Fine-scale tissue distribution of cadmium, inorganic mercury, and methylmercury in nymphs of the burrowing mayfly Hexagenia rigida studied by whole-body autoradiography

The distribution of inorganic Cd-109(II), inorganic Hg-203(II) and [Hg-203] methylmercury (MeHg) in nymphs of the burrowing mayfly Hexagenia rigida af ter exposure via water and sediments was studied. To better understand the mechanisms underlying the fate of Cd, Hg, and MeHg in this animal and to id entify target organs, autoradiography of whole-body cryosections was used t o obtain a detailed view of the distribution of the radiolabels. The gut an d exoskeleton were the only structures labeled in nymphs exposed to Cd via water or sediments. After exposure to inorganic Hg via water, the Malpighia n tubules exhibited a very high labeling, indicating that these organs may be a target for Hg toxicity. The distribution of Hg after exposure via sedi ments was similar, though the labeling of Malpighian tubules was less inten se. Distribution of MeHg strongly differed between treatment groups. Nymphs were rather uniformly labeled after exposure via water, whereas in those e xposed to MeHg in sediments, the intense labeling of all internal tissues c ontrasted with the very low labeling of the hemolymph, indicating that the translocation rate of the absorbed MeHg was faster in the latter group. Thi s may be related to the complexation of MeHg by small thiol ligands in the gut as a result of the digestion process. (C) 2001 Academic Press.