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© The European Society of Cardiology 2006. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Role of [Na⁺]_i and the emerging involvement of the late sodium current in the pathophysiology of cardiovascular disease

Lars S. Maier^* and Gerd Hasenfuss

Abt. Kardiologie and Pneumologie/Herzzentrum, Georg-August-Universität Göttingen, Robert-Koch-Str. 40, 37075 Göttingen, Germany

^* Corresponding author. Tel: +49 551 39 9481; fax: +49 551 39 8941. E-mail address: lmaier{at}med.uni-goettingen.de

	Abstract

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 
In recent years, it has become increasingly clear that, as well as abnormal intracellular calcium handling, changes in intracellular sodium homeostasis play an important role in the pathophysiology of heart failure. One key source of altered sodium homeostasis may be the slow inactivating sodium current. Altered intracellular sodium promotes alterations in intracellular calcium mainly through the sarcolemmal Na⁺/Ca²⁺ exchanger that can transport Ca²⁺ vs. Na⁺ in both directions. Changes in both calcium and sodium handling are the main factors associated with cardiac dysfunction and the propensity for cardiac arrhythmias. This article gives insight into the mechanisms involved in the pathophysiology of sodium homeostasis in heart failure.

Key Words: Late sodium current • Heart failure • Excitation–contraction coupling • Calcium/calmodulin-dependent protein kinase II

	Intracellular sodium homeostasis and cytosolic sodium concentration

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 
On the protein level of a single cardiac myocyte, several channels and transporters participate in the regulation of intracellular sodium homeostasis.¹ Briefly, influx and efflux mechanisms of Na⁺ can be differentiated, resulting in a relatively stable intracellular Na⁺ concentration ([Na⁺]_i) from beat to beat in steady-state conditions (i.e. without changes in stimulation rate). This is in contrast to the large (10-fold) changes occurring in intracellular Ca²⁺ concentration ([Ca²⁺]_i), which shows changes from 100 nM to 1 µM between diastole and systole.² Most importantly, Na⁺ influx into the myocytes through voltage-dependent Na⁺ channels (I_Na), as well as via the Na⁺/Ca²⁺ exchanger (NCX), represents the best studied sources for Na⁺ influx. In contrast, Na⁺/K⁺-ATPase and NCX (reverse mode) are mainly responsible for Na⁺ extrusion from the cytosol (Figure 1).

View larger version (23K): [in this window] [in a new window]   Figure 1 Schematic presentation of the most important Na+ influx and Na+ extrusion mechanisms in cardiac myocytes. Adapted from Bers et al.1

 
Interestingly, quantitative studies in rabbit myocytes show huge differences in the contribution of the various Na⁺ transport systems, depending on the stimulation rate. At rest (0 Hz), NCX accounts for most of the Na⁺ influx (in total 0.9 mM/min) with 0.27 mM/min or 32%, whereas all other mechanisms such as I_Na, Na⁺/H⁺ exchanger (NHE), or Na⁺/bicarbonate exchanger (NBC) only account for 0.14 mM/min or 15% each. In contrast, at 1 Hz stimulation frequency with a total Na⁺ influx of 3.3 mM/min, NCX represents by far the most important source for Na⁺ entry into the cell, with 2.2 mM/min or 66%, whereas I_Na accounts for 0.6 mM/min or 19%. All other mechanisms (e.g. NHE, NBC) do not show increase of Na⁺ influx with stimulation rate and stay similar to their values at rest with 0.15 mM/min or <5% each.¹ Knowing this, one also has to keep in mind that the physiological frequency range in animals can actually be as high as 10–12 Hz in small rodents, resulting in large Na⁺ fluxes passing through the sarcolemmal membrane by means of NCX and I_Na.

Although [Na⁺]_i in cardiac muscle is relatively stable between beats, absolute levels of [Na⁺]_i differ widely in the different species studied so far.³ Roughly, three different levels of [Na⁺]_i can be differentiated: rabbit, guinea-pig, and sheep being <8 mM; rat, ferret, and dog 10 mM; human and mouse >12 mM. Reasons for these differences are not completely clear, but they have consequences for excitation–contraction coupling itself. For example, let us compare rabbit (low [Na⁺]_i) with rat (high [Na⁺]_i) myocardium. In rabbit hearts, Ca²⁺ extrusion via NCX is thermodynamically favoured at rest, because the predicted reversal potential for NCX is positive to the membrane potential (E_m). This phenomenon leads to the well-known rest decay of twitch force because of continuous unloading of sarcoplasmic reticulum Ca²⁺ content. In contrast, the resting [Na⁺]_i in rat ventricle is high enough that the reversal potential for NCX is already near the resting E_m. In particular, after a train of stimuli, [Na⁺]_i would be higher still, such that the reversal potential for NCX would be negative to E_m and net Ca²⁺ uptake would be favoured, leading to a rest-dependent increase in sarcoplasmic reticulum Ca²⁺ load in rat ventricle that contributes to rest potentiation of twitch force.^2,4

The only two studies of [Na⁺]_i in human myocardium so far show stimulation-dependent increases in [Na⁺]_i.^5,6 At each stimulation frequency, human end-stage failing myocardium has even higher values for [Na⁺]_i (Figure 2).⁵ Unfortunately, the exact mechanism (i.e. the source of Na⁺ overload) remained unclear. Other authors, finding elevated [Na⁺]_i in a rabbit heart failure model, excluded the possibility that decreased Na⁺/K⁺-ATPase expression/function might be the mechanism and speculated about some prominent contribution of a very slowly inactivating I_Na component (late I_Na) in heart failure.⁷ These data underline that Na⁺ homeostasis is altered not only in failing human myocardium but also in animal models of heart failure. At slow heart rates, the higher [Na⁺]_i in failing myocytes appears to enhance Ca²⁺ influx through NCX and maintain sarcoplasmic reticulum Ca²⁺ load and force development. At faster rates, failing myocytes with high [Na⁺]_i cannot further increase sarcoplasmic reticulum Ca²⁺ load and are prone to diastolic Ca²⁺ overload. The consequence of increased [Na⁺]_i are therefore directly linked to increase in diastolic tension in failing human myocardium (Figure 2).^3,5

View larger version (16K): [in this window] [in a new window]   Figure 2 Major alterations in human heart failure. Stimulation frequency-dependent decrease in systolic twitch force and diastolic dysfunction, increased [Na+]i, and late INa.

 
This close correlation of [Na⁺]_i and contractile function can also be seen in a different methodological approach by increasing intracellular free oxygen radical production (reactive oxygen species, ROS) by adding H₂O₂. During H₂O₂ exposition, ventricular myocytes show a progressive increase in diastolic tension, and after 10 min they go into contracture. In parallel, [Na⁺]_i increases significantly.⁸ By blocking NCX with KB-R7943, myocyte contracture can be significantly reduced.⁹ Interestingly, we recently showed that ranolazine, a new anti-anginal and anti-ischaemic drug,¹⁰ delays development of ROS-mediated hypercontracture and reduces the increase in [Na⁺]_i, probably by inhibiting Na⁺ overload through selectively blocking late I_Na (Wagner and Maier, unpublished results).

	Cardiac Na+ channels and Na+ current (INa)

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 
The Na⁺ channel (260 kDa size) consists of various subunits, with the -subunit being required for function.¹¹ The isoform Nav1.5 (SCN5A) of the -subunit is the predominant isoform in the heart. However, other isoforms such as Nav1.1, Nav1.3, and Nav1.6, mainly expressed in the brain, were recently found in the heart. These isoforms seem to be localized in the transverse tubules, whereas cardiac Nav1.5 is preferentially localized in intercalated disks.¹² The cardiac -subunit consists of four homologous domains (I–IV), with each domain containing six transmembrane segments (S1–S6). The central pore is formed by the four domains, with the S5 and S6 transmembrane segments as putative pore centres that confer selectivity and conductance. The linkers between each fifth and sixth segment, also called the P (pore) segments, determine the Na⁺ selectivity of the pore relative to Ca²⁺ by a factor >1000:1 or even higher. The fourth transmembrane segment in each domain (S4) is covered with four to eight positively charged residues, thereby serving as voltage sensors. Its orientation within the membrane field allows it to move outward in response to depolarization, resulting in opening of the channel.¹¹

The function of Na⁺ channels is to generate a large and very brief inward I_Na and cause the rapid upstroke of the action potential (AP). This is accomplished by very brief channel openings with very short latency, normally without re-openings. That is, the channel inactivates quickly into an absorbing state, requiring recovery at negative E_m. However, cardiac Na⁺ channels can also exhibit some persistent late openings, and this may contribute to a background Na⁺ leak current.² An ultraslow component of I_Na inactivation (=600 ms) has also been reported in the human heart,¹³ which could contribute to early after-depolarizations (EADs); this is called late I_Na. Hypoxia and lysophospholipids also cause persistent Na⁺ channel openings, even at very negative E_m.²

In a dog heart failure model, increased late I_Na was described compared with dogs without heart failure; late I_Na was also increased in the rat heart after myocardial infarction.^14,15 Interestingly, dogs with chronic heart failure present with ventricular arrhythmias and, at the cellular level, increased AP durations, and also EADs. As mentioned earlier, late I_Na can also be found in human myocardium,¹³ and the anti-arrhythmic drug amiodarone at therapeutic concentrations was able to inhibit late I_Na in human myocardium without huge reductions in peak I_Na.¹⁶ Very recently, increased late I_Na has also been shown in a dog heart failure model as well as in human heart failure, compared with control conditions without heart failure (Figure 2). However, a reduced peak I_Na was also described by these authors, although there was no change in Na⁺ channel expression subunits.¹⁷ Reduced peak I_Na in human heart failure was also described by others,¹⁸ but not by all authors.¹⁹

Interestingly, it is well known that protein kinases (e.g. PKA and PKC) can regulate cardiac Na⁺ channels.² Surprisingly, PKA and PKC modulate Na⁺ channels divergently.¹¹ However, it is not known whether late I_Na can be regulated. As we recently described in a transgenic mouse model that overexpression of the cytosolic isoform of the Ca²⁺/calmodulin protein kinase (CaMKII_C) leads to heart failure, as well as changes in excitation–contraction coupling including AP prolongations,²⁰ we were interested in whether late I_Na can be affected by CaMKII_C overexpression. Therefore, we overexpressed CaMKII_C by adenoviral overexpression in rabbit cardiac myocytes and described for the first time that CaMKII_C modulates the availability of I_Na (Figure 3).²¹ We also subsequently showed that CaMKII_C increases late I_Na.²² However, there was no change in peak I_Na or Na⁺ channel expression. In parallel, increased [Na⁺]_i was found. Both increased late I_Na and [Na⁺]_i could be reversed by CaMKII inhibition. In localization studies using co-IPs and immunostaining with confocal imaging, we could show an association between Na⁺ channels and CaMKII_C.^2,22

View larger version (15K): [in this window] [in a new window]   Figure 3 CaMKII-dependent regulation of Na+ current. Adenovirus-mediated CaMKIIC overexpression resulted in a significant leftward-shift in the steady-state voltage dependence of inactivation (availability) compared with control group with ß-Gal (ß-galactosidase) expression (left). This reduced availability could be reversed to control values in the presence of CaMKII inhibition by using KN-93 (right). Adapted from Wagner et al.21

 

	Summary

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 
In summary, Na⁺ influx into cardiac myocytes occurs mainly through cardiac Na⁺ channels and NCX. In heart failure, [Na⁺]_i is increased, leading to Ca²⁺ overload and contractile dysfunction. One possible source for the elevated [Na⁺]_i may be increased late I_Na. This leads to AP prolongation promoting arrhythmias and increased [Na⁺]_i and [Ca²⁺]_i, resulting in contractile dysfunction. Interestingly, CaMKII_C increases late I_Na. With ranolazine, a selective inhibitor may be available to specifically inhibit Na⁺ entry and subsequently Ca²⁺ entry, leading to preserved contractile function.

	Acknowledgement

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 
L.S.M. is funded by the Deutsche Forschungsgemeinschaft (DFG) through an Emmy Noether-grant (MA 1982/1-4), by an Young Investigator Award of the GlaxoSmithKline Research Foundation, and by a grant from the Medical Faculty of the University of Göttingen (Anschubfinanzierung).

Conflict of interest: All authors have a collaboration/grant with CV Therapeutics.

	References

Top Abstract Intracellular sodium homeostasis... Cardiac Na+ channels and... Summary Acknowledgement References

 

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