Using the whole-cell recording mode of the patch-clamp technique we measured currents through mKv1.3 in L929 cells stably transfected with mKv1.3. Measurements were performed with either externally or internally applied d-Tubocurarine (dTC). The experiments with dTC_o showed a concentration- dependent block of mKv1.3 with a K_d of 1 mM. This block was voltage independent indicating that the positively charged dTC_o did not sense the membrane electrical field from the outside. When dTC was applied from inside currents were blocked in a concentration-and voltage- dependent manner. With 100 mM [dTC] _i we observed almost no block at membrane potentials more negative than -30 mV, block of current through mKv1.3 was almost complete at potentials more positive than +30 mV. This steep voltage- dependent block indicates that dTC_i is pushed into the channel from the inside by depolarized potentials. We suggest that dTC_i penetrates into the selectivity filter of the channel within the membrane electrical field.

Supported by grants from the Wihelm-Sander Stiftung (2004.046.1) and from 4SC AG (Martinsried)