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

1. We expressed the human eag-related gene (HERG), which is known to encode the delayed rectifier K+ current (IKr) in cardiac muscle, in Xenopus oocytes. Using a two-microelectrode voltage clamp technique, the effect of external Ca2+ and Mg2+ on the HERG current (IHERG) was investigated. 2. When [Ca2+]o was increased, the amplitude of outward IHERG 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 IHERG 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 IHERG were little affected by [Ca2+]o. 3. The effect of [Ca2+]o on IHERG can be interpreted using the channel blockade model. The blockade is voltage dependent; smaller dissociation constants (KM) at more negative potentials indicate that block is facilitated by hyperpolarization. KM changes e-fold for 14.5 mV and the fractional electrical distance of the binding site calculated from this value is 0.86. 4. 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. 5. The effect of [Mg2+]o on IHERG was qualitatively similar to that of [Ca2+]o, but the potency was lower. 6. 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.