Calcium control of zinc uptake in rainbow trout

Water hardness is known to control the uptake and toxicity of zinc and other metals through either chemical competition, biological acclimation, or both processes. The dominant process controlling zinc uptake has not been previously elucidated, nor has the effect of calcium been investigated independently of other cations. We determined zinc uptake by rainbow trout acclimated and exposed to four calcium treatments: (1) low calcium (6.5 mg Ca/l; 160 µM) acclimation and low calcium exposure (LL); (2) low acclimation and high calcium (131 mg Ca/l; 3300 µM) exposure (LH); (3) high acclimation and low exposure (HL); and (4) high acclimation and high exposure (HH). Trout were exposed to sublethal zinc (100 µg 65Zn/l; 1.5 µM) for 24 h, and whole body and gill 65Zn levels were determined. Zinc uptake was approximately linear during the 24 h exposure period and uptake was calcium dependent. 65Zn uptake by trout was 22 µg/kg per h in the LL treatment and 4.5 µg/kg per h in the HH treatment. 65Zn uptake by trout in the LH and HL treatments was 13 and 10 µg/kg per h, respectively. 65Zn uptake in gills was also significantly reduced by calcium acclimation or exposure. Trout in the LH and HH treatments had five fold lower 65Zn concentrations than LL treatment fish. The results of this study demonstrate that calcium reduces zinc uptake through both biological acclimation and chemical processes, and that the protective effects of calcium are approximately additive.