Insulin

Insulin is a hormone that is made in the beta cells of the pancreas and plays a critical role in the use of glucose (sugar) in the body. In the pancreas, insulin is made in an inactive form known as proinsulin. Once proinsulin is released, a portion of the compound known as c-peptide is cleaved off and the active form of insulin becomes available for the body to use. Insulin levels rise after a person eats and are low when a person has been fasting.

Insulin has many functions in the body. The primary function is to help bring sugar from the blood into muscle and fat tissue, two principal users of glucose. Insulin works by binding to a receptor on the outside of a cell and subsequently initiating a series of reactions inside the cell. The final step of these reactions is to activate glucose transporters, which are proteins that bring sugar inside the cell. Once inside the cell, the sugar can either be used to create energy by a process known as glycolysis or stored by a process known as glycogenesis. Insulin also promotes fatty acid synthesis, which is known as de novo synthesis. Insulin's role in this pathway is to activate enzymes that favor the creation of fatty acids. Additionally with fatty acids, insulin may also promote the storage of excess fatty acids as triglycerides. Insulin also regulates protein metabolism by increasing the uptake of amino acids, the building blocks of dietary protein, and by increasing protein synthesis. All of these metabolic pathways are classified as anabolic pathways because they promote growth in the body and thus insulin is classified an anabolic hormone.

Insulin also activates the enzyme lipoprotein lipase and inhibits the enzyme hormone sensitive lipase, which work to cleave and store fat in the body, respectively. More recently, it has been discovered that insulin may regulate gene expression. Some of the [Page 406]genes whose expression increases in the presence of insulin include SREBP 1-C, GLUT 1, GLUT 2, GLUT 3, GLUT 4, hexokinase II, glucokinase, glucose-3-phosphate dehydrogenase, and pyruvate kinase. Some of the genes whose expression is decreased in the presence of insulin are glucose-6-phosphotase, fructose-1, 6-bisphophotase, and PEP carboxykinase. These proteins that are regulated by insulin all play a role in glucose metabolism. Insulin has also been demonstrated to increase food intake. This is because insulin's main role is to bring glucose into the cells. If glucose levels are low but insulin levels are high, then insulin will send a signal to the brain to begin eating. Once a person eats, his or her blood glucose will rise and subsequently the extra insulin will be used.

Insulin is most commonly thought of in regard to diabetes (and shots). In nondiabetic individuals, insulin is produced in the pancreas, and is used to transport sugar from the bloodstream to muscle and fat tissue.

Glucagon is a catabolic hormone whose actions oppose insulin. To assess which metabolic pathways are currently occurring in the body, researchers examine the insulin-to-glucagon ratio. A low insulin-to-glucagon ratio means that there is more glucagon than insulin and thus catabolic pathways are favored. Conversely, a high insulin-to-glucagon ratio means that there is more insulin than glucagon and anabolic pathways. Other hormones that oppose insulin include epinephrine, norepinephrine, and ACTH. During stressful situations, insulin levels are decreased and these other hormones become elevated. This allows for our body to favor a catabolic response and provide a large amount of energy during a short period of time.

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