Fig. 1.

SOICR occurs in HEK293 cells expressing RyR2(wt) at elevated [Ca²⁺]_o. Stable inducible HEK293 RyR2(wt) cells were induced with tetracycline and loaded with 5 μM fura-2 acetoxymethyl ester in Krebs-Ringer-Hepes (KRH) buffer for 20 min at room temperature. Cells were perfused continuously with KRH buffer without (0 mM) or with 0.1, 0.2, 0.3, 0.5, or 1.0 mM CaCl₂ or 1.0 mM CaCl₂ plus 5 mM caffeine. (A) Single-cell fluorescent Ca²⁺ images in the presence (Upper) or absence (Lower) of 0.3 mM caffeine at various [Ca²⁺]_o (0-1.0 mM). (B) Fura-2 ratios of representative RyR2(wt) cells in the absence (green trace) and presence (blue trace) of 0.3 mM caffeine and a HEK293 parental cell expressing no RyR2 (pink trace). (C) The fraction (%, mean ± SEM) of cells that display Ca²⁺ oscillations in the presence (filled circle) and absence (open circle) of 0.3 mM caffeine. The total numbers of cells analyzed for Ca²⁺ oscillations were 563 (without 0.3 mM caffeine) and 237 (with 0.3 mM caffeine) from three to nine separate experiments. (D) Store Ca²⁺ content at various [Ca²⁺]_o, which was estimated by measuring the amplitude of caffeine-(5 mM) induced Ca²⁺ release from oscillating cells and normalized to the maximum level obtained at 1.0 mM [Ca²⁺]_o. Data shown are mean ± SEM from three to seven separate experiments.