Gill metal binding and stress gene transcription in brown trout (Salmo trutta) exposed to metal environments:: The effect of pre-exposure in natural populations

Brown trout (Salmo trutta) from two native populations from the Roros area in Central Norway, acclimated in mining-affected habitats to different levels of Cd/Zn and Cu, together with trout from a nearby unaffected river (reference) were transferred to a nearby lake with higher levels of Cu, Cd, and Zn than those in their respective native rivers. This experiment was conducted to gain information about the underlying resistance mechanisms developed in fish exposed to metal environments. The focus was on gill metal accumulation and transcription of the metal-responsive stress genes metallothionein-A (MT-A), Cu/Zn-superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), and heat shock protein 70 (HSP-70). The only shared response shown between the three groups after transfer were Cu accumulation and MT-A induction. The Cu-acclimated trout produced mucus to reduce the uptake of Cu into the gills. The MT-A levels were highest in the Cd/Zn-acclimated trout both before and after transfer. Before transfer, antioxidant transcription (SOD and GPx) was higher in gills of Cu-acclimated compared to the Cd/Zn-acclimated trout, but increased transcription of antioxidant stress genes was observed after transfer in both metal-acclimated groups. The metal-acclimated trout groups also showed an increase in the transcription of HSP-70. Compared to the reference population not previously exposed to metals, stress gene transcription increased faster in the metal-acclimated populations. The exception was induction of CAT, which appeared to be depressed after transfer in Cd/Zn-acclimated trout. The data indicate that acclimation to chronic metal exposure involves different strategies to cope with different metals and that these strategies involve both physiological mechanisms (mucus production) as well as metal-related stress gene transcription.