Class III antiarrhythmic drugs block HERG, a human cardiac delayed rectifier K+ channel. Open-channel block by methanesulfonanilides.
We recently reported that mutations in HERG, a potassium channel gene, cause long QT syndrome. Heterologous expression of HERG in Xenopus oocytes revealed that this channel had biophysical properties nearly identical to a cardiac delayed rectifier K+ current I(Kr), but had dissimilar pharmacological properties. Class III antiarrhythmic drugs such as E-4031 and MK-499 are potent and specific blockers of I (Kr) in cardiac myocytes. Our initial studies indicated that these compounds did not block HERG at a concentration of 1 micromol/L. In the present study, we used standard two-microelectrode voltage-clamp techniques to further characterize the effects of these drugs on HERG channels expressed in oocytes. Consistent with initial findings, 1 micromol/L MK-499 and E-4031 had not effect on HERG when oocytes were voltage clamped at a negative potential and not pulsed during equilibration with the drug. However, MK-499 did block HERG current if oocytes were repetitively pulsed, or clamped at a voltage positive to the threshold potential for channel activation. This finding is in contrast to previous studies that showed significant block of I(Kr) in isolated myocytes by similar drugs, even in the absence of pulsing. This apparent discrepancy may be due to differences in channel characteristics (HERG versus guinea pig and mouse I (Kr)), tissue (oocytes versus myocytes), or specific drugs. Under steady state conditions, block of HERG by MK-499 was half maximal at 123 +/- 12 nmol/L at a test potential of -20 mV. MK-499 (150 nmol/L) did not affect the voltage dependence of activation and rectification nor the kinetics of activation and deactivation of HERG. These data indicate that MK-499 preferentially blocks open HERG channels and further support the conclusion that HERG subunits form I(Kr) channels in cardiac myocytes.