Abstract

Long and short QT syndromes associated with loss and gain of human ether-a-go-go-related gene (hERG) channel activity, respectively, can cause life-threatening arrhythmias. As such, modulation of hERG channel activity is an important consideration in the development of all new therapeutic agents. In the present study, we investigated the mechanisms of action of 2-[2-(3,4-dichloro-phenyl)-2,3-dihydro-1H-isoindol-5-ylamino]-nicotinic acid (PD-307243), a known hERG channel activator, on hERG channels stably expressed in Chinese hamster ovary (CHO) cells using the patch-clamp technique. In the whole-cell recordings, the extracellular application of PD-307243 concentration-dependently increased the hERG current and markedly slowed hERG channel deactivation and inactivation. PD-307243 had no effect on the selectivity filter of hERG channels. The activity of PD-307243 was use-dependent. PD-307243 (3 and 10 μM) induced instantaneous hERG current with little decay at membrane potentials from -120 to -40 mV. At more positive voltages, PD-307243 induced an I_to-like upstroke of hERG current. The actions of PD-307243 on the rapid component of delayed rectifier K⁺ current (I_Kr) in rabbit ventricular myocytes were similar to those observed in hERG channel-transfected CHO cells. Inside-out patch experiments revealed that PD-307243 increased hERG tail currents by 2.1 ± 0.6 (n = 7) and 3.4 ± 0.3-fold (n = 4) at 3 and 10 μM, respectively, by slowing the channel deactivation but had no effect on channel activation. During a voltage-clamp protocol using a prerecorded cardiac action potential, 3 μM PD-307243 increased the total potassium ions passed through hERG channels by 8.8 ± 1.0-fold (n = 5). Docking studies suggest that PD-307243 interacts with residues in the S5-P region of the channel.