Mechanism of Block and Identification of the Verapamil Binding Domain to HERG Potassium Channels

Calcium channel antagonists have diverse effects on cardiac electrophysiology. We studied the effects of verapamil, diltiazem, and nifedipine on HERG K+ channels that encode IKr in native heart cells. In our experiments, verapamil caused high-affinity block of HERG current (IC50=143.0 nmol/L), a value close to those reported for verapamil block of L-type Ca2+ channels, whereas diltiazem weakly blocked HERG current (IC50=17.3 micromol/L), and nifedipine did not block HERG current. Verapamil block of HERG channels was use and frequency dependent, and verapamil unbound from HERG channels at voltages near the normal cardiac cell resting potential or with drug washout. Block of HERG current by verapamil was reduced by lowering pHO, which decreases the proportion of drug in the membrane-permeable neutral form. N-methyl-verapamil, a membrane-impermeable, permanently charged verapamil analogue, blocked HERG channels only when applied intracellularly. Verapamil antagonized dofetilide block of HERG channels, which suggests that they may share a common binding site. The C-type inactivation-deficient mutations, Ser620Thr and Ser631Ala, reduced verapamil block, which is consistent with a role for C-type inactivation in high-affinity drug block, although the Ser620Thr mutation decreased verapamil block 20-fold more than the Ser631Ala mutation. Our findings suggest that verapamil enters the cell membrane in the neutral form to act at a site within the pore accessible from the intracellular side of the cell membrane, possibly involving the serine at position 620. Thus, verapamil shares high-affinity HERG channel blocking properties with other class III antiarrhythmic drugs, and this may contribute to its antiarrhythmic mechanism.