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 3,5,30-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.