Metamorphic inhibition of Xenopus laevis by sodium perchlorate:: Effects on development and thyroid histology

The perchlorate anion inhibits thyroid hormone (TH) synthesis via inhibition of the sodium-iodide symporter. It is, therefore, a good model chemical to aid in the development of a bioassay to screen chemicals for affects on thyroid function. Xenopus laevis larvae were exposed to sodium perchlorate during metamorphosis, a period of TH-dependent development, in two experiments. In the first experiment, stage 51 and 54 larvae were exposed for 14 d to 16, 63, 250, 1,000, and 4,000 mu g perchlorate/ L. In the second experiment, stage 51 larvae were exposed throughout metamorphosis to 8, 16, 32. 63, and 125 mu g perchlorate/L. Metamorphic development and thyroid histology were the primary endpoints examined. Metamorphosis was retarded significantly in the first study at concentrations of 250 mu g/L and higher, but histological effects were observed at 16 mu g/L. In the second study, metamorphosis was delayed by 125 mu g/L and thyroid size was increased significantly at 63 mu g/L. These studies demonstrate that inhibition of metamorphosis readily can be detected using an abbreviated protocol. However, thyroid gland effects occur at concentrations below those required to elicit developmental delay, demonstrating the sensitivity of this endpoint and suggesting that thyroidal compensation is sufficient to promote normal development until perchlorate reaches critical concentrations.