Insulin Stimulation
Insulin is a peptide hormone, which means that it is a protein that is secreted into the bloodstream. Insulin is secreted by cells in the pancreas called beta cells. Although the beta cells secrete a constant low amount of insulin, insulin secretion is greatly increased. As Athena Diagnostics explains, when there is more sugar in the blood, the beta cells in the pancreas receive more glucose, which increases the amount of a molecule called ATP. This causes changes in the electric charge of the beta cells, which causes them to release more insulin. In this way insulin production can be coordinated with glucose levels in the blood.
Muscle and Fat
Insulin increases the uptake of glucose in cells in two different ways. According to the Colorado State Hypertext Library, muscle and adipose (fat) cells import glucose via special protein called GLUT4 glucose transporters. When there is not much insulin present, these GLUT4 transporters are kept on the inside of the cell (also known as the cytoplasm), where they have no function. However, when insulin is present in the blood, it binds to the outside of the muscle and adipose cells and causes them to send the GLUT4 transporters to the cell surface. Once they are at the surface of the cell, these transporters are able to pump glucose into the cells.
Liver
The liver does not have GLUT4 transporters, which means that insulin causes liver cells (hepatocytes) to take in glucose in a less direct way. The liver is constantly moving glucose in and also letting it out. Insulin activates an enzyme in the liver called hexokinase, which chemically modifies glucose in the hepatocytes, which traps the glucose inside the cells. Insulin also inhibits the action of an enzyme that essentially reverses the action of hexokinase. As a result, insulin causes more glucose to be stored inside the liver.
