State-dependent modification of ATP-sensitive K+ channels by phosphatidylinositol 4,5-bisphosphate

Midori Okamura, Masafumi Kakei, Koutaro Ichinari, Akihiro Miyamura, Naoya Oketani, Nobuyuki Koriyama, and Chuwa Tei

First Department of Internal Medicine, Faculty of Medicine, Kagoshima University, Kagoshima 890-8520, Japan


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

With inside-out patch recordings in ventricular myocytes from the hearts of guinea pigs, we studied ATP-sensitive K+ (KATP) channels activated by phosphatidylinositol 4,5-bisphosphate (PIP2) with respect to sensitivity to ATP when in either a rundown state (RS) or a non-rundown state (NRS). Rundown of KATP channels was induced by exposure either to ATP-free solution or to ATP-free solution containing 19 µM Ca2+. Exposure of membrane patches to 10 µM PIP2 reactivated channels with both types of rundown. The reactivation by PIP2 did not require ATP in the bath. The IC50 of channels recovered from RS and before the rundown was 37.1 and 31.1 µM, respectively. PIP2 irreversibly increased the mean current when the channel was in the NRS. This was associated with a shift of IC50 to 250.6 µM after PIP2 exposure. PIP2 activates NRS KATP channels by decreasing their sensitivity to ATP, whereas PIP2 reactivates RS-KATP channels independently of ATP without changing ATP sensitivity.

channel rundown; ventricular myocytes


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

ATP-SENSITIVE K+ (KATP) channels participate importantly in the physiology and pathophysiology of various tissues such as cardiac muscle cells, pancreatic beta -cells, skeletal muscle cells, smooth muscle cells, and cells of the ventromedial hypothalamic nucleus (1, 2, 18, 28). In the heart, the KATP channel has been implicated in action potential shortening during ischemia and may have a cardioprotective role in severe ischemic events (20, 34, 36). The opening probability of the KATP channel is substantially decreased by micromolar intracellular concentrations of ATP, which is now known to be a channel regulator in addition to functioning as a carrier of energy and a promoter of chemical reactions within the cell (20). The channel inhibition does not occur via phosphorylation of the channel protein, because a nonhydrolyzable analog of ATP inhibits KATP channel activity (1, 2, 20). KATP channel activity gradually decreases when the integrity of the cell is disrupted by excising a patch membrane (rundown phenomenon). Thus maintenance of the KATP channel activity also may depend on another regulatory process that may involve soluble elements as yet unidentified.

At the cellular level, phospholipids and their metabolites in the membrane have been shown to be essential in the regulation of cell functions. In addition to serving as important structural components of cell membranes, phospholipids generate signaling molecules that transduce the action of hormones or transmitters interacting with cell surface receptors to cytoplasmic effectors. Recently, a novel role of membrane phospholipids has been described concerning regulation of KATP channel activity: this has been reviewed by Hilgemann (10). Phosphatidylinositol 4,5-bisphosphate (PIP2) is a phospholipid that has been reported to maintain activity of KATP channels (11) undergoing Ca2+-induced rundown (5). Recent experiments have demonstrated that wortmannin blocked MgATP-dependent recovery of activity of channels reconstituted with Kir6.2/sulfonylurea (SUR)-2A, subunits of KATP channels, that ran down in a Ca2+-containing solution (33). Because exposure to PIP2 produced recovery of the rundown channels even in the presence of wortmannin, membrane lipid phosphorylation and production of PIP2 or phosphatidylinositol 3,4,5-trisphosphate appear to be principal regulators of rundown KATP channels (33). The effectiveness of anionic phospholipids, such as phosphatidylinositol 4-monophosphate and PIP2, in activating KATP channels has been found to be proportional to the number of negative charges on the head group of the phospholipid (4). PIP2 reduces the ATP sensitivity of the channel (3, 24). These findings suggest that PIP2 may determine ATP sensitivity of the channel by modulating ATP binding to positively charged amino acids in the COOH-terminal portion of Kir6.2, where ATP binding reportedly closes the channel (30). However, whether KATP channels reactivated from the rundown state (RS) by PIP2 had decreased sensitivity to ATP remains unclear. In the present work, we studied the effects of PIP2 on KATP channels in either an RS or a non-RS (NRS) with respect to changes in ATP sensitivity.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Isolation of single ventricular myocytes. Single ventricular myocytes were isolated from the hearts of guinea pigs (200-300 g) as previously described (27). Briefly, the animals were anesthetized by an intraperitoneal injection of pentobarbital sodium (25 mg/kg) and artificially ventilated. The chest was opened, the aorta was cannulated, and the coronary arteries were perfused with Tyrode solution. The heart then was removed, suspended on a Langendorff apparatus, and perfused with Tyrode solution containing (in mM): 136.9 NaCl, 5.4 KCl, 1.8 CaCl2, 0.5 MgCl2, 0.33 NaH2PO4, 5.5 glucose, and 5.0 HEPES-NaOH (pH 7.4). The perfusate was changed to nominally Ca2+-free Tyrode solution (100 ml) and a digestion solution containing 0.04% type II collagenase (Worthington) for 10 min. To wash out collagenase, the heart was perfused with 100 ml of Kraftbrühe (K-B) solution (12) containing (in mM): 40 KCl, 20 taurine, 50 glutamic acid, 20 KH2PO4, 3.0 MgCl2, 0.5 EGTA, 10 glucose, and 10 HEPES-KOH (pH 7.4). The left ventricle was removed and cut into small pieces (5 mm2) in K-B solution and dispersed mechanically into single cells. The dispersed myocytes were maintained in K-B solution at 4°C for at least 1 h before use. Next, single ventricular myocytes were placed in a recording chamber (500 µl) that was filled with Tyrode solution; the solution then was changed to the standard internal solution containing (in mM): 134.5 KCl, 0.5 KH2PO4, 2.0 MgCl2, 1.0 EGTA, and 5.0 HEPES-KOH (pH 7.2). The final K+ concentration of this solution was 140 mM, and the free Ca2+ concentration was calculated to be <0.1 nM. To evoke Ca2+-induced channel rundown, 1 mM CaCl2 was added to this solution resulting in a free Ca2+ concentration of 19 µM as calculated using the Chelator software package provided by van Heeswijk et al. (32). The Mg2+-free internal solution was nominally free of Mg2+ and contained 1 mM EDTA rather than equimolar EGTA [other constituents (in mM): 5 HEPES, 0.5 KH2PO4, and 134.5 KCl]. The pH was adjusted to 7.2 with KOH; the free Mg2+ concentration was <1 nM. ATP was added to these internal solutions as required.

Single-channel current recordings. Standard patch-clamp techniques (9) were used to record single KATP channel currents from inside-out membrane patches. Patch pipettes were pulled from hard glass tubing (Sutter Instruments), coated with silicon resin to reduce their electrical capacitance, and fire-polished immediately before use. The composition of the pipette solution was as follows (in mM): 140 KCl, 2.0 CaCl2, and 5.0 HEPES-NaOH (pH 7.4). The pipettes had resistances between 5 and 10 MOmega when filled with this solution. After the GOmega seal had been established on a cell exposed to standard internal solution containing 0.3 mM ATP, the patch membrane was excised. The patch membrane potential was held at -60 mV.

KATP channel current was recorded with an amplifier (AXOPATCH 200A; Axon Instruments, Foster City, CA), and the data were stored in a PCM digital data recorder (RD-101T; TEAC, Tokyo, Japan). Replayed data then were low-pass filtered (24 dB/octave; E-3201A; NF, Tokyo, Japan) at the cut-off frequency of 1 kHz and digitized at 5 kHz by using pCLAMP6 software (Axon Instruments) and an IBM computer (New York, NY). In some experiments, the single-channel current was recorded online.

To examine dose-response relationships between channel activity and ATP concentrations, we recorded the mean KATP channel current during superfusion with each test solution. We typically calculated the average current during a 20- to 30-s period under steady-state conditions. Relative channel activity was plotted as a function of ATP concentrations, and each point was fitted to the following Hill equation
I/I<SUB>c</SUB><IT>=1/</IT>{<IT>1+</IT>([ATP]<IT>/</IT>IC<SUB><IT>50</IT></SUB>)<SUP><IT>h</IT></SUP>}
where I is current, Ic is the control mean current recorded at 0 mM ATP, IC50 is the ligand (ATP) concentration producing half-maximal inhibition of channel activity, and h is the Hill coefficient. When openings of ATP-insensitive channels were observed, the mean current of this type of channel was measured in the presence of 3-10 mM ATP, and it was subtracted from the current amplitudes tested at various concentrations of ATP.

Chemicals and conditions. ATP-2Na, PIP2, and adenosine 5'-O-(3-thiotriphosphate) (ATPgamma S) were purchased from Boehringer (Mannheim, Germany). PIP2 was dissolved in DMSO (0.1%) diluted in the internal solution containing required ATP concentration by sonication on ice for 30 min. Stimulatory effects of PIP2 on activity of KATP channels seemed to be reduced ~5 h after newly making PIP2-containing solution. In that case, we prepared a new PIP2-containing solution every 4 h throughout the experiments. Glibenclamide was purchased from Sigma (St. Louis, MO). All experiments were performed at room temperature (22-25°C).

Statistical analysis. All data are expressed as means ± SE from the number of membrane patches. Data were analyzed with Wilcoxon's signed-rank test or Mann-Whitney's U test, with P < 0.05 accepted as statistically significant.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Effect of PIP2 on rundown KATP channels. A rundown state of the KATP channel (RS channel) was induced by superfusion with ATP-free solution containing either free Ca2+ concentrations <1 nM (spontaneous rundown) or 19 µM Ca2+ (Ca2+-induced rundown; Fig. 1, A and B). Activity of the KATP channel current, which was fully activated at initiation of recording without ATP, decreased with time during subsequent exposure of the membrane patch to the ATP-free solution (Fig. 1A). The time course of the channel rundown process in ATP-free solution varied among the membrane patches tested. In general, rundown of the channel was attained with ~7.6 ± 3.5 min (n = 10) of exposure to the ATP-free solution (when the rundown is defined by 90% reduction of the channel current levels in control). When rundown of the channel was studied with the membrane patch exposed to a high-Ca2+ concentration (Fig. 1B), rundown occurred much more rapidly than in the spontaneous rundown observed under low-Ca2+ concentration (Fig. 1, B vs. A). In membrane patches exposed to 19 µM Ca2+, we observed rundown of the channel activity within 20 s (n = 22). In both situations, exposure of the patch membrane to PIP2 (10 µM) slowly reactivated the KATP channel. The current reactivated by PIP2 was a result of KATP channel opening, because intermittent exposure to various concentrations of ATP dose dependently inhibited the channel, and single-channel unitary currents were identical to those observed in the control (Fig. 1B, see also inset). Dose-response relationships between channel activity and ATP concentrations before the rundown and after reactivation of channels by PIP2 were plotted (Fig. 1C). In these experiments, we used 19 µM Ca2+ exposure for 20-60 s to produce the channel rundown and subsequently superfused 10 µM PIP2 and 0 mM ATP solution intermittently for 1 min each to reactivate the channel. ATP concentration-dependent inhibition of the channel then was examined using the protocol illustrated in Fig. 1B. Once recovery of channel activity was established after exposure to PIP2, the channel openings were maintained after washout of PIP2. The IC50 of control and reactivated channels after Ca2+-induced rundown were 31.1 and 37.1 µM, respectively (n = 5). Reactivation of RS channels by PIP2 did not require the presence of ATP in the bathing solution (Fig. 1A).


View larger version (32K):
[in this window]
[in a new window]
 
Fig. 1.   Reactivation of rundown-state (RS) ATP-sensitive K+ channels (KATP) by phosphatidylinositol 4,5-biphosphate (PIP2) without a shift of ATP sensitivity. A and B: activity of channels ran down in either an ATP-free state with low-Ca2+ concentration [spontaneous rundown (A)] or an ATP-free state with high-Ca2+ concentration [Ca2+-induced rundown (B)]. The arrows indicate the zero-current level, and the bars above the current tracings indicate application of each test solution. The break in the current tracing in A represents deletion of current data for 5 or 20 min. In B, PIP2 at 10 µM was intermittently applied in the absence of ATP for 1 min, and the solution containing either 0.03 or 0.3 mM ATP was superfused to monitor the ATP sensitivity. The arrow indicates the zero-current level. The current tracings in insets a and b illustrate openings of KATP channels overlapping openings of inward rectifying K+ channels with a small conductance. Dotted lines in the inset indicate the baseline level. C: concentration-dependent inhibition of channel openings before rundown and after reactivation of the RS channel by 10 µM PIP2 was illustrated. Curves were drawn according to the Hill equation with the Hill coefficient of 1.2. Data points were plotted showing the lower half of error bars before rundown (n = 18, ), showing the upper half after reactivation after Ca2+-induced rundown (n = 5, ) and lower half after reactivation after spontaneous rundown (n = 4, open circle ). There is no significant difference among each result obtained at the same ATP concentration.

PIP2 may counteract Ca2+-induced channel rundown. The time course of rundown induced by 19 µM Ca2+ was extremely rapid (Figs. 1B and 2). After reactivation of rundown channels by 10 µM PIP2, the second application of 19 µM Ca2+ resulted in a much slower decline of channel current than was seen with the first application (Fig. 2). Thus PIP2 may be retained in the lipid membrane and may counteract the Ca2+-induced rundown process even after washout.


View larger version (15K):
[in this window]
[in a new window]
 
Fig. 2.   Comparison of the time course of rundown before and after exposure of the membrane patches to PIP2. After the KATP channel current in the absence of ATP was recorded, 19 µM Ca2+ was applied to induce rundown. Then, 10 µM PIP2 was superfused to reactivate the channel. When the current level reached a steady state, exposure to 19 µM Ca2+ was repeated.

Effect of PIP2 on non-rundown KATP channels. PIP2 activated the NRS KATP channels regardless of the presence of ATP. Figure 3A depicts current tracings recorded in the presence of 0.3 mM ATP showing activation of channels by exposure to 10 µM PIP2. Subsequent exposure to 3 mM ATP reduced channel activity that was greater than that at 0.3 mM ATP before PIP2 treatment. Thus loss of sensitivity of channels to ATP after PIP2 treatment was suggested. Because glibenclamide, a specific KATP channel blocker, inhibited the channels, the activation of channels observed involved opening of KATP channels. The blocking efficacy by 10 µM glibenclamide of the activity of the channels was 83.1 ± 8.0% (n = 5, P < 0.02) compared with 100% inhibition attainment without PIP2 treatment. Single-channel conductance under control conditions and after PIP2 treatment did not change. The slope conductance was 83.3 pS both under control conditions and with PIP2 treatment (data not shown). Mean open and closed times for the channels in the burstlike openings were, respectively, 2.11 ± 0.02 (n = 5) and 0.27 ± 0.02 ms (n = 5) under control conditions and 2.33 ± 0.02 (n = 5) and 0.23 ± 0.03 ms (n = 5) with PIP2 exposure when they were analyzed in open- and closed-time histograms. Thus kinetic properties during burstlike openings were not influenced by PIP2. These results indicated that PIP2 increased burst durations and reduced interburst intervals of channels and consequently increased opening probability in association with decreasing glibenclamide sensitivity. Dose-response relationships of channel inhibition with ATP before and after PIP2 treatment are presented in Fig. 3B. The curves fit IC50 of 31.1 µM under control conditions and 250.6 µM after exposure to PIP2. Thus activation of channels by PIP2 was due to reduction of the ATP sensitivity in the case of the NRS channel. The KATP channel was fully activated by decreasing the ATP concentration from 0.3 to 0 mM ATP (Fig. 4A), indicating that the channel was not run down in this state. Of course, subsequent application of PIP2 no longer increased the channel activity, and repetitive exposure to various concentrations of ATP resulted in reduction of sensitivity to ATP (Fig. 4A, bottom). Thus exogenously applied PIP2 may be kept in the lipid membrane to some extent even after washout and decreases the ATP sensitivity of NRS channels. SUR receptors have a domain where MgATP binds and is hydrolyzed (31). It has been reported that addition of MgATP in the presence of SUR led to an increase in ADP concentration by the hydrolysis (8). These results may suggest a mechanistic possibility for PIP2-induced reduction of ATP sensitivity of the channel that PIP2 might increase the rate of hydrolysis of MgATP at SUR. PIP2 activated the channel in the presence of 0.3 mM ATPgamma S, a nonhydrolyzable analog of ATP (Fig. 4B), indicating that reduction of ATP sensitivity by PIP2 did not involve hydrolysis of ATP. This was further confirmed in following experiments. After the ATP sensitivity of the channel was tested in the presence of Mg2+, Mg2+ was removed in the presence of 0.3 mM ATP. Channel opening at 0.3 mM ATP was increased by reducing Mg2+, and the channel was fully activated by 10 µM PIP2. Subsequent exposure to various concentrations of MgATP revealed decreased sensitivity of the channel to ATP (Fig. 4C). These results suggest that PIP2-induced reduction of ATP sensitivity did not depend on the presence of MgATP.


View larger version (30K):
[in this window]
[in a new window]
 
Fig. 3.   PIP2-induced activation of non-rundown channels. A: exposure to PIP2 in the presence of 0.3 mM MgATP activated the channel. Subsequent exposure to 3 mM ATP partially inhibited opening of the channels, which also was inhibited by 10 µM glibenclamide. B: concentration-dependent inhibition of the channel by ATP before () and after () exposure to 10 µM PIP2. Error bars indicate means ± SE. Normalized data for mean current obtained during exposure to various concentrations of ATP before (n = 18) and after (n = 5) exposure to PIP2 were plotted. The data at the same ATP concentrations under control conditions vs. PIP2 treatment differed significantly (P < 0.05).



View larger version (30K):
[in this window]
[in a new window]
 
Fig. 4.   Effects of PIP2 on non-rundown state KATP channels under dephosphorylated conditions. A: PIP2 was added to fully activated KATP channel current in ATP-free solution. Subsequent repetitive exposure to various concentrations of ATP showed reduced sensitivity of the channel to ATP. B: PIP2 increased openings of the channel in the presence of 0.3 mM adenosine 5'-O-(3-thiotriphosphate) (ATPgamma S). C: exposure to PIP2 in the presence of 0.3 mM ATP without Mg2+ decreased sensitivity to ATP. The channel was fully activated by 10 µM PIP2, and the following exposure to ATP showed attenuation of ATP sensitivity. Top and bottom current traces in A and C were continuous.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

In the present study, we demonstrated that PIP2 reactivated KATP channels after channel activity ran down in ATP-free bathing solution with high concentrations of Ca2+. ATP sensitivity of the reactivated channel was not changed compared with that of control. On the other hand, NRS KATP channels were similarly activated by PIP2 in association with much lower sensitivity to ATP. Thus PIP2 may influence gating of channels by mechanisms that differ for RS and NRS channels.

The time course of rundown of channel activity in the presence of 19 µM Ca2+ (Ca2+-induced rundown) was more rapid than that induced in ATP-free solution with low-Ca2+ concentration (spontaneous rundown). PIP2 reactivated both types of rundown channel (3, 4, 24, 33). After treatment of membrane patches with 10 µM PIP2 for 5 min, the time course of rundown during exposure to Ca2+ was slower than that seen before PIP2 treatment (Fig. 2). Thus we speculate that Ca2+ and PIP2 either allosterically interact to regulate channel activity, or both elements may compete at the same site. Recovery from spontaneous rundown of the KATP channel is known to occur after exposure to an MgATP-containing solution (21, 26). Because MgATP, but neither a nonhydrolyzable analog of ATP nor ATP4-, can mimic this phenomenon, a phosphorylation mechanism has been suspected (18, 21). However, a recent study demonstrated this MgATP-dependent reactivation of the channel to be mimicked by PIP2 in the absence of ATP, and MgATP-induced reactivation was prevented by an inhibitor of phosphatidylinositol 3- and 4-kinases, wortmannin (33). Thus lipid phosphorylation may be a principal mechanism for the reactivation of the rundown channel, as opposed to reactivation by protein phosphorylation involving the channel per se or related elements. Spontaneous or Ca2+-induced rundown of the channel may be due to deficiency of PIP2 in the membrane. Depolymerization of actin filaments (F actin) may be involved in the mechanism for KATP-channel rundown. F actin is disrupted when actin-binding proteins are activated by elevated cytosolic Ca2+ concentrations (19). Depolymerized actin (G actin) may not be able to maintain high opening probability of the channel in ATP-free solution (5). Furukawa et al. (5) have reported that actin-depolymerizing agents such as cytochalasins B and D or DNase I enhanced the rate of rundown of the KATP channel. In contrast, these authors found that phalloidin, an actin-filament stabilizer, as well as PIP2 prevented rundown of the channel. These results suggest that the channel is regulated by the actin system. Ca2+ activates actin-binding proteins that sever and cap F actin, and disruption of the network readily results (13). PIP2 has been reported to inhibit the activity of Ca2+-sensitive F actin-severing proteins (14, 17, 19, 25) and to remove these actin-binding proteins from the barbed ends of depolymerized F actin. Thus the presence of PIP2 may foster repolymerization of G actin.

Rundown may be enhanced by dissolution of PIP2 mediated by endogeneous Ca2+-dependent phospholipase C (33). Actin filament disrupters reportedly can attenuate ATP-dependent inhibitory gating of KATP channels (29, 35). The hypothesis that Ca2+-dependent depolymerization and PIP2-induced repolymerization of the actin filament network in the submembrane region may be a major mechanism underlying both KATP channel rundown and reactivation remains to be tested.

Fan and Makielski (4) have postulated an electrostatic interaction between the anionic head groups of PIP2 and the positively charged amino acid residues of the COOH terminus of Kir6.2 to explain channel activation by PIP2 associated with a shift of ATP sensitivity. Similar findings demonstrating that PIP2 reduced the ATP sensitivity of the channel have been reported consistently (3, 24). SUR has been suggested to have a domain of ATPase activity, and hydrolysis of ATP is needed to transmit the signaling of binding of potassium channel openers to pore subunits, Kir6.2, to open the channel (22, 31). When 1-3 mM ATP and membranes including SUR were present, ADP at the end of incubation was increased by 5-15 times ranging from 58 to 92 µM (8). Thus ATP hydrolysis at submembrane may produce micromolar levels of MgADP, and this concentration may be high enough to increase the activity of KATP channels because MgADP is a KATP channel stimulator (6, 15, 36). Whether these phenomena occur at a site close to channels and are involved in PIP2-induced loss of ATP sensitivity in KATP channels were tested in Fig. 4, B and C. Activation of the channel by PIP2 associated with decreased ATP sensitivity did not require the presence of MgATP or Mg2+. Thus hydrolysis of ATP is not involved in PIP2-induced reduction of ATP sensitivity. An expression of mutant phosphatidylinositol-4-phosphate 5-kinase (PIP5K), inactive form, did not influence the IC50 of wild-type KATP channels, whereas it increased ATP sensitivity of a mutant KATP channel that has lower ATP sensitivity (23). PIP5K may regulate the PIP2 concentration in the membrane and determine the ATP sensitivity of the channel. The findings that PIP2 did not lower the ATP sensitivity of the channel reactivated from the rundown state but lowered NRS channels suggest that an unidentified element distal to PIP2 may be involved in the PIP2-induced modification of the ATP sensitivity, and this may be lost in rundown process of the channel.

High affinity of tolbutamide for channel inhibition was reportedly impaired when the membrane patches were treated with PIP2 (16). We used 10 µM glibenclamide to test whether the channel openings stimulated by PIP2 were those of KATP channels (Fig. 3). Although the channel has sensitivity to glibenclamide, the channel was not closed completely even at 10 µM glibenclamide. The blocking efficacy of 10 µM glibenclamide on KATP channel after PIP2 treatment was 83.1 ± 8.0% (n = 5) compared with 100% inhibition for the channels without treatment of PIP2. It has been suggested that glibenclamide binds to the channels with high- and low-affinity sites (7). Low-affinity binding of glibenclamide might be impaired after PIP2 treatment. This hypothesis remains elucidated.


    ACKNOWLEDGEMENTS

This work was supported in part by a grant-in-aid for Scientific Research from the Ministry of Education, Science, and Culture of Japan (to M. Kakei).


    FOOTNOTES

Address for reprint requests and other correspondence: M. Kakei, First Dept. of Internal Medicine, Faculty of Medicine, Kagoshima Univ., 8-35-1 Sakuragaoka, Kagoshima 890-8520, Japan (E-mail: mkakei{at}med4.kufm.kagoshima-u.ac.jp).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 2 February 2000; accepted in final form 18 September 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Ashcroft, FM. Adenosine 5'-triphosphate-sensitive potassium channels. Annu Rev Neurosci 11: 97-118, 1988[ISI][Medline].

2.   Ashcroft, SJH, and Ashcroft FM. Properties and functions of ATP-sensitive K-channels. Cell Signal 2: 197-214, 1990[ISI][Medline].

3.   Baukrowitz, T, Schulte U, Oliver D, Herlitze S, Krauter T, Tucker SJ, Ruppersberg JP, and Fakler B. PIP2 and PIP as determinants for ATP inhibition of channels. Science 282: 1141-1144, 1998[Abstract/Free Full Text].

4.   Fan, Z, and Makielski JC. Anionic phospholipids activates ATP-sensitive potassium channels. J Biol Chem 272: 5388-5395, 1997[Abstract/Free Full Text].

5.   Furukawa, T, Yamane T, Terai T, Katayama Y, and Hiraoka M. Functional linkage of the cardiac ATP-sensitive K+ channel to the actin cytoskeleton. Pflügers Arch 431: 504-512, 1996[ISI][Medline].

6.   Gribble, FM, Tucker SG, and Ashcroft FM. The essential role of the Walker A motifs of SUR1 in K-ATP channel activation by Mg-ADP and diazoxide. EMBO J 16: 1145-1152, 1997[Abstract/Free Full Text].

7.   Gribble, FM, Tucker SG, Seino S, and Ashcroft FM. Tissue specificity of sulfonylureas. Studies on cloned cardiac and beta -cell channels. Diabetes 47: 1412-1418, 1998[Abstract].

8.   Hambrock, A, Lffler-Walz C, Kloor D, Delabar U, Horio Y, Kurachi Y, and Quast U. ATP-sensitive K+ channel modulator binding to sulfonylurea receptors SUR2A and SUR2B: opposite effects of MgADP. Mol Pharmacol 55: 832-840, 1999[Abstract/Free Full Text].

9.   Hamill, OP, Marty A, Neher E, Sakmann B, and Sigworth F. Improved patch-clamp techniques for high-resolution current recording from cells and cell-free membrane patches. Pflügers Arch 391: 85-100, 1981[ISI][Medline].

10.   Hilgemann, DW. Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers. Annu Rev Physiol 59: 193-220, 1997[ISI][Medline].

11.   Hilgemann, DW, and Ball R. Regulation of cardiac Na+, Ca2+ exchange and potassium channels by PIP2. Science 273: 956-959, 1996[Abstract].

12.   Isenberg, G, and Klöckner U. Calcium tolerant ventricular myocytes prepared by preincubation in a KB medium. Pflügers Arch 395: 6-18, 1982[ISI][Medline].

13.   Janmey, PA. Phosphoinositides and calcium as regulators of cellular actin assembly and disassembly. Annu Rev Physiol 56: 169-191, 1994[ISI][Medline].

14.   Janmey, PA, and Stossel TP. Modulation of gelsolin function by phosphatidylinositol 4,5-bisphosphate. Nature 325: 362-364, 1987[ISI][Medline].

15.   Kakei, M, Kelly RP, Ashcroft SJ, and Ashcroft FM. The ATP-sensitivity of K+ channels in rat pancreatic beta -cells is modulated by ADP. FEBS Lett 208: 63-66, 1986[ISI][Medline].

16.   Koster, JC, Sha Q, and Nichols CG. Sulfonylurea and K+-channel opener sensitivity of channels, functional coupling of Kir6.2 and SUR1 subunits. J Gen Physiol 114: 203-213, 1999[Abstract/Free Full Text].

17.   Lassing, I, and Lindberg U. Specific interaction between phosphatidylinositol 4,5-bisphosphate and profilactin. Nature 314: 472-474, 1985[ISI][Medline].

18.   Lazdunski, M. ATP-sensitive potassium channels: an overview. J Cardiovasc Pharmacol 24, Suppl 4: S1-S5, 1994[ISI][Medline].

19.   Nelson, TY, and Boyd AE, III. Gelsolin, a Ca2+-dependent actin-binding protein in a hamster insulin-secreting cell line. J Clin Invest 75: 1015-1022, 1985[ISI][Medline].

20.   Noma, A. ATP-regulated K+ channels in cardiac muscle. Nature 305: 147-148, 1983[ISI][Medline].

21.   Ohno-Shosaku, T, Zünkler BJ, and Trube G. Dual effects of ATP on K+ currents of mouse pancreatic beta -cells. Pflügers Arch 408: 133-138, 1987[ISI][Medline].

22.   Schwanstecher, M, Sieverding C, Drschner H, Gross I, Aguilar-Bryan L, Schwanstecher C, and Bryan J. Potassium channel openers require ATP to bind to and act through sulfonylurea receptors. EMBO J 17: 5529-5535, 1998[Abstract/Free Full Text].

23.   Shyng, S-L, Barbieri A, Gumusboga A, Cukras C, Pike L, Davis JN, Stahl PD, and Nichols CG. Modulation of nucleotide sensitivity of ATP-sensitive potassium channels by phosphatidylinositol-4-phosphate 5-kinase. Proc Natl Acad Sci USA 97: 937-941, 2000[Abstract/Free Full Text].

24.   Shyng, S-L, and Nichols CG. Membrane phospholipid control of nucleotide sensitivity of channels. Science 282: 1138-1141, 1998[Abstract/Free Full Text].

25.   Stossel, TP. From signal to pseudopod. How cells control cytoplasmic actin assembly. J Biol Chem 264: 18261-18264, 1989[Free Full Text].

26.   Takano, M, Qin D, and Noma A. ATP-dependent decay and recovery of K+ channels in guinea pig cardiac myocytes. Am J Physiol Heart Circ Physiol 258: H45-H50, 1990[Abstract/Free Full Text].

27.   Taniguchi, J, Kokubun S, Noma A, and Irisawa H. Spontaneously active cells isolated from the sino-atrial and atrio-ventricular nodes of the rabbit heart. Jpn J Physiol 31: 547-558, 1981[ISI][Medline].

28.   Terzic, A, Jahangir A, and Kurachi Y. Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs. Am J Physiol Cell Physiol 269: C525-C545, 1995[Abstract].

29.   Terzic, A, and Kurachi Y. Actin microfilament disrupters enhance channel opening in patches from guinea-pig cardiomyocytes. J Physiol (Lond) 492.2: 395-404, 1996[Abstract].

30.   Tucker, SJ, Gribble FM, Zhao C, Trapp S, and Ashcroft FM. Truncation of Kir6.2 produces ATP-sensitive K+ channels in the absence of the sulphonylurea receptor. Nature 387: 179-183, 1997[ISI][Medline].

31.   Ueda, K, Inagaki N, and Seino S. MgADP antagonism of Mg2+-independent ATP binding of the sulfonylurea receptor SUR1. J Biol Chem 272: 22983-22986, 1997[Abstract/Free Full Text].

32.   Van Heeswijk, MPE, Geertsen JAM, and van Os CH. Kinetic properties of the ATP-dependent Ca2+ pump and the Na+/Ca2+ exchange system in basolateral membranes from rat kidney cortex. J Membr Biol 79: 19-31, 1984[ISI][Medline].

33.   Xie, LH, Takano M, Kakei M, Okamura M, and Noma A. Wortmannin, an inhibitor of phosphatidylinositol kinases, blocks the MgATP-dependent recovery of Kir6.2/SUR2A channels. J Physiol (Lond) 514.3: 655-665, 1999[Abstract/Free Full Text].

34.   Yao, Z, Cavero I, and Gross GJ. Activation of cardiac channels: an endogenous protective mechanism during repetitive ischemia. Am J Physiol Heart Circ Physiol 264: H495-H504, 1993[Abstract/Free Full Text].

35.   Yokoshiki, H, Katsube Y, Sunagawa M, Seki T, and Sperelakis N. Disruption of actin cytoskeleton attenuates sulfonylurea inhibition of cardiac ATP-sensitive K+ channels. Pflügers Arch 434: 203-205, 1997[ISI][Medline].

36.   Yokoshiki, H, Sunagawa M, Seki T, and Sperelakis N. ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells. Am J Physiol Cell Physiol 274: C25-C37, 1998[Abstract/Free Full Text].


Am J Physiol Cell Physiol 280(2):C303-C308
0363-6143/01 $5.00 Copyright © 2001 the American Physiological Society