Effects of nitrate on metamorphosis, thyroid and iodothyronine deiodinases expression in Bufo gargarizans larvae

• Nitrate caused mortality increase in Bufo gargarizans larvae.
• Nitrate could delay metamorphosis and increase body size.
• Nitrate decrease thyroid hormone contents in Bufo gargarizans tadpole.
• Nitrate caused partial colloid depletion in the thyroid gland follicles.
• Nitrate induced down-regulation of Dio2 mRNA and up-regulation of Dio3 mRNA.

Chinese toad (Bufo gargarizans) tadpoles were exposed to nitrate (10, 50 and 100 mg/L NO3–N) from the beginning of the larval period through metamorphic climax. We examined the effects of chronic nitrate exposure on metamorphosis, mortality, body size and thyroid gland. In addition, thyroid hormone (TH) levels, type II iodothyronine deiodinase (Dio2) and type III iodothyronine deiodinase (Dio3) mRNA levels were also measured to assess disruption of TH synthesis. Results showed that significant metamorphic delay and mortality increased were caused in larvae exposed to 100 mg/L NO3–N. The larvae exposed to 100 mg/L NO3–N clearly exhibited a greater reduction in thyroxine (T4) and 3,5,3′-triiodothyronine (T3) levels. Moreover, treatment with NO3–N induced down-regulation of Dio2 mRNA levels and up-regulation of Dio3 mRNA levels, reflecting the disruption of thyroid endocrine. It seems that increased mass and body size may be correlated with prolonged metamorphosis. Interestingly, we observed an exception that exposure to 100 mg/L NO3–N did not exhibit remarkable alterations of thyroid gland size. Compared with control groups, 100 mg/L NO3–N caused partial colloid depletion in the thyroid gland follicles. These results suggest that nitrate can act as a chemical stressor inducing retardation in development and metamorphosis. Therefore, we concluded that the presence of high concentrations nitrate can influence the growth, decline the survival, impair TH synthesis and induce metamorphosis retardation of B. gargarizans larvae.