Endocrine Abstracts

We have previously used insulin-secreting MIN6 cells configured into islet-like structures (pseudoislets) to demonstrate that homotypic interactions between beta-cells are essential for normal insulin secretory responses. We have now investigated whether homotypic interactions are important in secretion of other pancreatic hormones. Glucagon-secreting alpha-TC1 cells were cultured as adherent monolayers, or as pseudoislets grown on gelatin, and hormone release was measured in perifusion experiments. RT-PCR and radioimmunoassay measurements confirmed that the alpha-TC1 cells expressed glucagon but not insulin. Pseudoislets formed from alpha-TC1 cells within 5-6 days culture on the gelatin substrate, and were morphologically similar to primary mouse islets and MIN6 pseudoislets. Electron micrographs revealed glucagon-containing secretory granules within the cells, and areas of close cell-cell apposition, typical of adherens junctions. Glucagon secretion from alpha-TC1 cells maintained as monolayers or as pseudoislets was significantly stimulated by exposure to 20mM arginine (416 plus/minus 60%, basal, P<0.01 vs control n=4; 480 plus/minus 50% basal, P<0.05 vs control n=4, respectively), but there was no significant difference between the two cell populations (P>0.2). In contrast, insulin secretion from MIN6 cells was enhanced upon pseudoislet formation. Thus, exposure to high extracellular K⁺(20mM) stimulated insulin secretion by 162 plus/minus 12% and 383 plus/minus 84% basal from monolayers and pseudoislets, respectively (P<0.05). Similarly, secretory responses of MIN6 pseudoislets to glucose (20mM) were significantly greater than those of monolayers (1182 plus/minus 270% and 234 plus/minus 58% basal, respectively P<0.001). These results demonstrate that the anatomical configuration of insulin-secreting cells into islet-like structures enhances their insulin secretory responses, but a similar effect is not seen with glucagon-secreting cells. These observations are consistent with the anatomical location of beta-cells as a central core of rodent primary islets of Langerhans with alpha-cells forming a mantle around the islet. Our results are also consistent with the improved secretory function of intact islets being primarily a consequence of interactions between beta-cells in the islets.