Published online before print July 18, 2002, doi: 10.1161/01.RES.0000029971.60214.49
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© 2002 American Heart Association, Inc.
Cellular Biology |
1-Adrenoceptor-Mediated Breakdown of Phosphatidylinositol 4,5-Bisphosphate Inhibits Pinacidil-Activated ATP-Sensitive K+ Currents in Rat Ventricular Myocytes
From the Departments of Cardiovascular Medicine (T.H., H.Y., H.O., T.N., M.H.) and Physiology and Biophysics (L.-H.X., M.T.), Kyoto University Graduate School of Medicine, Kyoto, Japan, and Pediatric Cardiology (T.Y.N., W.A.C.), NYU School of Medicine, New York, NY.
Correspondence to Dr Minoru Horie, Department of Cardiovascular Medicine, Graduate School of Medicine, Kyoto University, 54 Shogoin Kawahara-cho Sakyo-ku, Kyoto 606-8507, Japan. E-mail horie{at}kuhp.kyoto-u.ac.jp
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Abstract |
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Phosphatidylinositol 4,5-bisphosphate (PIP2) stimulates ATP-sensitive K+ (KATP) channel activity. Because phospholipase C (PLC) hydrolyzes membrane-bound PIP2, which in turn may potentially decrease KATP channel activity, we investigated the effects of the
1-adrenoceptor-Gq-PLC signal transduction axis on pinacidil-activated KATP channel activity in adult rat and neonatal mouse ventricular myocytes. The 1-adrenoceptor agonist methoxamine (MTX) reversibly inhibited the pinacidil-activated KATP current in a concentration-dependent manner (IC50 20.9±6.6 µmol/L). This inhibition did not occur when the specific 1-adrenoceptor antagonist, prazosin, was present. An involvement of G proteins is suggested by the ability of GDPßS to prevent this response. Blockade of PLC by U-73122 (2 µmol/L) or neomycin (2 mmol/L) attenuated the MTX-induced inhibition of KATP channel activity. In contrast, the MTX response was unaffected by protein kinase C inhibition or stimulation by H-7 (100 µmol/L) or phorbol 12,13-didecanoate. The MTX-induced inhibition became irreversible in the presence of wortmannin (20 µmol/L), an inhibitor of phosphatidylinositol-4 kinase, which is expected to prevent membrane PIP2 replenishment. In excised inside-out patch membranes, pinacidil induced a significantly rightward shift of ATP sensitivity of the channel. This phenomenon was reversed by pretreatment of myocytes with MTX. Direct visualization of PIP2 subcellular distribution using a PLC
pleckstrin homology domain-green fluorescent protein fusion constructs revealed reversible translocation of green fluorescent protein fluorescence from the membrane to the cytosol after 1-adrenoceptor stimulation. Our data demonstrate that 1-adrenoceptor stimulation reduces the membrane PIP2 level, which in turn inhibits pinacidil-activated KATP channels.
Key Words: ATP-sensitive K+ channels • phosphatidylinositol 4,5-bisphosphate • 1-adrenoceptors
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Introduction |
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During myocardial ischemia, cardiac ATP-sensitive K+ (KATP) channels open and shorten the duration of action potentials. Among numerous modulators of KATP channel activity, exogenously applied phosphatidylinositol 4,5-bisphosphate (PIP2) produces two remarkable effects on both native and reconstituted KATP channels: (1) it increases the channel open probability (even that of channels existing in a rundown state), and (2) it decreases the ability of the KATP channels to be blocked by cytosolic ATP.1–4 More recently, we demonstrated that phosphorylation of membrane phosphoinositol (PI) to PIP2 is an indispensable process in keeping the channel operative, both for Kir6.2/SUR2A channels reconstituted in COS-7 cells and for native KATP channels in guinea pig ventricular myocytes.5–7 Therefore, the membrane level of PIP2 is a potential key regulator of KATP channel activity.
Levels of PIP2 at the membrane are regulated by a subtle balance between PIP2 breakdown (hydrolysis by phospholipase C [PLC]) and replenishment (by PI kinases).8 PLC activity features in the transduction pathways of several hormonal receptors, including the M1 muscarinic, 1-adrenergic, endothelin (ET)-1, and angiotensin II (Ang II) receptors.9 Therefore, we hypothesize that the stimulation of PLC-linked receptors may change the level of membrane-bound PIP2 and thereby modulate the activity of KATP channels. In support of this hypothesis, 1-adrenoceptor or ET receptor stimulation has been reported to inhibit inward rectifier K+ channels, including KATP channels.10–12 We have recently demonstrated that M1 muscarinic receptor stimulation inhibits the Kir6.2/SUR2A channel current through depletion of membrane PIP2.6 The physiological relevance and exact pathway by which 1-adrenoceptor-mediated inhibition occurs remains unknown. Therefore, we used rat ventricular myocytes to examine whether 1-adrenoceptor stimulation modulates native KATP channel activity. We used a pharmacological approach to delineate the pathway through which this occurs. In addition, we directly measured PIP2 levels using a fluorescent PIP2 reporter protein. Our data show that (as is the case for reconstituted channels) the 1-adrenoceptor pathway inhibits cardiac KATP channels through a reduction of membrane PIP2 levels.
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Materials and Methods |
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Single-Cell Preparation and Electrophysiology
All experimental protocols were performed in accordance with institutional animal guidelines. Ventricular myocytes were isolated from the hearts of adult Sprague-Dawley rats (Shimizu Experimental Animal Supply, Kyoto, Japan) using a collagenase perfusion method. Single ventricular myocytes were placed in a recording chamber and superfused with Tyrode’s solution containing (mmol/L) NaCl 143, NaH2PO4 0.3, KCl 5.4, MgCl2 0.5, CaCl2 1.8, and HEPES/NaOH 5, pH 7.4, adjusted by NaOH. The pipette solution for whole-cell patch clamping contained (mmol/L) aspartic acid 110, NaCl 10, MgCl2 1, K2ATP 4, GTP 0.1, BAPTA 10, and HEPES 5, pH 7.4, with KOH. In some experiments, the GTP was replaced with GDPßS (1 mmol/L). KATP currents were usually evoked by extracellular application of the KATP channel opener pinacidil (100 µmol/L) and were recorded by applying ramp pulses (±100 mV/s) every 6 seconds from a -40-mV holding potential to obtain quasi-instantaneous current-voltage (I-V) relations (from 20 to -120 mV). Membrane current was recorded using an amplifier (Axopatch 200A, Axon Instruments), low-pass-filtered (Bessel response, 1 kHz) before being acquired online (pClamp 6.1 version, Axon), and displayed on a chart recorder (Linerecorder, WR 3320, Graphtec).
Single KATP channel activities were recorded in the inside-out mode. Glucose-free Tyrode’s solution was used as a pipette solution. For the internal solution bathing the cytoplasmic surface of the patch membrane, a K+-rich solution was used. Its composition was (mmol/L) KCl 150, EGTA 0.5, and HEPES 5, pH 7.4, adjusted by KOH. Patch pipettes were prepared by pulling borosilicate glass capillaries (Hilgenberg) at 2 to 3 M
for whole-cell recording and at 5 to 7.5 M for single-channel recording when they were filled with each pipette solution.
Cell Culture and Transfection
Ventricular myocytes were isolated from 2- to 3-day-old neonatal Swiss Webster mice (Taconic, Germantown, NY) and dissociated into single isolated cells by trypsinization, as described previously.13 To exclude nonmyocytes, cells were preplated at 37°C for 45 minutes in culture medium (DMEM containing 10% FBS and antibiotics). The cell suspension was passed through a 100-µm nylon mesh (Falcon) and plated (1x105 per dish) onto a collagen-coated (type VI, 100 µg/mL, Sigma Chemical Co) glass coverslip attached to the bottom of a Petri dish (glass bottom microwells, Corning). One day after incubation at 37°C, myocytes were transfected with a PLC pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion construct (1 µg)14 using FuGENE6 reagent (Boehringer). COS-7 cells were plated onto poly-L-lysine (100 µg/mL)-coated glass coverslips and cultured for 1 day. Cells were transfected with the PH-GFP construct and 1c-adrenoceptors (or pCDNA3 as a control) (1 µg each) as described above. One day after transfection, the culture medium was changed to the serum-free DMEM to prevent possible contamination of receptor agonists that might be present in the serum.
Confocal Microscopy
Cells on glass coverslips were transferred in a recording chamber continuously perfused with Ca2+-free Tyrode’s solution at 31°C to 33°C (by temperature controller TC-324B, Warner Instruments Corp). Images were obtained with a confocal microscope (Carl Zeiss LSM 510 and a Leica DM-IRE2 inverted microscope fitted with a TCS SP2 scan head) equipped with a plan-apochromat x60 oil objective lens. Fluorescence was detected using an argon laser (488-nm line). Series of confocal images were taken at 10- to 30-second intervals. The line intensity profiles and the ratios of the averaged fluorescence signals from membrane and cytosol area were analyzed with Leica confocal software.
Drugs
Methoxamine (MTX), phenylephrine, prazosin (Sigma), and neomycin (Nakalai) were freshly prepared in various test solutions immediately before each experiment. Propranolol (1 µmol/L) was present throughout each experiment to eliminate possible ß-adrenergic actions of the agonists used. Wortmannin, U-73122, pinacidil glibenclamide (Sigma), and bimakalim (a generous gift from Merck, Darmstadt, Germany) were dissolved in dimethyl sulfoxide at 10 or 100 mmol/L (stock solution). Phorbol 12,13-didecanoate (PDD) or H-7 was also dissolved in dimethyl sulfoxide as a stock solution before being diluted to its final concentration in the experimental solution.
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Results |
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Stimulation of
1-Adrenoceptor Inhibits Pinacidil-Activated KATP CurrentPinacidil15 (100 µmol/L) consistently elicited an outward current at a holding potential of -40 mV (Figure 1A, a
b). I-V relationships (Figure 1B, 1) show that the pinacidil-induced current reverses at -90 mV and is inhibited by glibenclamide (1 µmol/L, Figure 1A, fg). Difference currents for pinacidil-induced and glibenclamide-sensitive components exhibit reversal potentials of 
-85 mV, which is close to the estimated equilibrium potential for K+ under these experimental conditions (Figure 1C, b-a and f-g). These findings suggest that the pinacidil-induced current is caused by the activity of KATP channels.
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In the continued presence of propranolol (1 µmol/L), bath application of the 1-adrenoceptor agonist phenylephrine (100 µmol/L) or MTX (100 µmol/L) reversibly suppressed the pinacidil-induced current (Figure 1A, bc and de), without altering its reversal potential (Figure 1B, b-2 and b-3). The current component that is sensitive to the 1-adrenoceptor agonists (Figure 1C, b-c and d-e) also had a reversal potential close to the equilibrium potential for K+. Thus, agents that stimulate 1-adrenoceptors cause a reversible inhibition of the pinacidil-activated KATP channel current.
The dose dependence of MTX on pinacidil-activated KATP channel current is shown in Figure 2. Three different concentrations of MTX caused a cumulative inhibition of the pinacidil-induced KATP current (Figure 2A). We quantified the percentage of MTX-induced current inhibition (at steady state) as follows: 100x(IMTX/IKATP), where IMTX denotes the current component (at 0 mV) that is blocked by MTX (a and b in Figure 2A), and IKATP is defined as the glibenclamide-sensitive current component (a through c in Figure 2A). The MTX concentration-inhibition relationship was compiled from measurements made in different cells (n=35, Figure 2B), and the data points were fitted to a Hill equation as follows: % inhibition=100/{1+(IC50/[MTX])h}, where IC50 indicates the concentration for the half-maximal inhibition (20.9±6.6 µmol/L), [MTX] indicates MTX concentration, and h indicates the Hill coefficient (0.68±0.15).
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Involvement of 1-Adrenoceptors and Role of G Proteins in MTX-Induced Inhibition
The inhibition of pinacidil-induced current by 100 µmol/L MTX was largely prevented by pretreatment with prazosin (2.5 µmol/L), which is a specific 1-adrenoceptor antagonist (Figure 3A). Replacement of GTP in the pipette solution with GDPßS (1 mmol/L), a nonspecific inhibitor of G proteins, also prevented the MTX-induced inhibition (Figure 3B). Figure 3C summarizes the effects of these two interventions. The blockade of 1-adrenoceptors and G proteins significantly suppressed the MTX-induced current inhibition from 75.0±3.4% (control) to 17.8±5.4% or 22.7±1.7%, respectively (P<0.05). These findings suggest that the MTX-induced inhibition of the pinacidil response occurs via the 1-adrenoceptor pathway and illustrates an involvement of G proteins in this process.
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PLC Activity Is Required for MTX-Induced Inhibition
Stimulation of Gq proteins activates PLC activity.16 To examine the involvement of PLC activity in the inhibition of KATP channel activity by MTX, we used two types of PLC inhibitors, U-73122 and neomycin. After the reversible inhibition of pinacidil-induced current by MTX was confirmed in a given myocyte (Figure 4A), we applied U-73122 (2 µmol/L), which by itself caused a slight increase of the pinacidil-activated current. In the continued presence of U-73122, a subsequent application of MTX caused comparatively little inhibition of the pinacidil-activated current (Figure 4A). These data are summarized in Figure 4C, in which the magnitude of the first MTX-induced inhibition is compared with that of a second application (the latter being made after U-73122 treatment for >3 minutes). Treatment with U-73122 significantly attenuated the MTX-induced inhibition (from 68.0±5.0% to 40.1±4.9%, P<0.05). Using another PLC blocker (neomycin, 2 mmol/L), we essentially obtained an identical result (Figures 4B and 4C), except that neomycin produced a mild decrease in the pinacidil-induced current by itself. Neomycin attenuated the second MTX-induced inhibition from 78.4±4.5% (in the absence of the drug) to 39.5±6.4% (P<0.05, n=5). These findings demonstrate an involvement of PLC activation in the effects of 1-receptors on the MTX inhibition of the pinacidil-activated KATP current.
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Lack of Involvement of PKC
PLC-induced hydrolysis of PIP2 produces diacylglycerol (DAG) and inositol 1,4,5-phosphate (IP3).8 DAG, in turn, activates protein kinase C (PKC) and may increase cytosolic Ca2+ levels.9 Therefore, it was possible that activation of PKC (or increases in cytosolic Ca2+) may have mediated the MTX-induced inhibition of the current. An involvement of intracellular Ca2+ is unlikely in our experiments because the myocytes were dialyzed with pipette solutions containing 10 mmol/L BAPTA, which is expected to chelate the cytosolic [Ca2+] to subnanomolar levels. To exclude the possibility of an involvement of PKC activation, we used two types of PKC modulators. First, H-7, a nonspecific inhibitor of various PKC isoforms, did not affect the MTX-induced inhibition (Figure 5A). Second, we used a phorbol ester (PDD, 2 µmol/L), which activates PKC. PDD by itself had no statistically significant effect on the pinacidil-induced KATP current (n=6, 116.5±10.2% of control at 0 mV after 3 minutes). PDD pretreatment (for >3 minutes) did not prevent the MTX-induced inhibition of the KATP current (Figure 5B). These data are summarized in Figure 5C. Under control conditions, MTX inhibited the pinacidil response by 75.3±3.8%. This value was unchanged by H-7 (88.0±1.9%, n=4) or PDD (82.5±4.5%, n=6). Taken together, these data suggest that PKC activation is not involved in the response of pinacidil-activated KATP channel activity to 1-receptor stimulation.
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Recovery From MTX-Induced Inhibition Is Prevented by Wortmannin
After depletion of PIP2 by PLC activation, membrane PIP2 levels were restored principally by de novo synthesis, which in turn was mediated by PI kinases (PI-4 and PIP kinases).8 Wortmannin (an antifungal antibiotic isolated from a culture of a fungus, Penicillium) blocks PI-3 kinase17 but also inhibits some PI-4 kinase isoforms at higher concentrations.18,19 We used wortmannin as a PI-4 kinase inhibitor to examine whether replenishment of the membrane PIP2 is involved in the response of pinacidil-activated KATP channel activity to 1-receptor stimulation.
Pinacidil was again used to evoke the KATP current, and as before, we used a double application of MTX (100 µmol/L) to compare the first response with the second. The magnitudes of MTX-induced inhibition of pinacidil-activated current by both MTX applications were comparable, and both were readily reversible on MTX washout (Figure 6A). Double applications of MTX inhibited pinacidil-activated current by similar amounts. We then examined the effect of wortmannin at three different concentrations (2, 10, and 20 µmol/L) (Figures 6B through 6D). After the myocytes were exposed to >10 µmol/L wortmannin, however, the current did not return fully to the control level but instead declined gradually even in the maintained presence of pinacidil, after the second washout of MTX (Figures 6C and 6D). In contrast, at 2 µmol/L, it did not affect the recovery after MTX washout (Figure 6B).
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We grouped data from several experiments and plotted the current amplitude after the MTX applications as a percentage of the amplitude before MTX treatment (Figure 6E). In the absence of wortmannin (filled circles in Figure 6E), the current recovered up to 91.6±4.5% within 4 minutes after MTX washout (n=10). Similar recovery was observed in the presence of 2 µmol/L wortmannin (filled triangles, n=6). But, at higher wortmannin concentrations, the current level did not recover after MTX washout, and mean percent recovery at 3 minutes washout was 59.0±5.4% in 10 µmol/L wortmannin (open squares, n=5) and 42.2±2.9% in 20 µmol/L wortmannin (open circles, n=4).
Open triangles in the graph (Figure 6E) indicate the time course of relative current amplitude at 0 mV in the continued presence of 20 µmol/L wortmannin without MTX (n=5). The compound alone reduced the pinacidil-induced current slightly, suggesting the inhibition of basal level of PIP2 recruitment, but its suppression was statistically not significant. Thus, PI-4 kinase activity and membrane PIP2 replenishment are necessary for the recovery from the MTX-induced inhibition of the pinacidil-activated KATP current in rat ventricular myocytes.
MTX Reverses Pinacidil-Induced Alteration of ATP Sensitivity
Pinacidil is known to activate the KATP channel by altering its sensitivity to ATP.15 In the reconstituted channel experiment with Kir6.2 and SUR, PIP2 was shown to interact with the Kir6.2 subunit and to alter its ATP sensitivity.2 Taken together, the MTX-stimulated reduction in membrane PIP2 level might modulate the pinacidil-induced shift of ATP sensitivity of KATP channel in native cardiac myocytes too. In the excised patch experiment, we studied the hypothesis. As shown on the left of Figure 7A, on the formation of this mode in the artificial internal solution containing no ATP, vigorous KATP channel activities could be recorded as an upward deflection. They were completely inhibited by 2 mmol/L ATP (Figure 7A) or 1 µmol/L glibenclamide (data not shown), confirming that the single-channel events were indeed openings of KATP channels.
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K2ATP (100 µmol/L) applied to the inner surface largely suppressed mean patch currents from 17.7 to 1.1 pA. The inhibitory effect of K2ATP was determined as a relative current that was calculated by normalizing mean patch currents in K2ATP to the control measured immediately before its application. Relative currents thus calculated were 0.06. Subsequent application of 100 µmol/L pinacidil gradually opened the channel, even in the continued presence of K2ATP, increasing the relative currents to 0.96.
To produce the breakdown of PIP2, 50 µmol/L MTX was added to the pipette solution, and myocytes were preincubated with bathing solution containing 50 µmol/L MTX for 60 seconds before the formation of the inside-out patch. In the presence of MTX (Figure 7B), the stimulatory action of 100 µmol/L pinacidil was blunted: relative currents were not significantly increased by pinacidil (from 0.05 to 0.16). Direct application of MTX (50 µmol/L) to the inner surface of the patch membrane was without effect (data not shown).
Figures 7C and 7D summarizes the inhibitory action of K2ATP before (open circles) and after a 3-minute application of pinacidil (filled circles). Relative currents at a given concentration of K2ATP were averaged and are plotted as a function of ATP concentrations. Four smooth curves in the graphs are best fitted to the Hill equation as follows: relative current=1/{1+([K2ATP]/IC50)h}, where IC50 is the K2ATP concentration ([K2ATP]) for the half-maximal inhibition, and h is the Hill coefficient. In the control condition (Figure 7C), pinacidil significantly increased the IC50 from 49.0±0.5 to 264.7±11.1 µmol/L. However, after MTX treatment (Figure 7D), these values were not changed (47.5±0.4 and 49.0±0.5 µmol/L in the absence and the presence of 100 µmol/L pinacidil, respectively). Thus, MTX abolished the pharmacological action of pinacidil.
Stimulation of 1-Adrenoceptor Reduces Membrane PIP2 Levels
Our data presented until now are consistent with the notion that 1-adrenoceptor stimulation causes PIP2 breakdown (hydrolyzed by PLC) and that the decreased PIP2 levels are responsible, in part, for the inhibition of the pinacidil-activated KATP channel current. To confirm that 1-adrenoceptor stimulation indeed leads to decreases in membrane PIP2 levels in cardiac myocytes, we used a tool that visualizes PIP2 subcellular compartmentalization. To this end, we used the PLC PH-GFP construct, which has been previously used with success for this purpose.14 To characterize experimental conditions, pilot studies were performed in which COS cells were transiently transfected with PH-GFP cDNA alone or were cotransfected with 1c-receptor cDNA (Figure 8A). In both cases, strong GFP fluorescence was detected at the cell periphery. Bath application of phenylephrine (100 µmol/L) led to a rapid translocation of the fluorescent signal from the plasma membrane to the cytosol, but only in cells that coexpressed the 1c-receptor.
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To examine whether PIP2 translocation occurs in cardiac myocytes, we used mouse neonatal ventricular myocytes, which (unlike adult myocytes) can readily be transfected using commercial reagents (Figure 8B). Using patch-clamp techniques, we verified that pinacidil-activated KATP channels are present in this preparation and that phenylephrine blocks this pinacidil response (data not shown). On the left, Figure 8B shows two images of a myocyte that was transfected with the PH-GFP construct. As was the case in COS cells, GFP fluorescence was localized at the cell surface (top) and rapidly translocated to the cytosol after application of MTX (bottom). On the upper right of Figure 8B is the time course of the GFP signal at two regions of interest (cell membrane and cytosol), and the graph on the bottom indicates the temporal change of fluorescence ratios (PM/Cyt). PM/Cyt was obtained as the ratio of averaged fluorescence signals from plasma membrane (PM) against those from cytosol (Cyt) areas. Note that this phenomenon was fully reversible and that prazosin (2.5 µmol/L) completely inhibited the MTX-induced mobilization of GFP signal (bottom right). The percent PM/Cyt ratio measured 1 minute after MTX was reduced by 70.5±7.2% (n=9) and was 102.3±7.5% (n=2) in the presence of prazosin. These data are consistent with the idea that 1-adrenoceptor stimulation reduces membrane PIP2 levels and that membrane PIP2 levels are replenished after the cessation of receptor stimulation.
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Discussion |
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MTX-Induced Inhibition of KATP Current Is Due to Activation of PLC
Stimulation of
1-adrenoceptors with MTX led to a rapid and reversible inhibition of the KATP channel current that was activated by pinacidil. We found that activation of PLC is a key step in this pathway, inasmuch as inhibition of PLC with neomycin or U-73122 significantly attenuated the MTX-induced inhibition of the current. Because PLC-mediated PIP2 hydrolysis produces DAG and IP3 (and possible increase in cytosolic Ca2+), it is possible that KATP channels may have been modulated by 
1 of these events. It seems very unlikely that an increase in cytosolic Ca2+ activity was responsible for this effect of MTX, because myocytes were dialyzed with high concentrations of a high-affinity Ca2+ chelator (BAPTA). It is also unlikely that DAG mediates the MTX response, inasmuch as we found DAG to be without effect on reconstituted KATP channels (Kir6.2/SUR2A).6 Other studies have similarly found exogenous IP3 also to be without effect on KATP channel activity.3 Given the sensitivity of the MTX response to pharmacological interventions affecting PLC (and the comparative insensitivity to manipulation of the PKC pathway), we conclude that PLC activation is a key step in the 1-adrenoceptor pathway and in the effect that it has on cardiac KATP channels.
Effect of PLC Activation Is Mediated Through Alterations in Membrane PIP2 Levels
Our data suggest that the mechanism by which the 1-adrenoceptor pathway inhibits KATP channel activity involves alterations in membrane PIP2 levels. A proposed mechanism is that PLC-induced PIP2 hydrolysis leads to lower membrane PIP2 levels that, in turn, may inhibit KATP channel activity. This concept is supported by two main lines of complementary evidence. First, we directly demonstrated that 1-adrenoceptor stimulation produces a prompt depletion of the membrane PIP2 levels in cardiac myocytes, as visualized using a PLC PH-GFP fusion protein as a PIP2 indicator. This fusion protein has been widely used in the past by others to examine the subcellular distribution of PIP2 in a variety of cells.14,20 We have also characterized the use of this reporter protein in COS cells and found that 1-adrenoceptor stimulation altered only the subcellular fluorescence distribution when cells express the 1c-receptor.
A second main line of evidence comes from the wortmannin data. We found that preventing PIP2 regeneration by blockade of key PI-4 kinases by wortmannin prevents the KATP channel recovery after 1-adrenoceptor-mediated inhibition. Thus, we conclude that 1-adrenoceptor stimulation led to temporal reversible alterations in membrane PIP2 levels, which led to the observed changes in KATP channel activity. In addition, U-73122, a PLC inhibitor, produced a slight increase in basal pinacidil-induced KATP current. This would reflect the increase in baseline PIP2 level as a result of a subtle balance between its degradation by PLC and recruitment by PI kinases. In contrast, neomycin, another PLC inhibitor, reduced the pinacidil-induced current, suggesting that it has an additional action, ie, that of blocking the KATP channel as an aminoglycoside polycation.
Our data are fully consistent with the available literature. Using radioactivity measurements, another study found that in rat heart, the 1-adrenoceptor agonist stimulates PIP2 hydrolysis to a maximal rate within 60 seconds of application and that the total PIP2 content recovers to 70% of control values within 3 minutes of washout.17 With confocal microscopic techniques and PIP2 visualization (using the PLC PH-GFP construct also used in the present study), it has been directly demonstrated that PLC activation causes membrane PIP2 breakdown in living cells.18,19 The time course of membrane PIP2 breakdown reported in the latter study corresponds well with that of PIP2 hydrolysis as measured by radioactive techniques.18,19 These time courses were also very similar to those that we observed during the MTX-induced inhibition and the recovery of the KATP current in rat ventricular myocytes. Thus, taken altogether, our data are in full support of the idea that 1-adrenoceptor stimulation inhibits pinacidil-activated KATP channel current through a pathway mediated by decreased sarcolemmal PIP2 levels and the resultant change in ATP sensitivity of the channel.
Limitations of the Study
In the present study, we always used K+ channel openers to evoke the KATP channel activation. As suggested by our single-channel data in excised patches, pinacidil artificially opened the channel by altering its sensitivity to ATP. Therefore, it remains to be determined whether the PIP2 pathway is also involved in the inhibition of naturally opened currents. Stimulation of several membrane receptors activates events in diverse pathways that also involve the activation of PLC.9 We previously demonstrated in guinea pig ventricular myocytes21,22 that both ET-1 and Ang II inhibit the KATP channels activated by metabolic inhibition. We proposed that ET-1 and Ang-II inhibited KATP channel activity through an increased subsarcolemmal ATP concentration, an event that is mediated by blockade of adenylate cyclase and pertussis toxin-sensitive G proteins (Gi). However, both of these receptor pathways are also linked to PLC activation. Do ET-1 and Ang II also exert their modulation on KATP channel activity through PIP2 metabolism? Prevailing data argue against this possibility. In our previous studies, we used guinea pig ventricular myocytes and found that ET-1 and Ang II inhibited KATP channels only when they were activated by metabolic inhibition and not when they were activated by the KATP channel opener cromakalim.21,22 In contrast, we saw a reduction in pinacidil-activated current by ET-1 in rat ventricular myocytes. Thus, an involvement of PIP2 metabolism appears to depend on the species and the dominance of the type of G proteins through which each receptor is coupled.
In conclusion, we have demonstrated that PIP2 metabolism potentially plays a key role in regulating the activity of native KATP channels in cardiac myocytes. Hormonal regulation of KATP channel activity may occur under physiological conditions but is expected to have particular relevance to cardiac ischemia, in which drastic hormonal changes occur. PIP2 levels also regulate the activity of other types of inward rectifier K+ channels as well as the Na+-Ca2+ exchanger, 1,23,24 and receptor stimulation may lead to fundamental alterations in excitability under these conditions. Further investigation into the role of membrane PIP2 metabolism may lead to important new insights into ischemia-related phenomena.
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Acknowledgments |
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Dr Horie was supported by research grants from the Ministry of Education, Science, Sports, and Culture of Japan, by the Takeda Foundation for Science Promotion, and by a Japan Heart Foundation/Pfizer grant for cardiovascular disease research. Dr Takano was supported by a grant from the Japan Cardiovascular Research Foundation. The authors are grateful to K. Tsuji for technical assistance.
Received March 13, 2001; revision received July 8, 2002; accepted July 8, 2002.
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