Simulations of reverse mode activity for the parallel (A) and serial (B) exchange models. Simulated extracellular pH dependence of NHE activity at cytoplasmic Na concentrations of 140 and 10 mM are given in the top panels with data points replotted from Fig. 8 and scaled to the simulations. Simulated cytoplasmic Na dependence of reverse NHE activity at extracellular pH values of 6.0 and 8.2 are given in the bottom panels together with data points from Green et al. (1988) for the cytoplasmic Na dependence of reverse NHE1 activity, fitted to a Hill equation with a slope coefficient of 2.