Protein synthesis costs could account for the tissue-specific effects of sub-lethal copper on protein synthesis in rainbow trout (Oncorhynchus mykiss)

This study investigates protein synthesis, following exposure to sub-lethal Cu, in rainbow trout in vivo and in vitro. The investigation has two aims: to determine if perturbations in protein synthesis, compared with other ph ysiological changes, are a biomarker of Cu pollution and to evaluate the mo st productive role of cellular models in ecotoxicology. Protein synthesis r ates were measured by labelling with H-3-phenylalanine. In vivo this was ap plied by a single (i.p.) injection and in vitro by bathing the cells in H-3 -phenylalanine labelled culture media. The effects in vivo were tissue spec ific. After 3 weeks' exposure to 0.7 muM Cu only skin protein synthesis was reduced. Gills and liver from the same fish were unaffected. This reductio n in skin protein synthesis appears to be more sensitive than some other bi omarkers reported in the literature. However, Cu concentrations greater by orders of magnitude were required to reproduce this reduction in protein sy nthesis in skin cell explants (200 and 400 muM). Hepatocyte protein synthes is was unaffected by 10, 20 and 40 muM Cu and a separate investigation has also shown that 25 and 75 muM Cu does not effect protein synthesis in cultu red gill cells. Oxygen consumption rates were also measured in vitro by mon itoring the decline in O-2 partial pressure. The Cu concentrations given ab ove resulted in a decline in O-2 consumption rates in the respective cell t ypes. By measuring protein synthesis and O-2 consumption after treatment wi th a protein synthesis inhibitor (cycloheximide), the costs of protein synt hesis were also determined. Synthesis costs in hepatocytes are close to the theoretical minimum and are only marginally affected by Cu. Gill cell synt hesis costs are also minimal and are unaffected. In skin explants, the redu ction in protein synthesis was accompanied by greatly increased synthesis c osts. This in vitro result offers a hypothesis as to the tissue-specific ef fects in vivo; i.e. the energetic demand of protein synthesis may determine tissue sensitivity or susceptibility. Cell or tissue types with high prote in synthesis rates are able to avoid detrimental increases in the synthesis cost when exposed to Cu. In tissues with a low protein synthesis rate any further reduction is more likely to incur a potentially damaging increase i n protein synthesis costs. Thus, whilst in vitro models may have little pra ctical use in environmental monitoring, they may be best used as a mechanis tic tool in understanding susceptibility or tolerance to sub-lethal Cu. (C) 2001 Elsevier Science B.V. All rights reserved.