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 vitro exposure to high glucose on the release and content of proinsulin and 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 were perifused and acutely stimulated with 16-7 mM glucose, followed by 3.3 mM glucose. Compared with control islets, experimental islets showed a higher basal release of true insulin and proinsulin-like-morecules (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 significantly after 16-7 mM glucose. Moreover these islets showed a decreased true insulin content and an increased PLM/total insulin ratio. Quantitative ultrastructural analysis of granules, supported by double gold immunostaining with monoclonal antibodies against proinsulin and insulin, showed an increased proinsulin to insulin ratio in β-cells from experimental islets. These data support in vitro what was recently shown in vivo, and further confirm that culture in high glucose is a useful tool to mimic the effect of in vivo chronic hyperglycemia on human β-cell function.