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

Natriuretic peptides modulate ATP-sensitive K(+) channels in rat ventricular cardiomyocytes

B-type natriuretic peptide (BNP) and C-type natriuretic peptide (CNP), and (Cys-18)-atrial natriuretic factor (4–23) amide (C-ANF), are cytoprotective under conditions of ischemia–reperfusion, limiting infarct size. ATP-sensitive K⁺ channel (K_ATP) opening is also cardioprotective, and although the K_ATP activation is implicated in the regulation of cardiac natriuretic peptide release, no studies have directly examined the effects of natriuretic peptides on cardiac K_ATP activity. Normoxic cardiomyocytes were patch clamped in the cell-attached configuration to examine sarcolemmal K_ATP (sK_ATP) activity. The K_ATP opener pinacidil (200 μM) increased the open probability of the patch (NPo; values normalized to control) at least twofold above basal value, and this effect was abolished by HMR1098 10 μM, a selective K_ATP blocker (5.23 ± 1.20 versus 0.89 ± 0.18; P < 0.001). We then examined the effects of BNP, CNP, C-ANF and 8Br-cGMP on the sK_ATP current. Bath application of BNP (≥10 nM) or CNP (≥0.01 nM) suppressed basal NPo (BNP: 1.00 versus 0.56 ± 0.09 at 10 nM, P < 0.001; CNP: 1.0 versus 0.45 ± 0.16, at 0.01 nM, P < 0.05) and also abolished the pinacidil-activated current at concentrations ≥10 nM. C-ANF (≥10 nM) enhanced K_ATP activity (1.00 versus 3.85 ± 1.13, at 100 nM, P < 0.05). The cGMP analog 8Br-cGMP 10 nM dampened the pinacidil-activated current (2.92 ± 0.60 versus 1.53 ± 0.32; P < 0.05). Natriuretic peptides modulate sK_ATP current in ventricular cardiomyocytes. This may be at least partially associated with their ability to augment intracellular cGMP concentrations via NPR-A/B, or their ability to bind NPR-C with high affinity. Although the mechanism of modulation requires elucidation, these preliminary data give new insights into the relationship between natriuretic peptide signaling and sK_ATP in the myocardium.

ATP-sensitive potassium (K(ATP)) channel openers diazoxide and nicorandil lower intraocular pressure in vivo

PURPOSE

To evaluate the expression of ATP-sensitive potassium (K(ATP)) channel subunits and study the effect of K(ATP) channel openers diazoxide and nicorandil on intraocular pressure (IOP) in an in vivo mouse model.

METHODS

Expression of K(ATP) channel subunits in normal C57BL/6 mouse eyes was studied by immunohistochemistry and confocal microscopy. Wild-type C57BL/6 mice were treated with K(ATP) channel openers diazoxide (n = 10) and nicorandil (n = 10) for 14 days. Similar treatments with diazoxide were performed on K(ir)6.2((-/-)) mice (n = 10). IOP was recorded with a handheld tonometer 1 hour, 4 hours, and 23 hours following daily treatment. Posttreatment histology was examined by light and transmission electron microscopy.

RESULTS

The K(ATP) channel subunits SUR2B, K(ir)6.1, and K(ir)6.2 were identified in all tissues within mouse eyes. Treatment with diazoxide in wild-type mice decreased IOP by 21.5 ± 3.2% with an absolute IOP reduction of 3.9 ± 0.6 mm Hg (P = 0.002). Nicorandil also decreased IOP (18.9 ± 1.8%) with an absolute IOP reduction of 3.4 ± 0.4 mm Hg (P = 0.002). Treatment with diazoxide in K(ir)6.2((-/-)) mice had no effect on IOP. No morphological abnormalities were observed in diazoxide- or nicorandil-treated eyes.

CONCLUSIONS

K(ATP) channel openers diazoxide and nicorandil are effective regulators of IOP in mouse eyes. K(ir)6.2 appears to be a major K(ATP) channel subunit through which IOP is lowered following treatment with diazoxide.

Heterogeneity and function of K(ATP) channels in canine hearts

BACKGROUND

The concept that pore-forming Kir6.2 and regulatory SUR2A subunits form cardiac ATP-sensitive potassium (K(ATP)) channels is challenged by recent reports that SUR1 is predominant in mouse atrial K(ATP) channels.

OBJECTIVE

To assess SUR subunit composition of K(ATP) channels and consequence of K(ATP) activation for action potential duration (APD) in dog hearts.

METHODS

Patch-clamp techniques were used on isolated dog cardiomyocytes to investigate K(ATP) channel properties. Dynamic current clamp, by injection of a linear K(+) conductance to simulate activation of the native current, was used to study the consequences of K(ATP) activation on APD.

RESULTS

Metabolic inhibitor (MI)-activated current was not significantly different from pinacidil (SUR2A-specific)-activated current, and both currents were larger than diazoxide (SUR1-specific)-activated current in both the atrium and the ventricle. Mean K(ATP) conductance (activated by MI) did not differ significantly between chambers, although, within the ventricle, both MI-induced and pinacidil-induced currents tended to decrease from the epicardium to the endocardium. Dynamic current-clamp results indicate that myocytes with longer baseline APDs are more susceptible to injected K(ATP) current, a result reproduced in silico by using a canine action potential model (Hund-Rudy) to simulate epicardial and endocardial myocytes.

CONCLUSIONS

Even a small fraction of K(ATP) activation significantly shortens APD in a manner that depends on existing heterogeneity in K(ATP) current and APD.

Multiplicity of effectors of the cardioprotective agent, diazoxide

Diazoxide has been identified over the past 50years to have a number of physiological effects, including lowering the blood pressure and rectifying hypoglycemia. Today it is used clinically to treat these conditions. More recently, another important mode of action emerged: diazoxide has powerful protective properties against cardiac ischemia. The heart has intrinsic protective mechanisms against ischemia injury; one of which is ischemic preconditioning. Diazoxide mimics ischemic preconditioning. The purpose of this treatise is to review the literature in an attempt to identify the many effectors of diazoxide and discuss how they may contribute to diazoxide's cardioprotective properties. Particular emphasis is placed on the concentration ranges in which diazoxide affects its different targets and how this compares with the concentrations commonly used to study cardioprotection. It is concluded that diazoxide may have several potential effectors that may potentially contribute to cardioprotection, including KATP channels in the pancreas, smooth muscle, endothelium, neurons and the mitochondrial inner membrane. Diazoxide may also affect other ion channels and ATPases and may directly regulate mitochondrial energetics. It is possible that the success of diazoxide lies in this promiscuity and that the compound acts to rebalance multiple physiological processes during cardiac ischemia.

Acute regional left atrial ischemia causes acceleration of atrial drivers during atrial fibrillation

BACKGROUND

The mechanisms by which acute left atrial ischemia (LAI) leads to atrial fibrillation (AF) initiation and perpetuation remain unclear.

OBJECTIVE

To investigate the electrophysiological mechanisms of AF perpetuation in the presence of regional atrial ischemia.

METHODS

LAI (90-minute ischemia) was obtained in isolated sheep hearts by selectively perfusing microspheres into the left anterior atrial artery. Two charge-coupled device cameras and several bipolar electrodes enabled recording from multiple atrial locations: with a dual-camera setup (protocol 1, n = 10, and protocol 1', n = 4, for biatrial or atrioventricular camera setups, respectively), in the presence of propranolol/atropine (1 μM) added to the perfusate after LAI (protocol 2, n = 3) and after a pretreatment with glibenclamide (10 μM; protocol 3, n = 4).

RESULTS

Spontaneous AF occurred in 41.2% (7 of 17) of the hearts that were in sinus rhythm before LAI. LAI caused action potential duration shortening in both the ischemic (IZ) and nonischemic (NIZ) zones by 21% ± 8% and 34% ± 13%, respectively (pacing, 5 Hz; P<.05 compared to baseline). Apparent impulse velocity was significantly reduced in the IZ but not in the NIZ (-65% ± 19% and 9% ± 18%; P = .001 and NS, respectively). During LAI-related AF, a significant NIZ maximal dominant frequency increase from 7.4 ± 2.5 to 14.0 ± 5.5 Hz (P<.05) was observed. Glibenclamide, an ATP-sensitive potassium current (IKATP) channel blocker, averted LAI-related maximal dominant frequency increase (NIZ: LAI vs glibenclamide 14.0 ± 5.5 Hz vs 5.9 ± 1.3 Hz; P<.05). An interplay between spontaneous focal discharges and rotors, locating at the IZ-NIZ border zone, maintained LAI-related AF.

CONCLUSIONS

LAI leads to an IKATP conductance-dependent action potential duration shortening and spontaneous AF maintained by both spontaneous focal discharges and reentrant circuits locating at the IZ border zone.

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.

Mechanical effects on KATP channel gating in rat ventricular myocytes

Cardiac K_ATP channels link metabolism with electrical activity. They are implicated in arrhythmias, secretion of atrial natriuretic peptide and protection of the heart from hypertrophy and failure. These processes may involve mechanosensitivity. K_ATP channels can be activated by mechanical stimulation and disrupting the cortical actin increases the activity. We propose that K_ATP channels are modulated by local bilayer tension and this tension is affected by cortical F-actin. Here we measured K_ATP background activity and stretch sensitivity with inside-out patches of rat ventricular myocytes before and after disrupting F-actin. Disrupting F-actin potentiated background activity but did not influence the slope sensitivity in the semilog relationship of NP_o vs. suction that is a measure of the change in dimensions between closed and open states. Thus actin alters prestress on the channel probably by parallel elastic sharing of mean cortical tension with the bilayer.

Cardioprotective mechanism of diazoxide involves the inhibition of succinate dehydrogenase

BACKGROUND

The adenosine triphosphate-sensitive potassium (KATP) channel opener, diazoxide, preserves myocyte volume homeostasis and contractility during stress via an unknown mechanism. Pharmacologic overlap has been suggested between succinate dehydrogenase (SDH) activity and KATP channel modulators. Diazoxide may be cardioprotective due to the inhibition of SDH which may form a portion of the mitochondrial KATP channel. To determine the role of inhibition of SDH in diazoxide's cardioprotection, this study utilized glutathione to prevent the inhibition of SDH.

METHODS

SDH activity was measured in isolated mitochondria exposed to succinate (control), malonate (inhibitor of succinate dehydrogenase), diazoxide, and varying concentrations of glutathione alone or in combination with diazoxide. Enzyme activity was measured by spectrophotometric analysis. To evaluate myocyte volume and contractility, cardiac myocytes were superfused with Tyrode's physiologic solution (Tyrode's) (20 minutes), followed by test solution (20 minutes), including Tyrode's, hyperkalemic cardioplegia (stress), cardioplegia + diazoxide, cardioplegia + diazoxide + glutathione, or glutathione alone; followed by Tyrode's (20 minutes). Myocyte volume and contractility were recorded using image grabbing software.

RESULTS

Both malonate and diazoxide inhibited succinate dehydrogenase. Glutathione prevented the inhibition of succinate dehydrogenase by diazoxide in a dose-dependent manner. The addition of diazoxide prevented the detrimental myocyte swelling due to cardioplegia alone and this benefit was lost with the addition of glutathione. However, glutathione elicited an independent cardioprotective effect on myocyte contractility.

CONCLUSIONS

The ability of diazoxide to provide beneficial myocyte homeostasis during stress involves the inhibition of succinate dehydrogenase, which may also involve the opening of a purported mitochondrial adenosine triphosphate sensitive potassium channel.

Functional roles of KATP channel subunits in metabolic inhibition

ATP-sensitive potassium channel (KATP) activation can drastically shorten action potential duration (APD) in metabolically compromised myocytes. We showed previously that SUR1 with Kir6.2 forms the functional channel in mouse atria while Kir6.2 and SUR2A predominate in ventricles. SUR1 is more sensitive to metabolic stress than SUR2A, raising the possibility that KATP in atria and ventricles may respond differently to metabolic stress. Action potential duration (APD) and calcium transient duration (CaTD) were measured simultaneously in both atria and ventricles by optical mapping of the posterior surface of Langendorff-perfused hearts from C57BL wild-type (WT; n=11), Kir6.2(-/-) (n=5), and SUR1(-/-) (n=6) mice during metabolic inhibition (MI, 0mM glucose+2mM sodium cyanide). After variable delay, MI led to significant shortening of APD in WT hearts. On average, atrial APD shortened by 60.5 ± 2.7% at 13.1 ± 2.1 min (n=6, p<0.01) after onset of MI. Ventricular APD shortening (56.4 ± 10.0% shortening at 18.2 ± 1.8 min) followed atrial APD shortening. In SUR1(-/-) hearts (n=6), atrial APD shortening was abolished, but ventricular shortening (65.0 ± 15.4% at 25.33 ± 4.48 min, p<0.01) was unaffected. In Kir6.2(-/-) hearts, two disparate responses to MI were observed; 3 of 5 hearts displayed slight shortening of APD in the ventricles (24 ± 3%, p<0.05) and atria (39.0 ± 1.9%, p<0.05) but this shortening occurred later and to much less extent than in WT (p<0.05). Marked prolongation of ventricular APD was observed in the remaining hearts (327% and 489% prolongation) and was associated with occurrence of ventricular tachyarrhythmias. The results confirm that Kir6.2 contributes to APD shortening in both atria and ventricle during metabolic stress, and that SUR1 is required for atrial APD shortening while SUR2A is required for ventricular APD shortening. Importantly, the results show that the presence of SUR1-dependent KATP in the atria results in the action potential being more susceptible to metabolically driven shortening than the ventricle.

Electrocardiographic studies of romidepsin demonstrate its safety and identify a potential role for K(ATP) channel

PURPOSE

Romidepsin is a histone deacetylase inhibitor (HDI) approved for the treatment of both cutaneous and peripheral T-cell lymphoma (CTCL and PTCL). During development, a thorough assessment of cardiac toxicity was conducted.

EXPERIMENTAL DESIGN

A phase II single-agent nonrandomized study of romidepsin was conducted in patients with CTCL or PTCL who had progressed after at least 1 prior systemic therapy.

RESULTS

Results for the first 42 patients enrolled on the NCI 1312 phase II study of romidepsin in CTCL or PTCL showed no cardiac toxicity based on serial electrocardiograms (ECG), troponins, and MUGA scans/echocardiograms. The cardiac assessments reported herein confirm the safety of romidepsin among 131 enrolled patients, while supporting a role for electrolyte replacement. Heart rate increased an average 11 bpm following romidepsin infusion; there was no evidence of increased arrhythmia. Criteria for potassium/magnesium replacement were met before 55% of 1365 romidepsin doses; an association with hypoalbuminemia was confirmed. We propose a mechanism for ST segment flattening and depression, the most common ECG abnormalities observed: HDI-induced alteration of the activity or expression of KATP channels. In addition, examination of the variants of the active transporter of romidepsin, ABCB1, showed a trend toward smaller heart rate changes in the peri-infusion period among wild-type than variant diplotypes.

CONCLUSIONS

We conclude that in the context of appropriate attention to electrolyte levels, the data support the cardiac safety of romidepsin.

Inhibition of Succinate Dehydrogenase by Diazoxide Is Independent of the ATP-Sensitive Potassium Channel Subunit Sulfonylurea Type 1 Receptor

BACKGROUND

Diazoxide maintains myocyte volume and contractility during stress via an unknown mechanism. The mechanism of action may involve an undefined (genotype unknown) mitochondrial ATP-sensitive potassium channel and is dependent on the ATP-sensitive potassium channel subunit sulfonylurea type 1 receptor (SUR1). The ATP-sensitive potassium channel openers have been shown to inhibit succinate dehydrogenase (SDH) and a gene for a portion of SDH has been found in the SUR intron. Diazoxide may be cardioprotective via inhibition of SDH, which can form part of an ATP-sensitive potassium channel or share its genetic material. This study investigated the role of inhibition of SDH by diazoxide and its relationship to the SUR1 subunit.

STUDY DESIGN

Mitochondria were isolated from wild-type and SUR1 knockout mice. Succinate dehydrogenase activity was measured by spectrophotometric analysis of 2,6-dichloroindophenol reduction for 20 minutes as the relative change in absorbance over time. Mitochondria were treated with succinate (20 mM), succinate + 1% dimethylsulfoxide, succinate + malonate (8 mM) (competitive inhibitor of SDH), or succinate + diazoxide (100 μM).

RESULTS

Both malonate and diazoxide inhibit SDH activity in mitochondria of wild-type mice and in mice lacking the SUR1 subunit (p < 0.05 vs control).

CONCLUSIONS

The ability of DZX to inhibit SDH persists even after deletion of the SUR1 gene. Therefore, the enzyme complex SDH is not dependent on the SUR1 gene. The inhibition of SDH by DZX can play a role in the cardioprotection afforded by DZX; however, this role is independent of the ATP-sensitive potassium channel subunit SUR1.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

Comparative proteomic analysis of the ATP-sensitive K+ channel complex in different tissue types

ATP-sensitive K(+) (K(ATP)) channels are expressed ubiquitously, but have diverse roles in various organs and cells. Their diversity can partly be explained by distinct tissue-specific compositions of four copies of the pore-forming inward rectifier potassium channel subunits (Kir6.1 and/or Kir6.2) and four regulatory sulfonylurea receptor subunits (SUR1 and/or SUR2). Channel function and/or subcellular localization also can be modified by the proteins with which they transiently or permanently interact to generate even more diversity. We performed a quantitative proteomic analysis of K(ATP) channel complexes in the heart, endothelium, insulin-secreting min6 cells (pancreatic β-cell like), and the hypothalamus to identify proteins with which they interact in different tissues. Glycolysis is an overrepresented pathway in identified proteins of the heart, min6 cells, and the endothelium. Proteins with other energy metabolic functions were identified in the hypothalamic samples. These data suggest that the metabolo-electrical coupling conferred by K(ATP) channels is conferred partly by proteins with which they interact. A large number of identified cytoskeletal and trafficking proteins suggests endocytic recycling may help control K(ATP) channel surface density and/or subcellular localization. Overall, our data demonstrate that K(ATP) channels in different tissues may assemble with proteins having common functions, but that tissue-specific complex organization also occurs.

Evolving form to fit function: cardiomyocyte intercalated disc and transverse-tubule membranes

The vertebrate cardiac myocyte has evolved a highly organized cellular membrane architecture and cell-cell contacts in order to effectively transmit precisely timed and homogeneous depolarizing waves without failure (>2 billion times/human life span). Two unique specialized membrane domains, the intercalated disc and the transverse tubule (T-tubule), function to ensure the rapid and coordinated propagation of the action potential throughout the heart. Based on their critical roles in structure, signaling, and electric inter- and intracellular communication, it is not surprising that dysfunction in these membrane structures is associated with aberrant vertebrate physiology, resulting in potentially fatal congenital and acquired disease. This chapter will review the fundamental components of cardiomyocyte intercalated disc and transverse-tubule membranes with a focus on linking dysfunction in these membranes with human cardiovascular disease.

Activation of glibenclamide-sensitive ATP-sensitive K+ channels during β-adrenergically induced metabolic stress produces a substrate for atrial tachyarrhythmia

BACKGROUND

Cardiac ATP-sensitive K(+) channels have been suggested to contribute to the adaptive physiological response to metabolic challenge after β-adrenoceptor stimulation. However, an increased atrial K(+)-conductance might be expected to be proarrhythmic. We investigated the effect of ATP-sensitive K(+) channel blockade on the electrophysiological responses to β-adrenoceptor-induced metabolic challenge in intact atria.

METHODS AND RESULTS

Atrial electrograms were recorded from the left atrial epicardial surface of Langendorff-perfused rat hearts using a 5×5 electrode array. Atrial effective refractory period and conduction velocity were measured using an S(1)-S(2) protocol. The proportion of hearts in which atrial tachyarrhythmia was produced by burst-pacing was used as an index of atrial tachyarrhythmia-inducibility. Atrial nucleotide concentrations were measured by high performance liquid chromatography. Perfusion with ≥10(-9) mol/L of the β-adrenoceptor agonist, isoproterenol (ISO), resulted in a concentration-dependent reduction of atrial effective refractory period and conduction velocity. The ISO-induced changes produced a proarrhythmic substrate such that atrial tachyarrhythmia could be induced by burst-pacing. Atrial [ATP] was significantly reduced by ISO (10(-6) mol/L). Perfusion with either of the ATP-sensitive K(+) channel blockers, glibenclamide (10(-5) mol/L) or tolbutamide (10(-3) mol/L), in the absence of ISO had no effect on basal atrial electrophysiology. On the other hand, the proarrhythmic substrate induced by 10(-6) mol/L ISO was abolished by either of the sulfonylureas, which prevented induction of atrial tachyarrhythmia.

CONCLUSIONS

Atrial ATP-sensitive K(+) channels activate in response to β-adrenergic metabolic stress in Langendorff-perfused rat hearts, resulting in a proarrhythmic substrate.

NMR and fluorescence studies of drug binding to the first nucleotide binding domain of SUR2A

ATP sensitive potassium (K(ATP)) channels are composed of four copies of a pore-forming inward rectifying potassium channel (Kir6.1 or Kir6.2) and four copies of a sulfonylurea receptor (SUR1, SUR2A, or SUR2B) that surround the pore. SUR proteins are members of the ATP-binding cassette (ABC) superfamily of proteins. Binding of MgATP at the SUR nucleotide binding domains (NBDs) results in NBD dimerization, and hydrolysis of MgATP at the NBDs leads to channel opening. The SUR proteins also mediate interactions with K(ATP) channel openers (KCOs) that activate the channel, with KCO binding and/or activation involving residues in the transmembrane helices and cytoplasmic loops of the SUR proteins. Because the cytoplasmic loops make extensive interactions with the NBDs, we hypothesized that the NBDs may also be involved in KCO binding. Here, we report nuclear magnetic resonance (NMR) spectroscopy studies that demonstrate a specific interaction of the KCO pinacidil with the first nucleotide binding domain (NBD1) from SUR2A, the regulatory SUR protein in cardiac K(ATP) channels. Intrinsic tryptophan fluorescence titrations also demonstrate binding of pinacidil to SUR2A NBD1, and fluorescent nucleotide binding studies show that pinacidil binding increases the affinity of SUR2A NBD1 for ATP. In contrast, the KCO diazoxide does not interact with SUR2A NBD1 under the same conditions. NMR relaxation experiments and size exclusion chromatography indicate that SUR2A NBD1 is monomeric under the conditions used in drug binding studies. These studies identify additional binding sites for commonly used KCOs and provide a foundation for testing binding of drugs to the SUR NBDs.

Heterogeneity of ATP-sensitive K+ channels in cardiac myocytes: enrichment at the intercalated disk

Ventricular ATP-sensitive potassium (K(ATP)) channels link intracellular energy metabolism to membrane excitability and contractility. Our recent proteomics experiments identified plakoglobin and plakophilin-2 (PKP2) as putative K(ATP) channel-associated proteins. We investigated whether the association of K(ATP) channel subunits with junctional proteins translates to heterogeneous subcellular distribution within a cardiac myocyte. Co-immunoprecipitation experiments confirmed physical interaction between K(ATP) channels and PKP2 and plakoglobin in rat heart. Immunolocalization experiments demonstrated that K(ATP) channel subunits (Kir6.2 and SUR2A) are expressed at a higher density at the intercalated disk in mouse and rat hearts, where they co-localized with PKP2 and plakoglobin. Super-resolution microscopy demonstrate that K(ATP) channels are clustered within nanometer distances from junctional proteins. The local K(ATP) channel density, recorded in excised inside-out patches, was larger at the cell end when compared with local currents recorded from the cell center. The K(ATP) channel unitary conductance, block by MgATP and activation by MgADP, did not differ between these two locations. Whole cell K(ATP) channel current density (activated by metabolic inhibition) was ∼40% smaller in myocytes from mice haploinsufficient for PKP2. Experiments with excised patches demonstrated that the regional heterogeneity of K(ATP) channels was absent in the PKP2 deficient mice, but the K(ATP) channel unitary conductance and nucleotide sensitivities remained unaltered. Our data demonstrate heterogeneity of K(ATP) channel distribution within a cardiac myocyte. The higher K(ATP) channel density at the intercalated disk implies a possible role at the intercellular junctions during cardiac ischemia.

Distinct microRNA expression signatures in human right atrial and ventricular myocardium

Human atrial and ventricular myocardium has distinct structure and physiology. MicroRNAs (miRNAs) are the central players in the regulation of gene expression, participating in many physiological processes. A comprehensive knowledge of miRNA expression in the human heart is essential for the understanding of myocardial function. The aim of this study was to compare the miRNA signature in human right atrial and ventricular myocardium. Agilent human miRNA arrays were used to indicate the miRNA expression signatures of the right atrial (n = 8) and ventricular (n = 9) myocardium of healthy individuals. Quantitative reverse transcription-polymerase chain reactions (qRT-PCRs) were used to validate the array results. DIANA-mirPath was used to incorporate the miRNAs into pathways. MiRNA arrays showed that 169 miRNAs were expressed at different levels in human right atrial and ventricular myocardium. The unsupervised hierarchical clustering analysis based on the 169 dysregulated miRNAs showed that miRNA expression categorized two well-defined clusters that corresponded to human right atrial and ventricular myocardium. The qRT-PCR results correlated well with the microarray data. Bioinformatic analysis indicated the potential miRNA targets and molecular pathways. This study indicates that distinct miRNA expression signatures in human right atrial and ventricular myocardium. The findings provide a novel understanding of the molecular differences between human atrial and ventricular myocardium and may establish a framework for an anatomically detailed evaluation of cardiac function regulation.

Promoter DNA methylation regulates murine SUR1 (Abcc8) and SUR2 (Abcc9) expression in HL-1 cardiomyocytes

Two mammalian genes encode the SURx (SUR1, Abcc8 and SUR2, Abcc9) subunits that combine with Kir6.2 (Kcnj11) subunits to form the ATP-sensitive potassium (KATP) channel in cardiac myocytes. Different isoform combinations endow the channel with distinct physiological and pharmacological properties, and we have recently reported that the molecular composition of sarcolemmal KATP channels is chamber specific in the mouse heart. KATP channel composition is determined by what subunits are expressed in a cell or tissue. In the present study, we explore the role of CpG methylation in regulating SUR1 and SUR2 expression. In HL-1 cardiomyocytes, as in atrial myocytes, SUR1 expression is markedly greater than SUR2. Consistent with CpG methylation-dependent silencing of SUR2 expression, bisulfite sequencing of genomic DNA isolated from HL-1 cells demonstrates that 57.6% of the CpGs in the promoter region of the SUR2 gene are methylated, compared with 0.14% of the the CpG residues in the SUR1 sequence. Moreover, treatment with 10 µM 5-aza-2'-deoxycytidine (Aza-dC) significantly increased both the unmethylated fraction of the SUR2 CpG island and mRNA expression. However, we cannot rule out additional mechanisms of Aza-dC action, as Aza-dC also causes a decrease in SUR1 expression and lower doses of Aza-dC do not alter the unmethylated DNA fraction but do elicit a small increase in SUR2 expression. The conclusion that DNA methylation alone is not the only regulator of SUR subunit expression is also consistent with observations in native myocytes, where the CpG islands of both SUR genes are essentially unmethylated in both atrial and ventricular myocytes. Collectively, these data demonstrate the potential for CpG methylation to regulate SURx subunit expression and raises the possibility that regulated or aberrant CpG methylation might play a role in controlling channel structure and function under different physiological conditions or different species.

Mice expressing a human K(ATP) channel mutation have altered channel ATP sensitivity but no cardiac abnormalities

AIMS/HYPOTHESIS

Patients with severe gain-of-function mutations in the Kir6.2 subunit of the ATP-sensitive potassium (K(ATP)) channel, have neonatal diabetes, muscle hypotonia and mental and motor developmental delay-a condition known as iDEND syndrome. However, despite the fact that Kir6.2 forms the pore of the cardiac K(ATP) channel, patients show no obvious cardiac symptoms. The aim of this project was to use a mouse model of iDEND syndrome to determine whether iDEND mutations affect cardiac function and cardiac K(ATP) channel ATP sensitivity.

METHODS

We performed patch-clamp and in vivo cine-MRI studies on mice in which the most common iDEND mutation (Kir6.2-V59M) was targeted to cardiac muscle using Cre-lox technology (m-V59M mice).

RESULTS

Patch-clamp studies of isolated cardiac myocytes revealed a markedly reduced K(ATP) channel sensitivity to MgATP inhibition in m-V59M mice (IC(50) 62 μmol/l compared with 13 μmol/l for littermate controls). In vivo cine-MRI revealed there were no gross morphological differences and no differences in heart rate, end diastolic volume, end systolic volume, stroke volume, ejection fraction, cardiac output or wall thickening between m-V59M and control hearts, either under resting conditions or under dobutamine stress.

CONCLUSIONS/INTERPRETATION

The common iDEND mutation Kir6.2-V59M decreases ATP block of cardiac K(ATP) channels but was without obvious effect on heart function, suggesting that metabolic changes fail to open the mutated channel to an extent that affects function (at least in the absence of ischaemia). This may have implications for the choice of sulfonylurea used to treat neonatal diabetes.

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.

Oligomerization at the membrane: potassium channel structure and function

Cell membranes present a naturally impervious barrier to aqueous solutes, such that the physiochemical environment on either side of the lipid bilayer can substantially differ. Integral membrane proteins are embedded in this heterogeneous lipid environment, wherein the juxtaposition of apolar and polar molecular surfaces defines factors such as transverse orientation, the surface area available for oligomerisation and the symmetry of resultant assemblies. This chapter focuses on potassium channels -representative molecular pores that play a critical role in electrical signalling by enabling selective transport of K(+) ions across cell membranes. Oligomerization is central to K(+) channel action; individual subunits are nonfunctional and conduction, selectivity and gating involve manipulation of the common subunit interface of the tetramer. Regulation of channel activity can be viewed from the perspective that the pore of K(+) channels has coopted other proteins, utilizing a process of hetero-oligomerisation to absorb new functions that both enable the pore to respond to extrinsic signals and provide an electrical signature.

Novel pharmacological targets for the rhythm control management of atrial fibrillation

Atrial fibrillation (AF) is a growing clinical problem associated with increased morbidity and mortality. Development of safe and effective pharmacological treatments for AF is one of the greatest unmet medical needs facing our society. In spite of significant progress in non-pharmacological AF treatments (largely due to the use of catheter ablation techniques), anti-arrhythmic agents (AADs) remain first line therapy for rhythm control management of AF for most AF patients. When considering efficacy, safety and tolerability, currently available AADs for rhythm control of AF are less than optimal. Ion channel inhibition remains the principal strategy for termination of AF and prevention of its recurrence. Practical clinical experience indicates that multi-ion channel blockers are generally more optimal for rhythm control of AF compared to ion channel-selective blockers. Recent studies suggest that atrial-selective sodium channel block can lead to safe and effective suppression of AF and that concurrent inhibition of potassium ion channels may potentiate this effect. An important limitation of the ion channel block approach for AF treatment is that non-electrical factors (largely structural remodeling) may importantly determine the generation of AF, so that "upstream therapy", aimed at preventing or reversing structural remodeling, may be required for effective rhythm control management. This review focuses on novel pharmacological targets for the rhythm control management of AF.

The molecular genetics of sulfonylurea receptors in the pathogenesis and treatment of insulin secretory disorders and type 2 diabetes

Sulfonylurea receptors (SURs) form an integral part of the ATP-sensitive potassium (K(ATP)) channel complex that is present in most excitable cell types. K(ATP) channels couple cellular metabolism to electrical activity and provide a wide range of cellular functions including stimulus secretion coupling in pancreatic β cells. K(ATP) channels are composed of SURs and inward rectifier potassium channel (Kir6.x) subunits encoded by the ABCC8/9 and KCNJ8/11 genes, respectively. Recent advances in the genetics, molecular biology, and pharmacology of SURs have led to an increased understanding of these channels in the etiology and treatment of rare genetic insulin secretory disorders. Furthermore, common genetic variants in these genes are associated with an increased risk for type 2 diabetes. In this review we summarize the molecular biology, pharmacology, and physiology of SURs and K(ATP) channels, highlighting recent advances in their genetics and understanding of rare insulin secretory disorders and susceptibility to type 2 diabetes.

Unique properties of the ATP-sensitive K⁺ channel in the mouse ventricular cardiac conduction system

Background- The specialized cardiac conduction system (CCS) expresses a unique complement of ion channels that confer a specific electrophysiological profile. ATP-sensitive potassium (K(ATP)) channels in these myocytes have not been systemically investigated. Methods and Results- We recorded K(ATP) channels in isolated CCS myocytes using Cntn2-EGFP reporter mice. The CCS K(ATP) channels were less sensitive to inhibitory cytosolic ATP compared with ventricular channels and more strongly activated by MgADP. They also had a smaller slope conductance. The 2 types of channels had similar intraburst open and closed times, but the CCS K(ATP) channel had a prolonged interburst closed time. CCS K(ATP) channels were strongly activated by diazoxide and less by levcromakalim, whereas the ventricular K(ATP) channel had a reverse pharmacological profile. CCS myocytes express elevated levels of Kir6.1 but reduced Kir6.2 and SUR2A mRNA compared with ventricular myocytes (SUR1 expression was negligible). SUR2B mRNA expression was higher in CCS myocytes relative to SUR2A. Canine Purkinje fibers expressed higher levels of Kir6.1 and SUR2B protein relative to the ventricle. Numeric simulation predicts a high sensitivity of the Purkinje action potential to changes in ATP:ADP ratio. Cardiac conduction time was prolonged by low-flow ischemia in isolated, perfused mouse hearts, which was prevented by glibenclamide. Conclusions- These data imply a differential electrophysiological response (and possible contribution to arrhythmias) of the ventricular CCS to K(ATP) channel opening during periods of ischemia.

Remodeling of atrial ATP-sensitive K⁺ channels in a model of salt-induced elevated blood pressure

Hypertension is associated with the development of atrial fibrillation; however, the electrophysiological consequences of this condition remain poorly understood. ATP-sensitive K(+) (K(ATP)) channels, which contribute to ventricular arrhythmias, are also expressed in the atria. We hypothesized that salt-induced elevated blood pressure (BP) leads to atrial K(ATP) channel activation and increased arrhythmia inducibility. Elevated BP was induced in mice with a high-salt diet (HS) for 4 wk. High-resolution optical mapping was used to measure atrial arrhythmia inducibility, effective refractory period (ERP), and action potential duration at 90% repolarization (APD(90)). Excised patch clamping was performed to quantify K(ATP) channel properties and density. K(ATP) channel protein expression was also evaluated. Atrial arrhythmia inducibility was 22% higher in HS hearts compared with control hearts. ERP and APD(90) were significantly shorter in the right atrial appendage and left atrial appendage of HS hearts compared with control hearts. Perfusion with 1 μM glibenclamide or 300 μM tolbutamide significantly decreased arrhythmia inducibility and prolonged APD(90) in HS hearts compared with untreated HS hearts. K(ATP) channel density was 156% higher in myocytes isolated from HS animals compared with control animals. Sulfonylurea receptor 1 protein expression was increased in the left atrial appendage and right atrial appendage of HS animals (415% and 372% of NS animals, respectively). In conclusion, K(ATP) channel activation provides a mechanistic link between salt-induced elevated BP and increased atrial arrhythmia inducibility. The findings of this study have important implications for the treatment and prevention of atrial arrhythmias in the setting of hypertensive heart disease and may lead to new therapeutic approaches.

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.

Interventricular heterogeneity in rat heart responses to hypoxia: the tuning of glucose metabolism, ion gradients, and function

The matching of energy supply and demand under hypoxic conditions is critical for sustaining myocardial function. Numerous reports indicate that basal energy requirements and ion handling may differ between the ventricles. We hypothesized that ventricular response to hypoxia shows interventricular differences caused by the heterogeneity in glucose metabolism and expression and activity of ion transporters. Thus we assessed glucose utilization rate, ATP, sodium and potassium concentrations, Na, K-ATPase activity, and tissue reduced:oxidized glutathione (GSH/GSSG) content in the right and left ventricles before and after the exposure of either the whole animals or isolated blood-perfused hearts to hypoxia. The hypoxia-induced boost in glucose utilization was more pronounced in the left ventricle compared with the right one. ATP levels in the right ventricle of hypoxic heart were lower than those in the left ventricle. Left ventricular sodium content was higher, and hydrolytic Na, K-ATPase activity was reduced compared with the right ventricle. Administration of the Na, K-ATPase blocker ouabain caused rapid increase in the right ventricular Na(+) and elimination of the interventricular Na(+) gradients. Exposure of the hearts to hypoxia made the interventricular heterogeneity in the Na(+) distribution even more pronounced. Furthermore, systemic hypoxia caused oxidative stress that was more pronounced in the right ventricle as revealed by GSH/GSSG ratios. On the basis of these findings, we suggest that the right ventricle is more prone to hypoxic damage, as it is less efficient in recruiting glucose as an alternative fuel and is particularly dependent on the efficient Na, K-ATPase function.

Ursolic acid increases the secretion of atrial natriuretic peptide in isolated perfused beating rabbit atria

Ursolic acid is reported to have beneficial effects on the regulation of cardiovascular homeostasis. However, the effects of ursolic acid on cardiac hormone secretion are yet to be defined. The present study was designed to test the effects of ursolic acid on the secretory and contractile functions of the atria. Experiments were conducted in isolated perfused beating rabbit atria. We measured the changes in atrial dynamics, pulse pressure, stroke volume, cAMP efflux, as well as the secretion of atrial natriuretic peptide (ANP). Ursolic acid increased ANP secretion and mechanical dynamics in a concentration-dependent manner. The inhibition of L-type Ca(2+) channels with nifedipine attenuated the ursolic acid-induced increase in ANP secretion but not mechanical dynamics. The inhibition of K(+)(ATP) channels with glibenclamide attenuated the ursolic acid-induced increase in ANP secretion-but not atrial dynamics-in a concentration-dependent manner. The selective Na(+)-K(+)-ATPase inhibitor ouabain blocked the ursolic acid-induced increase in atrial dynamics but not ANP secretion. These findings show that ursolic acid increases ANP secretion via its activation of K(+)(ATP) channels and subsequent inhibition of Ca(2+) entry through L-type Ca(2+) channels in rabbit atria. These data also suggest that ursolic acid increases atrial dynamics via its inhibition of Na(+)-K(+)-ATPase activity.

HMR 1098 is not an SUR isotype specific inhibitor of heterologous or sarcolemmal K ATP channels

Murine ventricular and atrial ATP-sensitive potassium (K(ATP)) channels contain different sulfonylurea receptors (ventricular K(ATP) channels are Kir6.2/SUR2A complexes, while atrial K(ATP) channels are Kir6.2/SUR1 complexes). HMR 1098, the sodium salt of HMR 1883 {1-[[5-[2-(5-chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea}, has been considered as a selective sarcolemmal (i.e. SUR2A-dependent) K(ATP) channel inhibitor. However, it is not clear whether HMR 1098 would preferentially inhibit ventricular K(ATP) channels over atrial K(ATP) channels. To test this, we used whole-cell patch clamp techniques on mouse atrial and ventricular myocytes as well as (86)Rb(+) efflux assays and excised inside-out patch clamp techniques on Kir6.2/SUR1 and Kir6.2/SUR2A channels heterologously expressed in COSm6 cells. In mouse atrial myocytes, both spontaneously activated and diazoxide-activated K(ATP) currents were effectively inhibited by 10 μM HMR 1098. By contrast, in ventricular myocytes, pinacidil-activated K(ATP) currents were inhibited by HMR 1098 at a high concentration (100 μM) but not at a low concentration (10 μM). Consistent with this finding, HMR 1098 inhibits (86)Rb(+) effluxes through Kir6.2/SUR1 more effectively than Kir6.2/SUR2A channels in COSm6 cells. In excised inside-out patches, HMR 1098 inhibited Kir6.2/SUR1 channels more effectively, particularly in the presence of MgADP and MgATP (mimicking physiological stimulation). Finally, dose-dependent enhancement of insulin secretion from pancreatic islets and decrease of blood glucose level confirm that HMR 1098 is an inhibitor of Kir6.2/SUR1-composed K(ATP) channels.

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.

Voltage-dependent gating in a "voltage sensor-less" ion channel

The voltage sensitivity of voltage-gated cation channels is primarily attributed to conformational changes of a four transmembrane segment voltage-sensing domain, conserved across many levels of biological complexity. We have identified a remarkable point mutation that confers significant voltage dependence to Kir6.2, a ligand-gated channel that lacks any canonical voltage-sensing domain. Similar to voltage-dependent Kv channels, the Kir6.2[L157E] mutant exhibits time-dependent activation upon membrane depolarization, resulting in an outwardly rectifying current-voltage relationship. This voltage dependence is convergent with the intrinsic ligand-dependent gating mechanisms of Kir6.2, since increasing the membrane PIP2 content saturates Po and eliminates voltage dependence, whereas voltage activation is more dramatic when channel Po is reduced by application of ATP or poly-lysine. These experiments thus demonstrate an inherent voltage dependence of gating in a "ligand-gated" K+ channel, and thereby provide a new view of voltage-dependent gating mechanisms in ion channels. Most interestingly, the voltage- and ligand-dependent gating of Kir6.2[L157E] is highly sensitive to intracellular [K+], indicating an interaction between ion permeation and gating. While these two key features of channel function are classically dealt with separately, the results provide a framework for understanding their interaction, which is likely to be a general, if latent, feature of the superfamily of cation channels.

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Quaternary structure of KATP channel SUR2A nucleotide binding domains resolved by synchrotron radiation X-ray scattering

Heterodimeric nucleotide binding domains NBD1/NBD2 distinguish the ATP-binding cassette protein SUR2A, a recognized regulatory subunit of cardiac ATP-sensitive K(+) (K(ATP)) channels. The tandem function of these core domains ensures metabolism-dependent gating of the Kir6.2 channel pore, yet their structural arrangement has not been resolved. Here, purified monodisperse and interference-free recombinant particles were subjected to synchrotron radiation small-angle X-ray scattering (SAXS) in solution. Intensity function analysis of SAXS profiles resolved NBD1 and NBD2 as octamers. Implemented by ab initio simulated annealing, shape determination prioritized an oblong envelope wrapping NBD1 and NBD2 with respective dimensions of 168x80x37A(3) and 175x81x37A(3) based on symmetry constraints, validated by atomic force microscopy. Docking crystal structure homology models against SAXS data reconstructed the NBD ensemble surrounding an inner cleft suitable for Kir6.2 insertion. Human heart disease-associated mutations introduced in silico verified the criticality of the mapped protein-protein interface. The resolved quaternary structure delineates thereby a macromolecular arrangement of K(ATP) channel SUR2A regulatory domains.

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.

Molecular biology of K(ATP) channels and implications for health and disease

The ATP-sensitive potassium (K(ATP)) channel is expressed in most excitable tissues and plays a critical role in numerous physiological processes by coupling intracellular energetics to electrical activity. The channel is comprised of four Kir6.x subunits associated with four regulatory sulfonylurea receptors (SUR). Intracellular ATP acts on Kir6.x to inhibit channel activity, while MgADP stimulates channel activity through SUR. Changes in the cytosolic [ATP] to [ADP] ratio thus determine channel activity. Multiple mutations in Kir6.x and SUR genes have implicated K(ATP) channels in various diseases ranging from diabetes and hyperinsulinism to cardiac arrhythmias and cardiovascular disease. Continuing studies of channel physiology and pathology will bring new insights to the molecular basis of K(ATP) channel function, leading to a better understanding of the role that K(ATP) channels play in both health and disease.

5-Hydroxydecanoate and coenzyme A are inhibitors of native sarcolemmal KATP channels in inside-out patches

BACKGROUND

5-Hydroxydecanoate (5-HD) inhibits preconditioning, and it is assumed to be a selective inhibitor of mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channels. However, 5-HD is a substrate for mitochondrial outer membrane acyl-CoA synthetase, which catalyzes the reaction: 5HD + CoA + ATP --> 5-HD-CoA (5-hydroxydecanoyl-CoA) + AMP + pyrophosphate. We aimed to determine whether the reactants or principal product of this reaction modulate sarcolemmal K(ATP) (sarcK(ATP)) channel activity.

METHODS

Single sarcK(ATP) channel currents were measured in inside-out patches excised from rat ventricular myocytes. In addition, sarcK(ATP) channel activity was recorded in whole-cell configuration or in giant inside-out patches excised from oocytes expressing Kir6.2/SUR2A.

RESULTS

5-HD inhibited (IC(50) approximately 30 microM) K(ATP) channel activity, albeit only in the presence of (non-inhibitory) concentrations of ATP. Similarly, when the inhibitory effect of 0.2 mM ATP was reversed by 1 microM oleoyl-CoA, subsequent application of 5-HD blocked channel activity, but no effect was seen in the absence of ATP. Furthermore, we found that 1 microM coenzyme A (CoA) inhibited sarcK(ATP) channels. Using giant inside-out patches, which are weakly sensitive to "contaminating" CoA, we found that Kir6.2/SUR2A channels were insensitive to 5-HD-CoA. In intact myocytes, 5-HD failed to reverse sarcK(ATP) channel activation by either metabolic inhibition or rilmakalim.

GENERAL SIGNIFICANCE

SarcK(ATP) channels are inhibited by 5-HD (provided that ATP is present) and CoA but insensitive to 5-HD-CoA. 5-HD is equally potent at "directly" inhibiting sarcK(ATP) and mitoK(ATP) channels. However, in intact cells, 5-HD fails to inhibit sarcK(ATP) channels, suggesting that mitochondria are the preconditioning-relevant targets of 5-HD.

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.

Specificities of atrial electrophysiology: Clues to a better understanding of cardiac function and the mechanisms of arrhythmias

The electrical properties of the atria and ventricles differ in several aspects reflecting the distinct role of the atria in cardiac physiology. The study of atrial electrophysiology had greatly contributed to the understanding of the mechanisms of atrial fibrillation (AF). Only the atrial L-type calcium current is regulated by serotonine or, under basal condition, by phosphodiesterases. These distinct regulations can contribute to I(Ca) down-regulation observed during AF, which is an important determinant of action potential refractory period shortening. The voltage-gated potassium current, I(Kur), has a prominent role in the repolarization of the atrial but not ventricular AP. In many species, this current is based on the functional expression of K(V)1.5 channels, which might represent a specific therapeutic target for AF. Mechanisms regulating the trafficking of K(V)1.5 channels to the plasma membrane are being actively investigated. The resting potential of atrial myocytes is maintained by various inward rectifier currents which differ with ventricle currents by a reduced density of I(K1), the presence of a constitutively active I(KACh) and distinct regulation of I(KATP). Stretch-sensitive or mechanosensitive ion channels are particularly active in atrial myocytes and are involved in the secretion of the natriuretic peptide. Integration of knowledge on electrical properties of atrial myocytes in comprehensive schemas is now necessary for a better understanding of the physiology of atria and the mechanisms of AF.

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.