Figure 7. The effects of [Cs⁺]_o and [K⁺]_o on the voltage dependence of τ_inact of outward Cs⁺ current recorded with 130 mm Cs₁⁺.

A and B, HERG Cs⁺ currents elicited by the voltage protocol shown above the current traces in external solution containing 5 mm Cs₀⁺ (A) or 20 mm Cs₀⁺ (B). HERG channels were initially inactivated at the holding potential of +60 mV. As with data in Fig. 4, a 10 ms repolarizing step to −100 mV allows HERG channels to recover from inactivation, and steps to a range of test potentials allows the inactivation time course to be observed. C, voltage dependence of time constant of the decay of outward Cs⁺ current at different [Cs⁺]_o. Data were fitted (continuous lines) to eqn (5). Cs₀⁺ increased τ_inact and modulated the voltage sensitivity of the τ_inact−V relationship. The free parameters for each fit are shown in Table 1. The mean K_d(0) and δ values are 1.7 ± 0.2 mm and 0.66 ± 0.05, respectively (n = 4). D, as in C, but with varying [K⁺]_o. The mean K_d(0) and δ values are 6.8 ± 2.8 mm and 0.63 ± 0.04, respectively (n = 4). Thus, with Cs₀⁺ and K₀⁺ the location of the binding site in the electrical field is similar but the affinity of the site for K⁺ is slightly lower.