Figure 3.

Na⁺ and Ca²⁺ modulation is not required for proton regulation. (A) Representative outward currents recorded from oocytes expressing mutant K229Q-E516L, which lacks both Na⁺ and Ca²⁺ regulation. Acid pH inhibits K229Q-E516L ionic currents, indicating the pH modulation involves regions of NCX not associated with either Na⁺ or Ca²⁺ regulation. The lines below the traces indicate solution changes. (B and C) WT and K229Q-E516L currents were recorded after exposing the patch to the indicated pH values for 20 to 25 s before Na⁺ application. This allows us to investigate proton inhibition before (peak) and after (steady state) the development of Na⁺-dependent inactivation. Ca²⁺ is maintained constant throughout the recordings. Note that K229Q-E516L lacks the current decay caused by intracellular Na⁺ but still responds to changes in H⁺ concentration. Accordingly, K229Q-E516L peak and steady-state currents are essentially the same. (D) Dose–response curves for cytoplasmic H⁺ for WT peak and steady-state currents (peak, □; steady state, ■) and mutants K229Q (○) and K229Q-E5 16L (△). The K229Q-E516L H⁺ dose–response curve is superimposable with that of WT exchanger measured at peak, before the development of the Na⁺-dependent inactivation. Error bars represent SEM.