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

Heat-shock protects against transient environmental conditions that mi ght otherwise be lethal to the organism. Daphnia magna (2-day- and 7-d ay-old) exposed for 4h to 34 degrees C showed an increased tolerance t o a lethal malathion (0.1-0.2 ppm) challenge. Tolerance was measured u sing 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 d egrees C, and malathion, or in untreated (20 degrees C) animals. The 3 4 degrees C animals had a smaller decrease in AChE activity when compa red to the other pretreatments. A similar increase in tolerance was ob served 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 tr eatment. Aliesterase activity was reduced with heat-shock, but was not significantly reduced in the malathion challenge, regardless of the p retreatment. An apparently heat-inducible glutathione S-transferase (G ST) was detected after the 34 degrees C heat-shock pretreatment, and t his GST was present during the lethal malathion challenge.