Neutral red assay
The neutral red assay is based on the initial protocol described by Borenfreund and Puerner (1984) and determines the accumulation of the neutral red dye in the lysosomes of viable, uninjured cells.
Following exposure to CdCl2 cells were incubated for 2 h with neutral red dye (100 μg/ml) dissolved in serum free medium (DMEM or MEM for HTC and HepG2 cells, respectively). The pH of the neutral red solution was adjusted in all the experiments to 6.35 with the addition of KH2PO4 (1 M). Cells were then washed with Phosphate Buffered Saline (PBS) and the addition of 1 ml of elution medium (EtOH/AcCOOH, 50%/1%) followed by gentle shaking for 10 min so that complete dissolution was achieved. Aliquots of the resulting solutions were transferred to 96-well plates and absorbance at 540 nm was recorded using the microplate spectrophotometer system (Spectra max190-Molecular Devices). Results were analyzed with the Soft max pro software (version 2.2.1) and are presented as percentage of control values.