Characterization of cadmium and calcium fluxes along the gut, malpighian tubules, and anal papillae of the dipteran Chironomus riparius.

Chironomids are often one of the dominant organisms in significantly polluted freshwater. Many invertebrate studies have characterized whole-organism mechanisms of toxicity, for example, assessing cadmium (Cd) uptake via calcium (Ca) channels. However, with the use of the scanning ion-selective electrode technique and an innovative Cd-selective microelectrode, we analyze this relationship at the organ level using a realistic concentration of Cd and Ca in the hemolymph (blood). Generally, Cd fluxes follow the same directional pattern as Ca, although Ca fluxes are approximately 5 times higher than those of Cd. These results correlate well with previous studies indicating that chironomids have a higher affinity for Ca over Cd, which affords them tolerance to Cd toxicity. When saline Ca concentration was increased to 10 times physiological levels, Cd fluxes from the gut lumen into the cells of the midgut regions were reduced by 50 to 80%. Transport of Cd from hemolymph to tissue for the posterior midgut, Malpighian tubule, and proximal ceca was also reduced by approximately 50%. The present results indicate that Cd fluxes into or across the gut and Malpighian tubules are reduced by high Ca, suggesting that Cd may be transported in some cells by similar mechanisms. However, Cd was actively excreted at the anal papillae after a 48-h waterborne exposure to Cd, but this process was independent of Ca and instead may involve a P-glycoprotein-related pump to detoxify Cd. Environ Toxicol Chem 2018;37:2542-2549. © 2018 SETAC.

Effects of exposure to high concentrations of waterborne Tl on K and Tl concentrations in Chironomus riparius larvae.

Thallium (Tl) is a non-essential metal which is released into the environment primarily as the result of anthropogenic activities such as fossil fuel burning and smelting of ores. The ionic radius of monovalent Tl⁺ is similar to that of K⁺ and Tl⁺ may thus interfere with K⁺-dependent processes. We determined that the acute (48 h) lethal concentration where 50% of the organisms do not survive (LC50) of Tl for 4th instar Chironomus riparius larvae was 723 µmol L⁻¹. Accumulation of Tl by the whole animal was saturable, with a maximum accumulation (Jmax) of 4637 µmol kg⁻¹ wet mass, and K(D) of 670 µmol Tl l⁻¹. Tl accumulation by the gut appeared saturable at the lowest four Tl concentrations, with a Jmax of 2560 µmol kg⁻¹ wet mass and a K(D) of 54.5 µmol Tl l⁻¹. The saturable accumulation at the gut may be indicative of a limited capacity for intracellular detoxification, such as storage in lysosomes or complexation with metal-binding proteins. Tl accumulation by the hemolymph was found to be linear and Tl concentrations in the hemolymph were ~75% of the exposure concentration at Tl exposures >26.9 µmol L⁻¹. There was not a significant decrease in whole animal, gut or hemolymph K during exposure to waterborne Tl at any of the concentrations tested (up to 1500 µmol L⁻¹). The avoidance of hypokalemia by C. riparius larvae may contribute to survival during acute waterborne exposures to Tl.

Ion-selective microelectrode measurements of Tl⁺ and K⁺ transport by the gut and associated epithelia in Chironomus riparius.

Thallium (Tl) is a non-essential metal that is mobilized through industrial processes, subsequently entering aquatic environments where it can exert toxic effects. Although the aquatic larvae of the midge, Chironomus riparius, are exceptionally tolerant toward many waterborne non-essential metals, few studies have looked at the cellular mechanism of this tolerance. Tl⁺ and K⁺ share the same charge and have similar ionic radii, resulting in competition between these ions for K⁺ transporters. Using a recently developed Tl⁺-selective microelectrode in conjunction with the scanning ion selective electrode technique (SIET) and a two-microelectrode holder, measurements of K⁺ and Tl⁺ fluxes were made along the anal papillae and also along the isolated gut tract and Malpighian tubules (MTs) of C. riparius larvae. The MTs are a site of Tl⁺ secretion (i.e. from hemolymph into the tubule lumen). The major K⁺ transporting regions of the gut were the caecae, anterior midgut (AMG) and posterior midgut (PMG) in Tl⁺-naïve larvae, and Tl⁺ was also transported in the same direction at these locations. When the bathing saline concentration of Tl⁺ was increased to 50 µmol l⁻¹, K⁺ transport was inhibited at the AMG and PMG. Larvae exposed to 300 µmol l⁻¹ waterborne Tl⁺ for 48 h prior to ion flux measurements absorbed Tl⁺ (lumen to hemolymph) across the caecae, AMG and PMG. K⁺ secretion at the caecae was unaffected by Tl⁺ exposure, consistent with separate pathways for Tl⁺ and K⁺ transport across the caecae. By contrast, K⁺ flux at the AMG and PMG of Tl⁺-exposed larvae was impaired, suggesting that interference of Tl⁺ on K⁺ transport across these tissues may contribute to Tl⁺ toxicity.