PHYSIOLOGY AND PATHOPHYSIOLOGY OF DIABETES
Insulin is secreted by beta cells, which are one of four types of cells in the islets of Langerhans in the pancreas. Insulin is an anabolic, or storage, hormone. When a person eats a meal, insulin secre-tion increases and moves glucose from the blood into muscle, liver, and fat cells. In those cells, insulin:
• Transports and metabolizes glucose for energy
• Stimulates storage of glucose in the liver and muscle (in the form of glycogen)
• Signals the liver to stop the release of glucose
• Enhances storage of dietary fat in adipose tissue
• Accelerates transport of amino acids (derived from dietary protein) into cells
Insulin also inhibits the breakdown of stored glucose, protein, and fat.
During fasting periods (between meals and overnight), the pancreas continuously releases a small amount of insulin (basalinsulin); another pancreatic hormone called glucagon (secreted by the alpha cells of the islets of Langerhans) is released when blood glucose levels decrease and stimulate the liver to release stored glucose. The insulin and the glucagon together maintain a constant level of glucose in the blood by stimulating the release of glucose from the liver.Initially, the liver produces glucose through the breakdown of glycogen (glycogenolysis). After 8 to 12 hours without food, the liver forms glucose from the breakdown of noncarbohydrate substances, including amino acids (gluconeogenesis).