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THIRST AND VOLUME, ELECTROLYTE HOMEOSTASIS

Time course analysis of the mechanism by which silver inhibits active Na⁺ and Cl^– uptake in gills of rainbow trout

¹Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1; ³Department of Biology, Carleton University, Ottawa, Ontario K1S 5B6; ⁴Department of Biology, Wilfrid Laurier University, Waterloo, Ontario, Canada N2L 3C5; and ²Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149-1098

Submitted 7 August 2003 ; accepted in final form 9 March 2004

A time course analysis using ^110mAg, ²⁴Na⁺, and ³⁶Cl^– examined gill silver accumulation and the mechanism by which waterborne silver (4.0 x 10^–8 M; 4.3 µg/l) inhibits Na⁺ and Cl^– uptake in gills of freshwater rainbow trout. Analyses of gill and body fluxes allowed calculation of apical uptake and basolateral export rates for silver, Na⁺, and Cl^–. To avoid changes in silver bioavailability, flow-through conditions were used to limit the buildup of organic matter in the exposure water. For both Na⁺ and Cl^– uptake, apical entry, rather than basolateral export, was the rate-limiting step; Na⁺ and Cl^– uptake declined simultaneously and equally initially, with both uptakes reduced by 500 nmol·g^–1·h^–1 over the 1st h of silver exposure. There was a further progressive decline in Na⁺ uptake until 24 h. Carbonic anhydrase activity was inhibited by 1 h, whereas Na⁺-K⁺-ATPase activity was not significantly inhibited until 24 h of exposure. These results indicate that carbonic anhydrase inhibition can explain the early decline in Na⁺ and Cl^– uptake, whereas the later decline is probably related to Na⁺-K⁺-ATPase blockade. Contrary to previous reports, gill silver accumulation increased steadily to a plateau. Despite the rapid inhibition of apical Na⁺ and Cl^– uptake, apical silver uptake (and basolateral export) increased until 10 h, before decreasing thereafter. Thus silver did not inhibit its own apical uptake in the short term. These results suggest that reduced silver bioavailability is the mechanism behind the pattern of peak and decline in gill silver accumulation previously reported for static exposures to silver.

sodium-potassium-adenosinetriphosphatase; carbonic anhydrase; sodium channel; hydrogen-adenosinetriphosphatase; silver uptake

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