Silencing of NHE-1 blunts the slow force response to myocardial stretch

Induction of JAK2/STAT3 pathway contributes to protective effects of different therapeutics against myocardial ischemia/reperfusion

Insufficiency in coronary blood supply results in myocardial ischemia and consequently, various clinical syndromes and irreversible injuries. Myocardial damage occurs as a result of two processes during acute myocardial infarction (MI): ischemia and subsequent reperfusion. According to the available evidence, oxidative stress, excessive inflammation reaction, reactive oxygen species (ROS) generation, and apoptosis are crucial players in the pathogenesis of myocardial ischemia/reperfusion (IR) injury. There is emerging evidence that Janus tyrosine kinase 2 (JAK2) signal transducer and activator of the transcription 3 (STAT3) pathway offers cardioprotection against myocardial IR injury. This article reviews therapeutics that exert cardioprotective effects against myocardial IR injury through induction of JAK2/STAT3 pathway.

Disruption of Transverse-Tubules Eliminates the Slow Force Response to Stretch in Isolated Rat Trabeculae

Ventricular muscle has a biphasic response to stretch. There is an immediate increase in force that coincides with the stretch which is followed by a second phase that takes several minutes for force to develop to a new steady state. The initial increase in force is due to changes in myofilament properties, whereas the second, slower component of the stretch response (known as the “slow force response” or SFR) is accompanied by a steady increase in Ca²⁺ transient amplitude. Evidence shows stretch-dependent Ca²⁺ influx during the SFR occurs through some mechanism that is continuously active for several minutes following stretch. Many of the candidate ion channels are located primarily in the t-tubules, which are consequently lost in heart disease. Our aim, therefore, was to investigate the impact of t-tubule loss on the SFR in non-failing cardiac trabeculae in which expression of the different Ca²⁺ handling proteins was not altered by any disease process. For comparison, we also investigated the effect of formamide detubulation of trabeculae on β-adrenergic activation (1 μM isoproterenol), since this is another key regulator of cardiac force. Measurement of intracellular calcium ([Ca²⁺]_i) and isometric stress were made in RV trabeculae from rat hearts before, during and after formamide treatment (1.5 M for 5 min), which on washout seals the surface sarcolemmal t-tubule openings. Results showed detubulation slowed the time course of Ca²⁺ transients and twitch force, with time-to-peak, maximum rate-of-rise, and relaxation prolonged in trabeculae at optimal length (L_o). Formamide treatment also prevented development of the SFR following a step change in length from 90 to 100% L_o, and blunted the response to β-adrenergic activation (1 μM isoproterenol).

Stretch-Induced Biased Signaling in Angiotensin II Type 1 and Apelin Receptors for the Mediation of Cardiac Contractility and Hypertrophy

The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank-Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.

Control of cardiac contraction by sodium: Promises, reckonings, and new beginnings

Several generations of cardiac physiologists have verified that basal cardiac contractility depends strongly on the transsarcolemmal Na gradient, and the underlying molecular mechanisms that link cardiac excitation-contraction coupling (ECC) to the Na gradient have been elucidated in good detail for more than 30 years. In brief, small increases of cytoplasmic Na push cardiac (NCX1) Na/Ca exchangers to increase contractility by increasing the myocyte Ca load. Accordingly, basal cardiac contractility is expected to be physiologically regulated by pathways that modify the cardiac Na gradient and the function of Na transporters. Assuming that this expectation is correct, it remains to be elucidated how in detail signaling pathways affecting the cardiac Na gradient are controlled in response to changing cardiac output requirements. Some puzzle pieces that may facilitate progress are outlined in this short review. Key open issues include (1) whether the concept of local Na gradients is viable, (2) how in detail Na channels, Na transporters and Na/K pumps are regulated by lipids and metabolic processes, (3) the physiological roles of Na/K pump inactivation, and (4) the possibility that key diffusible signaling molecules remain to be discovered.

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

The lack of slow force response in failing rat myocardium: role of stretch-induced modulation of Ca-TnC kinetics

The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20-30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca2+ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca2+ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca2+ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca2+ utilized by TnC was indirectly evaluated using the methods of Ca2+ transient "bump" and "difference curve." The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca2+ transient "bump" and "difference curve." The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca2+ transient decay, evaluated by the methods of "bump" and "difference curve." We conclude that the intracellular mechanisms of Ca2+ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca-TnC interaction.

Epidermal Growth Factor Receptor Silencing Blunts the Slow Force Response to Myocardial Stretch

Background

Myocardial stretch increases force biphasically: the Frank‐Starling mechanism followed by the slow force response (SFR). Based on pharmacological strategies, we proposed that epidermal growth factor (EGF) receptor (EGFR or ErbB1) activation is crucial for SFR development. Pharmacological inhibitors could block ErbB4, a member of the ErbB family present in the adult heart. We aimed to specifically test the role of EGFR activation after stretch, with an interference RNA incorporated into a lentiviral vector (small hairpin RNA [shRNA]‐EGFR).

Methods and Results

Silencing capability of p‐shEGFR was assessed in EGFR‐GFP transiently transfected HEK293T cells. Four weeks after lentivirus injection into the left ventricular wall of Wistar rats, shRNA‐EGFR–injected hearts showed ≈60% reduction of EGFR protein expression compared with shRNA‐SCR–injected hearts. ErbB2 and ErbB4 expression did not change. The SFR to stretch evaluated in isolated papillary muscles was ≈130% of initial rapid phase in the shRNA‐SCR group, while it was blunted in shRNA‐EGFR–expressing muscles. Angiotensin II (Ang II)‐dependent Na+/H+ exchanger 1 activation was indirectly evaluated by intracellular pH measurements in bicarbonate‐free medium, demonstrating an increase in shRNA‐SCR–injected myocardium, an effect not observed in the silenced group. Ang II‐ or EGF‐triggered reactive oxygen species production was significantly reduced in shRNA‐EGFR–injected hearts compared with that in the shRNA‐SCR group. Chronic lentivirus treatment affected neither the myocardial basal redox state (thiobarbituric acid reactive substances) nor NADPH oxidase activity or expression. Finally, Ang II or EGF triggered a redox‐sensitive pathway, leading to p90RSK activation in shRNA‐SCR‐injected myocardium, an effect that was absent in the shRNA‐EGFR group.

Conclusions

Our results provide evidence that specific EGFR activation after myocardial stretch is a key factor in promoting the redox‐sensitive kinase activation pathway, leading to SFR development.

Sevoflurane post-conditioning reduces rat myocardial ischemia reperfusion injury through an increase in NOS and a decrease in phopshorylated NHE1 levels

The protective effects of sevoflurane post-conditioning against myocardial ischemia/reperfusion (I/R) injury (MIRI) have been previously reported. However, the mechanisms responsible for these protective effects remain elusive. In this study, in order to investigate the molecular mechanisms responsible for the protective effects of sevoflurane post-conditioning on isolated rat hearts subjected to MIRI, Sprague-Dawley rat hearts were randomly divided into the following 6 groups: i) the sham-operated control; ii) 2.5% sevoflurane; iii) ischemia/reperfusion (I/R); iv) 2.5% sevoflurane post-conditioning plus I/R; v) 2.5% sevoflurane post-conditioning + NG-nitro-L-arginine methyl ester (L-NAME) plus I/R; and vi) L-NAME plus I/R. The infarct size was measured using 2,3,5-triphenyl tetrazolium chloride (TTC) staining. Additionally, the myocardial nitric oxide (NO), NO synthase (NOS) and nicotinamide adenine dinucleotide (NAD⁺) levels were determined. Autophagosomes and apoptosomes in the myocardium were detected by transmission electron microscopy. The levels of Bcl-2, cleaved caspase-3, Beclin-1, microtubule-associated protein light chain 3 (LC3)-I/II, Na⁺/H⁺ exchanger 1 (NHE1) and phosphorylated NHE1 protein were measured by western blot analysis. NHE1 mRNA levels were measured by reverse transcription-quantitative polymerase chain reaction. Compared with the I/R group, 15 min of exposure to 2.5% sevoflurane during early reperfusion significantly decreased the myocardial infarct size, the autophagic vacuole numbers, the NHE1 mRNA and protein expression of cleaved caspase-3, Beclin-1 and LC3-I/II. Post-conditioning with 2.5% sevoflurane also increased the NO and NOS levels and Bcl-2 protein expression (P<0.05 or P<0.01). Notably, the cardioprotective effects of sevoflurane were partly abolished by the NOS inhibitor, L-NAME. The findings of the present study suggest that sevoflurane post-conditioning protects the myocardium against I/R injury and reduces the myocardial infarct size. The underlying protective mechanisms are associated with the inhibition of mitochondrial permeability transition pore opening, and with the attenuation of cardiomyoctye apoptosis and excessive autophagy. These effects are mediated through an increase in NOS and a decrease in phopshorylated NHE1 levels.

Experimental Study of the Effects of EIPA, Losartan, and BQ-123 on Electrophysiological Changes Induced by Myocardial Stretch

INTRODUCTION AND OBJECTIVES

Mechanical response to myocardial stretch has been explained by various mechanisms, which include Na(+)/H(+) exchanger activation by autocrine-paracrine system activity. Drug-induced changes were analyzed to investigate the role of these mechanisms in the electrophysiological responses to acute myocardial stretch.

METHODS

Multiple epicardial electrodes and mapping techniques were used to analyze changes in ventricular fibrillation induced by acute myocardial stretch in isolated perfused rabbit hearts. Four series were studied: control (n = 9); during perfusion with the angiotensin receptor blocker losartan (1 μM, n = 8); during perfusion with the endothelin A receptor blocker BQ-123 (0.1 μM, n = 9), and during perfusion with the Na(+)/H(+) exchanger inhibitor EIPA (5-[N-ethyl-N-isopropyl]-amiloride) (1 μM, n = 9).

RESULTS

EIPA attenuated the increase in the dominant frequency of stretch-induced fibrillation (control=40.4%; losartan=36% [not significant]; BQ-123=46% [not significant]; and EIPA=22% [P<.001]). During stretch, the activation maps were less complex (P<.0001) and the spectral concentration of the arrhythmia was greater (greater regularity) in the EIPA series: control=18 (3%); EIPA = 26 (9%) (P < .02); losartan=18 (5%) (not significant); and BQ-123=18 (4%) (not significant).

CONCLUSIONS

The Na(+)/H(+) exchanger inhibitor EIPA attenuated the electrophysiological effects responsible for the acceleration and increased complexity of ventricular fibrillation induced by acute myocardial stretch. The angiotensin II receptor antagonist losartan and the endothelin A receptor blocker BQ-123 did not modify these effects.

Cardiac hypertrophy reduction in SHR by specific silencing of myocardial Na+/H+ exchanger

We examined the effect of specific and local silencing of sodium/hydrogen exchanger isoform 1 (NHE1) with a small hairpin RNA delivered by lentivirus (L-shNHE1) in the cardiac left ventricle (LV) wall of spontaneously hypertensive rats, to reduce cardiac hypertrophy. Thirty days after the lentivirus was injected, NHE1 protein expression was reduced 53.3 ± 3% in the LV of the L-shNHE1 compared with the control group injected with L-shSCR (NHE1 scrambled sequence), without affecting its expression in other organs, such as liver and lung. Hypertrophic parameters as LV weight-to-body weight and LV weight-to-tibia length ratio were significantly reduced in animals injected with L-shNHE1 (2.32 ± 0.5 and 19.30 ± 0.42 mg/mm, respectively) compared with L-shSCR-injected rats (2.68 ± 0.06 and 21.53 ± 0.64 mg/mm, respectively). Histochemical analysis demonstrated a reduction of cardiomyocytes cross-sectional area in animals treated with L-shNHE1 compared with L-shSCR (309,81 ± 20,86 vs. 424,52 ± 21 μm2, P < 0.05). Echocardiography at the beginning and at the end of the treatment showed that shNHE1 expression for 30 days induced 9% reduction of LV mass. Also, animals treated with L-shNHE1 exhibited a reduced LV wall thickness without changing LV diastolic dimension and arterial pressure, indicating an increased parietal stress. In addition, midwall shortening was not modified, despite the increased wall tension, suggesting an improvement of cardiac function. Chronic shNHE1 expression in the heart emerges as a possible methodology to reduce pathological cardiac hypertrophy, avoiding potentially undesired effects caused from a body-wide inhibition of NHE1.

Lactosylceramide promotes hypertrophy through ROS generation and activation of ERK1/2 in cardiomyocytes

Hypertrophy is central to several heart diseases; however, not much is known about the role of glycosphingolipids (GSLs) in this phenotype. Since GSLs have been accorded several physiological functions, we sought to determine whether these compounds affect cardiac hypertrophy. By using a rat cardiomyoblast cell line, H9c2 cells and cultured primary neonatal rat cardiomyocytes, we have determined the effects of GSLs on hypertrophy. Our study comprises (a) measurement of [(3)H]-leucine incorporation into protein, (b) measurement of cell size and morphology by immunofluorescence microscopy and (c) real-time quantitative mRNA expression assay for atrial natriuretic peptide and brain natriuretic peptide. Phenylephrine (PE), a well-established agonist of cardiac hypertrophy, served as a positive control in these studies. Subsequently, mechanistic studies were performed to explore the involvement of various signaling transduction pathways that may contribute to hypertrophy in these cardiomyocytes. We observed that lactosylceramide specifically exerted a concentration- (50-100 µM) and time (48 h)-dependent increase in hypertrophy in cardiomyocytes but not a library of other structurally related GSLs. Further, in cardiomyocytes, LacCer generated reactive oxygen species, stimulated the phosphorylation of p44 mitogen activated protein kinase and protein kinase-C, and enhanced c-jun and c-fos expression, ultimately leading to hypertrophy. In summary, we report here that LacCer specifically induces hypertrophy in cardiomyocytes via an "oxygen-sensitive signal transduction pathway."

Endogenous endothelin 1 mediates angiotensin II-induced hypertrophy in electrically paced cardiac myocytes through EGFR transactivation, reactive oxygen species and NHE-1

Emerging evidence supports a key role for endothelin-1 (ET-1) and the transactivation of the epidermal growth factor receptor (EGFR) in angiotensin II (Ang II) action. We aim to determine the potential role played by endogenous ET-1, EGFR transactivation and redox-dependent sodium hydrogen exchanger-1 (NHE-1) activation in the hypertrophic response to Ang II of cardiac myocytes. Electrically paced adult cat cardiomyocytes were placed in culture and stimulated with 1 nmol l(-1) Ang II or 5 nmol l(-1) ET-1. Ang II increased ~45 % cell surface area (CSA) and ~37 % [(3)H]-phenylalanine incorporation, effects that were blocked not only by losartan (Los) but also by BQ123 (AT1 and ETA receptor antagonists, respectively). Moreover, Ang II significantly increased ET-1 messenger RNA (mRNA) expression. ET-1 similarly increased myocyte CSA and protein synthesis, actions prevented by the reactive oxygen species scavenger MPG or the NHE-1 inhibitor cariporide (carip). ET-1 increased the phosphorylation of the redox-sensitive ERK1/2-p90(RSK) kinases, main activators of the NHE-1. This effect was prevented by MPG and the antagonist of EGFR, AG1478. Ang II, ET-1 and EGF increased myocardial superoxide production (187 ± 9 %, 149 ± 8 % and 163.7 ± 6 % of control, respectively) and AG1478 inhibited these effects. Interestingly, Los inhibited only Ang II whilst BQ123 cancelled both Ang II and ET-1 actions, supporting the sequential and unidirectional activation of AT1, ETA and EGFR. Based on the present evidence, we propose that endogenous ET-1 mediates the hypertrophic response to Ang II by a mechanism that involves EGFR transactivation and redox-dependent activation of the ERK1/2-p90(RSK) and NHE-1 in adult cardiomyocytes.

Myocardial mineralocorticoid receptor activation by stretching and its functional consequences

Myocardial stretch triggers an angiotensin II-dependent autocrine/paracrine loop of intracellular signals, leading to reactive oxygen species-mediated activation of redox-sensitive kinases. Based on pharmacological strategies, we previously proposed that mineralocorticoid receptor (MR) is necessary for this stretch-triggered mechanism. Now, we aimed to test the role of MR after stretch by using a molecular approach to avoid secondary effects of pharmacological MR blockers. Small hairpin interference RNA capable of specifically knocking down the MR was incorporated into a lentiviral vector (l-shMR) and injected into the left ventricular wall of Wistar rats. The same vector but expressing a nonsilencing sequence (scramble) was used as control. Lentivirus propagation through the left ventricle was evidenced by confocal microscopy. Myocardial MR expression, stretch-triggered activation of redox-sensitive kinases (ERK1/2-p90(RSK)), the consequent Na(+)/H(+) exchanger-mediated changes in pHi (HEPES-buffer), and its mechanical counterpart, the slow force response, were evaluated. Furthermore, reactive oxygen species production in response to a low concentration of angiotensin II (1.0 nmol/L) or an equipotent concentration of epidermal growth factor (0.1 μg/mL) was compared in myocardial tissue slices from both groups. Compared with scramble, animals transduced with l-shMR showed (1) reduced cardiac MR expression, (2) cancellation of angiotensin II-induced reactive oxygen species production but preservation of epidermal growth factor-induced reactive oxygen species production, (3) cancellation of stretch-triggered increase in ERK1/2-p90(RSK) phosphorylation, (4) lack of stretch-induced Na(+)/H(+) exchanger activation, and (5) abolishment of the slow force response. Our results provide strong evidence that MR activation occurs after myocardial stretch and is a key factor to promote redox-sensitive kinase activation and their downstream consequences.

Effect of SR load and pH regulatory mechanisms on stretch-dependent Ca(2+) entry during the slow force response

When cardiac muscle is stretched, there is an initial inotropic response that coincides with the stretch followed by a slower increase in twitch force that develops over several minutes (the "slow force response", or SFR). Unlike the initial response to stretch, the SFR is produced by an increase in Ca(2+) transient amplitude, but the cellular mechanisms that give rise to the increased transients are still debated. We have examined the relationship between the SFR, intracellular [Ca(2+)] and the inotropic state of right ventricular trabeculae from rat hearts at 37°C. The magnitude of the SFR varied with [Ca(2+)]o and stimulation frequency, so that the SFR was greatest for conditions associated with a reduced SR Ca(2+) content. The SFR was not blocked by the AT1 receptor blocker losartan, but was reduced by SN-6, an inhibitor of reverse mode Na(+)/Ca(2+)-exchange (NCX). The Na(+)/H(+)-exchange (NHE) inhibitor HOE642 had no effect in HCO3(-)-buffered solutions, but blocked 50% of the SFR in HCO3(-)-free solution. Inhibition of HCO3(-) transport by DIDS increased the SFR and made it sensitive to HOE642. The addition of cross-bridge cycle inhibitors (20mM BDM or 20μM blebbistatin) to the superfusate reduced the SFR as monitored by changes in Ca(2+). In HCO3(-)-free conditions, the SFR was associated with a slow acidification that was inhibited by BDM, and by stopping electrical stimulation. These results can be explained by stretch increasing metabolic demand and stimulating Na(+) entry via both NHE and the Na(+)/HCO3(-) transporters. This mechanism provides a novel link between inotropic state and stretch, as well as a way for the cell to compensate for increased acid load. The feedback mechanism between force and Ca(2+) transient amplitude that we describe is also limited by the degree of SR Ca(2+) load.

Mitochondrial NHE1: a newly identified target to prevent heart disease

Mitochondrial damage has been associated with early steps of cardiac dysfunction in heart subjected to ischemic stress, oxidative stress and hypertrophy. A common feature for the mitochondrial deterioration is the loss of the mitochondrial membrane potential (ΔΨ m) with the concomitant irreversible opening of the mitochondrial permeability transition pore (MPTP) which follows the mitochondrial Ca²⁺ overload, and the subsequent mitochondrial swelling. We have recently characterized the expression of the Na⁺/H⁺ exchanger 1 (mNHE1) in mitochondrial membranes. This surprising observation provided a unique target for the prevention of the Ca²⁺-induced MPTP opening, based on the inhibition of the NHE1 m. In this line, inhibition of NHE1 m activity and/or reduction of NHE1 m expression decreased the Ca²⁺-induced mitochondrial swelling and the release of reactive oxygen species (ROS) in isolated cardiac mitochondria and preserved the ΔΨ m in isolated cardiomyocytes. Mitochondrial NHE1 thus represents a novel target to prevent cardiac disease, opening new avenues for future research.

Inhibition of carbonic anhydrase prevents the Na(+)/H(+) exchanger 1-dependent slow force response to rat myocardial stretch

Myocardial stretch is an established signal that leads to hypertrophy. Myocardial stretch induces a first immediate force increase followed by a slow force response (SFR), which is a consequence of an increased Ca(2+) transient that follows the NHE1 Na(+)/H(+) exchanger activation. Carbonic anhydrase II (CAII) binds to the extreme COOH terminus of NHE1 and regulates its transport activity. We aimed to test the role of CAII bound to NHE1 in the SFR. The SFR and changes in intracellular pH (pHi) were evaluated in rat papillary muscle bathed with CO2/HCO3(-) buffer and stretched from 92% to 98% of the muscle maximal force development length for 10 min in the presence of the CA inhibitor 6-ethoxzolamide (ETZ, 100 μM). SFR control was 120 ± 3% (n = 8) of the rapid initial phase and was fully blocked by ETZ (99 ± 4%, n = 6). The SFR corresponded to a maximal increase in pHi of 0.18 ± 0.02 pH units (n = 4), and pHi changes were blocked by ETZ (0.04 ± 0.04, n = 6), as monitored by epifluorescence. NHE1/CAII physical association was examined in the SFR by coimmunoprecipitation, using muscle lysates. CAII immunoprecipitated with an anti-NHE1 antibody and the CAII immunoprecipitated protein levels increased 58 ± 9% (n = 6) upon stretch of muscles, assessed by immunoblots. The p90(RSK) kinase inhibitor SL0101-1 (10 μM) blocked the SFR of heart muscles after stretch 102 ± 2% (n = 4) and reduced the binding of CAII to NHE1, suggesting that the stretch-induced phosphorylation of NHE1 increases its binding to CAII. CAII/NHE1 interaction constitutes a component of the SFR to heart muscle stretch, which potentiates NHE1-mediated H(+) transport in the myocardium.

The Anrep effect: 100 years later

Myocardial stretch elicits a rapid increase in developed force, which is mainly caused by an increase in myofilament calcium sensitivity (Frank-Starling mechanism). Over the ensuing 10-15 min, a second gradual increase in force takes place. This slow force response to stretch is known to be the result of an increase in the calcium transient amplitude and constitutes the in vitro equivalent of the Anrep effect described 100 years ago in the intact heart. In the present review, we will update and discuss what is known about the Anrep effect as the mechanical counterpart of autocrine/paracrine mechanisms involved in its genesis. The chain of events triggered by myocardial stretch comprises 1) release of angiotensin II, 2) release of endothelin, 3) activation of the mineralocorticoid receptor, 4) transactivation of the epidermal growth factor receptor, 5) increased formation of mitochondria reactive oxygen species, 6) activation of redox-sensitive kinases upstream myocardial Na(+)/H(+) exchanger (NHE1), 7) NHE1 activation, 8) increase in intracellular Na(+) concentration, and 9) increase in Ca(2+) transient amplitude through the Na(+)/Ca(2+) exchanger. We will present the experimental evidence supporting each of the signaling steps leading to the Anrep effect and its blunting by silencing NHE1 expression with a specific small hairpin interference RNA injected into the ventricular wall.

The zebrafish as a novel animal model to study the molecular mechanisms of mechano-electrical feedback in the heart

Altered mechanical loading of the heart leads to hypertrophy, decompensated heart failure and fatal arrhythmias. However, the molecular mechanisms that link mechanical and electrical dysfunction remain poorly understood. Growing evidence suggest that ventricular electrical remodeling (VER) is a process that can be induced by altered mechanical stress, creating persistent electrophysiological changes that predispose the heart to life-threatening arrhythmias. While VER is clearly a physiological property of the human heart, as evidenced by "T wave memory", it is also thought to occur in a variety of pathological states associated with altered ventricular activation such as bundle branch block, myocardial infarction, and cardiac pacing. Animal models that are currently being used for investigating stretch-induced VER have significant limitations. The zebrafish has recently emerged as an attractive animal model for studying cardiovascular disease and could overcome some of these limitations. Owing to its extensively sequenced genome, high conservation of gene function, and the comprehensive genetic resources that are available in this model, the zebrafish may provide new insights into the molecular mechanisms that drive detrimental electrical remodeling in response to stretch. Here, we have established a zebrafish model to study mechano-electrical feedback in the heart, which combines efficient genetic manipulation with high-precision stretch and high-resolution electrophysiology. In this model, only 90 min of ventricular stretch caused VER and recapitulated key features of VER found previously in the mammalian heart. Our data suggest that the zebrafish model is a powerful platform for investigating the molecular mechanisms underlying mechano-electrical feedback and VER in the heart.

Mineralocorticoid receptor activation is crucial in the signalling pathway leading to the Anrep effect

The increase in myocardial reactive oxygen species after epidermal growth factor receptor transactivation is a crucial step in the autocrine/paracrine angiotensin II/endothelin receptor activation leading to the slow force response to stretch (SFR). Since experimental evidence suggests a link between angiotensin II or its AT1 receptor and the mineralocorticoid receptor (MR), and MR transactivates the epidermal growth factor receptor, we thought to determine whether MR activation participates in the SFR development in rat myocardium. We show here that MR activation is necessary to promote reactive oxygen species formation by a physiological concentration of angiotensin II (1 nmol l(-1)), since an increase in superoxide anion formation of ~50% of basal was suppressed by blocking MR with spironolactone or eplerenone. This effect was also suppressed by blocking AT1, endothelin (type A) or epidermal growth factor receptors, by inhibiting NADPH oxydase or by targeting mitochondria, and was unaffected by glucocorticoid receptor inhibition. All interventions except AT1 receptor blockade blunted the increase in superoxide anion promoted by an equipotent dose of endothelin-1 (1 nmol l(-1)) confirming that endothelin receptors activation is downstream of AT1. Similarly, an increase in superoxide anion promoted by an equipotent dose of aldosterone (10 nmol l(-1)) was blocked by spironolactone or eplerenone, by preventing epidermal growth factor receptor transactivation, but not by inhibiting glucocorticoid receptors or protein synthesis, suggesting non-genomic MR effects. Combination of aldosterone plus endothelin-1 did not increase superoxide anion formation more than each agonist separately. We found that aldosterone increased phosphorylation of the redox-sensitive kinases ERK1/2-p90RSK and the NHE-1, effects that were eliminated by eplerenone or by preventing epidermal growth factor receptor transactivation. Finally, we provide evidence that the SFR is suppressed by MR blockade, by preventing epidermal growth factor receptor transactivation or by scavenging reactive oxygen species, but it is unaffected by glucocorticoid receptor blockade or protein synthesis inhibition. Our results suggest that MR activation is a necessary step in the stretch-triggered reactive oxygen species-mediated activation of redox-sensitive kinases upstream NHE-1.