Proinsulin Biosynthesis pCell Mass and Insulin Secretion

Insulin is synthesized in pancreatic |3 cells, where it is processed from its precursor, proinsulin, into c-peptide (connecting peptide) and insulin (9). Processed insulin is stored in insulin vesicles. There are —2000 P cells in an islet and ~ I

million islets in the pancreas of an adult human. The p cell is shaped like a truncated cone, with the base of the cell exposed first to the blood flowing into the islet capillary bed. The membrane at the base of the cell has the glucose-2-transporter proteins that permit glucose access to the cell. Metabolism and oxidation of this glucose to carbon dioxide and water generate ATP. The increased ATP prompts exocytosis of stored insulin vesicles that discharge insulin from the apex of the p cell into the capillary and then into the portal vein. Although the prevailing glucose concentration is the best-recognized stimulant for insulin secretion, fatty acids are also important regulators of insulin secretion.

Evidence is increasing that the p-cell mass is regulated in adult life with new islet formation from exocrine ducts as well as |3-cell replication within islets (10). Chronic insulin resistance leads to an adaptive increase in p-cell mass. Animals and humans with a partial decrease in p-cell mass as a consequence of a partial pancreatectomy have impaired insulin secretion, with an increased proinsulin-to-insulin ratio suggestive of inadequate processing of insulin. Leahy and colleagues (11) have shown somewhat paradoxically, that partial inhibition of insulin secretion can overcome diabetes under conditions of decreased p-cell mass. This intriguing observation implies that the pattern of insulin release may be more important than the absolute amount.

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