Calcium reduces the effects of cadmium on the responses of biomarkers in freshwater fish (Oreochromis niloticus).

The hardness of freshwaters varies depending on geographical locations and calcium is the most important element determining water hardness. Thus, the present study focused on responses of oxidative stress biomarkers in the liver and ATPase activities in the gill of freshwater fish (Oreochromis niloticus) after acute and chronic cadmium exposures in differing calcium levels (30, 60 and 120 mg Ca/L). Fish were exposed to Cd for acute (25 µM, 3 days) and chronic (5 µM, 30 days) durations. There were significant (p < 0.05) alterations in the oxidative stress parameters after cadmium exposures. In the liver, activities of catalase and superoxide dismutase increased in both durations, though activities of glutathione peroxidase, glutathione reductase and glutathione S-transferase decreased. Likewise, levels of glutathione and malondialdehyde increased significantly in both durations. In the gill, there were significant decreases in Na,K-ATPase activity and increases in Mg-ATPase activity. However, calcium alone did not cause any alteration (p > 0.05) in the responses of biomarkers. Data also showed that alterations in biomarkers were highest at the lowest calcium levels. Present data emphasised that calcium protected the fish against the toxic effects of cadmium and suggested that metal toxicity varies depending on the chemical quality of waters from different geographical areas. [ABSTRACT FROM AUTHOR]