The extracellular Na dependence of proton flux mediated by NHE1 in 25 CHO cells with a cytoplasmic pH of 6.0 (eight cells), 6.8 (five cells), 7.2 (six cells), and 7.6 (six cells). Data points in each experiment represent the average of 5–20 flux determinations. The lines connect the calculated values from the best fits of data points to Hill equations. The insets illustrate the two strikingly different wave forms obtained at pH 6.0 and 7.2 with the corresponding best fits to Hill equations. At pH 6.0, the extracellular Na dependence often appeared biphasic, showing an apparent high affinity component, such that the overall Hill slope <1. At pH 7.2, the extracellular Na concentration dependence of proton flux showed cooperativity with Hill slopes of ∼2. Composite data shown in Fig. 4 C and 5 document the significance of these differences.