Molecular form of islet amyloid polypeptide (amylin) released from isolated rat islets of Langerhans

Islet amyloid polypeptide (IAPP) is a 37-amino acid residue polypeptide, originally isolated from the pancreatic amyloid deposits of patients with type II diabetes mellitus. Subsequently, IAPP was found to be colocalised with insulin in beta-cell secretory granules of the normal mammalian pancreas. Recently, IAPP has been reported to inhibit glucose-stimulated insulin release from isolated rat islets and to be released in response to glucose and arginine. To investigate further the regulation of IAPP release from the islet, we used a previously developed specific radioimmunoassay for IAPP and measured IAPP secretion from isolated rat islets of Langerhans. Release of IAPP-like immunoreactivity (-LI) was stimulated by glucose: 3.3 +/- 0.3, 3.9 +/- 0.3, and 11.1 +/- 1.5 (n = 5, mean +/- SEM) fmol/islet/60 min at 2, 7, and 20 mM, respectively. Carbachol (0.1 mM) increased the release of IAPP-LI at the lower glucose concentrations: 8.1 +/- 0.9, 8.7 +/- 0.6, and 11.7 +/- 1.8 fmol/islet/60 m in at 2, 7, and 20 mM glucose. Somatostatin (1 microM) suppressed glucose-stimulated IAPP-LI release (17.5 +/- 1.5 vs. 5.1 +/- 0.5 fmol/islet/60 min). Chromatographic characterisation of the IAPP-LI released into the incubation medium revealed two immunoreactive forms: The major peak (74% of the total IAPP-LI) corresponded to synthetic IAPP-37, while a smaller form, comprising 26% IAPP-LI, eluted later. In acid extracts of islets, all (> 95%) immunoreactivity co-eluted with the synthetic IAPP.