Disparate Uptake of ^99mTcO₄ ^- and ¹²⁵I^- in Thyroid Cells in Culture

Summary

Pertechnetate (^99mTcO₄ ^-) is a large anion that is thought to be trapped but not organified by the thyroid. In order to determine its usefulness in discriminating trapping from organification in thyroid cell culture, the uptake of ^99mTcO₄ ^- has been compared to that of ¹²⁵I^- both in sheep thyroid cells and in FRTL5 cells, a functional rat thyroid cell line. We found that uptake of both isotopes was dependent on TSH or agents that raised intracellular cAMP levels and was displaceable with iodide in both cell types. However in the rat cell line ^99mTcO₄ ^- uptake was up to eight-fold higher than ¹²⁵I^- uptake, and ^99mTcO₄ ^- but not ¹²⁵I^- uptake was doubled by methimazole treatment. A considerable proportion of ^99mTcO₄ ^- but not ¹²⁵I^- taken up by the FRTL5 cell but not the sheep cells was precipitable with TCA. This proportion was increased by methimazole treatment. We also found marked differences in sensitivity to ouabain between the two cell types.

These results illustrate the differences between species in ¹²⁵I^- and ^99mTcO₄ ^- transport and support the concerns expressed by Socolow and Ingbar (1967) in using ^99mTcO₄ ^- as a direct and precise indicator of iodide transport activity.

Pertechnetate - Iodide - Thyroid - FRTL 5