Simulations of the cis–trans interactions of Na and H under near zero-trans conditions, as in Figs. 3–5,⁴⁵. (A) The dependence of half-maximal extracellular Na concentration of NHE activity on cytoplasmic pH under the conditions shown in Fig. 4 together with data points from Fig. 4 C. (B) The dependence of half-maximal cytoplasmic proton concentrations, based on fits of simulated data to Michaelis-Menton relations, on the extracellular Na concentration under the conditions of Fig. 4 together with data points from Fig. 4 D.