ABSTRACT
The hematological parameters and metallothionein (MT) levels in the liver, kidney and muscles were measured in bullfrog tadpoles, Lithobates catesbeianus, following exposures to 1 µg L-1 of zinc (Zn), copper (Cu) and cadmium (Cd) alone or in combination (1:1 and 1:1:1) for 2 and 16 days. Metal accumulation occurred in all organs, with the highest values found in the kidney, followed by the muscles and liver. After exposure to isolated metals, the accumulation was in the following order: Cd > Zn > Cu in the liver and muscles and Cd > Cu > Zn in the kidney. Exposure to combined metals (Zn + Cu, Zn + Cd, Cu + Cd and Zn + Cu + Cd) revealed complex responses, such as metal accumulation increased or decreased over the exposure periods, suggesting possible competion at the uptake sites and/or metabolization and elimination processes in each organ. The MT concentration increased in the organs of tadpoles following metal exposure alone, mainly in the liver, for both periods. After the combined exposures, the MT levels were higher in the liver and muscles at 16 days, suggesting that the interaction between metals was additive, and the level was decreased in the kidney after 2 and 16 days of exposure. The whole blood hemoglobin content (Hb), red blood cell count (RBCs) and mean corpuscular hemoglobin (MCH) differed from the control groups after 2 and 16 days of exposure, showing changes in the improvement of oxygen transport. The number of lymphocytes increased, and the levels of neutrophils, eosinophils, basophils and monocytes were reduced after exposure to the metals. The changes in blood cells suggested that tadpoles have a mechanism to improve oxygen transport probably because of the increased oxygen demand and a general reduction in defense cells. The exposure of L. catesbeianus to metals during the larval phase can generate long-term dysfunction to a degree, which could lead to alterations in their health status.
