© 1999 by European Society of Cardiology
Copyright © 1999, European Society of Cardiology
Norpropoxyphene-induced cardiotoxicity is associated with changes in ion-selectivity and gating of HERG currents
Laboratory of Toxicology, Faculty of Pharmaceutical Sciences, University of Leuven, Van Evenstraat 4, B-3000 Leuven, Belgium
* Corresponding author. Tel.: +32-16-32-3403; fax: +32-16-32-3405 jan.tytgat{at}farm.kuleuven.ac.be
Objective: Norpropoxyphene (NP) is a major metabolite of propoxyphene (P), a relatively weak µ-opioid receptor agonist. Toxic blood concentrations ranging from 3 to 180 µmol/l have been reported and the accumulation of NP in cardiac tissue leads to naloxone-insensitive cardiotoxicity. Since several lines of evidence suggest that not only block of INa but also IK block may contribute to the non-opioid cardiotoxic effects of P and NP, we investigated the effects of P and NP on HERG channels. HERG presumably encodes IKr, the rapidly-activating delayed rectifier K+ current, which is known to have an important role in initiating repolarization of action potentials in cardiac myocytes. Methods: Using the 2-microelectrode voltage clamp technique we investigated the interaction of P and NP with HERG channels, expressed in Xenopus oocytes. Results: Our experiments show that low drug concentrations (5 µmol/l) facilitate HERG currents, while higher drug concentrations block HERG currents (IC50-values of approx. 40 µmol/l) and dramatically shift the reversal potential to a more positive value because of a 30-fold increased Na+-permeability. P and NP also alter gating of HERG channels by slowing down channel activation and accelerating channel deactivation kinetics. The mutant S631C nullifies the effect of P and NP on the channel's K+-selectivity. Conclusion: P and NP show a complex and unique drug-channel interaction, which includes altering ion-selectivity and gating. Site-directed mutagenesis suggests that an interaction with S631 contributes to the drug-induced disruption of K+-selectivity. No specific role of the minK subunit in the HERG block mechanism could be determined.
KEYWORDS Gene expression; Ion channel, K-channel; Long QT syndrome; Gene expression; QT-dispersion
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