Effects of azinphos methyl and carbaryl on Rhinella arenarum larvae esterases and antioxidant enzymes.

Organophosphate (OP) and carbamate pesticides are anticholinesterasic agents also able to alter antioxidant defenses in different organisms. Amphibian larvae are naturally exposed to these pesticides in their aquatic environments located within agricultural areas. We studied the effect of the carbamate carbaryl (CB) and the OP azinphos methyl (AM), compounds extensively used in Northern Patagonian agricultural areas, on reduced glutathione (GSH) levels and the activities of esterases and antioxidant enzymes of the toad Rhinella arenarum larvae. Larvae were exposed 48 h to AM 3 and 6 mg/L or CB 10 and 20 mg/L. Cholinesterase and carboxylesterases were strongly inhibited by CB and AM. In insecticide-exposed larvae, carboxylesterases may serve as alternative targets protecting cholinesterase from inhibition. GSH-S-transferase (GST) activity was significantly increased by CB and AM. Superoxide dismutase activity increased in tadpoles exposed to 6 mg/L AM. Conversely, catalase (CAT) was significantly inhibited by both pesticides. GSH levels, GSH reductase and GSH peroxidase activities were not significantly affected by pesticide exposure. GST increase constitutes an important adaptive response to CB and AM exposure, as this enzyme has been related to pesticide tolerance in amphibian larvae. Besides, the ability to sustain GSH levels in spite of CAT inhibition indicates quite a good antioxidant response. In R. arenarum larvae, CAT and GST activities together with esterases could be used as biomarkers of CB and AM exposure.