Nitrate and sodium nitroprusside alter the development of Asian black-spined toads' embryos by inducing nitric oxide production.

Nitrate is the most stable and abundant form of inorganic nitrogen in water. However, owing to human activities, the nitrate concentration in aquatic ecosystems has notably increased worldwide. One of the mechanisms underlying nitrate toxicity in vertebrates includes the functional inhibition of the sodium iodide symporter, resulting in thyroid dysfunction. In this study, we aimed to determine the alternative mechanisms underlying the toxicological effects of nitrates on the Asian black-spined toad (Duttaphrynus melanostictus). Embryos of D. melanostictus were exposed to sodium nitroprusside (SNP, positive control) or 100 mg/L nitrate-nitrogen (NO 3 -N) for 184 h. We observed that both SNP and NO 3 -N significantly decreased body mass and length and delayed developmental processes. Teratogenic symptoms, including tumors, hyperplasia, and abdominal edema, were also observed in embryos exposed to SNP and NO 3 -N. Furthermore, SNP and NO 3 -N significantly increased nitric oxide levels in the embryos, altering the thyroid hormone, nitrogen, cytochrome P450-mediated drug, and xenobiotic metabolism signaling pathways, as well as the pathway involved in chemical carcinogenesis. The similar toxicological effects of SNP and NO 3 -N suggested that nitrate toxicity resulted from the generation of nitric oxide. Therefore, the present study provides insights into an alternative mechanism underpinning nitrate toxicity, which is useful for the conservation of amphibians in nitrate-rich environments.