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Intracellular Ca(2+) inhibits voltage-gated potassium channels of the ether à go-go (EAG) family. To identify the underlying molecular mechanism, we expressed the human version hEAG1 in Xenopus oocytes. The channels lost Ca(2+) sensitivity when measured in cell-free membrane patches. However, Ca(2+) sensitivity could be restored by application of recombinant calmodulin (CaM). In the presence of CaM, half inhibition of hEAG1 channels was obtained in 100 nM Ca(2+). Overlay assays using labelled CaM and glutathione S-transferase (GST) fusion fragments of hEAG1 demonstrated direct binding of CaM to a C-terminal domain (hEAG1 amino acids 673-770). Point mutations within this section revealed a novel CaM-binding domain putatively forming an amphipathic helix with both sides being important for binding. The binding of CaM to hEAG1 is, in contrast to Ca(2+)-activated potassium channels, Ca(2+) dependent, with an apparent K(D) of 480 nM. Co-expression experiments of wild-type and mutant channels revealed that the binding of one CaM molecule per channel complex is sufficient for channel inhibition.

ERG (ether-à-go-go-related gene) K+ channels are crucial in human heart physiology (h-ERG), but are also found in neuronal cells and are impaired in Drosophila 'seizure' mutants. Their biophysical properties include the relatively fast kinetics of the inactivation gate and much slower kinetics of the activation gate. In order to elucidate how the complex time- and voltage-dependent activation properties of ERG channels underlies distinct roles in excitability, we investigated different types of ERG channels intrinsically present in cells or heterologously expressed in mammalian cells or Xenopus oocytes. Voltage-dependent activation curves were highly dependent on the features of the eliciting protocols. Only very long preconditioning times produced true steady-state relationships, a fact that has been largely neglected in the past, hampering the comparison of published data on ERG channels. Beyond this technical aspect, the slow activation property of ERG can be responsible for unsuspected physiological roles. We found that around the midpoint of the activation curve, the time constant of ERG open-close kinetics is of the order of 10-15 s. During sustained trains of depolarizations, e.g. those produced in neuronal firing, this leads to the use-dependent accumulation of open-state ERG channels. Accumulation is not observed in a mutant with a fast activation gate. In conclusion, it is well established that other K+ channels (i.e. Ca2+-activated and M) control the spike-frequency adaptation, but our results support the notion that the purely voltage-dependent activation property of ERG channels would allow a slow inhibitory physiological role in rapid neuronal signalling.

The class III antiarrhythmic drug clofilium is known to block diverse delayed rectifier K+ channels at micromolar concentrations. In the present study we investigated the potency of clofilium and its tertiary analog LY97241 to inhibit K+ channels, encoded by the human ether-a-go-go related gene (HERG). Clofilium blocked HERG channels in a voltage-dependent fashion with an IC50 of 250 nM and 150 nM at 0 and +40 mV, respectively. LY97241 was almost 10-fold more potent (IC50 of 19 nM at +40 mV). Other cloned K+ channels which are also expressed in cardiac tissue, Kv1.1, Kv1.2, Kv1.4, Kv1.5, Kv4.2, Kir2.1, or I(Ks), were not affected by 100-fold higher concentrations. Block of HERG channels by LY97241 was voltage dependent and the rate of HERG inactivation was increased by LY97241. A rise of [K+]0 decreased both, rate of HERG inactivation and LY97241 affinity. The HERG S631A and S620T mutant channels which have a strongly reduced degree of inactivation were 7-fold and 33-fold less sensitive to LY97241 blockade, indicating that LY97241 binding is affected by HERG channel inactivation. In summary, the antiarrhythmic action of clofilium and its analog LY97241 appears to be caused by their potent, but distinct ability for blocking HERG channels.

1. The antipsychotic drug haloperidol can induce a marked QT prolongation and polymorphic ventricular arrhythmias. In this study, we expressed several cloned cardiac K+ channels, including the human ether-a-go-go related gene (HERG) channels, in Xenopus oocytes and tested them for their haloperidol sensitivity. 2. Haloperidol had only little effects on the delayed rectifier channels Kv1.1, Kv1.2, Kv1.5 and IsK, the A-type channel Kv1.4 and the inward rectifier channel Kir2.1 (inhibition < 6% at 3 microM haloperidol). 3. In contrast, haloperidol blocked HERG channels potently with an IC50 value of approximately 1 microM. Reduced haloperidol, the primary metabolite of haloperidol, produced a block with an IC50 value of 2.6 microM. 4. Haloperidol block was use- and voltage-dependent, suggesting that it binds preferentially to either open or inactivated HERG channels. As haloperidol increased the degree and rate of HERG inactivation, binding to inactivated HERG channels is suggested. 5. The channel mutant HERG S631A has been shown to exhibit greatly reduced C-type inactivation which occurs only at potentials greater than 0 mV. Haloperidol block of HERG S631A at 0 mV was four fold weaker than for HERG wild-type channels. Haloperidol affinity for HERG S631A was increased four fold at +40 mV compared to 0 mV. 6. In summary, the data suggest that HERG channel blockade is involved in the arrhythmogenic side effects of haloperidol. The mechanism of haloperidol block involves binding to inactivated HERG channels.