Copper uptake across rainbow trout gills: mechanisms of apical entry

Several components of branchial copper uptake were identified in juvenile freshwater rainbow trout (Oncorhynchus mykiss) using (64)Cu. On the basis of competitive interactions between sodium and copper uptake, inhibition of copper uptake by a proton pump inhibitor (bafilomycin A1, 2 micromol l(-1)) and a Na(+) channel blocker (phenamil, 100 micromol l(-1)), it appears that a proportion of the branchial copper uptake occurs via an apical Na(+) channel. This sodium-sensitive copper uptake demonstrates saturation kinetics, with a K(m) of 7.1 nmol l(-1) and a J(max) of 21.2 pmol g(-1) h(-1), and is characterized by an IC(50) of 104 micromol l(-1) sodium. On the basis of residual copper uptake in the presence of high sodium concentrations (20 mmol l(-1)) and differential inhibition of sodium and copper uptake by phenamil (100 micromol l(-1)), a sodium-insensitive component of copper uptake is also present in trout gills. It demonstrates saturation kinetics with a comparably low K(m) (9.6 nmol l(-1)) but a lower maximum transport capacity (J(max)=3.5 pmol g(-1) h(-1)) than the sodium-insensitive system. Sodium uptake exhibits saturation kinetics with a K(m) of 69 micromol l(-1). Copper reduced branchial sodium transport affinity but increased the maximal sodium transport capacity.