Interactions of waterborne and dietary cadmium on the expression of calcium transporters in the gills of rainbow trout: Influence of dietary calcium supplementation

Recent studies have shown that dietary Ca²⁺ supplementation strongly inhibits uptake of Ca²⁺ and Cd at the fish gill. To better understand the influence of dietary Ca²⁺ on branchial Ca²⁺ transport, we examined the expression of two trout gill calcium transporters during waterborne and dietary Cd exposure, at two different levels of dietary Ca²⁺. Quantitative polymerase chain reaction (PCR) was used to monitor epithelial calcium channel (ECaC) and sodium-calcium exchange (NCX) mRNA levels following 7-28 days of exposure to these treatments. In brief, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to control, 3 μg/L waterborne Cd, 500 mg/kg dietary Cd, or a combined 3 μg/L waterborne plus 500 mg/kg dietary Cd exposure, supplemented with either 20 mg/g or 60 mg/g dietary calcium (Ca²⁺). Two-way analysis of variance was used to discern the main effects of Cd exposure and dietary Ca²⁺ supplementation on ECaC and NCX mRNA levels. We found that dietary Ca²⁺ supplementation decreased significantly ECaC mRNA expression on days 14 and 21. In comparison, NCX mRNA levels were not influenced by dietary Ca²⁺ supplementation, but rather were significantly inhibited in the combined waterborne and dietary Cd exposure on day 7 alone. Statistical analysis found no interactive effects between Cd exposure and dietary Ca²⁺ exposure at any time point, except for day 28.This study provides evidence of the importance of nutritional status on the transcriptional regulation of ion transport at the fish gill. We discuss the importance of diet and nutritional status to the development of new regulatory approaches, such as the biotic ligand model, which currently do not account for the significance of diet on metal bioavailability in aquatic organisms.