Figure 4.

Steady-state pH_L in the presence of the V-ATPase proton pump. (A) Steady-state pH_L as a function of initial luminal concentration for four different counterion flows: ClC-7 antiporter, Cl^–, K⁺, or Na⁺ channel. For each curve, the initial concentration of the ion being transported was varied from 1 to 200 mM. Thus, for simulations including Cl^– channels, the initial [Cl^–]_L was varied from 1 to 200 mM, and the steady-state pH_L was determined for each concentration. (B) The steady-state membrane potentials for the simulations in A. (C) Steady-state pH_L as a function of cytosolic Cl^– concentration for models containing either Cl^– channels or ClC-7 antiporters. (D) The steady-state membrane potentials for the simulations in C. All simulations included P_H+ of 6 × 10⁻⁵ cm/s and 300 V-ATPases. Simulations with ClC-7 antiporters included 5,000 transporters, and the model lysosome was spherical with a diameter of 0.68 µm (Van Dyke, 1993). In all simulations, the initial luminal concentrations of ions were equal to typical extracellular values, except for the concentrations being explicitly varied along the x axes. All other parameter values are given in Table 1.