doi: 10.1161/01.RES.0000139434.26969.4f
© 2004 American Heart Association, Inc.
Editorials |
Is Modulation of Sodium-Calcium Exchange a Therapeutic Option in Heart Failure?
From the Georg-August Universität Göttingen, Herzzentrum, Kardiologie und Pneumologie, Göttingen, Germany.
Correspondence to Gerd Hasenfuss, MD, Georg-August-Universität Göttingen, Herzzentrum, Kardiologie und Pneumologie, Robert-Koch-Str. 40, 37099 Göttingen, Germany. E-mail hasenfus{at}med.uni-goettingen.de
Key Words: Na+-Ca2+ exchange • calcium • XIP • heart failure • SR function
Besides the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA), the sarcolemmal Na+-Ca2+ exchanger (NCX) is the most important Ca2+ transport protein responsible for maintaining the Ca2+ balance of the myocyte. It catalyzes the transport of Ca2+ across the membrane in exchange for Na+ in a reversible manner. Its activity is called "forward" when Na+ is transported inward and Ca2+ outward and "reversed" when ions are transported in the opposite directions. The driving force of NCX depends on Na+ and Ca2+ concentrations at either side of the plasma membrane and on the membrane potential. NCX is electrogenic and carries inward (depolarizing) current in forward mode and outward (repolarizing) current in reversed mode.1 NCX consists of 9 transmembrane helices and a large cytoplasmic loop. This loop has been shown to contain Ca2+- and Na+-binding regulatory sites, which are distinct from the transport sites. Thus, Na+ and Ca2+ ions are both transport substrates and modulators of activity. At the N-terminal end of the cytoplasmic loop near the membrane-lipid interface, there is a 20-amino acid segment, designated the endogenous XIP region. This region is considered to function as an autoinhibitory domain that plays a central role in NCX regulation. In addition, PIP2, protons, ATP, and PKC-dependent effects regulate NCX activity.1–3 More recently, it was shown that the Ca2+ binding protein sorcin exerts stimulatory actions on NCX.4
Altered expression and activity of the sarcolemmal NCX may play a key role for disturbed contractile function and arrhythmogenesis in hypertrophy and heart failure. It has become clear that disturbed excitation-contraction coupling attributable to altered SR Ca2+ accumulation significantly contributes to heart failure pathophysiology.3 Three major factors seem to contribute to disturbed SR Ca2+ accumulation in human heart failure: (1) increased leak of Ca2+ through ryanodine receptors, (2) reduced SERCA activity, and (3) increased transsarcolemmal elimination of Ca2+ by NCX. SR Ca2+ accumulation depends on the activity of SERCA relative to transsarcolemmal Ca2+ elimination by NCX. When protein levels of NCX were measured relative to SERCA, it was observed that this ratio was increased by a factor of 3 in endstage failing myocardium, indicating a relative dominance of NCX over SERCA Ca2+ transport.5 Interestingly, two different phenotypes were identified: (1) endstage failing hearts with a predominant increase in protein levels of NCX, and (2) endstage failing hearts with a predominant decrease in SERCA protein levels. In the former subgroup, diastolic function was preserved because overall cellular capacity to eliminate cytosolic Ca2+ is high. However, systolic function was impaired because Ca2+ is eliminated across the sarcolemmal membrane and therefore SR Ca2+ accumulation is decreased. In the latter group, both SR Ca2+ uptake and global cytosolic Ca2+ elimination are reduced and therefore systolic as well as diastolic function was severely compromised. Enhanced transsarcolemmal relative to SR Ca2+ cycling is most pronounced at high heart rates.5,6 As was indicated in clinical and experimental studies, increased forward mode exchange is arrhythmogenic because of delayed afterdepolarizations after inward current generation.1,3 Furthermore, it was shown that increased expression of NCX increases sensitivity to digitalis and predisposes to free radical induced myocyte dysfunction.3,7
From these considerations, modulation of NCX function in heart failure may be a therapeutic option. Stimulation of forward mode NCX activity would reduce cytosolic Ca2+, impair systolic, and improve diastolic function. Vice versa, stimulation of reversed mode NCX would increase intracellular Ca2+ and contractility with the risk of diastolic impairment. Inhibition of NCX function should have opposite effects. Apparently, the consequence of nonselective stimulation or inhibition of NCX function is quite complex and unpredictable. Several inhibitors of NCX function have been developed2: KB-R7943 is a well-characterized inhibitor. Intriguingly, KB-R7943 was suggested to exert a preferential effect on reverse-mode NCX activity. First studies also reported high selectivity of the drug; this was yet questioned by other authors. Synthetic peptides are considered as potent and highly selective NCX inhibitors. The NCX inhibiting peptide (XIP) is derived from the primary sequence of cardiac NCX1, binds at the large cytoplasmic loop, and decreases the Vmax of NCX activity. However, it does not appear to permeate through the cell membrane, which limits its use for therapeutic interventions in heart failure. In addition, other peptides, such as the cyclic hexapeptide FRCRCFa and its cell-permeant, N-myristylated derivative Myr-FRCRCFa, which are much smaller than XIP, have been reported to effectively inhibit NCX activity. Recently, new compounds with particularly high selectivity for NCX such as SEA0400 were synthesized.2,8
In this issue of Circulation Research, Hobai et al9 present an interesting study suggesting that inhibition of NCX may be a therapeutic option in heart failure by normalizing Ca2+ cycling and improving contractile function. They studied isolated myocytes from dog hearts with pacing-induced heart failure using single cell electrophysiology techniques with XIP added directly to the intracellular solution. The canine pacing tachycardia model was previously shown to present with decreased SERCA and increased NCX protein levels. The main findings are that NCX inhibition by XIP increases SR Ca2+ load and Ca2+ transients; an estimated 27% inhibition of NCX induced an 80% increase in the amplitude of the Ca2+ transient in nonfailing myocytes at 0.5 Hz. In failing myocytes with doubling of NCX function, a 27% inhibition of NCX induced a 3.86-fold increase in the Ca2+ transient amplitude. Quite surprisingly, inhibition of NCX was not associated with increased diastolic Ca2+ and rather accelerated relaxation kinetics. The finding of increased SR Ca2+ load with reduced NCX activity is consistent with findings using the opposite approach in a previous study.10 When protein levels of NCX were increased by adenovirus mediated gene transfer in isolated rabbit myocytes, this resulted in decreased SR Ca2+ content, decreased myocyte shortening, and blunted forced frequency relation, ie, findings similar to those observed in the failing human heart.5,6 However, it should be mentioned that in other models and experimental conditions, increased NCX expression and function was shown to upregulate SR Ca2+ levels. In these studies, high Na+ levels may have been the main cause promoting reversed mode NCX activity.11
What are the subcellular mechanisms coupling SR Ca2+ load to NCX function? Most likely, NCX inhibition transiently increases intracellular Ca2+, which subsequently results in stimulation of SERCA. It has been recognized that with increasing stimulation rates, a marked "frequency-dependent acceleration of relaxation" (FDAR) can be found in mammalian ventricular muscle.12 Apparently, FDAR is linked to the frequency-dependent increases in [Ca2+]i and is independent from ß-adrenergic activation. There are several mechanisms to explain SERCA stimulation by Ca2+. Involvement of phosphorylation of phospholamban (PLN) by Ca2+/calmodulin-dependent protein kinase (CaMKII) relieving the inhibitory action of PLN on SERCA has first been suggested. Accordingly, FDAR was abolished after inhibition of CaMKII with KN-62 or KN-93. However, it was also present in PLN knockout mice, implying that PLN is not essential.12 Interestingly, direct phosphorylation of SERCA by CaMKII resulting in stimulation of the Vmax of Ca2+ uptake has been described as an alternative pathway.13 However, there is still controversy and the appropriate target of CaMKII involved in accelerating Ca2+ transport during FDAR is not yet identified unequivocally. After SERCA stimulation, intracellular Ca2+ declines and a new steady state with higher SR Ca2+ levels and higher SR Ca2+ release and uptake develops. Of course, under steady state conditions, NCX Ca2+ elimination matches with L-type Ca2+ current influx.
Although the finding of increased SR load and Ca2+ transients after NCX inhibition is plausible for nonfailing as well as failing myocardium, the finding of improved rate of relaxation may depend on the experimental model. In the failing human heart, diastolic Ca2+ was shown to be increased.1,3 Furthermore, in muscle strip preparations from failing human hearts, diastolic function varied considerably between patients. Diastolic function was related to expression levels of SERCA and NCX. It was normal in patients with increased NCX and preserved SERCA levels. In those patients, NCX inhibition may improve function by increasing reduced SR Ca2+ load. In patients with reduced SERCA, diastolic function was impaired. In those patients, NCX inhibition may further deteriorate diastolic performance.5 This may be most pronounced at high heart rates with reduced time for SR Ca2+ transport. On the same line, intracellular Na+ is high in human heart failure.14 Because increased Na+ promotes reversed mode NCX activity, 1,3 the consequences of XIP induced NCX inhibition may critically depend on intracellular Na+.
Another aspect needs to be considered. In heart failure, Ca2+ leak from the SR is increased due to enhanced ryanodine receptor open probability.15 Because leak as measured by Ca2+ sparks is increased with higher SR Ca2+ load any intervention to increase SR Ca2+ load without reducing the leak decreases efficiency of excitation-contraction coupling with increased energy consumption and arrhythmias as potential side effects. Finally, XIP acts at the intracellular surface of the NCX, which was suggested to be involved in Na+-dependent inactivation of the transporter. This together with the finding that XIP did not inhibit NCX function during caffeine induced Ca2+ release in the study by Hobai et al9 may suggest that the observed functional changes are specific for XIP and may not be generally transferable to other nonspecific NCX inhibitors.
In summary, because NCX is a complex molecule, the regulation of which is incompletely understood, the effects of NCX modulation on intracellular Ca2+ and contractile function is rather unpredictable and may deeply depend on experimental conditions and models. The study by Hobai et al9 stimulates to consider inhibitors of NCX for improving excitation-contraction coupling in heart failure. However, more studies are needed to understand the effects of XIP and analogues under different conditions such as high stimulation rates, increased intracellular Na+, increased SR Ca2+ leak, and various SERCA and NCX expression levels.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
1. Bers DM. Excitation-Contraction Coupling and Cardiac Contractile Force, 2nd ed. Dordrecht, The Netherlands: Kluwer Academic Publishers; 2001.
2. Shigekawa M, Iwamoto T. Cardiac Na+-Ca2+ exchange: molecular and pharmacological aspects. Circ Res. 2001; 88: 864–876.
3. Schillinger W, Fiolet JWT, Schlotthauer K, Hasenfuss G. Relevance of Na/Ca exchange in heart failure. Cardiovasc Res. 2003; 57: 921–933.
4. Seidler T, Miller SL, Loughrey CM, Kania A, Burow A, Kettlewell S, Teucher N, Wagner S, Kogler H, Meyers MB, Hasenfuss G, Smith GL. Effects of adenovirus-mediated sorcin overexpression on excitation-contraction coupling in isolated rabbit cardiomyocytes. Circ Res. 2003; 93: 132–139.
5. Hasenfuss G, Schillinger W, Lehnart SE, Preuss M, Pieske B, Maier LS, Prestle J, Minami K, Just H. Relationship between Na+-Ca2+ exchanger protein levels and diastolic function of failing human myocardium. Circulation. 1999; 99: 641–648.
6. Pieske B, Maier LS, Bers DM, Hasenfuss G. Ca2+ handling and SR Ca2+ content in isolated failing and nonfailing human myocardium. Circ Res. 1999; 85: 38–46.
7. Schillinger W, Ohler A, Emami S, Muller F, Christians C, Janssen PM, Kogler H, Teucher N, Pieske B, Seidler T, Hasenfuss G. The functional effect of adenoviral Na+/Ca2+ exchanger overexpression in rabbit myocytes depends on the activity of the Na+/K+-ATPase. Cardiovasc Res. 2003; 57: 996–1003.
8. Tanaka H, Nishimaru K, Aikawa T, Hirayama W, Tanaka Y, Shigenobu K. Effect of SEA0400, a novel inhibitor of sodium-calcium exchanger, on myocardial ionic currents. Br J Pharmacol. 2002; 135: 1096–1100.[CrossRef][Medline] [Order article via Infotrieve]
9. Hobai IA, Maack C, O’Rourke B. Partial inhibition of sodium/calcium exchange restores cellular calcium handling in canine heart failure. Circ Res. 2004; 95: 292–299.
10. Schillinger W, Janssen PML, Emami S, Henderson SA, Ross Rs, Teucher N, Zeitz O, Philipson KD, Prestle J, Hasenfuss G. Impaired contractile performance of cultured rabbit ventricular myocytes after adenoviral gene transfer of Na/Ca exchanger. Circ Res. 2000; 87: 581–587.
11. Terracciano CMN, De Souza AI, Philipson KD, MacLeod KT. Na+-Ca2+exchange and sarcoplasmic reticular Ca2+ regulation in ventricular myocytes from transgenic mice overexpressing the Na+-Ca2+exchanger. J Physiol. 1998; 512: 651–667.
12. Maier LS, Bers DM. Calcium, calmodulin, and calcium-calmodulin kinase II: heartbeat to heartbeat and beyond. J Mol Cell Cardiol. 2002; 34. 919–939.[Medline] [Order article via Infotrieve]
13. Xu A, Hawkins C, Narayanan N. Phosphorylation and activation of the Ca2+-pumping ATPase of cardiac sarcoplasmic reticulum by Ca2+/calmodulin-dependent protein kinase. J Biol Chem. 1993; 268: 8394–8397.
14. Pieske B, Maier LS, Weisser J, Piacentino V, Hasenfuss G, Houser S. Rate dependence of [Na+]i and contractility in nonfailing and failing human myocardium. Circulation. 2002; 106: 447–453.
15. Marx SO, Reiken S, Hisamatsu Y, Jayaraman T, Burkhoff D, Rosemblit N, Marks AR. PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (RyR): defective regulation in failing hearts. Cell. 2000; 101: 365–376.[CrossRef][Medline] [Order article via Infotrieve]
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