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The content of islet amyloid polypeptide (IAPP) in isolated rat pancreatic islets was determined by a radioimmunoassay. Reverse-phase high-performance liquid chromatography analysis revealed that a main peak of IAPP immunoreactivity in the extracts from the islets corresponded to a synthetic rat IAPP. Secretion of IAPP from the cells is regulated by the extracellular glucose concentration. Thus, IAPP may be a novel regulator for glucose homeostasis and changes in the secretion perhaps relate to insular amyloid deposits and impaired glucose tolerance in type 2 diabetes mellitus.

To investigate the relationship between human islet amyloid polypeptide (IAPP)/amylin expression and islet amyloid deposits in the pathogenesis of human non-insulin-dependent diabetes mellitus (NIDDM), we developed transgenic mice using a human IAPP cDNA connected to an insulin promoter. Ribonucleic acid blotting and immunohistochemistry revealed the expression of the transgene in the pancreatic beta cells. Immunogold electron microscopy showed that beta-secretory granules contained the human C-terminal flanking peptide of the IAPP precursor. Reverse-phase HPLC demonstrated human and mouse IAPP amide in the pancreas. Electron microscopy showed the accumulation of fibril-like material in a considerable number of beta-secretory granules. These results suggest that in transgenic mice, the human IAPP precursor is expressed in beta cells and becomes normally sorted into beta-secretory granules in which normal conversion to mature human IAPP takes place. The human IAPP molecules, because of their amyloidogenesis, aggregate into amyloid fibrils in secretory granules. Glucose tolerance was normal at 7 months old and islet amyloid was not observed. A longer time may be required for islet amyloid deposits and hyperglycemia to develop in mice. Our working hypothesis is that in human NIDDM, IAPP aggregates into amyloid fibrils in beta-secretory granules, and that the fibrils are released into the extracellular space and islet amyloid deposits become substantial with time.

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To investigate the possible role of islet amyloid polypeptide (IAPP) in the development of type 2 diabetes mellitus, we examined the IAPP content and secretion in pancreatic islets isolated from ventromedial hypothalamic (VMH)-lesioned rats and genetically obese Zucker rats, using a specific RIA for IAPP. Obesity and hyperinsulinemia were observed in rats 21 days after VMH lesioning. IAPP content was increased in the islets of VMH-lesioned rats compared with findings in the sham-operated controls (100.9 +/- 6.6 vs. 72.8 +/- 3.85 fmol/islet; P less than 0.01). Isolated islets of VMH-lesioned rats secreted larger amounts of IAPP in the presence of 2.8 mM and 16.7 mM glucose (2.99 +/- 0.98 and 11.2 +/- 1.29 fmol.islet(-1).3 h-1) than was noted in sham-operated rats (ND and 6.65 +/- 0.78 fmol.islet(-1).3 h-1). In the obese Zucker rats, aged 14 weeks, IAPP concentrations in the islets were elevated compared with lean rats (133.3 +/- 10.6 vs. 84.4 +/- 8.5 fmol/islet; P less than 0.01). The isolated islets secreted larger amounts of IAPP in response to 2.8 mM and 16.7 mM glucose (2.83 +/- 0.88 and 15.81 +/- 1.35 fmol.islet(-1).3 h-1) than did those from lean control rats (0.36 +/- 0.19 and 12.49 +/- 1.20 fmol.islet(-1).3 h-1). These results strongly suggest that overproduction and hypersecretion of IAPP occur in animals with obesity and hyperinsulinemia.