The islets of Langerhans, the endocrine cells of the pancreas, produce at least three glucose-related hormones: (1) insulin, which is produced by the beta cells; (2) glucagon, which is produced by the alpha cells; and (3) somatostatin, which is produced by the delta cells.
The overall effect of insulin is to promote glucose utilization and energy storage. It accomplishes this by enhancing glucose and potassium entry into most body cells, stimulating glycogen synthesis in liver and muscle, promoting the conversion of glucose to fatty acids and triglycerides, and enhancing protein synthesis. It exerts its effects by interacting with cell surface receptors.
In contrast to insulin, glucagon increases blood glucose levels by stimulating the breakdown of glycogen and the release of glucose stored in the liver. Somatostatin inhibits secretion of both insulin and glucagon. It also inhibits release of growth hormone, thyroid-stimulating hormone, and adrenocorticotropic hormone by the adenohypophysis and may decrease production of parathormone, calcitonin, and renin. In addition, it is thought to inhibit secretion of gastric acid and gastrin. The exact physiological roles of glucagon and somatostatin are unknown.
Blood levels of insulin are measured by radioimmunoassay and can be determined in most laboratories. Samples for blood glucagon levels require special handling, and tests for its presence may not be routinely available in all laboratories. Somatostatin may be measured but this test is not routinely performed. C-peptide, a metabolically inactive peptide chain formed during the conversion of proinsulin to insulin, may be measured to provide an index of 
-cell activity not affected by exogenous insulin.
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