Mechanism of branchial apical silver uptake by rainbow trout is via the proton-coupled Na+ channel

The branchial uptake mechanism of the nonessential heavy metal silver from very dilute media by the gills of freshwater rainbow trout was investigated. At concentrations >36 nM AgNO3, silver rapidly entered the gills, reaching a peak at 1 h, after which time there was a steady decline in gill silver concentration and a resulting increase in body silver accumulation. Below 36 nM AgNO3, there was only a very gradual increase in gill and body silver concentration over the 48-h exposure period. Increasing water sodium concentration ([Na+]; 0.05 to 21 mM) significantly reduced silver uptake, although, in contrast, increasing ambient [Ca2+] or [K+] up to 10 mM did not reduce silver uptake. Kinetic analysis of silver uptake at varying [Na+] showed a significant decrease in maximal silver transport capacity (173 +/- 34 pmol.g(-1) h(-1) at 0.1 mM [Na+] compared with 35 +/- 9 at 13 mM [Na+]) and only a slight decrease in the affinity for silver transport (K-m; 55 +/- 27 nM at 0.1 mM [Na+] compared with 91 +/- 47 nM at 13 mM [Na+]). Phenamil (a specific blocker of Na+ channels), at a concentration of 100 mu M, blocked Na+ uptake by 78% of control values (58% after washout), and bafilomycin A(1) (a specific blocker of V-type ATPase), at a concentration of 2 mu M, inhibited Na+ uptake by 57% of control values, demonstrating the presence of a proton-coupled Na+ channel in the apical membrane of the gills. Phenamil (after washout) and bafilomycin A(1) also blocked silver uptake by 62 and 79% of control values, respectively, indicating that Ag+ is able to enter the apical membrane via the proton-coupled Na+ channel.