Voltage‐dependent blockade of HERG channels expressed in Xenopus oocytes by external Ca2+ and Mg2+

Abstract

We expressed the human eag‐related gene (HERG), which is known to encode the delayed rectifier K⁺ current (I_Kr) in cardiac muscle, in Xenopus oocytes. Using a two‐microelectrode voltage clamp technique, the effect of external Ca²⁺ and Mg²⁺ on the HERG current (I_HERG) was investigated. When [Ca²⁺]_o was increased, the amplitude of outward I_HERG elicited by depolarization decreased, and the rate of current onset slowed. The rate of current decay observed on repolarization was greatly accelerated. The threshold and fully activated potential of I_HERG shifted to a more positive potential. On the other hand, the inactivation property represented by the negative slope of the I‐V curve and the instantaneous conductance of I_HERG were little affected by [Ca²⁺]_o. The effect of [Ca²⁺]_o on I_HERG can be interpreted using the channel blockade model. The blockade is voltage dependent; smaller dissociation constants (K_M) at more negative potentials indicate that block is facilitated by hyperpolarization. K_M changes e‐fold for 14.5 mV and the fractional electrical distance of the binding site calculated from this value is 0.86. Blockade by a low concentration of Ca²⁺ (0.5 mM) was inhibited by increasing [K⁺]_o (from 2 to 20 mM), whereas blockade by a high concentration of Ca²⁺ (5 mM) was not affected by varying [K⁺]_o, indicating that there is competition between permeating ions and blocking ions. The effect of [Mg²⁺]_o on I_HERG was qualitatively similar to that of [Ca²⁺]_o, but the potency was lower. These results suggest that external Ca²⁺ and Mg²⁺ block the HERG channel in a voltage‐ and time‐dependent manner, resulting in a voltage dependence which has been regarded as a property of the activation gate.