Iodine Suppression of Iodide Uptake in FRTL-5 Thyroid Cells

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Abstract
Exposure of FRTL-5 cells to iodide (I-) in excess of 3 .mu.M suppresses the concentrative uptake of I-. The depression of I- uptake measured at the steady state is due to a decrease in the rate of I- influx and not to an effect on I- efflux. Exposure to NaI is associated with decreased T4 secretion and also depressed Na+-dependent amino acid accumulation. The depression in I- and amino acid transports increases proportionately with the duration of exposure and concentration of I- used but is not associated with alterations in FRTL-5 cell cAMP levels. The I- suppression effect is blocked, however, when methimazole is present during the incubation with NaI. In agreement with studies in vivo, I- suppression in FRTL-5 cells appears to depend on an intermediate in the organification process and to be independent of a TSH-induced cAMP-mediated action.