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Address correspondence to Paul B. Bennett, Department of Molecular Pharmacology, WP42-200, Merck Research Laboratories, 770 Sumneytown Pike, West Point, PA 19486. Fax: 215-993-5098; E-mail:Paul_bennett{at}merck.com
Potassium (K+) channels mediate numerous electrical events in excitable cells, including cellular membrane potential repolarization. The hERG K+ channel plays an important role in myocardial repolarization, and inhibition of these K+ channels is associated with long QT syndromes that can cause fatal cardiac arrhythmias. In this study, we identify saxitoxin (STX) as a hERG channel modifier and investigate the mechanism using heterologous expression of the recombinant channel in HEK293 cells. In the presence of STX, channels opened slower during strong depolarizations, and they closed much faster upon repolarization, suggesting that toxin-bound channels can still open but are modified, and that STX does not simply block the ion conduction pore. STX decreased hERG K+ currents by stabilizing closed channel states visualized as shifts in the voltage dependence of channel opening to more depolarized membrane potentials. The concentration dependence for steady-state modification as well as the kinetics of onset and recovery indicate that multiple STX molecules bind to the channel. Rapid application of STX revealed an apparent "agonist-like" effect in which K+ currents were transiently increased. The mechanism of this effect was found to be an effect on the channel voltage-inactivation relationship. Because the kinetics of inactivation are rapid relative to activation for this channel, the increase in K+ current appeared quickly and could be subverted by a decrease in K+ currents due to the shift in the voltage-activation relationship at some membrane potentials. The results are consistent with a simple model in which STX binds to the hERG K+ channel at multiple sites and alters the energetics of channel gating by shifting both the voltage-inactivation and voltage-activation processes. The results suggest a novel extracellular mechanism for pharmacological manipulation of this channel through allosteric coupling to channel gating.
Key Words: ion channel • neurotoxin • kinetic model • antiarrhythmic • pro-arrhythmia
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