Voltage-dependent profile of human ether-a-go-go-related gene channel block is influenced by a single residue in the S6 transmembrane domain

Many common medications block delayed rectifier K+ channels and prolong the duration of cardiac action potentials. Here we investigate the molecular mechanisms of voltage-dependent block of human ether-a-go-go-related gene (HERG) delayed rectifier K+ channels expressed in Xenopus laevis oocytes by quinidine, an antiarrhythmic drug, and vesnarinone, a cardiotonic drug. The IC50 values determined with voltage-clamp pulses to 0 mV were 4.6 muM and 57 muM for quinidine and quinine, respectively. Block of HERG by quinidine (and its isomer quinine) was enhanced by progressive membrane depolarization and accompanied by a negative shift in the voltage dependence of channel activation. As reported previously for other HERG blockers (e.g., MK-499, cisapride, terfenadine, chloroquine), the potency of quinidine was reduced >100-fold by the mutation of key aromatic residues (Y652, F656) located in the S6 domain. Mutations of Y652 eliminated (Y652F) or reversed (Y652A) the voltage dependence of HERG channel block by quinidine and quinine. These quinolines contain a charged N atom that might bond with Y652 by a cation-pi interaction. However, similar changes in the voltage-dependent profile for block of Y652F or Y652A HERG channels were observed with vesnarinone, a cardiotonic drug that is uncharged at physiological pH. Together, these results suggest that voltage-dependent block of HERG results from gating-dependent changes in the orientation of Y652, a critical component of the drug binding site, and not from a transmembrane field effect on a charged drug molecule.