Cytoplasmic Na dependence of NHE1 proton flux with 140 mM of extracellular Na, nominally no cytoplasmic Na, and with an extracellular pH of 8.2. Filled circles show average data from four individual experiments where pH_i was changed serially in the same cell via pipette perfusion (for details see Materials and methods). Open circles show data for 140 mM of extracellular Na obtained in the experiments presented in Fig. 3. The best Hill equation fit to all data points in the figure is given as a solid line.