Disopyramide block of K ATP channels is mediated by the pore-forming subunit

Abstract

The class Ia antiarrhythmic agent disopyramide blocks native ATP-sensitive K + (K ATP) channels at micromolar concentrations. The K ATP channel is a complex of a pore-forming inwardly rectifying K + channel (Kir6.2) and a sulfonylurea receptor (SUR). The aim of the present study was to further localize the site of action of disopyramide. We have used a C-terminal truncated form of Kir6.2 (Kir6.2Δ26), which — in contrast to Kir6.2 — expresses independently of SUR. Kir6.2Δ26 channels were expressed in African green monkey kidney COS-7 cells, and enhanced green fluorescent protein (EGFP) cDNA was used as a reporter gene. EGFP fluorescence was visualized by a laser scanning confocal microscope. Disopyramide applied to the cytoplasmic membrane surface of inside-out patches inhibited Kir6.2Δ26 channels half-maximally at 7.1 μM (at pH 7.15). Lowering the intracellular pH to 6.5 potentiated the inhibition of Kir6.2Δ26 channels by disopyramide. These observations suggest that disopyramide directly blocks the pore-forming Kir6.2 subunit, in particular at reduced intracellular pH values that occur under cardiac ischaemia.