The pancreas has two main functions: an exocrine and an endocrine one. The islets of Langerhans consist of four distinct endocrine (hormone-producing) cell types:
• alpha cells producing glucagon;
• beta cells producing insulin and amylin;
• delta cells producing somatostatin;
• F cells producing pancreatic polypeptide. Glucagon and insulin are hormones regulating glucose level in blood plasma.
Somatostatin performs function of a para-crine inhibitor of secretion. Amilin (islet amyloid polypeptide; IAPP) regulates appetite, gastric motor activity, and the secretion of insulin and glucagon.
Endocrine pancreatic insufficiency manifests predominantly as symptoms of insulin deficiency; therefore, insulin is deemed the main hormone of the pancreas.
Insulin is a polypeptide consisting of two peptide chains referred to as the A chain and B chain linked together by two disulfide bonds. The A chain consists of 21 amino acid residues and the B chain consists of 30 amino acid residues. Insulin is produced as preproinsulinin the Golgi apparatus of beta cells and converts to proinsulin. Proinsulin is made up of two chains of insulin bound to protein C chain (of 35 ami-no acid residues). After detachment of protein C followed by an uptake of four amino acid residues, insulin molecules are formed and packaged in granules and undergo exocytosis
(fig. 12.1).
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Fig. 12.1. Insulin molecule structure
The effect of insulin on metabolism in general terms can be characterized as anabolic (the
hormone enhances the synthesis of proteins, lipids, and glycogen). Of primary importance is the effect of insulin on carbohydrate metabolism. These metabolic effects of insulin are associated with а lowering of serum glucose level (hypoglycemia). One of the causes of hypogly-cemia is an increased glucose uptake in tissues.