Pre-Exposure of Common Carp to Ambient Copper and Microplastic Changes the Gill Ionoregulaion-Related Transcripts During Saltwater Exposure

In this study, common carp (Cyprinus carpio) were exposed to 0.25 mg/L copper alone (Cu treatment) or in combination with 0.5 mg/L polyvinyl chloride microparticles (Cu-MP treatment) for 14 days, followed by a 72-h challenge in brackish water (13 ppt NaCl). After 14 days of exposure, gill copper concentrations were significantly elevated in both the Cu and Cu-MP treatments relative to the control group (P = 0.008), with no significant difference between the two treatments. Gene expression analysis of gill tissue following the exposure period revealed significant upregulation of the glucocorticoid receptor (gr) and Na⁺/K⁺-ATPase α1a subunit (nakα1a) genes in the Cu treatment. In contrast, the Cu-MP treatment showed only modest upregulation of gr and significant downregulation of nakα1a. After the subsequent brackish-water challenge, nakα1a expression was significantly downregulated in control fish, whereas gr expression remained unchanged. Overall, these gill gene expression patterns suggest that copper exposure disrupts ionoregulatory homeostasis in common carp, but co-exposure to polyvinyl chloride microparticles does not exacerbate these effects.