Figure 3.

Measurement of Na⁺ efflux. Untransfected HeLa cells were loaded with SBFI, and then I₃₄₀/I₃₈₀ data pairs were acquired continuously. Cells were exposed first to zero-K⁺ buffer (solution 3; segment ab), and then to zero-Na⁺ buffer (solution 2; segment bc), and finally to two different calibration buffers (mixed from solutions 4 and 5; cd and de). [Na⁺]_i values were computed from I₃₄₀/ I₃₈₀ ratios, the calibration data from this experiment, and Eq. 4. Because the calibration buffers in this experiment contained two ionophores, we applied a correction (see methods) so that the computed [Na⁺]_i values were comparable with those in experiments with three ionophores.