Figure 5. Compared to its effect on the wild type HCN2 channel extracellular Cs+ blocks the T400 channel with greater sensitivity and lesser dependence on voltage.

(A) Plots of instantaneous I_f versus voltage in cells expressing HCN2 or T400, determined using the voltage protocol and analysis described in Fig. 1, before and during perfusion with 0.03 mM Cs⁺ with 135 mM K+ and 5.4 mM Na+ in the extracellular solution and 130 mM K+ and 10 mM Na+ in the intracellular solution (HCN2, filled squares, HCN2 + Cs⁺, filled circles, n = 5 cells; T400 open squares, T400 + Cs⁺, open circles, n = 6 cells). This type of experiment was carried out for a range of Cs+ concentrations and test voltages, from which the ratio of blocked current was determined and plotted against Cs+ concentration (example for −60 mV shown in “B”, below). (B) Plot of the ratio of blocked current at −60 mV, obtained from instantaneous I_f-V curves as shown in “A”, versus Cs+ concentration, for HCN2 (filled squares) and T400 (open circles). The values for the ratio of blocked current represent means ± s.e.m. Solid lines represent fits of the data with the Hill equation (Equation 2), which gave values for IC₅₀ and Hill factor (n); this analysis was carried out for a range of test voltages from which IC50 values were obtained and plotted against voltage (“C”, below). (C) Plot of IC₅₀ values, obtained from Hill plots as determined in “B”, versus test voltage for HCN2 (filled squares) and T400 (open circles) channels. The IC₅₀ values represent means ± s.e.m.