Can Cytotoxic Effects Induced by Industrial Chemicals be Time-dependent?

Short-term and delayed cytotoxic effects of selected water-soluble and water-insoluble industrial chemicals (dimethyl sulphoxide, ethyl alcohol, methyl alcohol, ammonium nitrate, benzalkonium chloride, butoxyethanol and propylene glycol) were tested on confluent 3T3-L1 mouse fibroblasts by using the neutral red uptake (NRU) assay and the MTT assay. The NRU and MTT assays were performed after exposure to a chemical for 10 minutes and 3 hours and then again 7 days later. The results indicate that the system of testing used permits the assessment of both early and delayed cytotoxic effects of different classes of chemicals. Our experiments revealed that three out of the seven substances tested (butoxyethanol, dimethyl sulphoxide and propylene glycol) exhibited similar cytotoxic effects when assessed after exposure for 10 minutes and 7 days later. The results for ammonium nitrate, ethyl alcohol and methyl alcohol, tested after exposure for 10 minutes and 7 days later, did not significantly differ from each other. However, we noted that the cytotoxic effects observed 7 days after exposure were more pronounced than those found for the same concentration, after exposure for 3 hours. One chemical tested (benzalkonium chloride) had almost no cytotoxic effect after exposure for 10 minutes and 3 hours, but showed a strong cytotoxic effect 7 days after exposure. Our study indicated that the cytotoxic effect might be time-dependent in some chemicals, thus inducing a delayed effect in vitro. In our opinion, it is advisable to assess the cytotoxicity of a chemical after exposure for 3 hours (for immediate effects) and again 7 days later (for delayed effects).