Fig. 2.

Rapid blockade of MthK channels by cytoplasmic divalent cations. (A) Representative currents at 120 mV with the indicated concentrations of divalent cation. In these experiments and subsequent experiments, channels are activated using 100 μM Cd²⁺, and control indicates recordings and results obtained without the addition of Mg²⁺, Ca²⁺, or Sr²⁺. (B) Unitary current amplitude vs. voltage over a range of [Mg²⁺], [Ca²⁺], and [Sr²⁺] as indicated. Smooth curves are drawn for display only. (C) K_app(V) vs. voltage for experiments with Mg²⁺, Ca²⁺, and Sr²⁺. Fractional current (relative to amplitude without added divalent) was measured over a range of depolarized voltages and fitted with Hill equations to estimate K_app(V). K_app(V) vs. voltage relations values were fit with an equilibrium block model, K_app(V) = K_app(0)exp(−zδVF/RT), to estimate K_app(0) and zδ. The similar zδ-values of ∼0.45 e₀ for [Mg²⁺]_i, [Ca²⁺]_i, and [Sr²⁺]_i suggest that the fast blocking site for these ions is at the same location in the conduction pathway, whereas the estimated K_app(0) values are consistent with a lower apparent affinity of Sr²⁺ and Ca²⁺ compared with Mg²⁺.