Insulin is a hormone produced by the pancreas (a gland that releases a digestive juice into the intestine). The pancreas is composed of acinar cells, which produce digestive enzymes, and the islet cells of Langerhans, which produce hormones.
What Insulin Does
Four hormones are produced by the Langerhans islet cells. Insulin is produced in the B cells, glucagon in the A cells, somatostatin in the D cells, and pancreatic polypeptide in the F cells. Insulin promotes anabolism (building up of tissues) and inhibits catabolism (breaking down of tissues) in muscle, liver, and fat cells. It increases the rate of synthesis (blending) of glycogen, fatty acids, and proteins. Lack of insulin causes diabetes mellitus (a disease characterized by excess sugar in the blood and other body fluids).
Insulin's most important feature is its ability to increase the rate of glucose (a crystalline sugar) absorption by cells. Glucose is the most efficient fuel used by and found in almost all cells. Insulin causes a decreased concentration of glucose in the blood and causes the cells to store glycogen (a starchlike substance), mostly in the liver. It also promotes the entry of other sugars and amino acids into the muscle and fat cells. Insulin is therefore responsible for promoting fat storage in fat cells and for the total quantity of protein in the body.
Insulin production is stimulated by high levels of glucose and inhibited (limited) by lower levels of glucose. Insulin regulates glucose with glucagon. Glucagon catabolizes (changes into a product of simpler composition) glycogen to glucose and also raises the blood sugar. Glucagon can be given to increase the blood sugar when intravenous (by needle) glucose cannot be given. Glucagon takes about twenty minutes to raise the blood sugar. Intravenous glucose raises it instantaneously, which is why it is preferred in treatment. Together insulin and glucagon ensure that the body stores and maintains the proper level of glucose for its energy needs.
Diabetes is from the Greek word meaning "siphon," and "mellitus" comes from melliferous, meaning "of or relating to honey." Diabetes has been recognized for centuries and was originally diagnosed by tasting the urine and finding it sweet (melliferous). The high sugar also causes the kidneys to excrete (or siphon) large amounts of water. In 1815, French chemist Michel Eugene Chevreul discovered that the sweetness came from grape sugar or glucose. Later discoveries showed how the body makes, stores, and uses glucose.
Injury to the pancreas was linked to diabetes beginning in the seventeenth century and confirmed by animal experiments, particularly those of the German physiologist Joseph von Mehring (1849-1908) and a Russian pathologist, Oscar Minkowski (1858-1931). The acinus cells were found in the seventeenth century by the Dutch anatomist Regnier de Graaf and the islet cells in 1869 by a German pathologist Paul Langerhans (1847-1888).
In 1905 English physiologists Ernest Starling and William Bayliss discovered hormones. Hormones are substances secreted (released) by glands and carried in the blood to control cell activity elsewhere. In 1916 an English physiologist named Edward Sharpey-Schafer proposed that a hormone produced by the pancreas lowered the level of glucose in the blood. He called the hormone "insuline," the Latin word for "island," because he believed it came from the islet cells of the pancreas.
Credit for discovering insulin is given to Canadian surgeon Frederick Grant Banting (1891-1941) and Canadian physiologist Charles Herbert Best (1899-1978). Banting and Scottish physiologist and professor John James Rikard Macleod (1876-1935) were jointly awarded the Nobel Prize for medicine in 1923. Banting gave half of his share to Best, and Macleod gave half of his share to James Bertram Collip, because of the men had contributed to the discovery.
The First Insulin Patient
Collip, a professor at the University of Alberta, had experience in the chemistry of hormones. Prior to January 1922, he had prepared an insulin pure enough to be used on human patients. The first patient to receive insulin was 14-year-old Leonard Thompson. Thompson was admitted to Toronto General Hospital with a high blood glucose level; he also was urinating between three and five liters of fluid per day. Despite his rigid diet of only 450 calories (the only known treatment at this time was a diet low in carbohydrates), Thompson continued to excrete (get rid of through bodily waste) large amounts of glucose. On January 11, 1922, he was given insulin. Within a fairly short time, his blood sugar level came down and he stopped urinating large amounts of liquid.
In 1982 insulin became available as a genetically-engineered product called Humulin. Humulin's structure is identical with human insulin. The A and B chains are produced separately in different strains of E. coli bacteria. The E. coli have been genetically encoded to produce each of these strains. The strains are separated from the bacteria and purified. The purified chains are combined chemically and repurified.