Human ether-a-go-go-related gene (hERG) channels play a critical role in cardiac action potential repolarization. The unintended block of hERG channels by compounds can prolong the cardiac action potential duration and induce arrhythmia. Several compounds not only block hERG channels but also enhance channel activation after the application of a depolarizing voltage step. This is referred to as facilitation. In this study, we tried to extract the property of compounds that induce hERG channel facilitation. We first examined the facilitation effects of structurally diverse hERG channel blockers in Xenopus oocytes. Ten of 13 assayed compounds allowed facilitation, suggesting that it is an effect common to most hERG channel blockers. We constructed a pharmacophore model for hERG channel facilitation. The model consisted of one positively ionizable feature and three hydrophobic features. Verification experiments suggest that the model well describes the structure-activity relationship for facilitation. Comparison of the pharmacophore for facilitation with that for hERG channel block showed that the spatial arrangement of features is clearly different. It is therefore conceivable that two different interactions of a compound with hERG channels exert two pharmacological effects, block and facilitation.
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