Responses of Cell Cultures to Insecticides. II. Chronic Toxicity and Induced Resistance.

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Abstract
Summary HeLa cell cultures were exposed to subtoxic concentrations of insecticides for 84 days. All insecticides tested induced cytotoxicity which was characterized by growth inhibition and cell destruction. The minimal toxic doses (MTD) in μ/ml were: Cygon 20, Dipterex 10, DI-Syston 5, Kelthane 5, Karathane 5, malathion 0.1, and chlordane 0.1. All insecticides, except Karathane, were significantly more toxic in the chronic than in the acute cytotoxicity tests. On continued exposure, the cells gradually adapted and grew in the presence of the insecticides at concentrations 2 to 200 times above the MTD. After cessation of exposure the malathion-, chlordane-, and DI-Syston-treated cultures were 1.7 to 3.0 times more resistant to the same insecticide than the untreated control cells. The induced resistance to DI-Syston and chlordane was associated with a growth stimulatory effect of each insecticide on the respective chronically treated culture. It is postulated that the cell resistance to insecticides may be associated with cell mutations or induction of enzyme systems capable of inactivating or metabolizing the insecticides.

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