Figure 4.

The effect of removal of divalent cations from the extracellular solution on HERG channel gating. (A) The voltage clamp protocol is shown at the top. The membrane potential was held at −80 mV, then stepped to +50 mV for 2 s before hyperpolarizing to −120 mV. Two recordings from the same cell are shown in control (1.8 mM Ca²⁺ and 1 mM Mg²⁺, dark solid line) and divalent free solutions (2 mM EGTA, dashed line). Similar results were obtained in four additional cells. (B) Deactivation persists in very low extracellular Ca²⁺ concentrations, and the deactivation rate is independent of Ca²⁺ concentration over five orders of magnitude. The voltage clamp protocol in A was used to measure deactivation. A monoexponential function was then fit to the deactivating tail current recorded at −120 mV. The resulting time constants are plotted versus extracellular Ca²⁺ concentration. The concentrations tested were: 5 mM (n = 5), 2 mM (n = 6), 1 mM (n = 5), 500 μM (n = 4), 100 μM (n = 2), 1 μM (n = 2), 100 nM (n = 4), and no added divalent cations that are estimated (see methods) to be 2.5 nM Ca²⁺ (n = 5). The error bars represent SEM. The solid curve was generated with a logistic equation with an IC₅₀ value of 2.5 mM. Mg²⁺ was omitted from all the external solutions used in B. Ca²⁺ concentrations were calculated as described in methods.