Mechanism of branchial apical silver uptake by rainbow trout is via the proton-coupled Na1 channel

Bury, Nicolas R., and Chris M. Wood. Mechanism of branchial apical silver uptake by rainbow trout is via the proton-coupled Na1 channel. Am. J. Physiol. 277 (Regulatory Integrative Comp. Physiol. 46): R1385–R1391, 1999.—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 ([Na1]; 0.05 to 21 mM) significantly reduced silver uptake, although, in contrast, increasing ambient [Ca21] or [K1] up to 10 mM did not reduce silver uptake. Kinetic analysis of silver uptake at varying [Na1] showed a significant decrease in maximal silver transport capacity (173 6 34 pmol ·g21 · h21 at 0.1 mM [Na1] compared with 35 6 9 at 13 mM [Na1]) and only a slight decrease in the affinity for silver transport (Km; 55 6 27 nM at 0.1 mM [Na1] compared with 91 6 47 nM at 13 mM [Na1]). Phenamil (a specific blocker of Na1 channels), at a concentration of 100 μM, blocked Na1 uptake by 78% of control values (58% after washout), and bafilomycin A1 (a specific blocker of V-type ATPase), at a concentration of 2 μM, inhibited Na1 uptake by 57% of control values, demonstrating the presence of a proton-coupled Na1 channel in the apical membrane of the gills. Phenamil (after washout) and bafilomycin A1 also blocked silver uptake by 62 and 79% of control values, respectively, indicating that Ag1 is able to enter the apical membrane via the proton-coupled Na1 channel.