- The pancreas is a long, slender organ that is mostly positioned posterior to the stomach’s bottom half. Although the pancreas is largely an exocrine gland that secretes a number of digestive enzymes, it also serves an endocrine function. Its pancreatic islets, formerly known as Langerhans islets, secrete the hormones glucagon, insulin, somatostatin, and pancreatic polypeptide (PP).
Cells and Secretions of the Pancreatic Islets
- Alpha Cell: The alpha cell, which accounts for around 20% of each islet, produces the hormone glucagon. Low blood glucose levels induce the release of glucagon, which plays a crucial role in blood glucose management.
- Beta Cell: The beta cell, which accounts for approximately 75% of each islet, produces the hormone insulin. Elevated blood glucose levels cause insulin to be released.
- Delta Cell: The delta cell makes up 4% of islet cells and secretes the peptide hormone somatostatin. Remember that somatostatin is also secreted by the hypothalamus (as GHIH) and the stomach and intestines. Pancreatic somatostatin is an inhibitory hormone that inhibits the release of both glucagon and insulin.
- PP Cell: The pancreatic polypeptide hormone is secreted by the PP cell, which accounts for around 1% of islet cells. It is hypothesised to play a function in appetite modulation as well as pancreatic exocrine and endocrine secretion modulation. Although pancreatic polypeptide released after a meal may limit subsequent food consumption, it is also secreted in response to fasting.
Regulation of Blood Glucose Levels
- Glucose is the primary fuel for all body cells and is essential for cellular respiration. The breakdown of carbohydrate containing foods and beverages provides glucose to the organism. Glucose that is not immediately taken up by cells for fuel can be stored as glycogen by the liver and muscles, or converted to triglycerides and stored in adipose tissue. Hormones control both the storage and consumption of glucose as needed. Pancreatic receptors detect blood glucose levels, and pancreatic cells release glucagon or insulin to maintain normal levels.
- The pancreas glucagon receptors can detect a drop in blood glucose levels, such as after fasting or extended work or exercise. In response, pancreatic alpha cells secrete the hormone glucagon, which has numerous effects: It stimulates the liver to convert glycogen reserves back into glucose. This is referred to as glycogenolysis.
- The glucose is subsequently released into the bloodstream for usage by the body’s cells. It causes the liver to absorb amino acids from the blood and convert them to glucose. This is referred to as gluconeogenesis. It promotes lipolysis, which is the breakdown of triglycerides into free fatty acids and glycerol. Some of the free glycerol released into the bloodstream is converted into glucose by the liver. This is a type of gluconeogenesis as well.
- Insulin’s principal purpose is to facilitate glucose uptake into bodily cells. Red blood cells, as well as brain, liver, kidney, and small intestinal lining cells, lack insulin receptors on their cell membranes and do not require insulin for glucose uptake. Although all other body cells require insulin to receive glucose from the bloodstream, skeletal muscle cells and adipose cells are insulin’s principal targets. Food in the intestine causes the release of gastrointestinal tract hormones such as glucose-dependent insulinotropic peptide.
- This, in turn, is the initial trigger for insulin synthesis and secretion by pancreatic beta cells. When nutrients are absorbed, the subsequent increase in blood glucose levels promotes insulin release even more. It’s not totally apparent how insulin promotes glucose uptake. Insulin, on the other hand, appears to activate a tyrosine kinase. Many substrates within the cell are phosphorylated as a result of the receptor. These several metabolic reactions come together.
- To help intracellular vesicles containing facilitative glucose transporters migrate to the cell membrane In the case of these transport proteins are ordinarily recycled slowly between the cell membrane and the cell interior in the absence of insulin. Insulin causes a pool of glucose transporter vesicles to rush to the cell membrane, where they fuse and expose the transporters of glucose to the extracellular fluid. Insulin also lowers blood glucose levels by activating glycolysis, which is the breakdown of glucose to produce ATP. Furthermore, it encourages the liver to convert excess glucose into glycogen for storage and inhibits glycogenolysis and gluconeogenesis enzymes.
- Finally, insulin stimulates the synthesis of triglycerides and proteins. Insulin secretion is controlled by a negative feedback mechanism. Further insulin release is inhibited as blood glucose levels fall.