Pretreatment with DTT does not prevent H2O2-mediated inhibition of SOICR. HEK293 cells stably expressing RyR2 were loaded with Fluo-4 in KRH buffer. The cells were then transiently superfused with KRH containing 0, 1, or 2 mM Ca2+ and 10 mM DTT (4.5 min) followed by 1 mM H2O2 (21.5 min). Some cells were superfused throughout the assay with 10 mM DTT (DTT). At the end of each experiment cells were perfused with 20 mM caffeine. Representative traces from cells where SOICR (A) was maintained or (B) was inhibited. (C) The frequency of SOICR events, per 5 min bin, normalized to the frequency of events before the application of DTT. Cells treated with both DTT and H2O2 are displayed as those where SOICR events were maintained or where SOICR ceased by the end of the experiment (Loss of SOICR). Control n = 234, Loss of SOICR n = 203, Maintained n = 82, Persistent DTT n = 203. Data shown are mean ± SE. ∗∗∗∗ Loss of SOICR, p < 0.0001; ### and #### Maintained SOICR, p < 0.001 and <0.0001, respectively, compared to control (no H2O2).