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 μmol L−1), exhibited classic Michaelis-Menten saturation kinetics for Ca influx, measured using 45Ca as a radio-tracer. The maximum rate of Ca influx (JmaxCa) was 0.39 μmol g−1 h−1, and the Ca concentration where the carrier reached half saturation (KMCa) was 289 μmol L−1. Cd influx was measured using 109Cd as a radio-tracer in larvae exposed to Cd (0-1400 μmol L−1) while the Ca concentration was set to the KMCa. This revealed a JmaxCd (2.26 μmol 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 KMCd (1133 μmol Cd L−1), when recalculated to account for the background Ca level, was still high (567 μmol 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 μmol Cd L−1 was added to Ca exposures (96-2410 μmol L−1). Michaelis-Menten analysis revealed a similar JmaxCa in Cd-exposed and control larvae (i.e. exposed only to Ca), but that the apparent KMCa was many-fold higher in larvae which were simultaneously exposed to Ca and Cd. Conversely, increasing Ca concentrations (96-2410 μmol L−1) progressively inhibited Cd uptake from a Cd exposure concentration (1200 μmol 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.