Sarcolemmal ATP-sensitive Potassium Channels Research Articles

Caveolin-3 negatively regulates endocytic recycling of cardiac KATP channels.

Sarcolemmal ATP-sensitive potassium (KATP) channels play a vital role in cardioprotection. Cardiac KATP channels are enriched in caveolae and physically interact with the caveolae structural protein caveolin-3 (Cav3). Disrupting caveolae impairs the regulation of KATP channels through several signaling pathways. However, the direct functional effect of Cav3 on KATP channels is still poorly understood. Here, we used the cardiac KATP channel subtype, Kir6.2/SUR2A, and showed that Cav3 greatly reduced KATP channel surface density and current amplitude in a caveolae-independent manner. A screen of Cav3 functional domains revealed that a 25 amino acid region in the membrane attachment domain of Cav3 is the minimal effective segment (MAD1). The peptide corresponding to the MAD1 segment decreased KATP channel current in a concentration-dependent manner with an IC50 of ∼5 μM. The MAD1 segment prevented KATP channel recycling, thus decreasing KATP channel surface density and abolishing the cardioprotective effect of ischemic preconditioning. Our research identified the Cav3 MAD1 segment as a novel negative regulator of KATP channel recycling, providing pharmacological potential in the treatment of diseases with KATP channel trafficking defects.NEW & NOTEWORTHY Cardiac KATP channels physically interact with caveolin-3 in caveolae. In this study, we investigated the functional effect of caveolin-3 on KATP channel activity and identified a novel segment (MAD1) in the C-terminus domain of Caveolin-3 that negatively regulates KATP channel surface density and current amplitude by impairing KATP channel recycling. The peptide corresponding to the MAD1 segment abolished the cardioprotective effect of ischemic preconditioning.

Role of sarcolemmal KATP channel in sevoflurane-induced maintenance of electrophysiological stability of ventricular myocardium in diabetic rats

Objective To evaluate the role of sarcolemmal ATP-sensitive potassium (sarcKATP) channel in sevoflurane-induced maintenance of electrophysiological stability of ventricular myocardium in diabetic rats. Methods Clean-grade healthy male Sprague-Dawley rats, aged 3 months, weighing 280-320 g, in which diabetes mellitus (DM) was induced by intraperitoneal streptozotocin 60 mg/kg and confirmed by blood glucose ≥16.7 mmol/L, were used in this study.Their hearts were excised after anesthesia and retrogradely perfused in a Langendorff apparatus at 4 weeks after establishing the DM model.Twenty-four Langendorff-perfused hearts were divided into 3 groups (n=8 each) using a random number table method: DM group (group D), DM plus sevoflurane group (group DS) and DM plus sevoflurane plus HMR-1098 group (group DSH). Another 8 Langendorff-perfused hearts of normal rats were selected as control group (group C). Hearts were perfused with 37 ℃ K-H solution via the aorta in each group, 15 min of equilibration later hearts were continuously perfused for 30 min with K-H solution in C and D groups, with K-H solution saturated with 2.5% sevoflurane in group DS, or with K-H solution saturated with 10 μmol/L HMR-1098 and 2.5% sevoflurane in group DSH.Monophasic action potential (MAP) duration at 50% and 90% repolarization (MAPD50 and MAPD90) in the endocardium and epicardium of the left ventricular anterior wall were recorded at 15 min of equilibration (T0) and 15 and 30 min of reperfusion (T1, 2), transmural dispersion of repolarization (TDR) was calculated.S1S2 program-controlled stimulation was performed at the end of perfusion to record the effective refractory period (ERP), ventricular arrhythmia (VA) induced and the longest pacing cycle length (PCL) of ventricular fibrillation threshold (VFT)induced.ERP/MAPD90 ratio was calculated. Results Compared with group C, TDR was significantly increased at T0, ERP/MADP90 ratio was decreased, the incidence of VA induced was increased, and the longest PCL of VFT induced was prolonged in group D (P<0.05). Compared with group D, TDR was significantly decreased at T2 in group DS (P<0.05), and ERP/MADP90 ratio was significantly increased, the incidence of VA induced was decreased, and the longest PCL of VFT induced was shortened in DS and DSH groups (P<0.05). TDR was significantly smaller at T2 in group DSH than in group DS (P<0.05). Conclusion sarcKATP channel is involved in sevoflurane-induced maintenance of electrophysiological stability of ventricular myocardium in diabetic rats. Key words: KATP channels; Anesthetics, inhalation; Diabetes mellitus; Cardiac electrophysiology

Myocardial ischemic preconditioning and the sarcolemmal ATP-sensitive potassium channels

Background In recent years, ischemic preconditioning(IPC) has been widely stuided. The ATP-sensitive potassium channel(KATP), which acts as a key end-effector in the pathways of intracellular signaling transduction, has gained wide attention because of its important role in the IPC. Objective To further clarify the pathophysiology of the IPC and provide theoretical basis for clinical work. Content This article reviews the research progress of the sarcolemmal ATP-sensitive potassium channel (sarcKATP) as well as its role in myocardial IPC. This article reviews the physiological characteristics of the sarcolemmal ATP-sensitive potassium channels (sarcKATP) channel, its role in myocardial IPC as well as its regulatory mechanism. Trend Regulating sarcKATP-targeted study is promising candidate for IPC. Key words: ATP-sensitive potassium channels; Ischemic preconditioning; Cardioprotection

KATP channel inhibition blunts electromechanical decline during hypoxia in left ventricular working rabbit hearts

Heart function is critically dependent upon the balance of energy production and utilization. Sarcolemmal ATP-sensitive potassium channels (KATP channels) in cardiac myocytes adjust contractile function to compensate for the level of available energy. Understanding the activation of KATP channels in working myocardium during high-stress situations is crucial to the treatment of cardiovascular disease, especially ischaemic heart disease. Using a new optical mapping approach, we measured action potentials from the surface of excised contracting rabbit hearts to assess when sarcolemmal KATP channels were activated during physiologically relevant workloads and during gradual reductions in myocardial oxygenation. We demonstrate that left ventricular pressure is closely linked to KATP channel activation and that KATP channel inhibition with a low concentration of tolbutamide prevents electromechanical decline when oxygen availability is reduced. As a result, KATP channel inhibition probably exacerbates a mismatch between energy demand and energy production when myocardial oxygenation is low. Sarcolemmal ATP-sensitive potassium channel (KATP channel) activation in isolated cells is generally understood, although the relationship between myocardial oxygenation and KATP activation in excised working rabbit hearts remains unknown. We optically mapped action potentials (APs) in excised rabbit hearts to test the hypothesis that hypoxic changes would be more severe in left ventricular (LV) working hearts (LWHs) than Langendorff (LANG) perfused hearts. We further hypothesized that KATP inhibition would prevent those changes. Optical APs were mapped when measuring LV developed pressure (LVDP), coronary flow rate and oxygen consumption in LANG and LWHs. Hearts were paced to increase workload and perfusate was deoxygenated to study the effects of myocardial hypoxia. A subset of hearts was perfused with 1μm tolbutamide (TOLB) to identify the level of AP duration (APD) shortening attributed to KATP channel activation. During sinus rhythm, APD was shorter in LWHs compared to LANG hearts. APD in both LWHs and LANG hearts dropped steadily during deoxygenation. With TOLB, APDs in LWHs were longer at all workloads and APD reductions during deoxygenation were blunted in both LWHs and LANG hearts. At 50% perfusate oxygenation, APD and LVDP were significantly higher in LWHs perfused with TOLB (199±16ms; 92±5.3mmHg) than in LWHs without TOLB (109±14ms, P=0.005; 65±6.5mmHg, P=0.01). Our results indicate that KATP channels are activated to a greater extent in perfused hearts when the LV performs pressure-volume work. The results of the present study demonstrate the critical role of KATP channels in modulating myocardial function over a wide range of physiological conditions.

SY4-1 - Beneficial Impact of Nicorandil on Mortality in Patients With Ischemic Heart Failure

Background: Effective treatments in heart failure (HF) patients with ischemic etiology have not been fully established. Nicorandil, combination of nitrate component and sarcolemmal ATP-sensitive potassium channel opener, is a potent vasodilator of coronary and peripheral vessels and has been used as an antianginal agent. Therefore, we examined impacts of nicorandil on mortality in ischemic HF patients. Methods and Results: We analyzed 334 HF patients with ischemic etiology. These patients were divided into 2 groups: nicorandil group (n = 116) and non-nicorandil group (n = 218). Baseline characteristics including age, gender, NYHA class, left ventricular ejection fraction, B-type natriuretic peptide, estimated glomerular filtration rate, presence of hypertension, diabetes, dyslipidemia, atrial fibrillation, chronic kidney disease, anemia and smoking did not differ between the two groups. In the Kaplan-Meier analysis (mean follow-up period 963 days), all-cause mortality was significantly lower in the nicorandil group than in the non-nicorandil group (24.1% vs. 35.8%, P = .018). In the Cox proportional hazard analysis after adjusting for potential confounding factors, use of nicorandil was an independent predictor of mortality in HF patients with ischemic etiology (hazard ratio 0.618, 95% confidence interval 0.400–0.953, P = .030). Conclusion: Nicorandil is potentially effective for reducing mortality in patients with ischemic heart failure.

Sarcolemmal ATP-sensitive potassium channel protects cardiac myocytes against lipopolysaccharide-induced apoptosis.

The sarcolemmal ATP-sensitive K+ (sarcKATP) channel plays a cardioprotective role during stress. However, the role of the sarcKATP channel in the apoptosis of cardiomyocytes and association with mitochondrial calcium remains unclear. For this purpose, we developed a model of LPS-induced sepsis in neonatal rat cardiomyocytes (NRCs). The TUNEL assay was performed in order to detect the apoptosis of cardiac myocytes and the MTT assay was performed to determine cellular viability. Exposure to LPS significantly decreased the viability of the NRCs as well as the expression of Bcl-2, whereas it enhanced the activity and expression of the apoptosis-related proteins caspase-3 and Bax, respectively. The sarcKATP channel blocker, HMR-1098, increased the apoptosis of NRCs, whereas the specific sarcKATP channel opener, P-1075, reduced the apoptosis of NRCs. The mitochondrial calcium uniporter inhibitor ruthenium red (RR) partially inhibited the pro-apoptotic effect of HMR-1098. In order to confirm the role of the sarcKATP channel, we constructed a recombinant adenovirus vector carrying the sarcKATP channel mutant subunit Kir6.2AAA to inhibit the channel activity. Kir6.2AAA adenovirus infection in NRCs significantly aggravated the apoptosis of myocytes induced by LPS. Elucidating the regulatory mechanisms of the sarcKATP channel in apoptosis may facilitate the development of novel therapeutic targets and strategies for the management of sepsis and cardiac dysfunction.

Disruption of ATP-sensitive potassium channel function in skeletal muscles promotes production and secretion of musclin

Sarcolemmal ATP-sensitive potassium (KATP) channels control skeletal muscle energy use through their ability to adjust membrane excitability and related cell functions in accordance with cellular metabolic status. Mice with disrupted skeletal muscle KATP channels exhibit reduced adipocyte size and increased fatty acid release into the circulation. As yet, the molecular mechanisms underlying this link between skeletal muscle KATP channel function and adipose mobilization have not been established. Here, we demonstrate that skeletal muscle-specific disruption of KATP channel function in transgenic (TG) mice promotes production and secretion of musclin. Musclin is a myokine with high homology to atrial natriuretic peptide (ANP) that enhances ANP signaling by competing for elimination. Augmented musclin production in TG mice is driven by a molecular cascade resulting in enhanced acetylation and nuclear exclusion of the transcription factor forkhead box O1 (FOXO1) – an inhibitor of transcription of the musclin encoding gene. Musclin production/secretion in TG is paired with increased mobilization of fatty acids and a clear trend toward increased circulating ANP, an activator of lipolysis. These data establish KATP channel-dependent musclin production as a potential mechanistic link coupling “local” skeletal muscle energy consumption with mobilization of bodily resources from fat. Understanding such mechanisms is an important step toward designing interventions to manage metabolic disorders including those related to excess body fat and associated co-morbidities.

Disruption of KATP Channel Expression in Skeletal Muscle by Targeted Oligonucleotide Delivery Promotes Activity-linked Thermogenesis

Despite the medical, social, and economic impact of obesity, only a few therapeutic options, focused largely on reducing caloric intake, are currently available and these have limited success rates. A major impediment is that any challenge by caloric restriction is counterbalanced by activation of systems that conserve energy to prevent body weight loss. Therefore, targeting energy-conserving mechanisms to promote energy expenditure is an attractive strategy for obesity treatment. Here, in order to suppress muscle energy efficiency, we target sarcolemmal ATP-sensitive potassium (KATP) channels which have previously been shown to be important in maintaining muscle energy economy. Specifically, we employ intramuscular injections of cell-penetrating vivo-morpholinos to prevent translation of the channel pore-forming subunit. This intervention results in significant reduction of KATP channel expression and function in treated areas, without affecting the channel expression in nontargeted tissues. Furthermore, suppression of KATP channel function in a group of hind limb muscles causes a substantial increase in activity-related energy consumption, with little effect on exercise tolerance. These findings establish a proof-of-principle that selective skeletal muscle targeting of sarcolemmal KATP channel function is possible and that this intervention can alter overall bodily energetics without a disabling impact on muscle mechanical function.

Early opening of sarcolemmal ATP-sensitive potassium channels is not a key step in PKC-mediated cardioprotection

ATP-sensitive potassium (KATP) channels are abundantly expressed in the myocardium. Although a definitive role for the channel remains elusive they have been implicated in the phenomenon of cardioprotection, but the precise mechanism is unclear. We set out to test the hypothesis that the channel protects by opening early during ischemia to shorten action potential duration and reduce electrical excitability thus sparing intracellular ATP. This could reduce reperfusion injury by improving calcium homeostasis. Using a combination of contractile function analysis, calcium fluorescence imaging and patch clamp electrophysiology in cardiomyocytes isolated from adult male Wistar rats, we demonstrated that the opening of sarcolemmal KATP channels was markedly delayed after cardioprotective treatments: ischemic preconditioning, adenosine and PMA. This was due to the preservation of intracellular ATP for longer during simulated ischemia therefore maintaining sarcolemmal KATP channels in the closed state for longer. As the simulated ischemia progressed, KATP channels opened to cause contractile, calcium transient and action potential failure; however there was no indication of any channel activity early during simulated ischemia to impart an energy sparing hyperpolarization or action potential shortening. We present compelling evidence to demonstrate that an early opening of sarcolemmal KATP channels during simulated ischemia is not part of the protective mechanism imparted by ischemic preconditioning or other PKC-dependent cardioprotective stimuli. On the contrary, channel opening was actually delayed. We conclude that sarcolemmal KATP channel opening is a consequence of ATP depletion, not a primary mechanism of ATP preservation in these cells.

Statin (Mevalotin) preconditioning decreases infarct size in senile rat myocardial infarction model

Although statins have been proved to have cardioprotective effects, it remains to be investigated whether “statin-induced preconditioning (PC)” can reduce the infarct size for senile animals. Senile Wistar rats (weighing 350–450 g) were randomly assigned to one of 14 experiment groups ( n ≥ 5 for each group). All hearts underwent 30 minutes of coronary artery occlusion and 2 hours of reperfusion (control procedure). Ischemic PC was elicited by three cycles of 5-minute occlusion and 5-minute reperfusion. Statin PC was performed by administering Mevalotin (1 mg/kg, the optimal dose being determined by a dose–effect study) 30 minutes prior to the control procedure. In addition, pretreatment with glibenclamide, 5-hydroxydecanoic acid, chelerythrine, genistein, and N G -nitro-4-arginine methyl ester, or their combination, was also performed to test possible signaling mechanisms. Statin PC decreased the infarct size significantly compared with the control group (22 ± 4% vs. 53 ± 4%, p < 0.01). Pretreatment with N G -nitro-4-arginine methyl ester completely blocked the effects of Mevalotin (infarct size: 48 ± 4% vs. 22 ± 4%, p < 0.01). The cardioprotective effects of statin PC could be blunted by antagonism of mitochondrial adenosine triphosphate (ATP)-sensitive potassium and sarcolemmal ATP-sensitive potassium channels (glibenclamide: 50 ± 4% vs. 22 ± 4%, p < 0.01; 5-hydroxydecanoic acid: 50 ± 4% vs. 22 ± 4%, p < 0.01). In addition, protein kinase C antagonist (chelerythrine) and tyrosine kinase antagonist (genisteine) also have similar effects (chelerythrine: 48 ± 4% vs. 22 ± 4%, p < 0.01; genistein: 45 ± 4% vs. 22 ± 4%, p < 0.01). It is established in this study the myocardial PC caused by statins, that is involved cellular mechanisms including mainly the pathway of endothelial nitrous oxide synthase (eNOS)–nitrous oxide donation–ATP-sensitive potassium channels. Protein kinase C and tyrosine kinase have regulatory effects.

“Roid-Rage” at the Cellular Level: Abolition of Endogenous Cardioprotection by Anabolic Steroids Reveals New Links Between the RAAS and Cardiac KATP Channels

Substantial evidence suggests that androgenic anabolic steroids (AAS) have deleterious effects on many body tissues, but their influence on endogenous mechanisms of cardioprotection is poorly understood. In the current issue of CDT, Marques-Neto et al. present a study examining the mechanisms by which AAS attenuate the protective effects of exercise training on myocardial ischemia/ reperfusion injury [1]. They chose to focus on the roles of angiotensin II and aldosterone, given the wellestablished stimulatory effect of AAS on the renin angiotensin-aldosterone system (RAAS) and the role of this system in the pathogenesis of myocardial injury. Exercise-trained rats were co-administered nandralone (AAS) along with losartan or spironolactone to block AT1 or aldosterone receptors, followed by evaluation of post-ischemic function and infarct size in isolated perfused hearts. Their results show that nandralone abolishes the benefits of exercise training on post-ischemic function and infarct size reduction, and that both RAAS blockers restore these benefits, confirming the hypothesized role of the RAAS in the AAS-induced suppression of exercise-induced cardioprotection. In this study, Marques-Neto et al. link the preservation of cardioprotection to maintenance of myocardial sarcolemmal ATP-sensitive potassium (sarcKATP) channel expression. sarcKATP channels are highly expressed in the cardiac sarcolemma, and open in response to local changes in nucleotide content (reviewed in [2–4]). A growing body of literature supports a positive correlation between sarcKATP expression and cardioprotection [5, 6]. Preconditioning conferred by exercise is associated with enhanced sarcKATP channel subunits [7], and genetic loss of sarcKATP channels confers exercise intolerance and poor adaptation of the heart to stress [8]. sarcKATP channel expression is also believed to mediate sex-dependent protection against cardiac injury. While women are known to exhibit reduced risk of cardiovascular events and associated mortality compared to males, the molecular mechanisms of this sex-dimorphism remains incompletely understood [9–11]. Animal studies have demonstrated an intrinsic resistance of the female heart to ischemia/ reperfusion injury, which has been linked to a higher expression of sarcKATP channel subunits in the female versus male heart [6, 12–14]. Estrogen upregulates sarcKATP channel subunit expression in cardiomyocytes [15], suggesting a potential mechanism for its protective effects in the heart. Clinical studies indicate that pharmacological interventions that block sarcKATP channels to manage disease symptoms are associated with increased cardiovascular mortality [16, 17], further suggesting an important protective role of these channels on cardiovascular health. Despite the relationship between cardiac tissue health and sarcKATP expression, the cellular mechanisms responsible for altering the expression of these channels are not clear. The study by Marques-Neto et al. implicates the RAAS in androgenic suppression of sarcKATP and exercise-induced A. J. Chicco (*) Departments of Health and Exercise Science and Biomedical Sciences, Colorado State University, Fort Collins, CO, USA e-mail: adam.chicco@colostate.edu

Modulation of Sarcolemmal ATP-Sensitive Potassium Channels by Atrial Natriuretic Peptide in Ventricular Cardiomyocytes

ATP-sensitive potassium (KATP) channels couple cell metabolic status to membrane excitability and are crucial for stress adaptation in the heart. Natriuretic peptides (NPs) produced by the heart, the vasculature and the kidney exert physiologically important actions such as natriuresis and inhibition of sympathetic tone and cardiomyocyte hypertrophy. Our recent study reveals that the gaseous messenger nitric oxide evokes cardiac KATP channel stimulation via activation of soluble guanylyl cyclase and subsequently a cGMP-dependent signaling pathway. Binding of NPs like atrial natriuretic peptide (ANP) to the type 1 NP receptor (NPR-A) also increases intracellular cGMP level, through activating guanylyl cyclase intrinsic to the receptor; however, how ANP modulates the function of cardiac KATP channels remains to be established. By performing cell-attached patch recordings in ventricular cardiomyocytes acutely isolated from adult rabbits, we found that the single-channel activity of sarcolemmal KATP (sarcKATP) channels induced by sodium azide through metabolic inhibition was potentiated by addition of ANP (100 nM) (n=16), whereas the potentiation by ANP was abolished when the NPR-A antagonist anantin (1 μM) was coapplied (n=5); these findings suggest that the stimulatory action of ANP is mediated by NPR-A activation, which may depend on generation of diffusible cGMP. Indeed, sarcKATP channel stimulation by ANP in intact ventricular myocytes was significantly abated by inhibition of cGMP-dependent protein kinase (PKG) with KT5823 (1 μM) (n=5). Moreover, the ANP effect was prevented by selective suppression of extracellular signal-regulated protein kinase (ERK)1/2 (n=6) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) (n=6). Collectively, here we demonstrate that the polypeptide hormone ANP positively modulates ventricular KATP channel function via activation of NPR-A, PKG, ERK1/2 and CaMKII. By opening myocardial KATP channels, this signaling pathway may regulate cardiac excitability and contribute to cytoprotection against ischemia-reperfusion injury.

Epoxyeicosatrienoic acids protect cardiac cells during starvation by modulating an autophagic response

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 epoxygenase metabolites of arachidonic acid involved in regulating pathways promoting cellular protection. We have previously shown that EETs trigger a protective response limiting mitochondrial dysfunction and reducing cellular death. Considering it is unknown how EETs regulate cell death processes, the major focus of the current study was to investigate their role in the autophagic response of HL-1 cells and neonatal cardiomyocytes (NCMs) during starvation. We employed a dual-acting synthetic analog UA-8 (13-(3-propylureido)tridec-8-enoic acid), possessing both EET-mimetic and soluble epoxide hydrolase (sEH) inhibitory properties, or 14,15-EET as model EET molecules. We demonstrated that EETs significantly improved viability and recovery of starved cardiac cells, whereas they lowered cellular stress responses such as caspase-3 and proteasome activities. Furthermore, treatment with EETs resulted in preservation of mitochondrial functional activity in starved cells. The protective effects of EETs were abolished by autophagy-related gene 7 (Atg7) short hairpin RNA (shRNA) or pharmacological inhibition of autophagy. Mechanistic evidence demonstrated that sarcolemmal ATP-sensitive potassium channels (pmKATP) and enhanced activation of AMP-activated protein kinase (AMPK) played a crucial role in the EET-mediated effect. Our data suggest that the protective effects of EETs involve regulating the autophagic response, which results in a healthier pool of mitochondria in the starved cardiac cells, thereby representing a novel mechanism of promoting survival of cardiac cells. Thus, we provide new evidence highlighting a central role of the autophagic response in linking EETs with promoting cell survival during deep metabolic stress such as starvation.

Ischemia reperfusion injury, KATPchannels, and exercise-induced cardioprotection against apoptosis

Exercise is a potent stimulus against cardiac ischemia reperfusion (IR) injury, although the protective mechanisms are not completely understood. The study purpose was to examine whether the mitochondrial or sarcolemmal ATP-sensitive potassium channel (mito K(ATP) or sarc K(ATP), respectively) mediates exercise-induced cardioprotection against post-IR cell death and apoptosis. Eighty-six, 4-mo-old male Sprague Dawley rats were randomly assigned to treadmill exercise (Ex; 30 m/min, 3 days, 60 min, ∼70 maximal oxygen uptake) and sedentary (Sed) treatments. Rats were exposed to regional cardiac ischemia (50 min) and reperfusion (120 min) or Sham (170 min; no ligation) surgeries. Exercise subgroups received placebo (saline), 5-hydroxydecanoate (5HD; 10 mg/kg ip), or HMR1098 (10 mg/kg ip) to inhibit mito K(ATP) or sarc K(ATP) channel. Comprehensive outcome assessments included post-IR ECG arrhythmias, cardiac tissue necrosis, redox perturbations, and autophagy biomarkers. No arrhythmia differences existed between exercised and sedentary hearts following extended-duration IR (P < 0.05). The sarc K(ATP) channel was confirmed essential (P = 0.002) for prevention of antinecrotic tissue death with exercise (percent infarct, Sed = 42%; Ex = 20%; Ex5HD = 16%; ExHMR = 42%), although neither the mito K(ATP) (P = 0.177) nor sarc K(ATP) (P = 0.274) channel provided post-IR protection against apoptosis (terminal deoxynucleotidyl transferase deoxy UTP-mediated nick-end labeling-positive nuclei/mm(2), Sham = 1.8 ± 0.5; Sed = 19.4 ± 6.7; Ex = 7.5 ± 4.6; Ex5HD = 14.0 ± 3.9; ExHMR = 11.1 ± 1.8). Exercise preconditioning also appears to preserve basal autophagy levels, as assessed by Beclin 1 (P ≤ 0.001), microtubule-associated protein-1 light-chain 3B ratios (P = 0.020), and P62 (P ≤ 0.001), in the hours immediately following IR. Further research is needed to better understand these findings and corresponding redox changes in exercised hearts.

Role of PI3Kα and sarcolemmal ATP-sensitive potassium channels in epoxyeicosatrienoic acid mediated cardioprotection

Epoxyeicosatrienoic acids (EETs) are cytochrome P450 epoxygenase metabolites of arachidonic acid that have known cardioprotective properties. While the mechanism(s) remains unknown, evidence suggests that phosphoinositide 3-kinase (PI3K) and sarcolemmal ATP-sensitive potassium channels (pmK(ATP)) are important. However the role of specific PI3K isoforms and corresponding intracellular mechanisms remains unknown. To study this, mice hearts were perfused in Langendorff mode for 40 min of baseline and subjected to 20 or 30 min of global no-flow ischemia followed by 40 min of reperfusion. C57BL6 mice perfused with 11,12-EET (1 μM) had improved postischemic recovery, whereas co-perfusion with PI3Kα inhibitor, PI-103 (0.1 μM), abolished the EET-mediated effect. In contrast, blocking of PI3Kβ or PI3Kγ isoforms failed to inhibit EET-mediated cardioprotection. In addition to the improved post-ischemic recovery, increased levels of p-Akt, decreased calcineurin activity and decreased translocation of proapoptotic protein BAD to mitochondria were noted in EET-treated hearts. Perfusion of 11,12-EET to Kir6.2 deficient mice (pmK(ATP)) failed to improve postischemic recovery, decrease calcineurin activity and translocation of proapoptotic protein BAD, however increased levels of p-Akt were still observed. Patch-clamp experiments demonstrated that 11,12-EET could not activate pmK(ATP) currents in myocytes pre-treated with PI-103. Mechanistic studies in H9c2 cells demonstrate that 11,12-EET limits anoxia-reoxygenation triggered Ca(2+) accumulation and maintains mitochondrial ΔΨm compared to controls. Both PI-103 and glibenclamide (10 μM, pmK(ATP) inhibitor) abolished EET cytoprotection. Together our data suggest that EET-mediated cardioprotection involves activation of PI3Kα, upstream of pmK(ATP), which prevents Ca(2+) overload and maintains mitochondrial function.

“Cardiac K ATP ”

Understanding the etiology of and development of appropriate treatment for myocardial ischemia and associated sudden cardiac death is a major goal of cardiovascular research. In a recent review,1 a compelling case is presented that sarcolemmal ATP-sensitive potassium (KATP) channels are an excellent target for antiarrhythmic therapy. Because opening of cardiac KATP channels shortens the action potential and reduces the refractory period, its activation can establish an arrhythmogenic substrate, supporting reentry. Hence, inhibition of KATP could be an ideal way to stop or even prevent arrhythmias: these channels tend to be closed under normal circumstances and to open only when cell metabolism is inhibited; therefore, any agents that inhibit KATP activity should specifically target channels only during ischemia, leaving nonischemic myocardium unaffected. On the other hand, activation of cardiac KATP channels has consistently been shown to protect the heart from damage during ischemia by limiting Ca entry.2 Finding a way to balance these opposing effects on the heart before, during, and after an ischemic event remains an intractable issue. Until recently, efforts to manipulate cardiac KATP have generally been conducted under a single underlying assumption, namely that cardiac KATP channels are structurally and functionally the same throughout the heart. However, recent studies, including the new report of Bao and colleagues3 in this issue, reveal that “cardiac KATP” is not a single entity and is more accurately a collection of structurally distinct channels. The relative density of each one varies regionally in the heart, may differ between species, and may be under dynamic control. Article see p 926 KATP channels are expressed in many different excitable tissues, where they couple cell energetic status with membrane excitability and function.4 In different tissues, KATP channels exhibit different properties. This …

An Open Sarcolemmal Adenosine Triphosphate-Sensitive Potassium Channel Is Necessary for Detrimental Myocyte Swelling Secondary to Stress

Stress (exposure to hyperkalemic cardioplegia, metabolic inhibition, or osmotic) results in significant myocyte swelling and reduced contractility. In contrast to wild-type mice, these detrimental consequences are not observed in mice lacking the Kir6.2 subunit of the sarcolemmal ATP-sensitive potassium (sK(ATP)) channel after exposure to hyperkalemic cardioplegia. The hypothesis for this study was that an open sK(ATP) channel (Kir6.2 and SUR2A subunits) is necessary for detrimental myocyte swelling to occur in response to stress. To investigate the role of the sK(ATP) channel in stress-induced myocyte swelling, high-dose pharmacological sK(ATP) channel blockade and genetic deletion (knockout of Kir6.2 subunit) were used. Myocytes were exposed sequentially to Tyrode control (20 minutes), test (stress) solution (20 minutes), and Tyrode control (20 minutes). To evaluate pharmacological channel blockade, myocytes were exposed to hyperkalemic cardioplegia (stress) with and without a K(ATP) channel blocker. To evaluate the effects of genetic deletion, wild-type and sK(ATP) knockout [Kir6.2(-/-)] myocytes were exposed to metabolic inhibition (stress). Myocyte volume was recorded using image-grabbing software. Detrimental myocyte swelling was prevented by high-dose sK(ATP) channel blockade (glibenclamide or HMR 1098) but not mitochondrial K(ATP) channel blockade (5-hydroxydecanoate) during exposure to hyperkalemic cardioplegia. Genetic deletion of the sK(ATP) channel prevented significant myocyte swelling in response to metabolic inhibition. K(ATP) channel openers prevent detrimental myocyte swelling and reduce contractility in response to stress through an unknown mechanism. Paradoxically, the present data support a role for sK(ATP) channel activation in myocyte volume derangement in response to stress.

Sarcolemmal K&lt;sub&gt;ATP&lt;/sub&gt; Channel Modulators and Cardiac Arrhythmias

Cardiac atrial and ventricular arrhythmias are major causes of mortality and morbidity. Ischemic heart disease is the most common cause underlying 1) the development of ventricular fibrillation that results in sudden cardiac death and 2) atrial fibrillation that can lead to heart failure and stroke. Current pharmacological agents for the treatment of ventricular and atrial arrhythmias exhibit limited effectiveness and many of these agents can cause serious adverse effects - including the provocation of lethal ventricular arrhythmias. Sarcolemmal ATP-sensitive potassium channels (sarcK(ATP)) couple cellular metabolism to membrane excitability in a wide range of tissues. In the heart, sarcK(ATP) are activated during metabolic stress including myocardial ischemia, and both the opening of sarcK(ATP) and mitochondrial K(ATP) channels protect the ischemic myocardium via distinct mechanisms. Myocardial ischemia leads to a series of events that promote the generation of arrhythmia substrate eventually resulting in the development of life-threatening arrhythmias. In this review, the possible mechanisms of the anti- and proarrhythmic effects of sarcK(ATP) modulation as well as the influence of pharmacological K(ATP) modulators are discussed. It is concluded that in spite of the significant advances made in this field, the possible cardiovascular therapeutic utility of current sarcK(ATP) channel modulators is still hampered by the lack of chamber-specific selectivity. However, recent insights into the chamber-specific differences in the molecular composition of sarcKATP in addition to already existing cardioselective sarcK(ATP) channel modulators with sarcK(ATP) isoform selectivity holds the promise for the future development of pharmacological strategies specific for a variety of atrial and ventricular arrhythmias.

Phenylephrine preconditioning involves modulation of cardiac sarcolemmal K ATP current by PKC delta, AMPK and p38 MAPK

Preconditioning of hearts with the α 1-adrenoceptor agonist phenylephrine decreases infarct size and increases the functional recovery of the heart following ischaemia–reperfusion. However, the cellular mechanisms responsible for this protection are not known. We investigated the role of protein kinase C ε and δ (PKCε and PKCδ), AMP-activated protein kinase (AMPK), p38 MAPK (p38) and sarcolemmal ATP-sensitive potassium ( sarcK ATP) channels in phenylephrine preconditioning using isolated rat ventricular myocytes. Preconditioning of ventricular myocytes with phenylephrine increased the recovery of contractile activity following metabolic inhibition and re-energisation from 30.1 ± 1.9% to 66.5 ± 5.2% (P < 0.01) and increased the peak sarcK ATP current activated during metabolic inhibition from 32.1 ± 1.8 pA/pF to 46.0 ± 5.0 pA/pF (P < 0.05), which was required for protection. Phenylephrine preconditioning resulted in a sustained activation of PKCε and PKCδ, and transient activation of AMPK, which was dependent upon activation of PKCδ but not PKCε. P38 was also activated by phenylephrine preconditioning and this was blocked by inhibitors of PKCε, PKCδ or AMPK. Inhibition of PKCδ, AMPK or p38 was sufficient to prevent the increase in current, suggesting that these kinases are involved in modulation of sarcK ATP channel current by phenylephrine preconditioning. However, whilst inhibition of AMPK and p38 prevented the protection from phenylephrine preconditioning, PKCδ inhibition paradoxically had no effect. The increase in sarcK ATP current induced by phenylephrine preconditioning requires PKCδ, AMPK and p38 and may contribute to the observed improvement in contractile recovery.

Exercise-induced cardiac preconditioning: how exercise protects your achy-breaky heart

The ability of exercise to protect the heart against ischemia-reperfusion (I/R) injury is well known in both human epidemiological studies and experimental animal models. In this review article, we describe what is currently known about the ability of exercise to precondition the heart against infarction. Just 1 day of exercise can protect the heart against ischemia/reperfusion damage, and this protection is upheld with months of exercise, making exercise one of the few sustainable preconditioning stimuli. Exercise preconditioning depends on the model and intensity of exercise, and appears to involve heightened oxidant buffering capacity, upregulated subunits of sarcolemmal ATP-sensitive potassium channels, and adaptations to cardiac mitochondria. We review the putative mechanisms involved in exercise preconditioning and point out many areas where future research is necessary to advance our understanding of how this stimulus confers resistance against I/R damage.