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THE PANCREAS The pancreas is made up of two functionally different parts: the exocrine pancreas, the major digestive gland of the body; and the endocrine pancreas. The endocrine pancreas consists of a large number (about 1 million) groups (clusters) of cells scattered within the tissue of the exocrine pancreas called islets of Langerhans. The islets form less than 2% of the pancreatic tissue. Two main types of cell have been identified in the endocrine pancreas, the A- and B-cells (α- and β-cells) that synthesize, store, and secrete the hormones glucagon and insulin, respectively, while D- and F- cells are found in much smaller numbers. D-cells (δ-cells) secrete somatostatin and F-cells produce pancreatic polypeptide. Insulin Insulin molecule consists of a 21 amino acid α- chain and a 30 amino acid β-chain, with the two chains linked together by disulphide bridges. Insulin is a dominant metabolic hormone primarily involved in the control of the blood glucose concentration. It is the only hormone that directly lowers blood glucose levels. Although the release of insulin from β-cells is regulated by many factors, the principal stimulus is an increased blood glucose concentration. The main target organs of insulin are the liver and almost all cells of the body. In the liver and skeletal muscle cells insulin stimulates glucose uptake and glycogenesis (formation of glycogen, i.e storage of glucose), while in the body cells it stimulates glucose uptake for immediate energy use, for the storage of energy in fats and for the synthesis of proteins. Glucagon It is a polypeptide synthesized in the α-cells of the islets of Langerhans. Glucagon exerts actions on carbohydrate, protein, and fat metabolism, however its principal effect is to raise the blood glucose concentration. It is very important in maintaining blood glucose and energy substrates in the circulation during fasting. The target organs of the hormone are the liver (the main target) and adipose tissue where it 1. Stimulates breakdown of stored glycogen (glycogenolysis) 2. Stimulates gluconeogenesis 3. Stimulates ketogenesis 4. Stimulates lipolysis in adipose tissue Somatostatin In the pancreas, somatostatin is synthesized by the D-cells. Since it was identified in the hypothalamus where it was found to have the ability to inhibit release of growth hormone from the pituitary, somatostatin has also been identified in a number of tissues, including many areas of the brain, the gastrointestinal tract, and the pancreas. Generally, somatostatin is an inhibitory paracrine hormone, and in the pancreas it inhibits the release of both insulin and glucagon. The hormone has been implicated in the regulation of nutrient concentrations in the blood where it appears to prevent the exaggerated responses following a meal. It has been found to retard the entry of nutrients into the blood by inhibiting various digestive events such as gastric emptying, acid, pepsin and gastrin secretions, sugar and fat absorption, and duodenal motility. Somatostatin therefore, may function as a hormone that regulates the movement of nutrients from the gut to the internal environment. Pancreatic Polypeptide (PP) It is a peptide found in the F-cells of the islets of Langerhans in the pancreas. PP is released after a high protein meal and in case of hypoglycemia. Its major effect is the inhibition of the secretion of enzymes by the pancreas and the bile by the gall-bladder. Amylin Recent literature reveals that this peptide is secreted from B-cells of the pancreas that secretes insulin. Although it is cosecreted with insulin in response to glucose and other B-cell stimulators, it was found to have opposing metabolic effects to those of insulin. Its metabolic role is unclear. DIABETES MELLITUS It is a prevalent disease characterized by excessive excretion of urine and above normal blood glucose level. This is one of the few diseases that have been well documented and described since 1500B.C. It is a prevalent serious hormonal disease in which the body cells are unable to absorb glucose from the blood. This disease occurs when there is not enough insulin in the blood or when the body cells do not respond normally to blood insulin. In either case, the cells cannot obtain enough glucose from the blood, and thus, starved for fuel, they are forced to burn the body’s supply of fats and proteins. There are two types of diabetes Type I: Insulin-Dependent Diabetes Mellitus (IDDM) It is characterized by progressive marked decrease in the number of insulin secreting B-cells. It is an autoimmune disease where destruction of insulin secreting cells by islet-cell antibodies occurs. As a result the pancreas does not produce enough insulin. It is a catabolic disorder in which due to the absence of circulating insulin, the three main target tissues of insulin (liver, muscle and fat) fail to take up absorbed nutrients and continue to spew glucose, amino acids and fatty acids into the bloodstream from their respective storage depots. Furthermore, continued fat breakdown lead to the production and accumulation of ketones. Type II: NonInsulin-Dependent Diabetes Mellitus (NIDDM). This is the more prevalent type and occurs predominantly in adults. In contrast to type I diabetes, circulating insulin is always present. It may be due to several factors, either the insulin secreted is not sufficient to lower the blood glucose to the normal level (insufficient insulin secretion), or the tissues do not respond to the hormone (insulin resistance). Meanwhile, since the digestive system continue to absorb glucose from the diet, the glucose concentration in the blood becomes extremely high and glucose is excreted in the urine.
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