Figure 1.

Cytosolic protons regulate NCX current. (A–C) Giant-patch recordings from oocytes expressing the cardiac NCX1.1 in the presence of the indicated intracellular pH before (A and B) and after (C) chymotrypsin digestion. The lines below traces indicate solution changes. Outward currents were generated by rapidly replacing 100 mM intracellular Cs⁺, pH 7, with 100 mM Na⁺, pH 7.5. Ca²⁺ is maintained constant during the recording (contaminant Ca²⁺ measured at 4 µM). WT current peaked and remains stable until Na⁺ was removed from the intracellular side of the patch (A). In the presence of pH 7.5 the WT exchanger does not show Na⁺-dependent inactivation, as alkalization abrogates it (Hilgemann et al., 1992b). A step decrease in pH from 7.5 to 6.5 evoked a slow inhibition of the current (B), which could be reversed by restoring pH 7.5. Cytoplasmic perfusion of 1 mg/ml chymotrypsin removes the regulatory properties of NCX (Hilgemann, 1990). After treatment, cytoplasmic protons are less effective in inhibiting NCX currents, suggesting that proteolytically accessible cytoplasmic components are involved in pH regulation (C). (D) Steady-state currents measured before (■) and after (□) chymotrypsin digestion were plotted versus pH values. Each point corresponds to the mean of five to nine experiments. Error bars represent SEM.