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 1 mM cyanide produced a time-dependent shortening of the action potential duration (APD) in the current-clamp mode. Cibenzoline (30 microM) slowed the development of APD shortening (APD90 to approximately 91% vs. approximately 55% control 16 min after metabolic inhibition) at pHo 7.4, but not at pHo 6.4 (to approximately 60%). The pinacidil (30 microM)-induced K(ATP) currents were inhibited by cibenzoline in a pHo-dependent manner: the higher the pHo, the stronger the blocking effect of cibenzoline. The binding of [3H]-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.
Horie
M
M
Department of Cardiovascular Medicine, Kyoto University, Graduate School of Medicine, Japan. horie@kuhp.kyoto-u.ac.jp
Watanuki
M
M
Tsuji
K
K
Ishida
H
H
Ishida-Takahashi
A
A
Yuzuki
Y
Y
Seino
Y
Y
Sasayama
S
S
D000200:Action Potentials / Q000187:drug effects*; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology*; D000959:Antihypertensive Agents / Q000494:pharmacology; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D004347:Drug Interactions; D004594:Electrophysiology; D005260:Female; D005905:Glyburide / Q000494:pharmacology*; D006168:Guinea Pigs; D006352:Heart Ventricles / Q000187:drug effects; D006863:Hydrogen-Ion Concentration; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007093:Imidazoles / Q000494:pharmacology*; D008297:Male; D018408:Patch-Clamp Techniques; D020110:Pinacidil / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects*
D000889:Anti-Arrhythmia Agents; D000959:Antihypertensive Agents; D007004:Hypoglycemic Agents; D007093:Imidazoles; D015221:Potassium Channels; D020110:Pinacidil; D005905:Glyburide; C032151:cifenline