In vitro characterization of cadmium and zinc uptake via the gastrointestinal tract of rainbow trout (Oncorhynchus mykiss): interactive effects and the influence of calcium
An in vitro gut sac technique was employed to study whether Cd and Zn uptake mechanisms in the
gastro-intestinal tract of the rainbow trout are similar to those at the gills, where both metals are taken
up via the Ca transport pathway. Metal accumulation in surface mucus, in the mucosal epithelium, and
transport into the blood space were assayed using radiolabelled Cd or Zn concentrations of 50 mol L−1
in the luminal (internal) saline. Elevated luminal Ca (10 or 100 mmol L−1 versus 1 mmol L−1) reduced Cd
uptake into all three phases by approximately 60% in the stomach, but had no effect in the anterior,
mid, or posterior intestine. This finding is in accordance with recent in vivo evidence that Ca is taken
up mainly via the stomach, and that high [Ca] diets inhibit Cd accumulation from the food specifically
in this section of the tract. In contrast, 10 mmol L−1 luminal Ca had no effect on Zn transport in any
section, whereas 100 mmol L−1 Ca stimulated Zn uptake, by approximately threefold, into all three phases
in the stomach only. There was no influence of elevated luminal Zn (10 mmol L−1) on Cd uptake in the
stomach or anterior intestine, or of high Cd (10 mmol L−1) on Zn uptake in these sections. However, high
[Zn] stimulated Cd transport into the blood space but inhibited accumulation in the mucosal epithelium
and/or mucus-binding in the mid and posterior intestine, whereas high [Cd] exerted a reciprocal effect
in the mid-intestine only. We conclude that Cd uptake occurs via an important Ca-sensitive mechanism
in the stomach which is different from that at the gills, while Cd transport mechanisms in the intestine
are not directly Ca-sensitive. Zn uptake does not appear to involve Ca uptake pathways, in contrast to the
gills. These results are discussed in the context of other possible Cd and Zn transport pathways, and the
emerging role of the stomach as an organ of divalent metal uptake.Natural Sciences and Engineering Research Council of Canada CRD Program, the International Lead
Zinc Research Organization, the International Zinc Association, the
Nickel Producers Environmental Research Association, the International Copper Association, the Copper Development Association,
Teck-Cominco, Xstrata (Noranda-Falconbridge), and Inc