ATP-sensitive K+ channels and mitochondrial permeability transition pore mediate effects of hydrogen sulfide on cytosolic Ca2+ homeostasis and insulin secretion in β-cells

Hydrogen sulfide (H2S) is endogenously produced in pancreatic ss cells and its level is elevated in diabetes. Here, we report that H2S affects insulin secretion via two mechanisms that converge on cytosolic free Ca2+ ([Ca2+](i)), a key mediator of insulin exocytosis. Cellular calcium imaging, using Fura-2 or Fluo-4, showed that exposure of INS-1E cells to H2S (30-100 mu M) reduced both [Ca2+](i) levels (by 21.7 +/- 2.3%) and oscillation frequency (p < 0.01, n = 4). Consistent with a role of plasma membrane K-ATP channels (plasma-K-ATP), the effects of H2S on [Ca2+](i) were blocked by gliclazide (a blocker of plasma-K-ATP channels), but were mimicked by diazoxide (an activator of plasma-K-ATP channels). Surprisingly, when Ca2+ entry via plasma membrane was inhibited using Ca2+-free external solutions, H2S increased [Ca2+](i) by 39.7 +/- 3.6% suggesting Ca2+ release from intracellular stores. H2S-induced [Ca2+](i) increases were abolished by either FCCP (which depletes Ca2+ stored in mitochondria) or cyclosporine A (an inhibitor of mitochondrial permeability transition pore, mPTP) suggesting that H2S induces Ca2+ release from mitochondria. Measurement of mitochondrial membrane potential (MMP) suggested that H2S causes MMP depolarization, which was blocked by cyclosporine A. Finally, insulin measurements by ELISA indicated that H2S decreased insulin release from INS-1E cells, but after plasma membrane Ca2+ entry was blocked by nifedipine, H2S-induced mitochondrial Ca2+ release is able to increase insulin release. Together, our results indicate that H2S has dual effects on insulin release suggesting that, with different metabolic conditions, H2S may differentially modulate the insulin release from pancreatic ss cells and play a role in ss cell dysfunction.