LONG-TERM IN-VITRO EXPOSURE TO HIGH GLUCOSE INCREASES PROINSULIN-LIKE-MOLECULES RELEASE BY ISOLATED HUMAN ISLETS

The aim of this study was to determine the effect of long-term in vitr o exposure to high glucose on the release and content of proinsulin an d insulin in human islets. After 48 h culture in CMRL medium at 5.5 mM (control islets) and 16.7 mM glucose (experimental islets), islets we re perifused and acutely stimulated with 16.7 mM glucose, followed by 3.3 mM glucose. Compared with control islets, experimental islets show ed a higher basal release of true insulin and proinsulin-like-molecule s (PLM), with no increase of true insulin and PLM release in response to 16.7 mM glucose, and a paradoxical true insulin release in response to 3.3 mM glucose; the PLM/total insulin ratio increased significantl y after 16.7 mM glucose. Moreover these islets showed a decreased true insulin content and an increased PLM/total insulin ratio. Quantitativ e ultrastructural analysis of granules, supported by double gold immun ostaining with monoclonal antibodies against proinsulin and insulin, s howed an increased proinsulin to insulin ratio in beta-cells from expe rimental islets. These data support in vitro what was recently shown i n vivo, and further confirm that culture in high glucose is a useful t ool to mimic the effect of in vivo chronic hyperglycemia on human beta -cell function.