Published online before print March 3, 2008, doi: 10.1161/CIRCULATIONAHA.107.745539
(Circulation. 2008;117:1405-1413.)
© 2008 American Heart Association, Inc.
Molecular Cardiology |
Role of Sulfonylurea Receptor Type 1 Subunits of ATP-Sensitive Potassium Channels in Myocardial Ischemia/Reperfusion Injury
From the Department of Medicine and Pathology (J.W.E., S.G., D.J.L.), Albert Einstein College of Medicine, New York, NY; Department of Pediatrics (M.H., W.A.C.), NYU School of Medicine, New York, NY; Department of Cell Biology and Physiology (T.P.F., C.G.N.), Washington University School of Medicine, St Louis, Mo; and Department of Molecular Physiology and Biophysics (M.A.M.), Vanderbilt University School of Medicine, Nashville, Tenn.
Correspondence to Dr William A. Coetzee, Pediatric Cardiology, NYU School of Medicine, 560 First Ave, TCH-521, New York, NY 10016. E-mail william.coetzee{at}med.nyu.edu
Received October 11, 2007; accepted January 22, 2008.
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Abstract |
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Background— Opening of cardiac ATP-sensitive potassium channels (KATP channels) is a well-characterized protective mechanism against ischemia and reperfusion injury. Evidence exists for an involvement of both sarcolemmal and mitochondrial KATP channels in such protection. Classically, cardiac sarcolemmal KATP channels are thought to be composed of Kir6.2 (inward-rectifier potassium channel 6.2) and SUR2A (sulfonylurea receptor type 2A) subunits; however, the evidence is strong that SUR1 (sulfonylurea receptor type 1) subunits are also expressed in the heart and that they may have a functional role. The aim of this study, therefore, was to examine the role of SUR1 in myocardial infarction.
Methods and Results— We subjected mice lacking SUR1 subunits to in vivo myocardial ischemia/reperfusion injury. Interestingly, the SUR1-null mice were markedly protected against the ischemic insult, displaying a reduced infarct size and preservation of left ventricular function, which suggests a role for this KATP channel subunit in cardiovascular function during conditions of stress.
Conclusions— SUR1 subunits have a high sensitivity toward many sulfonylureas and certain KATP channel–opening drugs. Their potential role during ischemic events should therefore be considered both in the interpretation of experimental data with pharmacological agents and in the clinical arena when the cardiovascular outcome of patients treated with antidiabetic sulfonylureas is being considered.
Key Words: KATP channels • ischemia • ion channels • infarction • myocardial infarction
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Introduction |
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Even in the initial description of the ATP-sensitive potassium (KATP) channel in the heart,1 it was postulated that the opening of these channels during hypoxic or ischemic conditions might be cardioprotective. In general, this has proved to be the case, and a multitude of in vivo and in vitro studies have demonstrated that KATP channel opening generally protects against ischemic insults.2 KATP channel opening has also been demonstrated to play a key role in the events that lead to the protection afforded by ischemic preconditioning. A role for KATP channels present both at the sarcolemma and in mitochondria has been reported in the phenomenon of ischemic preconditioning.3
Clinical Perspective p 1413
Many of the arguments about the roles of KATP channels in ischemia are derived from experiments using putatively specific pharmacological compounds, the action of which may depend on the metabolic state of the cell.4 The nonspecificity of these agents is a cause for concern in the interpretation of experimental results. The molecular characterization of subunits of KATP channels (inward-rectifier potassium channels 6.1 and 6.2 [Kir6.1 and Kir6.2] and sulfonylurea receptor [SUR] type 1 [SUR1] and type 2 [SUR2]) allowed genetic approaches to be used to assess the roles of KATP channels in a variety of physiological and pathophysiological processes.5 Mouse models have been created that are genetically deficient for each of these respective subunits.5 The cardiac KATP channel has been proposed to be composed of Kir6.2 subunits in combination with sulfonylurea receptor type 2A (SUR2A) subunits5,6; indeed, mouse hearts deficient in Kir6.2 subunits lack functional cardiac KATP channel activity.7 These mice also lack the protective effect of ischemic preconditioning,8 which is consistent with a protective role for cardiac KATP channels during ischemia. Mice deficient in SUR2 subunits have an elevated blood pressure and elevated coronary perfusion pressure and exhibit spontaneous coronary vasospasm,9 which is consistent with expression of SUR type 2B (SUR2B) subunits in the vasculature5 and the known functional role of KATP channels in the maintenance of vascular tone and blood flow.10 Although the vascular phenotype may complicate interpretations, it has recently been reported that mouse hearts deficient in SUR2 subunits are protected against global ischemia.11 This surprising finding is not predicted from the simple view of SUR2-containing KATP channels being involved in ischemic protection, because loss of the KATP channel–mediated protection would be predicted to exacerbate ischemia.
Several reports discuss the expression of (and a possible role for) SUR1 subunits in the heart.12–15 To assess the possible contribution of SUR1 subunits in protection during myocardial ischemia, we used a genetic mouse model that lacks SUR1 subunits16 and subjected the mice to an in vivo ischemia/reperfusion protocol. The data indicate a novel and unexpected role for SUR1 subunits in the heart.
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Methods |
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Generation of Genetic Mouse Models
Mice deficient in the ABCC8 gene (SUR1-null mice) have been described previously.16 All animals received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society of Medical Research and the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health (NIH publication No. 85-23, revised 1996). The experimental protocol was reviewed and approved by the institutional animal care and use committees of Albert Einstein College of Medicine and NYU School of Medicine.
Myocardial Ischemia and Reperfusion Protocol
All surgical procedures were performed with aseptic techniques as described in detail previously.17 Briefly, mice were anesthetized via an intraperitoneal injection of sodium pentobarbital (50 mg/kg) and ketamine (60 mg/kg). Mice were intubated and ventilated with 100% oxygen by a rodent ventilator (Minivent, model 845, Hugo Sachs Elektronik, March-Hugstetten, Germany) at a rate of 110 strokes/min and 230-µL tidal volume. A thoracotomy was performed, and the left coronary artery was visually identified and ligated with a 7-0 silk suture to render the left ventricle (LV) ischemic.
Assessment of Myocardial Infarct Size
After 24 hours of reperfusion, infarct size was determined as described previously.17 Briefly, Evan’s blue dye was injected into the carotid artery catheter to delineate the ischemic zone from the nonischemic zone. The heart was rapidly excised and cross-sectioned into 1-mm-thick slices, which were then incubated in 1.0% 2,3,5-triphenyl-tetrazolium chloride solution (TTC) to demarcate the viable and nonviable myocardium within the risk zone. Images of each side of each section were acquired and weighed. The areas of infarction, risk, and nonischemic LV were assessed in a blinded fashion with computer-assisted planimetry (NIH Image 1.57, National Institutes of Health, Bethesda, Md).
Echocardiographic Assessment of LV Structure and Function
Mice were lightly anesthetized with isoflurane in 100% O2 with continuous monitoring of ECG, core body temperature, and respiration rate. Transthoracic high-resolution echocardiography of the LV with a 30-MHz RMV (real-time microvisualization) scanhead (probe) interfaced with a Vevo 770 (VisualSonics, Toronto, Canada) was performed 1 week before myocardial infarction. Parasternal long- and short-axis B-mode and M-mode images were acquired. Offline measurements and calculations were made with VisualSonics software (version 2.2.3) for assessment of LV structure and function. Seven days after myocardial infarction, mice again underwent echocardiography just before the evaluation of infarct size.
Histological Analysis of Myocardial Infarct Size
Hearts were excised rapidly and fixed in conventional fixing solutions (10% buffered formalin) after 30 minutes of left coronary artery ischemia and 7 days reperfusion. Hearts were cross-sectioned into 1-mm-thick slices with a tissue chopper. Hearts were embedded in a standard fashion and stained with hematoxylin and eosin. Digital images of the slides were then captured and analyzed in a blinded fashion with computer-assisted planimetry with Image J software (version 1.37, National Institutes of Health) to measure the area of infarct or scar relative to the LV. For each heart, we analyzed 4 sections taken from each 1-mm–thick slice and then averaged those numbers to obtain the size of infarct or scar per LV for each animal.
Isolation of Ventricular Cardiac Myocytes
LV myocytes were isolated from adult mice (aged 4 to 6 months; weight 25 g to 35 g) by a standard enzymatic technique described previously18 with some minor modifications. Briefly, mice were anesthetized by intraperitoneal injection (0.01 mL/g) that contained ketamine (1.5 mg/mL) and xylazine (15 mg/mL) diluted in PBS. The heart was excised, cannulated within 2 minutes, and perfused retrogradely at 3 mL/min with PBS containing (in mmol/L) NaCl 113, KCl 4.7, KH2PO4 0.6, Na2HPO4 0.6, MgSO4.7H20 1.2, phenol red 0.032, NaHCO3 12, KHCO3 10, HEPES 10, taurine 30, 2,3-butanedione monoxine (BDM) 10, and glucose 10 (pH 7.2 at 37°C). After a 5-minute wash, saline containing 20 mg of collagenase (356 U/mg; type II, Worthington Biochemicals, Lakewood, NJ), 3 mg of protease (5.2 U/mg; type XIV, Sigma-Aldrich, St Louis, Mo), and trypsin (0.14 mg/mL; 10X solution; GIBCO/Invitrogen, Carlsbad, Calif) was perfused. After 13 to 15 minutes of perfusion, the heart was removed from the cannula and gently minced in enzyme-free, Ca2+-free perfusion buffer containing BSA (1 mg/mL) and BDM (0.2 mg/mL). The cell suspension was filtered though 250-µm nylon mesh (Small Parts, Inc, Miami, Fla), and myocytes were separated into pellets by gravity and subsequently washed with perfusion buffer without BDM and BSA. The myocytes were plated in 50-mm nonstick Valmark Petri dishes (Midwest Scientific, St Louis, Mo) and subjected to stepwise Ca2+ reintroduction. Myocytes were stored at room temperature (20°C to 22°C) in perfusion buffer containing 1 mmol/L Ca2+ and used within 8 hours after isolation.
Patch-Clamp Recordings
Recordings of single-KATP channel activity were made by standard patch-clamp techniques.19 Patch electrodes were made from thick-walled glass capillaries (outside diameter 1.5 mm, inside diameter 1.12 mm; World Precision Instruments Inc, Sarasota, Fla) with a horizontal puller (Zeitz Instrumente Universal puller, Augsburg, Germany) and heat-polished. The pipette solution (extracellular medium) contained (in mmol/L) gluconate 110, KCl 30, EGTA 1, MgCl2 1.2, and HEPES 10 (pH 7.2 with KOH). The bath solution (intracellular solution) contained (in mmol/L) gluconate 110, KCl 30, CaCl2 2, MgCl2 1, and HEPES 10 (pH 7.4 with KOH). When filled, electrode resistances ranged between 4 and 9 M
. Recordings were made with a patch-clamp amplifier (Axopatch 200, Axon Instruments, Foster City, Calif). Solution changes were made within with a rapid solution changer (<100 ms; RSC-160, Biological, Cliax, France). The current signals were amplified (20 to 200 mV/pA), low-pass filtered at 1 kHz, and digitized at 5 kHz with pClamp software (Axon Instruments). Unless otherwise stated, the pipette potential was kept at 100 mV. Data were analyzed with pClamp (Axon Instruments) or Origin for Windows (MicroCal Software, Northampton, Mass) software. The unitary current amplitude was determined from an amplitude histogram of 15 to 20 seconds of recorded data. The histogram was fitted to a sum of gaussian distributions. The difference between 2 adjacent gaussian peaks was taken as a measure of the unitary current amplitude. Because most recordings contained many more than a single KATP channel, no attempts were made to study the distribution of channel dwell times. Where possible, results are expressed as mean±SEM. Results were repeated in a minimum of 3 patches to ensure reproducibility.
Reverse-Transcription Polymerase Chain Reaction Expression Assays
Real-time reverse-transcription polymerase chain reaction (RT-PCR) was performed essentially as described previously.20 Total RNA was extracted from ventricular tissue by the acid phenol guanidinium method (TriReagent, Sigma). RNA quality was determined by agarose electrophoresis (intact 18S and 28S RNA bands), and the concentration was determined spectrophotometrically. Total RNA was reverse transcribed with random hexamer primers according to the manufacturer’s guidelines (Superscript III; Invitrogen). PCR reactions were performed with an ABI Prism 7900HT sequence detection system (PerkinElmer Applied Biosystems, Foster City, Calif) with a SYBR green master mix (PerkinElmer Applied Biosystems). The thermal cycling conditions comprised an initial denaturing step at 95°C for 10 minutes and 40 cycles at 95°C for 5 seconds, 60°C for 15 seconds, and 72°C for 15 seconds, followed by a melting curve analysis.20 The primers used in the PCR reactions are shown in the Table. Because of the nature of the primer design used for real-time RT-PCR (amplicons to be within 80 to 120 bp), it was not possible to specifically amplify SUR2B. Conventional RT-PCR experiments were therefore performed to determine the relative expression of SUR2A and SUR2B (30 cycles with an annealing temperature of 55°C) with primers that produce different amplicon sizes for SUR2A and SUR2B21 (Table).
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Statistical Analysis
All data in the present study are expressed as mean±SEM. Differences in data between groups were compared with Prism 4 software (GraphPad Software, Inc, San Diego, Calif) with Student t test or 1-way ANOVA where appropriate. For the ANOVA, if a significant variance was found, the Tukey test was used as the post hoc analysis. P<0.05 was considered significant.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
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Results |
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Genetic Ablation of SUR1 Significantly Reduces Myocardial Ischemia-Reperfusion Injury
Wild-type (WT) and SUR1-null mice were subjected to 30 minutes of left coronary artery ischemia and 24 hours of reperfusion. Representative images of TTC-stained midmyocardial sections from WT and SUR1-null hearts qualitatively demonstrated the reduced infarct size observed in SUR1 knockout animals (Figure 1A). Quantitatively, WT mice displayed a mean infarct area of 37.2±3.2% per area at risk (Figure 1B). SUR1-null mice were found to have a 62% reduction in mean infarct area per area at risk compared with WT mice (14.0±7%, P<0.001). Mean infarct area per LV was also significantly reduced in SUR1-null mice compared with WT littermates (P<0.01). The percent of area at risk relative to the LV was similar between groups.
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LV Structure and Function Are Preserved in SUR1-Null Mice After Myocardial Infarction
Mice were next subjected to 30 minutes of left coronary artery ischemia followed by 7 days of reperfusion, after which myocardial injury was assessed with hematoxylin and eosin staining for calculation of scar formation (mean infarct area per LV; Figure 2A). SUR1-null hearts displayed a 55% reduction in scar size compared with WT mice 7 days after myocardial infarction (P<0.01; Figure 2B). The evaluation of LV dimensions by echocardiography revealed complete preservation of both LV end-diastolic diameter (Figure 3A) and LV end-systolic diameter (Figure 3B) in SUR1-null mice compared with WT littermates, which displayed a significant increase in LV chamber dilation. Additionally, SUR1-null animals were found to have a 21.5% decrease in fractional shortening 7 days after myocardial infarction compared with WT mice, which displayed a 44.2% reduction in fractional shortening (P<0.01; Figure 3C). In conjunction with this, SUR1-null mice displayed a minimal 17.4% reduction in ejection fraction after myocardial infarction compared with WT mice, which displayed a 39.3% reduction (Figure 3D). No significant difference in LV function was noted between the groups at baseline.
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SUR1-Null Mice Display Reduced Fibrosis and Inflammation After Myocardial Ischemia/Reperfusion Injury
We assessed the degree of cardiac tissue damage and fibrosis using histological techniques (hematoxylin and eosin staining) after 30 minutes of left coronary artery ischemia and 7 days of reperfusion (Figure 4). SUR1-null hearts displayed a reduced degree of myocardial neutrophilic infiltrate, necrosis, hemorrhage, and spindle-shaped interstitial cells compared with WT mice.
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SUR1-Null Hearts Displayed No Compensatory Alteration in Other KATP Subunits
To examine whether mRNA expression levels of other subunits of KATP channels were affected by genetic ablation of SUR1, we performed real-time RT-PCR using RNA extracted from WT or SUR1-null heart ventricles. Relative to the expression of GAPDH, the expression levels of Kir6.1, Kir6.2, and SUR2 subunits were similar between the 2 groups (Figure 5). SUR1 mRNA expression was low in the WT ventricle and undetectable in the SUR1-null hearts (not shown). Similar data were obtained when data were expressed relative to 18S (online-only Data Supplement). No differences in the ratio of SUR2A/SUR2B mRNA expression were observed between controls and SUR1-null hearts (Figure 5).
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Patch Clamping
Experiments were performed to assess whether functional KATP channels could be recorded in ventricular myocytes isolated from SUR1-null mouse hearts. Indeed, inside-out patches contained robust KATP channels that opened with a bursting behavior with a unitary conductance of 85.5±1.24 pS (n=6; Figure 6). Application of 30 µmol/L ATP reduced the open probability to 27±6.1% (n=4) of the values in the absence of ATP, whereas 1 mmol/L ATP reduced the open probability to near zero. These properties were similar to those of KATP channels recorded from WT myocytes under identical experimental conditions (not illustrated). Thus, ventricular myocytes of SUR1-null mouse hearts expressed KATP channels with biophysical properties that resembled those of KATP channels, as also reported previously in normal hearts.22
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Discussion |
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This study was designed to examine the role of SUR1 subunits of KATP channels in cardiac function, specifically with regard to the sequelae of myocardial ischemia and reperfusion. We used an animal model of mice deficient in SUR1 subunits in combination with a well-characterized in vivo model of myocardial ischemia/reperfusion injury to investigate this question. Surprisingly, these mice were strongly protected from ischemic insults and displayed reduced infarct size and preservation of LV structure and function after myocardial infarction.
SUR1 Expression in the Heart
The sulfonylurea receptor gene (Abcc8) encodes the high-affinity sulfonylurea receptor, SUR1.23 Transcripts of SUR1 are most abundant in neurons, brain, and pancreatic β-cells.24,25 In the brain, channels composed of SUR1 subunits may have diverse roles, such as regulating the activity of glucose-receptive neurons in the ventromedial hypothalamus,26 whereas in β-cells of the pancreas, SUR1-containing KATP channels have a well-defined role as a trigger for the first phase of insulin release.27 However, SUR1 transcripts are also detected in other cell types, including the heart20,24,28–30 (albeit at lower abundance). We have previously detected SUR1 protein in mouse and rat cardiac myocytes, with the stated reservation that the result should be interpreted within the limitations of available antibodies.14 Here, we found low levels of SUR1 mRNA expression by comparative real-time RT-PCR methods, which may imply low expression levels of SUR1 protein or restricted expression distribution in the heart. A potential role for SUR1 in KATP channel activity in rat atrial myocytes has also been suggested based on a functional and pharmacological profile that resembles that of the pancreatic β-cell KATP channel (high affinity for glyburide and diazoxide).30 Interestingly, human atrial cells are also reported to have high sensitivity to blockade by tolbutamide.31 More direct evidence for a role of SUR1 subunits in forming cardiac KATP channels was provided by the finding that antisense oligonucleotides against SUR1 inhibit KATP channel currents in cultured neonatal rat ventricular cells.12 In the present study, we were able to record KATP channels from ventricular myocytes isolated from SUR1-null hearts that were blocked by cytosolic ATP in the micromolar range and with a unitary conductance similar to that previously reported for ventricular KATP channels.32 Although we did not perform a full biophysical or pharmacological analysis of these channels in SUR1-null mice, the present data are consistent with the prevailing notion that SUR1 subunits are not essential subunits of ventricular sarcolemmal KATP channels. Further studies are warranted to identify the localization and mechanistic role of SUR1 subunits in the heart and cardiovascular system.
SUR Subunits and Protection From Ischemia
KATP channels open during myocardial ischemia and partially contribute to K+ loss and arrhythmias.33 Opening of KATP channels with pharmacological agents has been clearly associated with improvement of myocardial function on reperfusion and with a reduction in infarct size.34–36 Conversely, KATP channel blockers are associated with a worse prognostic outcome.37 The concept that KATP channels are protective during ischemia has been strengthened by genetic studies. In Kir6.2-null mice, for example, the increase in LV end-diastolic pressure during ischemia was more marked than in WT littermates, whereas an ischemic preconditioning protocol reduced the infarct size in WT but not Kir6.2-null mice.38 Transgenic overexpression of SUR2A (driven by the ubiquitous cytomegalovirus promoter), which is reported to increase the number of functional KATP channels (but see Flagg et al39), increases the resistance to hypoxia in isolated cardiac myocytes and decreases infarct size in Langendorff-perfused isolated hearts after a period of global ischemia in these transgenic mice.40 In contrast to these studies, a recent study demonstrated that SUR2-null mice are resistant to acute cardiovascular stress.11 In the latter study, infarct size was reduced in hearts of SUR2-null mice when Langendorff-perfused and subjected to global ischemia. Interestingly, these SUR2-null mice had hypertension and developed cardiac hypertrophy, but they did not develop significant fibrosis.11 Paralleling the findings of the latter study, we now report that loss of SUR1 subunits also results in increased protection from ischemia/reperfusion insults.
Mechanisms Responsible for Cardiac Protection
Pharmacological approaches to study the role of KATP channels in protection from myocardial ischemia/reperfusion injury have pivoted on the use of agents that are putatively selective for sarcolemmal versus mitochondrial KATP channels. For example, diazoxide and 5-hydroxydecanoate, which respectively are reported to selectively open or block mitochondrial KATP channels, protect against ischemic preconditioning,41 and this has been taken to argue a role for these channels in mediating protection. The argument is complicated by reports that 5-hydroxydecanoate can in fact block cardiac KATP channels,42 possibly in relation to an effect mediated by intracellular cAMP,43 or that it directly fuels mitochondrial respiration.44 Similarly, diazoxide may also directly affect mitochondrial respiration.44 Although more selective agents may now be available, a further complication is that the pharmacology of cardiac KATP channels may depend on the specific tissue location in the heart and the metabolic state of the cell. For example, diazoxide effectively opens cardiac KATP channels when intracellular ADP levels are elevated4 (as would happen during ischemia). Atrial KATP channels also show increased sensitivity to diazoxide compared with those recorded in ventricular myocytes,30 and ventricular KATP channels show increased sensitivity during remodeling after myocardial ischemia/reperfusion.29
Given the issues raised above, it was hoped that genetic approaches would clarify the role of KATP channels in myocardial ischemia. Unfortunately, they have brought their own set of challenges. First, mitochondrial KATP channels cannot be studied genetically until and unless their molecular composition has been identified. Although consensus does not exist in this regard, the majority of studies have indicated that conventional KATP channel subunits are not expressed in mitochondria.14 If this is correct, then the genetic models of KATP channel subunit ablation or overexpression should address the question of the role(s) of sarcolemmal KATP channels (or any other functions that the KATP channel subunits may have) during ischemia.
Consistent with the pharmacological data discussed above, transgenic overexpression of SUR2A subunits is reported to increase the KATP channel number and to provide better protection against ischemia.40 Furthermore, Kir6.2-null mice lack cardiac KATP channels and have a decreased resistance to ischemia.38 It is not clear, however, how the loss of SUR2 subunits (in SUR2-null mice), which is expected to result in loss of cardiac KATP channels, leads to an opposite phenotype of enhanced resistance to ischemia.11 One possibility is that these mice lack both SUR2A and SUR2B (components of vascular KATP channels), which could account in part for the observed differences. SUR1-null mice, in which KATP channels with biophysical characteristics resembling known KATP channels can be measured, similarly are better protected against ischemia. Reciprocal compensation in expression of other KATP channel subunits is unlikely, because we have not observed any evidence of altered Kir6.1, Kir6.2, SUR2A, or SUR2B mRNA expression. There may, however, be other adaptive changes in these mice, such as different posttranslational modifications or trafficking events of subunits such as SUR2. These questions have not been examined in the present study. In addition to these possibilities, it is plausible that KATP channels in different tissue compartments within the heart or extracardiac tissues, such as the ventricle, atrium, coronary vasculature, or cardiac neurites,6,9,21,30 may be composed of different SUR subunit isoforms and that SUR1 may not be primarily expressed in ventricular myocytes. It is also possible that SUR subunits have unanticipated functions. For example, we recently identified a role for KATP channels in the release of the vasoconstrictor endothelin-1 from the coronary endothelium,45 and although the cellular mechanisms remain to be identified, this observation suggests a role for KATP channels in exocytosis in the cardiovascular system. KATP channels similarly have been linked to release of atrial natriuretic peptide from atrial myocytes46,47 and the release of norepinephrine and acetylcholine from peripheral nerve endings.48,49 Interestingly, a high sensitivity to drugs that act on SUR1 subunits (diazoxide) has been reported,50 and the KATP channels in dorsal vagal neurons have been suggested to be composed of Kir6.2/SUR1 channels.51 The extent to which processes such as these contribute to the gain of protection in SUR1-null mice remains to be determined. Finally, the existence of a nonselective cation channel (distinct from the KATP channel) that has the SUR1 subunit as a component has been reported.52 It appears that SUR1 is upregulated in tissues in which it was previously only expressed at low levels and that this newly formed channel contributes to ischemic damage in the brain and endothelium.53,54 Although the present data obtained with SUR1-null mice would be consistent with this idea, no such channel with these properties has been described in the heart to the best of our knowledge.
Conclusions
Although SUR1 expression has been noted in the heart previously, this is the first direct study to define the potential role of SUR1 subunits in the cardiovascular system. The present study provides evidence for protection from cardiac ischemia/reperfusion in a genetic model of SUR1 deletion. That loss of SUR1 subunits leads to ischemic protection is interesting and is suggestive of a previously unrecognized biological role of this subunit in regulating the function of the cardiovascular system under pathophysiological conditions. The tissue-expression patterns and exact function of the SUR1 subunit remain to be identified. Nevertheless, these data may have important therapeutic implications, because low concentrations of sulfonylureas and certain KATP channel–opening drugs that are not normally thought to act on conventional ventricular KATP channels may modulate the ischemic outcome through yet-to-be-understood processes that are mediated by SUR1 subunits. This can have important considerations in basic research when "selective" KATP channel drugs are used to identify roles for particular classes of channels in biological processes. Additionally, the present study has important implications in the clinic, such as with regard to defining the relationship between sulfonylurea treatment and heart function in diabetes mellitus and ischemic events in this important group of patients.55–57
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Acknowledgments |
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The authors are grateful to Laura L. Carihill for assistance with some of the RT-PCR experiments.
Sources of Funding
This study was supported by grants from the National Institutes of Health (HL060849 to Dr Lefer and HL45742 to Dr Nichols), the American Diabetes Association (7-04-RA-59 to Dr Lefer), and the Masonic Seventh Manhattan District Association (to Dr Coetzee).
Disclosures
None.
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The online-only Data Supplement, consisting of a table and a figure, is available with this article at http://circ.ahajournals.org/cgi/content/ full/CIRCULATIONAHA.107.745539/DC1.
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