Intracellular Ca²⁺ concentration change of T24 cell under irradiation in the presence of TiO₂ ultrafine particles

We respond that malignant cells are inactivated by photo-excited TiO₂ particles (Cai, R.-X., et al. (1992) Cancer Res. 52, 2346-2348). In the present study, the process of cell death of a human bladder cell line T24 with the irradiated TiO₂ was investigated by monitoring the time course change in intracellular calcium concentration ([Ca²⁺]_i) under UV irradiation using calcium fluorescence indicator, fura-2. In the presence of photo-excited TiO₂ particles (100 μg/ml), [Ca²⁺]_i showed a rapid two-step increase, while in the absence of TiO₂, it exhibited only a slight and monotonous increase or maintained a constant level. The rapid elevation of [Ca²⁺]_i in the presence of photo-excited TiO₂ particles was caused by the influx of extracellular Ca²⁺ through the plasma membrane and cell death occurred only after the second rapid elevation of [Ca²⁺]_i. These results suggested that the cell membrane permeability to Ca²⁺ was promoted prior to cell death.