Juvenile rainbow trout were exposed to 0, 0. 1, 1, 3, and 5 mug/l silver (Ag, as AgNO3) for 23 days. Specific growth rate, cumulative food consumption, food-conversion efficiency, and critical swimming speed (U-crit) were significantly reduced during 5 mug/l Ag exposure, demonstrating a physiological cost of silver acclimation. Only the 5 mug/l Ag treatment had significant cumulative mortality (5.2%). Fish were most susceptible to silver on days 5 and 15. Exposure to 5 mug/l Ag significantly lowered plasma Na+ and Cl- on days 5 and 10, but plasma ion concentration recovered thereafter. Unidirectional Na+ uptake and gill Na/K-ATPase activity were significantly inhibited by 3 and 5 mug/l Ag exposure. Na+ uptake was inhibited by 3 mug/l Ag at day 5 alone, whereas the effects at the highest Ag exposure persisted until day 15. Gill Na/K-ATPase was inhibited on day 5 in both the 3 and 5 mug/l Ag treatments but increased to approx. 1.5 times of control levels by day 23. Only the 3 and 5 mug/l Ag treatments produced toxicological acclimation (at least twofold elevations in 168-h LC50 values in fish subsampled on days 15 and 23). We conclude that physiological acclimation results from compensatory changes in Na+ transport at the gills, and that these changes may eventually lead to toxicological acclimation.
