Waterborne iron acquisition by a freshwater teleost fish, zebrafish Danio rerio

Waterborne iron accumulation by the gills of the zebrafish Danio rerio was assessed in ion-poor water. Branchial iron uptake, which comprises both the iron that has entered the gill cells and iron that is strongly bound to the epithelia, has high- and low-affinity components. At low nominal [Fe] (<40 nmol l^-1) the high-affinity component demonstrated saturation kinetics, with an apparent K_m of 5.9 nmol l^-1 Fe and V_max of 2.1 pmol g^-1 h^-1. Over a range of higher nominal [Fe] (40-200 nmol l^-1), branchial uptake was linear. In the presence of 2μmol l^-1 of the reducing agent dithiothreitol (DTT), branchial iron accumulation was significantly enhanced at [Fe]>15 nmol l^-1. The proton pump inhibitor bafilomycin A significantly reduced iron uptake in the presence of DTT. On the basis of these observations we conclude that branchial iron uptake at low [Fe] shows characteristics similar to those of other iron-transporting epithelia, coupling an apical membrane ferric reductase to a Fe²⁺/H⁺ symporter. Zebrafish branchial iron transport at 18.6 nmol l^-1 was inhibited by 200 nmol l^-1 Cd²⁺. But, unlike other Fe²⁺/H⁺ symporters, iron uptake was not affected by other divalent metals (Co²⁺, Ni²⁺, Pb²⁺,Cu²⁺, Zn²⁺ and Mn²⁺). Zebrafish loaded with ⁵⁹Fe from the water showed a loss of 7.9 pmol Fe g^-1body mass over the first day and a further loss of 5.7 pmol Fe g^-1body mass over the following 28 days. The depuration kinetics followed a two-component exponential model; for the short-lived component, t_1/2=0.31 days, and for the long-lived component, t_1/2=13.2 days. The daily iron loss by zebrafish can be compensated by iron uptake at exceedingly low water iron concentrations(uptake rate at 1.625 nmol l^-1 Fe=0.425 pmol g^-1h^-1), demonstrating that uptake of iron from the water is potentially an important source of this nutritive metal in freshwater teleost fish.