The influence of metal exposure history on rates of aqueous Cd accumulation, elimination, and subcellular distribution was examined in the aquatic insect Hydropsyche californica. Specimens were obtained from a reference site and a metal-contaminated site and returned to the laboratory where they were continuously exposed to aqueous Cd (518 ng/L, nominal) for 6 d, followed by 9 d of depuration. Rates of Cd accumulation and elimination were similar in insects from the two sites. Efflux rate constants, ke, ranged from 0.20 to 0.24/d (t1/2 ≈ 3 d). Immediately following exposure, the cytosol accounted for 40% of the body burden in insects from both sites; however, 89 ± 2% of the cytosolic Cd was associated with metallothionein-like proteins (MTLP) in insects from the contaminated site, compared to 60 ± 0% in insects from the reference site. The concentration of Cd bound to non-MTLPs (representing potentially Cd-sensitive proteins) was significantly greater in the insects from the reference site (134 ± 7 ng/g) than in those from the contaminated site (42 ± 2 ng/g). At the end of the depuration period, 90% of the accumulated Cd body burden had been eliminated, and Cd concentrations in MTLPs and non-MTLPs were similar between the sites. Results suggested that differences in exposure history had no influence on the bioaccumulation of Cd, but did affect the concentrations of Cd bound to MTLP during Cd exposure in these insects.
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