Comparing the effects and potencies of perchlorate and nitrate on amphibian metamorphosis using a modified amphibian metamorphosis assay (AMA)

A modified amphibian metamorphosis assay was performed in which Nieuwkoop and Faber (NF) stage 47 Xenopus laevis larvae were exposed to different concentrations of either perchlorate (ClO₄ ^-) or nitrate (NO₃ ^-) for 32 days. Larvae were exposed to 0.0 (control), 5, 25, 125, 625, and 3125 μg/L ClO₄ ^-, or 0 (control), 23, 71, 217, 660, and 2000 mg/L NO₃ ^-. The primary endpoints were survival, hind limb length (HLL), forelimb emergence and development, developmental stage (including time to NF stage 62 [MT₆₂]), thyroid histopathology, wet weight, and snout-vent length (SVL). Developmental delay as evidenced by altered stage distribution and increased MT₆₂, a higher degree of thyroid follicular cell hypertrophy, and an increase in the prevalence of follicular cell hyperplasia was observed at concentrations ≥125 μg/L ClO₄ ^-. The no observed effect concentration (NOEC) for developmental endpoints was 25.0 μg/L ClO₄ ^- and the NOEC for growth endpoints was 3125 μg/L ClO₄ ^-. Exposure to nitrate did not adversely affect MT₆₂, but a decreasing trend in stage distribution and median developmental stage at ≥217 mg/L NO₃ ^- was observed. No histopathologic effects associated with nitrate exposure were observed. An increasing trend in SVL-normalized HLL was observed at 2000 mg/L NO₃ ^-. Nitrate did not alter larval growth. The NOEC for developmental endpoints was 71 mg/L NO₃ ^-, and 2000 mg/L NO₃ ^- for growth endpoints. The present study provided additional evidence that the effects and potency of nitrate and perchlorate on metamorphosis and growth in X. laevis are considerably different.