Extracellular Na and cytoplasmic proton dependence of NHE1 in voltage-clamped CHO cells. (A) Composite data from Fig. 3 plotted with standard errors. (B) Average proton fluxes at different Na concentrations plotted in dependence on the cytoplasmic free proton concentration. Lines in the plot represent the best fit of data to a Michaelis-Menton relation. (C) Parameters of the Hill equations fitted to the individual records in Fig. 3. Top plot, Hill coefficients; middle plot, half maximal extracellular Na concentrations; bottom plot, maximal proton fluxes. The open circle gives the Hill coefficient for the maximal fluxes extrapolated in B for the different Na concentration (i.e., the bottom plot in D). (D) Extracellular Na dependence of parameters from the Michaelis-Menton fits given in B. Top plot, half-maximal cytoplasmic proton concentration determined for different extracellular Na concentrations; bottom plot, extrapolated maximal proton fluxes.