Insulins

Insulin is a type of peptide hormone that reduces the amount of glucose in the blood. It is produced in the pancreas by beta cells. These cells are found within clusters of endocrine cells called the Islets of Langerhans, which are distributed across the pancreas. If the body is unable to produce enough insulin, then insulin therapy is used to keep the blood glucose low.

Insulin’s main function is to facilitate the transport of glucose from the blood into the various insulin-responsive tissues like muscle cells and adipose tissue. This hormone binds to insulin receptors on the surface of the cell membrane. Now, these receptors have two alpha and two beta subunits. Alpha subunits are located outside of the cell and they bind insulin; while two beta subunits are located within the cell and they have tyrosine kinase activity which carries signals into the cell. Once stimulated, insulin receptors cause intracellular storage vesicles, which contain glucose transport proteins called GLUT4, to fuse with the cell membrane. Next, the GLUT4 proteins embed themselves into the membrane and allow glucose to move into the cell.

As a result, insulin promotes glucose uptake and glycogenesis, which is the conversion of glucose to glycogen. Glycogenesis is the process that takes place in the liver and skeletal muscles. When glycogen storage capacity is reached, insulin promotes glycolysis, which is the breakdown of glucose to pyruvate. It also stimulates lipogenesis, the synthesis of fatty acids and triglycerides in the liver and adipose tissue; and amino acid uptake and protein synthesis in skeletal muscles.

Finally, insulin activates Na+/K+- ATPase pumps and shifts potassium into intracellular space, thereby decreasing potassium levels in the blood. On the flip side, insulin inhibits glycogenolysis, which stands for the breakdown of glycogen; and gluconeogenesis, which is glucose production from lactic acids and noncarbohydrate molecules. Finally, insulin inhibits lipolysis, the breakdown of lipids; and proteolysis, the breakdown of proteins.