Secretion of Insulin and Glucagon

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As I have already mentioned, the pancreas contains clusters of cells known as the Islets of Langerhans. They contain three cell types: alpha cells that produce glucagon, beta cells that produce insulin, and delta cells where somatostatin is synthesized. Together, these cells and their hormone products are responsible for the minute-to-minute regulation of metabolism. Metabolism in this case includes storage and release of carbohydrates and lipids, rates of energy production, protein synthesis and even the regulation of hunger. Seemingly minor aberrations in function of these cells have large and often devastating effects on an individual's health.

Insulin secretion is stimulated by glucose, some amino acids and fatty acids. Let us take these up individually.

Monitoring Blood Glucose.
The basic functions and physiology of the beta cell are relatively well understood. A model of the beta cell showing the basic components for insulin secretion is presented below. A glucose "sensor" mechanism, a metabolic coupling to potassium channels to control plasma membrane potential and a voltage dependent Ca++ channel are required to link blood glucose levels to insulin secretion. Insulin containing granules are found in a reserve pool and a "readily released" pool.

Let us look at the "glucose sensor" system first. The beta cell's primary function is to correlate release of insulin with changes in blood glucose concentration. Obviously, these cells must have a sensitive glucose-measuring device. Nature has achieved this by equipping the beta cell with a glucose transport protein (GLUT2) and a kinase (glucokinase) both of which have low affinities for glucose. GLUT2 is quite active, but the Km for glucose is around 5 mmol/l. Therefore, transport of glucose into the beta cell is rapid, but only when the blood glucose concentration exceeds post-meal levels.

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