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 transfected 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 than in FRTL-5 cells and that at single cell level was similar to 1000-fold higher. ClO4- and SCN- dose-dependently inhibited iodide uptake in a competitive manner. The K-i was 1.5 and 16 mu M. respectively. Iodide efflux from CHO-4J cells was apparently slower (t(1/2) = 15 min) than FRTL-5 cells (t(1/2) = 2 min). Electrophysiological characteristics were examined using the whole cell patch clamp technique. Rapid inward 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 cells will function as a new tool for sensitive analysis of iodide uptake. (C) 1996 Academic Press, Inc.