Utilization of Iodine by the Thyroid of the Platyfish, Xiphophorus (Platypoecilus) maculatus

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Abstract
One hundred platyfish, X. (Platypoecilus) maculatus, 12 pretreated with K iodide, were injd. with a tracer dose of carrier free I131 and sacrificed at intervals up to 144 hrs. after injn. Groups of 4 fish were processed together. The lower jaws, containing the thyroidal tissue, were removed, hydrolyzed, and measured for I131 uptake. Such hydrolysates then were analyzed by chromatographic techniques and the relative rates of synthesis of monoiodotyrosine, diiodotyrosine and thyroxine were detd. The thyroid tissue accumulated about one-third of the injd. isotope. Uptake was rapid but loss was slow, so that 144 hrs. after injn. 15% of the I131 was still held in the thyroid. Thyroid tissue of fish kept in 60 ppm. KI soln. accumulated only 3% of the injd. dose of I131. The loss of radioidoine into the water was much more rapid in fish treated with KI than in the others. As in mammals, the thyroid first synthesized monoiodotyrosine, then diiodotyrosine, and finally thyroxine. The synthesis of thyroxine was relatively slow. A max. value of 15% of the radioiodine in the thyroid was converted to thyroxine under the exptl. conditions employed.

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