Investigating a potential mechanism of Cd resistance in Chironomus riparius larvae using kinetic analysis of calcium and cadmium uptake.

The uptake kinetics of waterborne Ca and Cd, both independently and in combination, were examined in C. riparius larvae, which are extremely Cd tolerant. Larvae exposed to Ca (100-2500 micromol L(-1)), exhibited classic Michaelis-Menten saturation kinetics for Ca influx, measured using (45)Ca as a radio-tracer. The maximum rate of Ca influx (J(max)(Ca)) was 0.39 micromol g(-1)h(-1), and the Ca concentration where the carrier reached half saturation (K(M)(Ca)) was 289 micromol L(-1). Cd influx was measured using (109)Cd as a radio-tracer in larvae exposed to Cd (0-1400 micromol L(-1)) while the Ca concentration was set to the K(M)(Ca). This revealed a J(max)(Cd) (2.26 micromol g(-1)h(-1)) which was nearly 6-fold higher that of Ca. This unusually high capacity for Cd uptake is in accordance with the huge tissue Cd burdens that chironomid larvae are able to accumulate during high level exposures. The apparent K(M)(Cd) (1133 micromol Cd L(-1)), when recalculated to account for the background Ca level, was still high (567 micromol Cd L(-1)), suggesting that this organism has a low affinity for Cd relative to most aquatic animals, indeed lower or comparable to its affinity for Ca. In consequence, even well above environmentally relevant Cd exposures, C. riparius does not accumulate Cd at the expense of Ca, thereby avoiding internal hypocalcaemia, in contrast to most other organisms which are much more sensitive to Cd. However, Ca influx was significantly reduced when 1200 micromol Cd L(-1) was added to Ca exposures (96-2410 micromol L(-1)). Michaelis-Menten analysis revealed a similar J(max)(Ca) in Cd-exposed and control larvae (i.e. exposed only to Ca), but that the apparent K(M)(Ca) was many-fold higher in larvae which were simultaneously exposed to Ca and Cd. Conversely, increasing Ca concentrations (96-2410 micromol L(-1)) progressively inhibited Cd uptake from a Cd exposure concentration (1200 micromol L(-1)), providing additional support for a common transport system. These results suggest that the interaction of Cd and Ca in C. riparius is one of simple competitive interaction, and that the unusual Cd transport kinetics (low affinity, high capacity) relative to fairly standard Ca transport kinetics help explain the unusual tolerance that this organism has to acute Cd exposure.