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

Nutrient stimulation of pancreatic beta-cells increases the cellular reduce d pyridine nucleotide content, but the specific role of cytosolic redox sta te in glucose-induced insulin release (GIIR) remains undetermined. The role of cytosolic redox state has been assessed (as reflected by the lactate/py ruvate ratio) in nutrient- and non-nutrient-induced insulin release using a recently established glucose-sensitive clonal beta-cell line (BRIN-BD11). Long-term exposure to the NAD(+) precursor vitamin nicotinic acid (NA, 100 mu M) 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 whic h was linearly related to the extent of GIIR. NA suppressed the glucose-ind uced rise in the NADH/NAD(+) ratio and concomitantly reduced GIIR by 44%. I t also inhibited, by 47%, the average glucose-induced rise in cytosolic fre e Ca2+ concentration ([Ca2+](i), assessed by fura-2 microfluorometry from s ingle cells). The latter effect was not accounted for by a reduction in the activity of voltage-sensitive Ca2+ channels, inasmuch as both high K+- and tolbutamide-induced [Ca2+](i) rises remained insensitive to NA exposure. N A did not affect insulin release evoked by any of the depolarizing agents, indicating that steps in the stimulus-secretion coupling cascade distal to Ca2+ influx are insensitive to changes in the cytosolic redox state. It is concluded that GIIR is partially controlled by the cytosolic redox state. M oreover, the impairment in GIIR caused by a shift toward a more oxidized st ate in the cytosol, originates from an attenuated [Ca2+](i) response. The l atter is likely mediated by the influence of cytosolic redox state on speci fic metabolic pathways (NADH shuttle systems and/or the malonyl-CoA pathway ), leading ultimately to enhancement of the activity of ATP-sensitive K+ ch annels. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.