CHANNEL CATFISH Ictalurus punctatus SHOWED NO CROSS-TOLERANCE TO NITRITE AND HYPOXIA BUT MAY SHARE SOME COMMON REGULATION MECHANISMS TO THESE TWO STRESSORS

Nitrite is a toxicant which can interfere with multiple physiological functions of fish highlighted with the accumulation of methemoglobin leading to tissue anoxia. Although usually do not cause mortality directly in practice, nitrite stress results in increased susceptibility of fish to pathogens or other stress and can cause large economic or academic loss especially in intensive farming systems and recirculating systems. Is spite of its toxicity at high concentration, nitrite is a natural constituent with important biological functions in the body at low concentration. Nitrite could be a potential donor of nitric oxide and the latter is a key physiological messenger molecule induced under hypoxia for regulating hypoxic vasodilation and energetic metabolism. In the light of the inner correlations between nitrite and hypoxic regulations through nitric oxide and their similar anoxia consequence, here we sought to investigate the cross tolerance of channel catfish on nitrite and low dissolved oxygen. Channel catfish fingerlings were pre-treated with 0, 0.7, 1, 3.3, 4.7, 6ppm nitrite for 24h and then exposed to acute hypoxia (started at 0.5ppm and gradually decreased to 0), another group acclimated with sublethal hypoxia (1.5ppm) was also included. In another experiment, fish acclimated with sublethal hypoxia (1.5ppm) for 24h were subjected to acute nitrite challenge at a concentration of 26.4 ppm, a control group without acclimation was also included. Time of losing equilibrium for each fish was recorded. The results showed that neither the sublethal hypoxia acclimation provided cross protection to acute nitrite challenge nor the pre-treatment of nitrite improved acute hypoxia resistance at any of the tested nitrite levels based on the survival analysis. On the contrary, the susceptibility of fish to acute hypoxia increased when nitrite pre-treatment levels went up. While acclimation of sublethal hypoxia provided protection to fish when subjected to acute hypoxia. Interestingly, significant (p-value < 0.05) or marginally significant (p-value < 0.06) positive correlation between the survival time and body weight was observed on the low nitrite level groups (0.7ppm and 1ppm) and the sublethal hypoxia acclimated groups no matter subjected to acute nitrite or hypoxia challenge. While this correlation was not observed on the control groups and high nitrite level groups. This difference indicated that both the nitrite and hypoxia acclimation can provide protection that benefits larger fish, which is probably related to glycolysis, but this protection was counteracted by other effects of nitrite toxicity in addition to methemoglobinemia.