Influence of temperature on biomarker responses and histology of the liver of American bullfrog tadpoles (Lithobates catesbeianus, Shaw, 1802) exposed to the herbicide Tebuthiuron.

Tebuthiuron (TBU) is a phenylurea herbicide that is extensively used in sugarcane fields. Owing to the low degradation rate, high water solubility, and leaching potential, TBU is believed to have harmful effects on aquatic organisms, such as anuran tadpoles. Contaminant effects can be influenced by temperature since increases in temperature are often associated with increased metabolic reactions. In this study, we evaluated the influence of temperature on the negative effects of TBU in bullfrog tadpoles (Lithobates catesbeianus) through a multi-biomarker approach. Tadpoles were exposed to 0 (control) 10, 50, and 200 ng L-1 of TBU for 16 days at 25 and 32 °C. TBU increased the transcript levels of genes involved in biotransformation (glutathione S-transferase, GST, and sulfotransferase, SULT) and antioxidant (superoxide dismutase, SOD, and catalase, CAT) enzymes. TBU exposure also increased CAT and glutathione peroxidase (GPx) activities, whereas SOD and carboxylesterase activities were decreased. The highest temperature caused a decrease in the activities of ethoxyresorufin-O-deethylase and SOD but increased the activities of GST, GPx, glucose 6-phosphate dehydrogenase, and acetylcholinesterase. No effects of temperature or TBU exposure were observed in genotoxic markers (frequencies of micronucleous and nuclear abnormalities) or in lipid peroxidation levels. Tadpoles exposed to TBU at all tested concentrations presented a higher index of biomarker responses than that of the control groups. Higher values of severity scores from histological analyses were found in the liver of tadpoles exposed to 50 and 200 ng L-1 of TBU at 32 °C compared with those of the control group at the same temperature. These results indicate that TBU and temperature increases are able to disturb the metabolic homeostasis of L. catesbeianus tadpoles after 16 days of exposure, causing substantial alterations in biomarker responses and liver morphology.