doi: 10.1161/01.RES.0000143419.87518.9e
© 2004 American Heart Association, Inc.
Editorials |
Sodium Calcium Exchange in the Heart
Necessity or Luxury?
From the Unit of Cardiac Physiology (D.A.E.), Oxford Rd, Manchester, UK; and the Laboratory of Experimental Cardiology (K.R.S.), KUL, Campus Gasthuisberg, Leuven, Belgium.
Correspondence to D.A. Eisner, Unit of Cardiac Physiology, 1.524 Stopford Bldg, Oxford Rd, Manchester M13 9PT, UK. E-mail Eisner{at}man.ac.uk
See related article, pages 604–611
Key Words: sodium calcium exchange • calcium • arrhythmia
The sodium calcium exchange (NCX) was first discovered in cardiac muscle1 and squid axon2 and has since been found in most cell types (see reviews3,4). It accounts for the previously observed effects of sodium on cardiac contractility.5 The exchanger transports 
3 Na+ ions per Ca2+.6–8 This stoichiometry has three important consequences.1 Ca2+ fluxes and hence intracellular Ca2+ concentration ([Ca2+]i) are very sensitive to intracellular Na+ concentration ([Na+]i),9 and therefore, even small changes of [Na+]i have large effects on contractility. In the case of vascular smooth muscle, the [Na+]i-dependence of NCX has been suggested to account for aspects of hypertension.102 The activity of NCX is affected by membrane potential with depolarization hindering Ca2+ efflux and increasing Ca2+ influx. This voltage dependence may produce net Ca2+ entry into the cell at the start of the action potential and contribute to triggering Ca2+-induced Ca2+ release from the sarcoplasmic reticulum (SR)11 (although the NCX is a much weaker trigger of Ca2+ release than is the L-type Ca2+ current12).3 Changes in the activity of NCX attributable to an increase in [Ca2+]i activate inward current. Specifically, (1) inward current activated by the systolic Ca2+ transient will contribute to maintaining the action potential plateau,13 and (2) current activated by abnormal Ca2+ release in diastole generates14,15 the delayed afterdepolarizations known to be a cause of triggered arrhythmias.16–18
Effects of NCX on Systolic [Ca2+]i
NCX does not only control the cytoplasmic Ca2+ concentration but, indirectly, also regulates the amount of Ca2+ stored in the SR. This occurs because, on any one beat, the change of total Ca2+ in the cell is the difference between the influx of Ca2+ into the cell (largely via the L-type Ca2+ current) and the efflux (largely on NCX). In the steady state, influx and efflux must be equal and therefore if the influx is maintained constant from beat to beat so must the efflux. If there is less NCX, then a larger systolic Ca2+ transient will be required to activate the same efflux. Until this larger transient is achieved, influx will exceed influx and SR content will increase until they are equal.19,20
One consequence of this is that SR Ca2+ content is decreased by maneuvers that decrease the ratio of the activities of SERCA to NCX as has been reported to occur in human heart failure when SERCA activity decreases and that of NCX increases.21
Ca2+ Removal by Mechanisms Other Than NCX
The only other known mechanism that can pump Ca2+ out of the cardiac cell is the plasma membrane Ca2+-ATPase (PMCA).22 Lack of specific inhibitors has made it difficult to obtain a quantitative estimate of the activity of the PMCA compared with that of NCX. One approach is to release Ca2+ from the SR using the rapid application of caffeine. The Ca2+ cannot be taken back into the SR and the rate constant of its subsequent decay represents the rate of Ca2+ removal by other systems including NCX, PMCA, and possibly, mitochondria. When the experiment is repeated with NCX inhibited (Na+-free solution or application of an inhibitor such as Nickel) then any contribution from NCX should be eliminated and the relative contributions of NCX versus other mechanisms can be calculated. In rat cells, the non-NCX mechanisms contribute between 9 and 32%.23,24 Relevant to the present article, in mice cells, 19% is via non-NCX mechanisms.25 Another approach is to examine the physiology of the cell in the absence of NCX and see whether stable contraction can be observed. In single cells, Na+ can be eliminated from the intra- and extracellular milieu resulting in full block of NCX. [Ca2+]i rises to high levels, SR Ca2+ content increases and repeated stimulation leads to spontaneous Ca2+ oscillations,26,27 unless Ca2+ influx is severely reduced by decreasing external Ca2+.
NCX Knockout
From this one would expect that cardiac function would be grossly abnormal in animals in which NCX is knocked out, and indeed, full knockout of NCX1 is embryonically lethal.28–30 The article in this issue of Circulation Research by Henderson et al31 uses the Cre/LoxP system to knock out NCX in 80% to 90% of cells in the ventricle at a slightly later point in development and restricted to the ventricle. Despite this gross insult, the animals live to adulthood and display only a 30% or so decrease of contractility, suggesting that the embryonic death in the whole animal knockout results from early developmental problems (possibly failure of the heart beat) or from some tissue other than the heart. The authors checked that the reduction of NCX was not compensated for by increased expression of PMCA. Perhaps most striking are the results from single cells that showed that not only was the amplitude of the Ca2+ transient the same in control and knockout cells, but there was also no difference in its modulation by changing stimulus frequency or adding isoprenaline. Indeed, the reader is challenged to cut out the traces from the article and attempt to identify which is from control versus knockout! There was also no difference in SR content between control and knockout cells. Interestingly, the amplitude of the L-type Ca2+ current was decreased to 50% of control and it was suggested that this reduction of Ca2+ entry might allow the PMCA alone to provide the required efflux. Although qualitatively in the right direction, there is a quantitative problem with this hypothesis. If in control cells the activity of the PMCA was equal to that of the NCX, then knocking out NCX could be compensated for exactly by decreasing Ca2+ entry to 50%. Even allowing for the possibility that the shorter action potential of the KO may further decrease Ca2+ entry, the required PMCA activity is greater than that found in previous work. A rough estimate of the activity of the PMCA can be obtained from the half times of decay of [Ca2+]i after applying caffeine. The decay rate in knockouts was 20% of that in controls, suggesting that, in controls, the PMCA accounts for 20% of efflux. This value is comparable to the amount of extrusion by PMCA calculated from the decay of caffeine transients with NCX blocked by Ni in normal mouse myocytes25 and is consistent with the absence of upregulation of PMCA. To gain further insight then into the remarkably unaltered Ca2+ transients during field stimulation additional experiments will be needed. As indicated by the authors the Ca2+ influx, SR content and Ca2+ transients were measured in separate experiments. A comprehensive analysis of fluxes during voltage clamp may more clearly delineate the actual changes in Ca2+ influx and efflux, and might quantify the changes in fraction of Ca2+ entry versus Ca2+ cycling across the SR. The nature of the efflux pathways can then also be further characterized. Currently, it is also unclear what causes the significant decrease in cardiac function in vivo. A further study of Ca2+ transients at more physiological stimulation rates and temperature might reveal a more pronounced cellular deficit.
By showing that the heart can survive remarkably well in the absence of NCX, the work of Henderson et al emphasizes the need to better characterize fluxes carried by the PMCA in the heart. As pointed out by the authors, the data indicate the remarkable potential for adaptation to reduced NCX at the cellular level. Whereas in mice the NCX apparently is not a necessity, it is certainly no luxury either as evidenced by the shorter lifespan and mortality of females after giving birth. It is also important to keep in mind that the limited Ca2+ influx during the very brief action potential of the mouse myocyte is different from the Ca2+ influx during the action potential in larger mammals, including human. The consequences of reduced NCX may then be quite different, and not as well tolerated. With the development of pharmacological blockers of NCX well under way4,32 the role of the NCX in Ca2+ homeostasis will remain an important issue.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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Circ. Res. 2004 95: 604-611.
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