Responses of growth, malformation, and thyroid hormone-dependent genes expression in Bufo gargarizans embryos following chronic exposure to Pb 2+

Abstract

The aim of this study was to examine the adverse effects of lead (Pb) exposure on Bufo gargarizans embryos. The 96 h-LC₅₀ of Pb²⁺ for B. gargarizans embryos was determined to be 26.6 mg L⁻¹ after an acute test. In the chronic test, B. gargarizans embryos at Gosner stage 3 were exposed to 10~2000 μg Pb²⁺ L⁻¹ during embryogenesis. Total length, weight, developmental stage, and malformation were monitored. In addition, the transcript levels of type II and type III iodothyronine deiodinase (Dio2 and Dio3) and thyroid hormone receptors (TRα and TRβ) were determined to assess the thyroid-disrupting effects of Pb²⁺. Slightly increased growth and development of B. gargarizans embryos were observed at low concentrations of Pb²⁺ (10, 50, and 100 μg L⁻¹), while retarded growth and development were found at high concentrations of Pb²⁺ (1000 and 2000 μg L⁻¹). In addition, Pb²⁺ exposure induced morphological abnormalities, which were characterized by edema at tail, wavy fin, abdominal edema, stunted growth, hyperplasia, and axial flexures in B. gargarizans embryos. Furthermore, our results showed that exposure to 2000 μg Pb²⁺ L⁻¹ decreased the transcript levels of Dio2, TRα, and TRβ, but it increased Dio3 mRNA level. In contrast, exposure to 50 μg Pb²⁺ L⁻¹ increased TRα mRNA level and decreased Dio3 mRNA level. These results suggested that Pb²⁺ might have thyroid-disrupting effects, leading to the disruption of growth and development in B. gargarizans embryos.