Characterization of dietary Ni uptake in the rainbow trout, Oncorhynchus mykiss

We characterized dietary Ni uptake in the gastrointestinal tract of rainbow trout using both in vivo and in vitro techniques. Adult trout were fed a meal (3% of body mass) of uncontaminated commercial trout chow, labeled with an inert marker (ballotini beads). In vivo dietary Ni concentrations in the supernatant (fluid phase) of the gut contents averaged from 2 μmol l−1 to 24 μmol l−1, and net overall absorption efficiency of dietary Ni was approximately 50% from the single meal, similar to that for the essential metal Cu, adding to the growing evidence of Ni essentiality. The stomach and mid-intestine emerged as important sites of Ni uptake in vivo, accounting for 78.5% and 18.9% of net absorption respectively, while the anterior intestine was a site of net secretion. Most of the stomach uptake occurred in the first 4 h. In vitro gut sac studies using radiolabeled Ni (at 30 μmol l−1) demonstrated that unidirectional uptake occurred in all segments, with area-weighted rates being highest in the anterior intestine. Differences between in vivo and in vitro results likely reflect the favourable uptake conditions in the stomach, and biliary secretion of Ni in the anterior intestine in vivo. The concentration-dependent kinetics of unidirectional Ni uptake in vitro were biphasic in nature, with a saturable Michaelis–Menten relationship observed at 1–30 μmol l−1 Ni (Km = 11 μmol l−1, Jmax = 53 pmol cm−2 h−1 in the stomach and Km = 42 μmol l−1, Jmax = 215 pmol cm−2 h−1 in the mid-intestine), suggesting mediation by a channel or carrier process. A linear uptake relationship was seen at higher concentrations, indicative of simple diffusion. Ni uptake (at 30 μmol l−1) into the blood compartment was significantly reduced in the stomach by high Mg (50 mmol l−1), and in the mid-intestine by both Mg (50 mmol l−1) and Ca (50 mmol l−1). In both regions, kinetic analysis demonstrated reductions in Jmax with unchanged Km, suggesting non-competitive interactions. Therefore the Mg and Ca content of the food will be an important consideration affecting the availability of Ni.