Figure 2. Effects of external H⁺ and Ni²⁺ on Kv1.5 current.

In all panels, superimposed grey traces represent current recorded during 5 s voltage steps to +50 mV from −80 mV following prolonged exposure (20–40 s) to low pH or [Ni²⁺]. Black lines represent the fits of the current decay to a mono-exponential function with a time constant denoted by τ_inact. Each panel represents an experiment on a different cell. A, in 0 mm, low pH decreases both peak Kv1.5 current and τ_inact. Pooled data from 4 cells gave a pK_a of 6.9 and a Hill coefficient of 1.4. B, increasing [K⁺]_o antagonizes the effect of low pH on peak test current. In 3.5 mm the pK_a for the decrease in peak current was 6.2, with a Hill coefficient of 1.8 (n = 4 cells). C, Ni²⁺ also causes a concentration-dependent decrease in peak current but this is associated with relatively smaller decreases of the test current decay rate. From 4 cells, the fit of the [Ni²⁺] dependence of the mean normalized peak current amplitude to the Hill equation gave K_d and Hill coefficient values of 0.034 mm and 0.85, respectively. D, in 3.5 mm the K_d for the [Ni²⁺]-dependent decrease of peak current amplitude was 0.52 mm; the Hill coefficient was 1.2 (n = 4 cells). For reasons unclear to us, these K_d values for the Ni²⁺ effect are lower than those previously reported (Kwan et al. 2004).