Cardiac Contractility Modulation Electrical Signals Normalize Activity, Expression, and Phosphorylation of the Na +Ca2 + Exchanger in Heart Failure (original) (raw)
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European journal of pharmacology, 2017
Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) transporter in cardiac myocytes. Its inhibition could be expected to exert positive inotropic action by accumulation of cytosolic Ca(2+) ([Ca(2+)]i). However, we have observed only a marginal positive inotropic effect upon selective inhibition of NCX, which was enhanced when forward activity was facilitated. Here we attempted to clarify the underlying mechanism of the limited inotropic action of selective NCX inhibition by a novel inhibitor ORM-10962 on canine ventricular myocytes. 1µM ORM-10962 reduced the Ca(2+) content of sarcoplasmic reticulum (SR) when the reverse NCX was favoured, while SR Ca(2+) content was increased by ORM-10962 under conditions favouring the forward activity, like elevation of [Ca(2+)]i. L-type Ca(2+) current (ICa) was not affected by 1µM ORM-10962 in the absence of SR Ca(2+) release, while ICa was suppressed by ORM-10962 during normal Ca(2+) cycling. The apparent degree of forward NCX inhibition was dependent...
Myocardial Function With Reduced Expression of the Sodium-Calcium Exchanger
Journal of Cardiac Failure, 2010
Background-The complete removal of the cardiac sodium-calcium exchanger (NCX1) is associated with embryonic lethality while its overexpression is linked to heart failure. To determine whether or not a reduced expression of NCX1 is compatible with normal heart structure and function, we studied two knockout mouse models with reduced levels of NCX1: a heterozygous global knockout (HG-KO) with a 50% level of NCX1 expression in all myocytes, and a ventricular specific KO (V-KO) with NCX1 expression in only 10-20% of the myocytes.
Increased cardiomyocyte function and Ca2+ transients in mice during early congestive heart failure
Journal of Molecular and Cellular Cardiology, 2007
End-stage heart failure is believed to involve depressed cardiomyocyte contractility and Ca 2+ transients. However, the time course of these alterations is poorly understood. We examined alterations in myocyte excitation-contraction coupling in a mouse model of early congestive heart failure (CHF) following myocardial infarction. One week after myocardial infarction was induced by ligation of the left coronary artery, CHF mice were selected based on established criteria (increased left atrial diameter, increased lung weight). Sham-operated animals (SHAM) served as controls. Echocardiographic measurements showed decreased global function in early CHF relative to SHAM, but increased local function in viable regions of the myocardium which deteriorated with time. Cardiomyocytes isolated from the non-infarcted septum also exhibited larger contractions in early CHF than SHAM (CHF = 219.6 ± 15.3% of SHAM values, P b 0.05; 1 Hz field stimulation), and relaxation was more rapid (time to 50% relaxation = 82.9 ± 5.5% of SHAM values, P b 0.05). Ca 2+ transients (fluo-4 AM) were larger and decayed more rapidly in CHF than SHAM during both field stimulation (1 Hz) and voltage-clamp steps. Sarcoplasmic reticulum (SR) Ca 2+ content was increased. Western blots showed that while SR Ca 2+ ATPase (SERCA) expression was unaltered in CHF, phospholamban (PLB) was downregulated (60 ± 11% of SHAM values, P b 0.05). Thus, an increased SERCA/PLB ratio in CHF may promote SR Ca 2+ re-uptake. Additionally, peak L-type Ca 2+ current and Na + / Ca 2+ exchanger expression were increased in CHF, suggesting increased sarcolemmal Ca 2+ flux. Thus, in early CHF, alterations in Ca 2+ homeostasis improve cardiomyocyte contractility which may compensate for loss of function in the infarction area. release. Following release, Ca 2+ is recycled into the SR by the SR Ca 2+ ATPase (SERCA) and extruded from the cell by the Na + / Ca 2+ exchanger (NCX). SERCA activity is modulated by its inhibitor phospholamban (PLB). Phosphorylation of PLB relieves the inhibition, and promotes cardiomyocyte relaxation by increasing the Ca 2+ affinity of SERCA.
Regulation of Ncx1 Gene Expression in the Normal and Hypertrophic Heart
Annals of the New York Academy of Sciences, 2007
The Na + /Ca 2+ exchanger (NCX1) is crucial in the regulation of [Ca 2+ ] i in the cardiac myocyte. The exchanger is upregulated in cardiac hypertrophy, ischemia, and failure. This upregulation can have an effect on Ca 2+ transients and possibly contribute to diastolic dysfunction and an increased risk of arrhythmias. Studies from both in vivo and in vitro model systems have provided an initial skeleton of the potential signaling pathways that regulate the exchanger during development, growth, and hypertrophy. The Ncx1 gene is upregulated in response to ␣-adrenergic stimulation. We have shown that this is via p38␣ activation of transcription factors binding to the Ncx1 promotor at the-80 CArG element. Interestingly, most of the elements, including the CArG element, which we have demonstrated to be important for regulation of Ncx1 expression are in the proximal 184 bp of the promotor. Using a transgenic mouse, we have shown that the proximal 184 bp is sufficient for expression of reporter genes in adult cardiomyocytes and for the correct spatiotemporal pattern of Ncx1 expression in development but not for upregulation in response to pressure overload.
Cardiovascular Research, 2008
Time for primary review: 16 days Aims The Na þ /K þ-ATPase (NKA) a 2-isoform is preferentially located in the t-tubules of cardiomyocytes and is functionally coupled to the Na þ /Ca 2þ-exchanger (NCX) and Ca 2þ regulation through intracellular Na þ concentration ([Na þ ] i). We hypothesized that downregulation of the NKA a 2-isoform during congestive heart failure (CHF) disturbs the link between Na þ and Ca 2þ , and thus the control of cardiomyocyte contraction. Methods and results NKA isoform and t-tubule distributions were studied using immunocytochemistry, confocal and electron microscopy in a post-infarction rat model of CHF. Sham-operated rats served as controls. NKA and NCX currents (I NKA and I NCX) were measured and a 2-isoform current (I NKA,a2) was separated from total I NKA using 0.3 mM ouabain. Detubulation of cardiomyocytes was performed to assess the presence of a 2-isoforms in the t-tubules. In CHF, the t-tubule network had a disorganized appearance in both isolated cardiomyocytes and fixed tissue. This was associated with altered expression patterns of NKA a 1-and a 2-isoforms. I NKA,a2 density was reduced by 78% in CHF, in agreement with decreased protein expression (74%). When I NKA,a2 was blocked in Sham cardiomyocytes, contractile parameters converged with those observed in CHF. In Sham, abrupt activation of I NKA led to a decrease in I NCX , presumably due to local depletion of [Na þ ] i in the vicinity of NCX. This decrease was smaller when the a 2-isoform was downregulated (CHF) or inhibited (ouabain), indicating that the a 2-isoform is necessary to modulate local [Na þ ] i close to NCX. Conclusion Downregulation of the a 2-isoform causes attenuated control of NCX activity in CHF, reducing its capability to extrude Ca 2þ from cardiomyocytes.
Journal of Biological Chemistry, 2006
The Na ؉-Ca 2؉ exchanger (NCX1) is up-regulated in hypertrophy and is often found up-regulated in end-stage heart failure. Studies have shown that the change in its expression contributes to contractile dysfunction. We have previously shown that the 1831-bp Ncx1 H1 (1831Ncx1) promoter directs cardiac-specific expression of the exchanger in both development and in the adult, and is sufficient for the up-regulation of Ncx1 in response to pressure overload. Here, we utilized adenoviral mediated gene transfer and transgenics to identify minimal regions and response elements that mediate Ncx1 expression in the heart. We demonstrate that the proximal 184 bp of the Ncx1 H1 (184Ncx1) promoter is sufficient for expression of reporter genes in adult cardiomyocytes and for the correct spatiotemporal pattern of Ncx1 expression in development but not for up-regulation in response to pressure overload. Mutational analysis revealed that both the ؊80 CArG and the ؊50 GATA elements were required for expression in isolated adult cardiomyocytes. Chromatin immunoprecipitation assays in adult cardiocytes demonstrate that SRF and GATA4 are associated with the proximal region of the endogenous Ncx1 promoter. Transgenic lines were established for the 1831Ncx1 promoter-luciferase containing mutations in the ؊80 CArG or ؊50 GATA element. No luciferase activity was detected during development, in the adult, or after pressure overload in any of the ؊80 CArG transgenic lines. The Ncx1 ؊50 GATA mutant promoter was sufficient for driving the normal spatiotemporal pattern of Ncx1 expression in development and for up-regulation in response to pressure overload but importantly, expression was no longer cardiac restricted. This work is the first in vivo study that demonstrates which cis elements are important for Ncx1 regulation.
Functional Adult Myocardium in the Absence of Na+-Ca2+ Exchange: Cardiac-Specific Knockout of NCX1
Circulation Research, 2004
The excitation-contraction coupling cycle in cardiac muscle is initiated by an influx of Ca 2ϩ through voltage-dependent Ca 2ϩ channels. Ca 2ϩ influx induces a release of Ca 2ϩ from the sarcoplasmic reticulum and myocyte contraction. To maintain Ca 2ϩ homeostasis, Ca 2ϩ entry is balanced by efflux mediated by the sarcolemmal Na ϩ -Ca 2ϩ exchanger. In the absence of Na ϩ -Ca 2ϩ exchange, it would be expected that cardiac myocytes would overload with Ca 2ϩ . Using Cre/loxP technology, we generated mice with a cardiac-specific knockout of the Na ϩ -Ca 2ϩ exchanger, NCX1. The exchanger is completely ablated in 80% to 90% of the cardiomyocytes as determined by immunoblot, immunofluorescence, and exchange function. Surprisingly, the NCX1 knockout mice live to adulthood with only modestly reduced cardiac function as assessed by echocardiography. At 7.5 weeks of age, measures of contractility are decreased by 20% to 30%. We detect no adaptation of the myocardium to the absence of the Na ϩ -Ca 2ϩ exchanger as measured by both immunoblots and microarray analysis. Ca 2ϩ transients of isolated myocytes from knockout mice display normal magnitudes and relaxation kinetics and normal responses to isoproterenol. Under voltage clamp conditions, the current through L-type Ca 2ϩ channels is reduced by 50%, although the number of channels is unchanged. An abbreviated action potential may further reduce Ca 2ϩ influx. Rather than upregulate other Ca 2ϩ efflux mechanisms, the myocardium appears to functionally adapt to the absence of the Na ϩ -Ca 2ϩ exchanger by limiting Ca 2ϩ influx. The magnitude of Ca 2ϩ transients appears to be maintained by an increased gain of sarcoplasmic reticular Ca 2ϩ release. The myocardium of the NCX1 knockout mice undergoes a remarkable adaptation to maintain near normal cardiac function. (Circ Res. 2004;95: 604-611.)
Cardiovascular Research, 2013
The complex molecular mechanisms underlying spontaneous cardiac pacemaking are not fully understood. Recent findings point to a coordinated interplay between intracellular Ca 2+ cycling and plasma membrane-localized cation transport determining the origin and periodicity of pacemaker potentials. The sodium-calcium exchanger (NCX1) is a key sarcolemmal protein for the maintenance of calcium homeostasis in the heart. Here, we investigated the contribution of NCX1 to cardiac pacemaking. Methods and results We used an inducible and sinoatrial node-specific Cre transgene to create mice lacking NCX1 selectively in cells of the cardiac pacemaking and conduction system (cpNCX1 KO). RT-PCR and immunolabeling experiments confirmed the precise tissue-specific and temporally controlled deletion. Ablation of NCX1 resulted in a progressive slowing of heart rate accompanied by severe arrhythmias. Isolated sinoatrial tissue strips displayed a significantly decreased and irregular contraction rate underpinning a disturbed intrinsic pacemaker activity. Mutant animals displayed a gradual increase in the heart-to-body weight ratio and developed ventricular dilatation; however, their ventricular contractile performance was not significantly affected. Pacemaker cells from cpNCX1 KO showed no NCX1 activity in response to caffeine-induced Ca 2+ release, determined by Ca 2+ imaging. Regular spontaneous Ca 2+ discharges were frequently seen in control, but only sporadically in knockout (KO) cells. The majority of NCX1 KO cells displayed an irregular and a significantly reduced frequency of spontaneous Ca 2+ signals. Furthermore, Ca 2+ transients measured during electrical field stimulation were of smaller magnitude and decelerated kinetics in KO cells. Conclusions Our results establish NCX1 as a critical target for the proper function of cardiac pacemaking.