MANZON, R.G.*; YOUSON, J.H.: In vitro Inhibition of Thyroidal Activity in the Larval Lamprey Endostyle by Potassium Perchlorate.
The larval lamprey is unique among vertebrates because it lacks a follicular thyroid gland. In larval lampreys, thyroid hormones are produced in the endostyle, a subpharyngeal gland. Moreover, contrary to the amphibian paradigm, lamprey metamorphosis coincides with a decline in serum thyroid hormone (TH) titers and goitrogens (anti-thyroid agents), which depress TH titers, can induce precocious metamorphosis in lampreys. In this study, an in vitro experimental system was devised to assess the direct effects of the goitrogen potassium perchlorate (KClO4) on radioiodide uptake and organification by the larval lamprey endostyle. Organification refers to the incorporation of iodide into lamprey thyroglobulin (Tg), an essential step in TH synthesis. A single iodoprotein, designated as lamprey Tg, was identified in the endostylar homogenates with polyacrylamide gel electrophoresis followed by autoradiography and Western blotting. Lamprey Tg was immunoreactive with a rabbit anti-human Tg antibody and had an electrophoretic mobility similar to porcine Tg. When KClO4 was added to the incubation medium, both iodide uptake and organification by the endostyle were inhibited, as determined by gamma counting and gel-autoradiography, respectively. Western blotting showed that KClO4 lowered the total amount of lamprey Tg in the endostyle. Based on the results of this in vitro investigation, we conclude that KClO4 acts directly on the larval lamprey endostyle to inhibit thyroidal activity, thus suppressing the synthesis of thyroxine and triiodothyronine and resulting in a decrease in the serum levels of these two hormones. Funded by NSERC.
