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2 McMaster University, Department of Biology, Hamilton, Ontario, Canada L8S 4K1; and 1 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
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 · h1 at 0.1 mM
[Na+] compared with 35 ± 9 at 13 mM
[Na+])
and only a slight decrease in the affinity for silver transport (Km; 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 µM, blocked
Na+ 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
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
A1 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.
silver transport; teleost; sodium channel; phenamil; bafilomycin A1
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