Studies on the Accumulation of Thyroxine in Isolated Bovine Thyroid Cells

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By the use of isolated bovine thyroid cells (ITC), accumulation of T4 in thyroid tissue has been studied. ITC were chosen to disclose this thyroid cell function per se by eliminating the effect of follicular structure and contaminated serum. Cell/medium ratio (C/M) of 125I-T4, used as an indicator of T4 accumulation, was 17.7 when ITC were incubated in Eagles solution. On the other hand, the ratio in the thyroid slice and erythrocytes was 1.1 and 3.0 respectively. By equilibration chamber method, T4 accumulated more in ITC than in erythrocytes. The observed accumulation of T4 in ITC was reversible when re-incubated with normal human serum. Displacement of 125I-T4 by non-labelled T4 was not observed at the range of added T4 from 10 to 1000 ital. Essentially the same result were obtained when the liver, thyroid slice or erythrocytes were studied. Therefore, the number of T4-binding sites appears to be very large. Increased deiodination of 125I-T4 was observed when 125I-T4 was incubated with ITC, whereas conversion of 125I-T4 to 125I-T3 was not remarkable. It is concluded that ITC obviously concentrate T4 by an entirely different mechanism from that in serum TBP.
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