Figure 5. pH-dependent changes in macroscopic currents, P_open and single channel conductance.

Whereas P_open (▵, n = 4) decreases with low pH_i, the Kir4.1 currents (□, n = 5) increase with a modest intracellular acidification (pH 6.6, 7.0), and are strongly inhibited by further acidification. At pH 5.4–6.6, current response to low pH is almost the same as the P_open, both of which can be expressed with the same Hill equation (continuous line): P_open = 1/{1 + (pK/pH_i)^n_H}, where pK is the midpoint pH value for channel inhibition, and n_H the Hill coefficient. The pK and n_H here are pH 5.95 and 2.7, respectively. In contrast to P_open, single channel conductance (g,^, n = 5) increases with low pH_i. Its change can be also expressed using a Hill equation (dashed line) with pK 6.8 and n_H 2.3. Based on the percentage changes in conductance and P_open, theoretical currents (⋆) are calculated at each pH point using Ohm's law and an equation: I = i P_openN, where I is the macroscopic current, i single channel currents, and N the number of channels. Data are presented as means ±s.e.m.