Modulation of glucose-induced insulin secretion by cytosolic redox state in clonal β-cells

Abstract

Nutrient stimulation of pancreatic β-cells increases the cellular reduced pyridine nucleotide content, but the specific role of cytosolic redox state in glucose-induced insulin release (GIIR) remains undetermined. The role of cytosolic redox state has been assessed (as reflected by the lactate/pyruvate ratio) in nutrient- and non-nutrient-induced insulin release using a recently established glucose-sensitive clonal β-cell line (BRIN-BD11). Long-term exposure to the NAD + precursor vitamin nicotinic acid (NA, 100 μM) was used to promote a more oxidized state in the cytosol. Glucose (2–16 mM) evoked a dose-dependent rise in the cytosolic NADH/NAD + ratio which was linearly related to the extent of GIIR. NA suppressed the glucose-induced rise in the NADH/NAD + ratio and concomitantly reduced GIIR by 44%. It also inhibited, by 47%, the average glucose-induced rise in cytosolic free Ca 2+ concentration ([Ca 2+] i , assessed by fura-2 microfluorometry from single cells). The latter effect was not accounted for by a reduction in the activity of voltage-sensitive Ca 2+ channels, inasmuch as both high K +- and tolbutamide-induced [Ca 2+] i rises remained insensitive to NA exposure. NA did not affect insulin release evoked by any of the depolarizing agents, indicating that steps in the stimulus-secretion coupling cascade distal to Ca 2+ influx are insensitive to changes in the cytosolic redox state. It is concluded that GIIR is partially controlled by the cytosolic redox state. Moreover, the impairment in GIIR, caused by a shift toward a more oxidized state in the cytosol, originates from an attenuated [Ca 2+] i response. The latter is likely mediated by the influence of cytosolic redox state on specific metabolic pathways (NADH shuttle systems and/or the malonyl-CoA pathway), leading ultimately to enhancement of the activity of ATP-sensitive K + channels.