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

Abstract

Summary

1. One hundred platyfish, Xiphophorus (Platypoecilus) maculatus, 12 pretreated with potassium iodide, were injected with a tracer dose of carrier-free I¹³¹ and sacrificed at intervals up to 144 hours after injection. Groups of 4 fish were processed together. The lower jaws, containing the thyroidal tissue, were removed, hydrolysed, and measured for I¹³¹ uptake. Such hydrolysates then were analyzed by chromatographic technics and the relative rates of synthesis of monoiodotyrosine, diiodotyrosine, and thyroxine were determined. 2. The thyroid tissue accumulated about one-third of the injected isotope. Uptake was rapid but loss was slow so that 144 hours after injection 15% of the I¹³¹ was still held in the thyroid. Thyroid tissue of fish kept in 60 p.p.m. KI solution accumulated only 3% of the injected dose of I¹³¹. 3. The loss of radioiodine into the water was much more rapid in fish treated with KI than in the others. 4. As in mammals, the thyroid first synthesized monoiodotyrosine, then diiodotyrosine, and finally, thyroxine. The synthesis of thyroxine was relatively slow. A maximum value of 15% of the radioiodine in the thyroid was converted to thyroxine under the experimental conditions employed.