Pancreatic Hormones and Glucose Regulation

The pancreatic islets continually balance glucose use with glucose supply. Their two clinically dominant hormones have opposing effects: - Beta cells release insulin, the principal hormone that lowers blood glucose. - Alpha cells release glucagon, which raises blood glucose by signalling the liver to release or make glucose. After a meal, rising glucose stimulates insulin release. Insulin helps skeletal muscle and adipose tissue take up glucose and tells the liver to stop releasing stored glucose. The liver is especially important: it normally acts as a glucose buffer, storing excess glucose after meals and releasing glucose between meals. During fasting, exercise, or physiologic stress, insulin secretion falls and glucagon rises. Glucagon directs the liver to break down glycogen (glycogenolysis) and synthesize new glucose (gluconeogenesis). This preserves plasma glucose for the brain and other glucose-dependent tissues. Insulin is not simply a key that opens cells. It also tells the liver that fuel is available. When insulin is absent or ineffective, the liver may continue producing glucose even while plasma glucose is already high. That mismatch is a major driver of diabetes-related hyperglycemia....