External nickel blocks human Kv1.5 channels stably expressed in CHO cells

We have investigated the actions of Nickel (Ni(2+)) on a human cardiac potassium channel (hKv1.5), the main component of human atrial ultra-rapid delayed rectifier current, stably expressed in Chinese hamster ovary cell line using the whole-cell voltage-clamp technique. External Ni(2+) reversibly decreased the amplitude of the current in a concentration-dependent manner. The concentration for half-maximum inhibition of the current at +50 mV was 568 microm. The activation, deactivation, reactivation kinetics of the current were not affected by Ni(2+). Block was not voltage-dependent but frequency-dependent block was apparent. The extent of channel block during the first pulse increased when the duration of exposure to Ni(2+), prior to channel activation, was prolonged indicating that Ni(2+) interacted with hKv1.5 in the closed state. The percentage of current remaining in presence of Ni(2+) decreased steeply over the range of steady-state channel inactivation, consistent with an enhanced block with increased inactivation. This suggests that Ni(2+) preferentially blocks nonconducting hKv1.5 channels, either in the resting or inactivated state in a concentration-dependent manner. The data indicate that the mechanisms of hKv1.5 channel inhibition by Ni(2+) are distinct from those of other K(+) channels.