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Am J Physiol. 1999 Nov;277(5):R1385-91. doi: 10.1152/ajpregu.1999.277.5.R1385.

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

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Department of Biological Sciences, Hatherly Laboratories, University of Exeter, Exeter, United Kingdom EX4 4PS.


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 AgNO(3), 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 AgNO(3), 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 [Ca(2+)] 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 microM, 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 microM, 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.

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