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Description:
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Xenopus laevis and native anuran embryos and larvae were exposed to ammonium perchlorate (AP ) in a series of experiments . Five - and seventy -day median lethal concentrations (LC50S ) for Xenopus laevis embryos were 510 ± 36 mg /L and 223 ±13 mg /L AP ; 4 - and 7 -d LC50S for embryonic Rana utriculata were 15 .6 mg /L and 3 .12 mg /L . Exposure of X . laevis to sodium perchlorate (SP ; or ammonium chloride (AC ) showed AC to be significantly more lethal than SP , with 5 -d LC50S of 98 .3 mg ammonium /L and 3177 .2 mg perchlorate /L . Although AC (14 mg /L ) reduced snout -vent length , there was no effect observed on thyroid -sensitive indices of metamorphosis while similar concentrations of SP and AP inhibited metamorphosis . AP (14 mg /L ) reduced whole -body thyroxine content and caused significant thyroid follicular epithelium hypertrophy in Xenopus larvae , both of which were reversed during a 28 -d recovery period . AP exposure also skewed the sex ratio , reducing the percentage of males at metamorphosis .
Rana catesbeiana tadpoles exposed to perchlorate (14 or 118 mg /L ) for up to 264 h demonstrated a continual linear increase in whole -body uptake , with perchlorate content after 96 h of recovery no different from control animals . Individual tissue accumulation was as follows : thyroid (1072 mg /L ) > whole -body (765 mg /L ) > liver (725 mg /L ) > kidney (551 mg /L ) > stomach (447 mg /L ) . Utilizing the U .S . Environmental Protection Agency Endocrine Disrupter Screening and Testing Committee (EDSTAC ) Tier I tail resorption assay , AP (14 mg /L ) significantly inhibited tail resorption in X . laevis . A modified EDSTAC assay using surface waters collected from Longhorn Army Ammunition Plant (LHAAP ) found no difference in tail length . These waters were found to have less perchlorate than previously reported , as well as a significant amount of iodide . A later study found that the simultaneous exposure of X . laevis to AP (14 mg /L ) and sodium iodide (Nal ; 14 mg /L ) reversed the antimetamorphic effects of AP .
Although AP was not teratogenic , concentrations reported in contaminated surface waters did inhibit metamorphosis . Even though these effects are negated by an adequate supply of iodide , the continued linear uptake of perchlorate , coupled with the relatively slow elimination , suggests that even episodic exposures have the potential to alter thyroid function in developing anurans . |