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  • Our experimental results demonstrated http www

    2022-01-21

    Our experimental results demonstrated marked variability in antioxidant defenses and oxidative damage in Crassostrea gigas between the visceral mass and the gill after the treatments by rifampicin and verapamil through determining the changes of the activity of SOD and POD and MT content. It has been confirmed that the lowest concentrations of Cd among different tissues was in the Crassostrea gigas adductor muscle (Sun et al., 2018). Furthermore, in our study, soft tissues of the Crassostrea gigas were divided into 4 parts: gill, mantle, adductor muscle and remaining part which was called visceral mass in the current study, and Cd content in Crassostrea gigas in three different tissues followed this order: gill > visceral mass > adductor muscle during exposure to 50 μg/L CdCl2 for a week. Therefore, we focused on the oxidative damage of gills and visceral masses with a higher level of Cd. Antioxidant Sotalol are effective to protect tissues against the oxidative damage by eliminating ROS or blocking the activity of ROS. An elevation in the activity of antioxidant enzymes or up-regulated expression of these enzymes has been broadly found in aquatic animals exposed to Cd, while the expression level of genes involved in antioxidant defenses exhibited a recovery trend during decontamination (Arini et al., 2015; Karray et al., 2015; Das et al., 2018). In the present study, the descending activity of SOD and POD and MDA content with concomitant decrease in Cd level for the oysters transferring to natural seawater implies probable correlation between Cd concentration and antioxidant responses and oxidative damage in Crassostrea gigas. It also verified that the changing activity of SOD and POD could be an indicator of Cd toxicity. When comparing oysters decontaminated with rifampicin with oysters in natural seawater, the decrease in activity of SOD and POD and MDA content was more evident in the visceral mass than in the gill, which may be explained by different detoxication processes of various tissues under metal exposure and tissue-dependent accumulation level of Cd. It has been put forward that the digestive gland was stronger than the gill in oxidative responses in another oyster Crassostrea madrasensis exposed to Pb (Shenai-Tirodkar et al., 2017). The biological responses are tissue-specific under Pb exposure. Pb had an adverse effect on protein metabolism in the gill of Crassostrea gigas, whereas Pb mainly induced oxidation and altered fatty acid profile in the digestive gland (Meng et al., 2018). Tissue-specific antioxidant responses were also investigated in marine mussels Mytilus coruscus. Interestingly, a higher activity of SOD was shown in the gill of the mussels even at the end of depuration period than at the end of exposure to contaminants (Chen et al., 2018). In our study, the gill also maintained a high activity of POD even during depuration by rifampicin. This might be attributed to a higher accumulation of Cd in the gill, which was an important interphase between environmental contaminants and the internal tissues and oxidative damage difficult to reverse due to continuous metal exposure. It was noteworthy that in the two investigated parts of soft tissues, 25 μg/L verapamil can lead to significant decline in the activity of SOD and POD and MDA content compared with 1 μg/L verapamil. This result is opposite to the assumption that the highest concentration of Cd occurring in the oysters treated by 25 μg/L verapamil is likely to cause the maximum level of oxidative damage or antioxidant responses. However, the studies regarding the antioxidant role of verapamil in bivalve molluscs are limited. The protective effect of verapamil against oxidative stress has been found in rat, and similar to our study, the levels of antioxidant enzymes were lower after verapamil treatment (Sarica et al., 2016). An inhibition of MDA generation by verapamil was suggested in other studies (Tan et al., 2012; Li et al., 2017).