4-Aminopyridine (4-AP) is used extensively in the field of cardiac muscle electrophysiology to study the role (s) in cardiac tissue of transient outward K⁺ current (I_to). Although millimolar 4-AP is widely considered to be selective for I_to, results of a previous study from this laboratory suggested that 4-AP might, in addition, exert an inhibitory effect on the rapid delayed rectifier K⁺ current, I_Kr (Mitcheson & Hancox, 1999). The aim of the present study was to investigate further this possibility, by characterising effects of 4-AP on heterologously expressed HERG channels, since HERG (human-ether-a-go-go-related gene) encodes the pore-forming subunit of channels responsible for I_Kr (Sanguinetti & Keating, 1997).

Measurements of whole-cell HERG current (I_HERG) were made at 37 ± 1 °C from a mammalian cell line (HEK-293) stably expressing HERG (Zhou et al. 1998). I_HERG ‘tail’ amplitude was monitored at -40 mV, following activating pulses to +30 mV. 4-AP was found to exert a concentration-dependent inhibitory effect on I_HERG, with a half-maximal inhibitory concentration (IC₅₀) of 4.4 ± 0.5 mM (mean ± S.E.M.; n = minimum of 5 cells for each of seven 4-AP concentrations between 0.1 and 100 mM). Positive control experiments under similar conditions with the methanesulphonanilide HERG blocker E-4031 gave an IC₅₀ value near to 12 nM, which is close to an IC₅₀ value (7.7 nM) observed previously for E-4031 with this cell line (Zhou et al. 1998). Inhibition of I_HERG by 5 mM 4-AP was observed to be voltage dependent; 4-AP also shifted voltage-dependent activation of I_HERG towards more negative voltages, producing a mean shift in half-maximal activation of -7.4 ± 0.9 mV (n = 8 cells). In experiments utilising a ventricular AP waveform as the voltage command, 5 mM 4-AP suppressed the peak outward I_HERG elicited during the AP repolarisation phase by 58.5 ± 4.1 % (n = 5 cells).

We conclude that 4-AP is a low-affinity blocker of cardiac HERG potassium channels, blocking I_HERG with a potency similar to that suggested for I_Kr from previous experiments on rabbit native cardiomyocytes (Mitcheson & Hancox, 1999). Since 4-AP concentrations in the millimolar range are used in investigations of cardiac I_to, our results confirm that an accessory inhibitory effect on I_Kr may need to be taken into account in interpreting data that might otherwise be ascribed to selective inhibition of I_to.

This work was supported by the British Heart Foundation and The Wellcome Trust.

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