Overexpression of SUR2A generates a cardiac phenotype resistant to ischemia

Diazoxide attenuates DOX-induced cardiotoxicity in cultured rat myocytes

Doxorubicin (DOX)-induced cardiotoxicity is a well known clinical problem, and many investigations have been made of its possible amelioration. We have investigated whether diazoxide (DIA), an agonist at mitochondrial ATP-sensitive potassium channels (mitoKATP), could reverse DOX-induced apoptotic myocardial cell loss, in cultured rat cardiomyocytes. The role of certain proteins in this pathway was also studied. The rat cardiomyocyte cell line (H9c2) was treated with DOX, and also co-treated with DOX and DIA, for 24 h. Distribution of actin filaments, mitochondrial membrane potential, superoxide dismutase (SOD) activity, total oxidant and antioxidant status (TOS and TAS, respectively), and some protein expressions, were assessed. DOX significantly decreased SOD activity, increased ERK1/2 protein levels, and depolarised the mitochondrial membrane, while DIA co-treatment inhibited such changes. DIA co-treatment ameliorated DOX-induced cytoskeletal changes via F-actin distribution and mitoKATP structure. Co-treatment also decreased ERK1/2 and cytochrome c protein levels. Cardiomyocyte loss due to oxidative stress-mediated apoptosis is a key event in DOX-induced cytotoxicity. DIA had protective effects on DOX-induced cardiotoxicity, via mitoKATP integrity, especially with elevated SUR2A levels; but also by a cascade including SOD/AMPK/ERK1/2. Therefore, DIA may be considered a candidate agent for protecting cardiomyocytes against DOX chemotherapy.

Glutathione Upregulates the Expression of KATP Channels and Vasorelaxation Responses and Inhibits mPTP Opening and Oxidative Stress in the Heart Mitochondria of Old Rats

Background

In the present work, we investigated the effect of exogenous glutathione in old rats on the expression of ATP-sensitive potassium (K_ATP) channels, the mitochondrial permeability transition pore (mPTP) opening in the heart, and the vasorelaxation responses of isolated aortic rings to activation of K_ATP channels.

Methods

Experiments were performed on adult (6 months) and old (24 months) male Wistar rats, which were divided into three groups: adult, old, and glutathione-treated old rats. Glutathione was injected intraperitoneally at a dose of 52 mg/kg 1 hour before the studies. The mRNA expression of K_ATP channels was determined using reverse transcription and real-time polymerase chain reaction analysis. The effect of glutathione administration on mPTP opening, relaxation responses of isolated aortic rings, and oxidative stress markers was studied.

Results

It was shown that the expression levels of Kir6.1, Kir6.2, and SUR1 subunits of K_ATP channels and levels of reduced glutathione were significantly increased in glutathione-treated old rats (by 8.3, 2.8, 13.1, and 1.5-fold, respectively), whereas the levels of oxidative stress markers (hydrogen peroxide, diene conjugates, malondialdehyde, and rate of superoxide generation) in heart mitochondria and mPTP opening were significantly reduced. Relaxation of aortic rings was significantly increased in response to the actions of K_ATP channel openers flocalin and pinacidil in glutathione-treated animals, which was prevented by glibenclamide.

Conclusions

Thus, the administration of exogenous glutathione to old rats resulted in a significant increase in the expression levels of the Kir6.1, Kir6.2, and SUR1 subunits of K_ATP channels and a decrease in oxidative stress. This was accompanied by inhibition of mPTP opening and enhancement of vasorelaxation responses to activation of K_ATP channels.

Upregulation of ATP-Sensitive Potassium Channels as the Potential Mechanism of Cardioprotection and Vasorelaxation Under the Action of Pyridoxal-5-Phosphate in Old Rats

Background: The aging process is accompanied by the weakening of the protective systems of the organism, in particular by the decrease in the expression of ATP-sensitive potassium (KATP) channels and in the synthesis of H2S. The aim of our work was to investigate the role of KATP channels in the cardioprotection induced by pyridoxal-5-phosphate (PLP) in aging. Methods: Experiments were performed on adult and old (aged 24 months) male Wistar rats, which were divided into 3 groups: adults, old, and old PLP-treated rats. PLP was administered orally once a day for 14 days at a dose of 0.7 mg/kg. The levels of mRNA expression of subunits KATP channels were determined by reverse transcription and real-time polymerase chain reaction analysis. Protein expression levels were determined by the Western blot. Cardiac tissue morphology was determined using transverse 6 μm deparaffinized sections stained with picrosirius red staining. Vasorelaxation responses of isolated aortic rings and the function of Langendorff-perfused isolated hearts during ischemia-reperfusion, H2S levels, and markers of oxidative stress were also studied. Results: Administration of PLP to old rats reduces cardiac fibrosis and improves cardiac function during ischemia-reperfusion and vasorelaxation responses to KATP channels opening. At the same time, there was a significant increase in mRNA and protein expression of SUR2 and Kir6.1 subunits of KATP channels, H2S production, and reduced markers of oxidative stress. The specific KATP channel inhibitor-glibenclamide prevented the enhancement of vasodilator responses and anti-ischemic protection in PLP-treated animals. Conclusions: We suggest that this potential therapeutic effect of PLP in old animals may be a result of increased expression of KATP channels and H2S production.

SUR2A as a base for cardioprotective therapeutic strategies

BACKGROUND

ATP-sensitive K⁺ (K_ATP) channels link the metabolic state of the cell with membrane excitability and SUR2A serves as a regulatory subunit of sarcolemmal K_ATP channels. The aim of the present study was to review SUR2A-mediated cardioprotection.

METHODS AND RESULTS

A related literature search in PubMed, Scopus, Web of Science, Google Scholar, and Science direct was performed. Levels of SUR2A regulate number of fully assembled K_ATP channels in the sarcolemma. Increased numbers of sarcolemmal K_ATP channels protect cardiomyocytes against different types of stress by improving the timing of K_ATP channels opening, but, also, by catalyzing ATP production in subsarcolemmal space. Fully-assembled sarcolemmal K_ATP channels protein complex contain ATP-producing enzymes in addition to channel subunits, SUR2A and Kir6.2. An increase in the number of fully-assembled channels results in increased levels of ATP-producing enzymes and subsarcolemmal ATP, which is beneficial in ischemia. Expression of SUR2A is regulated by diverse mechanisms, including AMPK, PI3K/Akt, and ERK1/2 as well as intracellular levels of NAD+/NADH and ATP. There are many compounds and treatments that can be used to regulate SUR2A and some of them seem to be clinically viable options. The most suitable medication to use to increase SUR2A and confer cardioprotection in the clinical setting seems to be nicotinamide. It is one of the safest compounds used in clinical practice and all pre-clinical studies demonstrated that it is an efficient cardioprotective agent.

CONCLUSIONS

Taken all together, SUR2A-based cardioprotection is a likely efficient and safe cardioprotective strategy that can be quickly introduced into clinical practice.

Pyridoxal-5-phosphate induced cardioprotection in aging associated with up-expression of cystathionine-γ-lyase, 3-mercaptopyruvate sulfurtransferase, and ATP-sensitive potassium channels

BACKGROUND

In the present work, we investigated the cardioprotective potential of pyridoxal-5-phosphate (PLP) in old rats as a cofactor of enzymes that synthesize hydrogen sulphide (H₂ S).

MATERIALS AND METHODS

PLP was administered per os in a dose of 0.7 mg per kg daily for 2 weeks. Rats were divided into three groups (adult, old and old +PLP) of 20 animals. The cardiac mRNA levels of genes encoding H₂ S-synthesizing enzymes cystathionine-γ-lyase (CSE) and 3-mercaptopyruvate sulfurtransferase (3-MST), uncoupling proteins (UCP3), subunits of ATP-sensitive potassium (K_ATP ) channels were determined using real-time polymerase chain reaction analysis. We also studied the effect of PLP-administration on the content of H₂ S, oxidative stress, the activities of inducible and constitutive NO-synthase (iNOS, cNOS), arginase and nitrate reductase in the heart homogenates as well as cardiac resistance to ischemia-reperfusion in Langendorff-isolated heart model.

RESULTS

It was shown that PLP restored mRNA levels of CSE, 3-MST and UCP3 genes, and H₂ S content and also significantly increased the expression of SUR2 and Kir6.1 (2.2 and 3.3 times, respectively) in the heart of old rats. PLP significantly reduced the formation of superoxide, malondialdehyde, diene conjugates as well as the activity of iNOS and arginase. PLP significantly increased constitutive synthesis of NO and prevented reperfusion disturbances of the heart function after ischemia.

CONCLUSIONS

Thus, PLP-administration in old rats was associated with up-expression of CSE, 3-MST, UCP3 and SUR2 and Kir6.1 subunits of K_ATP channels, and also increased cNOS activity and reduced oxidative stress and prevented reperfusion dysfunction of the heart in ischemia-reperfusion.

Increase in cardioprotective SUR2A does not alter heart rate and heart rate regulation by physical activity and diurnal rhythm

OBJECTIVES

SUR2A is an ABC protein serving as a regulatory subunit of ATP-sensitive (K_ATP) channels. An increase in SUR2A levels is cardioprotective and it is a potential therapeutic strategy against ischaemic heart disease, heart failure and other diseases. However, whether overexpression of this protein has any adverse effects is yet to be fully understood. Here, we examined the heart rate and the heart rate diurnal variation in mice overexpressing SUR2A (SUR2A+) and their littermate controls (WT) using ECG telemetry that was continuously recorded for 14 days (days 8-23 post-radiotransmitter implantation).

METHODS

Using SigmaPlot 14.0 and Microsoft Excel, Area Under the Curve (AUC) for each parameter was calculated and plotted in a graph.

RESULTS

Both WT and SUR2A+ mice were more physically active during nights and there were no significant differences between two phenotypes. Physical activity was associated with increased heart rate in both phenotypes, but there were no differences in heart rate between phenotypes irrespective of physical activity or time of the day. A diurnal heart rate variation was preserved in the SUR2A+ mice. As area under the curve (AUC) analysis has the potential to reveal differences that are invisible with other statistical methods, we compared AUC of heart rate in SUR2A+ and WT mice. This analysis did not yield anything different from traditional analysis.

CONCLUSIONS

We conclude that increased SUR2A levels are not associated with changes in physical activity, heart rate and/or circadian rhythm influence on the heart rate. This lack of adverse effects supports a notion that manipulation with SUR2A levels is a promising cardioprotective strategy.

Cardiac dysfunction in spontaneously hypertensive old rats is associated with a significant decrease of SUR2 expression

ATP-sensitive potassium (K_ATP) channels are participants of mechanisms of pathological myocardial remodeling containment. The aim of our work was to find the association of changes in the expression of Kir6.1, Kir6.2, SUR1, and SUR2 subunits of K_ATP channels with changes in heart function and structure during aging under conditions of the constant increase of vascular pressure. The experiments were carried out on young and old spontaneously hypertensive rats (SHR) and Wistar rats. The expression levels of K_ATP channels subunits were determined using reverse transcription and quantitative PCR. It is shown that the mRNA expression level of Kir6.1 in young SHR rats is significantly lower (6.3-fold, p = 0.035) than that of young Wistar rats that may be one of the causes of arterial hypertension in SHR. At the same time, mRNA expression of both Kir6.1 and Kir6.2 in old SHR rats was significantly higher (6.8-fold, p = 0.003, and 5.9-fold, p = 0.006, respectively) than in young hypertensive animals. In both groups of old animals, SUR2 expression was significantly reduced compared to young animals, in Wistar rats at 3.87-fold (p = 0.028) and in SHR rats at 48.2-fold (p = 0.033). Changes in SUR1 expression were not significant. Thus, significant changes in the cardiovascular system, including impaired function and structure of the heart in old SHR rats, were associated with a significant decrease in SUR2 expression that may be one of the mechanisms of heart failure decompensation. Therefore, it can be assumed that increased expression of SUR2 may be one of the protective mechanisms against pathological myocardial remodeling.

Novel cholesterol-dependent regulation of cardiac KATP subunit expression revealed using histone deacetylase inhibitors

We recently discovered that the histone deacetylase inhibitor, trichostatin A (TSA), increases expression of the sulfonylurea receptor 2 (SUR2; Abcc9) subunit of the ATP-sensitive K+ (KATP ) channel in HL-1 cardiomyocytes. Interestingly, the increase in SUR2 was abolished with exogenous cholesterol, suggesting that cholesterol may regulate channel expression. In the present study, we tested the hypothesis that TSA increases SUR2 by depleting cholesterol and activating the sterol response element binding protein (SREBP) family of transcription factors. Treatment of HL-1 cardiomyocytes with TSA (30 ng/ml) caused a time-dependent increase in SUR2 mRNA expression that correlates with the time course of cholesterol depletion assessed by filipin staining. Consistent with the cholesterol-dependent regulation of SREBP increasing SUR2 mRNA expression, we observe a significant increase in SREBP cleavage and translocation to the nucleus following TSA treatment that is inhibited by exogenous cholesterol. Further supporting the role of SREBP in mediating the effect of TSA on KATP subunit expression, SREBP1 significantly increased luciferase reporter gene expression driven by the upstream SUR2 promoter. Lastly, HL-1 cardiomyocytes treated with the SREBP inhibitor PF429242 significantly suppresses the effect of TSA on SUR2 gene expression. These results demonstrate that SREBP is an important regulator of KATP channel expression and suggest a novel method by which hypercholesterolemia may exert negative effects on the cardiovascular system, namely, by suppressing expression of the KATP channel.

Cardioprotection by isosteviol derivate JC105: A unique drug property to activate ERK1/2 only when cells are exposed to hypoxia-reoxygenation

In the present study, we have investigated potential cardioprotective properties of Isosteviol analogue we recently synthesized and named JC105. Treatment of heart embryonic H9c2 cells with JC105 (10 μM) significantly increased survival of cells exposed to hypoxia‐reoxygenation. JC105 (10 μM) activated ERK1/2, DRP1 and increased levels of cardioprotective SUR2A in hypoxia‐reoxygenation, but did not have any effects on ERK1/2, DRP1 and/or SUR2A in normoxia. U0126 (10 μM) inhibited JC105‐mediated phosphorylation of ERK1/2 and DRP1 without affecting AKT or AMPK, which were also not regulated by JC105. Seahorse bioenergetic analysis demonstrated that JC105 (10 μM) did not affect mitochondria at rest, but it counteracted all mitochondrial effects of hypoxia‐reoxygenation. Cytoprotection afforded by JC105 was inhibited by U0126 (10 μM). Taken all together, these demonstrate that (a) JC105 protects H9c2 cells against hypoxia‐reoxygenation and that (b) this effect is mediated via ERK1/2. The unique property of JC105 is that selectively activates ERK1/2 in cells exposed to stress, but not in cells under non‐stress conditions.

Embryonic cardioprotection by hydrogen sulphide: studies of isolated cardiac function and ischaemia-reperfusion injury in the chicken embryo

KEY POINTS

In mammals, pregnancy complications can trigger an embryonic or fetal origin of cardiac dysfunction. However, underlying mechanisms remain uncertain because the partial contributions of the challenge on the mother, placenta or offspring are difficult to disentangle. The avian embryo permits isolation of the direct effects of suboptimal conditions during development on the cardiac function of the offspring, independent of additional effects on the mother and/or the placenta. Therefore, the objectives of this work were to adapt the isolated Langendorff technique using the chicken embryo to study the physiology of the developing heart. Here, we introduce a novel technique and show the utility of the technique for exploring cardioprotective roles of H₂ S in the chicken embryo heart. This work lays the foundation for studying the direct effects of H₂ S therapy on the embryonic heart independent of effects on the mother and the placenta in adverse development.

ABSTRACT

This study adapted the isolated Langendorff preparation to study the chicken embryo heart in response to ischaemia-reperfusion (IR) injury. The utility of the technique was tested by investigating cardioprotective effects of hydrogen sulphide (H₂ S) and underlying mechanisms. Embryonic hearts (19 out of 21 days of incubation) mounted on a Langendorff preparation were exposed to IR (30 min ischaemia) after 4 treatments administered randomly, all as a 1 mm bolus, into the perfusate: saline vehicle (control); sodium hydrogen sulphide (NaHS); NaHS plus glibenclamide, an antagonist of K_ATP opening (NaHS Glib), and Glib alone (Glib). Relative to controls, NaHS treatment improved cardiac function after ischaemia (mean ± SD for area under the curve, AUC, for left ventricular developed pressure, LVDP: 1767.3 ± 929.5 vs. 492.7 ± 308.1; myocardial contractility, dP/dt_max : 2748.9 ± 1514.9 vs. 763.7 ± 433.1) and decreased infarct size (22.7 ± 8.0 vs. 43.9 ± 4.2%) and cardiac damage (% change in creatinine kinase, 49.3 ± 41.3 vs. 214.6 ± 155.1; all P < 0.05). Beneficial effects of NaHS were blocked by Glib. Glib alone had no effects. NaHS increased coronary flow rate (CFR) during baseline (mean ± SD for AUC: 134.3 ± 91.6 vs. 92.2 ± 35.8) and post IR (1467 ± 529.5 vs. 748.0 ± 222.1; both P < 0.05). However, this effect was not prevented by Glib. Therefore, the chicken embryo heart is amenable for study via the Langendorff preparation under basal conditions and during IR. The data show that H₂ S confers embryonic cardiac protection via opening of myocardial K_ATP channels and not via increasing CFR. H₂ S may prove a useful therapeutic agent to protect the human fetal heart against IR injury, as may occur in complicated labour.

Improved adaptation to physical stress in mice overexpressing SUR2A is associated with changes in the pattern of Q-T interval

The purpose of this study was to determine whether increased expression of SUR2A, a regulatory subunit of sarcolemmal ATP-sensitive K⁺ (K_ATP) channels, improves adaptation to physical stress and regulates cardiac electrophysiology in physical stress. All experiments have been done on transgenic mice in which SUR2A expression was controlled by cytomegalovirus immediate-early (CMV) promoter (SUR2A) and their littermate wild-type controls (WT). The levels of mRNA in heart tissue were measured by real-time RT-PCR. Electrocardiogram (ECG) was monitored with telemetry. The physical adaptation to stress was elucidated using treadmill. We have found that SUR2A mice express 8.34 ± 0.20 times more myocardial SUR2A mRNA than WT (n = 8–18). The tolerated workload on exercise stress test was more than twofold higher in SUR2A than in WT (n = 5–7; P = 0.01). The pattern of Q-T interval from the beginning of the exercise test until drop point was as follows in the wild type: (1) increase in Q-T interval, (2) decrease in Q-T interval, (3) steady stage with a further decrease in Q-T interval, and (4) a sharp increase in Q-T interval. The pattern of Q-T interval was different in transgenic mice and the following stages have been observed: (1) increase in Q-T interval, (2) decrease in Q-T interval, and (3) prolonged steady-state stage with a slight decrease in Q-T interval. In SUR2A mice, no stage 4 (a sharp increase in Q-T interval) was observed. Based on the obtained results, we conclude that an increase in the expression of SUR2A improves adaptation to physical stress and physical endurance by increasing the number of sarcolemmal K_ATP channels and, by virtue of their channel activity, improving Ca²⁺ homeostasis in the heart.

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

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

Isosteviol prevents the development of isoprenaline‑induced myocardial hypertrophy

Isosteviol sodium (STVNa), which is a derivate of the natural sweet-tasting glycoside stevioside, has recently been developed and it has been determined that this compound exhibits neuro- and cardio-protective properties. In the current study, whether STVNa interferes with the development of cardiac hypertrophy, which is induced by isoprenaline (Iso), was investigated in an experimental rat model. Rats were treated with a vehicle (0.9% NaCl; control), isoprenaline (Iso; 5 mg/kg) or Iso (5 mg/kg) with STVNa (4 mg/kg; Iso + STVNa). Cardiomyocytes were isolated using enzymatic dissociation and were treated with 5 µM Iso for 24 h and co-treated with 5 µM STVNa. Brain natriuretic peptide (BNP) mRNA expression was determined using PCR analysis. Cell surface area, intracellular reactive oxygen species (ROS), mitochondrial transmembrane potential (ΔΨm), cytoplasmic Ca²⁺ and Ca²⁺ and contractile function were examined using a laser scanning confocal microscope. The current study demonstrated that STVNa inhibited Iso-induced cardiac hypertrophy by inhibiting cardiomyocyte size. STVNa significantly reduced cell surface area and decreased BNP mRNA expression in ventricular cardiomyocyte Iso-induced hypertrophy. STVNa was also revealed to restore ΔΨm and reduce ROS generation and intracellular Ca²⁺ concentration when compared with the Iso-treated group. Additionally, STVNa preserved Ca²⁺ transients in hypertrophic cardiomyocytes. In conclusion, the present study demonstrated that STVNa protects against Iso-induced myocardial hypertrophy by reducing oxidative stress, restoring ΔΨm and maintaining Ca²⁺ homeostasis.

Cardiomyocyte-specific deletion of Sirt1 gene sensitizes myocardium to ischaemia and reperfusion injury

Aims

A longevity gene, Sirtuin 1 (SIRT1) and energy sensor AMP-activated protein kinase (AMPK) have common activators such as caloric restriction, oxidative stress, and exercise. The objective of this study is to characterize the role of cardiomyocyte SIRT1 in age-related impaired ischemic AMPK activation and increased susceptibility to ischemic insults.

Methods and results

Mice were subjected to ligation of left anterior descending coronary artery for in vivo ischemic models. The glucose and fatty acid oxidation were measured in a working heart perfusion system. The cardiac functions by echocardiography show no difference in young wild-type C57BL/6 J (WT, 4-6 months), aged WT C57BL/6 J (24-26 months), and young inducible cardiomyocyte-specific SIRT1 knockout (icSIRT1 KO) (4-6 months) mice under physiological conditions. However, after 45 mins ischaemia and 24-h reperfusion, the ejection fraction of aged WT and icSIRT1 KO mice was impaired. The aged WT and icSIRT1 KO hearts vs. young WT hearts also show an impaired post-ischemic contractile function in a Langendorff perfusion system. The infarct size of aged WT and icSIRT1 KO hearts was larger than that of young WT hearts. The immunoblotting data demonstrated that aged WT and icSIRT1 KO hearts vs. young WT hearts had impaired phosphorylation of AMPK and downstream acetyl-CoA carboxylase during ischaemia. Intriguingly, AMPK upstream LKB1 is hyper-acetylated in both aged WT and icSIRT1 KO hearts; this could blunt activation of LKB1, leading to an impaired AMPK activation. The working heart perfusion results demonstrated that SIRT1 deficiency significantly impaired substrate metabolism in the hearts; fatty acid oxidation is augmented and glucose oxidation is blunted during ischaemia and reperfusion. Adeno-associated virus (AAV9)-Sirt1 was delivered into the aged hearts via a coronary delivery approach, which significantly rescued the protein level of SIRT1 and the ischemic tolerance of aged hearts. Furthermore, AMPK agonist can rescue the tolerance of aged heart and icSIRT1 KO heart to ischemic insults.

Conclusions

Cardiac SIRT1 mediates AMPK activation via LKB1 deacetylation, and AMPK modulates SIRT1 activity via regulation of NAD+ level during ischaemia. SIRT1 and AMPK agonists have therapeutic potential for treatment of aging-related ischemic heart disease.

Pyrazinamide may possess cardioprotective properties

Pyrazinamide is an anti-tubercular agent, used as a part of a three-drug regime (any three of the following: rifampicin, isoniazid, pyrazinamide, streptomycin or ethambutol) for the initial phase of treatment. One of the effects pyrazinamide has on mammalian cells is to regulate NAD⁺/NADH levels. We have recently found that changes in NAD⁺/NADH are associated with regulation of expression levels of SUR2A, a cardioprotective protein serving as a regulatory subunit of cardiac ATP-sensitive K⁺ (K_ATP) channels. Here, we have tested whether pyrazinamide regulate expression of SUR2A/K_ATP channel subunits and resistance to metabolic stress in embryonic heart-derived H9c2 cells. We have found that 24-h-long treatment with pyrazinamide (3 mcg/ml) increased mRNA levels of SUR2A, SUR2B and Kir6.1 without affecting mRNA levels of other K_ATP channel subunits. This treatment with pyrazinamide (3 mcg/ml) protected H9c2 cells against stress induced by 10 mM 2,4-dinitrophenol (DNP). The survival rate of DNP-treated cells was 45.6 ± 2.3% (n = 5) if not treated with pyrazinamide and 90.8 ± 2.3% (n = 5; P < 0.001) if treated with pyrazinamide. We conclude that pyrazinamide increases resistance to metabolic stress in heart H9c2 cells probably by increasing SUR2A and SUR2B expression. Our results of this study indicate that pyrazinamide should be seriously considered as a drug of choice for patients with tuberculosis and ischaemic heart disease.

Area under the curve analysis of blood pressure reveals increased spontaneous locomotor activity in SPAK knock-in mice: relevance for hypotension induced by SPAK inhibition?

SPAK (Ste20/SPS1-related proline/alanine-rich kinase) has been recently identified as a protein kinase which targets the electroneutral cation-coupled chloride cotransporters and it stands out as a target for inhibition in novel anti-hypertensive agents. From this prospective, any information about physiological consequences of SPAK inhibition would be useful to better understand the efficacy and potential adverse effects of the SPAK-based anti-hypertensive therapy. Radiotelemetry was employed to continuously measure the parameters of blood pressure (mean arterial blood pressure, systolic blood pressure, and diastolic blood pressure), heart rate, and physical activity in SPAK knock-in (KI) mice and littermate controls for 24 h. For each parameter, the area under the curve (AUC) was calculated and compared between the SPAK KI mice and wild type mice. There was no statistically significant difference in the AUC of blood pressure parameters between SPAK KI and littermate mice. When mice were physically inactive, the AUCs for blood pressures were significantly lower in SPAK KI than in littermates. When physically active, blood pressures were significantly higher in SPAK KI than in littermates. The heart rate followed a similar pattern. The AUC of physical activity was significantly increased in SPAK KI mice when compared to littermates and the SPAK KI mice spent significantly less time in an inactive state and significantly more time in active states. Comparison between SPAK KI mice and unrelated wild type mice yielded similar results to the comparison between SPAK KI mice and littermates. We conclude that SPAK inhibition increases spontaneous locomotor activity, which has a significant effect on blood pressure.

Insulin down-regulates cardioprotective SUR2A in the heart-derived H9c2 cells: A possible explanation for some adverse effects of insulin therapy

Some recent studies associated insulin therapy with negative cardiovascular events and shorter lifespan. SUR2A, a K_ATP channel subunit, regulate cardioprotection and cardiac ageing. Here, we have tested whether glucose and insulin regulate expression of SUR2A/K_ATP channel subunits and resistance to metabolic stress in heart H9c2 cells. Absence of glucose in culture media decreased SUR2A mRNA, while mRNAs of Kir6.2, Kir6.1, SUR1 and IES SUR2B were increased. 2-deoxyglucose (50 mM) decreased mRNAs of SUR2A, SUR2B and SUR1, did not affect IES SUR2A and IES SUR2B mRNAs and increased Kir6.2 mRNA. No glucose and 2-deoxyglucose (50 mM) decreased resistance to an inhibitor of oxidative phosphorylation, DNP (10 mM). 50 mM glucose did not alter K_ATP channel subunits nor cellular resistance to DNP (10 mM). Insulin (20 ng/ml) in both physiological and high glucose (50 mM) down-regulated SUR2A while upregulating Kir6.1 and Kir6.2 (in high glucose only). Insulin (20 ng/ml) in physiological and high glucose decreased cell survival in DNP (10 mM). As opposed to Kir6.2, infection with SUR2A resulted in titre-dependent cytoprotection. We conclude that insulin decreases resistance to metabolic stress in H9c2 cells by decreasing SUR2A expression. Lower cardiac SUR2A levels underlie increased myocardial susceptibility to metabolic stress and shorter lifespan.

Alterations of Cardiac KATP Channels and Autophagy Contribute in the Late Cardioprotective Phase of Exercise Preconditioning

The cardiac effects of exercise preconditioning (EP) are well established; however, the mechanisms involving cardiac ATP-sensitive potassium channel (K_ATP channel) subunits and autophagy are yet to be fully established. The present work aims to investigate the alterations of cardiac K_ATP channel subunits Kir6.2, SUR2A, and autophagy-related LC3 during the late cardioprotective phase of EP against exhaustive exercise-induced myocardial injury. Rats run on treadmill for four running time intervals, each with 10 minutes running and rest. Exhaustive exercise was performed 24 h after EP. Cardiac biomarkers, cTnI and NT-proBNP, along with the histological stain, were served as indicators of myocardial injury. Cardiac K_ATP channel subunits Kir6.2 and SUR2A were analyzed in this study, and autophagy was evaluated by LC3. The results revealed that EP reduced the exhaustive exercise-induced high level of serum cTnI and myocardial ischemia/hypoxia; however, it did not reveal any changes in the serum NT-proBNP level or cardiac BNP. Cardiac SUR2A mRNA significantly upregulated during the exhaustive exercise. The high levels of Kir6.2, SUR2A, LC3IIpuncta and LC3II turnover observed after exhaustive exercise were significantly mitigated by EP in the late phase. These results suggest that EP alleviates myocardial injury induced by exhaustive exercise through the downregulation of cardiac K_ATP channels and autophagy.

Ectopic overexpression of Kir6.1 in the mouse heart impacts on the life expectancy

We recently reported the reduced ATP-sensitive potassium (K_ATP) channel activities in the transgenic mouse heart overexpressing the vascular type K_ATP channel pore-forming subunit (Kir6.1). Although dysfunction of cardiac K_ATP channel has been nominated as a cause of cardiomyopathy in human, these transgenic mice looked normal as wild-type (WT) during the experiment period (~20 weeks). Extended observation period revealed unexpected deaths beginning from 30 weeks and about 50% of the transgenic mice died by 55 weeks. Surface ECG recordings from the transgenic mice at rest demonstrated the normal sinus rhythm and the regular ECG complex as well as the control WT mice except for prolonged QT interval. However, the stress ECG test with noradrenaline revealed abnormal intraventricular conduction delay and arrhythmogeneity in the transgenic mouse. Fibrotic changes in the heart tissue were remarkable in aged transgenic mice, and the cardiac fibrosis developed progressively at least from the age of 30 weeks. Gene expression analyses revealed the differentiation of cardiac fibroblasts to myofibroblasts with elevated cytokine expressions was initiated way in advance before the fibrotic changes and the upregulation of BNP in the ventricle. In sum, Kir6.1TG mice provide an electro-pathological disease concept originated from K_ATP channel dysfunction.

Limb remote ischemia per-conditioning protects the heart against ischemia–reperfusion injury through the opioid system in rats

Remote ischemia per-conditioning (RPerC) has been demonstrated to have cardiac protection, but the underlying mechanism remains unclear. This study aimed to investigate the mechanism underlying cardiac protection of RPerC. Adult male Sprague–Dawley rats were used in this study. Cardiac ischemia/reperfusion (I/R) was induced by 30 min of occlusion and 3 h of reperfusion of the left anterior descending coronary artery. RPerC were performed by 5 min of occlusion of the right femoral artery followed by 5 min of reperfusion for three times during cardiac ischemia. The hemodynamics, left ventricular function, arrhythmia, and infarct area were measured. Protein expression levels of endothelial nitric oxide synthase (eNOS), inducible NOS (iNOS), protein kinase C-ε (PKCε), and PKCδ in the myocardium were assayed. During I/R, systolic artery pressure and left ventricular function were decreased, infarct area was increased, and arrhythmia score was increased (P < 0.05). However, changes of the above parameters were significantly attenuated in RPerC-treated rats compared with control rats (P < 0.05). The cardiac protective effects of RPerC were prevented by naloxone or glibenclamide. Also, RPerC increased the protein expression levels of eNOS, iNOS, PKCε, and PKCδ in the myocardium compared with control rats. These effects were blocked by naloxone, an opioid receptor antagonist, and glibenclamide, an ATP-sensitive K+ channel blocker (KATP). In summary, this study suggests that RPerC protects the heart against I/R injury through activation of opioid receptors and the NO–PKC–KATP channel signaling pathways.

RNA SEQ Analysis Indicates that the AE3 Cl-/HCO3- Exchanger Contributes to Active Transport-Mediated CO2 Disposal in Heart

Loss of the AE3 Cl⁻/HCO₃⁻ exchanger (Slc4a3) in mice causes an impaired cardiac force-frequency response and heart failure under some conditions but the mechanisms are not known. To better understand the functions of AE3, we performed RNA Seq analysis of AE3-null and wild-type mouse hearts and evaluated the data with respect to three hypotheses (CO₂ disposal, facilitation of Na⁺-loading, and recovery from an alkaline load) that have been proposed for its physiological functions. Gene Ontology and PubMatrix analyses of differentially expressed genes revealed a hypoxia response and changes in vasodilation and angiogenesis genes that strongly support the CO₂ disposal hypothesis. Differential expression of energy metabolism genes, which indicated increased glucose utilization and decreased fatty acid utilization, were consistent with adaptive responses to perturbations of O₂/CO₂ balance in AE3-null myocytes. Given that the myocardium is an obligate aerobic tissue and consumes large amounts of O₂, the data suggest that loss of AE3, which has the potential to extrude CO₂ in the form of HCO₃⁻, impairs O₂/CO₂ balance in cardiac myocytes. These results support a model in which the AE3 Cl⁻/HCO₃⁻ exchanger, coupled with parallel Cl⁻ and H⁺-extrusion mechanisms and extracellular carbonic anhydrase, is responsible for active transport-mediated disposal of CO₂.

The beneficial effect of repaglinide on in vitro maturation and development ability of immature mouse oocytes

Repaglinide is a hypoglycemic drug, causing depolarization of the cell membrane, opening the voltage-gated calcium channels, and then increasing intracellular calcium in the pancreatic B cells by inhibition of the K-ATP-sensitive channels. Oocyte in vitro maturation (IVM) is influenced by different factors such as calcium signaling. In this study, we examined the effects of repaglinide on in vitro maturation and fertilization ability of mouse oocyte. Immature oocytes were isolated from female Naval Medical Research Institute mice which are 6-8 wk old mechanically and then cultured in 30 μl droplets of T6 medium with different concentrations of repaglinide. The control group did not receive repaglinide (R₀). Treatment groups received different concentrations (5, 10, and 100 nM and 1 and 10 μM) of repaglinide (R₁, R₂, R₃, R₄, and R₅, respectively). Oocyte in vitro maturation rate was assessed after 24 h. In vitro fertilization was performed using metaphase II oocytes obtained from R₀ and R₄ treatments. Embryo cleavage rate was calculated at 48 h post-IVF. Chi-square test was used for evaluating difference between control and treatment groups (p < 0.05). Oocyte maturation rate after 24 h in treatment groups R₂, R₃, R₄, and R₅ was significantly higher than that in the control (p < 0.05). Supplementation of medium with 1 μM of repaglinide (R₄) during IVM significantly improved outcome of embryo cleavage rate than control at 48 h post-IVF (p < 0.05). In conclusion, repaglinide can be considered as an effective agent for in vitro oocyte maturation and embryo cleavage.

Exposure to 15% oxygen in vivo up-regulates cardioprotective SUR2A without affecting ERK1/2 and AKT: a crucial role for AMPK

SUR2A is an 'atypical' ABC protein that forms sarcolemmal ATP-sensitive K⁺ (K_ATP ) channels by binding to inward rectifier Kir6.2. Manipulation with SUR2A levels has been suggested to be a promising therapeutic strategy against ischaemic heart diseases and other diseases where increased heart resistance to stress is beneficial. Some years ago, it has been reported that high-altitude residents have lower mortality rates for ischaemic heart disease. The purpose of this study was to determine whether SUR2A is regulated by mild-to-severe hypoxic conditions (15% oxygen; oxygen tension equivalent to 3000 m above sea level) and elucidate the underlying mechanism. Mice were exposed to either to 21% (control) or 15% concentration of oxygen for 24 hrs. Twenty-four hours long exposure to 15% oxygen decreased partial pressure of O2 (PO₂ ), but did not affect blood CO₂ (PCO₂ ), haematocrit nor levels of ATP, lactate and NAD+/NADH in the heart. Cardiac SUR2A levels were significantly increased while Kir6.2 levels were not affected. Hypoxia did not induce phosphorylation of extracellular signal-regulated kinases (ERK1/2) or protein kinase B (Akt), but triggered phosphorylation of AMP activated protein kinase (AMPK). AICAR, an activator of AMPK, increased the level of SUR2A in H9c2 cells. We conclude that oxygen increases SUR2A level by activating AMPK. This is the first account of AMPK-mediated regulation of SUR2A.

Anti-Apoptotic and Pro-Survival Effect of Alpinate Oxyphyllae Fructus (AOF) in a d-Galactose-Induced Aging Heart

Aging, a natural biological/physiological phenomenon, is accelerated by reactive oxygen species (ROS) accumulation and identified by a progressive decrease in physiological function. Several studies have shown a positive relationship between aging and chronic heart failure (HF). Cardiac apoptosis was found in age-related diseases. We used a traditional Chinese medicine, Alpinate Oxyphyllae Fructus (AOF), to evaluate its effect on cardiac anti-apoptosis and pro-survival. Male eight-week-old Sprague–Dawley (SD) rats were segregated into five groups: normal control group (NC), d-Galactose-Induced aging group (Aging), and AOF of 50 (AL (AOF low)), 100 (AM (AOF medium)), 150 (AH (AOF high)) mg/kg/day. After eight weeks, hearts were measured by an Hematoxylin–Eosin (H&E) stain, Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL)-assays and Western blotting. The experimental results show that the cardiomyocyte apoptotic pathway protein expression increased in the d-Galactose-Induced aging groups, with dose-dependent inhibition in the AOF treatment group (AL, AM, and AH). Moreover, the expression of the pro-survival p-Akt (protein kinase B (Akt)), Bcl-2 (B-cell lymphoma 2), anti-apoptotic protein (Bcl-xL) protein decreased significantly in the d-Galactose-induced aging group, with increased performance in the AOF treatment group with levels of p-IGFIR and p-PI3K (Phosphatidylinositol-3′ kinase (PI3K)) to increase by dosage and compensatory performance. On the other hand, the protein of the Sirtuin 1 (SIRT1) pathway expression decreased in the aging groups and showed improvement in the AOF treatment group. Our results suggest that AOF strongly works against ROS-induced aging heart problems.

KATP Channels in the Cardiovascular System

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

Phenylephrine preconditioning in embryonic heart H9c2 cells is mediated by up-regulation of SUR2B/Kir6.2: A first evidence for functional role of SUR2B in sarcolemmal KATP channels and cardioprotection

ATP-sensitive K⁺ (K_ATP) channels were originally described in cardiomyocytes, where physiological levels of intracellular ATP keep them in a closed state. Structurally, these channels are composed of pore-forming inward rectifier, Kir6.1 or Kir6.2, and a regulatory, ATP-binding subunit, SUR1, SUR2A or SUR2B. SUR1 and Kir6.2 form pancreatic type of K_ATP channels, SUR2A and Kir6.2 form cardiac type of K_ATP channels, SUR2B and Kir6.1 form vascular smooth muscle type of K_ATP channels. The presence of SUR2B has been described in cardiomyocytes, but its functional significance and role has remained unknown. Pretreatment with phenylephrine (100 nM) for 24 h increased mRNA levels of SUR2B and Kir6.2, without affecting those levels of SUR1, SUR2A and Kir6.1 in embryonic heart H9c2 cells. Such increase was associated with increased K⁺ current through K_ATP channels and Kir6.2/SUR2B protein complexes as revealed by whole cell patch clamp electrophysiology and immunoprecipitation/Western blotting respectively. Pretreatment with phenylephrine (100 nM) generated a cellular phenotype that acquired resistance to chemical hypoxia induced by 2,4-dinitrophenol (DNP; 10 mM), which was accompanied by increased in K⁺ current in response to DNP (10 mM). Cytoprotection afforded by phenylephrine (100 nM) was abolished by infection of H9c2 cells with adenovirus containing Kir6.2AFA, a mutant form of Kir6.2 with largely reduced K⁺ conductance. Taking all together, the present findings demonstrate that the activation of α1-adrenoceptors up-regulates SUR2B/Kir6.2 to confer cardioprotection. This is the first account of possible physiological role of SUR2B in cardiomyocytes.

Sirtuins, aging, and cardiovascular risks

The sirtuins comprise a highly conserved family proteins present in virtually all species from bacteria to mammals. Sirtuins are members of the highly conserved class III histone deacetylases, and seven sirtuin genes (sirtuins 1-7) have been identified and characterized in mammals. Sirtuin activity is linked to metabolic control, apoptosis, cell survival, development, inflammation, and healthy aging. In this review, we summarize and discuss the potential mutual relations between each sirtuin and cardiovascular health and the impact of sirtuins on oxidative stress and so age-related cardiovascular disorders, underlining the possibility that sirtuins will be novel targets to contrast cardiovascular risks induced by aging.

Mild hypoxia in vivo regulates cardioprotective SUR2A: A role for Akt and LDH

High-altitude residents have lower mortality rates for ischaemic heart disease and this is ascribed to cardiac gene remodelling by chronic hypoxia. SUR2A is a cardioprotective ABC protein serving as a subunit of sarcolemmal ATP-sensitive K⁺ channels. The purpose of this study was to determine whether SUR2A is regulated by mild hypoxia in vivo and to elucidate the underlying mechanism. Mice were exposed to either 21% (control) or 18% (mild hypoxia) oxygen for 24 h. Exposure to 18% oxygen did not affect partial pressure of O₂ (PO₂) and CO₂ (PCO₂) in the blood, haematocrit or level of ATP in the heart. However, hypoxia increased myocardial lactate dehydrogenase (LDH) and lactate as well as NAD⁺ without affecting total NAD. SUR2A levels were significantly increased as well as myocardial resistance to ischaemia–reperfusion. Exposure to 18% oxygen did not phosphorylate extracellular signal regulated kinases (ERK1/2) or AMP activated protein kinase (AMPK), but it phosphorylated protein kinase B (Akt). An inhibitor of phosphoinositide 3-kinases (PI3K), LY294002 (0.2 mg/mouse), abolished all observed effects of hypoxia. LDH inhibitors, galloflavin (50 μM) and sodium oxamate (80 mM) significantly decreased levels of SUR2A in heart embryonic H9c2 cells, while inactive mutant LDH form, gly193-M-LDH increased cellular sensitivity towards stress induced by 2,4-dinitrophenol (10 mM). Treatment of H9c2 cells with sodium lactate (30 mM) increased intracellular lactate, but did not affect LDH activity or SUR2A levels. We conclude that PI3K/Akt signalling pathway and LDH play a crucial role in increase of cardiac SUR2A induced by in vivo exposure to 18% oxygen.

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Mild hypoxia increases levels of cardioprotective SUR2A in the heart.

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Phosphorylation of Akt mediates mild hypoxia-induced increase in SUR2A.

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Phosphorylation of ERK1/2 and AMPK is not involved in observed increase in SUR2A.

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PI3K/Akt target LDH to regulate SUR2A levels in the myocardium.

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LDH mediates regulation of SUR2A in a lactate-independent manner.

The role of ATP-sensitive potassium channels in cellular function and protection in the cardiovascular system

ATP-sensitive potassium channels (K(ATP)) are widely distributed and present in a number of tissues including muscle, pancreatic beta cells and the brain. Their activity is regulated by adenine nucleotides, characteristically being activated by falling ATP and rising ADP levels. Thus, they link cellular metabolism with membrane excitability. Recent studies using genetically modified mice and genomic studies in patients have implicated K(ATP) channels in a number of physiological and pathological processes. In this review, we focus on their role in cellular function and protection particularly in the cardiovascular system.

Upregulation of cardioprotective SUR2A by sub-hypoxic drop in oxygen

The effects of hypoxia on gene expression have been vigorously studied, but possible effects of small changes in oxygen tension have never been addressed. SUR2A is an atypical ABC protein serving as a regulatory subunit of sarcolemmal ATP-sensitive K⁺ (K_ATP) channels. Up-regulation of SUR2A is associated with cardioprotection and improved physical endurance. Here, we have found that a 24 h-long exposure to slightly decreased ambient fractional concentration of oxygen (20% oxygen), which is an equivalent to oxygen tension at 350 m above sea level, significantly increased levels of SUR2A in the heart despite that this drop of oxygen did not affect levels of O₂, CO₂ and hematocrit in the blood or myocardial levels of ATP, lactate and NAD/NADH/NAD⁺. Hearts from mice exposed to 20% oxygen were significantly more resistant to ischaemia-reperfusion when compared to control ones. Decrease in fractional oxygen concentration of just 0.9% was associated with phosphorylation of ERK1/2, but not Akt, which was essential for up-regulation of SUR2A. These findings indicate that a small drop in oxygen tension up-regulates SUR2A in the heart by activating ERK signaling pathway. This is the first report to suggest that a minimal change in oxygen tension could have a profound signaling effect.

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Mice were exposed for 24 h to 20% oxygen (oxygen tension at sea level is 20.9%).

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Exposure to 20% oxygen did not produce measurable in vivo signs of hypoxia.

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However, 20% of oxygen up-regulated cardioprotective SUR2A.

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Phosphorylation of ERK1/2, but not Akt, mediated observed increase in SUR2A.

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Thus, a small drop in oxygen up-regulates cardiac SUR2A by activating ERK1/2.

Cholesterol-enriched diet inhibits cardioprotection by ATP-sensitive K+ channel activators cromakalim and diazoxide

It has been previously shown that hyperlipidemia interferes with cardioprotective mechanisms. Here, we investigated the interaction of hyperlipidemia with cardioprotection induced by pharmacological activators of ATP-sensitive K(+) (KATP) channels. Hearts isolated from rats fed a 2% cholesterol-enriched diet or normal diet for 8 wk were subjected to 30 min of global ischemia and 120 min of reperfusion in the presence or absence of KATP modulators. In normal diet-fed rats, either the nonselective KATP activator cromakalim at 10(-5) M or the selective mitochondrial (mito)KATP opener diazoxide at 3 × 10(-5) M significantly decreased infarct size compared with vehicle-treated control rats. Their cardioprotective effect was abolished by coadministration of the nonselective KATP blocker glibenclamide or the selective mitoKATP blocker 5-hydroxydecanoate, respectively. However, in cholesterol-fed rats, the cardioprotective effect of cromakalim or diazoxide was not observed. Therefore, we further investigated how cholesterol-enriched diet influences cardiac KATP channels. Cardiac expression of a KATP subunit gene (Kir6.1) was significantly downregulated in cholesterol-fed rats; however, protein levels of Kir6.1 and Kir6.2 were not changed. The cholesterol diet significantly decreased cardiac ATP, increased lactate content, and enhanced myocardial oxidative stress, as shown by increased cardiac superoxide and dityrosine formation. This is the first demonstration that cardioprotection by KATP channel activators is impaired in cholesterol-enriched diet-induced hyperlipidemia. The background mechanism may include hyperlipidemia-induced attenuation of mitoKATP function by altered energy metabolism and increased oxidative stress in the heart.

Aortic aneurysm and craniosynostosis in a family with Cantu syndrome

Cantu syndrome is an autosomal dominant overgrowth syndrome associated with facial dysmorphism, congenital hypertrichosis, and cardiomegaly. Some affected individuals show bone undermodeling of variable severity. Recent investigations revealed that the disorder is caused by a mutation in ABCC9, encoding a regulatory SUR2 subunit of an ATP-sensitive potassium channel mainly expressed in cardiac and skeletal muscle as well as vascular smooth muscle. We report here on a Japanese family with this syndrome. An affected boy and his father had a novel missense mutation in ABCC9. Each patient had a coarse face and hypertrichosis. However, cardiomegaly was seen only in the boy, and macrosomia only in the father. Skeletal changes were not evident in either patient. Craniosynostosis in the boy and the development of aortic aneurysm in the father are previously undescribed associations with Cantu syndrome.

Kir6.2 limits Ca(2+) overload and mitochondrial oscillations of ventricular myocytes in response to metabolic stress

ATP-sensitive K(+) (KATP) channels are abundant membrane proteins in cardiac myocytes that are directly gated by intracellular ATP and form a signaling complex with metabolic enzymes, such as creatine kinase. KATP channels are known to be essential for adaption to cardiac stress, such as ischemia; however, how all the molecular components of the stress response interact is not fully understood. We examined the effects of decreasing the KATP current density on Ca(2+) and mitochondrial homeostasis and ischemic preconditioning. Acute knockdown of the pore-forming subunit, Kir6.2, was achieved using adenoviral delivery of short hairpin RNA targeted to Kir6.2. The acute nature of the knockdown of Kir6.2 accurately shows the effects of Kir6.2 depletion without any compensatory effects that may arise in transgenic studies. We also investigated the effect of reducing the KATP current while maintaining KATP channel protein in the sarcolemmal membrane using a nonconducting Kir6.2 construct. Only 50% KATP current remained after Kir6.2 knockdown, yet there were profound effects on myocyte responses to metabolic stress. Kir6.2 was essential for cardiac myocyte Ca(2+) homeostasis under both baseline conditions before any metabolic stress and after metabolic stress. Expression of nonconducting Kir6.2 also resulted in increased Ca(2+) overload, showing the importance of K(+) conductance in the protective response. Both ischemic preconditioning and protection during ischemia were lost when Kir6.2 was knocked down. KATP current density was also important for the mitochondrial membrane potential at rest and prevented mitochondrial membrane potential oscillations during oxidative stress. KATP channel density is important for adaption to metabolic stress.

Real-time RT-PCR threshold cycles value for Kir6.1 from the blood correlates with parameters of vascular function: a potential for the vascular function biomarker?

Abstract We examined the presence of KATP channel subunits, Kir6.1 and SUR2B, mRNAs in the blood and vascular function in healthy volunteers (41 males, 34 females). Real-time reverse transcriptase (RT)-PCR threshold cycles (Ct) was used as an indicator of mRNA levels. Baseline skin perfusion and the post-occlusion reactive hyperemia response exhibited a significant positive correlation with Ct for Kir6.1. There was no correlation between Kir6.1 Ct and brachial artery flow-mediated dilatation. Gender had no influence on relationships between blood Kir6.1 Ct and vascular function. We conclude that blood Kir6.1 mRNA levels could be potentially used as a biomarker of the vascular function.

Valsartan attenuates oxidative stress and NF-κB activation and reduces myocardial apoptosis after ischemia and reperfusion

Myocardial apoptosis is primarily triggered during reperfusion. Various mechanisms are involved, including oxidative stress which activates the translocation of nuclear factor-kappa B (NF-κB) and stimulates the release of tumor necrosis factor-alpha (TNF-α). However, the relative contribution of the renin angiotensin system (RAS) to the development of myocardial apoptosis during reperfusion remains unknown. In the present study, we examined whether inhibition of RAS with Valsartan, an Angiotensin II 1 receptor (AT1) antagonist, could reduce apoptosis during reperfusion. We constructed a rat model of myocardial ischemia reperfusion injury. Rats were pretreated with Valsartan for 2 weeks, and then subjected to 30 min ischemia and 4h reperfusion. Apoptosis was detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL). Levels of malondialdehyde (MDA), superoxide dismutase (SOD), TNF-α, and caspase-3 were detected by ELISA. NF-κB, Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase expression was assessed by Western blot analysis. Valsartan inhibited apoptosis (TUNEL staining) in ischemic myocardium (P<0.05), consistent with reduced caspase-3 activity. Valsartan also inhibited of NF-κB translocation to nucleus (P<0.05), and decreased plasma TNF-α levels (P<0.05). Valsartan pretreatment suppressed MDA content and preserved SOD activity, consistent with reduced NADPH oxidase expression (P<0.01). These data provided substantial evidence that RAS was involved in NF-κB activation, mediated by AT1 dependent oxidative stress; thus, RAS might ultimately promote myocardial apoptosis during reperfusion pathogenesis.

Testosterone protects female embryonic heart H9c2 cells against severe metabolic stress by activating estrogen receptors and up-regulating IES SUR2B

A recent clinical study demonstrated that a testosterone supplementation improves functional capacity in elderly female patients suffering from heart failure. These findings prompted us to consider possible mechanisms of testosterone-induced cardioprotection in females. To address this question we have used a pure female population of rat heart embryonic H9c2 cells. Pre-treatment of cells with testosterone for 24h significantly increased survival of H9c2 cells exposed to 2,4-dinitrophenol (DNP), an inhibitor of oxidative phosphorylation. These cells expressed low level of androgen receptors and the effect of testosterone was not modified by hydroxyflutamide, an antagonist of androgen receptor. In contrast, cyclohexamide, an inhibitor of protein biosynthesis, and tamoxifene, a partial agonist of estrogen receptors, abolished cardioprotection afforded by testosterone. In addition, finasteride, an inhibitor of 5α-reductase, and anastrazole, an inhibitor of α-aromatase, also blocked testosterone-induced cytoprotection. Real time RT-PCR revealed that testosterone did not regulate the expression of nine subunits and accessory proteins of sarcolemmal ATP-sensitive K(+) (K(ATP)) channels. On the other hand, testosterone, as well as 17β-estradiol, up-regulated a putative mitochondrial K(ATP) channel subunit, mitochondrial sulfonylurea receptor 2B intraexonics splice variant (IES SUR2B), without affecting expression of IES SUR2A. Tamoxifene inhibited testosterone-induced up-regulation of IES SUR2B without affecting IES SUR2A. In conclusion, this study has shown that testosterone protect female embryonic heart H9c2 cells against severe metabolic stress by its conversion into metabolites that activate estrogen receptors and up-regulate IES SUR2B.

KATP channels are up-regulated with increasing age in human myometrium

It is well established that ageing is associated with decrease in myometrial efficiency and higher incidence of labour complications. In myometrium, the presence of ATP-sensitive K+ (KATP) channels has been detected and they could be a factor in regulating uterine quiescence in pregnancy and contractions during labour. Here, we have examined a possibility of ageing-mediated regulation of KATP channels in the human myometrium. Myometrial samples were taken from non-pregnant women undergoing hysterectomy (n=34) and from women undergoing caesarean section in late pregnancy (n=36). Real time RT-PCR revealed that mRNAs of all known KATP channel subunits were present in the human myometrium. In non-pregnant myometrium, ageing up-regulated SUR2B/Kir6.1, subunits forming KATP channels in this tissue, without affecting the expression of other channel subunits. In the late pregnant myometrium, the level of subunits that do not form functional KATP channels was not affected by age within 20-41 age range. However, uterine SUR2B and Kir6.1 were up-regulated in parturient over 35 years. An ageing-induced increase in those channel subunits was confirmed by Western blotting. Thus, this study suggests that KATP channels are up-regulated with increasing age in human myometrium. This may help explain, at least partially, increased rate of birth complications in women aged over 35 years.

Properties and functions of KATP during mouse perinatal development

BACKGROUND

Prevailing data suggest that ATP-sensitive potassium channels (K(ATP)) contribute to a surprising resistance to hypoxia in mammalian embryos, thus we aimed to characterize the developmental changes of K(ATP) channels in murine fetal ventricular cardiomyocytes.

METHODS

Patch clamp was applied to investigate the functions of K(ATP). RT-PCR, Western blot were used to further characterize the molecular properties of K(ATP) channels.

RESULTS

Similar K(ATP) current density was detected in ventricular cardiomyocytes of late development stage (LDS) and early development stage (EDS). Molecular-biological study revealed the upregulation of Kir6.1/SUR2A in membrane and Kir6.2 remained constant during development. Kir6.1, Kir6.2, and SUR1 were detectable in the mitochondria without marked difference between EDS and LDS. Acute hypoxia-ischemia led to cessation of APs in 62.5% of tested EDS cells and no APs cessation was observed in LDS cells. SarcK(ATP) blocker glibenclamide rescued 47% of EDS cells but converted 42.8% of LDS cells to APs cessations under hypoxia-ischemic condition. MitoK(ATP) blocker 5-HD did not significantly influence the response to acute hypoxia-ischemia at either EDS or LDS. In summary, sarcK(ATP) played distinct functional roles under acute hypoxia-ischemic condition in EDS and LDS fetal ventricular cardiomyocytes, with developmental changes in sarcK(ATP) subunits. MitoK(ATP) were not significantly involved in the response of fetal cardiomyocytes to acute hypoxia-ischemia and no developmental changes of K(ATP) subunits were found in mitochondria.

Nicotinamide-rich diet improves physical endurance by up-regulating SUR2A in the heart

SUR2A is an ATP-binding protein that serves as a regulatory subunit of cardioprotective ATP-sensitive K(+) (K(ATP) ) channels. Based on signalling pathway regulating SUR2A expression and SUR2A role in regulating numbers of fully assembled K(ATP) channels, we have suggested that nicotinamide-rich diet could improve physical endurance by stimulating SUR2A expression. We have found that mice on nicotinamide-rich diet significantly improved physical endurance, which was associated with significant increase in expression of SUR2A. Transgenic mice with solely overexpressed SUR2A on control diet had increased physical endurance in a similar manner as the wild-type mice on nicotinamide-rich diet. The experiments focused on action membrane potential and intracellular Ca(2+) concentration have demonstrated that increased SUR2A expression was associated with the activation of sarcolemmal K(ATP) channels and steady Ca(2+) levels in cardiomyocytes in response to β-adrenergic stimulation. In contrast, the same challenge in the wild-type was characterized by a lack of the channel activation and rise in intracellular Ca(2+) . Nicotinamide-rich diet was ineffective to increase physical endurance in mice lacking K(ATP) channels. This study has shown that nicotinamide-rich diet improves physical endurance by increasing expression of SUR2A and that this is a sole mechanism of the nicotinamide-rich diet effect. The obtained results suggest that oral nicotinamide is a regulator of SUR2A expression and has a potential as a drug that can improve physical endurance in conditions where this effect would be desirable.

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.

Ageing-induced decline in physical endurance in mice is associated with decrease in cardiac SUR2A and increase in cardiac susceptibility to metabolic stress: therapeutic prospects for up-regulation of SUR2A

Ageing is characterized by decline in physical endurance which has been suggested to be partly due to diminished functional and adaptive reserve capacity of the heart. Ageing is associated with decrease in numbers of sarcolemmal ATP-sensitive K(+) (K(ATP)) channels, but whether this has anything to do with ageing-induced decline in physical endurance is yet to be determined. We have previously shown that the numbers of sarcolemmal K(ATP) channels are controlled by the level of expression of SUR2A, a K(ATP) channel regulatory subunit. Here, we have found that ageing decreases the level of SUR2A mRNA in the heart without affecting expression of pore-forming K(ATP) channel subunits, Kir6.1 and Kir6.2. This effect of ageing was associated with decrease in levels of fully-assembled sarcolemmal K(ATP) channels. At the same time, ageing was associated with decreased physical endurance. In order to determine whether increased expression of SUR2A would counteract ageing-induced decrease in physical endurance, we have taken advantage of mice which SUR2A levels are regulated by more efficient CMV promoter. These mice had increased resistance of cardiomyocytes to metabolic stress/hypoxia and increased physical endurance when compared to the wild type. In transgenic mice, ageing did not affect the level of SUR2A mRNA in the heart and the level of fully-assembled sarcolemmal K(ATP) channels. The effect of increased SUR2A to resistance of cardiomyocytes to hypoxia and physical endurance was retained in old mice. The magnitude of these effects was such that they were significantly increased even when compared to those in wild type young mice. We conclude that (1) the level of SUR2A expression in the heart is important factor in regulating physical endurance, (2) ageing-induced decrease in cardiac SUR2A is, at least in part, responsible for ageing-induced decline in physical fitness and (3) up-regulation of SUR2A could be a viable strategy to counteract ageing-induced decline in physical endurance.

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K(ATP)) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K(ATP) channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K(ATP) channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K(ATP) channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K(ATP) channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.

A patient suffering from hypokalemic periodic paralysis is deficient in skeletal muscle ATP-sensitive K channels

Hypokalemic periodic paralysis (HOPP) is a rare disease associated with attacks of muscle weakness and hypokalemia. In the present study, immunoprecipitation/Western blotting has shown that a HOPP patient was deficient in sarcolemmal K(ATP) channels. Real-time RT-PCR has revealed that HOPP has decreased mRNA levels of Kir6.2, a pore-forming K(ATP) channel subunit, without affecting the expression of other K(ATP) channel-forming proteins. Based on these findings, we conclude that HOPP could be associated with impaired expression of Kir6.2 which leads to deficiency in skeletal muscle K(ATP) channels, which may explain the symptoms and clinical signs of this disease.

Human oocytes express ATP-sensitive K(+) channels

BACKGROUND

ATP-sensitive K(+) (K(ATP)) channels link intracellular metabolism with membrane excitability and play crucial roles in cellular physiology and protection. The K(ATP) channel protein complex is composed of pore forming, Kir6.x (Kir6.1 or Kir6.2) and regulatory, SURx (SUR2A, SUR2B or SUR1), subunits that associate in different combinations. The objective of this study was to determine whether mammalian oocytes (human, bovine, porcine) express K(ATP) channels.

METHODS

Supernumerary human oocytes at different stages of maturation were obtained from patients undergoing assisted conception treatments. Bovine and porcine oocytes in the germinal vesicle (GV) stage were obtained by aspirating antral follicles from abattoir-derived ovaries. The presence of mRNA for K(ATP) channel subunits was determined using real-time RT-PCR with primers specific for Kir6.2, Kir6.1, SUR1, SUR2A and SUR2B. To assess whether functional K(ATP) channels are present in human oocytes, traditional and perforated patch whole cell electrophysiology and immunoprecipitation/western blotting were used.

RESULTS

Real-time PCR revealed that mRNA for Kir6.1, Kir6.2, SUR2A and SUR2B, but not SUR1, were present in human oocytes of different stages. Only SUR2B and Kir6.2 mRNAs were detected in GV stage bovine and porcine oocytes. Immunoprecipitation with SUR2 antibody and western blotting with Kir6.1 antibody identified bands corresponding to these subunits in human oocytes. In human oocytes, 2,4-dinitrophenol (400 µM), a metabolic inhibitor known to decrease intracellular ATP and activate K(ATP) channels, increased whole cell K(+) current. On the other hand, K(+) current induced by low intracellular ATP was inhibited by extracellular glibenclamide (30 µM), an oral antidiabetic known to block the opening of K(ATP) channels.

CONCLUSIONS

In conclusion, mammalian oocytes express K(ATP) channels. This opens a new avenue of research into the complex relationship between metabolism and membrane excitability in oocytes under different conditions, including conception.

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.