The electrophysiological characteristics of the Na+/I- symporter were examined using the Chinese hamster ovary (CHO) cell line, which was transfected with the rat Na+/I- symporter gene and stably expressed the Na+/I- symporter. In this cell line, iodide uptake was dependent on Na+, and kinetic studies revealed that the K(m) for iodide was 35 microM, similar to that of FRTL-5 cells. The maximal velocity at the cell protein level was 6- to 10-fold higher than in FRTL-5 cells. ClO-4 and SCN- dose-dependently inhibited iodide uptake in a competitive manner. Electrophysiological characteristics were examined using the whole-cell patch-clamp technique. The holding current at-40 mV rapidly shifted inwardly when the cells were perfused with 1 mEq I- or SCN-. The inward current induced by 1 mEq I- did not increase when bathing solution was replaced with a Tyrode solution with 10 mEq I-, indicating that 1 mEq I- was a saturating amount. The inward current induced by 1 mEq I- increased 1.5-fold by changing the bathing solution to a Tyrode solution containing 1 mEq I- and 1 mEq SCN-. The inward current induced by 0.5 mEq SCN- decreased when the bathing solution was changed to a Tyrode solution containing 0.5 mEq SCN- and 10 mEq I-. These findings indicated that the I- ion and the SCN- ion were carried by the NA+/ I- symporter with at least two Na+ ions. The current induced by the transport of SCN- was larger than that induced by the transport of I-, possibly because the number of Na+ ions that was carried with one SCN- ion was larger than the number of Na+ ions carried with one I- ion. Surprisingly, the perfusion of ClO-4 did not induce an inward current, indicating that ClO-4 bound to the Na+/I- symporter, but was not carried by it, or that one ClO-4 ion was carried with one Na+ ion.