Torsades de pointes (TdP) is a potentially fatal form of ventricular arrhythmia that occurs under conditions where cardiac repolarization is delayed (as indicated by prolonged QT intervals from electrocardiographic recordings). A likely mechanism for QT prolongation and TdP is blockade of the rapid component of the cardiac delayed rectifier K⁺ current (I_Kr), which is encoded by HERG (human ether-a-go-go-related gene). The gastroprokinetic agent cisapride is a potent blocker of HERG currents and serious cardiac arrhythmias and deaths from TdP and ventricular fibrillation have been reported in patients taking cisapride. The aim of the present study was to compare the effects of the gastroprokinetic agents domperidone and metoclopramide on HERG channels transiently expressed in human embryonic kidney (HEK 293) cells using the whole-cell configuration of the patch-clamp technique. Both domperidone and metoclopramide concentration-dependently blocked HERG currents, and the following values were calculated for IC₅₀ (the concentrations causing half-maximal inhibition) and n (the Hill coefficient): 57.0 nmol/l and 0.99 for domperidone, 5.4 µmol/l and 0.95 for metoclopramide. The observation that the extent of block of HERG currents by domperidone increased at more positive membrane potentials whereas block of HERG currents by metoclopramide displayed a smaller degree of voltage dependency seems to indicate that domperidone and metoclopramide have distinct binding sites on HERG channels. In conclusion, the potency for block of HERG currents is about 100-fold lower for metoclopramide when compared to domperidone.

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HERG (human ether-a-go-go-related gene) encodes the Kv11.1 protein alpha-subunit that underlies the rapidly activating delayed rectifier K+ current (IKr) in the heart. Alterations in the functional properties or membrane incorporation of HERG channels, either by genetic mutations or by administration of drugs, play major roles in the development of life-threatening torsades de pointes cardiac arrhythmias. Visualization of ion channel localization is facilitated by enhanced green fluorescent protein (EGFP) tagging, but this process can alter their properties. The aim of the present study was to characterize the electrophysiological properties and the cellular localization of HERG channels in which EGFP was tagged either to the C terminus (HERG/EGFP) or to the N terminus (EGFP/HERG). These fusion constructs were transiently expressed in human embryonic kidney (HEK) 293 cells, and the whole-cell patch-clamp configuration and a confocal laser scanning microscope with primary anti-HERG antibodies and fluorescently labeled secondary antibodies were used. For EGFP/HERG channels the deactivation kinetics were faster and the peak tail current density was reduced when compared to both wild-type HERG channels and HERG/EGFP channels. Laser scanning microscopic studies showed that both fusion proteins were localized in the cytoplasm and on discrete microdomains in the plasma membrane. The extent of labeling with anti-HERG antibodies of HEK 293 cells expressing EGFP/HERG channels was less when compared to HERG/EGFP channels. In conclusion, both electrophysiological and immunocytochemical studies showed that EGFP/HERG channels themselves have a protein trafficking defect. HERG/EGFP channels have similar properties as untagged HERG channels and, thus, might be especially useful for fluorescence microscopy studies.