ESTABLISHMENT AND CHARACTERIZATION OF A CHINESE-HAMSTER OVARY CELL-LINE, CHO-4J, STABLY EXPRESSING A NUMBER OF NA+ I- SYMPORTERS/

The cDNA of the Na+/I- symporter playing a key role in thyroid iodide transport was cloned very recently. To characterize its function, we t ransfected the Na+/I- symporter gene into Chinese hamster ovary (CHO) cells and established a cell Line stably expressing a number of nt Na/I- symporters, named CHO-45. Iodide uptake was dependent on [Na+] and reached a plateau within 30 min. Kinetic studies revealed that the K- m for iodide was 35 mu M, similar to that of FRTL-5 thyroid cells. The maximal velocity (V-max) at cell protein level was 6-10-fold higher t han in FRTL-5 cells and that at single cell level was similar to 1000- fold higher. ClO4- and SCN- dose-dependently inhibited iodide uptake i n a competitive manner. The K-i was 1.5 and 16 mu M. respectively. Iod ide efflux from CHO-4J cells was apparently slower (t(1/2) = 15 min) t han FRTL-5 cells (t(1/2) = 2 min). Electrophysiological characteristic s were examined using the whole cell patch clamp technique. Rapid inwa rd current was observed when CHO-U cells were perfused with 50-1000 mu M NaI, suggesting a transport stoichiometry of at least 2 Na+ per I-. The current-voltage relation revealed that this current was membrane potential-dependent. The reversal potential was very close to that of Na+ in agreement with dependency on the Na+ electrochemical gradient. CHO-4J cells with a slow iodide efflux, expressing a number of Na+/I- symporters whose characteristics are identical to those of FRTL-5 cell s will function as a new tool for sensitive analysis of iodide uptake. (C) 1996 Academic Press, Inc.