Summary of apparent discrepancies between this study and previous studies of NHE. (A) Symbols reiterate the cytoplasmic pH dependence obtained with 140 mM of extracellular Na from composite data using different cells (open squares) and from experiments in which the cytoplasmic pH was changed (open circles). Simulated results are given as gray lines for 140 and 2 mM of extracellular Na, whereby the fluxes are scaled and plotted as a percentage of maximum. Dotted lines give the range of “typical” pH–flux relations from the literature. (B) A simple Michaelis-Menton curve is given as pseudo-data points. The line that fits the data points is the sum of two Hill equations with slope coefficients of two, as given in the figure. (C) Data points give the maximal proton fluxes obtained from analysis of the cytoplasmic proton concentration dependence of forward NHE activity in Fig. 4. The curve describing the relation is the sum of two Hill equations with slope coefficients of two, as given in the figure.