting angiotensin II in the central failed to encourage intake of water or release of vasopressin.

According to some scientists, this type of anomalies in release of secretin may possibly explain the aberrations that are fundamental to type D syndrome of inappropriate anti-diuretic hormone hypersecretion (also known as SIADH). In such persons, release and response of vasopressin are normal, even though anomalous translocation or aquaporin 2, renal expression or both are usually found. It is recommended that since secretin is a neurosecretory hormone released by the posterior pituitary, it may possibly be that the vasopressin independent mechanism, which has been long sought after, will solve the puzzle that has confused physiologists as well as clinicians for several decades.

In the hypothalamus, both secretin as well as secretin receptors are present in the discrete nuclei, including the arcuate nucleus and the paraventricular nucleus. Arcuate nucleus and the paraventricular nucleus are the two main sites in the brain that regulates the body energy homeostasis (the dynamic equilibrium in a cell or body). During animal studies it was discovered that injection of secretin in the central as well as peripheral nervous system made mice to intake lesser food. Hence, this finding is an indication that the peptide also plays an anorectic role. The central melanocortin system mediates this specific function of secretin.


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