In order to find out whether the steep voltage-dependent block of current through mKv1.3 is connected to the C-type inactivation of the channels, we used inside-out patch clamp recordings of COS7 cells transfected with hKv1.3 H399T mutant channels. The H399T mutant was used because of its strongly reduced C-type inactivated state (Dreker & Grissmer 2005, Mol Pharm 68:966). With 100 mM d-Tubocurarine [dTC] in the bath solution, i.e. at the cytosolic side of the mutant channel, we observed a similar block compared to earlier measurements on the wt channel: the block was negligible at membrane potentials more negative than -30 mV and became stronger at more positive potentials. The steepness of the voltage-dependent block was identical to the wt channel about 13 mV per e-fold change suggesting that the voltage-dependence of the dTC block is independent of the C- type inactivated state of the channel and that two charges (one molecule) could penetrate all the way through the mutant channel. Other explanations for the steep voltage dependent dTC block include the binding and/or interaction of the dTC with K+ in the selectivity filter or the assumption that two dTC molecules have to bind the channel in order to block it.

Supported by Wilhelm-Sander Stiftung (2004.046.1), the 4SC AG (Martinsried) and theLand Baden-Württemberg (1423/74).