Cellular Mechanisms of Insulin Release: Effects of Retinoids on Rat Islet Cell-to-Cell Adhesion, Reaggregation, and Insulin Release

We studied the effects of retinoids on islet cell-to-cell adhesiveness and glucose-induced insulin release from rat islets. For adhesion studies, islets were dispersed using low concentrations of trypsin. Thirteen cis-retinoic acid (13 cis-RA) was added to a suspension of 15 × 105 islet cells and adhesion of cells was quantitated using a hemocytometer. For functional studies, we measured biphasic insulin release from collagenase-isolated perifused islets, dispersed cells, and single large aggregates (clumps) of islet cells. Thirteen cis-RA (10−4 M) stimulated insulin secretion at 9.7, 12.5, 16.7, and 27.7 mM glucose. Maximal effects of 13 cis-RA (174% of control) were evident during second phase release at 9.7 mM glucose. Thirteen cis-RA (10−7 and 10−6 M) caused cells to adhere to each other, and at higher concentrations, 13 cis-RA caused dispersed cells to reaggregate into a single clump. These retinoid-induced clumps were perifused in a Bio-Gel P-2 gel column. Secretion from the clump was twofold greater than from an equal number of perifused dispersed cells. Electron microscopic and freeze-fracture examination of the clump showed reaggregated cells to be intact and the presence of gap junctions between cells. In conclusion, 13 cis-RA has marked effects on islet cell-to-cell adhesiveness. Trypsin-dispersed cells reaggregated by 13 cis-RA have anatomical contacts and secrete more insulin as an aggregate than as dispersed cells. Thirteen cis-RA increases insulin release possibly by increasing adhesion or interactions between β-cellS.