HEAT-SHOCK REDUCES THE TOXICITY OF MALATHION IN DAPHNIA-MAGNA

Heat-shock protects against transient environmental conditions that might otherwise be lethal to the organism. Daphnia magna (2-day- and 7-day-old) exposed for 4h to 34 degrees C showed an increased tolerance to a lethal malathion (0.1-0.2 ppm) challenge. Tolerance was measured using two toxicity endpoints, survival and acetylcholinesterase (AChE) activity. Survival among daphniids previously exposed to 34 degrees C was greater than in daphniids previously exposed to 32 degrees C, 30 degrees C, and malathion, or in untreated (20 degrees C) animals. The 34 degrees C animals had a smaller decrease in AChE activity when compared to the other pretreatments. A similar increase in tolerance was observed among 2-day-old daphniids. Other endpoints were examined during the various treatments. Elevated levels of heat-shock proteins (HSPs) were present in daphniids exposed to 34 degrees C, but in no other treatment. Aliesterase activity was reduced with heat-shock, but was not significantly reduced in the malathion challenge, regardless of the pretreatment. An apparently heat-inducible glutathione S-transferase (GST) was detected after the 34 degrees C heat-shock pretreatment, and this GST was present during the lethal malathion challenge.