Stress-induced dissociations between intracellular calcium signaling and insulin secretion in pancreatic islets

Abstract

In healthy pancreatic islets, glucose-stimulated changes in intracellular calcium ([Ca2+]i) provide a reasonable reflection of the patterns and relative amounts of insulin secretion. We report that [Ca2+]i in islets under stress, however, dissociates with insulin release in different ways for different stressors. Islets were exposed for 48h to a variety of stressors: cytokines (low-grade inflammation), 28mM glucose (28G, glucotoxicity), free fatty acids (FFAs, lipotoxicity), thapsigargin (ER stress), or rotenone (mitochondrial stress). We then measured [Ca2+]i and insulin release in parallel studies. Islets exposed to all stressors except rotenone displayed significantly elevated [Ca2+]i in low glucose, however, increased insulin secretion was only observed for 28G due to increased nifedipine-sensitive calcium-channel flux. Following 3–11mM glucose stimulation, all stressors substantially reduced the peak glucose-stimulated [Ca2+]i response (first phase). Thapsigargin and cytokines also substantially impacted aspects of calcium influx and ER calcium handling. Stressors did not significantly impact insulin secretion in 11mM glucose for any stressor, although FFAs showed a borderline reduction, which contributed to a significant decrease in the stimulation index (11:3mM glucose) observed for FFAs and also for 28G. We also clamped [Ca2+]i using 30mM KCl+250μM diazoxide to test the amplifying pathway. Only rotenone-treated islets showed a robust increase in 3–11mM glucose-stimulated insulin secretion under clamped conditions, suggesting that low-level mitochondrial stress might activate the metabolic amplifying pathway. We conclude that different stressors dissociate [Ca2+]i from insulin secretion differently: ER stressors (thapsigargin, cytokines) primarily affect [Ca2+]i but not conventional insulin secretion and ‘metabolic’ stressors (FFAs, 28G, rotenone) impacted insulin secretion.