Objective Physiological, antioxidant system, and gene expression responses of Silurus lanzahouensis to acute hypoxia followed by reoxygenation were studied for optimal aquaculture and fish breeding program.

Methods The dynamic changes in the respiratory physiology, such as tissue pathology, oxidative stress response, and gene expressions in the gills and liver, of juvenile S. lanzahouensis under hypoxic stress for 0, 6, 24, and 48 h followed by 6 h of reoxygenation were monitored.

Results Imposed by an artificial hypoxic stress, the fish responded morphologically with an increased blood cell count, swollen epithelial cells, and damaged lamella in the gills as well as reduced vacuoles, disordered cell arrangement, and local necrosis in the liver. The subsequent 6h reoxygenation brought about some degrees of recovery on the spacing, length, and width of the gill lamellae. The activities of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px), as well as malondialdehyde (MDA) content in the gills, increased initially and then decreased as the stress prolonged. Meanwhile, those of SOD and CAT in the liver exhibited an opposite trend. The stress-related genes, such as hypoxia inducible factor-1α (HIF-1α), prolyl hydroxylase domain 1 (PHD1), and heat shock protein 90α (HSP90α), in the fish gills were significantly upregulated within 48 h under hypoxia (P＜0.05). Whereas in the liver, their expressions rose at first, peaked at 24h, and were followed by significant downregulation at the end (P＜0.05). The reoxygenation significantly downregulated the expressions in the gills (P＜0.05), but in the liver the HIF-1α and PHD1 increased significantly over the level under the 48h of hypoxic treatment, while the HSP90α decreased significantly but still higher than prior to the treatment (P＜0.05).

Conclusion S. lanzahouensis adapted to hypoxic environments by adjusting the physical conditions of the gills, activating the antioxidant system associated with the respiratory functions, and regulating the expression of hypoxia-related genes. Being able to reverse some of the tissue damage induced by the imposed short-term oxygen deprivation, it was considered moderately tolerant to hypoxic stress.