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Journal of Pharmacology And Experimental Therapeutics Fast Forward
First published on May 23, 2007; DOI: 10.1124/jpet.107.121129

0022-3565/07/3222-871-878$20.00
JPET 322:871-878, 2007
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ENDOCRINE AND DIABETES

Iptakalim, a Vascular ATP-Sensitive Potassium (KATP) Channel Opener, Closes Rat Pancreatic beta-Cell KATP Channels and Increases Insulin Release

Naoko Misaki, Xia Mao, Yu-Fung Lin, Sechiko Suga, Guo-Hui Li, Qiang Liu, Yongchang Chang, Hai Wang, Makoto Wakui, and Jie Wu

Department of Physiology, Hirosaki University School of Medicine, Zaifucho, Japan (N.M., S.S., M.W.); Department of Physiology and Membrane Biology, School of Medicine, University of California, Davis, California (X.M., Y.-F.L.); Divisions of Neurology (G.-H.L., Q.L., J.W.) and Neurobiology (Y.C.), Barrow Neurological Institute, Phoenix, Arizona; and Department of Cardiovascular Pharmacology, Beijing Institute of Pharmacology and Toxicology, Beijing, People's Republic of China (H.W., J.W.)

Received February 7, 2007; accepted April 26, 2007.


   Abstract
Top
 Abstract
Materials and Methods
 Results
 Discussion
 References
 
Sulfonylureas have been the leading oral antihyperglycemic agents, and they presently continue to be the most popular antidiabetic drugs prescribed for treatment of type 2 diabetes. However, concern has arisen over the side effects of sulfonylureas on the cardiovascular system. Here, we tested the hypothesis that iptakalim, a novel vascular ATP-sensitive potassium (KATP) channel opener, closes rat pancreatic beta-cell KATP channels and increases insulin release. Rat pancreatic beta-cell KATP channels and heterologously expressed KATP channels in both human embryonic kidney (HEK) 293 cells and Xenopus oocytes were used to test the pharmacological effects of iptakalim. Patch-clamp recordings, Ca2+ imaging, and measurements of insulin release were applied. Patch-clamp whole-cell recordings revealed that iptakalim depolarized beta-cells, induced action potential firing, and reduced KATP channel-mediated currents. Single-channel recordings revealed that iptakalim reduced the open probability of KATP channels without changing channel sensitivity to ATP. By closing beta-cell KATP channels, iptakalim elevated intracellular Ca2+ concentrations and increased insulin release. In addition, iptakalim decreased the open probability of recombinant Kir6.2FL4A (a trafficking mutant of the Kir6.2) KATP channels heterologously expressed in HEK 293 cells, suggesting that iptakalim suppressed the function of beta-cell KATP channels by directly inhibiting the Kir6.2 subunit. Finally, iptakalim inhibited Kir6.2/SUR1, but it activated Kir6.1/SUR2B (vascular-type), KATP channels heterologously expressed in Xenopus oocytes. Iptakalim bidirectionally regulated pancreatic-type and vascular-type KATP channels, and this unique pharmacological property suggests the potential use of iptakalim as a new therapeutic strategy for treating type 2 diabetes with the additional benefit of alleviating vascular disorders.

Type 2 diabetes is characterized by high levels of blood glucose due to decreased secretion of insulin from pancreatic beta-cells and/or decreased insulin action (Nourparvar et al., 2004Go). More than half of type 2 diabetes patients require medication, and sulfonylureas have been the leading oral antihyperglycemic agents used over the past half-century, and they continue to be the most popular antidiabetic drugs prescribed today. Sulfonylureas trigger insulin release from pancreatic beta-cells by binding to SUR1, the regulatory subunit of pancreatic beta-cell KATP channels, which leads to closure of KATP channels; plasma membrane depolarization; opening of voltage-gated Ca2+ channels; and consequently, the release of insulin. Alternatively, sulfonylureas also interact with KATP channels in extrapancreatic tissues, such as cardiac, vascular, and smooth muscles, and in various brain regions (Gribble and Reimann, 2003Go). Such general actions of sulfonylureas have raised concerns that they may close cardiovascular KATP channels when used to treat patients with type 2 diabetes, thereby diminishing the endogenous, protective responses to ischemia that require KATP channel activation. Indeed, it has been recognized that type 2 diabetes is associated with increased incidences of cardiovascular disease risk factors, including hypertension, dyslipidemia, microalbuminuria, and altered homeostasis of glucose (Shin et al., 2003Go). Meanwhile, the application of KATP channel isoform-selective drugs, which close pancreatic beta-cell KATP channels without clearly affecting KATP channels in the cardiovascular system, has been strongly recommended (Lawrence et al., 2001Go).

Iptakalim was initially designed and synthesized as an antihypertensive drug, and it has exhibited remarkable antihypertensive effects in a variety of hypertensive animal models using in vivo and in vitro preparations (Wang, 1998Go, 2003Go; Wang et al., 2005aGo,bGo). The molecular mechanisms underlying its antihypertensive effect include the activation of vascular KATP channels (Wang, 1998Go, 2003Go). However, whether iptakalim affects KATP channel function in pancreatic beta-cells and/or alters insulin release was heretofore unknown.

In the present study, we tested the hypothesis that iptakalim induces beta-cell excitability, elevates intracellular Ca2+ concentrations, and enhances insulin release by closing beta-cell KATP channels. To determine both the possible target and subunit selectivity of iptakalim, we also examined its effect on heterologously expressed Kir6.2RKRR368/369/370/371 AAAA (i.e., Kir6.2FL4A, a Kir6.2 trafficking mutant capable of functional expression without SUR) KATP channels in HEK 293 cells as well as on heterologously expressed Kir6.2/SUR1 and Kir6.1/SUR2B KATP channels in Xenopus oocytes.


   Materials and Methods
Top
 Abstract
 Materials and Methods
Results
 Discussion
 References
 
This study was carried out in accordance with the Guidelines for Animal Experimentation; Hirosaki University (Zaifucho, Japan); University of California (Davis, CA); and Barrow Neurological Institute (Phoenix, AZ).

Patch-Clamp Recordings
Cultured beta-Cells. Detailed protocols for isolation of rat pancreatic islets and for culturing isolated beta-cells have already been described in a previous study (Suga et al., 1997Go). In brief, the standard external solution contained 135 mM NaCl, 5.6 mM KCl, 1.2 mM MgCl2, 1 mM CaCl2, 5 mM glucose, and 10 mM HEPES, pH 7.3 adjusted with NaOH. For perforated patch recordings (to measure the resting and action potentials of beta-cells), the pipette solution contained 100 mM K-gluconate, 35 mM KCl, 5 mM glucose, 0.5 mM EGTA, 10 mM HEPES, and 240 µg/ml amphotericin B (Sigma-Aldrich, St. Louis, MO), pH 7.2. For cell-attached and inside-out single-channel recordings, the pipette solution contained 135 mM KCl, 1.2 mM MgCl2, 5 mM glucose, 0.5 mM EGTA, and 10 mM HEPES, pH 7.3. The ionic composition of the bath solution during inside-out recordings was the same as the pipette solution, but the pH was 7.2. The resistance of recording electrodes, when filled with pipette solution, ranged from 2 to 4 M{Omega}. To measure whole-cell membrane current, voltage-ramp pulses from –90 to –50 mV were repeatedly applied using a ramp pulse generator (SET-2100; Nihon Kohden, Tokyo, Japan). The membrane capacitance ranged from 8 to 14 pF. Series resistance below 12 M{Omega} was accepted. Single-channel current recordings were carried out in cell-attached and inside-out configurations. All electrophysiological experiments were performed at room temperature (22 ± 1°C). Data of single-channel currents were low-pass filtered at 1 kHz, digitized at 10 kHz, and analyzed using a single-channel current analysis program (Clampfit 9.0; Molecular Devices, Sunnyvale, CA).

HEK 293 Cells. The recording electrodes were prepared as described in a previous study (Lin et al., 2000Go). The intracellular (bath) solution consisted of 110 mM KCl, 1.44 mM MgCl2, 30 mM KOH, 10 mM EGTA, and 10 mM HEPES, pH 7.2. The extracellular (intrapipette) solution consisted of 140 mM KCl, 1.2 mM MgCl2, 2.6 mM CaCl2, and 10 mM HEPES, pH 7.4. All salts were obtained from Sigma-Aldrich. The equilibrium potential for potassium ions was around 0 mV as determined from the current-voltage relationship. Cell-attached single-channel recordings were performed at room temperature 48 to 72 h after transfection. All patches were voltage-clamped at –60 mV intracellularly. Single-channel currents were recorded with an Axopatch 200B amplifier and Clampex 9.0 software (Molecular Devices), and they were low-pass filtered (3 dB; 2 kHz) and digitized at 20 kHz. Single-channel events were detected using Fetchan 6.05 (pCLAMP; Molecular Devices) and analyzed with Intrv5 (Dr. Barry S. Pallotta, University of North Carolina, Chapel Hill, NC) as described previously (Lin et al., 2000Go).

Xenopus Oocytes. Patch pipettes were pulled from Narishige G-1.5 glass (Narishige, Tokyo, Japan) and had resistances of 1 to 2 M{Omega} when filled with pipette solution. Macroscopic currents were recorded from giant excised inside-out patches at a holding potential of 0 mV (Gribble et al., 2000Go). Currents were evoked by repetitive 3-s voltage ramps from –110 to +100 mV, and they were recorded with an Axopatch 200B patch-clamp amplifier. Clampex 9.0 and Clampfit 9.0 were used for data acquisition and analysis, respectively. The pipette (external) solution contained 140 mM KCl, 1.2 mM MgCl2, 2.6 mM CaCl2, and 10 mM HEPES, pH 7.4 with KOH. The intracellular (bath) solution contained 107 mM KCl, 2 mM MgCl2,1mM CaCl2, 10 mM EGTA, 10 mM HEPES, and 3 mM ATP, pH 7.2 with KOH (final K+ 140 mM). Solutions were made fresh each day.

Fura-2 Ca2+ Imaging
Isolated islets were placed in a glass-bottomed culture dish, and then they were loaded with a HEPES buffer solution (140 mM NaCl, 4.7 mM KCl, 1.2 mM MgCl2, 1.0 mM CaCl2, 10 mM glucose, and 10 mM HEPES) containing 1 µM fura-2/AM (Dojin, Kumamoto, Japan) for 20 min at room temperature. Ca2+ images were captured using an inverted microscope with 40x Plan-Neofluar objectives (Axiovert 135; Carl Zeiss, Oberkochen, Germany) and a silicon intensifier target camera. Images were recorded on an Argus 50/CA fluorescence-imaging system (Hamamatsu Photonics, Hamamatsu, Japan). Excitation wavelengths were 340 and 380 nm, selected from a xenon light source, and the emission wavelength was 510 nm (Grynkiewicz et al., 1985Go). All microfluorometric experiments were carried out at room temperature.

Measurement of Insulin Release
The amount of insulin released from islets was measured as described previously (Nakano et al., 2002Go). In brief, isolated islets were hand-picked under a microscope. Twenty islets were relocated using a polypropylene syringe filter (0.45-mm filter; Corning Glassworks, Corning, NY), and they were continuously perfused with a control solution of Hanks' balanced salt solution containing 5.5 mM glucose, 10 mM HEPES, and 2% bovine serum albumin at a rate of 1 ml/min. After preincubation for 30 min, the islets were perfused with control solution for another 12 min, and then they were stimulated by a high glucose concentration (17.5 mM) or with 100 µM iptakalim for 12 min. Before and during stimulation with glucose or iptakalim, the perfusate was collected and stored at –20°C until assay. The amount of insulin was measured by an enzyme-linked immunosorbent assay using a microplate reader (Bio-Rad, Hercules, CA). Insulin measurement kits were purchased from Morinaga Seikagaku Institute (Yokohama, Japan).

Preparation of Drugs
For studying ATP sensitivity, 10 µM ATP was added to the bath solution when performing inside-out recordings. Iptakalim hydrochloride [N-(1-methylethyl)-1,1,2-trimethyl-propylamine hydrochloride] was kindly provided by Dr. H. Wang (Institute of Pharmacology and Toxicology, Beijing, People's Republic of China), and diazoxide, nifedipine, and tolbutamide were purchased from Sigma-Aldrich.

Statistics
Data are expressed as mean ± S.E.M., and statistical significance was evaluated by two-tailed paired, unpaired, or one-sample Student's t-tests. p values less than 0.05 are considered to be significant.


   Results
Top
 Abstract
 Materials and Methods
 Results
Discussion
 References
 
Iptakalim-Induced Excitation of Single Pancreatic beta-Cells of Rats. Using perforated whole-cell recordings (amphotericin B) in current-clamp mode, the resting membrane potential of rat beta-cells was –52.9 ± 1.1 mV (n = 28), and the cells were electrically silent with 5.5 mM glucose in the external solution. Bath application of 22.5 mM glucose induced a slowly developed membrane depolarization and action potential firing (Fig. 1A). Likewise, application of 100 µM iptakalim also slowly depolarized beta-cell plasma membrane and elicited action potential firing (Fig. 1B). In the presence of 10 µM nifedipine, an L-type Ca2+ channel blocker that suppresses action potential firing, iptakalim depolarized, whereas 100 µM diazoxide (a classic SUR1-selective KATP channel opener) hyperpolarized, pancreatic beta-cell plasma membrane (Fig. 1C). These results suggest that iptakalim is capable of regulating beta-cell excitability.


Figure 1
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  		Fig. 1. Effects of iptakalim on the excitability of single rat pancreatic beta-cells. A, a high glucose concentration (22.5 mM) induced plasma membrane depolarization and action potential firing. B, 100 µM iptakalim depolarized the plasma membrane and induced action potential firing. C, in the presence of 10 µM nifedipine, iptakalim depolarized, but diazoxide hyperpolarized, the plasma membrane. The traces in A to C are representative of recordings from six to eight cells.

 
Iptakalim Reduced Whole-Cell Currents of KATP Channels in Rat Pancreatic beta-Cells. To determine whether the currents permeating through pancreatic beta-cell KATP channels were inhibited by iptakalim, whole-cell currents induced by repetitive ramp pulses from –90 to –50 mV (at 0.2 Hz) were recorded in voltage-clamp mode (external glucose 5.5 mM). The classic KATP channel blocker tolbutamide at 500 µM reversibly suppressed (Fig. 2A), whereas 100 µM diazoxide (Fig. 2B) increased, these ramp-evoked currents, suggesting that these currents were KATP channel-mediated currents. Bath application of 100 µM iptakalim reduced these beta-cell KATP channel-mediated currents (Fig. 2C), and the inhibition was concentration-dependent (Fig. 2D). The IC50 and Hill coefficient of iptakalim were 136.4 µM and 0.7, respectively (n = 7). These results indicate that iptakalim inhibits the function of beta-cell KATP channels in a concentration-dependent manner.


Figure 2
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  		Fig. 2. Inhibitory effects of iptakalim on KATP channel-like currents in single pancreatic beta-cells. Ramp-pulse-induced KATP channel-like currents were inhibited by 500 µM tolbutamide (A) and 100 µM iptakalim (C), but they were enhanced by 100 µM diazoxide (B). D, concentration-response relationship of iptakalim-mediated inhibition of KATP channel-like currents. Each symbol represents the average from seven cells (mean ± S.E.M.). The effects were normalized to ramp-pulse-induced KATP channellike currents without iptakalim.

 

Iptakalim Decreased the Open Probability of Pancreatic beta-Cell KATP Channels in Cell-Attached and Inside-Out Patches. Single-channel currents in pancreatic beta-cells were recorded in cell-attached patches clamped at a pipette potential of 0 mV. Bath application of 500 µM tolbutamide abolished, whereas 100 µM diazoxide enhanced, the single-channel currents (Fig. 3A), indicating that these currents were KATP channel-mediated currents. Iptakalim at 100 µM reduced the single-channel currents of these KATP channels (Fig. 3B). The normalized NPo value during application of iptakalim was 0.50 ± 0.02 (16 patches; p < 0.0001; control as 1). Total KATP channel current amplitude distribution obtained before (Fig. 3Ca) and during (Fig. 3Cb) iptakalim exposure indicated that iptakalim reduced numbers of multiple and singular KATP channel openings. However, mean single-channel amplitudes before and during exposure to iptakalim were similar (control, 3.31 ± 0.16; iptakalim, 3.26 ± 0.14 pA; 16 patches; p > 0.05).


Figure 3
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  		Fig. 3. Iptakalim inhibited KATP single-channel currents in single rat pancreatic beta-cells recorded in the cell-attached configuration. A, application of 500 µM tolbutamide suppressed, whereas 100 µM diazoxide enhanced, single-channel currents, suggesting that these currents were conducted through KATP channels. B, 100 µM iptakalim reduced KATP single-channel currents. Underlined (a, b) sections of the trace in B (top) were expanded and are shown as corresponding traces at higher temporal resolution (bottom). C, total current amplitude distribution histograms indicate that iptakalim decreased the number of beta-cell KATP channel openings without affecting single-channel conductance. c, closed; o, opened.

 
To determine whether the inhibition of KATP channels by iptakalim was mediated via altering channel sensitivity to intracellular ATP concentrations, inside-out patch experiments were performed. In the absence (Fig. 4A) or presence (Fig. 4B) of intracellular 10 µM ATP, the normalized NPo values were 0.49 ± 0.08 (eight patches; p < 0.01) and 0.52 ± 0.06 (10 patches; p < 0.01; Fig. 4C), respectively, during perfusion of 100 µM iptakalim. The difference between NPo values when comparing 0 and 10 µM ATP was not significant (p > 0.05). These results suggest that the inhibition of rat pancreatic beta-cell KATP channels by iptakalim is independent of intracellular ATP concentrations.


Figure 4
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  		Fig. 4. Iptakalim inhibited beta-cell KATP channel activity without altering channel sensitivity to ATP. A, single-channel current recordings in the inside-out patch recording configuration. The intrapipette voltage was clamped at +60 mV (the transmembrane potential was –60 mV). The solution inside the membrane contained no ATP. B, the solution inside the membrane contained 10 µM ATP. Traces in A and B are representative of recordings from 8 and 10 cells, respectively. C, bar graph comparing the inhibitory effects of 100 µM iptakalim in the absence or presence of 10 µM ATP; the inhibitory effects were normalized to KATP channel activity before iptakalim application as represented by the horizontal dashed line. **, p < 0.01 (compared with normalized NPo values before iptakalim application).

 
Iptakalim Elevated Intracellular Ca2+ Concentrations of Pancreatic beta-Cells. To determine whether iptakalim-induced membrane depolarization would trigger Ca2+ influx (through voltage-gated Ca2+ channels), intracellular Ca2+ concentrations were measured by fura-2 fluorescence imaging. An increase in glucose concentration from 5.5 to 17.5 mM induced a marked elevation of intracellular Ca2+ concentrations (Fig. 5A). The changes were diazoxide-sensitive (100 µM; six cells), suggesting that the increase in intracellular Ca2+ concentrations resulted from glucose-induced closure of KATP channels. The closure of KATP channels by 500 µM tolbutamide caused a similar elevation of intracellular Ca2+ concentrations (six cells; Fig. 5B). Likewise, 100 µM iptakalim also increased intracellular Ca2+ concentrations, which was sensitive to both nifedipine (1 µM; five cells; Fig. 5C) and diazoxide (100 µM; eight cells; Fig. 5D). From seven cells tested, 100 µM iptakalim elevated intracellular Ca2+ concentrations from 171.1 ± 19.5 to 775.0 ± 100.6 nM (p < 0.001). These results suggest that in rat pancreatic beta-cells, iptakalim increased intracellular Ca2+ concentrations by closing KATP channels, which led to cellular membrane depolarization and consequently to the activation of L-type voltage-gated Ca2+ channels.


Figure 5
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  		Fig. 5. Iptakalim elevated intracellular Ca2+ levels. A, 17.5 mM glucose increased intracellular Ca2+ concentrations, which was reversed by 100 µM diazoxide. B, 500 µM tolbutamide increased intracellular Ca2+ concentrations, which was antagonized by 100 µM diazoxide. Iptakalim at 100 µM increased intracellular Ca2+ concentrations, which was restored by 1 µM nifedipine (C) or 100 µM diazoxide (D). Each trace (A–D) is representative from six to eight cell recordings.

 

Iptakalim Increased Insulin Release from Rat Pancreatic Islets. With 5.5 mM glucose in the external solution, basal insulin secretion was measured. The application of 17.5 mM glucose increased insulin secretion from a basal level of 21.5 ± 0.9 to 59.2 ± 8.6 pg/islet/min (p < 0.01; five cells). Tolbutamide at 500 µM increased insulin secretion from 20.1 ± 1.2 to 43.7 ± 2.1 pg/islet/min (p < 0.01; five cells), and 100 µM iptakalim increased insulin secretion from 22.5 ± 1.2 to 34.5 ± 1.8 pg/islet/min (p < 0.05; seven cells; Fig. 6). These data indicate that iptakalim increased insulin secretion, presumably by closing KATP channels in rat pancreatic beta-cells.


Figure 6
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  		Fig. 6. Iptakalim increased insulin release. The rates of insulin secretion from pancreatic islets were measured. Islets were separated into three treatment groups: 17.5 mM glucose, 500 µM tolbutamide, or 100 µM iptakalim. The rates of insulin secretion were measured and compared with 5.5 mM glucose as a baseline (open columns). Averaged data were obtained from five to seven cells for each group, and they are presented as mean ± S.E.M.

 
Iptakalim Inhibited Kir6.2FL4A KATP Channels Heterologously Expressed in HEK 293 Cells. To determine the minimal subunit requirement for iptakalim to inhibit beta-cell-type KATP channels, we investigated the effect of iptakalim on the tetrameric Kir6.2FL4A KATP channel functionally expressed in the absence of the SUR subunit. The opening pattern of Kir6.2FL4A KATP channels in cell-attached patches exhibited numerous brief openings (Fig. 7A), a feature also observed in Kir6.2{Delta}C36 channels, another mutant of Kir6.2 that is capable of functional expression without the SUR subunit (Tucker et al., 1997Go; Lin et al., 2000Go). Iptakalim (100 and 500 µM) reduced the single-channel currents of Kir6.2FL4A KATP channels (Fig. 7, A–C), and the normalized NPo values were 0.65 ± 0.04 (p < 0.0001; eight patches) and 0.38 ± 0.05 (p < 0.001; five patches), respectively (Fig. 7C; control as 1; two-tailed, one-sample t test). There was no change in single-channel conductance. These results suggest that iptakalim suppresses the function of beta-cell KATP channels by directly inhibiting the Kir6.2 subunit, or alternatively, by interacting with some closely associated regulatory proteins of the channel.


Figure 7
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  		Fig. 7. Iptakalim inhibited the single-channel currents of recombinant Kir6.2FL4A KATP channels in transfected HEK 293 cells. Traces in A and B are representative recordings obtained from cell-attached patches. The single-channel activity of tetrameric Kir6.2FL4A KATP channels expressed in the absence of the SUR subunit was inhibited by iptakalim in a concentration-dependent manner (100 µM, A; 500 µM, B). The horizontal scale bar represents 1 s, 300 ms, and 100 ms for traces from top to bottom in each panel, and the vertical scale bar represents 4 pA.C, iptakalim reduced the normalized NPo values of Kir6.2/SUR1 and Kir6.2FL4A KATP channels. NPo values obtained from five to eight patches during the application of iptakalim at 100 or 500 µM were normalized to the corresponding control obtained before iptakalim application in individual patches (control as 1). Data are presented as mean ± S.E.M. ***, p < 0.001 (two-tailed, one-sample t tests).

 
Iptakalim Bidirectionally Modulated beta-Cell-Type (Kir6.2/SUR1) and Vascular-Type (Kir6.1/SUR2B) KATP Channels Heterologously Expressed in Xenopus Oocytes. Our data thus far indicate a clear inhibitory effect by iptakalim on KATP channels natively expressed in rat pancreatic beta-cells or heterologously expressed in HEK 293 cells. It was of interest to determine whether iptakalim opened vascular-type (Kir6.1/SUR2B) KATP channels under the same experimental conditions because iptakalim has been reported to be a novel antihypertensive drug that opens vascular KATP channels (Wang, 1998Go, 2003Go; Wang et al., 2005aGo,bGo). To address this question, we expressed vascular-type and pancreatic-type (Kir6.2/SUR1) KATP channels in Xenopus oocytes, and we examined the effect of iptakalim on these channels using inside-out patch recordings. In the presence of bath-applied 3 mM ATP, 3 to 300 µM iptakalim failed to open Kir6.2/SUR1 KATP channels, whereas 300 µM diazoxide opened these channels (Fig. 8Aa). The diazoxide-induced currents were inhibited by 300 µM iptakalim (seven patches; Fig. 8A, b and c). In contrast, iptakalim opened Kir6.1/SUR2B channels in a concentration-dependent manner (eight patches; Fig. 8B, a and b), and the stimulatory effect induced by iptakalim was blocked by the classic KATP channel blocker glibenclamide (1 µM; data not shown). Collectively, these results suggest that at concentrations (e.g., 100 µM) that block pancreatic beta-cell KATP channels, iptakalim opens vascular-type KATP channels.


Figure 8
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  		Fig. 8. Iptakalim bidirectionally modulated Kir6.2/SUR1 and Kir6.1/SUR2B KATP channels heterologously expressed in Xenopus oocytes. Aa, iptakalim failed to open diazoxide-sensitive Kir6.2/SUR1 KATP channels. Ab, iptakalim inhibited diazoxide-activated Kir6.2/SUR1 KATP channels. Ac, bar graph summarizes the inhibition of diazoxide-activated current by iptakalim. Ba and Bb, iptakalim opened Kir6.1/SUR2B KATP channels in a concentration-dependent manner. All recordings were performed in the inside-out patch configuration with 3 mM ATP in the bath (inside of cell membrane) solution. Horizontal dashed lines in Ac and Bb indicate the control level before drug application (as 100%). *, p < 0.05; **, p < 0.01; ***, p < 0.001; ****, p < 0.0001.

 


   Discussion
Top
 Abstract
 Materials and Methods
 Results
 Discussion
References
 
The experimental data support the hypothesis that iptakalim, a vascular KATP channel opener, closes rat pancreatic beta-cell KATP channels, which in turn increases beta-cell excitability, elevates intracellular Ca2+ concentrations, and enhances insulin release. Furthermore, by examining the effect of iptakalim on truncated Kir6.2FL4A KATP channels heterologously expressed in HEK 293 cells, we provide evidence that iptakalim suppressed the function of beta-cell KATP channels by directly inhibiting the Kir6.2 subunit (Fig. 7). Finally, we confirmed that iptakalim opened recombinant vascular-type (Kir6.1/SUR2B), but closed pancreatic-type (Kir6.2/SUR1), KATP channels heterologously expressed in Xenopus oocytes. The findings that iptakalim closed beta-cell (Figs. 1, 2, 3, 4, 5, 6, 7), but opened vascular, KATP channels (Wang, 1998Go, 2003Go; Wang et al., 2005aGo,bGo) (Fig. 8B), suggest the potential use of iptakalim for treatment of patients afflicted with type 2 diabetes because iptakalim seems promising and beneficial in relieving the symptoms of vascular disorders (or even in improving vascular function) of patients with type 2 diabetes.

Iptakalim was initially designed as a new vascular-type KATP channel opener with a unique structure and antihypertensive-inducing effects; its antihypertensive effects are abolished by the KATP channel blocker glibenclamide (Wang, 1998Go, 2003Go; Wang et al., 2005aGo,bGo). In the present study, however, iptakalim failed to open pancreatic beta-cell KATP channels, whereas the beta-cell KATP channel opener diazoxide clearly opened these channels. Although the precise mechanisms are unclear, a distinct effect related to the subunit composition of KATP channels expressed in beta-cells may explain the inability of iptakalim to open beta-cell KATP channels.

It is well known that KATP channels are expressed in a variety of tissues with different SUR subunits (Mannhold, 2004Go; Ashcroft, 2006Go; Hansen, 2006Go). For example, vascular KATP channels are formed by Kir6.1 and SUR2B, cardiac KATP channels are formed by Kir6.2 and SUR2A (Chutkow et al., 1996Go), and pancreatic beta-cell KATP channels are formed by Kir6.2 and SUR1 (Aguilar-Bryan et al., 1995Go), whereas neuronal KATP channel isoforms are Kir6.2/SUR1 (Inagaki et al., 1996Go; Liss et al., 1999aGo,bGo; Avshalumov et al., 2005Go) and/or Kir6.2/SUR2B (Liss et al., 1999aGo; Avshalumov et al., 2005Go). This tissue-specific expression of SUR renders different sensibilities of KATP channels to metabolic stress, sulfonylureas, and KATP channel openers. It has also been reported that in the presence of ATP, pinacidil effectively opens Kir6.2/SUR2A, but it fails to open Kir6.2/SUR1, KATP channels in Xenopus oocytes (Ashfield et al., 1999Go; Gribble and Ashcroft, 2000bGo). In the present study, the presence of SUR1 resulted in tolbutamide- and iptakalim-sensitive KATP channels in pancreatic beta-cells being sensitive to diazoxide (Figs. 2 and 3). In addition, we have shown that iptakalim was incapable of opening recombinant Kir6.2/SUR1 KATP channels heterologously expressed in both the HEK 293 cell line (Wu et al., 2006Go) and Xenopus oocytes, whereas diazoxide clearly opened these channels (Fig. 8A). In contrast, iptakalim was able to open vascular-type KATP channels heterologously expressed in Xenopus oocytes at relatively low concentrations (Fig. 8B), suggesting a relatively high-affinity opening of vascular-type KATP channels by iptakalim. Therefore, it is likely that the SUR2B, but not the SUR1, subunit of KATP channels is the target of iptakalim that mediates KATP channel activation.

Iptakalim was unable to open pancreatic beta-cell KATP channels, perhaps due to the presence of the SUR1, instead of the SUR2, subunit in these cells. Moreover, an intriguing finding was that iptakalim closed pancreatic beta-cell-type KATP channels. It has been reported that PNU-99963, a nonsulfonylurea-based KATP channel inhibitor that has a structure similar to the KATP channel opener pinacidil, inhibits beta-cell KATP channels (Cui et al., 2003Go). Structurally, iptakalim is also similar to the core portion of pinacidil; therefore, it is possible that KATP channel opener analogs with such a structure can inhibit KATP channels. Iptakalim may directly block beta-cell KATP channels by acting on the Kir6.2 subunit (or some closely associated regulatory proteins). It is well documented that some KATP channel modulators, such as nicorandil, pinacidil, and glibenclamide, regulate KATP channel activity by targeting the regulating subunit SUR (Gribble and Ashcroft, 2000aGo; Hansen, 2006Go), whereas others (e.g., phentolamine and cibenzoline) directly inhibit the pore-forming subunit Kir6.2 (Proks and Ashcroft, 1997Go; Mukai et al., 1998Go). Moreover, tolbutamide has been shown to act on both SUR1 and Kir6.2 (Gribble et al., 1997Go). The best model to test this hypothesis is to use KATP channels composed of the heterologously expressed truncated or mutant Kir6.2 subunit in the absence of the SUR subunit (Mukai et al., 1998Go). Our data obtained using tetrameric Kir6.2 (i.e., Kir6.2FL4A) KATP channels expressed in the absence of the SUR subunit strongly suggest that the Kir6.2 subunit mediates iptakalim-induced pancreatic beta-cell KATP channel inhibition.

In pancreatic beta-cells, KATP channels play pivotal roles in maintaining plasma membrane potential and in regulating cell excitability, and these channels have been considered a key target for treatment of type 2 diabetes (Mannhold, 2004Go). Indeed, many KATP channel blockers, including tolbutamide, glyburide, gliclazide, nateglinide, and repaglinide, have been used for many years for treatment of type 2 diabetes (Gromada et al., 1995Go; Malaisse, 1995Go; Hu et al., 1999Go). Conversely, because KATP channels are widely expressed in a variety of different tissues, including cardiac muscle, smooth muscle, skeletal muscle, and the nervous system (Mannhold, 2004Go), blockade of KATP channels in nonpancreatic tissues during treatment of type 2 diabetes using KATP channel blockers may cause some severe side effects (Engler and Yellon, 1996Go). For example, the opening of sarcolemmal KATP channels protects cardiac myocytes against ischemic injury by energy sparing, whereas the opening of both sarcolemmal and mitochondrial KATP channels is necessary for ischemia preconditioning (Zhuo et al., 2005Go). In patients with type 2 diabetes treated with KATP channel blockers, the major cause of death is cardiovascular disease, which has been argued to be, at least in part, related to the side effects of sulfonylureas in blocking cardiovascular KATP channels (Brady and Terzic, 1998Go; Klamann et al., 2000Go). Therefore, there is a substantial need to develop novel types of pancreatic beta-cell KATP channel blockers that exhibit little blocking effects on, or even open, vascular KATP channels. In the present study, we show, for the first time, that iptakalim, a vascular KATP channel opener, closed beta-cell KATP channels, depolarized beta-cells, elevated beta-cell intracellular Ca2+ concentrations, and increased insulin release. The finding that iptakalim blocked pancreatic beta-cell KATP channels indicates that iptakalim is a highly promising compound that may potentially satisfy therapeutic demands. Evidence has indicated that iptakalim exerts remarkable protective effects against hypertension without exhibiting clear effects on cardiac function in a variety of in vivo and in vitro animal models (Wang, 1998Go, 2003Go; Wang et al., 2005aGo). Although iptakalim exhibited high-affinity opening of vascular KATP channels but low-affinity closure of beta-cell KATP channels, our data showed that at a concentration (100 µM) that closed beta-cell KATP channels, iptakalim opened vascular-type KATP channels (Fig. 8B). As a small molecule, being water-soluble, having the ability to freely penetrate the blood-brain barrier, and considering its unique property of bidirectional regulation of pancreatic beta-cell and vascular KATP channels, iptakalim exhibits great potential to serve as, and stimulate the development of, a new generation of antidiabetic (type 2 diabetes) drugs, and it is particularly desirable for treatment of type 2 diabetes in patients that have accompanying vascular disorders.


   Acknowledgements
 
We thank Kevin Ellsworth for assistance in preparing this manuscript, Dr. H. Wang for providing iptakalim as a gift, and Dr. L. Y. Jan for provision of cDNA for the expression of KATP channels in Xenopus oocytes.


   Footnotes
 
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.

doi:10.1124/jpet.107.121129.

ABBREVIATIONS: KATP channel, ATP-sensitive potassium channel; SUR, sulfonylurea receptor; HEK, human embryonic kidney; NPo, product of the total number of functional channels present in the patch membrane (N) and the probability that a particular channel is open under steady-state conditions (Po).

Address correspondence to: Dr. Jie Wu, Neurology Research, Barrow Neurological Institute, 350 West Thomas Rd., Phoenix, AZ 85013-4496. E-mail: jie.wu{at}chw.edu


   References
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