Antioxidant Defense System in Tadpoles of the American Bullfrog (Lithobates catesbeianus) Exposed to Paraquat

There is a great need for proactive approaches to avoid amphibian declines. We investigated the possibility that antioxidant stress markers might serve as a proactive measure of physiological stress in anuran tadpoles. Commercially purchased American Bullfrog tadpoles (Lithobates catesbeianus, Gosner stage 36-37) were subjected to 0- (control), 0.1-, 0.5-, 1.0-, and 2.0-mg/L paraquat for 24 h. Liver and muscle (tail clip) tissues were removed and analyzed for catalase, superoxide dismutase (SOD), general peroxidase, and glutathione reductase (GR) activities. In the controls, there was no significant difference in GR activity in tissues collected from the liver and the tail; however, peroxidase, SOD, and catalase activities ranged from two- to 20-fold higher in the liver than in the muscle tissue. Treatment with paraquat resulted in significant increases in SOD, general peroxidase, and GR activities in the liver tissue, whereas the high constitutively expressed catalase activity remained unchanged. GR activity also increased significantly in the muscle tissue when the tadpoles were treated with 2-mg/L paraquat, but the activities of the other three antioxidant enzymes did not vary significantly from the control values in this tissue regardless of the paraquat treatment. After 24 h of paraquat treatment, all tadpoles at all treatment levels were alive and appeared to be vigorous, suggesting that the bullfrog is very tolerant to paraquat toxicity. It is proposed that this tolerance is caused by the stress-induced increases of antioxidant enzyme activity such as SOD, general peroxidases, and GR, as well as, the high constitutive activity of catalase. These findings suggest that, with refinement and further studies, antioxidant markers may be important indicators of sublethal environmental stress in amphibians.