Inactivation gating determines nicotine blockade of human HERG channels

We have previously found that nicotine blocked multiple K+ currents, includ ing the rapid component of delayed rectifier K+ currents (I-Kr), by interac ting directly with the channels. To shed some light on the mechanisms of in teraction between nicotine and channels, we performed detailed analysis on the human ether-h-gogo-related gene (HERG) channels, which are believed to be equivalent to the native I-Kr when expressed in Xenopus oocytes. Nicotin e suppressed the HERO channels in a concentration-dependent manner with gre ater potency with voltage protocols, which favor channel inactivation. Nico tine caused dramatic shifts of the voltage-dependent inactivation curve to more negative potentials and accelerated the inactivation process. Converse ly, maneuvers that weakened the channel inactivation gating considerably re lieved the blockade. Elevating the extracellular K+ concentration from 5 to 20 mM increased the nicotine concentration (by similar to 100-fold) needed to achieve the same degree of inhibition. Moreover, nicotine lost its abil ity to block the HERG channels when a single mutation was introduced to a r esidue located after transmembrane domain 6 (S631A) to remove the rapid cha nnel inactivation. Our data suggest that the inactivation gating determines nicotine blockade of the HERG channels.