Diverse effects of hydrogen peroxide on cytosolic Ca2+ homeostasis in rat pancreatic beta-cells

Oxygen-free radicals are thought to be a major cause of p-cen dysfunction i n diabetic animals induced by alloxan or streptozotocin, We evaluated the e ffect of H2O2 On cytosolic Ca2+ concentration ([Ca2+](i)) and the activity of ATP-sensitive potassium (K-ATP(+)) channels in isolated rat pancreatic b eta-cells using microfluorometry and patch clamp techniques. Exposure to 0. 1 mM H2O2 in the presence of 2.8 mM glucose increased [Ca2+](i) from 114.3/-15.4 nM to 531.1+/-71.9 nM (n=6) and also increased frequency of K-ATP(+) channel openings. The intensity of NAD(P)H autofluorescence was conversely reduced, suggesting that H2O2 inhibited the cellular metabolism. These thr ee types of cellular parameters were reversed to the control level on washo ut of H2O2 followed by a transient increase in [Ca2+](i), the transient inh ibition of K-ATP(+) channels associated with action currents and increase o f the NAD(P)H intensity with an overshoot. In the absence of external Ca2+, 0.1 mM H2O2 increased [Ca2+](i) from 88.8+/-7.2 nM to 134.6+/-8.3 nM. Magn itude of [Ca2+](i) increase induced by 0.1 mM H2O2 was decreased after trea tment of cells with 0.5 mM thapsigargin, an inhibitor of endoplasmic reticu lum Ca2+ pump (45.8+/-4.9 nM vs 15.0+/-4.8 nM). Small increase in [Ca2+](i) in response to an increase of external Ca2+ from zero to 2 mM was further facilitated by 0.1 mM H2O2 (330.5+/-122.7 nM), We concluded that H2O2 not o nly activates K-APT(+) channels in association with metabolic inhibition, b ut also increases partly the Ca2+ permeability of the thapsigargin-sensitiv e intracellular stores and of the plasma membrane in pancreatic beta-cells.