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Cardiovascular Research 2005 65(1):83-92; doi:10.1016/j.cardiores.2004.09.024
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Copyright © 2004, European Society of Cardiology

Chronic inhibition of Na+/H+-exchanger attenuates cardiac hypertrophy and prevents cellular remodeling in heart failure

Antonius Baartscheera,*, Cees A. Schumachera, Marcel M.G.J. van Borrenb, Charly N.W. Beltermana, Ruben Coronela, Tobias Opthofa and Jan W.T. Fioleta

aExperimental Cardiology, Room M-0-052, Academic Medical Center, University of Amsterdam, P.O. Box 22700, 1100 DE Amsterdam, The Netherlands
bLaboratory of Physiology, Academic Medical Center, University of Amsterdam, P.O. Box 22700, 1100 DE, Amsterdam, The Netherlands

* Corresponding author. Tel.: +31 20 5663265; fax: +31 20 6975458. Email address: A.Baartscheer{at}AMC.UVA.NL

Received 14 May 2004; revised 26 August 2004; accepted 23 September 2004


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: In patients with heart disease, the transition from compensatory hypertrophy to heart failure (HF) is associated with altered calcium handling. Up-regulated Na+/H+-exchanger (NHE-1) activity underlies increased [Na+]i and disturbance of cellular calcium handling in HF. We hypothesize that chronic inhibition of NHE-1 activity prevents the hypertrophic response, cellular remodeling, and development of HF.

Methods: Rabbits received a control or cariporide (inhibitor of NHE-1) diet for 3 months, starting after induction of combined volume and pressure overload. Age-matched animals served as control. Development of HF was examined echocardiographically and electrocardiographically after 3 months. [Na+]i, [Ca2+]i, pHi, and action potentials were measured in left ventricular midmural myocytes with SBFI, indo-1, SNARF, and di-4-anepps. Sarcoplasmic reticulum calcium content was calculated from the response of [Ca2+]i to rapid cooling. Calcium after-transients were elicited by cessation of rapid stimulation (3 Hz) in the presence of 100 nmol/l noradrenalin.

Results: Chronic treatment of rabbits with the specific Na+/H+-exchanger activity inhibitor cariporide greatly attenuated development of hypertrophy and entirely abolished development of HF; the heart/body weight ratio increased only little, no change in lung weight occurred, left ventricular dimensions and fractional shortening changed mildly, ascites was not present, QT duration did not increase, and sudden death did not occur. Chronic cariporide treatment also prevented cellular electrical and ionic remodeling. Myocyte dimensions were unaltered, action potentials were not prolonged, cytoplasmic sodium and NHE-1 activity did not increase, cytoplasmic and SR calcium handling remained undisturbed, and no increase of the incidence of calcium after-transient dependent delayed after depolarizations (DADs) occurred.

Conclusion: We conclude that enhanced activity of NHE-1 underlies cardiac cellular electrical and ionic remodeling in experimental heart failure, and that chronic dietary treatment with cariporide attenuates hypertrophy, development of HF, and cellular remodeling.

KEYWORDS Heart failure; Remodeling; Calcium (cellular); Na/H-exchanger; Na/Ca-exchanger; SR function; Myocytes

This article is referred to in the Editorial by M.A. Stagg and C.M.N. Terracciano (pages 10–12) in this issue.


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
The incidence of heart diseases leading to development of hypertrophy and heart failure (HF) progressively increases with aging of the population. The transition from compensated hypertrophy into heart failure is associated with poor prognosis and decreased life expectancy [1]. The transition from hypertrophy to heart failure is not fully understood. Disturbed cytoplasmic and sarcoplasmic reticular (SR) calcium handling in HF may underlie contractile dysfunction and arrhythmogenesis [2,3]. Altered calcium handling in HF, evidenced by increased diastolic and decreased systolic cytoplasmic calcium ([Ca2+]i) and SR calcium content [2,4,5], is related to altered expression and or activities of the Na+/Ca2+-exchanger (NCX) [4,6], the SR Ca2+-ATPase (SERCA2) [7,8], and the SR Ca-release channels (RyR) [9,10].

In cardiac hypertrophy and HF, [Na+]i is increased [4,11,12]. The elevation of [Na+]i in HF is mediated by increased activity of the Na+/H+-exchanger (NHE-1) [13]. From the perspective of adaption, elevation of [Na+]i favours positive inotropy through an NCX-mediated increase of diastolic calcium. We have shown that acute inhibition of NHE-1 in HF normalized not only [Na+]i but also cytoplasmic and SR calcium handling and the propensity to develop delayed after depolarizations (DADs) [13].

We hypothesize that altered NHE-1 and sodium homeostasis play an important part in the remodeling process. Both angiotensin- and endothelin-dependent activation [14,15] and enhanced expression of NHE [16] may contribute to increased activity of NHE-1 in HF. NHE-1 is a common effector of mitogen-activated protein kinases (MAPKs) responsible for protein phosphorylation, including several transcription factors [17–19]. Increased pH and [Ca2+]i secondary to increased NHE-1 activity affects protein synthesis; an increase of pH by 0.1 accelerated protein synthesis by 40% [20], and elevated [Ca2+]i activated MAPKs and RAF-1 kinase [21]. Inhibition of NHE-1 reduced cellular pH [20] and attenuated mechanical stretch-induced activation of MAPKs and RAF-1 [22].

Several recent studies suggest that chronic inhibition of NHE-1 might favorably interfere with development of hypertrophy and HF [23–29]. However, none of these studies dealt with the effects of chronic inhibition of NHE-1 on sodium homeostasis and calcium handling. We therefore tested whether chronic NHE inhibition interferes with development of HF and prevents disturbance of cellular sodium homeostasis and cytoplasmic and SR calcium handling.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
Animal care and handling conformed to the Guide for Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996), and the study was approved by the local institutional ethical committee.

2.1. Design of the study
New Zealand white male rabbits, bodyweight of 3.4 ± 0.05 kg (mean ± S.E.M., n=27), were fed with a chow with or without 0.3% cariporide during 16 weeks. In 17 rabbits, combined volume and pressure overload was induced [30]. In a first surgical procedure (P1), volume overload was produced by rupture of the aortic valve until pulse pressure increased by about 100% (101 ± 7.3; mean ± S.E.M.), and after 3 weeks, pressure overload was created (P2) by suprarenal abdominal aortic constriction of 50%. Four groups of animals were studied: (1) control animals fed with normal chow (Ctrl, n=5), (2) animals with pressure and volume overload fed with normal chow (HF, n=10), (3) control animals fed with cariporide chow (Ctrl-car, n=5) and (4) animals with pressure and volume overload fed with cariporide chow (HF-car, n=7). There was no difference in the increase of pulse pressure between HF and HF-car groups (101 ± 6.2% and 105 ± 8.8%) at the time of surgery. Cariporide chow feeding in the HF-car group started immediately after P1. Animals were sacrificed after 16 weeks on diet. Heart rate, QT and QRS duration, and echocardiographic M-mode long axis LV dimensions recordings were obtained before P1 and termination. Hemodynamic measurements were made during P1 and before sacrifice. Presence of ascites was assessed at autopsy, and heart and lung weights were measured relative to body weight. In the chronically treated animals, the blood cariporide concentration was 5.0 ± 1.4 µmol/l (mean ± S.E.M., n=12), measured just before sacrifice.

Myocytes were isolated from the midmural left ventricular wall [31] and stored at room temperature in vials, each containing about 105 myocytes in 5 ml solution containing (mmol/l): [Na+] 156, [K+] 4.7, [Ca2+] 2.6, [Mg2+] 2.0, [Cl–] 150.6, [HCO3–] 4.3, [HPO42–] 1.4, [HEPES] 17, [Glucose] 11 supplied with 1% fatty acid free albumin (pH 7.3).

2.2. Measurement of action potential, [Na+]i, [Ca2+]i, pHi, and SR calcium content
Action potentials were measured in myocytes stained during 1 min with 0.5 µmol/l di-4-anepps. Anepps fluorescence was excited at 480 nm and measured in dual wavelength emission mode (510–570)/(590–640) nm. Fluorescence emission ratio was used as a measure of membrane potential. [Na+]i, [Ca2+]i, and pHi were measured in SBFI, indo-1, and SNARF loaded myocytes, preincubated in HEPES-buffered solutions containing 10 µmol/l SBFI-AM or 5 µmol/l indo-1-AM or 10 µmol/l SNARF-AM and 0.01% pluronic. Incubation durations were 120, 30, and 10 min, respectively. Myocytes were washed twice before use with fresh HEPES solution (without albumin) and kept for 15 min to ensure complete deesterification. Excitation wavelengths were 340, 340, and 515 nm, respectively. Fluorescence of all indicators was measured in dual wavelength emission mode at 410/590, (405–440)/(505–540), and 580/640 nm, respectively.

Myocytes were attached to a polylysine (0.1 g/l)-treated cover slip on the stage of a microscope (Nikon Diaphot). A perfusable chamber (height 0.4 mm, diameter 10 mm, volume 30 µl), containing two platinum electrodes for field stimulation (8 mm distance, 40 V/cm bipolar square pulses of 0.5 ms duration), was pressed onto the cover slip. The microscope stage and perfusion chamber were maintained at 37 °C. The measuring window was adjusted to the cellular surface of one quiescent rod-shaped myocyte with a rectangular diaphragm. For every experimental condition, fluorescence measurements were taken in at least three myocytes per heart in fresh HEPES solution without albumin.

Fluorescence signals of SBFI and indo-1 were corrected for background recorded from probe-free myocytes and calibrated to obtain values for Rmax, Rmin, β, and kd with the procedures described in detail previously [32–35]. [Na+]i and [Ca2+]i were calculated with the formulation of Grynkiewicz et al. [36]. [Na+]i was averaged over the entire cardiac cycle because it does not vary on a beat to beat basis [32]. Cytosolic free [Ca2+]i was obtained by correction of measured overall cellular fluorescence signals for contributions of mitochondrially compartmentalized indo-1 [33]. pHi was determined from calibration curves obtained with nigericin, and NHE-1-dependent proton flux was calculated from the rate of recovery of pHi following acid loading using data on cellular buffer capacity obtained, as described previously [35].

Diastolic SR calcium content was calculated from the response of cytoplasmic [Ca2+]i to rapid cooling, which releases all calcium from SR into the cytoplasm [37]. There was no difference between calculated SR calcium content measured with rapid cooling and with caffeine application (data not shown); rapid cooling was the preferred technique because caffeine greatly quenched indo-1 fluorescence signals (to a similar extent at both wavelengths). Literature data on cytoplasmic calcium buffer characteristics [38] were used to convert the cytoplasmic [Ca2+]i response to total calcium released into the cytoplasm [33]. Systolic SR calcium content was calculated as the difference between diastolic SR calcium content and total cytoplasmic calcium associated with the amplitude of the calcium transient. Fractional SR depletion was defined as the ratio of systolic to diastolic SR calcium content.

Calcium after-transients were elicited by cessation of stimulation following 10 s of rapid pacing (3 Hz) in the presence of 100 nmol/l noradrenaline [5].

2.3. Statistics
Data are expressed as mean ± S.E.M. Two-way ANOVA (with a post-hoc test according to Student–Newman–Keuls) or Student's t-test was used where appropriate to test for statistical significance at a level of significance of p<0.05.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
Table 1 summarizes cardiac dimensions and echocardiographic, electrocardiographic, and hemodynamic data. In the HF-car group, a mild hypertrophic response without signs of heart failure was found relative to Ctrl and Ctrl-car. A small increase in relative heart weight, a small increase in myocyte length, LV dimensions, and QT interval were measured. No differences were found between Ctrl and Ctrl-car. In the HF group, most parameters specific for development of hypertrophy and failure (relative heart and lung weight, LV diameters, QT interval, QRS duration, LVEDP, myocyte dimensions, presence of ascites) were substantially increased, and fractional shortening was decreased relative to HF-car and the two control groups. In addition, in HF, three out of 10 animals died untimely (after about 2 months), and no such deaths occurred in HF-car. Thus, chronic cariporide treatment prevented or attenuated development of hypertrophy and heart failure. Chronic cariporide treatment had no effects in control animals.


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  		Table 1 Morphological, echocardiographical, electrocardiographical, and hemodynamic data of the experimental groups

 
3.1. Activity of NHE-1
Fig. 1A shows the activity of NHE-1 measured as proton efflux (JNHE) in nonstimulated myocytes from the rate of recovery of pHi after acid loading. In the HF-car group, JNHE was a little enhanced below pHi 7.0 but was not different at the higher pHi values. In the HF group, JNHE was significantly larger than in all other groups at all pHi. No difference was found between Ctrl and Ctrl-car. Steady state resting pHi was about 0.15 units higher in HF than in Ctrl. In HF-car, steady-state resting pHi was not different from Ctrl.


Figure 1
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  		Fig. 1 Activity of the Na+/H+-exchanger. (Upper panel) The rate of proton efflux (JNHE) as a function of pHi (calculated from the derivative of the pHi recovery curve following acid loading induced by washout of NH4CL, [34]). Ctrl ({circ}-{circ}; four rabbits, six cells), Ctrl-car ({square}-{square};three rabbits six cells), HF (bullet-bullet; four rabbits, seven cells), and HF-car (lhblk-lhblk; four rabbits, eight cells). (Lower panel) The rate of sodium influx (JNa) in the absence (left) and presence (middle) of 10 µmol/l cariporide and the cariporide sensitive part (difference between left and middle) of JNa (right) following inhibition of the Na+/K+ ATPase with 100 µmol/l ouabain. [Na+]i increased linearly in all groups (JNa) after application of ouabain (see Ref. [13]). Ctrl (white; 5 rabbits, 15 cells), Ctrl-car (dashed; 5 rabbits, 15 cells), HF (gray; 7 rabbits, 21 cells) and HF-car (black; 7 rabbits, 21 cells). All data are expressed as mean ± S.E.M. *p<0.05 versus ctrl, {dagger}p<0.05 versus HF (ANOVA).

 
Fig. 1B shows the activity of NHE-1 measured as cariporide sensitive sodium influx (JNa) in stimulated myocytes (2 Hz) at physiological pH after sodium pump inhibition (see also Ref. [13]). In the absence of cariporide, JNa was the same in HF-car, Ctrl, and Ctrl-car, but in the HF-group, it was significantly higher. In the presence of cariporide, JNa was reduced to the same value in all groups. Thus, in HF-car, the cariporide sensitive part of JNa was the same as in Ctrl and significantly less than in HF.

3.2. Action potential, [Na+]i, cytoplasmic and SR calcium handling, and calcium after-transients
Fig. 2 shows representative examples of action potential, calcium transient, and response to rapid cooling (SR calcium content) in 2 Hz stimulated myocytes of all groups. In HF-car myocytes, none of these parameters clearly differed from control. Without chronic cariporide treatment, however, clear differences in all parameters were observed in HF myocytes. This is summarized in Table 2 and Figs. 3 to 4Go.


Figure 2
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  		Fig. 2 Action potentials and calcium transients. Representative examples of action potentials (upper panel), calcium transients (middle panel), and SR calcium content (lower panel) in 2 Hz stimulated myocytes of the Ctrl and Ctrl-car groups (left) and the HF and HF-car groups (right). Rapid cooling (indicated by arrows) caused the release of all calcium from sarcoplasmic reticulum (SR). The cytoplasmic response was used to calculate SR calcium content (see Methods).

 

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  		Table 2 Action potential duration at 90% repolarization (APD90) and calcium transient duration at 80% recovery (CTD80)

 

Figure 3
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  		Fig. 3 [Na+]i, diastolic [Ca2+]i, and calcium transient amplitude. Steady state levels of [Na+]i, (upper panels), diastolic [Ca2+]i (middle panels), and calcium transient amplitude (lower panels) were measured in myocytes of Ctrl and Ctrl-car (left) and of HF and HF-car (right) after 2 min of conditioning at each stimulation frequency in Ctrl ({circ}-{circ}; 5 rabbits, 15 cells), Ctrl-car ({square}-{square}; 5 rabbits, 15 cells), HF (bullet-bullet; 7 rabbits, 21 cells), and HF-car (lhblk-lhblk; 7 rabbits, 21 cells). All data are expressed as mean ± S.E.M. *p<0.05 HF versus Ctrl, {dagger}p<0.05 versus HF (ANOVA).

 

Figure 4
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  		Fig. 4 SR calcium handling during steady state 2-Hz stimulation. Diastolic SR calcium content (upper panel), systolic SR calcium content (middle panel), and fractional systolic SR calcium depletion (lower panel). Ctrl (white; 5 rabbits, 15 cells), Ctrl-car (dashed; 5 rabbits, 15 cells), HF (grey; 7 rabbits, 21 cells), and HF-car (black; 7 rabbits, 21 cells). All data are expressed as mean ± S.E.M. *p<0.05 versus ctrl, {dagger}p<0.05 versus HF. Diastolic, systolic SR calcium content, and fractional systolic SR calcium release were measured using the rapid cooling technique and calculated as detailed in Methods.

 
Table 2 shows the action potential and calcium transient duration. Action potential prolongation in HF-car was not different from Ctrl-car, but some prolongation was found relative to Ctrl. The calcium transient duration in HF-car was not different from Ctrl and Ctrl-car. In HF, the action potential and the calcium transient were significantly prolonged relative to HF-car and the control groups at all stimulation rates.

Fig. 3 summarizes steady state [Na+]i, diastolic [Ca2+]i, and calcium transient amplitude as a function of stimulation rate. All parameters increased with frequency in all groups, except the calcium transient amplitude in the HF group, which decreased with frequency. In HF-car, myocytes [Na+]i, diastolic [Ca2+]i, and calcium transient amplitude were in general not different from those in Ctrl and in Ctrl-car at any frequency; only at the lower frequencies diastolic [Ca2+]i was a little elevated. [Na+]i, diastolic [Ca2+]i, and calcium transient amplitudes were significantly different between HF-car and HF except for the calcium transient amplitude at the lower frequencies.

Fig. 4 summarizes calculated diastolic and systolic SR calcium content and the fractional depletion of SR during systole in myocytes stimulated at 2 Hz. In the HF-car group, diastolic and systolic SR calcium content were the same as those in the Ctrl and Ctrl-car groups. In the HF group, diastolic SR calcium content was significantly reduced by about 50%, and during systole, SR became almost entirely depleted. Consequently, the fractional systolic SR calcium release was the same in HF-car, Ctrl, and Ctrl-car groups (about 55%), but in the HF group, it was significantly enhanced (about 80%).

We previously demonstrated that HF myocytes exhibit a high incidence of spontaneous calcium after-transients and DADs after cessation of rapid pacing in the presence of 100 nM noradrenalin [5]. Table 3 shows that the incidence of after-transients in HF-car myocytes was significantly reduced compared to HF, and that the incidence in HF-car was not different from Ctrl and Ctrl-car.


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  		Table 3 Incidence of calcium after-transients

 
3.3. Residual cariporide in myocyte preparations cannot explain results
We previously demonstrated that in HF myocytes, NHE-1 activity is increased, and that acute application of cariporide partly normalized ionic remodeling, but had minor effects on sodium and calcium homeostasis in control animals [13]. Residual cariporide present in the myocyte preparations of HF-car animals could thus mask cellular ionic remodeling. Therefore, it is imperative to show that the results described so far cannot be attributed to residual cariporide present in the myocyte preparations used. Therefore, we studied washout kinetics of cariporide, acutely applied to myocytes of nontreated HF animals, which have elevated NHE-1 activity. Fig. 5 (upper panel) shows that presence of cariporide after acid loading prevented the recovery of pHi, which was relieved immediately upon washout of cariporide; pHi recovered to baseline within 5 min. Fig. 5 (lower panel) shows that the application of cariporide caused reduction of cellular [Na+]i (c.f. also Ref. [13]), and that [Na+]i recovered to its original elevated value within a few minutes after washout of cariporide. These results substantiate that presence of cariporide in our myocyte preparations is highly unlikely because the duration of washout during the isolation procedure of myocytes is more than 20 times longer than required to completely remove cariporide.


Figure 5
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  		Fig. 5 Time course of cariporide inhibition and washout in HF myocytes. (Upper panel) An example of time course of pHi after acid loading during onset and relieve of NHE-1 inhibition by cariporide. (Lower panel) Time course of [Na+]i during application and washout of cariporide in HF myocytes.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
We demonstrate that chronic inhibition of NHE-1 with cariporide after induction of volume and pressure overload in the rabbit attenuates development of cardiac hypertrophy and prevents development of heart failure. At the cellular level, chronic cariporide treatment maintains normal cellular dimensions and precludes ionic and electrical remodeling; low [Na+]i and normal diastolic and systolic calcium handling are preserved, prolongation of action potential and development of calcium after-transient related DADs are prohibited.

In a well-characterized rabbit model of pressure and volume overload-induced heart failure [30,39,40], we previously demonstrated that NHE-1 activity is enhanced. This is the cause of increased cytoplasmic [Na+], which secondarily leads to disturbed cellular calcium handling. Inhibition of NHE-1 in HF myocytes (partly) reversed these ionic alterations [13]. Recent evidence indicates that NHE-1 becomes up-regulated molecularly in HF [41].

Protective effects of acute inhibition of NHE-1 have been well established in experimental ischemia and reperfusion [42]. Short-term (2 to 7 days) oral cariporide treatment failed to show positive effects in patients at risk for ischemia and reperfusion damage, except in patients undergoing coronary artery bypass surgery (guardian trial [43]). In studies with chronic inhibition of NHE-1, a role for NHE-1 has been implicated in development of fibrosis [27,29], hypertrophy [23–27,29], and survival rates after myocardial infarction [23].

Several mechanisms related to enhanced NHE-1 activity may be operative in the development of HF. First, in neonatal cardiac myocytes, a direct relation between enhanced NHE-1-dependent Na+-influx and activation of PKC has been found, which supports the idea of involvement with signaling pathways leading to a hypertrophic response [44,45]. Second, NHE-1-dependent elevation of [Na+]i causes NCX-mediated increase of cellular [Ca2+]i [4,11], which activates mitogen-activated protein kinase and RAF-1 kinase [21] and, thus, may function as a cellular growth signal [46]. Third, NHE-1-dependent increase of pHi may directly contribute to a hypertrophic response; an increase of 0.1 pH unit readily observable in HEPES-buffered HF myocytes [13] can accelerate protein synthesis by about 40% [20]. It should be realized, however, that pHi changes might be less in vivo as it is modulated also by activity of other proton transporters [34,47].

Therefore, we hypothesized that increase of activity of NHE-1 may be instrumental to cellular hypertrophic and ionic remodeling, eventually leading to heart failure. We tested this hypothesis by evaluating the effects of chronic in vivo inhibition of NHE-1 in rabbits after induction of pressure and volume overload. Steady-state cariporide plasma levels of about 5 µmol/l was achieved by feeding the animals a 0.3% cariporide-supplemented diet from the first surgical treatment onward, which is five times higher than needed for complete inhibition of NHE-1 [26,29].

With completely inhibited NHE-1, the animals in the HF-car group did not show functional signs of developing heart failure despite the infliction of pressure and volume overload and largely maintained normal contractile function. Therefore, the (mal)adaptive remodeling processes initiated by pressure and volume overload leading apparently cannot proceed without increased activity of NHE-1. Thus, it seems rather likely that elevation of cytoplasmic [Na+] and/or pH is a necessary condition for cellular remodeling that compromises contractile performance. In this respect, it is of relevance to speculate that increased Na-channel-related sodium influx also underlies cellular remodeling in rapid pacing induced heart failure.

Diastolic and systolic contractile dysfunction is generally ascribed to disturbed cytoplasmic and SR calcium handling and associated with altered molecular expression and/or activity of calcium handling proteins, such as NCX SERCA-2, Phospholamban, and RyR [2,3,7,8]. Key aspects of disturbance of calcium handling are increased diastolic [Ca2+]i, decreased calcium transient amplitude, increased calcium transient duration, decreased SR Ca content, and a negative force frequency relationship [5]. The absence of major changes of in situ contractile disturbance (Table 1) and of cellular calcium handling (Figs. 3 and 4Go and Table 2) in the chronically cariporide-treated HF-car group does not suggest substantial changes in molecular expression and/or activity of the proteins involved.

Electrical remodeling in heart failure is characterized by increased incidence of DADs and EADs, which are triggers for reentrant arrhythmias, the substrate for which is provided by increased fibrosis in HF [48] or surviving myocardial bundles after myocardial infarction [49]. Genesis of DADs is related to spontaneous calcium release from SR, which, in HF, may be enhanced by altered FKBP binding [9] or increased diastolic Ca [5]. In HF, genesis of EADs is related to action potential prolongation [50] caused by altered expression of K-channels, and possibly, NCX activity is involved as well [51]. In contrast to nontreated HF animals, in the HF-car group, the incidence of spontaneous calcium release (Table 3) was only little enhanced, and action potential duration was not prolonged neither in vivo nor in vitro (Tables 1 and 2Go) compared to control animals. Therefore, it is expected that the incidence of DAD- and EAD-related arrhythmias in vivo is reduced in the HF-car group. This potentially explains why no animals died untimely in the HF-car group, whereas in the untreated HF group, three out of 10 rabbits died suddenly.

In conclusion, during pressure and volume overload, chronic inhibition of NHE-1 attenuates hypertrophy, prevents development of HF, and prevents ionic and electrical cellular remodeling and is antiarrhythmic. Chronic treatment with an inhibitor of NHE-1 might prove beneficial in patients at risk to develop HF.


   Acknowledgement
 
We gratefully acknowledge support of this study by Aventis Pharma (Frankfurt, Germany), who kindly supplied the cariporide-supplemented chow and performed the measurement of plasma concentrations of cariporide.


   Notes
 
Time for primary review 28 days


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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