To our surprise, insulin was the first hormone identified (late 1920's) which won the doctor and medical student who discovered it the Nobel Prize (Banting and Best). They discovered insulin by tying a string around the pancreatic duct of several dogs. When they examined the pancreas of these dogs several weeks later, all of the pancreas digestive cells were gone (died and were absorbed by the immune system) and the only thing left was thousands of pancreatic islets. They then isolated the protein from these islets and behold, they discovered insulin. Note that there are other hormones produced by different types of cells within pancreatic islets (glucagon, somatostatin, etc) but insulin is produced in far greater amounts under normal conditions making the simple approach used by Banting and Best quite successful.
Insulin is a hormone. And like many hormones, insulin is a protein. Insulin is secreted by groups of cells within the pancreas called islet cells. The pancreas is an organ that sits behind the stomach and has many functions in addition to insulin production. The pancreas also produces digestive enzymes and other hormones. Carbohydrates (or sugars) are absorbed from the intestines into the bloodstream after a meal. Insulin is then secreted by the pancreas in response to this detected increase in blood sugar. Most cells of the body have insulin receptors which bind the insulin which is in the
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