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The Pancreas and Energy Metabolism The pancreas is a combined exocrine and endocrine gland. The exocrine part of the pancreas secretes digestive enzymes in response to gastrointestinal hormones such as CCK. It also produces bicarbonate to neutralise stomach acid in response to secretin. The endocrine cells form clusters called the islets of Langerhans which are scattered throughout the gland. Four peptide hormones are secreted by these cells - insulin from beta cells, glucagon from alpha cells, somatostatin from delta cell sand pancreatic polypeptide from PP cells. Most of the cells in the islets of Langerhans are insulin producing beta cells.
The capillaries in the pancreas transport the secreted hormones to the portal vein, which conducts blood from the digestive tract to the liver. Glucagon exerts its effects primarily on the cells of the liver, whilst insulin affects many cells throughout the body.
1. Insulin Insulin secretion is enhanced after meals in response to an increase in glucose and amino acids in the blood. Like other peptide hormones, insulin is first produced as a pro-hormone. Pro-insulin is converted to insulin immediately before secretion by removal of an inactive peptide called Cpeptide. Active insulin consists of two polypeptide chains of different lengths connected by two disulphide bridges. Insulin is metabolised in the liver and kidneys by cleavage of the disulphide bonds and shortening of the polypeptide chains.
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