Function of the various parts of the gastrointestinal tract is tightly regulated by hormones that are secreted into the circulation in an endocrine fasion by specialized cells all over the gastrointestinal system. Secretions other than hormones and which assist in the digestive process occurs in many parts of the digestive system.
As an example, the esophagus usually secretes mucus from its epithelial cells that function to lubricate its inner surface. Thus helping in the propulsion of food into the stomach. The stomach secretes acid and other several compounds such as the enzyme pepsin and the hormone gastrin which assist in the digestive process.
The pancreas is an important organ of the digestive system. It secretes watery solution that contains bicarbonate ions in addition to enzymes that act in degrading the proteins and carbohydrates and lipids that are present in the intestine.
The gallbladder also secretes material into the intestine which helps in the digestion process of lipids by dissolving it. The gallbladder contraction and relaxation is under hormonal regulation from a hormone which is called cholecystokinin. In addition to this function it can stimulate the pancreas to secrete its enzymes into the duodenum. It was discovered in 1928 and is found in the duodenum and jejunal lining epithelium. It is also found in the cells of the nervous system.
Cholecystikinin release is inhibited by the hormone somatostatin and which is by itself an important inhibitory hormone that will be mentioned shortly. The increased amount of bile in the intestine also inhibits its secretion from the gallbladder.
The other hormone that I discuss here is called secretin. This hormone is a polypeptide hormone that is composed of 27 amino acids. Its function centers around stimulation of the pancreas to secrete a solution of water with bicarbonate ions. It is usually secreted in respose to the presence of acid in the duodenum which in turn can be neutralized by the basic solution of the pancreas. Cholecystokinin increases or stimulates the action of secretin. Secretin is usually used clinically for the evaluation of pancreatic function.
The other hormone that I discuss here is somatostatin. This hormone is secreted by cells in the hypothalamus as well as cells of the gastrointestinal tract. It is a known inhibitor of growth hormone secretion. Thus it is an antagonist to this hormone. As far as gastrointestinal hormones are concerned so soamtostatin is an inhibitor of several hormones. Among these hormones are gastrin inhibitory peptide and insulin and glucagon in addition to vasoactive intestinal peptide and motilin which will be discussed later,
Somatostatin is known for its inhibitory effect on pancreatic secretions of enzymes and bicarbonate ions. In addition, it inhibits the secretion of gastric acid in the stomach. On the other hand eating would stimulate the release of somatostatin. Gastric inhibitory peptide is a 42 amino acids polypeptide hormone which functions to reduce the amount of acid in the atomach. In addition, it stimulates the function of insulin.
The motility of the gastrointestinal tract is reguated by the hormone motilin. Motilin is a 22 amino acids polypeptide hormone which is found in the cells of the duodenum and the stomach. Drugs that can increase the motility of the gastrointestine are present such as prepulsid which is a prokinetic drug that stimulates the peristalsis of the digestive system.
The last hormone that I will discuss here is called vasoactive intestinal peptide. It functions by relaxation of smooth muscles especially of the esophageal sphincter and the anal sphincter.