What is "insulin"

1922 (earlier insuline, 1914), coined in English from Latin insula "island," so called because the hormone is secreted by the islets of Langerhans in the pancreas. Insuline was coined independently in French in 1909.

n. A polypeptide hormone that regulates carbohydrate metabolism.

n. hormone secreted by the isles of Langerhans in the pancreas; regulates storage of glycogen in the liver and accelerates oxidation of sugar in cells

Insulin (medication) is the use of insulin and similar proteins as a medication to treat disease. Insulin comes in a number of different types including short acting (such as regular insulin) and long acting (such as NPH insulin). Insulin is used to treat a number of diseases including diabetes and its acute complications such as diabetic ketoacidosis and hyperosmolar hyperglycemic states. It is also used along with glucose to treat high blood potassium levels. Side effects may include: low blood sugar levels, skin reactions at the site of injection and low potassium levels among others. Insulin was first used as a medication in Canada by Charles Best and Frederick Banting in January 1922.

Insulin (from the Latin, insula meaning island) is a peptide hormone produced by beta cells of the pancreatic islets, and by the Brockmann body in some teleost fish. It has important effects on the metabolism of carbohydrates, fats and protein by promoting the absorption of, especially, glucose from the blood into fat, liver and skeletal muscle cells. In these tissues the absorbed glucose is converted into either glycogen or fats ( triglycerides), or, in the case of the liver, into both. Glucose production (and excretion into the blood) by the liver is strongly inhibited by high concentrations of insulin in the blood. Circulating insulin also affects the synthesis of proteins in a wide variety of tissues. In high concentrations in the blood it is therefore an anabolic hormone, promoting the conversion of small molecules in the blood into large molecules inside the cells. Low insulin levels in the blood have the opposite effect by promoting widespread catabolism.

The pancreatic beta cells (β cells) are known to be sensitive to the glucose concentration in the blood. When the blood glucose levels are high they secrete insulin into the blood; when the levels are low they cease their secretion of this hormone into the general circulation. Their neighboring alpha cells, probably by taking their cues from the beta cells, secrete glucagon into the blood in the opposite manner: high secretion rates when the blood glucose concentrations are low, and low secretion rates when the glucose levels are high. High glucagon concentrations in the blood plasma powerfully stimulate the liver to release glucose into the blood by glycogenolysis and gluconeogenesis, thus having the opposite effect on the blood glucose level to that produced by high insulin concentrations. The secretion of insulin and glucagon into the blood in response to the blood glucose concentration is the primary mechanism responsible for keeping the glucose levels in the extracellular fluids within very narrow limits at rest, after meals, and during exercise and starvation.

When the pancreatic beta cells are destroyed by an autoimmune process, insulin can no longer be synthesized or be secreted into the blood. This results in type 1 diabetes mellitus, which is characterized by very high blood sugar levels, and generalized body wasting, which is fatal if not treated. This can only be corrected by injecting the hormone, either directly into the blood if the patient is very ill and confused or comatosed, or subcutaneously for routine maintenance therapy, which must be continued for the rest of the person’s life. The exact details of how much insulin needs to be injected, and when during the day, has to be adjusted according to the patient’s daily routine of meals and exercise, in order to mimic the physiological secretion of insulin as closely as is practically possible.

In type 2 diabetes mellitus the destruction of beta cells is less pronounced than in type 1 diabetes, and probably not due to an autoimmune process. Instead there is an accumulation of amyloid in the pancreatic islets, which disrupts the anatomy and physiology of the pancreatic islets. What causes this amyloid deposition is unknown, and precisely how it affects the secretion of insulin and glucagon is not known. What is known is that type 2 diabetes is characterized by high rates of glucagon secretion into the blood which are unaffected by, and unresponsive to the blood glucose levels. Insulin is still secreted into the blood in response to the blood glucose level, but there seems to be a “resistance” to its actions, which may be, at least partly, due to the high glucagon concentrations in the blood. As a result, the insulin levels, even when the blood sugar level is normal, are much higher than they are in healthy persons. There are a variety of treatment regimens, very few of which are entirely satisfactory. When the pancreas’ capacity to secrete insulin can no longer keep the blood sugar level within normal bounds, insulin injections are given. Over 40% of patients with type 2 diabetes require insulin injections as part of their diabetes management plan.

Insulin may have originated more than a billion years ago. The molecular origins of insulin go at least as far back as the simplest unicellular eukaryotes. Apart from animals, insulin-like proteins are also known to exist in the Fungi and Protista kingdoms. The human insulin protein is composed of 51 amino acids, and has a molecular mass of 5808 Da. It is a dimer of an A-chain and a B-chain, which are linked together by disulfide bonds. Insulin's structure varies slightly between species of animals. Insulin from animal sources differs somewhat in effectiveness (in carbohydrate metabolism effects) from human insulin because of these variations. Porcine insulin is especially close to the human version, and was widely used to treat type 1 diabetics before human insulin could be produced in large quantities by recombinant DNA technologies.

The crystal structure of insulin in the solid state was determined by Dorothy Hodgkin; she was awarded the Nobel Prize in Chemistry in 1964. It is on the WHO Model List of Essential Medicines, the most important medications needed in a basic health system.