Na⁺ transport by the mouse Ae4 exchanger. Intracellular Na⁺ levels were estimated using the Na⁺ indicator SBFI. A marked increase in intracellular [Na⁺] was observed when the external [Cl⁻] was reduced in Ae4-transfected (black circles; n = 15; changing solution A to B [Table 1]) but not in nontransfected (green circles; n = 13) cells. The increase in intracellular [Na⁺] in Ae4-transfected cells was reversed by increasing the external [Cl⁻] (changing solution B to A [Table 1]). Furthermore, addition of the Na⁺-permeable ionophore gramicidin A at the end of the experiment demonstrated that SBFI was responsive to changes in intracellular Na⁺. Results are presented as the mean ± SEM.