Mitochondrial superoxide production decreases upon glucose-stimulated insulin secretion in pancreatic β-cells due to decreasing mitochondrial matrix NADH/NAD + ratio.

AIMS Glucose-stimulated insulin secretion (GSIS) in pancreatic β-cells was expected to enhance mitochondrial superoxide formation. Hence, we elucidated relevant redox equilibria. RESULTS Unexpectedly, INS-1E cells at transitions from 3 (11 mM; pancreatic islets from 5 mM) to 25 mM glucose decreased matrix superoxide release rates (MitoSOX Red monitoring validated by MitoB) and H2O2 (mitoHyPer, subtracting mitoSypHer emission). Novel double-channel fluorescence lifetime imaging, approximating free mitochondrial matrix NADHF indicated its ~20% decrease. Matrix NAD+F increased upon GSIS, indicated by the FAD-emission lifetime decrease, reflecting higher quenching of FAD by NAD+F. The participation of pyruvate/malate and pyruvate/citrate redox shuttles, elevating cytosolic NADPHF (iNAP1 fluorescence monitoring) at the expense of matrix NADHF, was indicated, using citrate (2-oxoglutarate) carrier inhibitors and cytosolic malic enzyme silencing: all changes vanished upon these manipulations. 13C-incorporation from 13C-1-glutamine into 13C-citrate reflected the pyruvate/isocitrate shuttle. Matrix NADPHF, (iNAP3 monitored) decreased. With decreasing glucose, the S3QEL suppressor caused a higher Complex I IF site contribution, but a lower superoxide fraction ascribed to the Complex III site IIIQo. Thus the diminished matrix NADHF/NAD+F decreased Complex I flavin site IF superoxide formation upon GSIS. INNOVATION Mutually validated methods showed decreasing superoxide release into the mitochondrial matrix in pancreatic β-cells upon GSIS, due to the decreasing matrix NADHF/NAD+F (NADPHF/NADP+F) at increasing cytosolic NADPHF levels. The developed innovative methods enable real-time NADH/NAD+ and NADPH/NADP+ monitoring in any distinct cell compartment. CONCLUSION The export of reducing equivalents from mitochondria adjusts lower mitochondrial superoxide production upon GSIS, but does not prevent oxidative stress in pancreatic β-cells.