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.

Cystic fibrosis transmembrane conductance regulator mediates sulphonylurea block of the inwardly rectifying K+ channel Kir6.1

1. Recombinant ATP-sensitive K+ channels (KATP channels) were heterologously expressed in the NIH3T3 mouse cell line, and the electrophysiological properties were studied using patch-clamp techniques. 2. The NIH3T3 cell lines transfected with the inwardly rectifying K+ channel Kir6.1 alone or with both Kir6.1 and cystic fibrosis transmembrane conductance regulator (CFTR) exhibited time-independent K+ currents with weak inward rectification. In contrast, no measurable K+ conductance was observed in mock-transfected cells or in cells transfected with CFTR alone. Regardless of co-transfection with Kir6.1, the transfection with CFTR produced a Cl- conductance that was activated by cell dialysis with cAMP (1 mM). The conductance was reversibly suppressed by glibenclamide (30 microM). 3. Whole-cell currents at +60 mV were blocked in a concentration-dependent manner by Ba2+ ions with similar IC50 values: 89.3 +/- 23.3 microM (Kir6.1 alone) and 67.3 +/- 24.9 microM (Kir6.1-CFTR). 4. The currents recorded from Kir6. 1-transfected cells were not affected by glibenclamide, whereas glibenclamide did inhibit the conductance expressed in cells co-transfected with CFTR (IC50 = 35.9 +/- 6.6 microM). 5. In the cell-attached mode with a 150 mM K+ pipette solution, both Kir6.1- and Kir6.1-CFTR-transfected cells displayed a class of K+ channels showing weak inward rectification and a slope conductance of 50.7 +/- 1.0 and 52.4 +/- 4.9 pS, respectively. 6. In the inside-out mode, the single-channel currents recorded from both types of cells were not inhibited by intracellular ATP (1 mM). However, glibenclamide was found to block the single-channel activities in the co-transfected cells.

Metabolic inhibition impairs ATP-sensitive K+ channel block by sulfonylurea in pancreatic beta-cells

The effect of metabolic inhibition on the blocking of beta-cell ATP-sensitive K+ channels (KATP channels) by glibenclamide was investigated using a patch-clamp technique. Inhibition of KATP channels by glibenclamide was attenuated in the cell-attached mode under metabolic inhibition induced by 2,4-dinitrophenol. Under a low concentration (0.1 microM) of ATP applied in the inside-out mode, KATP channel activity was not fully abolished, even when a high dose of glibenclamide was applied, in contrast to the dose-dependent and complete KATP channel inhibition under 10 microM ATP. On the other hand, cibenzoline, a class Ia antiarrhythmic agent, inhibits KATP channel activity in a dose-dependent manner and completely blocks it, even under metabolic inhibition. In sulfonylurea receptor (SUR1)- and inward rectifier K+ channel (Kir6.2)-expressed proteins, cibenzoline binds directly to Kir6.2, unlike glibenclamide. Thus, KATP channel inhibition by glibenclamide is impaired under the condition of decreased intracellular ATP in pancreatic beta-cells, probably because of a defect in signal transmission between SUR1 and Kir6.2 downstream of the site of sulfonylurea binding to SUR1.

Insulin secretion and its modulation by antiarrhythmic and sulfonylurea drugs

Cardiovascular drugs such as antiarrhythmic agents with Vaughan Williams class Ia action have been found to induce a sporadic hypoglycemia. Recent investigation has revealed that these drugs induce insulin secretion from pancreatic beta-cells by inhibiting ATP-sensitive K+ (KATP) channels in a manner similar to sulfonylurea drugs. The mechanism underlying block of KATP channels by antiarrhythmic drugs was different, however, from that of sulfonylureas: firstly, because binding of radioactive glibenclamide could not be inhibited by unlabelled antiarrhythmic agents, and vice versa; secondly, because the two compounds differ in the kinetics and sidedness of drug action-antiarrhythmic drugs act on the channel from the inner surface of the cell membrane, whereas glibenclamide binds through the intramembrane pathway; finally, it was shown that functional KATP channels in beta-cells are composed of two distinct molecules-a sulfonylurea receptor (SUR) and a channel pore-forming subunit, an inwardly-rectifying K channel with two transmembrane regions (Kir6.2). Antiarrhythmic drugs reversibly inhibit the K+ conductance displayed by the Kir6.1 (a putative KATP channel clone)-transfected NIH3T3 cells. Therefore they appear to interact directly with the pore-forming subunit, thereby inhibiting KATP channel currents and exerting an insulinotrophic effect.

Block of pancreatic ATP-sensitive K+ channels and insulinotrophic action by the antiarrhythmic agent, cibenzoline

1. We investigated the effect of cibenzoline (a class Ia antiarrhythmic drug) on basal insulin secretory activity of rat pancreatic islets and ATP-sensitive K+ channels (KATP) in single pancreatic beta cells of the same species, using radioimmunoassay and patch clamp techniques. 2. Micromolar cibenzoline had a dose-dependent insulinotrophic action with an EC50 of 94.2 +/- 46.4 microM. The compound inhibited the activity of the KATP channel recorded from a single beta-cell in a concentration-dependent manner. The IC50 was 0.4 microM in the inside-out mode and 5.2 microM in the cell-attached mode, at pH 7.4. 3. In the cell-attached mode, alkalinization of extracellular solution increased the inhibitory action of cibenzoline and the IC50 was reduced from 26.8 microM at pH 6.2 to 0.9 microM at pH 8.4. On the other hand, the action of cibenzoline in the excised inside-out mode was acute in onset with a small IC50, indicating that the drug attains its binding site from the cytoplasmic side of the cell membrane. 4. In the inside-out mode, micromolar ADP reactivated the cibenzoline-blocked KATP channels in a manner similar to that by which ADP restored ATP-dependent block of the channel. 5. The binding of [3H]-glibenclamide to pancreatic islets was inhibited by glibenclamide but not by cibenzoline. In contrast, the [3H]-cibenzoline binding was displaced by unlabelled cibenzoline but not by glibenclamide. It is concluded that cibenzoline blocks pancreatic KATP channels via a binding site distinct from the sulphonylurea receptor.

Phosphorylation and subcellular distribution of calpastatin in human hematopoietic system cells

Calpastatin is an endogenous inhibitor protein acting specifically on calpain (EC 3.4.22.17; Ca2(+)-dependent cysteine proteinase). The phosphorylation of calpastatin was investigated in human hematopoietic system cell lines. Microheterogeneity of calpastatin was observed, in which 118- and 116-kDa forms were named calpastatin a and b, respectively. The phosphorylation of both calpastatins was identified in all cell lines examined and occurred mainly at serine residues with trace amounts of phosphothreonine in vivo. The incubation of cells with 12-O-tetradecanoylphorbol-13-acetate increased the incorporation of 32P-orthophosphate into calpastatin a. Two-dimensional maps of 32P-labeled phosphopeptide from both calpastatins were identical except for additional minor spots for calpastatin a. [35S]methionine-labeled calpastatins a and b were localized mainly in the cytosol, and only 6% of cellular calpastatins were detected in the membrane fraction. By contrast, more than 30% of the 32P-labeled calpastatins a and b were distributed in the membrane fraction. Thus, the phosphorylation of calpastatin may be involved in regulating the calpain-calpastatin protein kinase system by its subcellular distribution.