Bioavailability of waterborne strontium to the common carp, Cyprinus carpio, in complexing environments

The uptake of strontium (Sr) and calcium (Ca) in the common carp, Cyprinus carpio, was studied in chemically defined freshwater in the presence of the complexing ligands, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA). The uptake rates were measured in the whole body, gills, and blood of the fish after an exposure period of 3 h. The uptake rates were determined by using the radiotracers (85)Sr and (45)Ca, and analyzed as a function of the free-ion activity of Sr and Ca in water. Although Sr(2+) activity decreased, the uptake of Sr showed an increase at relatively low concentrations of EDTA and NTA, and a decrease at relatively high concentrations. This can be explained by the decreased competition between Sr(2+) and Ca(2+) at the gill uptake sites due to approximately 30-140-fold higher affinity of EDTA and NTA for Ca(2+) than Sr(2+). With decreasing Ca(2+) activity, Ca uptake rates decreased in the presence of EDTA and NTA, but the effect of NTA was less pronounced. A Michaelis-Menten type competitive inhibition model was derived that could predict the whole-body Sr and Ca uptake rates, taking into account the ambient Sr(2+) and Ca(2+) activities in the presence of EDTA. In case of NTA, the uptake rates were found to be 1.5-3.2 times higher than what was predicted by the model. When the fish were exposed to complexing environments in the complete absence of Ca, an increased uptake of Sr was still observed in case of NTA, but not EDTA. The increased uptake in the presence of NTA is attributed to the direct uptake of SrNTA(-) and CaNTA(-) complexes from water. The results reveal that the uptake of Sr and Ca in carp is not merely a function of the free metal-ion activity but that certain complex species may contribute significantly to overall uptake.