Simulations of forward transport by a single NHE1 dimer using the parallel (A) and serial (B) coupling models. The top panels give the simulated extracellular Na dependence of NHE activity at cytoplasmic pH values of 6.0, 6.8, 7.2, and 7.6. Data points are from the insets in Fig. 3. Bottom panels give the simulated cytoplasmic pH dependencies of NHE activity at extracellular Na concentrations of 140, 40, 20, 10, and 5 mM. Data points are replotted from the linear proton concentration plot in Fig. 4 B and scaled to the simulations. For the parallel model in A, the contributions of Mode 1 and Mode 2 to total proton transport are given.