Mechanism of terfenadine block of ATP-sensitive K+ channels

1 The ATP-sensitive K+ (K-ATP) channel is a complex of a pore-forming inwar dly rectifying K+ channel (Kir6.2) and a sulphonylurea receptor (SUR). The aim of the present study was to gain further insight into the mechanism of block of K-ATP channels by terfenadine. 2 Channel activity was recorded both from native K-ATP channels from the cl onal insulinoma cell line RINm5F and from a C-terminal truncated form of Ki r6.2 (Kir6.2 Delta 26), which - in contrast to Kir6.2 - expresses independe ntly of SUR. Kir6.2 Delta 26 channels were expressed in COS-7 cells, and en hanced green fluorescent protein (EGFP) cDNA was used as a reporter gene. E GFP fluorescence was visualized by a laser scanning confocal microscope. 3 Terfenadine applied to the cytoplasmic side of inside-out membrane patche s concentration-dependently blocked both native K-ATP channel and Kir6.2 De lta 26 channel activity, and the following values were calculated for IC50 (the terfenadine concentration causing half-maximal inhibition) and n (the Hill coefficient): 1.2 mu M and 0.7 for native K-ATP channels, 3.0 mu M and 1.0 for Kir6.2 Delta 26 channels. 4 Terfenadine had no effect on slope conductance of either native K-ATP cha nnels or Kir6.2 Delta 26 channels. Intraburst kinetics of Kir6.2 Delta 26 c hannels were not markedly affected by terfenadine and, therefore, terfenadi ne acts as a slow channel blocker on Kir6.2 Delta 26 channels. Terfenadine- induced block of Kir6.2 Delta 26 channels demonstrated no marked voltage de pendence, and lowering the intracellular pH to 6.5 potentiated the inhibiti on of Kir6.2 Delta 26 channels by terfenadine. 5 These observations indicate that terfenadine blocks pancreatic B-cell K-A TP channels via binding to the cytoplasmic side of the pore-forming subunit . The presence of the pancreatic SUR1 has a small, but significant enhancin g effect on the potency of terfenadine.