Acclimation of rainbow trout, Salmo gairdneri Richardson, to zinc: kinetics and mechanism of enhanced tolerance induction

Pre‐exposure of rainbow trout to 0.31–0.53 of the lethal concentration of zinc increased their tolerance of the metal by a factor of 2.5. This acclimation was fully induced within 5 days, and completely lost within 7 days when fish were returned to zinc‐free water. The changes in tolerance were closely paralleled by changes, in both liver and gill tissue, of heat‐stable, sulfhydryl‐rich protein (an estimate of levels of metal lothionein, MT). This strongly suggests that MT is involved in the induction of enhanced tolerance of zinc. Zinc residues in liver and gill tissue did not parallel the changes in MT levels and tolerance. Thus MT does not appear to bind and store excess zinc; however, it may increase the ability of acclimated fish to regulate the levels of zinc in their gills. This hypothesis is supported by the observation that the rate of accumulation of zinc in gill tissue was slower in acclimated fish than in control fish.