It appears that in HTC cells CdCl2 has a profound effect on lysosomes without affecting the cell membrane or the mitochondria. This localized effect on lysosomes could account for the low cell viability observed in HTC cells with the neutral red assay. The lysosomal effect of cadmium chloride in HTC cells was further characterized (Fotakis et al., 2005) and it appears that it is one of the primary effects in this cell line and may account for the DNA damage observed at a later stage. Cadmium is among the heavy metals that have the potential to cause lysosomal damage (George and Pirie, 1979 and Etxeberria et al., 1994). Therefore, neutral red assay is a useful tool to detect lysosomal damage when used in conjunction with other tests in order to distinguish between cytotoxicity and organelle damage.
The fact that in HepG2 cells the MTT assay revealed toxicity before any effect with the LDH leakage assay or the neutral red assay is observed can be attributed to the fact that CdCl2 may have an effect on mitochondria in this cell line. It has been suggested that cadmium disrupts mitochondrial function both in vivo (Belyaeva et al., 2002) and in vitro (Pourahmad and Brien, 2000). In addition there is lysosomal damage in HepG2 cells before any significant LDH leakage is observed.