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

A time course analysis using ^110mAg, ²⁴Na ⁺, and ³⁶Cl^- examined gill silver accumulation and the mechanism by which waterborne silver (4.0 × 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.