The influence of acute ammonia stress on intestinal oxidative stress, histology, digestive enzymatic activities and PepT1 activity of grass carp (Ctenopharyngodon idella)

This study was conducted to evaluate the effect of acute ammonia stress on intestinal oxidative stress levels, histology, and activity of digestive enzymes and oligopeptide transporter 1 (PepT1) in grass carp. Three concentrations of ammonia nitrogen: 0 mg/L total ammonia nitrogen (TA-N) (the control group), 1.7 mg/L TA-N were used in the present study. Fish were sampled at 0 h, 12 h, 24 h, and 48 h after exposure to ammonia. The results showed that the activities of reactive oxygen species (ROS), superoxide dismutase (SOD), catalase (CAT) and the content of cortisol were all increased in fish exposed to the two concentrations of TA-N. Compared with the control group, the activities of intestinal trypsin and chymotrypsin significantly increased in the 1.7 mg/L TA-N group, while amylase activity was significantly suppressed in the 50 mg/L TA-N group. After ammonia exposure, the expression of intestinal Nrf2 gene significantly increased in the 1.7 mg/L TA-N group while the expression of Keap1 gene significantly increased in the 50 mg/L TA-N group. Moreover, the expression of intestinal PepT1, Sp1 and CDX2 significantly increased after fish exposed to 1.7 mg/L TA-N. The width of midintestine villi of fish from the 50 mg/L TA-N group was significantly higher than fish from the other two groups. Meanwhile, the area density of PepT1 in the 50 mg/L TA-N group was significantly smaller than that in the control and 1.7 mg/L TA-N groups. The results suggested that ammonia stress induces intestinal oxidative stress, and 1.7 mg/L TA-N increased the expressions of PepT1-related genes and the activities of the intestinal trypsin and chymotrypsin. This conclusion indicates that ammonia nitrogen stress affects the intestinal digestion and absorption performance of aquatic animals, and reducing ammonia nitrogen to a suitable level can improve the protein digestion ability of animals.