Abstract
In freshwater teleosts, the primary mechanism of acute silver toxicity is inhibition of Na
+/K
+ ATPase and carbonic anhydrase at the gill, leading to net Na
+ and Cl
− loss due to the continued diffusion of these ions into the hypoosmotic external environment. External Cl
− has been shown to protect rainbow trout (
Oncorhychus mykiss) against silver toxicity presumably by complexation to form AgCl. However, Cl
− does not appear to greatly influence silver toxicity to at least two other species, the European eel (
Anguilla Anguilla) and the fathead minnow (
Pimephales promelas). We hypothesized that differences in protective effects of Cl
− at the gill were due to differing requirements or mechanisms for Cl
− uptake among fish species. To test this hypothesis, we exposed
Fundulus heteroclitus, which does not take up Cl
− across the gills, and
Danio rerio and
P. promelas, which do rely on Cl
− uptake across the gills, to Ag
+ in waters of varying Cl
− concentration. The 96-h LC50s of
F. heteroclitus exposed to Ag
+ in soft water with 10
μM Cl
−, 1
mM KCl, and 0.5
mM MgCl
2 were 3.88, 1.20, and 3.20
μg/L, respectively, and not significantly different. The 96-h LC50s for
D. rerio exposed to Ag
+ in soft water with 10
μM Cl
− and 1
mM KCl were 10.3 and 11.3
μg/L, respectively and
P. promelas exposed under the same conditions were 2.32 and 2.67
μg/L, respectively. Based on these results, increasing external Cl
− concentration by as much as 1
mM (35.5
mg/L) did not offer protection against Ag
+ toxicity to any fish species tested. Although previous results in our laboratory have demonstrated that
P. promelas do take up Cl
− at the gill, a mechanism of uptake has not been identified. Additional experiments, investigating the mechanisms of Na
+ and Cl
− influx at the gill of
P. promelas and the influence of silver, demonstrated that Cl
− uptake in
P. promelas acclimated to soft water occurs through both a Na
+:K
+:2Cl
− co-transporter and a Cl
−/HCO
3
− exchanger, but is not dependent on carbonic anhydrase. Further, acclimation water chemistry was found to greatly influence subsequent branchial silver accumulation, but Cl
− uptake was not sensitive to 10
μg/L Ag
+.
