Role of sulfonylurea receptor type 1 subunits of ATP-sensitive potassium channels in myocardial ischemia/reperfusion injury

Expression of SUR1 isoforms in the brain and heart after ischemia/reperfusion

The sulfonylurea receptor 1 (SUR1) has been classified as a member of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter superfamily. SUR1, unlike the classic ABC transporters, assembles with Kir6.2, forming KATP channels to regulate the flux of potassium ions. In the central nervous system, SUR1 is weakly expressed in some brain regions but is induced by pathological conditions in the different cell types of the neurovascular unit. Therefore, we first analyzed the expression of SUR1 in various rat tissues and brain regions to identify SUR1 isoforms and their mRNA exon composition under physiological conditions. Later, we focused on the SUR1 expression in the brain and heart after ischemia/reperfusion. We observed two SUR1 isoforms (170 and 60-75 kDa) abundantly expressed in most rat tissues, except for the testis and brain, where basal expression of these isoforms was relatively low and exhibit a band of 100 kDa. Every exons coding for the functional domains of SUR1 mRNA were amplified from the tissues and brain regions analyzed. Results from in vitro and in vivo experiments indicated that SUR1 isoforms previously identified (170 and 60-75 kDa) were dramatically overexpressed in the brain after middle cerebral artery occlusion followed by reperfusion. In contrast, myocardial infarction followed by reperfusion significantly reduced SUR1 isoform expression in the heart. This study demonstrates the expression of at least two SUR1 isoforms in various tissues and suggests that ischemic processes may differentially regulate SUR1 expression depending on the tissue injured.

Cardiac PET Imaging of ATP Binding Cassette (ABC) Transporters: Opportunities and Challenges

Adenosine triphosphate binding cassette (ABC) transporters are a broad family of membrane protein complexes that use energy to transport molecules across cells and/or intracellular organelle lipid membranes. Many drugs used to treat cardiac diseases have an affinity for these transporters. Among others, P-glycoprotein (P-gp) plays an essential role in regulating drug concentrations that reach cardiac tissue and therefore contribute to cardiotoxicity. As a molecular imaging modality, positron emission tomography (PET) has emerged as a viable technique to investigate the function of P-gp in organs and tissues. Using PET imaging to evaluate cardiac P-gp function provides new insights for drug development and improves the precise use of medications. Nevertheless, information in this field is limited. In this review, we aim to examine the current applications of ABC transporter PET imaging and its tracers in the heart, with a specific emphasis on P-gp. Furthermore, the opportunities and challenges in this novel field will be discussed.

SUR1 as a New Therapeutic Target for Pulmonary Arterial Hypertension

Mutations in ABCC8 have been identified in pulmonary arterial hypertension (PAH). ABCC8 encodes SUR1, a regulatory subunit of the ATP-sensitive-potassium channel Kir6.2. However, the pathophysiological role of the SUR1/Kir6.2 channel in PAH is unknown. We hypothesized that activation of SUR1 could be a novel potential target for PAH. We analysed the expression of SUR1/Kir6.2 in the lungs and pulmonary artery (PA) in human PAH or experimental pulmonary hypertension (PH). The contribution of SUR1 in human or rat PA tone was evaluated, and we measured the consequences of in vivo activation of SUR1 in control and PH rats. SUR1 and Kir6.2 protein expression was not reduced in the lungs or human pulmonary arterial endothelial cells and smooth muscle cells (hPAECs and hPASMCs) from PAH or experimentally induced PH. We showed that pharmacological activation of SUR1 by 3 different SUR1 activators (diazoxide, VU0071063, and NN414) leads to PA relaxation. Conversely, the inhibition of SUR1/Kir6.2 channels causes PA constriction. In vivo, long- and short-term activation of SUR1 with diazoxide reversed monocrotaline-induced PH in rats. Additionally, in vivo diazoxide application (short protocol) reduced the severity of PH in chronic-hypoxia rats. Moreover, 3 weeks of diazoxide exposure in control rats had no cardiovascular effects. Finally, in vivo, activation of SUR1 with NN414 reduced monocrotaline-induced PH in rats. In PAH and experimental PH, the expression of SUR1/Kir6.2 was still presented. In vivo pharmacological SUR1 activation by two different molecules alleviated experimental PH, providing proof-of-concept that SUR1 activation should be considered for PAH and evaluated more thoroughly.

Genetic Discovery of ATP-Sensitive K+ Channels in Cardiovascular Diseases

The ATP-sensitive K+ (KATP) channels are hetero-octameric protein complexes comprising 4 pore-forming (Kir6.x) subunits and 4 regulatory sulfonylurea receptor (SURx) subunits. They are prominent in myocytes, pancreatic β cells, and neurons and link cellular metabolism with membrane excitability. Using genetically modified animals and genomic analysis in patients, recent studies have implicated certain ATP-sensitive K+ channel subtypes in physiological and pathological processes in a variety of cardiovascular diseases. In this review, we focus on the causal relationship between ATP-sensitive K+ channel activity and pathophysiology in the cardiovascular system, particularly from the perspective of genetic changes in human and animal models.

The Slo(w) path to identifying the mitochondrial channels responsible for ischemic protection

Mitochondria play an important role in tissue ischemia and reperfusion (IR) injury, with energetic failure and the opening of the mitochondrial permeability transition pore being the major causes of IR-induced cell death. Thus, mitochondria are an appropriate focus for strategies to protect against IR injury. Two widely studied paradigms of IR protection, particularly in the field of cardiac IR, are ischemic preconditioning (IPC) and volatile anesthetic preconditioning (APC). While the molecular mechanisms recruited by these protective paradigms are not fully elucidated, a commonality is the involvement of mitochondrial K⁺ channel opening. In the case of IPC, research has focused on a mitochondrial ATP-sensitive K⁺ channel (mitoK_ATP), but, despite recent progress, the molecular identity of this channel remains a subject of contention. In the case of APC, early research suggested the existence of a mitochondrial large-conductance K⁺ (BK, big conductance of potassium) channel encoded by the Kcnma1 gene, although more recent work has shown that the channel that underlies APC is in fact encoded by Kcnt2 In this review, we discuss both the pharmacologic and genetic evidence for the existence and identity of mitochondrial K⁺ channels, and the role of these channels both in IR protection and in regulating normal mitochondrial function.

KATP channels in high glucose-induced rat mesangial cell proliferation and release of MMP-2 and fibronectin

ATP-sensitive potassium (KATP) channels are well characterized in cardiac, pancreatic and many other muscle cells. The purpose of this study was to determine if KATP channels play a role in diabetic nephropathy (DN). In the present study, functional expression of the KATP channel was examined in rat mesangial cells with or without high glucose (HG) stimulation. The mesangial cell proliferation and the release of matrix metalloproteinase (MMP)-2 and fibronectin in response to high glucose with a selective opener of KATP (diazoxide, DZX), or with a selective inhibitor of KATP (5-hydroxydecanoate, 5-HD) were also measured. The cell proliferation was observed using Cell Counting Kit-8 assay, and the mRNA expressions of KATP subunit, including Kir6.1, Kir6.2, sulfonylurea receptor 1 (SUR1), SUR2A and SUR2B, were assessed using quantitative real-time PCR. MMP-2 and fibronectin release was measured by ELISA. The present study clarified expression of SUR subunit of KATP in plasma. HG treatment could cause increased cell proliferation and release of MMP-2 and fibronectin in a dose-dependent manner. HG also significantly decreased the expression of Kir6.1, SUR2A and SUR2B. Pretreatment of DZX markedly decreased the expression of SUR1, SUR2A and SUR2B, but had no effect on Kir6.1 expression compared with HG alone, while these changes were inhibited by 5-HD pretreatment. Moreover, DZX also inhibited cell proliferation and release of MMP-2 and fibronectin in HG-induced rat mesangial cells, and that was corrected by 5-HD. These data suggest that HG stimulates mesangial cell proliferation and cellular matrix release via inhibiting KATP channel activity, leading us to propose that KATP channel dysfunction may be involved in the development of DN.

Adenosine Triphosphate-Sensitive Potassium Currents in Heart Disease and Cardioprotection

The subunit makeup of the family of adenosine triphosphate-sensitive potassium channel (KATP) channels is more complex and labile than thought. The growing association of Kir6.1 and SUR2 variants with specific cardiovascular electrical and contractile derangements and the clear association with Cantu syndrome establish the importance of appropriate activity in normal function of the heart and vasculature. Further studies of such patients will reveal new mutations in KATP subunits and perhaps in proteins that regulate KATP synthesis, trafficking, or location, all of which may ultimately benefit therapeutically from the unique pharmacology of KATP channels.

Loss of ATP-Sensitive Potassium Channel Surface Expression in Heart Failure Underlies Dysregulation of Action Potential Duration and Myocardial Vulnerability to Injury

The search for new approaches to treatment and prevention of heart failure is a major challenge in medicine. The adenosine triphosphate-sensitive potassium (K_ATP) channel has been long associated with the ability to preserve myocardial function and viability under stress. High surface expression of membrane K_ATP channels ensures a rapid energy-sparing reduction in action potential duration (APD) in response to metabolic challenges, while cellular signaling that reduces surface K_ATP channel expression blunts APD shortening, thus sacrificing energetic efficiency in exchange for greater cellular calcium entry and increased contractile force. In healthy hearts, calcium/calmodulin-dependent protein kinase II (CaMKII) phosphorylates the Kir6.2 K_ATP channel subunit initiating a cascade responsible for K_ATP channel endocytosis. Here, activation of CaMKII in a transaortic banding (TAB) model of heart failure is coupled with a 35–40% reduction in surface expression of K_ATP channels compared to hearts from sham-operated mice. Linkage between K_ATP channel expression and CaMKII is verified in isolated cardiomyocytes in which activation of CaMKII results in downregulation of K_ATP channel current. Accordingly, shortening of monophasic APD is slowed in response to hypoxia or heart rate acceleration in failing compared to non-failing hearts, a phenomenon previously shown to result in significant increases in oxygen consumption. Even in the absence of coronary artery disease, failing myocardium can be further injured by ischemia due to a mismatch between metabolic supply and demand. Ischemia-reperfusion injury, following ischemic preconditioning, is diminished in hearts with CaMKII inhibition compared to wild-type hearts and this advantage is largely eliminated when myocardial K_ATP channel expression is absent, supporting that the myocardial protective benefit of CaMKII inhibition in heart failure may be substantially mediated by K_ATP channels. Recognition of CaMKII-dependent downregulation of K_ATP channel expression as a mechanism for vulnerability to injury in failing hearts points to strategies targeting this interaction for potential preventives or treatments.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Novel human ABCC9/SUR2 brain-expressed transcripts and an eQTL relevant to hippocampal sclerosis of aging

ABCC9 genetic polymorphisms are associated with increased risk for various human diseases including hippocampal sclerosis of aging. The main goals of this study were 1 > to detect the ABCC9 variants and define the specific 3' untranslated region (3'UTR) for each variant in human brain, and 2 > to determine whether a polymorphism (rs704180) associated with risk for hippocampal sclerosis of aging pathology is also associated with variation in ABCC9 transcript expression and/or splicing. Rapid amplification of ABCC9 cDNA ends (3'RACE) provided evidence of novel 3' UTR portions of ABCC9 in human brain. In silico and experimental studies were performed focusing on the single nucleotide polymorphism, rs704180. Analyses from multiple databases, focusing on rs704180 only, indicated that this risk allele is a local expression quantitative trait locus (eQTL). Analyses of RNA from human brains showed increased ABCC9 transcript levels in individuals with the risk genotype, corresponding with enrichment for a shorter 3' UTR which may be more stable than variants with the longer 3' UTR. MicroRNA transfection experiments yielded results compatible with the hypothesis that miR-30c causes down-regulation of SUR2 transcripts with the longer 3' UTR. Thus we report evidence of complex ABCC9 genetic regulation in brain, which may be of direct relevance to human disease. ABCC9 gene variants are associated with increased risk for hippocampal sclerosis of aging (HS-Aging--a prevalent brain disease with symptoms that mimic Alzheimer's disease). We describe novel ABCC9 variants in human brain, corresponding to altered 3'UTR length, which could lead to targeting by miR-30c. We also determined that the HS-Aging risk mutation is associated with variation in ABCC9 transcript expression.

Abcc9 is required for the transition to oxidative metabolism in the newborn heart

The newborn heart adapts to postnatal life by shifting from a fetal glycolytic metabolism to a mitochondrial oxidative metabolism. Abcc9, an ATP-binding cassette family member, increases expression concomitant with this metabolic shift. Abcc9 encodes a membrane-associated receptor that partners with a potassium channel to become the major potassium-sensitive ATP channel in the heart. Abcc9 also encodes a smaller protein enriched in the mitochondria. We now deleted exon 5 of Abcc9 to ablate expression of both plasma membrane and mitochondria-associated Abcc9-encoded proteins, and found that the myocardium failed to acquire normal mature metabolism, resulting in neonatal cardiomyopathy. Unlike wild-type neonatal cardiomyocytes, mitochondria from Ex5 cardiomyocytes were unresponsive to the KATP agonist diazoxide, consistent with loss of KATP activity. When exposed to hydrogen peroxide to induce cell stress, Ex5 neonatal cardiomyocytes displayed a rapid collapse of mitochondria membrane potential, distinct from wild-type cardiomyocytes. Ex5 cardiomyocytes had reduced fatty acid oxidation, reduced oxygen consumption and reserve. Morphologically, Ex5 cardiac mitochondria exhibited an immature pattern with reduced cross-sectional area and intermitochondrial contacts. In the absence of Abcc9, the newborn heart fails to transition normally from fetal to mature myocardial metabolism.-Fahrenbach, J. P., Stoller, D., Kim, G., Aggarwal, N., Yerokun, B., Earley, J. U., Hadhazy, M., Shi, N.-Q., Makielski, J. C., McNally, E. M. Abcc9 is required for the transition to oxidative metabolism in the newborn heart.

Blocking of the ATP sensitive potassium channel ameliorates the ischaemia-reperfusion injury in the rat testis

There is increasing evidence that the effects of administered ATP sensitive potassium (KATP ) channel openers or blockers during ischaemia are still controversial in many organs/tissues. Testicular torsion detorsion which causes ischaemia-reperfusion (IR) injury, cannot be predicted, thus an effective drug should be administered during or after the ischaemia. The aim of this study was to examine whether the administration of KATP channel openers or blockers during ischaemia ameliorates IR injury in the testis. Eight-week-old male Sprague-Dawley rats were subjected to 2 h right testicular ischaemia followed by 24 h reperfusion. The selective mitochondrial (mito) KATP channel blocker, 5-hydroxydecanoate (5-HD) (40 mg/kg), the non-selective KATP channel blocker glibenclamide (5 mg/kg), the selective mito KATP channel opener diazoxide (10 mg/kg) and the non-selective KATP channel opener cromakalim (300 μg/kg) were administered intraperitoneally 15 min prior to the ischaemia or 75 min after the induction of ischaemia. Tissue damage was evaluated by malondialdehyde concentration, myeloperoxidase activity, histological evaluation and TdT-mediated dUTP nick end labelling assay in the testis. There was a significant increase in oxidative stress, neutrophil infiltration, histological damage and apoptosis in the testicular IR model. A significant reduction in the testicular IR injury was observed with the administration of glibenclamide, but not 5-HD, diazoxide or cromakalim during ischaemia. The administration of non-selective KATP channel blocker glibenclamide ameliorated the testicular IR injury. On the other hand, the selective mito KATP channel blocker, 5-HD and KATP channel openers did not reduce the testicular IR injury. These data suggest that blocking of the membrane KATP channel may have a protective effect during the testicular ischaemia. Glibenclamide could be an effective drug to manage the post-ischaemic injury caused by the testicular torsion-detorsion.

Kir6.2-containing ATP-sensitive K(+) channel is required for cardioprotection of resveratrol in mice

Background

Resveratrol is a natural compound that affects energy metabolism and is also known to possess an array of cardioprotective effects. However, its overall effects on energy metabolism and the underlying mechanism involved in cardioprotection require further investigation. Herein we hypothesize that ATP-sensitive potassium (K-ATP) channels as molecular sensors of cellular metabolism may mediate the cardioprotective effects of resveratrol.

Methods

Kir6.2 knockout, Kir6.1 heterozygous and wild-type (WT) mice were subjected to ischemia/reperfusion injury and were injected with resveratrol (10 mg/kg, i.p). Myocardial infarct size, serum lactate dehydrogenase (LDH) and creatine kinase (CK) activities were determined. Neonatal cardiomyocytes were used in in vitro assays to investigate the underlying mechanism of resveratrol in cardioprotection.

Results

Resveratrol treatment significantly reduced myocardial infarct size and serum LDH and CK activity and inhibited oxygen-glucose deprivation/reoxygenation – induced cardiomyocyte apoptosis in WT and Kir6.1 heterozygous mice, but Kir6.2 deficiency can abolish the cardioprotective effects of resveratrol in vivo and in vitro. We further found that resveratrol enhanced 5′-AMP-activated protein kinase (AMPK) phosphorylation and promoted the association of AMPK with Kir6.2. Suppression of AMPK attenuated and activation of AMPK mimicked the cardioprotective effects of resveratrol in cardiomyocytes. Notably, Kir6.2 knockout also reversed the cardioprotection of AMPK activator.

Conclusions

Our study demonstrates that resveratrol exerts cardioprotective effects through AMPK -Kir6.2/K-ATP signal pathway and Kir6.2-containing K-ATP channel is required for cardioprotection of resveratrol.

KATP channels and cardiovascular disease: suddenly a syndrome

ATP-sensitive potassium (KATP) channels were first discovered in the heart 30 years ago. Reconstitution of KATP channel activity by coexpression of members of the pore-forming inward rectifier gene family (Kir6.1, KCNJ8, and Kir6.2 KCNJ11) with sulfonylurea receptors (SUR1, ABCC8, and SUR2, ABCC9) of the ABCC protein subfamily has led to the elucidation of many details of channel gating and pore properties. In addition, the essential roles of Kir6.x and SURx subunits in generating cardiac and vascular KATP(2) and the detrimental consequences of genetic deletions or mutations in mice have been recognized. However, despite this extensive body of knowledge, there has been a paucity of defined roles of KATP subunits in human cardiovascular diseases, although there are reports of association of a single Kir6.1 variant with the J-wave syndrome in the ECG, and 2 isolated studies have reported association of loss of function mutations in SUR2 with atrial fibrillation and heart failure. Two new studies convincingly demonstrate that mutations in the SUR2 gene are associated with Cantu syndrome, a complex multi-organ disorder characterized by hypertrichosis, craniofacial dysmorphology, osteochondrodysplasia, patent ductus arteriosus, cardiomegaly, pericardial effusion, and lymphoedema. This realization of previously unconsidered consequences provides significant insight into the roles of the KATP channel in the cardiovascular system and suggests novel therapeutic possibilities.

Blockers of sulfonylureas receptor 1 subunits may lead to cardiac protection against isoprenaline-induced injury in obese rats

Recent studies have found that blockers of sulfonylureas receptor 1(SUR1) might have cardiac ischemic protective effects. We evaluated the effects of a selective SUR1 blocker gliclazide on cardiac function and arrhythmia after isoprenaline-induced myocardial injury in obese rats. Diet-induced obese rats received isoprenaline or saline shots subcutaneously. Gliclazide or saline was given q12 h for 48 h to rats received isoprenaline. We measured ECG and hemodynamic parameters and collected blood samples for CK-MB, glucose and lipid profile determination, and then harvested hearts for water content, histological and immunohistochemical analysis and infarct size measurements. The obese rats' hearts receiving isoprenaline-induced myocardial injury showed up-regulated SUR-1 expression in the peri-microvascular area. Obese rats receiving gliclazide lavage had less severe arrhythmia (ASI: 4.00 ± 0.61 vs. 2.14 ± 0.39, P<0.05) and myocardial edema (water percentage: 85.16 ± 0.46% vs. 81.56 ± 0.57%, P<0.05). Less infarct size (47.6 ± 12.8% vs. 32.7 ± 9.1%, P<0.05) and improved diastolic function (LVEDP: 6.86 ± 0.85% vs. 2.51 ± 1.09%, P<0.05;-(dp/dt)(max): -1663.6 ± 387.91 mmHg/s vs. -2834.8 ± 290.76 mmHg/s, P<0.05) were also observed in rats receiving gliclazide lavage. Blocking of the SUR1 thus exerts a protective effect on the isoprenaline-induced myocardial injury in obese rats. That SUR1 blocker leads to ischemic protection suggesting a critical biological role of SUR1 in regulating the function of the cardiovascular system than previously recognized under pathophysiological conditions.

Measuring and evaluating the role of ATP-sensitive K+ channels in cardiac muscle

Since ion channels move electrical charge during their activity, they have traditionally been studied using electrophysiological approaches. This was sometimes combined with mathematical models, for example with the description of the ionic mechanisms underlying the initiation and propagation of action potentials in the squid giant axon by Hodgkin and Huxley. The methods for studying ion channels also have strong roots in protein chemistry (limited proteolysis, the use of antibodies, etc.). The advent of the molecular cloning and the identification of genes coding for specific ion channel subunits in the late 1980s introduced a multitude of new techniques with which to study ion channels and the field has been rapidly expanding ever since (e.g. antibody development against specific peptide sequences, mutagenesis, the use of gene targeting in animal models, determination of their protein structures) and new methods are still in development. This review focuses on techniques commonly employed to examine ion channel function in an electrophysiological laboratory. The focus is on the K(ATP) channel, but many of the techniques described are also used to study other ion channels.

An abundant, truncated human sulfonylurea receptor 1 splice variant has prodiabetic properties and impairs sulfonylurea action

An alternatively spliced form of human sulfonylurea receptor (SUR) 1 mRNA lacking exon 2 (SUR1Δ2) has been identified. The omission of exon 2 caused a frame shift and an immediate stop codon in exon 3 leading to translation of a 5.6-kDa peptide that comprises the N-terminal extracellular domain and the first transmembrane helix of SUR1. Based on a weak first splice acceptor site in the human SUR1 gene (ABCC8), RT-PCR revealed a concurrent expression of SUR1Δ2 and SUR1. The SUR1Δ2/(SUR1 + SUR1Δ2) mRNA ratio differed between tissues, and was lowest in pancreas (46%), highest in heart (88%) and negatively correlated with alternative splice factor/splicing factor 2 (ASF/SF2) expression. In COS-7 cells triple transfected with SUR1Δ2/SUR1/Kir6.2, the SUR1Δ2 peptide co-immunoprecipitated with Kir6.2, thereby displacing two of four SUR1 subunits on the cell surface. The ATP sensitivity of these hybrid ATP-sensitive potassium channels (K(ATP)) channels was reduced by about sixfold, as shown with single-channel recordings. RINm5f rat insulinoma cells, which genuinely express SUR1 but not SUR1Δ2, exhibited a strongly increased K(ATP) channel current upon transfection with SUR1Δ2. This led to inhibition of glucose-induced depolarization, calcium flux, insulin release and glibenclamide action. A non-mutagenic SNP on nucleotide position 333 (Pro69Pro) added another exonic splicing enhancer sequence detected by ASF/SF2, reduced relative abundance of SUR1Δ2 and slightly protected from non-insulin dependent diabetes in homozygotic individuals. Thus, SUR1Δ2 represents an endogenous K(ATP)-channel modulator with prodiabetic properties in islet cells. Its predominance in heart may explain why high-affinity sulfonylurea receptors are not found in human cardiac tissue.

Vasodilation induced by oxygen/glucose deprivation is attenuated in cerebral arteries of SUR2 null mice

Physiological functions of arterial smooth muscle cell ATP-sensitive K(+) (K(ATP)) channels, which are composed of inwardly rectifying K(+) channel 6.1 and sulfonylurea receptor (SUR)-2 subunits, during metabolic inhibition are unresolved. In the present study, we used a genetic model to investigate the physiological functions of SUR2-containing K(ATP) channels in mediating vasodilation to hypoxia, oxygen and glucose deprivation (OGD) or metabolic inhibition, and functional recovery following these insults. Data indicate that SUR2B is the only SUR isoform expressed in murine cerebral artery smooth muscle cells. Pressurized SUR2 wild-type (SUR2(wt)) and SUR2 null (SUR2(nl)) mouse cerebral arteries developed similar levels of myogenic tone and dilated similarly to hypoxia (<10 mmHg Po(2)). In contrast, vasodilation induced by pinacidil, a K(ATP) channel opener, was ∼71% smaller in SUR2(nl) arteries. Human cerebral arteries also expressed SUR2B, developed myogenic tone, and dilated in response to hypoxia and pinacidil. OGD, oligomycin B (a mitochondrial ATP synthase blocker), and CCCP (a mitochondrial uncoupler) all induced vasodilations that were ∼39-61% smaller in SUR2(nl) than in SUR2(wt) arteries. The restoration of oxygen and glucose following OGD or removal of oligomycin B and CCCP resulted in partial recovery of tone in both SUR2(wt) and SUR2(nl) cerebral arteries. However, SUR(nl) arteries regained ∼60-82% more tone than did SUR2(wt) arteries. These data indicate that SUR2-containing K(ATP) channels are functional molecular targets for OGD, but not hypoxic, vasodilation in cerebral arteries. In addition, OGD activation of SUR2-containing K(ATP) channels may contribute to postischemic loss of myogenic tone.

Effects of KATP channel openers diazoxide and pinacidil in coronary-perfused atria and ventricles from failing and non-failing human hearts

This study compared the effects of ATP-regulated potassium channel (K(ATP)) openers, diazoxide and pinacidil, on diseased and normal human atria and ventricles. We optically mapped the endocardium of coronary-perfused right (n=11) or left (n=2) posterior atrial-ventricular free wall preparations from human hearts with congestive heart failure (CHF, n=8) and non-failing human hearts without (NF, n=3) or with (INF, n=2) infarction. We also analyzed the mRNA expression of the K(ATP) targets K(ir)6.1, K(ir)6.2, SUR1, and SUR2 in the left atria and ventricles of NF (n=8) and CHF (n=4) hearts. In both CHF and INF hearts, diazoxide significantly decreased action potential durations (APDs) in atria (by -21±3% and -27±13%, p<0.01) and ventricles (by -28±7% and -28±4%, p<0.01). Diazoxide did not change APD (0±5%) in NF atria. Pinacidil significantly decreased APDs in both atria (-46 to -80%, p<0.01) and ventricles (-65 to -93%, p<0.01) in all hearts studied. The effect of pinacidil on APD was significantly higher than that of diazoxide in both atria and ventricles of all groups (p<0.05). During pinacidil perfusion, burst pacing induced flutter/fibrillation in all atrial and ventricular preparations with dominant frequencies of 14.4±6.1 Hz and 17.5±5.1 Hz, respectively. Glibenclamide (10 μM) terminated these arrhythmias and restored APDs to control values. Relative mRNA expression levels of K(ATP) targets were correlated to functional observations. Remodeling in response to CHF and/or previous infarct potentiated diazoxide-induced APD shortening. The activation of atrial and ventricular K(ATP) channels enhances arrhythmogenicity, suggesting that such activation may contribute to reentrant arrhythmias in ischemic hearts.

Comparison of two sulfonylureas with high and low myocardial K(ATP) channel affinity on myocardial infarct size and metabolism in a rat model of type 2 diabetes

AIMS/HYPOTHESIS

Sulfonylureas (SUs) may impair outcome in patients with acute coronary syndrome. Most experimental studies of the myocardial effects of SU treatment are performed in non-diabetic models. We compared the effect of two widely used SUs, glibenclamide (gb) and gliclazide (gc), with high and low myocardial K(ATP) channel affinity, respectively, at therapeutic concentrations on infarct size, left ventricular (LV) function and myocardial glycogen, lactate and alanine content before and after ischaemia/reperfusion (I/R).

METHODS

Non-diabetic Wistar and diabetic Goto-Kakizaki rat hearts were investigated in a Langendorff preparation. Gb (0.1 μmol/l) and gc (1.0 μmol/l) were administrated throughout the study. Infarct size was evaluated after 120 min of reperfusion. Myocardial metabolite content was measured before and after ischaemia.

RESULTS

Infarct size was smaller in diabetic hearts than in non-diabetic hearts (0.33 ± 0.03 vs 0.51 ± 0.05, p < 0.05). Gb increased infarct size (0.54 ± 0.04 vs 0.33 ± 0.03, p < 0.05) and reduced post-ischaemic LV developed pressure (60 ± 3 vs 76 ± 3 mmHg, p < 0.05) and coronary flow (4.9 ± 0.5 vs 7.1 ± 0.4 ml min(-1) g(-1), p < 0.05) in gb-treated diabetic rats compared with untreated diabetic rats. On comparing gb-treated diabetic rats with untreated diabetic rats, glycogen content was reduced before (9.1 ± 0.6 vs 13.6 ± 1.0 nmol/mg wet weight, p < 0.01) and after ischaemia (0.9 ± 0.2 vs 1.8 ± 0.2 nmol/mg wet weight, p < 0.05), and lactate (4.8 ± 0.4 vs 3.2 ± 0.3 nmol/mg wet weight, p < 0.01) and alanine (1.38 ± 0.12 vs 0.96 ± 0.09 nmol/mg wet weight, p < 0.05) contents were increased during reperfusion. Gc-treatment of diabetic and non-diabetic rats did not affect any of the measured variables.

CONCLUSIONS/INTERPRETATIONS

Gb, but not gc, exacerbates I/R injury and deteriorates LV function in diabetic hearts. These effects of gb on diabetic hearts may be due to detrimental effects on myocardial carbohydrate metabolism.

Diazoxide maintenance of myocyte volume and contractility during stress: evidence for a non-sarcolemmal K(ATP) channel location

OBJECTIVE

Animal and human myocytes demonstrate significant swelling and reduced contractility during exposure to stress (metabolic inhibition, hyposmotic stress, or hyperkalemic cardioplegia), and these detrimental consequences may be inhibited by the addition of diazoxide (adenosine triphosphate-sensitive potassium channel opener) via an unknown mechanism. Both SUR1 and SUR2A subunits have been localized to the heart, and mouse sarcolemmal adenosine triphosphate-sensitive potassium channels are composed of SUR2A/Kir6.2 subunits in the ventricle and SUR1/Kir6.2 subunits in the atria. This study was performed to localize the mechanism of diazoxide by direct probing of sarcolemmal adenosine triphosphate-sensitive potassium channel current and by genetic deletion of channel subunits.

METHODS

Sarcolemmal adenosine triphosphate-sensitive potassium channel current was recorded in isolated wild-type ventricular mouse myocytes during exposure to Tyrode's solution, Tyrode's + 100 μmol/L diazoxide, hyperkalemic cardioplegia, cardioplegia + diazoxide, cardioplegia + 100 μmol/L pinacidil, or metabolic inhibition using whole-cell voltage clamp (N = 7-12 cells per group). Ventricular myocyte volume was measured from SUR1(-/-) and wild-type mice during exposure to control solution, hyperkalemic cardioplegia, or cardioplegia + 100 μmol/L diazoxide (N = 7-10 cells per group).

RESULTS

Diazoxide did not increase sarcolemmal adenosine triphosphate-sensitive potassium current in wild-type myocytes, although they demonstrated significant swelling during exposure to cardioplegia that was prevented by diazoxide. SUR1(-/-) myocytes also demonstrated significant swelling during exposure to cardioplegia, but this was not altered by diazoxide.

CONCLUSIONS

Diazoxide does not open the ventricular sarcolemmal adenosine triphosphate-sensitive potassium channel but provides volume homeostasis via an SUR1-dependent pathway in mouse ventricular myocytes, supporting a mechanism of action distinct from sarcolemmal adenosine triphosphate-sensitive potassium channel activation.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Gain-of-function mutation S422L in the KCNJ8-encoded cardiac K(ATP) channel Kir6.1 as a pathogenic substrate for J-wave syndromes

BACKGROUND

J-wave syndromes have emerged conceptually to encompass the pleiotropic expression of J-point abnormalities including Brugada syndrome (BrS) and early repolarization syndrome (ERS). KCNJ8, which encodes the cardiac K(ATP) Kir6.1 channel, recently has been implicated in ERS following identification of the functionally uncharacterized missense mutation S422L.

OBJECTIVE

The purpose of this study was to further explore KCNJ8 as a novel susceptibility gene for J-wave syndromes.

METHODS

Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, comprehensive open reading frame/splice site mutational analysis of KCNJ8 was performed in 101 unrelated patients with J-wave syndromes, including 87 with BrS and 14 with ERS. Six hundred healthy individuals were examined to assess the allelic frequency for all variants detected. KCNJ8 mutation(s) was engineered by site-directed mutagenesis and coexpressed heterologously with SUR2A in COS-1 cells. Ion currents were recorded using whole-cell configuration of the patch-clamp technique.

RESULTS

One BrS case and one ERS case hosted the identical missense mutation S422L, which was reported previously. KCNJ8-S422L involves a highly conserved residue and was absent in 1,200 reference alleles. Both cases were negative for mutations in all known BrS and ERS susceptibility genes. K(ATP) current of the Kir6.1-S422L mutation was increased significantly over the voltage range from 0 to 40 mV compared to Kir6.1-WT channels (n = 16-21; P <.05).

CONCLUSION

These findings further implicate KCNJ8 as a novel J-wave syndrome susceptibility gene and a marked gain of function in the cardiac K(ATP) Kir6.1 channel secondary to KCNJ8-S422L as a novel pathogenic mechanism for the phenotypic expression of both BrS and ERS.

A novel mitochondrial K(ATP) channel assay

RATIONALE

The mitochondrial ATP sensitive potassium channel (mK(ATP)) is implicated in cardioprotection by ischemic preconditioning (IPC), but the molecular identity of the channel remains controversial. The validity of current methods to assay mK(ATP) activity is disputed.

OBJECTIVE

We sought to develop novel methods to assay mK(ATP) activity and its regulation.

METHODS AND RESULTS

Using a thallium (Tl(+))-sensitive fluorophore, we developed a novel Tl(+) flux based assay for mK(ATP) activity, and used this assay probe several aspects of mK(ATP) function. The following key observations were made. (1) Time-dependent run down of mK(ATP) activity was reversed by phosphatidylinositol-4,5-bisphosphate (PIP(2)). (2) Dose responses of mK(ATP) to nucleotides revealed a UDP EC(50) of approximately 20 micromol/L and an ATP IC(50) of approximately 5 micromol/L. (3) The antidepressant fluoxetine (Prozac) inhibited mK(ATP) (IC(50)=2.4 micromol/L). Fluoxetine also blocked cardioprotection triggered by IPC, but did not block protection triggered by a mK(ATP)-independent stimulus. The related antidepressant zimelidine was without effect on either mK(ATP) or IPC.

CONCLUSIONS

The Tl(+) flux mK(ATP) assay was validated by correlation with a classical mK(ATP) channel osmotic swelling assay (R(2)=0.855). The pharmacological profile of mK(ATP) (response to ATP, UDP, PIP(2), and fluoxetine) is consistent with that of an inward rectifying K(+) channel (K(IR)) and is somewhat closer to that of the K(IR)6.2 than the K(IR)6.1 isoform. The effect of fluoxetine on mK(ATP)-dependent cardioprotection has implications for the growing use of antidepressants in patients who may benefit from preconditioning.

Differential K(ATP) channel pharmacology in intact mouse heart

Classically, cardiac sarcolemmal K(ATP) channels have been thought to be composed of Kir6.2 (KCNJ11) and SUR2A (ABCC9) subunits. However, the evidence is strong that SUR1 (sulfonylurea receptor type 1, ABCC8) subunits are also expressed in the heart and that they play a significant functional role in the atria. To examine this further, we have assessed the effects of isotype-specific potassium channel-opening drugs, diazoxide (specific to SUR1>SUR2A) and pinacidil (SUR2A>SUR1), in intact hearts from wild-type mice (WT, n=6), SUR1(-/-) (n=6), and Kir6.2(-/-) mice (n=5). Action potential durations (APDs) in both atria and ventricles were estimated by optical mapping of the posterior surface of Langendorff-perfused hearts. To confirm the atrial effect of both openers, isolated atrial preparations were mapped in both WT (n=4) and SUR1(-/-) (n=3) mice. The glass microelectrode technique was also used to validate optical action potentials. In WT hearts, diazoxide (300 microM) decreased APD in atria (from 33.8+/-1.9 ms to 24.2+/-1.1 ms, p<0.001) but was without effect in ventricles (APD 60.0+/-7.6 ms vs. 60.8+/-7.5 ms, respectively, NS), consistent with an atrial-specific role for SUR1. The absence of SUR1 resulted in loss of efficacy of diazoxide in SUR1(-/-) atria (APD 36.8+/-1.9 ms vs. 36.8+/-2.8 ms, respectively, NS). In contrast, pinacidil (300 microM) significantly decreased ventricular APD in both WT and SUR1(-/-) hearts (from 60.0+/-7.6 ms to 29.8+/-3.5 ms in WT, p<0.001, and from 63.5+/-2.1 ms to 24.8+/-3.8 ms in SUR1(-/-), p<0.001), but did not decrease atrial APD in either WT or SUR1(-/-) hearts. Glibenclamide (10 microM) reversed the effect of pinacidil in ventricles and restored APD to control values. The absence of Kir6.2 subunits in Kir6.2(-/-) hearts resulted in loss of efficacy of both openers (APD 47.2+/-2.2 ms vs. 47.6+/-2.1 ms and 50.8+/-2.4 ms, and 90.6+/-5.7 ms vs. 93.2+/-6.5 ms and 117.3+/-6.4 ms, for atria and ventricle in control versus diazoxide and pinacidil, respectively). Collectively, these results indicate that in the same mouse heart, significant differential K(ATP) pharmacology in atria and ventricles, resulting from SUR1 predominance in forming the atrial channel, leads to differential effects of potassium channel openers on APD in the two chambers.

Sulfonylurea receptor 1 subunits of ATP-sensitive potassium channels and myocardial ischemia/reperfusion injury

K(ATP) channels are generally cardioprotective under conditions of metabolic impairment, consisting of pore-forming (Kir6.1 and/or Kir6.2) and sulphonylurea-binding, modulatory subunits [sulfonylurea receptor (SUR) 1, 2A, or 2B]. Cardiovascular K(ATP) channels are generally thought to consist of Kir6.2/SUR2A subunits (in the case of heart muscle) or Kir6.1/SUR2B subunits (smooth muscle), whereas SUR1-containing channels have well-documented roles in pancreatic insulin release. Recent data, however, demonstrated the presence of SUR1 subunits in mouse cardiac tissue (particularly in atria) and a surprising protection from myocardial ischemia/reperfusion in SUR1-null mice. Here, we review some of the extra-pancreatic roles assigned to SUR1 subunits and consider whether these might be involved in the sequelae of ischemia/reperfusion.

Cardiac sarcolemmal K(ATP) channels: Latest twists in a questing tale!

Reconstitution of K(ATP) channel activity from coexpression of members of the pore-forming inward rectifier gene family (Kir6.1, KCNJ8, and Kir6.2 KCNJ11) with sulfonylurea receptors (SUR1, ABCC8, and SUR2, ABCC9) of the ABCC protein sub-family, has led to the elucidation of many details of channel gating and pore properties, as well as the essential roles of Kir6.2 and SUR2 subunits in generating cardiac ventricular K(ATP). However, despite this extensive body of knowledge, there remain significant holes in our understanding of the physiological role of the cardiac K(ATP) channel, and surprising new findings keep emerging. Recent findings from genetically modified animals include the apparent insensitivity of cardiac sarcolemmal channels to nucleotide levels, and unenvisioned complexities of the subunit make-up of the cardiac channels. This topical review focuses on these new findings and considers their implications.

SURA2 targeting for cardioprotection?

SUR2A is an ATP-binding protein known to serve as a regulatory subunit of metabolic-sensing, cardioprotective sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. It has been recently found that a moderate increase in expression of SUR2A protects the heart against different types of metabolic stresses, including ischaemia/reperfusion and hypoxia. Although the sarcolemmal K(ATP) channel is a multiprotein complex composed of many proteins in vivo, it seems that an increase in SUR2A levels is sufficient to increase the number of sarcolemmal K(ATP) channels. This effect of SUR2A could be due to SUR2A being the rate-limiting factor in generating fully composed sarcolemmal K(ATP) channels. An increased number of sarcolemmal K(ATP) channels seems to protect the heart by regulating action membrane potential, inhibiting Ca(2+) influx and preventing Ca(2+) overload, although an additional yet to be recognised mechanism independent of K(ATP) channels activity cannot be excluded.

Differential structure of atrial and ventricular KATP: atrial KATP channels require SUR1

The isoform-specific structure of the ATP-sensitive potassium (K(ATP)) channel endows it with differential fundamental properties, including physiological activation and pharmacology. Numerous studies have convincingly demonstrated that the pore-forming Kir6.2 (KCNJ11) and regulatory SUR2A (ABCC9) subunits are essential elements of the sarcolemmal K(ATP) channel in cardiac ventricular myocytes. Using a novel antibody directed against the COOH terminus of SUR1 (ABCC8), we show that this K(ATP) subunit is also expressed in mouse myocardium and is the dominant SUR isoform in the atrium. This suggests differential sarcolemmal K(ATP) composition in atria and ventricles, and, to test this, K(ATP) currents were measured in isolated atrial and ventricular myocytes from wild-type and SUR1(-/-) animals. K(ATP) conductance is essentially abolished in SUR1(-/-) atrial myocytes but is normal in SUR1(-/-) ventricular myocytes. Furthermore, pharmacological properties of wild-type atrial K(ATP) match closely the properties of heterologously expressed SUR1/Kir6.2 channels, whereas ventricular K(ATP) properties match those of heterologously expressed SUR2A/Kir6.2 channels. Collectively, the data demonstrate a previously unappreciated K(ATP) channel heterogeneity: SUR1 is an essential component of atrial, but not ventricular, K(ATP) channels. Differential molecular make-up of the 2 channels underlies differential pharmacology, with important implications when considering sulfonylurea therapy or dissecting the role of cardiac K(ATP) pharmacologically, as well as for understanding of the role of diazoxide in preconditioning.