Characterisation of recombinant HERG K+ channel blockade by the Class Ia antiarrhythmic drug procainamide

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Abstract

Class Ia antiarrhythmic drugs, including procainamide (PROC), are associated with cardiac sodium channel blockade, delayed ventricular repolarisation and with a risk of ventricular pro-arrhythmia. The HERG K+ channel is frequently linked to drug-induced pro-arrhythmia. Therefore, in this study, interactions between PROC and HERG K+ channels were investigated, with particular reference to potency and mechanism of drug action. Whole-cell patch-clamp recordings of HERG current (IHERG) were made at 37 °C from human embryonic kidney (HEK 293) cells stably expressing the HERG channel. Following activating pulses to +20 mV, IHERG tails were inhibited by PROC with an IC50 value of ∼139 μM. IHERG blockade was found to be both time- and voltage-dependent, demonstrating contingency upon HERG channel gating. However, IHERG inhibition by PROC was relieved by depolarisation to a highly positive membrane potential (+80 mV) that favoured HERG channel inactivation. These data suggest that PROC inhibits the HERG K+ channel by a primarily ‘open’ or ‘activated’ channel state blocking mechanism and that avidity of drug-binding is decreased by extensive IHERG inactivation. The potency of IHERG blockade by PROC is much lower than for other Class Ia agents that have been studied previously under analogous conditions (quinidine and disopyramide), although the blocking mechanism appears similar. Thus, differences between the chemical structure of PROC and other Class Ia antiarrhythmic drugs may help provide insight into chemical determinants of blocking potency for agents that bind to open/activated HERG channels.

Section snippets

Methods

Maintenance of mammalian cell line stably expressing HERG. Measurements of HERG current (IHERG) were made from a cell line (human embryonic kidney; HEK 293) stably expressing HERG that was generously donated by Professor Craig January, University of Wisconsin [24]. Cells were maintained in culture as described previously [17].

Electrophysiological recordings. Coverslip fragments on which cultured HEK 293 cells were plated were placed in a bath mounted on an inverted microscope (Nikon Diaphot)

Results and discussion

In order to assess IHERG blockade by PROC, the voltage protocol shown in Fig. 1A (lower trace) was employed and IHERG tail amplitude was measured on repolarisation to −40 mV after voltage commands to +20 mV. A range of drug concentrations were examined; Fig. 1A shows representative data with 100 μM PROC. Fig. 1B shows the concentration-dependence of the inhibitory action of PROC. For each cell and each drug concentration studied, blockade was quantified by determining the fractional inhibition of

Acknowledgements

J.M.R. was supported by a University of Bristol Ph.D. studentship, H.J.W. and J.T.M. were supported by the British Heart Foundation (PG/2001104 and PG/2000123). J.C.H. also acknowledges research fellowship support from the Wellcome Trust. The authors also gratefully appreciate technical assistance from Mrs. Lesley Arberry.

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