Minimising aerobic respiratory demands could form the basis to sub-lethal copper tolerance by rainbow trout gill epithelial cells in vitro

Mechanisms of Cu tolerance were investigated in respiratory epithelial cell cultures, from rainbow trout gills, by studying O-2 consumption and protein synthesis rates, intracellular Na concentration and TER. The lowest concentration found to reduce O-2 consumption was 25 muM Cu. This did not affect either protein synthesis rate or intracellular Na concentration and was interpreted in terms of copper tolerance; i.e., how these two energetically demanding processes are maintained despite a reduction in aerobic ATP supply. The relationship between protein synthesis rate and synthesis cost is exponential and the cost of protein synthesis in gill cells was found to be minimal (i.e., this cell occupies a position on the asymptotic section of the protein synthesis rate/synthesis cost model) and unaffected by 25 muM Cu. Thus protein synthesis rates could be maintained since any reduction would represent an insignificant energy saving. Intracellular Na concentrations and O-2 consumption rates were linearly correlated suggesting reducing intracellular maintenance costs would have a greater significance in terms of overall energetic conservation. Intracellular Na maintenance costs, calculated from O-2 consumption rates and intracellular Na concentrations, were found to decline after exposure to 25 muM Cu. Since TER was unaffected this implied the reduced costs arose from membrane `channel arrest'. Thus the Na/K ATPase energy demands, associated with maintaining intracellular Na concentration, could be reduced by decoupling metabolic demand and membrane function. Therefore this study may demonstrate how the flexibility of cellular energetics enables gill epithelial cells to tolerate sub-lethal Cu.