Influence of gender on ethanol-induced ventricular myocyte contractile depression in transgenic mice with cardiac overexpression of alcohol dehydrogenase

Acute ethanol exposure depresses ventricular contractility and contributes to alcoholic cardiomyopathy in both men and women chronically consuming ethanol. However, a gender-related difference in the severity of myopathy exists with female being more sensitive to ethanol-induced tissue damage. Acetaldehyde (ACA), the major oxidized product of ethanol, has been implicated to play a role in the pathogenesis and gender-related difference of alcoholic cardiomyopathy, possibly due to its direct cardiac effect and interaction with estrogen. This study was designed to compare the effects of cardiac overexpression of alcohol dehydrogenase (ADH), which converts ethanol into ACA, on the cardiac contractile response to ethanol in ventricular myocytes isolated from age-matched adult male and female transgenic (ADH) and wild-type (FVB) mice. Mechanical properties were measured with an IonOptix SoftEdge system. ACA production was assessed by gas chromatography. The ADH myocytes from both genders exhibited similar mechanical properties but a higher efficacy to produce ACA compared to FVB myocytes. Exposure to ethanol (80-640 mg/dl) for 60 min elicited concentration-dependent decrease of cell shortening in both FVB and ADH groups. The ethanol-induced depression on cell shortening was significantly augmented in female but not male ADH group. ADH transgene did not exacerbate the ethanol-induced inhibition of maximal velocity of shortening/relengthening in either gender. In addition, neither ethanol nor ADH transgene affect the duration of shortening and relengthening in male or female mice. These data suggest that females may be more sensitive to ACA-induced cardiac contractile depression than male, which may attribute to the gender-related difference of alcoholic cardiomyopathy.

Anti-oxidant effects of estrogen reduce [Ca2+]i during metabolic inhibition

We previously reported that 17beta-estradiol (betaE2) inhibits the rise in [Ca(2+)](i) and [Na(+)](i) during metabolic inhibition (MI) in mouse cardiomyocytes, but the mechanism has not yet been clarified. Estrogen has been reported to have anti-oxidant properties. We, therefore, have investigated whether interaction with the estrogen receptor (ER) is involved, or whether estrogen reduces free-radical-induced impairment of Na(+)-K(+) ATPase in cardiac myocytes, and whether this effect reduces [Ca(2+)](i) rise. Male mouse ventricular myocytes were studied. Flow cytometry was used with fluo-3 for [Ca(2+)](i) measurement. Dead cells were excluded from analysis by propidium iodide fluorescence. betaE2 reduced the increase in [Ca(2+)](i) during MI even in the presence of the ER blocker tamoxifen. A similar effect on [Ca(2+)](i) was produced by its non-estrogenic isomer, betaE2-estradiol. Other hormones (estrone and estriol) with a phenolic structure also inhibited Ca(2+) overload during MI, but testosterone without the structure did not. The betaE2 effect was attenuated by inhibition of Na(+)-Ca(2+) exchanger (KB-R7943) or Na(+)-K(+) ATPase (low K(+) or ouabain), but not by block of L-type Ca(2+) channel (nifedipine). Tiron (4,5-dihydroxy-1,3-benzenedisulfonic acid), a superoxide scavenger, decreased the rise in [Ca(2+)](i) and abolished the betaE2 effect during MI. We conclude that the acute cardioprotective effect of estrogen during MI may be mediated by an ER-independent anti-oxidant action, which results in improved function of Na(+)-K(+) ATPase.

Anticonvulsant valproic acid inhibits cardiomyocyte differentiation of embryonic stem cells by increasing intracellular levels of reactive oxygen species

BACKGROUND

The anticonvulsant valproic acid (VPA) exerts teratogenic properties and has been demonstrated to cause neural tube defects and malformations of the heart. The effect of VPA on the differentiation of cardiomyocytes from pluripotent murine embryonic stem cells (ES cells) was investigated.

METHODS

Embryoid bodies derived from ES cells were treated with different concentrations of VPA and the differentiation of cardiomyocytes was monitored by immunohistochemical staining for sarcomeric alpha-actinin. Cytotoxicity was evaluated by the use of the dead cell stain SYTOX green. Intracellular levels of reactive oxygen species (ROS) within the tissue were evaluated by the use of the redox-sensitive dye dichlorodihydrofluorescein diacetate (H2DCFDA).

RESULTS

VPA retarded the growth of ES cell-derived embryoid bodies but did not exert cytotoxic effects. The compound dose-dependently inhibited the development of spontaneously beating clusters of cardiomyocytes within embryoid bodies grown from ES cells and reduced the extension of beating areas of cardiac cells. Furthermore, VPA significantly increased ROS levels, indicating that VPA altered the intracellular redox balance. To investigate whether the inhibition of cardiomyocyte differentiation by VPA was owing to increased ROS overwhelming the intracellular antioxidative defense, the compound was coadministered with the free radical scavenger vitamin E.

CONCLUSIONS

This treatment significantly restored cardiomyogenic differentiation, indicating that VPA inhibits cardiomyogenesis of ES cells by increasing intracellular ROS levels.

[Effect of ginkgolide B on L-type calcium current and cytosolic [Ca2+]i in guinea pig ischemic ventricular myocytes]

With whole-cell variant patch-clamp and laser scanning confocal microscope technique, we examined the effect of ginkgolide B (GB) from ginkgo leaves on L-type calcium current and cytosolic [Ca(2+)](i) in guinea pig ischemic ventricular myocytes. The results showed that under normal conditions, at a test voltage of 0 mV, GB had no significant effect on I(Ca,L); and during ischemia, the peak Ca(2+) current reduced by 37.71%, and the I-V curve of I(Ca,L) was shifted upward. 1 micromol/L GB reversed the change induced by ischemia, a result being significantly different from those of the ishemia group (P<0.05).Under control condition, 0.1,1,10 micromol/L GB decreased intracellular calcium concentration by 10.58%, 17.27% and 16.35% (n=12, 12, 10, P<0.01-0.001), respectively. With perfusion of ischemic solution for 12 min, intracellular calcium concentration increased by 20.15%. After a 12 min-perfusion of ischemic solution containing 1 micromol/L nifedipine or 5 mmol/L NiCl2, intracellular calcium concentration increased by 18.18% (P>0.05 vs ischemia) and 11% (P<0.05 vs ischemia), respectively. After 12 min of perfusion with ischemic solution containing 1 micromol/L GB, intracellular calcium concentration increased by 9.6% (P<0.05 vs ischemia). It is shown that GB could reverse the decrease of I(Ca,L) and partially inhibit calcium overload during ischemia.

[Adenosine protects cardiomyocytes from hypoxia/reoxygenation injury]

The aim of this study was to investigate the protective effect of adenosine (ADO) on cardiomyocytes following hypoxia/reoxygenation (H/R) and its molecular mechanism. Primary cultured cardiomyocytes of neonatal rats were divided into two groups, namely H/R (control) and ADO (1.0 micromol/L) groups. The morphologic changes in cardiomyocytes were observed under an inverted phase-contrast microscope. The following parameters of the two groups were determined: lactate dehydrogenase (LDH) activity, intracellular calcium concentration and malondialdehyde (MDA) content. Tumor necrotic factor (TNF-alpha) assay was performed using an ELISA kit and NF-kappaB in the nucleus was analyzed by electrophoretic mobility shift assay (EMSA). The results are as follows: (1) after H/R injury, cardiomyocytes contracted, tending to get round in shape and its pseudopods decreased, while marked morphological changes were not observed in ADO group; (2) LDH leakage maintained at a lower level in ADO group than that in the control group during H/R (both P<0.01); (3) ADO significantly reduced the concentration of calcium in cells and prevented calcium overload during H/R (both P<0.01); (4) ADO markedly reduced the content of MDA during H/R (both P<0.01); (5) ADO inhibited the production of TNF-alpha during H/R (both P<0.01); and (6) ADO down-regulated NF-kappaB binding activity of cardiomyocytes during H/R (both P<0.01) The results suggest that (1) exogenous ADO attenuates H/R injury of cultured cardiomyocytes; (2) exogenous ADO inhibits the production of TNF-alpha after H/R injury; (3) exogenous ADO prevents the activation of NF-kappaB, which may be the molecular mechanism of down-regulation of TNF-alpha expression.

Biodegradable elastomeric scaffolds for soft tissue engineering

Elastomeric copolymers of 1,3-trimethylene carbonate (TMC) and epsilon-caprolactone (CL) and copolymers of TMC and D,L-lactide (DLLA) have been evaluated as candidate materials for the preparation of biodegradable scaffolds for soft tissue engineering. TMC-DLLA copolymers are amorphous and degrade more rapidly in phosphate-buffered saline (PBS) of pH 7.4 at 37 degrees C than (semi-crystalline) TMC-CL copolymers. TMC-DLLA with 20 or 50 mol% TMC loose their tensile strength in less than 5 months and are totally resorbed in 11 months. In PBS, TMC-CL copolymers retain suitable mechanical properties for more than a year. Cell seeding studies show that rat cardiomyocytes and human Schwann cells attach and proliferate well on the TMC-based copolymers. TMC-DLLA copolymers with either 20 or 50 mol% of TMC are totally amorphous and very flexible, making them excellent polymers for the preparation of porous scaffolds for heart tissue engineering. Porous structures of TMC-DLLA copolymers were prepared by compression molding and particulate leaching techniques. TMC-CL (co)polymers were processed into porous two-ply tubes by means of salt leaching (inner layer) and fiber winding (outer layer) techniques. These grafts, seeded with Schwann cells, will be used as nerve guides for the bridging of large peripheral nerve defects.

Mechanism of pacemaking in I(K1)-downregulated myocytes

Biological pacemakers were recently created by genetic suppression of inward rectifier potassium current, I(K1), in guinea pig ventricular cells. We simulated these cells by adjusting I(K1) conductance in the Luo-Rudy model of the guinea pig ventricular myocyte. After 81% I(K1) suppression, the simulated cell reached steady state with pacemaker period of 594 ms. Pacemaking current is carried by the Na+-Ca2+ exchanger, I(NaCa), which depends on the intracellular calcium concentration [Ca2+]i. This [Ca2+]i dependence suggests responsiveness (increase in rate) to beta-adrenergic stimulation (betaAS), as observed experimentally. Simulations of betaAS demonstrate such responsiveness, which depends on I(NaCa) expression. However, a simultaneous betaAS-mediated increase in the slow delayed rectifier, I(Ks), limits betaAS sensitivity.

Spironolactone and its main metabolite, canrenoic acid, block human ether-a-go-go-related gene channels

BACKGROUND

It has been demonstrated that spironolactone (SP) decreases the QT dispersion in chronic heart failure. In this study, the effects of SP and its metabolite, canrenoic acid (CA), on human ether-a-go-go-related gene (HERG) currents were analyzed.

METHODS AND RESULTS

HERG currents elicited in stably transfected Chinese hamster ovary cells were measured with the whole-cell patch-clamp technique. SP decreased HERG currents in a concentration-dependent manner (IC50=23.0+/-1.5 micromol/L) and shifted the midpoint of the activation curve to more negative potentials (Vh=-13.1+/-3.4 versus -18.9+/-3.6 mV, P<0.05) without modifying the activation and deactivation kinetics. SP-induced block (1 micromol/L) appeared at the range of membrane potentials coinciding with that of channel activation, and thereafter, it remained constant, reaching 24.7+/-3.8% at +60 mV (n=6, P<0.05). CA (0.01 nmol/L to 500 micromol/L) blocked HERG channels in a voltage- and frequency-independent manner. CA at 1 nmol/L shifted the midpoint of the activation curve to -19.9+/-1.8 mV and accelerated the time course of channel activation (tau=1064+/-125 versus 820+/-93 ms, n=11, P<0.01). The envelope of the tail test demonstrated that at the very beginning of the pulses to +40 mV (25 ms), a certain amount of block was apparent (31.3+/-9.9%). CA did not modify the voltage-dependence of HERG channel inactivation (Vh=-60.8+/-5.6 versus -62.9+/-3.1 mV, n=6, P>0.05) or the kinetics of the reactivation process at any potential tested. CA and aldosterone also blocked the native I(Kr) in guinea-pig ventricular myocytes.

CONCLUSIONS

At concentrations reached after administration of therapeutic doses of SP, CA blocked the HERG channels by binding to both the closed and open states of the channel.

Inhibitory effect of natriuretic peptides on aldosterone synthase gene expression in cultured neonatal rat cardiocytes

BACKGROUND

Although previously thought to be synthesized solely in adrenal cortex, we have recently showed that aldosterone is also produced in and the expression of CYP11B2 mRNA was induced in the failing or hypertensive human ventricle. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones with wide biological effects, including inhibition of renin and aldosterone production. We hypothesized that natriuretic peptides reduce the expression of CYP11B2 mRNA in the heart.

METHODS AND RESULTS

To test this hypothesis, we examined whether endogenous or exogenous natriuretic peptides reduce the expression of CYP11B2 mRNA using real-time reverse transcription-polymerase chain reaction. By using HS 142-1, a functional guanylyl cyclase-A type receptor antagonist, we showed that angiotensin II (AngII) pretreated with HS 142-1 increased CYP11B2 mRNA expression (1.62+/-0.12-fold, HS 142-1+AngII 10(-7) mol/L versus AngII 10(-7) mol/L alone, P<0.0001). The treatment with exogenous (10(-6) mol/L) ANP and BNP reduced CYP11B2 mRNA expression (ANP, P=0.0042; BNP, P=0.0012).

CONCLUSIONS

We showed that endogenous and exogenous natriuretic peptides reduced CYP11B2 mRNA expression in cultured neonatal rat cardiocytes. This may inhibit the cardiac renin-angiotensin-aldosterone system by suppressing the gene expression of CYP11B2 and restraining cardiac hypertrophy and fibrosis.

Induction of coxsackievirus-adenovirus-receptor expression during myocardial tissue formation and remodeling: identification of a cell-to-cell contact-dependent regulatory mechanism

BACKGROUND

The coxsackievirus-adenovirus receptor (CAR) was cloned as a receptor for both viruses, but its primary biological functions and regulatory mechanisms are unknown. CAR was low in healthy adult myocardium, whereas strong CAR reexpression was observed in human dilated cardiomyopathy. The molecular mechanisms of CAR induction in cardiomyocytes are unknown.

METHODS AND RESULTS

We report on CAR regulation during development, CAR induction after myocardial infarction, and cell-to-cell contact-dependent CAR regulation in the rat. The high CAR expression during development in various organs decreased up to 190-fold after birth. After infarction resulting in severe cardiac dysfunction (dP/dt(max), -53%; dP/dt(min), -58%; left ventricular pressure, -45%), CAR was induced locally in cardiomyocytes of the infarct zone, where it was also expressed by capillary-like CD31+ structures and CD18+ interstitial cells, whereas it remained confined to subendothelial layers of arterioles and venules. In cultured cardiomyocytes, endothelin-1, cardiotrophin-1, leukemia-inhibiting factor, and cyclic stretch had no effect on CAR, whereas at high versus low cell density, CAR was suppressed up to 10-fold (P=0.006). Conditioned media from low- or high-density cardiomyocytes or cardiofibroblasts had no effect.

CONCLUSIONS

The locally confined CAR upregulation after infarction makes induction by various humoral factors unlikely, because cardiac dysfunction results in high activities of sympathetic and renin-angiotensin systems and cytokines. The cell culture experiments identify a cell-to-cell contact-dependent mechanism of CAR regulation. Further characterization of the signals linking cell-to-cell interactions to CAR gene expression may provide insight into mechanisms and functional consequences of the generalized CAR induction in dilated cardiomyopathy, and of its local induction after myocardial infarction.

Cardioprotective actions of an N-terminal fragment of annexin-1 in rat myocardium in vitro

We have previously shown that the glucocorticoid dexamethasone prevents the cardiodepressant actions of interferon-gamma plus lipopolysaccharide in cardiac tissue in vitro. We now demonstrate that an N-terminal fragment of annexin-1 (Ac2-26, 1 microM), a putative mediator of glucocorticoid actions, completely protects against interferon-gamma+lipopolysaccharide-induced depression of the inotropic response to isoprenaline in rat isolated papillary muscles. However, Ac2-26 does not preserve resting contractile function. Fifteen hours incubation with interferon-gamma+lipopolysaccharide also markedly induced mRNA expression (by real time polymerase chain reaction, PCR) of both the nitric oxide synthase 2 (NOS2) isoform of nitric oxide synthase (by 6.7 +/- 1.7-fold, P < 0.01) and cyclo-oxygenase-2 (by 3.4 +/- 0.6-fold, P < 0.05) in cardiomyocytes. Pretreatment with Ac2-26 (1 microM) prevented the induction of cyclo-oxygenase-2 mRNA, but not NOS2 mRNA, whereas dexamethasone (1 microM) suppressed the expression of both NOS2 mRNA and cyclo-oxygenase-2 mRNA. Co-incubation of dexamethasone with an anti-annexin-1 antibody did not attenuate the suppression of NOS2 mRNA. Thus, Ac2-26 reproduces some, but not all, of the cardioprotective effects of glucocorticoids in vitro in the absence of neutrophils. These protective actions are independent of changes in NOS2 expression.

Hepatocyte growth factor induces GATA-4 phosphorylation and cell survival in cardiac muscle cells

Hepatocyte growth factor (HGF) is released in response to myocardial infarction and may play a role in regulating cardiac remodeling. Recently, HGF was found to inhibit the apoptosis of cardiac muscle cells. Because GATA-4 can induce cell survival, the effects of HGF on GATA-4 activity were investigated. Treatment of HL-1 cells or primary adult rat cardiac myocytes with HGF, at concentrations that can be detected in the human serum after myocardial infarction, rapidly enhances GATA-4 DNA-binding activity. The enhanced DNA-binding activity is associated with the phosphorylation of GATA-4. HGF-induced phosphorylation and activation of GATA-4 is abolished by MEK inhibitors or the mutation of the ERK phosphorylation site (S105A), suggesting that HGF activates GATA-4 via MEK-ERK pathway-dependent phosphorylation. HGF enhances the expression of anti-apoptotic Bcl-x(L), and this is blocked by dominant negative mutants of MEK or GATA-4. Forced expression of wild-type GATA-4, but not the GATA-4 mutant (S105A) increases the expression of Bcl-x(L). Furthermore, expression of the GATA-4 mutant (S105A) suppresses HGF-mediated protection of cells against daunorubicin-induced apoptosis. These results demonstrate that HGF protects cardiac muscle cells against apoptosis via a signaling pathway involving MEK/ERK-dependent phosphorylation of GATA-4.

Insulin-like growth factor-II induces hypertrophy of adult cardiomyocytes via two alternative pathways

Insulin-like growth factor (IGF)-II is known to induce hypertrophy of isolated adult rat ventricular cardiomyocytes cultured in the absence of serum. However, it is not known how the growth factor exerts this hypertrophic effect. We show here that IGF-II induces hypertrophy of the cultured cardiomyocytes via two alternative pathways: (1) an IGF-I receptor-dependent pathway, or (2) a lysosome-dependent pathway when the IGF-I receptor-dependent pathway is blocked.

Beta-adrenergic receptor-stimulated apoptosis in cardiac myocytes is mediated by reactive oxygen species/c-Jun NH2-terminal kinase-dependent activation of the mitochondrial pathway

Stimulation of beta-adrenergic receptors (betaARs) causes apoptosis in adult rat ventricular myocytes (ARVMs). The role of reactive oxygen species (ROS) in mediating betaAR-stimulated apoptosis is not known. Stimulation of betaARs with norepinephrine (10 micromol/L) in the presence of prazosin (100 nmol/L) for 24 hours increased the number of apoptotic myocytes as determined by TUNEL staining by 3.6- fold. The superoxide dismutase/catalase mimetics Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin pentachloride (MnTMPyP; 10 micromol/L) and Euk-134 decreased betaAR-stimulated apoptosis by 89+/-6% and 76+/-10%, respectively. Infection with an adenovirus expressing catalase decreased betaAR-stimulated apoptosis by 82+/-15%. The mitochondrial permeability transition pore inhibitor bongkrekic acid (50 micromol/L) decreased betaAR-stimulated apoptosis by 76+/-8%, and the caspase inhibitor zVAD-fmk (25 micromol/L) decreased betaAR-stimulated apoptosis by 62+/-11%. betaAR-stimulated cytochrome c release was inhibited by MnTMPyP. betaAR stimulation caused c-Jun NH2-terminal kinase (JNK) activation, which was abolished by MnTMPyP. Transfection with an adenovirus expressing dominant-negative JNK inhibited betaAR-stimulated apoptosis by 81+/-12%, and the JNK inhibitor SP600125 inhibited both betaAR-stimulated apoptosis and cytochrome c release. Thus, betaAR-stimulated apoptosis in ARVMs involves ROS/JNK-dependent activation of the mitochondrial death pathway.

Insulin-like growth factor-1 exerts Ca2+-dependent positive inotropic effects in failing human myocardium

Myocardial generation of insulin-like growth factor-1 (IGF-1) is altered in hypertrophy and heart failure, but there are no reports on acute functional effects of IGF-1 in human cardiac muscle. We examined inotropic responses and signal transduction mechanisms of IGF-1 in human myocardium. Experiments were performed in isolated trabeculae or cardiomyocytes from 46 end-stage failing hearts. The effect of IGF-1 (0.001 to 0.2 micromol/L) on isometric twitch force (37 degrees C, 1 Hz), intracellular Ca2+ transients (aequorin method), sarcoplasmic reticulum (SR) Ca2+ content (rapid cooling contractures), L-type Ca2+ current (whole-cell voltage clamp), and cAMP concentrations was assessed. In addition, the effects of blocking IGF-1 receptors, phosphoinositide 3-kinase (PI3-kinase), protein kinase C (PKC), or transsarcolemmal Ca2+ entry were tested. IGF-1 exerted concentration-dependent positive inotropic effects (twitch force increased to maximally 133+/-4% of baseline values at 0.1 micromol/L; P<0.05). The IGF-1 receptor antibody alphaIR3 or the PI3-kinase inhibitor wortmannin prevented the functional effects. The inotropic response was paralleled by increases in Ca2+ transients and SR Ca2+ content. IGF-1 (0.1 micromol/L) increased L-type Ca2+ current amplitude by 24+/-7% (P<0.05). Blockade of SR function did not affect the inotropic response to IGF-1. In contrast, L-type Ca2+ channel blockade with diltiazem partially prevented ( approximately 50%) the inotropic response to IGF-1. Inhibition of PKC (GF109203X), Na+-H+ exchange (HOE642), or reverse-mode Na+-Ca2+ exchange (KB-R7943) reduced the response to IGF-1 by approximately 60% to 70%. IGF-1 exerts Ca2+-dependent positive inotropic effects through activation of IGF-1 receptors and a PI3-kinase-dependent pathway in failing human myocardium. The increased [Ca2+]i with IGF-1 originates from both enhanced L-type Ca2+ currents and enhanced Na+-H+ exchange-dependent reverse-mode Na+-Ca2+ exchange. These nongenomic functional effects of IGF-1 may be of clinical relevance.

Mechanistically distinct steps in the mitochondrial death pathway triggered by oxidative stress in cardiac myocytes

Oxidative stress plays an important role in the pathogenesis of cardiovascular diseases. In the present study, we characterize three distinct phases of the H2O2-induced response, which leads to loss of mitochondrial membrane potential (DeltaPsi(m)) and subsequent cell death in cultured cardiac myocytes. (1) Priming: After H2O2 exposure (100 micromol/L), cells maintain a constant DeltaPsi(m) for the cell-to-cell specific latency but at the same time undergo progressive changes in inner mitochondrial membrane structure (swelling and loss of cristae by electron microscopy). An increase of matrix calcium is required, but not sufficient, for this process. (2) Depolarization: Priming is followed by sudden depolarization of DeltaPsi(m), which is mediated by mitochondrial permeability transition pore opening, as evidenced by the concomitant release of calcein from mitochondria. This process is rapid (<4 minutes), complete, and irreversible. The duration of depolarization is constant and does not depend on the length of the priming process in any given cell. (3) Fragmentation: Along with massive mitochondrial swelling and release of cytochrome c into the cytoplasm, cells undergo surface membrane alterations, such as exposure of phosphatidylserine and eventual loss of membrane integrity and cellular fragmentation. Thus, oxidant stress elicits reproducible and stereotyped responses in cardiac cells. The priming phase, during which mitochondria undergo major ultrastructural alterations but remain functional, represents a particularly attractive target for intervention in the prevention of cell death.

Load-induced transcriptional activation of c-jun in rat myocardium: regulation by myocyte enhancer factor 2

The increased expression of immediate-early genes is a key feature of the myocardial response to hypertrophic stimuli. In this study, we investigated whether pressure overload or phenylephrine treatment stimulated myocyte enhancer factor 2 (MEF2)-dependent transcriptional activation of c-jun in cardiac myocytes. Western blotting and immunohistochemical analysis of rat myocardium demonstrated that p70(MEF2) is highly expressed in the rat heart and is predominantly located at the nuclei of cardiac myocytes. Electrophoretic mobility shift assays of myocardial nuclear extracts revealed a consistent DNA binding activation of MEF2 after 1 and 2 hours of pressure overload. We further showed that pressure overload induced a progressive nuclear translocation and activation of extracellular signal-regulated kinase 5 (ERK5). Coimmunoprecipitation and in vitro kinase assays indicated that the activation of ERK5 was paralleled by increased association of ERK5/p70(MEF2) and by enhanced ability of ERK5 to phosphorylate p70(MEF2). Experiments with in vivo transfection of the left ventricle with the c-jun promoter reporter gene showed that pressure overload induced a consistent increase of c-jun transcriptional activity in the rat myocardium. Rendering the MEF2 site of the c-jun plasmid inactive by mutation abolished the load-induced activation of the c-jun promoter reporter gene. Mutation of the MEF2 site also abolished the phenylephrine-induced c-jun promoter activation in neonatal rat ventricular myocytes. In addition, we demonstrated that neonatal rat ventricular myocyte transfection with ERK5-antisense oligodeoxynucleotide inhibited the phenylephrine-induced c-jun promoter activation. These findings identify MEF2 as a potential regulator of c-jun transactivation and suggest that ERK5 might be an important mediator of MEF2 and c-jun promoter activation in response to hypertrophic stimuli in cardiac myocytes.

Mimicking phosphorylation of alphaB-crystallin on serine-59 is necessary and sufficient to provide maximal protection of cardiac myocytes from apoptosis

AlphaB-crystallin (alphaBC), a small heat shock protein expressed in high levels in the heart, is phosphorylated on Ser-19, 45, and 59 after stress. However, it is not known whether alphaBC phosphorylation directly affects cell survival. In the present study, constructs were prepared that encode forms of alphaBC harboring Ser to Ala (blocks phosphorylation) or Ser to Glu (mimics phosphorylation) mutations at positions 19, 45, and 59. The effects of each form on apoptosis of cultured cardiac myocytes after hyperosmotic or hypoxic stress were assessed. Compared with controls, cells that expressed alphaBC with Ser to Ala substitutions at all three positions, alphaBC(AAA), exhibited more stress-induced apoptosis. Cells expressing either alphaBC(AAE) or (EEE) exhibited 3-fold less apoptosis than cells expressing alphaBC(AAA), indicating that phosphorylation of Ser-59 confers protection. alphaBC is known to bind to procaspase-3 and to decrease caspase-3 activation. Compared with cells expressing alphaBC(AAA), the activation of caspase-3 was decreased by 3-fold in cells expressing alphaBC(AAE). These results demonstrate that mimicking the phosphorylation of alphaBC on Ser-59 is necessary and sufficient to confer caspase-3 inhibition and protection of cardiac myocytes against hyperosmotic or hypoxic stress. These findings provide direct evidence that alphaBC(S59P) contributes to the cardioprotection observed after physiologically relevant stresses, such as transient hypoxia. Identifying the targets of alphaBC(S59P) will reveal important details about the mechanism underlying the cytoprotective effects of this small heat shock protein.

Human MYO18B, a novel unconventional myosin heavy chain expressed in striated muscles moves into the myonuclei upon differentiation

We have characterized a novel unconventional myosin heavy chain, named MYO18B, that appears to be expressed mainly in human cardiac and skeletal muscles and, at lower levels, in testis. MYO18B transcript is detected in all types of striated muscles but at much lower levels compared to class II sarcomeric myosins, and it is up regulated after in vitro differentiation of myoblasts into myotubes. Phylogenetic analysis shows that this myosin belongs to the recently identified class XVIII, however, unlike the other member of this class, it seems to be unique to Vertebrate since it contains two large amino acid domains of unknown function at the N and C-termini. Immunolocalization of MYO18B protein in skeletal muscle cells shows that this myosin heavy chain is located in the cytoplasm of undifferentiated myoblasts. After in vitro differentiation into myotubes, a fraction of this protein is accumulated in a subset of myonuclei. This nuclear localization was confirmed by immunofluorescence experiments on primary cardiomyocytes and adult muscle sections. In the cytoplasm MYO18B shows a punctate staining, both in cardiac and skeletal fibers. In some cases, cardiomyocytes show a partial sarcomeric pattern of MYO18B alternating that of alpha-actinin-2. In skeletal muscle the cytoplasmic MYO18B results much more evident in the fast type fibers.

The effect of myosin light chain 2 dephosphorylation on Ca2+ -sensitivity of force is enhanced in failing human hearts

OBJECTIVE

Phosphorylation of the myosin light chain 2 (MLC-2) isoform expressed as a percentage of total MLC-2 was decreased in failing (21.1+/-2.0%) compared to donor (31.9+/-4.8%) hearts. To assess the functional implications of this change, we compared the effects of MLC-2 dephosphorylation on force development in failing and non-failing (donor) human hearts.

METHODS

Cooperative effects in isometric force and rate of force redevelopment (K(tr)) were studied in single Triton-skinned human cardiomyocytes at various [Ca(2+)] before and after protein phosphatase-1 (PP-1) incubation.

RESULTS

Maximum force and K(tr) values did not differ between failing and donor hearts, but Ca(2+)-sensitivity of force (pCa(50)) was significantly higher in failing myocardium (Deltap Ca(50)=0.17). K(tr) decreased with decreasing [Ca(2+)], although this decrease was less in failing than in donor hearts. Incubation of the myocytes with PP-1 (0.5 U/ml; 60 min) decreased pCa(50) to a larger extent in failing (0.20 pCa units) than in donor cardiomyocytes (0.10 pCa units). A decrease in absolute K(tr) values was found after PP-1 in failing and donor myocytes, while the shape of the K(tr)-Ca(2+) relationships remained unaltered.

CONCLUSIONS

Surprisingly, the contractile response to MLC-2 dephosphorylation is enhanced in failing hearts, despite the reduced level of basal MLC-2 phosphorylation. The enhanced response to MLC-2 dephosphorylation in failing myocytes might result from differences in basal phosphorylation of other thin and thick filament proteins between donor and failing hearts. Regulation of Ca(2+)-sensitivity via MLC-2 phosphorylation may be a potential compensatory mechanism to reverse the detrimental effects of increased Ca(2+)-sensitivity and impaired Ca(2+)-handling on diastolic function in human heart failure.

Calcineurin transgenic mice have mitochondrial dysfunction and elevated superoxide production

Introduction of the constitutively active calcineurin gene into neonatal rat cardiomyocytes by adenovirus resulted in decreased mitochondrial membrane potential (P < 0.05). Infection of H9c2 cells with calcineurin adenovirus resulted in increased superoxide production (P < 0.001). Transgenic mice with cardiac-specific expression of a constitutively active calcineurin cDNA (CalTG mice) exhibit a two- to threefold increase in heart size that progresses to heart failure. We prepared mitochondria enriched for the subsarcolemmal population from the hearts of CalTG mice and transgene negative littermates (control). Intact, well-coupled mitochondria prepared from one to two mouse hearts at a time yielded sufficient material for functional studies. Mitochondrial oxygen consumption was measured with a Clark-type oxygen electrode with substrates for mitochondrial complex II (succinate) and complex IV [tetramethylpentadecane (TMPD)/ascorbate]. CalTG mice exhibited a maximal rate of electron transfer in heart mitochondria that was reduced by approximately 50% (P < 0.002) without a loss of respiratory control. Mitochondrial respiration was unaffected in tropomodulin-overexpressing transgenic mice, another model of cardiomyopathy. Western blotting for mitochondrial electron transfer subunits from mitochondria of CalTG mice revealed a 20-30% reduction in subunit 3 of complex I (ND3) and subunits I and IV of cytochrome oxidase (CO-I, CO-IV) when normalized to total mitochondrial protein or to the adenine nucleotide transporter (ANT) and compared with littermate controls (P < 0.002). Impaired mitochondrial electron transport was associated with high levels of superoxide production in the CalTG mice. Taken together, these data indicate that calcineurin signaling affects mitochondrial energetics and superoxide production. The excessive production of superoxide may contribute to the development of cardiac failure.

Microtubules mobility affects the modulation of L-type I(Ca) by muscarinic and beta-adrenergic agonists in guinea-pig cardiac myocytes

To investigate the interaction of cytoskeleton with the receptor modulation of ionic currents, we studied the effect of muscarinic and beta-adrenergic stimulation in adult guinea-pig ventricular cardiac myocytes treated with paclitaxel and colchicine, two drugs that respectively stabilize or destabilize microtubules. We observed that the stabilization of microtubules with paclitaxel (1 microM for 1-4 h) did not markedly affect either the kinetics of I(Ca), or the stimulatory effect of isoproterenol (Iso, 1 microM); however paclitaxel significantly blunted the response to carbachol (CCh, 1 microM). In agreement with the electrophysiological measurements, Iso induced a similar enhancement of intracellular cAMP levels in both control and paclitaxel-treated cells, while the response to CCh 1 microM was significantly reduced in paclitaxel-treated cells. The reduction of muscarinic response induced by paclitaxel was also evident in atrial cells, in which the stimulation of I(KACh) by CCh 1 microM was reduced to about 10%. Compared to the muscarinic response, paclitaxel did not have significant effect on the purinergic (adenosine 1-10 microM) modulation of I(Ca). In contrast to paclitaxel, in colchicine-treated cells, I(Ca) was not enhanced by beta-adrenergic stimulation, but instead reduced by CCh, even in the absence of previous stimulation. In conclusion, our data suggest that microtubule stabilization significantly affects the muscarinic modulation of I(Ca), by interacting with the receptor or the G-protein rather than on the intracellular signaling cascade.

Ethanol reduces cardiac myocyte function through activation of the nitric oxide-cyclic GMP pathway

We tested the hypothesis that low-dose ethanol would reduce cardiac myocyte function through increased production in the nitric oxide/cyclic GMP signal transduction pathway, rather than reduced degradation. Ventricular myocytes were isolated from the hearts of 9 rabbits. Myocyte function was studied using a video-edge detector and cyclic GMP levels were measured by radioimmunoassay. Cells were administered 5 and 10 mmol/l ethanol alone or after 10(-6) mol/l N(G)-nitro-L-arginine methyl ester (L-NAME, nitric oxide synthase inhibitor), 10(-6) mol/l 1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one (ODQ, soluble guanylyl cyclase inhibitor) or 10(-5) mol/l zaprinast (cyclic GMP phosphodiesterase inhibitor). Ethanol (10 mmol/l) significantly decreased percent shortening from 10.0 +/- 0.9 to 6.0 +/- 0.2%. Similar decrements occurred in the maximum rate of shortening and relaxation. After L-NAME or ODQ, the decrements in percent shortening, maximum rate of shortening and relaxation caused by ethanol were not significant. After zaprinast, ethanol significantly decreased the maximum rate of shortening and relaxation and percent shortening to 4.3 +/- 0.5. Ethanol (10 mmol/l) significantly increased cyclic GMP from 403 +/- 121 to 529 +/- 128 fmol/10(5) myocytes. Both L-NAME and ODQ lowered cyclic GMP, and ethanol did not affect cyclic GMP after either. Zaprinast raised cyclic GMP, as did its combination with 10 mmol/l ethanol (653 +/- 120). Thus, ethanol both reduced myocyte function and increased cyclic GMP. Blocking nitric oxide production or guanylyl cyclase activity prevent these effects of ethanol, while blocking cyclic GMP degradation did not. This suggests that ethanol acts as a nitric oxide stimulator in ventricular myocytes leading to reduced function and increased cyclic GMP.

Differential sensitivity of atrial and ventricular K(ATP) channels to metabolic inhibition

OBJECTIVE

The aim is to compare the activation of ATP-sensitive potassium channels (K(ATP) channels) in intact and metabolically impaired atrial and ventricular myocytes.

METHODS

The K(ATP) channel current is measured by whole cell and gramicidin-perforated patch clamp recordings in 164 cultured neonate rat cardiomyocytes.

RESULTS

In whole cell recordings with 84 micromol/l ADP in pipette, spontaneous activity is significantly higher in atrium than ventricle, and EC(50) for the K(ATP) channel opener diazoxide is 0.13 micromol/l (atrium) versus 3.1 micromol/l (ventricle). With an ATP-regenerating system in pipette, EC(50) for diazoxide is 19.7 micromol/l (atrium) versus 54.9 micromol/l (ventricle). In gramicidin-perforated patch recordings, atrial myocytes respond significantly to 100 nmol/l of the mitochondrial protonophore CCCP, while ventricular myocytes do not. EC(50) for diazoxide is 129 micromol/l (atrium) versus >2500 micromol/l (ventricle) for myocytes exposed to CCCP, and 676 versus >2500 micromol/l, respectively, without CCCP.

CONCLUSIONS

(1) K(ATP) channels are significantly more sensitive to metabolic inhibition in atrial than ventricular myocytes. (2) Sensitivity of atrium versus ventricle to the channel opener diazoxide increases from 3:1 to > or = 24:1 with ADP or metabolic inhibition. If extended to intact hearts, the results would predict a higher atrial sensitivity to ischemia, and a high sensitivity of the ischemic atrium to K(ATP) channel openers.

Antiapoptotic effect of heat adaptation in cultured cells

Heat adaptation of cultured rat cardiomyoblasts and mouse myoblasts protected these cells from damage produced by staurosporine and attenuated metabolic disturbances. Heat adaptation inhibited apoptotic, but not necrotic cell death. The mechanism of antiapoptotic adaptive protection was not associated with prevention of mitochondrial depolarization, because heat adaptation not only induced depolarization of mitochondria, but also intensified this process under conditions of staurosporine-induced damage.

Role of calcium in metabolic signaling between cardiac sarcoplasmic reticulum and mitochondria in vitro

The role of Ca(2+) as a cytosolic signaling molecule between porcine cardiac sarcoplasmic reticulum (SR) ATPase and mitochondrial ATP production was evaluated in vitro. The Ca(2+) sensitivity of these processes was determined individually and in a reconstituted system with SR and mitochondria in a 0.5:1 protein-to-cytochrome aa(3) ratio. The half-maximal concentration (K(1/2)) of SR ATPase was 335 nM Ca(2+). The ATP synthesis dependence was similar with a K(1/2) of 243 nM for dehydrogenases and 114 nM for overall ATP production. In the reconstituted system, Ca(2+) increased thapsigargin-sensitive ATP production (maximum approximately 5-fold) with minimal changes in mitochondrial reduced nicotinamide adenine dinucleotide (NADH). NADH concentration remained stable despite graded increases in NADH turnover induced over a wide range of Ca(2+) concentrations (0 to approximately 500 nM). These data are consistent with a balanced activation of SR ATPase and mitochondrial ATP synthesis by Ca(2+) that contributes to a homeostasis of energy metabolism metabolites. It is suggested that this balanced activation by cytosolic Ca(2+) is partially responsible for the minimal alteration in energy metabolism intermediates that occurs with changes in cardiac workload in vivo.

Trimetazidine effect on phospholipid synthesis in ventricular myocytes: consequences in alpha-adrenergic signaling

The anti-anginal drug trimetazidine (TMZ) has been shown to increase the synthesis of phospholipids in ventricular myocytes, including phosphatidyl-inositol (PI). This study focused on the consequences of increasing PI metabolism on alpha-adrenergic signaling pathway in cultured rat cardiomyocytes. In the cells treated with TMZ, the synthesis of PI from inositol was largely increased as compared with the control (+55% in 60 min). The stimulation of alpha-adrenergic receptors by phenylephrine (PE) induced a dose-dependent production of inositide phosphates (IPs) by phospholipase C (PLC) activation. However, the amount of available IPs was significantly lower in TMZ-treated cells, in a dose-dependent manner. This effect was observed in the presence and absence of the IP1-phosphatase inhibitor LiCl. The in vitro determination of PLC activity revealed that this effect could not be attributed to the direct inhibition of the enzyme by TMZ. The TMZ-induced reduction of IPs in the PE-stimulated cardiomyocytes should be attributed to the increase of inositol recycling and incorporation in membrane structures, elicited by increased phospholipid synthesis. The consequences of this reduction in IPs availability were investigated on the cardiomyocyte hypertrophy induced by alpha-adrenergic chronic stimulation. Acute stimulation with PE increased protein synthesis (+50%), but this increase was largely prevented by TMZ. In conclusion, TMZ reduces cell available IPs, by accelerating their recycling in membranes as PI. This effect results in a cytoprotection in the pathological process of hypertrophy elicited by chronic alpha-adrenergic stimulation.

[Effect of homocysteine on injury of cardiomyocytes and its signal transduction mechanism]

AIM

To observe the injured effect of homocysteine (HCY) on cardiomyocytes and investigate its signal transduction mechanism as well as the key regulatory link.

METHODS

Cardiomyocytes were isolated from neonatal Wistar rats. After incubation with HCY, the survival rate of cardiomyocytes was determined by trypan blue stained assay, while the apoptosis rate was measured by TUNEL and FCM. Western blot and EMSA were used to tested ERK2 protein phosphorylation and NF-kappaB active expression in cardiomyocytes, respectively.

RESULTS

The survival rate of cardiomyocytes treated with HCY was reduced significantly in dose- and time- dependent manner. It was found that 10(-3) mol/L HCY could increase the apoptosis rate of cardiomyocytes to the peak (7.65%) at 4 h stress. Several HCY levels revealed the strong inhibitory effect on ERK2 protein phosphorylation, especially, 10(-3) mol/L HCY decreased the level of active ERK2 expression to 3.04% of control at 4 h (P < 0.01). NF-kappaB activation was also inhibited significantly by several HCY level for different time in cardiomyocytes.

CONCLUSION

HCY plays an important role in injury of cardiomyocytes and apoptosis is a form of HCY-induced injury to cardiomyocytes. HCY can block ERK2 protein phosphorylation and NF-kappaB activation, which contribute to the injury of cardiomyocytes.

Cardiac overexpression of monocyte chemoattractant protein-1 in transgenic mice mimics ischemic preconditioning through SAPK/JNK1/2 activation

OBJECTIVE AND METHODS

Although a beneficial association between innate immunity and ischemic preconditioning has recently been proposed, the mechanisms responsible for this link are poorly understood. To test the hypothesis that pro-inflammatory cytokines have a beneficial role in the activation of the cell survival pathway mediated by ischemic preconditioning, we have studied transgenic mice with cardiac myocyte specific overexpression of murine monocyte chemoattractant protein-1 (MCP-1). The resistance to ischemia was studied by performing 45-min (with or without injection of the SAPK/JNKs inhibitor D-JNKI1) and 3-day left coronary artery occlusions as well as 45-min left coronary artery occlusion followed by 3 days of reperfusion. In addition, quantitative Western blot analyses for TNF-alpha, and SAPK/JNK1/2, ERK1/2 and p38 activity were performed.

RESULTS

Infarct size, expressed in percent of either the risk area or the left ventricle, was reduced in transgenic mice when compared with control after both, 45-min (14.7+/-2.6% vs. 52.0+/-2.4%; P<0.05) and 45-min occlusion followed by 3 days of reperfusion (23.2+/-1.8% vs. 30.0+/-1.8%; P<0.05) but it was not significantly different for 3-day occlusion. Western blot analyses showed significantly increased levels of TNF-alpha (1.8-fold) and phosphorylated-SAPK/JNK1/2 (1.5-fold) in transgenic hearts. Phosphorylated-ERK1/2, and phosphorylated-p38 levels were unchanged. Immunohistochemistry revealed that in transgenic mice monocytes/macrophages, lymphocytes, and fibroblasts are the source of TNF-alpha, whereas myocytes have increased phosphorylated-SAPK/JNK1/2 levels. In addition, injection of the SAPK/JNKs inhibitor D-JNKI1 partially abrogated the cardioprotective effect observed in untreated transgenic mice.

CONCLUSION

Overexpression of MCP-1 by cardiomyocytes causes chronic infiltration and activation of leukocytes, resulting in elevated TNF-alpha secretion and SAPK/JNK1/2 activation. The activation of this pathway is in part responsible for the preconditioning effect of MCP-1 overexpression. These results show a possible beneficial link between innate immunity and ischemic preconditioning through MAP-kinase activation.

Increased susceptibility of hypertrophied hearts to ischemic injury

Cardiac hypertrophy is an adaptive response that compensates for increased workload by normalizing wall stress and preserving cardiac contractile function. In advanced stages, however, clinical and experimental studies have shown that when the high workload is maintained, hypertrophy progresses to ventricular dilatation, contractile dysfunction, and decreased tolerance to ischemia/reperfusion. Development of hypertrophy is accompanied by distinct qualitative and quantitative changes in contractile protein expression and isoform switching, cytosolic calcium regulation, and substrate delivery and use. We have focused our investigations on changes in substrate delivery and capillary density in pressure overload hypertrophy and on the effects that these changes have on tolerance to ischemia/reperfusion. This report summarizes our work in this area using a model of aortic banding in 10-day-old rabbits, which exhibits the same pattern of concentric hypertrophy early, followed by ventricular dilatation and contractile dysfunction that is clinically apparent.

Direct and cross-protective effects of heat adaptation in cultured cells

The model of 6-day dosed heat adaptation was developed on cultured rat H9c2 cardiomyoblasts and mouse C2c12 myoblasts. Heat adaptation produced a direct protective effect and increased the resistance of cells to heat shock. The cross-protective effect of heat adaptation was manifested in an increase in the resistance to staurosporine-induced damage. Heat adaptation did not protect cells from oxidative damage. As differentiated from adaptation of the organism, heat adaptation produces direct and cross-protective effects that develop due to the action of environmental factors in the absence of neurohumoral agents. It should be emphasized that the cross-protective effect of heat adaptation in cultured cells is specific to a certain type of damage.

Is there an A-type K+ current in guinea pig ventricular myocytes?

A unique transient outward K(+) current (I(to)) has been described to result from the removal of extracellular Ca(2+) from ventricular myocytes of the guinea pig (15). This study addressed the question of whether this current represented K(+)-selective I(to) or the efflux of K(+) via L-type Ca(2+) channels. This outward current was inhibited by Cd(2+), Ni(2+), Co(2+), and La(3+) as well as by nifedipine. All of these compounds were equally effective inhibitors of the L-type Ca(2+) current. The current was not inhibited by 4-aminopyridine. Apparent inhibition of the outward current by extracellular Ca(2+) was shown to result from the displacement of the reversal potential of cation flux through L-type Ca(2+) channels. The current was found not to be K(+) selective but also permeant to Cs(+). The voltage dependence of inactivation of the outward current was identical to that of the L-type Ca(2+) current. It is concluded that extracellular Ca(2+) does not mask an A-type K(+) current in guinea pig ventricular myocytes.

Morphological and stereological characteristics of myocardial remodeling in aged spontaneously hypertensive SHR rats

Myocardial remodeling in SHR rats with age-related hypertrophy was characterized by elimination of cardiomyocytes, their hypertrophy, and marked increase in the volume of the connective tissue. The count of cardiomyocytes with contracture injuries and subsegmental contractures increased, and pronounced perivascular and interstitial sclerosis developed in the hypertrophic myocardium of SHR rats. Damage to the microcirculatory bed manifested in degenerative changes and destruction of some endotheliocytes. Signs of atypical intracellular regeneration in myofibrils and impairment of their longitudinal orientation were revealed in cardiomyocytes in the late stage of compensatory hypertrophy.

Matrix metalloproteinases and coronary artery diseases

Matrix metalloproteinases (MMPs) play an important role in cardiovascular remodeling by degrading the extracellular matrix. Enhanced MMP expression has been detected in the atherosclerotic plaque, and activation of MMPs appears to be involved in the vulnerability of the plaque. Circulating MMP levels are elevated in patients with acute myocardial infarction and unstable angina. Increased MMP expression is also observed after coronary angioplasty, which is related to late loss index after the procedure. These observations suggest that MMP expression may be not only related to instability of the plaque, but also to the formation of restenotic lesions. The development of therapeutic drugs targeted specifically against MMPs may be useful in the prevention of atherosclerotic lesion development, plaque rupture, and restenosis.

A functional activating protein 1 (AP-1) site regulates matrix metalloproteinase 2 (MMP-2) transcription by cardiac cells through interactions with JunB-Fra1 and JunB-FosB heterodimers

Enhanced synthesis of a specific matrix metalloproteinase, MMP-2, has been demonstrated in experimental models of ventricular failure and in cardiac extracts from patients with ischaemic cardiomyopathy. Cultured neonatal rat cardiac fibroblasts and myocytes were used to analyse the determinants of MMP-2 synthesis, including the effects of hypoxia. Culture of rat cardiac fibroblasts for 24 h in 1% oxygen enhanced MMP-2 synthesis by more than 5-fold and augmented the MMP-2 synthetic responses of these cells to endothelin-1, angiotensin II and interleukin 1beta. A series of MMP-2 promoter-luciferase constructs were used to map the specific enhancer element(s) that drive MMP-2 transcription in cardiac cells. Deletion studies mapped a region of potent transactivating function within the 91 bp region from -1433 to -1342 bp, the activity of which was increased by hypoxia. Oligonucleotides from this region were cloned in front of a heterologous simian-virus-40 (SV40) promoter and mapped the enhancer activity to a region between -1410 and -1362 bp that included a potential activating protein 1 (AP-1)-binding sequence, C(-1394)CTGACCTCC. Site-specific mutagenesis of the core TGAC sequence (indicated in bold) eliminated the transactivating activity within the -1410 to -1362 bp sequence. Electrophoretic mobility shift assays (EMSAs) using the -1410 to -1362 bp oligonucleotide and rat cardiac fibroblast nuclear extracts demonstrated specific nuclear-protein binding that was eliminated by cold competitor oligonucleotide, but not by the AP-1-mutated oligonucleotide. Antibody-supershift EMSAs of nuclear extracts from normoxic rat cardiac fibroblasts demonstrated Fra1 and JunB binding to the -1410 to -1362 bp oligonucleotide. Nuclear extracts isolated from hypoxic rat cardiac fibroblasts contained Fra1, JunB and also included FosB. Co-transfection of cardiac fibroblasts with Fra1-JunB and FosB-JunB expression plasmids led to significant increases in transcriptional activity. These studies demonstrate that a functional AP-1 site mediates MMP-2 transcription in cardiac cells through the binding of distinctive Fra1-JunB and FosB-JunB heterodimers. The synthesis of MMP-2 is widely considered, in contrast with many members of the MMP gene family, to be independent of the AP-1 transcriptional complex. This report is the first demonstration that defined members of the Fos and Jun transcription-factor families specifically regulate this gene under conditions relevant to critical pathophysiological processes.

Ageing exacerbates the cardiotoxicity of hydrogen peroxide through the Fenton reaction in rats

Reactive oxygen species (ROS) are involved in the post-ischemic reperfusion syndrome of the myocardium. Moreover, ageing is associated with an increased cardiac sensitivity to both ischemia and reperfusion. The aim of the present study was to determine whether the lower tolerance of aged hearts to reperfusion could be due to an increased sensitivity to the ROS that are produced during the early phase of reperfusion. For this purpose isolated perfused hearts from adult (4 months) and aged (24 months) rats were perfused with a buffer containing 150 microM of hydrogen peroxide (H(2)O(2)) in presence or absence of deferoxamine mesylate (150 microM), an iron chelator. H(2)O(2) perfusion was continued until left ventricular developed pressure had decreased up to 20% of its initial value. Ageing led to a significant reduction of the duration of the H(2)O(2) perfusion required for inducing a 80% functional alteration. Although deferoxamine did not affect this parameter in adult rats, it significantly increased the duration of H(2)O(2)-perfusion in senescent hearts (control: 14.0+/-0.9 min vs. deferoxamine: 18.1+/-1.0, P<0.05). Similarly, ageing aggravated cardiac contracture induced by H(2)O(2)-perfusion. Again, deferoxamine, which had no effect on this parameter in young adult hearts, significantly reduced the contracture of senescent rat hearts. To conclude, our data clearly show that ageing is associated with an increased sensitivity of the myocardium to hydrogen peroxide, which is partly reversed by iron chelation. These results suggest that the iron-catalyzed Fenton reaction producing hydroxyl radicals might be greater in hearts from senescent rats than in hearts from young adults.

Angiotensin II effects on STAT3 phosphorylation in cardiomyocytes: evidence for Erk-dependent Tyr705 dephosphorylation

Experiments were performed to define the basis for negative regulation of STAT3 activation (i.e., Tyr705 phosphorylation) by angiotensin II in cardiomyocytes. Treatment of cardiomyocytes with angiotensin II resulted in rapid and sustained phosphorylation of STAT3 on Ser727; in contrast, STAT3 Tyr705 phosphorylation was decreased, with dephosphorylation being most pronounced at 30 minutes. Angiotensin II-induced STAT3 Tyr705 dephosphorylation was not prevented by inhibiting protein synthesis, but was blocked by vanadate or the MEK inhibitor PD98059. PD98059 was found to inhibit angiotensin II-induced Erk activation and STAT3 Ser727 phosphorylation. Angiotensin II also attenuated LIF-induced STAT3 Tyr705 phosphorylation, and this effect could be blocked with PD89059. These results are consistent with Erk-mediated STAT3 Ser727 phosphorylation leading to STAT3 Tyr705 dephosphorylation, and accounting for angiotensin II-mediated STAT3 inhibition in cardiomyocytes. We propose that Erk serves as a scaffolding protein in recruiting either a protein tyrosine or MAP kinase phosphatase to STAT3.

Regional upregulation of Kv2.1-encoded current, IK,slow2, in Kv1DN mice is abolished by crossbreeding with Kv2DN mice

Overexpression of a truncated Kv1.1 channel transgene in the heart (Kv1DN) resulted in mice with a prolonged action potential duration due to marked attenuation of a rapidly activating, slowly inactivating potassium current (I(K,slow1)) in ventricular myocytes. Optical mapping and programmed electrical stimulation revealed inducible ventricular tachycardia due to spatial dispersion of repolarization and refractoriness. Here we show that a delayed rectifier with slower inactivation kinetics (I(K,slow2)) was selectively upregulated in Kv1DN cardiocytes. This electrical remodeling was spatially restricted to myocytes derived from the apex of the left ventricle. Biophysical and pharmacological studies of I(K,slow2) indicate that it resembles Kv2-encoded currents. Northern blot analyses and real-time PCR revealed upregulation of Kv2.1 transcript in Kv1DN mice. Crossbreeding of Kv1DN mice with mice expressing a truncated Kv2.1 polypeptide (Kv2DN) eliminated I(K,slow2). In summary, our data indicate that the spatially restrictive upregulation of Kv2.1-encoded currents underlies the increased dispersion of the repolarization observed in Kv1DN mice.

Effect of glucose and fatty acid availability on neonatal and adult heart contractility

Changes in contractility of newborn and adult hearts with different substrates were studied under oxygenation and hypoxia. Under oxygenation, insulin (50 mU/l) and low doses of fatty acids (sodium palmitate 0.12 mM) increased the differential ventricular pressure (DVP), while higher doses of fatty acids (from 0.3 to 1.5 mM) decreased it. However, when both low doses of fatty acids and insulin were added simultaneously, tension development decreased. Hypoxia reduced DVP, and low doses of fatty acids restored cardiac force. The contractile response to extracellular glucose concentrations changed during development, and sensitivity to high doses of fatty acids increased with age. In adult cardiomyocytes, glucose uptake was also inhibited by sodium palmitate under oxygenation when cardiac metabolism is fatty acid dependent, but not under hypoxia, when it consumes carbohydrates. Newborn cardiomyocytes consumed more glucose than adult cells, they did not respond to insulin, and palmitate did not completely inhibit glucose uptake neither under oxygenation nor under hypoxia. Substrate availability modified glucose uptake and contractility by independent mechanisms.

Characterization of sabiporide, a new specific NHE-1 inhibitor exhibiting slow dissociation kinetics and cardioprotective effects

Sabiporide, a new benzoguanidine, was characterized on fibroblasts stably expressing the Na(+)/H(+) exchanger isoforms NHE-1, NHE-2 and NHE-3. 22Na(+) uptake experiments show that this compound possesses a K(i) of 5+/-1.2 x 10(-8) M for NHE-1, and discriminates efficiently between the NHE-1, -2 and -3 isoforms (K(i) for NHE-2: 3+/-0.9 x 10(-6) M, and K(i)>1 mM for NHE-3). Similar K(i) values are obtained on rat cardiomyocytes and human platelets expressing NHE-1 (K(i)'s of 7+/-1 x 10(-9) and 2.7+/-0.4 x 10(-8) M respectively). Interestingly, when compared with amiloride and cariporide, sabiporide inhibition persists even after this molecule had been rinsed out (half time of 7 h for sabiporide, and of 1 and 2.5 min for amiloride and cariporide, respectively), the decay of all these molecules exhibiting a complex multiexponential behavior. Thus, sabiporide, which possesses remarkable cardioprotective properties, is a specific NHE-1 inhibitor possessing unique binding kinetics.

Acute exposure to thyroid hormone increases Na+ current and intracellular Ca2+ in cat atrial myocytes

Whole-cell recording methods and fluorescence microscopy were used to study the effects of acute exposure to thyroid hormone (T(3)) on cat atrial myocytes. Acute exposure ( approximately 5 min) to 10 nM T(3) significantly increased tetrodotoxin (TTX)-sensitive inward Na(+) current (I(Na)) at voltages between -40 and +20 mV. At maximal I(Na) activation (-40 mV) T(3) increased peak I(Na) by 32 %. T(3) had no effect on the time course of I(Na) decay, voltage dependence of activation, inactivation, or recovery from inactivation. Comparable exposures to reverse T(3) (rT(3)) or T(4) had no effect on I(Na). L-type Ca2+ current was unaffected by acute exposure to T(3). T(3)-induced increases in I(Na) were unaffected by 50 microM nickel, a blocker of T-type Ca2+ current. T(3) significantly increased cell shortening (+62 %) and could elicit spontaneous action potentials arising from Ca2+ -mediated after-depolarizations. T(3) (but not rT(3)) significantly increased baseline intracellular Ca2+, release of Ca2+ from sarcoplasmic reticulum (SR) and caffeine (10 mM)-induced release of SR Ca2+. We conclude that acute T(3) exposure increases Na(+) influx via I(Na) and thereby stimulates reverse-mode Na(+)-Ca2+ exchange to increase intracellular Ca2+ content and release. As a result, T(3) increases contraction strength, and can initiate Ca2+ -mediated arrhythmic activity. Acute non-genomic effects of T(3) can contribute to the positive inotropy and sinus (atrial) tachycardia traditionally attributed to chronic, genomic effects of elevated thyroid hormone on atrial muscle.

Abnormal Na channel gating in murine cardiac myocytes deficient in myotonic dystrophy protein kinase

DMPK is a serine/threonine kinase implicated in the human disease myotonic muscular dystrophy (DM). Skeletal muscle Na channels exhibit late reopenings in Dmpk-deficient mice and peak current density is reduced, implicating DMPK in regulation of membrane excitability. Since complete heart block and sudden cardiac death occur in the disease, we tested the hypothesis that cardiac Na channels also exhibit abnormal gating in Dmpk-deficient mice. We made whole cell and cell-attached patch clamp recordings of ventricular cardiomyocytes enzymatically isolated from wild-type, Dmpk+/-, and Dmpk-/- mice. Recordings from membrane patches containing one or a few Na channels revealed multiple Na channel reopenings occurring after the macroscopic Na current had subsided in both Dmpk+/- and Dmpk-/- muscle, but only rare reopenings in wild-type muscle (>3-fold difference, P < 0.05). This resulted in a plateau of non-inactivating Na current in Dmpk-deficient muscle. The magnitude of this plateau current was independent on the magnitude of the test potential from -40 to 0 mV and was also independent of gene dose. Macroscopic Na current density was similar in wild-type and Dmpk-deficient muscle, as was steady-state Na channel gating. Decay of macroscopic currents was slowed in Dmpk-/- muscle, but not in Dmpk+/- or wild-type muscle. Entry into, and recovery from, inactivation were similar at multiple test potentials in wild-type and Dmpk-deficient muscle. Resting membrane potential was depolarized, and action potential duration was significantly prolonged in Dmpk-deficient muscle. Thus in cardiac muscle, Dmpk deficiency results in multiple late reopenings of Na channels similar to those seen in Dmpk-deficient skeletal muscle. This is reflected in a plateau of non-inactivating macroscopic Na current and prolongation of cardiac action potentials.

Protein kinase A phosphorylation of the cardiac calcium release channel (ryanodine receptor) in normal and failing hearts. Role of phosphatases and response to isoproterenol

The cardiac ryanodine receptor/calcium release channel (RyR2) on the sarcoplasmic reticulum (SR) comprises a macromolecular complex that includes a kinase and two phosphatases that are bound to the channel via targeting proteins. We previously found that the RyR2 is protein kinase A (PKA)-hyperphosphorylated in end-stage human heart failure. Because heart failure is a progressive disease that often evolves from hypertrophy, we analyzed the RyR2 macromolecular complex in several animal models of cardiomyopathy that lead to heart failure, including hypertrophy, and at different stages of disease progression. We now show that RyR2 is PKA-hyperphosphorylated in diverse models of heart failure and that the degree of RyR2 PKA phosphorylation correlates with the degree of cardiac dysfunction. Interestingly, we show that RyR2 PKA hyperphosphorylation can be lost during perfusion of isolated hearts due to the activity of the endogenous phosphatases in the RyR2 macromolecular complex. Moreover, infusion of isoproterenol resulted in PKA phosphorylation of RyR2 in rat, indicating that systemic catecholamines can activate phosphorylation of RyR2 in vivo. These studies extend our previous analyses of the RyR2 macromolecular complex, show that both the kinase and phosphatase activities in the macromolecular complex are regulated physiologically in vivo, and suggest that RyR2 PKA hyperphosphorylation is likely a general feature of heart failure.

Attenuation of myocardial ischemia/reperfusion injury in mice with myocyte-specific overexpression of endothelial nitric oxide synthase

OBJECTIVE

The role of nitric oxide (NO) in myocardial ischemia/reperfusion injury remains controversial as both NO donors and NO synthase (NOS) inhibitors have shown to be protective. We generated transgenic (TG) mice that overexpress endothelial NOS (eNOS) exclusively in cardiac myocytes to determine the effects of high cardiac NO levels on ischemia/reperfusion injury and cellular Ca(2+) homeostasis. Wild-type (WT) mice served as controls.

METHODS

Hearts were perfused in vitro and subjected to 20 min of total no-flow ischemia and 30 min of reperfusion (n=5 per group). Left ventricular function, cGMP levels and intracellular Ca(2+) transients (Ca(2+)(i)) were determined.

RESULTS

Left ventricular pressure was reduced (maximum, -33%) and basal cardiac cGMP was increased (twofold) in TG hearts, and the changes were reversed by NOS blockade with N(G)-nitro-L-arginine methyl ester (L-NAME). Relative to baseline, recovery of reperfusion contractile function was significantly better in hearts from TG (98%) than WT (51%) mice, and L-NAME abolished this effect. Heart rate and coronary perfusion pressure were not different between groups. Systolic and diastolic Ca(2+)(i) concentrations were similar in WT and TG hearts, but Ca(2+)(i) overload during early reperfusion tended to be less in TG hearts. Kinetic analysis of pressure curves and Ca(2+)(i) transients revealed a faster left ventricular diastolic relaxation and abbreviated aequorin light signals in TG hearts at baseline and during reperfusion.

CONCLUSIONS

High levels of NO/cGMP strongly protect against ischemia/reperfusion injury, the protection is largely independent of changes in Ca(2+)(i) modulation, but relates to reduced preischemic performance. Myocyte-specific NO augmentation may aid in studies of the (patho)physiological roles of cardiac-derived NO.

Protective effects of SEA0400, a novel and selective inhibitor of the Na+/Ca2+ exchanger, on myocardial ischemia-reperfusion injuries

The Na(+)/Ca(2+) exchanger (NCX) is involved in myocardial ischemia-reperfusion injuries. We examined the effects of 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a potent and selective inhibitor of NCX, on myocardial ischemia-reperfusion injury models. In canine cardiac sarcolemmal vesicles and rat cardiomyocytes, SEA0400 potently inhibited the Na(+)-dependent 45Ca(2+) uptake with an IC(50) value of 90 and 92 nM, compared with 2-[2-[4-(4-nitrobenzyloxy)phenyl]isothiourea (KB-R7943, 7.0 and 9.5 microM), respectively. In rat cardiomyocytes, SEA0400 (1 and 3 microM) attenuated the Ca(2+) paradox-induced cell death. In isolated rat Langendorff hearts, SEA0400 (0.3 and 1 microM) improved the cardiac dysfunction induced by low-pressure perfusion followed by normal perfusion. In anesthetized rats, SEA0400 (0.3 and 1 mg/kg, i.v.) reduced the incidence of ventricular fibrillation and mortality induced by occlusion of the left anterior descending coronary artery followed by reperfusion. These results suggest that SEA0400 is a most potent NCX inhibitor in the heart and that it has protective effects against myocardial ischemia-reperfusion injuries.

Role of nitric oxide in the pathophysiology of heart failure

Nitric oxide (NO) plays critical roles in the regulation of integrated cardiac and vascular function and homeostasis. An understanding of the physiologic role and relative contribution of the three NO synthase isoforms (neuronal--NOS1, inducible--NOS2, and endothelial--NOS3) is imperative to comprehend derangements of the NO signaling pathway in the failing cardiovascular system. Several theories of NO and its regulation have developed as explanations for the divergent observations from studies in health and disease states. Here we review the physiologic and pathophysiologic influence of NO on cardiac function, in a framework that considers several theories of altered NO signaling in heart failure. We discuss the notion of spatial compartmentalization of NO signaling within the myocyte in an effort to reconcile many controversies about derangements in the influences of NO in the heart and vasculature.

Demonstration of functional M3-muscarinic receptors in ventricular cardiomyocytes of adult rats

1 Muscarinic receptors (M-receptors) in the mammalian heart are predominantly of the M(2)-subtype. The aim of this study was to find out whether there might exist an additional myocardial non-M(2)-receptor. 2 For this purpose, we assessed, in adult rat isolated ventricular cardiomyocytes, carbachol-induced [(3)H]-inositol phosphate (IP) formation, and its inhibition by M-receptor antagonists. 3 Carbachol (10(-7)-10(-3) mol l(-1)) increased IP-formation (maximal increase: 14+/-3% above basal, n=6). This increase was significantly enhanced by pretreatment with pertussis toxin (PTX, 250 ng ml(-1) for 20 h): maximal increase was 31+/-5%, pEC(50)-value was 5.08+/-0.33 (n=6). 4 In PTX-pretreated cardiomyocytes 100 micromol l(-1) carbachol-induced IP-formation was inhibited by atropine (pK(i)-value: 8.89+/-0.10) and by the M(3)-receptor antagonist darifenacin (pK(i)-value: 8.67+/-0.23) but was not significantly affected by the M(1)-receptor antagonist pirenzepine (1 micromol l(-1)) or the M(2)-receptor antagonists AF-DX 116 and himbacine (1 micromol l(-1)). 5 In conclusion, in adult rat cardiomyocytes there exists an additional, non-M(2)-receptor, that is coupled to activation of the phospholipase C/IP(3)-pathway; this receptor is very likely of the M(3)-subtype.

Negative feedback regulation of MKK6 mRNA stability by p38alpha mitogen-activated protein kinase

p38 mitogen-activated protein (MAP) kinases play an important role in the regulation of cellular responses to all kinds of stresses. The most abundant and broadly expressed p38 MAP kinase is p38alpha, which can also control the proliferation, differentiation, and survival of several cell types. Here we show that the absence of p38alpha correlates with the up-regulation of one of its upstream activators, the MAP kinase kinase MKK6, in p38alpha(-/-) knockout mice and in cultured cells derived from them. In contrast, the expression levels of the p38 activators MKK3 and MKK4 are not affected in p38alpha-deficient cells. The increase in MKK6 protein concentration correlates with increased amounts of MKK6 mRNA in the p38alpha(-/-) cells. Pharmacological inhibition of p38alpha also up-regulates MKK6 mRNA levels in HEK293 cells. Conversely, reintroduction of p38alpha into p38alpha(-/-) cells reduces the levels of MKK6 protein and mRNA to the normal levels found in wild-type cells. Moreover, we show that the MKK6 mRNA is more stable in p38alpha(-/-) cells and that the 3'untranslated region of this mRNA can differentially regulate the stability of the lacZ reporter gene in a p38alpha-dependent manner. Our data indicate that p38alpha can negatively regulate the stability of the MKK6 mRNA and thus control the steady-state concentration of one of its upstream activators.

Basic biology and pharmacology of the cardiac sarcolemmal sodium/hydrogen exchanger

The Na+/H+ exchangers are a family of membrane proteins that transport sodium and hydrogen ions in opposite directions on a one-to-one basis, and play important roles in regulating cytoplasmic pH and cell volume and mediating sodium reabsorption in various tissues. In the myocardium, the physiological role of the exchanger is pH regulation. However, ischemic activation of the Na+/H+ exchanger in myocardium ultimately leads to intracellular calcium overload, a key mediator of ischemia and reperfusion injury. Studies in a wide variety of animal models have clearly shown that selective inhibition of the sarcolemmal Na+/H+ exchanger can delay progression of injury during ischemia, thereby reducing myocardial necrosis and improving recovery of ventricular function upon reperfusion. Furthermore, this inhibition does not adversely affect either the rate or degree of acidosis during ischemia. To be efficacious, Na+/H+ inhibition must be initiated before or during early ischemia; inhibition only during late ischemia and reperfusion has minimal to no beneficial effects. These preclinical data suggest that selective sodium hydrogen exchanger (NHE) inhibition may provide a new, efficacious treatment for acute myocardial ischemia in appropriate settings in humans.