Effect of Chronic Hydrogen Peroxide Exposure on Ion Transport in Gills of Common Carp (Cyprinus carpio)

High environmental hydrogen peroxide (H2O2) has been demonstrated to be toxic for fish. However, the response mechanism of fish to chronic H2O2 exposure is not yet well understood. Therefore, this study aimed to investigate the alteration in ion transport in gills and analyzed the potential response mechanism after chronic H2O2 exposure. The common carps were exposed to 0, 0.25, 0.50, and 1.00 mM of H2O2 for 14 days. The histopathological evaluation results indicated that H2O2 exposure caused incomplete gill filament structure. In the plasma, H2O2 exposure suppressed the potassium (K+) concentration but increased sodium (Na+) concentration. In the gills, the calcium (Ca2+) level was raised, but the K+ and chlorine (Cl−) levels were decreased after H2O2 exposure. After 14 days of exposure, H2O2 prompted the activities of Ca2+/Mg2+-ATPase and H+/K+-ATPase but suppressed Na+/K+-ATPase activity in the gills. Gene transcription analysis showed that the ion-regulation-related genes including nkaa and rhbg were downregulated after H2O2 exposure. In addition, H2O2 exposure upregulated the mRNA levels of cam and camk II, indicating that the Ca2+ singling pathway was activated. In conclusion, our data showed that chronic H2O2 exposure altered gill structure and disturbed ion transport, which further negatively affected the equilibrium of ions and osmotic pressure.