Physiological modulation of iron metabolism in rainbow trout (Oncorhynchus mykiss) fed low and high iron diets

Iron (Fe) is an essential element, but Fe metabolism is poorly described in fish and the role of ferrireductase and transferrin in iron regulation by teleosts is unknown. The aim of the present study was to provide an overview of the strategy for Fe handling in rainbow trout, Oncorhynchus mykiss. Fish were fed Fe-deficient, normal and high-Fe diets (33, 175, 1975 mg Fe kg(-1) food, respectively) for 8 weeks. Diets were chosen so that no changes in growth, food conversion ratio, haematology, or significant oxidative stress (TBARS) were observed. Elevation of dietary Fe caused Fe accumulation particularly in the stomach, intestine, liver and blood. The increase in total serum Fe from 10 to 49 mumol l(-1) over 8 weeks was associated with elevated total Fe binding capacity and decreased unsaturated Fe binding capacity, so that in fish fed a high-Fe diet transferrin saturation increased from 15 % at the start of the experiment to 37 %. Fish on the 75 high-Fe diet increased Fe accumulation in the liver, which was correlated with elevation of hepatic ferrireductase activity and serum transferrin saturation. Conversely, fish on the low-Fe diet did not show tissue Fe depletion compared with normal diet controls and did not change Fe binding to serum transferrin. Instead, these fish doubled intestinal ferrireductase activity which may have contributed to the maintenance of tissue Fe status. The absence of clear treatment-dependent changes in branchial Fe accumulation and ferrireductase activity indicated that the gills do not have a major role in Fe metabolism. Some transient changes in Cu, Zn and Mn status of tissues occurred.