Responses of hemocyanin and energy metabolism to acute nitrite stress in juveniles of the shrimp Litopenaeus vannamei

Nitrite is a common toxic substance in culture systems of Litopenaeus vannamei, and the stress may disturb hemocyanin synthesis and energy metabolism and result in shrimp death. In the present study, nitrite at concentrations of 0 (control), 3.3 (46.2 NO2-N mg/L), 6.6 (92.4) and 9.9 mM (138.6) was used to evaluate the responses of hemocyanin level and energy metabolism in L. vannamei (5.80 +/- 0.44 cm, 1.88 +/- 0.38 g) for 96 h. The mortality rate at 96 h increased with nitrite concentration (50% at 9.9 mM, 40% at 6.6 mM, 30% at 3.3 mM, and 10% at 0 mM). In general, HIF-1 alpha and hemocyanin mRNA expression in the nitrite stress groups was up-regulated from 6 to 12 h and down-regulated from 24 to 96 h. In the hemolymph, nitrite levels were significantly elevated in a dose-dependent manner, and exposure to nitrite stress significantly decreased the oxyhemocyanin content from 24 to 96 h. The glucose and lactate levels in the hemolymph in the nitrite stress groups were higher than those in the control group from 12 to 96 h. Compared with the control group, the shrimp in the nitrite stress groups exhibited decreased glycogen concentrations in the hepatopancrets. The triglyceride (TG) levels in the nitrite stress groups were all higher than those in the control group from 48 to 96 h. The hexokinase (HK) activity in the hepatopancreas and muscle increased in the nitrite stress groups from 48 to 96 h. In general, nitrite stress enhanced the activities of pyruvate kinase (PK), phosphofructokinase (PFK) and lactate dehydrogenase (LDH) in muscle from 24 to 96 h. In addition, nitrite stress decreased the activities of succinate dehydrogenase (SDH) and fatty acid synthase (FAS) from 24 to 96 h in the hepatopancreas and muscle. This study indicates that exposure to nitrite stress can enhance the accumulation of nitrite in the hemolymph and then reduce oxygenation and hemocyanin synthesis, leading to tissue hypoxia and thereby resulting in accelerated anaerobic metabolism and the inhibition of aerobic metabolism. The effects of nitrite stress on hemocyanin synthesis and energy metabolism may be one of the reasons for the mortality of L. vannamei in culture systems.