INHIBITION OF IODIDE TRANSPORT IN RAT-THYROID CELLS USING N-SUBSTITUTED ANTHRANILIC ACID-DERIVATIVES

The purpose of this study was to test the effects of chloride channel blockers on iodide uptake in thyroid cells, in the hope of eventually using these blockers to identify and isolate a putative iodide transpo rter, The chloride channel blockers used in this report are derivative s of N-substituted anthranilic acid and were synthesized using publish ed procedures, For these studies FRTL-5 cells, a line of continuous-gr owing rat thyroid cells, were used as a model system to study effects on iodide transport. In these cells, there are at least two ways for t ransmembrane iodide movements, a sodium-dependent influx step and a pr oposed channel that normally mediates iodide efflux, Two derivatives s tudied decreased iodide accumulation in FRTL-5 cells, but were found a lso to lower intracellular pH and ATP levels, To simplify interpretati on of the effect of the drugs on iodide transport, we extended the stu dies using plasma membrane vesicles made from pig thyroid, Iodide entr y in these vesicles depended on a sodium gradient and was independent of ATP levels, Iodide transport in plasma membrane vesicles and FRTL-5 cells was measured at 30 sec when the uptake was nearly linear and th erefore likely to reflect iodide entry, The uptake was measured using three concentrations of iodide and three of drug. Kinetic analysis of the data described a competitive inhibition by the drugs with a K-i of approximately 250 mu M. In summary, N-substituted anthranilic acid de rivatives reversibly inhibit iodide entry in FRTL-5 cells and pig plas ma membrane vesicles, Because of their ease of synthesis and modificat ion, these derivatives are potentially useful probes for isolation of the NaI symporter in thyroid.