Abstract

hERG potassium channel activity helps shape the cardiac action potential and influences its duration. In this study, we report upon the discovery of ICA-105574 (3-nitro-N-(4-phenoxyphenyl) benzamide), a potent and efficacious hERG channel activator with a unique mechanism of action. In whole cell patch-clamp studies of recombinant hERG channels, ICA-105574 steeply potentiated current amplitudes over 10-fold with an EC₅₀ value of 0.5 ± 0.1 μM and a Hill slope of 3.3 ± 0.2. The effect on hERG channels was confirmed as the known hERG channel blockers, E-4031 and BeKm-1, potently blocked the stimulatory effects of ICA-105574. The primary mechanism by which ICA-105574 potentiates hERG channel activity is by removing hERG channel inactivation as ICA-105574 (2 μM) shifts the mid-point of the voltage-dependence of inactivation by >180 mV from -86 mV to +96 mV. In addition to the effects on inactivation, greater concentrations of ICA-105574 (3 μM) produced comparatively small hyperpolarizing shifts (up to 11 mV) in the voltage dependence of channel activation as well as a 2-fold slowing of channel deactivation. In isolated guinea pig ventricular cardiac myocytes, ICA-105574 induced a concentration-dependent shortening of action potential duration (>70%, 3 μM) that could be prevented by pre-incubation with E-4031. In conclusion, we have identified a novel agent that can prevent inactivation of hERG potassium channels. This compound may provide a useful tool to further understand the mechanism by which hERG channels inactivate and affect cardiac function in addition to the role of hERG channels in other cell systems.