Calcium/cadmium interactions at uptake surfaces in rainbow trout: Waterborne versus dietary routes of exposure

Abstract

Juvenile rainbow trout (Oncorhynchus mykiss) were exposed to control, 3 microg/L waterborne Cd, or 500 mg/kg dietary Cd in combination with either a control (20 mg/g Ca2+ as CaCO3) or elevated (60 mg/g Ca2+) Ca2+ diet for 28 d. No mortality or growth effects were observed in response to either route of Cd exposure, although fish fed Ca2+-supplemented diets exhibited minor reductions in growth within the first few days of feeding. Waterborne and dietary Cd resulted in significant Cd accumulation in most tissues, with dietary uptake being far in excess of waterborne under the exposure conditions used. The order of Cd accumulation strongly reflected the exposure pathway, being gill and kidney > liver > gut > carcass (waterborne Cd); gut > kidney > liver > gill > carcass > bone (dietary Cd). On a whole-body basis, the net retention of Cd from the diet was < 1%, indicating that the gut wall forms an important protective barrier reducing Cd accumulation into internal tissues. Dietary Ca2+ supplementation reduced short-term whole-body uptake rates of waterborne Ca2+ and Cd by >50% and resulted in much lower chronic accumulation of Cd (via the water and diet) in target tissues. Results suggest that Ca2+ and Cd share common pathway(s)/transport mechanism(s) in the gill and gut and that increased gastrointestinal Ca2+ uptake likely caused downregulation of branchial and gastrointestinal Ca2+ and therefore Cd uptake pathways. Because nutrient metals other than Ca2+ may also influence Cd (and other metal) uptake, new regulatory approaches to metal toxicity (e.g., biotic ligand model) require understanding of the influence of dietary status on metal accumulation.

Uptake pathways and subcellular fractionation of Cd in the polychaete Nereis diversicolor

“…As a non-essential element, Cd has been assumed to be taken up by transporters for essential elements as a consequence of lack of specificity of the transporters. It has been demonstrated that Cd follows Ca transport pathways, and Cd-Ca interactions have been reported previously (Rainbow 1995;Franklin et al 2005;Kiewiet and Ma 1991;Li et al 2008a). In polychaetes, there are few studies on the involvement of Ca channels in transporting metals.…”

Significance of subcellular metal distribution in prey in influencing the trophic transfer of metals in a marine fish

“…As a nonessential metal, Cd is considered to be taken up through the Ca transport pathway in fish gills (Niyogi and Wood 2003), and at least part of this pathway is in the fish intestine (Schoenmakers et al 1992;Franklin et al 2005). Thus, Cd assimilation could be inhibited by competition of Ca, as the Cd concentration in organism tissues is usually negligible compared with the Ca concentration (Franklin et al 2005). The low AE of Cd was unlikely caused by excretion, which was very low in marine fish (,0.1 d 21 ; Xu and Wang 2002).…”

Intraspecific comparisons of metal bioaccumulation in the juvenile Atlantic silverside Menidia menidia

“…Low Am AEs in fish can be attributed to a low Am AE in zooplankton, which in turn is attributed to the low percentage of Am associated with cytoplasm in phytoplankton , 1994b. Also, Am is not known to share cellular uptake channels with other elements, unlike Cd, another biologically non-essential element which can be taken up through Ca and Zn uptake channels (Brzóska & Moniuszko-Jakoniuk 2001, Franklin et al 2005.…”

“…Why some fish are better protected from Cd uptake across the intestine is not fully understood. Studies using rainbow trout Oncorhynchus mykiss have shown that Cd shares the same uptake pathway as Ca 2+ in the gills and intestinal tract, and a diet enriched in Ca 2+ can inhibit Cd uptake at these exposure sites by downregulating the Ca 2+ uptake pathway in the gills and intestine (Franklin et al 2005, Wood et al 2006). However, we did not measure Ca 2+ levels in our prey item to conclude whether this could have an influence in our study.…”