Cytotoxic effects of iodide on thyroid cells: difference between rat thyroid FRTL-5 cell and primary dog thyrocyte responsiveness.

In order to investigate whether the cytotoxic effect of iodide observed in the thyroid gland represented an apoptotic phenomenon, in vitro experiments were performed using the FRTL-5 thyroid cell line and dog thyrocytes in primary culture. These cells were exposed to iodide under various incubation conditions. Apoptosis was assessed through the analysis of DNA breakdown, i.e. the electrophoresis of internucleosomal DNA fragments generating a typical "ladder" and quantification of prelabelled DNA cleavage products. The FRTL-5 cells appeared to be sensitive only to high doses of iodide, far in excess of physiological levels. They exhibited the different characteristics of two different cell death phenomena: apoptosis and necrosis. The toxicity of iodide appeared to be partially relieved by anti-thyroid agents. This effect constitutes an additional example of the general paradigm of iodide action through oxidized intermediates. In contrast dog thyrocytes in primary culture did not appear to be sensitive to iodide under similar incubation conditions; species differences and/or types of culture could account for these discrepant effects.