Chronic mild hypoxia protects heart-derived H9c2 cells against acute hypoxia/reoxygenation by regulating expression of the SUR2A subunit of the ATP-sensitive K+ channel

A dual mechanism of cytoprotection afforded by M-LDH in embryonic heart H9C2 cells

Muscle form of lactate dehydrogenase (M-LDH), a minor LDH form in cardiomyocytes, physically interacts with ATP-sensitive K+ (K ATP) channel-forming subunits. Here, we have shown that expression of 193gly-M-LDH, an inactive mutant of M-LDH, inhibit regulation of the K ATP channels activity by LDH substrates in embryonic rat heart H9C2 cells. In cells expressing 193gly-M-LDH chemical hypoxia has failed to activate K ATP channels. The similar results were obtained in H9C2 cells expressing Kir6.2AFA, a mutant form of Kir6.2 with largely decreased K+ conductance. Kir6.2AFA has slightly, but significantly, reduced cellular survival under chemical hypoxia while the deleterious effect of 193gly-M-LDH was significantly more pronounced. The levels of total and subsarcolemmal ATP in H9C2 cells were not affected by Kir6.2AFA, but the expression of 193gly-M-LDH led to lower levels of subsarcolemmal ATP during chemical hypoxia. We conclude that M-LDH regulates both the channel activity and the levels of subsarcolemmal ATP and that both mechanism contribute to the M-LDH-mediated cytoprotection.

SURA2 targeting for cardioprotection?

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

Decreased expression of aortic KIR6.1 and SUR2B in hypertension does not correlate with changes in the functional role of K(ATP) channels

ATP-dependent potassium (K(ATP)) channels are the target of multiple vasoactive factors and drugs. Changes in the functional role of ATP-dependent (K(ATP)) potassium channels in hypertension are controversial. The aim of the present study was to analyze the possible changes of ATP-sensitive potassium channels (K(ATP)) expression and function during hypertension. For this purpose, we used endothelium-denuded aorta segments from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) to analyze the 1) expression of K(ATP) subunits Kir6.1, Kir6.2 and SUR2B by immunohistochemistry and Western blot, 2) the K(ATP) currents recorded in the whole cell configuration of the patch-clamp technique and 3) the vasodilator response to the K(ATP) channel openers, pinacidil and cromakalim. Kir6.1 and SUR2B were expressed in the medial layer of the aorta from WKY rats and SHR rats, while Kir6.2 was not detected in aorta from either strain. Kir6.1 and SUR2B expression were decreased in hypertension. However, the vasodilator responses of pinacidil and cromakalim were similar in WKY rats and SHR rats. Moreover, pinacidil induced increase in K+ currents was also similar in WKY rats and SHR rats and also similarly inhibited by glybenclamide. Our data demonstrate for the first time direct evidence of decreased aortic Kir6.1/SUR2B subunit expression in hypertension, but preserved functional responses to K(ATP) channel openers.

Mice lacking sulfonylurea receptor 2 (SUR2) ATP-sensitive potassium channels are resistant to acute cardiovascular stress

Adenosine triphosphate-sensitive potassium (K(ATP)) channels are thought to mediate the stress response by sensing intracellular ATP concentration. Cardiomyocyte K(ATP) channels are composed of the pore-forming Kir6.2 subunit and the regulatory sulfonylurea receptor 2 (SUR2). We studied the response to acute isoproterenol in SUR2 null mice as a model of acute adrenergic stress and found that the episodic coronary vasospasm observed at baseline in SUR2 null mice was alleviated. Similar results were observed following administration of a nitric oxide donor consistent with a vasodilatory role. Langendorff-perfused hearts were subjected to global ischemia, and hearts from SUR2 null mice exhibited significantly reduced infarct size (54+/-4 versus 30+/-3%) and improved cardiac function compared to control mice. SUR2 null mice have hypertension and develop cardiac hypertrophy. However, despite longstanding hypertension, fibrosis was absent in SUR2 null mice. SUR2 null mice were administered nifedipine to block baseline coronary vasospasm, and hearts from nifedipine-treated SUR2 null mice exhibited increased infarct size compared to untreated SUR2 null mice (42+/-3% versus 54+/-3%). We conclude that conventional sarcolemmal cardiomyocyte K(ATP) channels containing full-length SUR2 are not required for mediating the response to acute cardiovascular stress.

Localization of sulfonylurea receptor subunits, SUR2A and SUR2B, in rat heart

To understand the possible functions and subcellular localizations of sulfonylurea receptors (SURs) in cardiac muscle, polyclonal anti-SUR2A and anti-SUR2B antisera were raised. Immunoblots revealed both SUR2A and SUR2B expression in mitochondrial fractions of rat heart and other cellular fractions such as microsomes and cell membranes. Immunostaining detected ubiquitous expression of both SUR2A and SUR2B in rat heart in the atria, ventricles, interatrial and interventricular septa, and smooth muscles and endothelia of the coronary arteries. Electron microscopy revealed SUR2A immunoreactivity in the cell membrane, endoplasmic reticulum (ER), and mitochondria. SUR2B immunoreactivity was mainly localized in the mitochondria as well as in the ER and cell membrane. Thus, SUR2A and SUR2B are not only the regulatory subunits of sarcolemmal K(ATP) channels but may also function as regulatory subunits in mitochondrial K(ATP) channels and play important roles in cardioprotection.

Cardiac adaptation to chronic high-altitude hypoxia: beneficial and adverse effects

This review deals with the capability of the heart to adapt to chronic hypoxia in animals exposed to either natural or simulated high altitude. From the broad spectrum of related issues, we focused on the development and reversibility of both beneficial and adverse adaptive myocardial changes. Particular attention was paid to cardioprotective effects of adaptation to chronic high-altitude hypoxia and their molecular mechanisms. Moreover, interspecies and age differences in the cardiac sensitivity to hypoxia-induced effects in various experimental models were emphasized.

AMP-activated protein kinase mediates preconditioning in cardiomyocytes by regulating activity and trafficking of sarcolemmal ATP-sensitive K(+) channels

Brief periods of ischemia and reperfusion that precede sustained ischemia lead to a reduction in myocardial infarct size. This phenomenon, known as ischemic preconditioning, is mediated by signaling pathway(s) that is complex and yet to be fully defined. AMP-activated kinase (AMPK) is activated in cells under conditions associated with ATP depletion and increased AMP/ATP ratio. In the present study, we have taken advantage of a cardiac phenotype overexpressing a dominant negative form of the alpha2 subunit of AMPK to analyze the role, if any, that AMPK plays in preconditioning the heart. We have found that myocardial preconditioning activates AMPK in wild type, but not transgenic mice. Cardiac cells from transgenic mice could not be preconditioned, as opposed to cells from the wild type. The cytoprotective effect of AMPK was not related to the effect that preconditioning has on mitochondrial membrane potential as revealed by JC-1, a mitochondrial membrane potential-sensitive dye, and laser confocal microscopy. In contrast, experiments with di-8-ANEPPS, a sarcolemmal-potential sensitive dye, has demonstrated that intact AMPK activity is required for preconditioning-induced shortening of the action membrane potential. The preconditioning-induced activation of sarcolemmal K(ATP) channels was observed in wild type, but not in transgenic mice. HMR 1098, a selective inhibitor of sarcolemmal K(ATP) channels opening, inhibited preconditioning-induced shortening of action membrane potential as well as cardioprotection afforded by AMPK. Immunoprecipitation followed by Western blotting has shown that AMPK is essential for preconditioning-induced recruitment of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that AMPK mediates preconditioning in cardiac cells by regulating the activity and recruitment of sarcolemmal K(ATP) channels without being a part of signaling pathway that regulates mitochondrial membrane potential.

Short time exposure to hypoxia promotes H9c2 cell growth

The effects of short time (15 min) exposure to hypoxia on rat cardiomyocytes (H9c2) were examined. Exposure to hypoxia inhibited cell death via activation of MEK/extracellular signal-regulated kinase (ERK). Further, exposure to hypoxia promoted cell growth by down-regulation of p27 and phosphorylation of cyclin-dependent kinase 2 (CDK2) and retinoblastoma protein (Rb).

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

Characterization of adenosine transport in H9c2 cardiomyoblasts

Adenosine plays a significant role in various physiological processes including cardioprotection. Nucleoside transporters modulate adenosine levels in the vicinity of adenosine receptors, which in turn modulate adenosine functional efficacy. In the current study, adenosine transport in the rat heart myoblast cell line H9c2 was characterized. Kinetic analysis of adenosine transport in H9c2 cells revealed a Km of 8.9+/-0.001 microM and a Vmax of 32.1+/-0.65 pmol/mg protein/min. Adenosine transport in H9c2 cells was Na+-independent. About 6% of the total adenosine uptake was sensitive to nitrobenzylmercaptopurine riboside (NBMPR); however, 94% was insensitive, suggesting that adenosine uptake by H9c2 cells was predominantly mediated by the equilibrative nucleoside transporter (ENT)-2 and only mildly by ENT-1. Results of RT-PCR demonstrated the presence of mRNA for ENT-1, ENT-2 and ENT-3. Upon culture in a cell differentiation medium containing fetal bovine serum (1%) and retinoic acid (10 nM), both the activity and mRNA expression of ENT-1 increased 3-fold, however, ENT-2 was unaffected. Pharmacological studies revealed that ENT-1 activity was stimulated by PKA and PKC-delta/epsilon, however, ENT-2 activity was unaffected. Taken together, the exceptionally high expression level of ENT-2 in H9c2 cells raises questions regarding the use of H9c2 cells as a model for physiological adenosine activity in the heart. Furthermore, this study may form the basis for further investigation into the effect of cell differentiation and protein kinases on the regulation of nucleoside transporters.

Overexpression of SUR2A generates a cardiac phenotype resistant to ischemia

ATP-sensitive K+ (K(ATP)) channels are present in the sarcolemma of cardiac myocytes where they link membrane excitability with the cellular bioenergetic state. These channels are in vivo composed of Kir6.2, a pore-forming subunit, SUR2A, a regulatory subunit, and at least four accessory proteins. In the present study, real-time RT-PCR has demonstrated that of all six sarcolemmal K(ATP) channel-forming proteins, SUR2A was probably the least expressed protein. We have generated mice where the SUR2A was under the control of a cytomegalovirus promoter, a promoter that is more efficient than the native promoter. These mice had an increase in SUR2A mRNA/protein levels in the heart whereas levels of mRNAs of other channel-forming proteins were not affected at all. Imunoprecipitation/Western blot and patch clamp electrophysiology has shown an increase in K(ATP) channel numbers in the sarcolemma of transgenic mice. Cardiomyocytes from transgenic mice responded to hypoxia with shortening of action membrane potential and were significantly more resistant to this insult than cardiomyocytes from the wild-type. The size of myocardial infarction in response to ischemia-reperfusion was much smaller in hearts from transgenic mice compared to those in wild-type. We conclude that overexpression of SUR2A generates cardiac phenotype resistant to hypoxia/ischemia/reperfusion injury due at least in part to increase in levels of sarcolemmal K(ATP) channels.

Effects of KR-31378, a novel ATP-sensitive potassium channel activator, on hypertrophy of H9c2 cells and on cardiac dysfunction in rats with congestive heart failure

The present study was performed to evaluate the effects of (2S, 3S, 4R)-N"-cyano-N-(6-amino-3, 4-dihydro-2-dimethoxymethyl-3-hydroxy-2-methyl-2H-1-benzopyran-4yl)-N'-benzylguanidine (KR-31378), a novel mitochondrial ATP-sensitive potassium channel activator, on hypertrophy of H9c2 cells and on cardiac dysfunction in rats with congestive heart failure. In rat heart-derived H9c2 cells treated with hypertrophic agonists, such as angiotensin II, phenylephrine, isoproterenol, and urotensin II, cell size was significantly increased by 27-47%. The increases in cell size induced by the hypertrophic agonists were inhibited by treatment of KR-31378 in a concentration-dependent manner. This was confirmed by the results showing that KR-31378 inhibited the angiotensin II-induced increase in cell protein content. The effect of KR-31378 on the angiotensin II-induced increase in cell size was reversed by mitochondrial ATP-sensitive potassium channel blockers, 5-hydroxydecanoate or glibenclamide. In rats with congestive heart failure, induced by permanent coronary artery occlusion for 8 weeks, KR-31378 significantly reversed the cardiac dysfunction (increase in ratios of stroke volume or cardiac output to body weight) induced by myocardial infarction without reducing infarct size. In addition, KR-31378 significantly inhibited atrial hypertrophy (decrease in ratio of right atrium to body weight) and decreased the serum pro-atrial natriuretic peptide level, a biochemical marker of heart failure. These results suggest that KR-31378 suppresses hypertrophy induced by hypertrophic agonists in H9c2 cells and improves cardiac dysfunction in rats with congestive heart failure induced by myocardial infarction, and that the effects may be mediated by the activation of mitochondrial ATP-sensitive potassium channels.

Glyceraldehyde 3-phosphate dehydrogenase serves as an accessory protein of the cardiac sarcolemmal K(ATP) channel

Cardiac sarcolemmal ATP-sensitive K+ (K(ATP)) channels, composed of Kir6.2 and SUR2A subunits, are regulated by intracellular ATP and they couple the metabolic status of the cell with the membrane excitability. On the basis of previous studies, we have suggested that glyceraldehyde 3-phosphate dehydrogenase (GAPDH) may be a part of the sarcolemmal K(ATP)-channel protein complex. A polypeptide of approximately 42 kDa was immunoprecipitated with an anti-SUR2A antibody from guinea-pig cardiac membrane fraction and identified as GAPDH. Immunoprecipitation/western blotting analysis with anti-Kir6.2, anti-SUR2A and anti-GAPDH antibodies showed that GAPDH is a part of the sarcolemmal K(ATP)-channel protein complex in vivo. Further studies with immunoprecipitation/western blotting and the membrane yeast two-hybrid system showed that GAPDH associates physically with the Kir6.2 but not the SUR2A subunit. Patch-clamp electrophysiology showed that GAPDH regulates K(ATP)-channel activity irrespective of high intracellular ATP, by producing 1,3-bisphosphoglycerate, a K(ATP)-channel opener. These results suggest that GAPDH is an integral part of the sarcolemmal K(ATP)-channel protein complex, where it couples glycolysis with the K(ATP)-channel activity.

A novel Val734Ile variant in the ABCC9 gene associated with myocardial infarction

BACKGROUND

Alterations in coronary vasomotor tone are deemed to play an important role in myocardial infarction (MI), and the ATP-binding cassette transporter C9-ABCC9-may be involved in the regulation of coronary artery vasomotility. We sought to determine whether genetic variations in the coding sequence of ABCC9 gene could be associated with precocious MI (myocardial infarction before the age of 60 years) in humans.

METHODS

In this study, we screened using PCR-SSCP analysis the entire coding region of the ABCC9 gene in 45 patients with precocious MI and 45 age- and gender-matched controls.

RESULTS

A novel missense mutation, Val734Ile in exon 17, was detected in one MI patient. We therefore analyzed by PCR-RFLPs the frequency of this nonsynonymous change in a large Italian cohort of precocious MI patients (n=584) and healthy comparison subjects (n=873). After allowance for the potential confounding effects of age, gender, and established cardiovascular risk factors, multivariate logistic regression analysis revealed that carriers of the rare 734Ile allele would have a 6.40-fold risk of suffering MI before the age of 60 years as compared to controls (95% CI=1.58-25.90, P=0.009).

CONCLUSIONS

Taken together, our results provide the first important evidence that the newly discovered 734Ile allele in ABCC9 might influence susceptibility to precocious MI in our population.

Activation of ATP-sensitive K channels protects hippocampal CA1 neurons from hypoxia by suppressing p53 expression

Oxygen-sensing and responses to changes in oxygen concentration is a fundamental property of cellular physiology. In the central nervous system (CNS), hippocampal CA1 neurons are known to be extremely vulnerable to low oxygen concentrations or anoxia. Understanding the mechanisms governing tolerance to oxygen depletion is vital for developing strategies to protect the brain from hypoxic-ischemic insult. Our current study demonstrates the protective mechanism of KATP channels on hippocampal CA1 neurons subjected to hypoxic or anoxic conditions. Specifically, we show that CA1 neurons undergo apoptosis when depleted of oxygen for 12 or 24 h. A KATP channels agonist diazoxide inhibits the observed apoptosis. The inhibition of apoptosis is mediated through diazoxide's ability to reduce p53 expression. On the other hand, tolbutamide, a KATP channels antagonist which blocks the cellular sulphonylureas receptor, significantly increases p53 expression and apoptosis under hypoxic/anoxic conditions. Trichostatin (TSA), a p53 inhibitor, can block the effects of tolbutamide, lending further support for a role of p53 in mediating this process. These studies demonstrate that KATP channels act as an upstream antagonist of p53 in hippocampal CA1 neurons, and suggests their protective role in cerebral hypoxia.

High glucose protects single beating adult cardiomyocytes against hypoxia

In the heart, the opening of sarcolemmal ATP-sensitive K(+) (K(ATP)) channels seems to be crucial for the cardiac protection against hypoxia/ischaemia. In the present study, we have exposed cardiomyocytes under hypoxia to high extracellular glucose (30 mM). Under these conditions, intracellular concentration of 1,3-bisphosphoglycerate has increased confirming stimulation of glycolysis. Perforated patch-clamp electrophysiology revealed that hypoxia induces whole-cell K(+) current in cardiomyocytes more efficiently in the presence than in the absence of high glucose. Glucose significantly promoted survival of cardiomyocytes exposed to hypoxia. HMR 1098, an antagonist of sarcolemmal K(ATP) channels, inhibited glucose-induced activation of whole-cell K(+) current during hypoxia as well as glucose-mediated cytoprotection. An inhibitor of glyceraldehyde 3-phosphate dehydrogenase, iodoacetate, inhibited glycolysis in hypoxia and blocked the activation of sarcolemmal K(ATP) channels. Based on the obtained results, we conclude that the activation of sarcolemmal K(ATP) channels is involved in glucose-mediated cardioprotection.

Oxygen tension modulates the expression of pulmonary vascular BKCa channel alpha- and beta-subunits

At birth, the lung environment changes from low to relatively high O(2) tension. Pulmonary blood flow increases and pulmonary artery pressure decreases. Recent data suggest that pulmonary vascular calcium-sensitive K(+) channel (BK(Ca)) activation mediates perinatal pulmonary vasodilation. Although BK(Ca) channel expression is developmentally regulated, the molecular mechanisms responsible for BK(Ca) expression remain unknown. We tested the hypothesis that the low-O(2) tension environment of the normal fetus modulates BK(Ca) channel expression. We analyzed BK(Ca) expression under conditions of hypoxia and normoxia both in vitro and in vivo. BK(Ca) alpha-subunit mRNA expression increased twofold in ovine pulmonary artery smooth muscle cell (PASMC) primary cultures maintained in hypoxia. In vivo, BK(Ca) expression was similarly affected by hypoxia. When adult Sprague-Dawley rats were placed in hypobaric hypoxic chambers for 3 wk, hypoxic animals showed an increase of threefold in the expression of BK(Ca) alpha- and more than twofold in the expression of BK(Ca) beta(1)-subunit mRNA. Immunochemical staining was consistent with the genetic data. To assess transcriptional activation of the beta-subunit of the BK(Ca), both BK(Ca) beta(1)- and beta(2)-subunit luciferase (K(Ca) beta:luc(+)) reporter genes were constructed. Hypoxia increased PASMC K(Ca) beta(1):luc(+) reporter expression by threefold and K(Ca) beta(2):luc(+) expression by 35%. Fetal PASMC treated with the hypoxia-inducible factor-1 mimetic deferoxamine showed a 63 and 41% increase in BK(Ca) channel alpha- and beta(1)-subunit expression, respectively. Together, these results suggest that oxygen tension modulates BK(Ca) channel subunit mRNA expression, and the regulation is, at least in part, at the transcriptional level.

Cardioprotection afforded by chronic exercise is mediated by the sarcolemmal, and not the mitochondrial, isoform of the KATP channel in the rat

This study was conducted to examine the role of myocardial ATP-sensitive potassium (K(ATP)) channels in exercise-induced protection from ischaemia-reperfusion (I-R) injury. Female rats were either sedentary (Sed) or exercised for 12 weeks (Tr). Hearts were excised and underwent a 1-2 h regional I-R protocol. Prior to ischaemia, hearts were subjected to pharmacological blockade of the sarcolemmal K(ATP) channel with HMR 1098 (SedHMR and TrHMR), mitochondrial blockade with 5-hydroxydecanoic acid (5HD; Sed5HD and Tr5HD), or perfused with buffer containing no drug (Sed and Tr). Infarct size was significantly smaller in hearts from Tr animals (35.4 +/- 2.3 versus 44.7 +/- 3.0% of the zone at risk for Tr and Sed, respectively). Mitochondrial K(ATP) blockade did not abolish the training-induced infarct size reduction (30.0 +/- 3.4 versus 38.0 +/- 2.6 in Tr5HD and Sed5HD, respectively); however, sarcolemmal K(ATP) blockade completely eradicated the training-induced cardioprotection. Infarct size was 71.2 +/- 3.3 and 64.0 +/- 2.4% of the zone at risk for TrHMR and Sed HMR. The role of sarcolemmal K(ATP) channels in Tr-induced protection was also supported by significant increases in both subunits of the sarcolemmal K(ATP) channel following training. LV developed pressure was better preserved in hearts from Tr animals, and was not influenced by addition of HMR 1098. 5HD decreased pressure development regardless of training status, from 15 min of ischaemia through the duration of the protocol. This mechanical dysfunction was likely to be due to a 5HD-induced increase in myocardial Ca2+ content following I-R. The major findings of the present study are: (1) unlike all other known forms of delayed cardioprotection, infarct sparing following chronic exercise was not abolished by 5HD; (2) pharmacological blockade of the sarcolemmal K(ATP) channel nullified the cardioprotective benefits of exercise training; and (3) increased expression of sarcolemmal K(ATP) channels was observed following chronic training.

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.

High glucose regulates the activity of cardiac sarcolemmal ATP-sensitive K+ channels via 1,3-bisphosphoglycerate: a novel link between cardiac membrane excitability and glucose metabolism

Because we were interested in assessing glucose-mediated regulation of the activity of sarcolemmal ATP-sensitive K(+) channels (K(ATP) channels) (which are closed by physiological levels of intracellular ATP and serve to couple intracellular metabolism with the membrane excitability in the heart) during ischemia, we performed experiments designed to test whether high extracellular glucose would have effects on sarcolemmal K(ATP) channels per se. Surprisingly, we found that high extracellular glucose (50 mmol/l) activates sarcolemmal K(ATP) channels in isolated guinea pig cardiomyocytes. To activate K(ATP) channels, glucose had to be transported into cardiomyocytes and subjected to glycolysis. The activation of these channels was independent of ATP production and intracellular ATP levels. The effect of glucose on sarcolemmal K(ATP) channels was mediated by the catalytic activity of glyceraldehyde-3-phosphate dehydrogenase and consequent generation of 1,3-bisphosphoglycerate. The 1,3-bisphosphoglycerate (20 mmol/l), an intermediate product of glycolysis, directly targeted and activated K(ATP) channels, despite physiological levels of intracellular ATP (5 mmol/l). We conclude that glucose, so far exclusively viewed as a metabolic fuel in the heart important only during ischemia/hypoxia, may serve a signaling role in the nonstressed myocardium by producing an agent that regulates cardiac membrane excitability independently of high-energy phosphates.

Hypoxia-induced preconditioning in adult stimulated cardiomyocytes is mediated by the opening and trafficking of sarcolemmal KATP channels

The opening of sarcolemmal and mitochondrial ATP-sensitive K(+) (KATP) channels in the heart is believed to mediate ischemic preconditioning, a phenomenon whereby brief periods of ischemia/reperfusion protect the heart against myocardial infarction. Here, we have applied digital epifluorescent microscopy, immunoprecipitation and Western blotting, perforated patch clamp electrophysiology, and immunofluorescence/laser confocal microscopy to examine the involvement of KATP channels in cardioprotection afforded by preconditioning. We have shown that adult, stimulated-to-beat, guinea-pig cardiomyocytes survived in sustained hypoxia for approximately 17 min. An episode of 5-min-long hypoxia/5-min-long reoxygenation before sustained hypoxia dramatically increased the duration of cellular survival. Experiments with different antagonists of KATP channels, applied at different times during the experimental protocol, suggested that the opening of sarcolemmal KATP channels at the beginning of sustained hypoxia mediate preconditioning. This conclusion was supported by perforated patch clamp experiments that revealed activation of sarcolemmal KATP channels by preconditioning. Immunoprecipitation and Western blotting as well as immunofluorescence and laser confocal microscopy showed that the preconditioning is associated with the increase in KATP channel proteins in sarcolemma. Inhibition of trafficking of KATP channel subunits prevented preconditioning without affecting sensitivity of cardiomyocytes to hypoxia in the absence of preconditioning. We conclude that the preconditioning is mediated by the activation and trafficking of sarcolemmal KATP channels.

Pore loop-mutated rat KIR6.1 and KIR6.2 suppress KATP current in rat cardiomyocytes

Cardiomyocytes express mRNA for all major subunits of ATP-sensitive potassium (K(ATP)) channels: KIR6.1, KIR6.2, SUR1A, SUR2A, and SUR2B. It has remained controversial as to whether KIR6.1 may associate with KIR6.2 to form the tetrameric pore of K(ATP) channels in cardiomyocytes. To explore this possibility, cultured rat cardiomyocytes were examined for an inhibition of K(ATP) current by overexpression of pore loop-mutated (inactive) KIR6.x. Bicistronic plasmids were constructed encoding loop-mutated (AFA or SFG for GFG) rat KIR6.x followed by EGFP. In ventricular myocytes, the overexpression of KIR6.1SFG-pIRES(2)-EGFP or KIR6.2AFA-pIRES(2)-EGFP DNA caused, after 72 h, a major decrease of K(ATP) current density of 85.8% and 82.7%, respectively (P < 0.01), relative to EGFP controls (59 +/- 9 pA/pF). In atrial myocytes, overexpression of these pore-mutated KIR6.x by 6.0-fold and 10.6-fold, as assessed by quantitative immunohistochemistry, caused a decrease of K(ATP) current density of 73.7% and 58.5%, respectively (P < 0.01). Expression of wild-type rat KIR6.2 increased the ventricular and atrial K(ATP) current density by 58.3% and 42.9%, respectively (P < 0.01), relative to corresponding EGFP controls, indicating a reserve of SUR to accommodate increased KIR6.x trafficking to the sarcolemma. The results favor the view that KIR6.1 may associate with KIR6.2 to form heterotetrameric pores of native K(ATP) channels in cardiomyocytes.