Sustained exposure to high glucose concentrations modifies glucose signaling and the mechanics of secretory vesicle fusion in primary rat pancreatic beta-cells.

The mechanism(s) by which chronic hyperglycemia impairs glucose-stimulated insulin secretion is poorly defined. Here, we compare the "nanomechanics" of single exocytotic events in primary rat pancreatic beta-cells cultured for 48 h at optimal (10 mmol/l) or elevated (30 mmol/l) glucose concentrations. Cargo release was imaged by total internal reflection fluorescence microscopy of lumen-targeted probes (neuropeptide Y [NPY]-pH-insensitive yellow fluorescent protein [NPY-Venus] or NPY-monomeric red fluorescent protein), while the fate of the vesicle membrane was reported simultaneously with phosphatase-on-the-granule-of-insulinoma-enhanced green fluorescent protein. Under all conditions studied, exocytosis proceeded via a "cavity recapture" mechanism in which the vesicle and plasma membranes fused transiently. While essentially complete release of NPY-Venus was observed in 24 +/- 1% of glucose-stimulated exocytotic events in cells maintained at 10 mmol/l glucose, this value was reduced reversibly to 5 +/- 2% of events by culture at 30 mmol/l glucose, in line with decreases in Glut2 and glucokinase gene expression, and attenuated glucose-stimulated increases in NADPH and intracellular [Ca2+]. Since vesicle release in response to cell depolarization with KCl was not affected by culture at 30 mmol/l glucose, we conclude that hyperglycemia causes the abnormal termination of individual insulin release events principally by inhibiting glucose signaling.