Juvenile rainbow trout were exposed to 25–400 μg copper∙L−1 for 24 h. Water hardness, pH, and alkalinity were varied independently at a constant [Na+]. Net and unidirectional sodium fluxes were measured in hard and soft, low-alkalinity water and in hard, high-alkalinity water at neutral pH and pH 5.0. In low alkalinity water, Na+ uptake (Jin) was inhibited at copper concentrations as low as 25 μg∙L−1, and sodium efflux (Jout) was stimulated above 100 μg∙L−1. High-alkalinity water significantly reduced the effects of copper on Jin and Jout, but there was no significant effect of increasing water hardness. The effects of pH 5.0 and copper were additive from 25 to 100 μg∙L−1, but a pure copper effect was found from 200 to 400 μg∙L−1. Fish died when they had lost about 50–55% of their exchangeable Na+ pool. Water hardness and alkalinity had no effect on the apparent uptake of copper, but copper uptake was reduced by about 50% at pH 5.0.

The structure and function of the gills of freshwater fish are briefly summarized and the responses to low pH are reviewed, with particular reference to the salmonid fishes. Major influences are seen upon ion and acid–base regulatory mechanisms at the gills and upon mucous secretion and gill structure. Ionic imbalances which can be responsible for death at low pH are caused by disturbances to both the active transport and the diffusional losses of Na+ and Cl−. The disturbances, while not identical for the two ions, are similar to the extent that the undirectional fluxes of both are dependent upon the severity and duration of the acid exposure and upon the external calcium concentration. Calcium also has an important influence on the net flux of H+ across the gills. A model is proposed for the interaction of Ca2+ and H+ on gill function. The physiological importance of mucous secretion at low pH is discussed and the issue of whether acid-intolerant species can be successfully adapted for life at low pH is examined.

Whole body, gill, and liver copper uptake, gill Na+-K+-ATPase specific activity, and gill and liver acid-soluble thiols (AST), glutathione, and cysteine of rainbow trout (Salmo gairdneri) were measured during 28 d of exposure to 55 μg copper∙L−1. Na+-K+-ATPase specific activity was inhibited by 33% within 24 h of copper exposure, but this was compensated by a significant increase in microsomal protein so that the total Na+-K+-ATPase activity per milligram of gill tissue returned to normal by day 14. There was no accumulation of copper and no increase in AST, glutathione, or cysteine in the gill. However, after 7 d of exposure, hepatic AST and glutathione had increased by about 2 times, and a sulfhydryl-rich, acid-soluble protein, tentatively identified as metallothionein, increased by 2.8 times. Copper accumulation was highest in the liver, but other tissues also accumulated copper.

Juvenile rainbow trout (Salmo gairdneri) were exposed to 55 μg copper∙L−1 for 28 d and then transferred to uncontaminated water for 7 d. Whole body sodium concentration and sodium uptake (Jin) were measured at weekly intervals; Jin was measured at various Na+ concentrations and kinetic parameters estimated. After 24 h of copper exposure, the maximum rate of sodium uptake (Jmax) was inhibited by 55%, the affinity for sodium (Km) reduced by 49%, and whole body Na+ decreased by about 12.5%. After 7 d of exposure, whole body Na+ had returned to control values, but Jmax was still inhibited by 41%. Recovery of whole body Na+ occurred largely by a reduction of sodium efflux (Jout). Both Jmax and Km continued to recover until day 28, at which time Jin had returned to control values. We conclude that acclimation to sublethal copper depends on changes in both Na+ transport and permeability.

SUMMARYOurs is the first study to demonstrate an influence of dietary sodium on waterborne copper uptake in fish. We examined possible interactions between dietary sodium and the response of freshwater rainbow trout (Oncorhynchus mykiss) to waterborne copper in light of recent evidence of interactions between sodium and copper metabolism in the gills. Trout were maintained for 6 days on one of four diets of increasing sodium concentration (0.25 mmol g-1, 0.51 mmol g-1, 0.76 mmol g-1 and 1.27 mmol g-1, which corresponds to 0.6%, 1.2%, 1.8% and 3% sodium by mass, respectively). At the end of 7 days, fish were exposed for 6 h to waterborne copper spiked with 64Cu to determine if the dietary sodium affected responses to a subsequent short-term waterborne copper exposure. The radiotracer allowed us to distinguish between Cu occurring in fish tissues before the experiment and `newly accumulated' Cu arising from the experimental exposure. Dietary sodium concentrations of 1.8% or 3% reduced newly accumulated copper concentrations in gill (from 93.9 ng g-1in control to 38.9 ng g-1 and 20.0 ng g-1 in fish fed 1.8% or 3% Na+-supplemented diets, respectively), liver (from 64.3 ng g-1 to 23.1 ng g-1 and 7.5 ng g-1,respectively), kidney (from 29.3 ng g-1 to 11.7 ng g-1and 7.8 ng g-1, respectively), plasma (from 64.7 ng g-1to 21.5 ng g-1 and 10.7 ng g-1, respectively) and gut(from 6.8 ng g-1 to 3.4 ng g-1 and 2.2 ng g-1, respectively) by 50.0-88.2%. The 3%Na+-supplemented diets also increased plasma and gut sodium concentrations by 38.1% (from 137.1 μmol g-1 to 189.3 μmol g-1) and 104.3% (from 56.5 μmol g-1 to 115.4 μmol g-1), respectively, relative to fish maintained on untreated diets. Whole body uptake rates of both sodium and copper were significantly reduced,and highly correlated (r=0.97) with one another, in fish fed high-sodium diets relative to controls. Moreover, sodium efflux was 12% and 38% higher in fish fed 1.8% and 3% sodium-enriched diets, respectively. Fish fed high-sodium diets also drank more water, but the contribution of drinking to waterborne copper uptake was negligible. From these results, we speculate that, at least in part, aqueous sodium and copper share a common branchial uptake route, probably through an apical sodium channel. According to this hypothesis, as the channel is downregulated with increasing internal sodium concentrations, both sodium and copper uptake from the water are inhibited.