The influence of dietary Na on Cu accumulation in juvenile rainbow trout exposed to combined dietary and waterborne Cu in soft water

Fish inhabiting metal-contaminated environments can take up metals such as Cu via the gills as well as via the gut. Previous research on rainbow trout (Oncorhynchus mykiss) has indicated that dietary Na can reduce the accumulation of waterborne Cu; however, in hard water, dietary Na does not reduce the accumulation of dietary Cu. In this study, we exposed juvenile rainbow trout in soft water with slightly elevated [Cu] for 28 days to control or high levels of dietary Cu (6 and 580 mu g Cu/g food, respectively) at low (1.5%), intermediate (3%), or high (4.5%) levels of dietary Na, for a total of six experimental groups. A separate gastrointestinal sampling experiment demonstrated that these levels resulted in moderately elevated Na concentrations in the gastrointestinal fluid, which declined between 6 h and 12 h post-feeding. Growth and condition indices were not affected by the dietary Cu and Na exposure. Among the control dietary Cu groups, those that received the highest amount of dietary Na had significantly higher whole-body [Cu] on days 18 and 28. In contrast, among the high-Cu groups, fish that were fed the highest amount of Na tended to have significantly lower whole-body [Cu] on days 9 and 18. Tissue Na concentrations did not differ among any of the groups, and unidirectional Na flux measurements demonstrated that Na homeostasis was not impaired by dietary or waterborne Cu. Our results suggest that elevated dietary Na stimulates Cu uptake via the gut under low-Cu conditions, thereby increasing whole-body [Cu], whereas under Cu-loaded conditions, downregulation of Cu uptake at the gills, and/or competitive inhibition of gut Cu uptake as a result of increased dietary Na, leads to decreased whole-body [Cu].