Acute exposure to waterborne copper inhibits both the excretion and uptake of ammonia in freshwater rainbow trout (Oncorhynchus mykiss)

In freshwater fish, exposure to sub-lethal concentrations of waterborne copper (Cu) results in inhibitions of ammonia excretion (J_amm) and Na⁺ uptake (J^Na _in), yet the mechanisms by which these occur are not fully understood. In the present study, rainbow trout (Oncorhynchus mykiss) fry exposed to 50 μg/l Cu for 24 h displayed a sustained 40% decrease in J_amm and a transient 60% decrease in J^Na _in. Previously, these effects have been attributed to inhibitions of gill Na⁺/K⁺-ATPase and/or carbonic anhydrase (CA) activities by Cu. Trout fry did not display significant reductions in the branchial activities of these enzymes or H⁺-ATPase over 24 h Cu exposure. Recently, Rhesus (Rh) glycoproteins, bi-directional NH₃ gas channels, have been implicated in the mechanismof Cu toxicity. Juvenile trout were exposed to nominal 0, 50, and 200 μg/l Cu for 3-6 h under control conditions (ammonia-free water) followed by 6 h exposure to high environmental ammonia (HEA; 1.5 mmol/l NH₄HCO₃). HEA led to significant ammonia uptake in control fish (0 μg/l Cu), and exposure to 50 and 200 μg/l Cu resulted in significant reductions of ammonia uptake during HEA exposure. This is the first evidence that Cu inhibits both the excretion and uptake of ammonia, implicating bi-directional Rh glycoproteins as a target for Cu toxicity. We propose a model whereby Rh blockade by Cu causes the sustained inhibition of Jamm and transient inhibition of J^Na _in, with H⁺-ATPase potentially aiding in J^Na _in recovery. More work is needed to elucidate the role of Rh proteins in sub-lethal Cu toxicity.