Voltage-Dependent Profile of HumanEther-a-go-go-Related Gene Channel Block Is Influenced by a Single Residue in the S6 Transmembrane Domain

Abstract

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 IC(50) values determined with voltage-clamp pulses to 0 mV were 4.6 microM and 57 microM 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.

Topological Mapping of the Asymmetric Drug Binding to the Human Ether-à-go-go-Related Gene Product (HERG) Potassium Channel by Use of Tandem Dimers

“…The current amplitudes were depressed slightly as the membrane potential was depolarized further. This pattern is an indication of the presence of voltagedependent block (Walker et al, 1999;Sá nchez-Chapula et al, 2003). At each voltage, the K i values were calculated from the concentration-dependence curves (Fig.…”

Role of the activation gate in determining the extracellular potassium dependency of block of HERG by trapped drugs

“…One explanation for the observed difference in extracellular potassium sensitivity between on the one hand, block of HERG by bepridil, terfenadine and dofetilide, and on the other hand, block of HERG by quinidine and cisapride, is that bepridil, terfenadine and dofetilide can be trapped inside the channel after the channel closes 14,16 whereas quinidine and cisapride cannot be trapped inside the channel after channel closure. 16,17 Consistent with a drug trapping mechanism at negative voltages, the HERG point mutation D540K, which results in a demonstrates that the non-conducting conformational change measured in 0 mM K is not seen in 20 mM NH 4 , possibly because NH 4 , an ion that shows some permeability through the HERG channel (P NH4 /P K = 0.15), binds to one or more sites in the HERG channel at or near the selectivity filter and prevents collapse of the selectivity filter. 5 Since block of either WT HERG or the HERG mutant D540K by bepridil is similar in 0 mM K and 20 mM NH 4 (see Fig.…”

“…10 Both bepridil and terfenadine are trapped inside the channel after channel closure, whereas quinidine and cisapride cannot be trapped inside the channel after the channel closes. 16,17 We show here that this trapping mechanism may be partly responsible for the lack of extracellular potassium dependency of block of HERG by bepridil and terfenadine.…”

“…[14][15][16] Other drugs, in particular quinidine and chloroquine, have been shown to slow channel closing. 17,18 Finally, although the vast majority of HERG blockers are sensitive to mutations in the HERG channel at residue F656 19,20 a few compounds do not show large reductions in block of HERG channels with mutations at residue F656 (i.e., fluvoxamine, dronedarone, amiodarone). 21,22 In this paper we show that block of HERG by two drugs, bepridil and terfenadine, is not sensitive to extracellular potassium.…”

“…It has been shown that bepridil and terfenadine can be trapped in the HERG channel after channel closure, 16 whereas quinidine and cisapride cannot be trapped in the channel after channel closure. 16,17 In order to determine if the lack of extracellular potassium dependent block of WT HERG by bepridil or terfenadine is related to the ability of these drugs to be trapped in the channel after the channel closes, block of the HERG mutant D540K by bepridil and terfenadine in low and high extracellular potassium was measured. The HERG mutant D540K shows an unusual gating behavior in that it opens both with membrane depolarization (similar to WT HERG) and with membrane hyperpolarization (unlike WT HERG).…”