Simulations of the inhibition of forward transport by counter-transported ions on the “cis” membrane side for the parallel (A) and serial (B) exchange models. Simulated inhibition of forward transport with ion concentrations as in Fig. 7 are given in the top panels, together with data points replotted from Fig. 7. Also scaled to the simulations are data from Dascalu et al. (1991) mentioned in the text. Bottom panels give the simulated inhibition of forward transport by cytoplasmic Na with 140 mM of extracellular Na, an extracellular pH of 8.2, and cytoplasmic pH values of 6.0, 6.8, 7.2, and 7.6. The data points are from experiments described previously for a cytoplasmic pH of 6.8 (Hilgemann et al., 2006).