E-4031

Abstract: An envelope of tails test was used to show that the delayed rectifier K+ current (IK) of guinea pig ventricular myocytes results from the activation of two outward K+ currents. One current was specifically blocked by the benzenesulfonamide antiarrhythmic agent, E-4031 (IC50 = 397 nM). The drug-sensitive current, "IKr" exhibits prominent rectification and activates very rapidly relative to the slowly activating drug-insensitive current, "IKs." IKs was characterized by a delayed onset of activation that occurs over a voltage range typical of the classically described cardiac IK. Fully activated IKs, measured as tail current after 7.5-s test pulses, was 11.4 times larger than the fully activated IKr. IKr was also blocked by d-sotalol (100 microM), a less potent benzenesulfonamide Class III antiarrhythmic agent. The activation curve of IKr had a steep slope (+7.5 mV) and a negative half-point (-21.5 mV) relative to the activation curve of IKs (slope = +12.7 mV, half-point = +15.7 mV). The reversal potential (Erev) of IKr (-93 mV) was similar to EK (-94 mV for [K+]o = 4 mM), whereas Erev of IKs was -77 mV. The time constants for activation and deactivation of IKr made up a bell-shaped function of membrane potential, peaking between -30 and -40 mV (170 ms). The slope conductance of the linear portion of the fully activated IKr-V relation was 22.5 S/F. Inward rectification of this relation occurred at potentials greater than -50 mV, resulting in a voltage-dependent decrease in peak IKr at test potentials greater than 0 mV. Peak IKr at 0 mV averaged 0.8 pA/pF (n = 21). Although the magnitude of IKr was small relative to fully activated IKs, the two currents were of similar magnitude when measured during a relatively short pulse protocol (225 ms) at membrane potentials (-20 to +20 mV) typical of the plateau phase of cardiac action potentials.

Abstract: In contrast to other members of the Eag family of voltage-gated, outwardly rectifying potassium channels, the human eag-related gene (HERG) has now been shown to encode an inwardly rectifying potassium channel. The properties of HERG channels are consistent with the gating properties of Eag-related and other outwardly rectifying, S4-containing potassium channels, but with the addition of an inactivation mechanism that attenuates potassium efflux during depolarization. Because mutations in HERG cause a form of long-QT syndrome, these properties of HERG channel function may be critical to the maintenance of normal cardiac rhythmicity.

Abstract: Class III antiarrhythmic agents act by selective prolongation of cardiac action potential duration (APD). Methanesulfonanilide class III agents (e.g., E-4031 and dofetilide) are extremely potent and lengthen action potentials in a "reverse" rate-dependent manner; i.e., effects are greater at low compared with high rates of stimulation. By using the whole-cell current-clamp technique in isolated guinea pig ventricular myocytes, APD was shortened by rapid pacing (244 +/- 16 msec at 30 pulses per minute, 166 +/- 8 msec at 240 pulses per minute; n = 8). Dofetilide (1 microM) prolonged APD more when cells were stimulated at the rate of 30 pulses per minute (44 +/- 10-msec increase) than at 240 pulses per minute (21 +/- 5-msec increase). We investigated the mechanism of APD prolongation using voltage-clamp techniques. Dofetilide selectively inhibited IKr (IC50, 31.5 nM), defined as the rapidly activating inward rectifying component of net delayed rectifier K+ current (IK), without effects on the larger but more slowly activating component of IK (IKs) or on the inward rectifier K+ current (IK1). To examine the rate-dependent effects of dofetilide on APD, trains of conditioning pulses to 0 mV (200-msec duration) were applied at either 30 or 240 pulses per minute to mimic the action potential experiments. Test pulses or ramps were given after the conditioning train to quantitate changes in IK1, IKr, or IKs. The magnitude of neither IK1 nor IKr was dependent on the rate of the preceding train of depolarizations. Sensitivity to block of IKr by dofetilide was rate independent.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract: 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, IKr, but had dissimilar pharmacological properties. Class III antiarrhythmic drugs such as E-4031 and MK-499 are potent and specific blockers of IKr in cardiac myocytes. Our initial studies indicated that these compounds did not block HERG at a concentration of 1 μmol/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 μmol/L MK-499 and E-4031 had no 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 th...