Bo-Jian Chen, Wen-Yi Zhang, Cui-Juan Niu, Wen-Jie Li, Hui Jia, and Kenneth B. Storey
The antioxidant defense protects turtles from oxidative stress caused by adverse environment conditions, such as acute thermal fluctuations. However, it remains unclear how these defenses work. The present study examined changes in key enzymes of the enzymatic antioxidant system and the glutathione (GSH) system at both the mRNA and enzyme activity levels during acute cold exposure and recovery in juvenile Chinese soft-shelled turtles, Pelodiscus sinensis. Transcript levels of the upstream regulator NF-E2-related factor 2 (Nrf2) were also measured. Turtles were acclimated at 28°C (3 weeks), then given acute cold exposure (8°C, 12 h) and finally placed in recovery (28°C, 24 h). The mRNA levels of cerebral and hepatic Nrf2 and of genes encoding downstream antioxidant enzymes did not change, whereas levels of nephric Nrf2, manganese superoxide dismutase (MnSOD) and glutathione peroxidase 4 (GPx4) mRNA decreased upon cold exposure. During recovery, Nrf2 mRNA remained stable in all three tissues, hepatic Cu/ZnSOD, MnSOD and catalase (CAT) mRNA levels increased, and nephric MnSOD and GPx4 mRNAs did not change from the values during cold exposure. In the GSH system, mRNA levels of most enzymes remained constant during cold exposure and recovery. Unmatched with changes in mRNA level, high and stable constitutive antioxidant enzyme activities were maintained throughout, whereas GPx activity significantly reduced in the kidney during cold exposure, and in liver and kidney during recovery. Our results suggest that the antioxidant defense regulation in response to acute cold exposure in P. sinensis may not be achieved at the transcriptional level, but may rely mainly on high constitutive antioxidant enzyme activities.