Accumulation of nitrite and responses of energy metabolism exposed to nitrite stress in Litopenaeus vannamei

Litopenaeus vannamei is one of the most important breeding economic varieties in the world because of its fast growth, strong environmental adaptability, and short growth cycle. Along with the rapid promotion of the intensive culture model of L. vannamei, large quantities of harmful substances accumulate in the culture environment which seriously affect the growth and health of L. vannamei. Nitrite enters through gills and accumulates in tissues, which has severely toxic effects on cultured L. vannamei. In the present study, to explore the accumulation of nitrite and changes in enzymes activities related to energy metabolism exposed to nitrite in L. vannamei body length (6.8 ± 0.3) cm, weight (4.0 ± 0.6 g), shrimps were exposed to four nitrite concentrations of 0 (control), 0.8 (11.2 mg/L NO₂-N), 4.0 (56 mg/L NO₂-N) and 8.0 mmol/L (112 mg/L NO₂-N) for 96 h and then recovered for 12 h. Each experimental treatment included six replicates. Three replicates were used for sample collection, and the other three replicates were used to calculate the cumulative mortality of L. vannamei. At 0, 6, 12, 24, 48 and 96 h of the nitrite stress, the hemolymph, hepatopancreas, muscle, gill and intestine of 9 L. vannamei in each group were randomly selected for the determination of the index. Additionally, death number were recorded every 12 h. The results indicated that the shrimp mortality rate increased with nitrite concentrations. At the end of nitrite stress, shrimp mortality in control group, 0.8 mmol/L group, 4.0 mmol/L group and 8.0 mmol/L group were 10.7%, 32.0%, 42.7% and 52.0%, respectively. Within 6 hours of exposure, nitrite accumulated significantly in the gill, hemolymph, intestine, hepatopancreas, and muscle tissues of L. vannamei, and was positively correlated with stress concentrations. The maximum accumulation of nitrite in gills, hemolymph, hepatopancreas, intestine and muscle were 50.1 mg/kg, 43.2 mg/L, 20.7 mg/kg, 33.5 mg/kg and 14.9 mg/kg. In the same stress concentration group, nitrite accumulated the most in gill and the least in muscle, and the accumulation in gill was about 3 times greater than that in muscle. The activity of Na⁺-K⁺-ATPase in hepatopancreas and muscle of L. vannamei were significantly increased at 0.8 and 4.0 mmol/L, but significantly decreased in muscle of 8.0 mmol/L during the exposure period. The activity of AMPK in the hepatopancreas in the stress groups increased compared to the control group, and showed a positive correlation with the stress concentration. During the recovery period, except for hemolymph (8.0 mmol/L group), 1-hour recovery rate of nitrite in L. vannamei tissues in the stress groups were above 50%, and the hepatopancreas and gill showed the highest recovery efficiency exceeding 74%. The recovery time of hemolymph, gill and intestine were the shortest within 6 hours. In addition, the content of nitrite in water increased significantly. This study indicates that nitrite can be accumulated to shrimp tissues in a short time and can accelerate the process of energy metabolism. Nitrite would excrete from the body rapidly during recovery in order to reduce the toxic effects. The results of this study will provide reference for alleviating the toxic effects of nitrite on cultured shrimp.