Blockade of cardiac ATP-sensitive k+ channel by cibenzoline targets its pore-forming subunit

Several antiarrhythmic agents with Na-channel blocking action have been shown to inhibit cardiac K ATP channels. We used cibenzoline to examine its precise target site using patch-clamp techniques and receptor binding assays in guinea-pig ventricular myocytes. Exposure of myocytes to a glucose-free perfusate containing I mM cyanide produced a time-dependent shortening of the action potential duration (APD) in the current-clamp mode. Cibenzoline (30 μM) slowed the development of APD shortening (APD 90 to ∼91% vs. ∼55% control 16 min after metabolic inhibition) at pH o 7.4, but not at pH o 6.4 (to ∼60%). The pinacidil (30 μM)-induced K ATP currents were inhibited by cibenzoline in a pH o -dependent manner: the higher the pH o , the stronger the blocking effect of cibenzoline. The binding of [ 3 H]-labeled cibenzoline was prevented by cibenzoline, but not by glibenclamide. Alkalinization produces a higher concentration of the uncharged form of cibenzoline, which can more easily permeate the cell membrane than the charged form. In NIH3T3 cells stably expressing Kir6.1, a putative pore-forming subunit of K ATP channel, cibenzoline but not glibenclamide inhibited the K conductance. Thus cibenzoline interacts with the channel pore-forming subunit of the K ATP channel (Kir6.2), but not the sulfonylurea receptor, from the cytosolic side after it permeates into the cell interior via the membrane lipid bilayer.