St. Zhang et al., Mechanism of block and identification of the verapamil binding domain to HERG potassium channels, CIRCUL RES, 84(9), 1999, pp. 989-998
Calcium channel antagonists have diverse effects on cardiac electrophysiolo
gy. We studied the effects of verapamil, diltiazem, and nifedipine on HERG
K+ channels that encode I-Kr in native heart cells. In our experiments, ver
apamil caused high-affinity block of HERO 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 mu mol/L), and nif
edipine did not block HERG current. Verapamil block of HERG channels was us
e and frequency dependent, and verapamil unbound from HERG channels at volt
ages near the normal cardiac cell resting potential or with drug washout. B
lock of HERG current by verapamil was reduced by lowering pH(o), which decr
eases the proportion of drug in the membrane-permeable neutral form. N-meth
yl-verapamil, a membrane-impermeable permanently charged verapamil analogue
, blocked HERG channels only when applied intracellularly. Verapamil antago
nized dofetilide block of HERG channels, which suggests that they may share
a common binding site. The C-type inactivation-deficient mutations, Ser620
Thr and Ser631Ala, reduced verapamil block, which is consistent with a role
for C-type inactivation in high-affinity drug block, although the Ser620Th
r mutation decreased verapamil block 20-fold more than the Ser631Ala mutati
on. Our findings suggest that verapamil enters the cell membrane in the neu
tral form to act at a site within the pore accessible from the intracellula
r 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 antia
rrhythmic mechanism.