Histopathological, hematological, and biochemical changes in high-latitude fish Phoxinus lagowskii exposed to hypoxia

Hypoxia is one of the most significant threats to biodiversity in aquatic systems. The ability of high-latitude fish to tolerate hypoxia with histological and physiological responses is mostly unknown. We address this knowledge gap by investigating the effects of exposures to different oxygen levels using Phoxinus lagowskii (a high-latitude, cold-water fish) as a model. Fish were exposed to different oxygen levels (0.5 mg/L and 3 mg/L) for 24 h. The loss of equilibrium (LOE), an indicator of acute hypoxia tolerance, was 0.21 ± 0.01 mg/L, revealing the ability of fish to tolerate low-oxygen conditions. We sought to determine if, in P. lagowskii, the histology of gills and liver, blood indicators, enzyme activities of carbohydrate and lipid metabolism, and antioxidants changed to relieve stress in response to acute hypoxia. Notably, changes in vigorous jumping behavior under low oxygen revealed the exceptional hypoxia acclimation response compared with other low-latitude fish. A decrease in blood parameters, including RBC, WBC, and Hb, as well as an increase in MCV was observed compared to the controls. The increased total area in lamella and decreased ILCM volume in P. lagowskii gills were detected in the present study. Our results also showed the size of vacuoles in the livers of the hypoxic fish shrunk. Interestingly, an increase in the enzyme activity of lipid metabolism but not glucose metabolism was observed in the groups exposed to hypoxia at 6 h and 24 h. After combining histology and physiology results, our findings provide evidence that lipid metabolism plays a crucial role in enhancing hypoxia acclimation in P. lagowskii. Additionally, SOD activity significantly increased during hypoxia, suggesting the presence of an antioxidant response of P. lagowskii during hypoxia. High expression levels of lipogenesis and lipolysis-related genes were detected in the 6 h 3 mg/L and 24 h 3 mg/L hypoxia group. Enhanced expression of lipid-metabolism genes (ALS4, PGC-1, and FASN) was detected during hypoxia exposure. Together, these data suggest that P. lagowskii's ability to tolerate hypoxic events is likely mediated by a comprehensive strategy.