Open Channel Block of HERG K+ Channels by Vesnarinone

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Abstract
Vesnarinone, a cardiotonic agent, blocks IKr and, unlike other IKr blockers, produces a frequency-dependent prolongation of action potential duration (APD). To elucidate the mechanisms, we studied the effects of vesnarinone on HERG, the cloned human IKr channel, heterologously expressed inXenopus laevis oocytes. Vesnarinone caused a concentration-dependent inhibition of HERG currents with an IC50 value of 17.7 ± 2.5 μM at 0 mV (n = 6). When HERG was coexpressed with the β-subunit MiRP1, a similar potency for block was measured (IC50: 15.0 ± 3.0 μM at 0 mV, n= 5). Tonic block of the HERG channel current was minimal (n = 5). The rate of onset of block and the steady-state value for block of current were not significantly different for test potentials ranging from −40 to +40 mV [time constant (τ) = 372 ± 76 ms at +40 mV,n = 4]. Recovery from block at −60, −90, and −120 mV was not significantly different (τ = 8.5 ± 1.5 s at −90 mV, n = 4). Vesnarinone produced similar effects on inactivation-removed mutant (G628C/S631C) HERG channels. The IC50 value was 10.7 ± 3.7 μM at 0 mV (n = 5), and the onset and recovery from block of current findings were similar to those of wild-type HERG. Amino acids important for the binding of vesnarinone were identified using alanine-scanning mutagenesis of residues believed to line the inner cavity of the HERG channel. Six important residues were identified, including G648, F656, and V659 located in the S6 domain and T623, S624, and V625 located at the base of the pore helix. These residues are similar but not identical to those determined previously for MK-499, an antiarrhythmic drug. In conclusion, vesnarinone preferentially blocks open HERG channels, with little effect on channels in the rested or inactivated state. These actions may contribute to the favorable frequency-dependent prolongation in APD.