IN OVO EXPOSURE TO ENDOSULFAN ALTERS THYROID HORMONE HOMEOSTASIS IN JUVENILE FEMALE BROAD-SNOUTED CAIMAN (Caiman latirostris)

SCHUERI, F.D.; TAVALIERI, Y.E.; LUQUE, E.H.; MUÑOZ DE TORO, M.; GALOPPO, G.H.

Buenos Aires

Congreso; XXIV ANNUAL MEETING OF THE ARGENTINEAN SOCIETY OF BIOLOGY (SAB); 2022

Sociedad Argentina de Biología

In vertebrates, thyroid hormones, triiodothyronine (T3) and thyroxine (T4) have a role in the regulation of growth, development, metabolism, and reproduction. Circulating levels of T3 and T4 are, in turn, regulated by the hepatic deiodinases dio1 and dio2 (that catalyze the conversion of T4 into T3) and dio3 (that converts T3 into reverse-T3). Endosulfan (END), a persistent pesticide reported to be present in broad-snouted caiman eggs, has been described as a potential thyroid-disrupting compound. The aim of this study was to assess the effects of in-ovo exposure to a single dose of 20 ppm of END on thyroid hormone homeostasis in juvenile female caimans. Sixteen animals (8 for the CONTROL group, and 8 for the END group) were used in this work. For the thyroid histoarchitecture study, the percentage of the gland section represented by follicular epithelium, colloid, reabsorption vacuoles, and stroma was assessed by using an orthogonal grid. T3 and T4 plasma levels were assessed by electrochemiluminescence, and hepatic dio1 gene expression was assessed by qPCR using L8 as a housekeeping gene. Our results (presented as mean ± SEM) show that, although exposure to END did not induce changes in thyroid histoarchitecture, T4 circulating levels were higher in exposed animals (CONTROL 1.01 ± 0.08 μg/dL vs. END 1.19 ± 0.03 μg/dL; P = 0.017), which significantly modified the T3:T4 ratio (CONTROL 144.4 ± 5.0 vs. END 129.2 ± 3.3; P = 0.030). Regarding hepatic dio1 gene expression, although a decreasing trend was observed in END-exposed animals (CONTROL 40.85 ± 17.7 vs. END 9.91 ± 3.10), no statistical significance was achieved (P = 0.456). Our results demonstrate that exposure to END during key stages of embryo development increased the T4 plasma levels, possibly as a consequence of T4-altered hepatic metabolism. Although subtle decreased mRNA levels of dio1 could account for increased T4 circulating levels, other deiodinases, possibly dio2 could have a more relevant role in this effect. Our findings suggest that modified T4 circulating levels because of natural exposure to END could affect female broad-snouted caiman metabolism, growth, and reproduction. This, in turn, could negatively affect the caiman population dynamic and wetland ecosystem health.