Wk. Ho et al., VOLTAGE-DEPENDENT BLOCKADE OF HERG CHANNELS EXPRESSED IN XENOPUS OOCYTES BY EXTERNAL CA2+ AND MG2+, Journal of physiology, 507(3), 1998, pp. 631-638
1. We expressed the human eag-related gene (HERG), which is known to e
ncode the delayed rectifier K+ current (I-Kr) in cardiac muscle, in Xe
nopus oocytes. Using a two-microelectrode voltage clamp technique, the
effect of external Ca2+ and Mg2+ on the HERG current (I-HERG) was inv
estigated. 2. When [Ca2+](o) was increased, the amplitude of outward I
-HERG elicited by depolarization decreased, and the rate of current on
set slowed. The rate of current decay observed on repolarization was g
reatly accelerated. The threshold and fully activated potential of I-H
ERG shifted to a more positive potential. On the other hand, the inact
ivation property represented by the negative slope of the I-V curve an
d the instantaneous conductance of I-HERG were little affected by [Ca2
+](o). 3. The effect of [Ca2+](o) on I-HERG can be interpreted using t
he 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 calcul
ated from this value is 0.86. 4. Blockade by a low concentration of Ca
2+ (0.5 mM) was inhibited by increasing [K+](o) (from 2 to 20 mM), whe
reas blockade by a high concentration of Ca2+ (5 mM) was not affected
by varying [K+](o), indicating that there is competition between perme
ating ions and blocking ions. 5. The effect of [Mg2+](o) on I-HERG 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 ga
te.