Exposure to nitrate alters the histopathology and gene expression in the liver of Bufo gargarizans tadpoles.

Nitrate is a ubiquitous component in aquatic environment and the concentrations of anthropogenic nitrate-nitrogen (NO3N) can exceed 25 mg/L in surface waters and 100 mg/L in ground waters. The exceed nitrate has adverse effects on survival, development, and metamorphosis of amphibian. Liver is the hub of many biological processes, including lipid metabolism and bile salts secretion. However, there is little information about the effects of nitrate on the liver in amphibians during metamorphosis. In this study, B. gargarizans was exposed to different concentrations of nitrate from embryo to metamorphosis climax to investigate the effects of nitrate on the liver. The survival rate, metamorphosis percent, body mass, total length, and hind-limb length were measured. The histopathological changes and transcriptome responses in the liver of B. gargarizans to nitrate were examined. Results indicated exposure to 50 and 100 mg/L NO3N delayed the metamorphosis and decreased the metamorphosis percent of B. gargarizans. The body size of B. gargarizans at 10 and 50 mg/L NO3N groups were decreased while it was increased at 100 mg/L NO3N group. In addition, exposure to 100 mg/L NO3N caused severe histopathological changes, including cellular atrophy, increased intercellular areas, degraded lipid droplets, hepatic fibrosis, bile canaliculus contraction and degraded mitochondria in liver. The results of RNA-seq and qRT-PCR interpreted the molecular responses, which might be the factors to induce histopathological changes in the liver of B. gargarizans under the pressure of nitrate exposure.