RESUMEN
We investigated stage-dependent changes in sensitivity of the thyroid gland to perchlorate during development of African clawed frog tadpoles (Xenopus laevis) in relation to non-thyroidal iodide transporting tissues. Perchlorate-induced increases in thyroid follicle cell size and colloid depletion were blunted when exposures began at Nieuwkoop-Faber (NF) stage 55 compared to when exposures began at NF stages 49 or 1-10. To determine if the development of other iodide transporting tissues may contribute to this difference we first examined which tissues expressed transcripts for the sodium dependent iodide symporter (NIS). RT-PCR analysis revealed that NIS was expressed in stomach and small intestine in addition to the thyroid gland of X. laevis tadpoles. NIS mRNA was not detected in lung, kidney, skin, gill, muscle, heart or liver. Perchlorate sensitive (125)I uptake was found in stomach, lung, kidney, gill, and small intestine but not muscle, liver, or heart. Perchlorate-sensitive (125)I uptake by stomach was 6-10 times greater than in any other non-thyroidal tissue in tadpoles. While NF stage 49 tadpoles exhibited perchlorate-sensitive uptake in stomach it was roughly 4-fold less than that observed in NF stage 55 tadpoles. Although abundance of NIS gene transcripts was greater in stomachs from NF stage 55 compared to NF stage 49 tadpoles this difference was not statistically significant. We conclude that gastric iodide uptake increases between NF stages 49 and 55, possibly due to post-translational changes in NIS glycosylation or trafficking within gastric mucosal cells. These developmental changes in gastric NIS gene expression may affect iodide availability to the thyroid gland.
Asunto(s)
Larva/metabolismo , Metamorfosis Biológica/genética , Percloratos/metabolismo , Simportadores/genética , Glándula Tiroides/metabolismo , Xenopus laevis/metabolismo , Animales , Femenino , MasculinoRESUMEN
Ammonium perchlorate (AP) is a widespread military waste product in the United States and is a potent thyroid function disruptor in all vertebrates tested thus far. To determine the relative contribution of ammonium ions to the toxicity of AP, we exposed embryonic and larval Xenopus laevis to various concentrations of sodium perchlorate (SP) or ammonium chloride (AC). Ammonium perchlorate was significantly more lethal than SP; 5-d LC50s were 83 and 2,780 mg/L, respectively. To determine whether ammonium ions contribute to the antithyroid effects of AP, we exposed embryonic and larval X. laevis to two sublethal, environmentally relevant concentrations of AP or identical concentrations of AC or SP. At the smaller concentration, only AP delayed metamorphosis and reduced hindlimb growth. Sodium perchlorate and AP, but not AC, prevented metamorphosis and reduced hindlimb growth at the greater concentration. Although AP was slightly more effective in reducing outward manifestations of thyroid disruption, both perchlorate salts, unlike AC, caused profound histopathologic changes in the thyroid. Exposure to the higher concentration of either perchlorate salt produced a feminizing effect, resulting in a skewed sex ratio. We conclude that ammonium ions contribute significantly to the toxicity of AP but not to the direct antithyroid effects of perchlorate.
Asunto(s)
Percloratos/toxicidad , Compuestos de Amonio Cuaternario/química , Compuestos de Amonio Cuaternario/toxicidad , Animales , Conducta Animal , Diferenciación Celular/efectos de los fármacos , Edema , Femenino , Gónadas/citología , Gónadas/efectos de los fármacos , Iones/química , Masculino , Percloratos/química , Glándula Tiroides/efectos de los fármacos , Glándula Tiroides/patología , Factores de Tiempo , Xenopus laevis/embriología , Xenopus laevis/crecimiento & desarrolloRESUMEN
Larval Xenopus laevis were exposed to one of four concentrations of atrazine (0, 1, 10, or 25 microg/L, 11 replicate tanks per treatment, 60-65 larvae per replicate) dissolved in an artificial pond water (frog embryo teratogenesis assay- Xenopus [FETAX]) medium beginning 48 h after hatching until the completion of metamorphosis. Separate groups of larvae (six replicate tanks per treatment, 60-65 larvae per replicate) were exposed to estradiol (100 microg/L), dihydrotestosterone (100 microg/L), or ethanol vehicle control dissolved in FETAX medium. None of the treatments affected posthatch mortality, larval growth, or metamorphosis. There were no treatment effects on sex ratios except for estradiol, which produced a greater percentage of female offspring. Exposure to either estradiol or 25 microg atrazine/L increased the incidence of intersex animals based on assessment of gonadal morphology. Atrazine did not reduce the size of the laryngeal dilator muscle, a sexually dimorphic muscle in this species. We conclude that environmentally relevant concentrations of atrazine do not influence metamorphosis or sex ratios and do not inhibit sexually dimorphic larynx growth in X. laevis. The incidence of atrazine-induced intersex animals was small (<5%) and occurred only at the greatest concentration of atrazine tested, a concentration that is rarely observed in surface waters in the United States.
Asunto(s)
Atrazina/toxicidad , Gónadas/crecimiento & desarrollo , Laringe/crecimiento & desarrollo , Metamorfosis Biológica/efectos de los fármacos , Contaminantes Químicos del Agua/toxicidad , Animales , Dihidrotestosterona/toxicidad , Trastornos del Desarrollo Sexual/inducido químicamente , Estradiol/toxicidad , Femenino , Gónadas/efectos de los fármacos , Larva/efectos de los fármacos , Larva/crecimiento & desarrollo , Laringe/efectos de los fármacos , Masculino , Músculos/efectos de los fármacos , Razón de Masculinidad , Xenopus laevisRESUMEN
Embryos and larvae of the South African frog Xenopus laevis were exposed to ammonium perchlorate (AP) or control medium for 70 d. The dosage levels (59 ppb, 14,140 ppb) bracketed a range of perchlorate concentrations measured in surface waters at the Longhorn Army Ammunition Plant (LHAAP) in Karnack, Texas, USA. The experiment also included a 28-d nontreatment recovery period to assess the reversibility of AP effects. There were no significant effects of AP on mortality or hatching success. There were no effects of AP on developmental abnormalities such as bent/asymmetric tails or edema. Ammonium perchlorate inhibited forelimb emergence, the percentage of animals completing tail resorption, and hindlimb development during the 70-d exposure period. Only the upper AP concentration reduced whole-body thyroxine content, whereas both concentrations caused significant hypertrophy of the thyroid follicular epithelium. Both concentrations of AP caused a skewed sex ratio, significantly reducing the percentage of males at metamorphosis. The effects of AP on metamorphosis and thyroid function were reversed during the 28-d nontreatment recovery period. We conclude that AP inhibits thyroid activity and alters gonadal differentiation in developing X. laevis. These effects were observed at concentrations at or below concentrations reported in surface waters contaminated with ammonium perchlorate, suggesting that this contaminant may pose a threat to normal development and growth in natural amphibian populations.
Asunto(s)
Metamorfosis Biológica/efectos de los fármacos , Percloratos/efectos adversos , Compuestos de Amonio Cuaternario/efectos adversos , Diferenciación Sexual/efectos de los fármacos , Razón de Masculinidad , Glándula Tiroides/efectos de los fármacos , Glándula Tiroides/patología , Contaminantes Químicos del Agua/efectos adversos , Animales , Femenino , Gónadas/efectos de los fármacos , Gónadas/embriología , Larva/crecimiento & desarrollo , Masculino , Pruebas de Función de la Tiroides , Xenopus laevis/embriología , Xenopus laevis/crecimiento & desarrolloRESUMEN
We determined whether environmentally relevant concentrations of ammonium perchlorate alter development and metamorphosis in Xenopus laevis. Eggs and larvae were exposed to varying concentrations of ammonium perchlorate or control medium for 70 d. Most treatment-related mortality was observed within 5 d after exposure and was due in large part to reduced hatching success. The 5- and 70-d median lethal concentrations (LC50s) were 510 +/- 36 mg ammonium perchlorate/L and 223 +/- 13 mg ammonium perchlorate/L, respectively. Ammonium perchlorate did not cause any concentration-related developmental abnormalities at concentrations below the 70-d LC50. Ammonium perchlorate inhibited metamorphosis in a concentration-dependent manner as evident from effects on forelimb emergence, tail resorption, and hindlimb growth. These effects were observed after exposure to ammonium perchlorate concentrations in the parts-per-billion range, at or below concentrations reported in surface waters contaminated with ammonium perchlorate. Ammonium perchlorate significantly inhibited tail resorption after a 14-d exposure in the U.S. Environmental Protection Agency (U.S. EPA) Endocrine Disruptor Screening and Testing Committee (EDSTAC) Tier I frog metamorphosis assay for thyroid disruption in amphibians. We believe that ammonium perchlorate may pose a threat to normal development and growth in natural amphibian populations.