Experimental Physiology
90.1 pp 3-12
DOI: 10.1113/expphysiol.2004.029231
© The Physiological Society 2005
Stability and instability of regulation of intracellular calcium
D. A. Eisner1,
M. E. Diaz2,
Y. Li1,
S. C. O'Neill1 and
A. W. Trafford1
1 Unit of Cardiac Physiology, University of Manchester, 1.524 Stopford Building, Oxford Road, Manchester M13 9PT, UK
2
Veterinary Biomedical Sciences, Royal (Dick) School of Veterinary Studies, The University of Edinburgh, Summerhall, Edinburgh EH9 1QH, UK
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Abstract
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[Ca2+]i is used as a signal in many tissues. In this review we discuss the mechanisms that regulate [Ca2+]i and, importantly, what determines their stability. Brief mention is made of the effects of feedback gain and delays on stability. The control of cytoplasmic Ca concentration is shown to be generally stable as Ca pumping is essentially an instantaneous function of [Ca2+]i. In contrast, regulation of the Ca content of intracellular stores may be less stable. One example of this is instability in the control of sarcoplasmic reticulum (SR) Ca content in cardiac muscle. An increase of SR Ca content increases the systolic Ca transient amplitude. This in turn decreases Ca influx into the cell and increases efflux, thereby restoring SR Ca to control levels. This feedback system has an inherent delay and is potentially unstable if the gain is increased beyond a certain level. This instability produces Ca transients of alternating amplitude and may contribute to the clinical syndrome of pulsus alternans.
(Received 4 October 2004;
accepted after revision 2 November 2004; first published online 30 November 2004)
Corresponding author D. A. Eisner: Unit of Cardiac Physiology, University of Manchester, 1.524 Stopford Building, Oxford Road, Manchester M13 9PT, UK. Email: eisner{at}man.ac.uk
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Introduction
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An increase of cytoplasmic [Ca2+]i underlies many important physiological activities, including synaptic transmission, muscle contraction and hormonal secretion. Much attention has been paid to how [Ca2+]i is controlled. In general, the increases of [Ca2+]i that underlie activity can result from either influx from the extracellular fluid or release from intracellular stores. For these processes to work effectively, it is important that the concentrations of Ca in both the cytoplasm and intracellular stores are regulated. The purpose of this review is to consider how this regulation occurs and what determines its stability. Although much of the discussion concerns cardiac muscle, many of the issues and concepts are generally applicable.
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Non-technical overview of stability conditions
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Throughout this review we will be concerned with the stability of the various feedback processes that regulate [Ca2+]i and it is essential to consider briefly the factors that determine whether a feedback system is stable. In general, feedback systems measure the level of the parameter that is to be controlled. If the value is too high corrective action is taken to reduce it; if too low it is increased. In this context two parameters are important: (1) the feedback gain and (2) whether there are delays in the system. We consider these in turn.
The gain of a feedback system can be defined as the change in output produced by a given change of input. The greater the gain the more tightly a negative feedback system controls the input. However, too high a gain can result in instability if there are delays in the system. The presence of a delay means that the feedback measures the input at a certain time but corrective action is only taken after a delay when the value of the signal may well have changed. A good example of this is provided by the example of a shower. Here the object is to control the temperature of the water emerging. This is sensed by the person in the shower who takes corrective action by adjusting the setting of the mixer valve. However, there is usually a length of pipe between the mixer valve and the showerhead and this produces a delay before the water emerges and its temperature is sensed. It is common experience that, particularly in a shower to which one is not accustomed, the temperature can oscillate between too hot and too cold. Experience also shows that the best way to stop the temperature oscillating is to make small, gradual adjustments of the mixer valve rather than large changes. In other words, applying low feedback gain mitigates against the problems produced by delays. We will return to this point in the context of cell calcium regulation later.
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The control of cytoplasmic Ca concentration
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[Ca2+]o is much greater than [Ca2+]i. This, coupled with the negative membrane potential, means that there is a large electrochemical driving force for Ca to enter the cell. This Ca influx can occur through a variety of channels, including the various voltage-gated channels, receptor-operated and store-operated channels. Influx is compensated by active transport of Ca out of the cell on both Na–Ca exchange (NCX) and the Ca-ATPase (PMCA). The dependence of efflux on [Ca2+]i is shown in Fig. 1A for two cases: (1) a system with a low Kd and Vmax and (2) one with a high Kd and Vmax. In the steady state the pump flux must exactly balance the influx (here called leak) of Ca into the cell. Figure 1B shows steady-state [Ca2+]i plotted as a function of leak influx. It is clear that a given fractional change of leak (from an initial value of 1.0) has a larger effect on [Ca2+]i when [Ca2+]i is elevated above Km. Furthermore, the low Kd curve displays a greater sensitivity of [Ca2+]i to influx than does the high Kd curve.
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Figure 1. Control of [Ca2+]i by a simple pump leak balance
A, dependence of pump efflux on [Ca2+]i. The continuous line is for a pump with a low Kd and Vmax (Kd = 0.1 µM; Vmax = 2 arbitrary units). The dashed line represents a higher Kd pump (Kd = 1.9 µM; Vmax = 20 arbitrary units) The two curves have the same rate at a [Ca2+]i of 0.1 µM. B, the steady-state level of [Ca2+]i achieved when the pump is balanced by a constant leak influx of the specified magnitude. In both cases a leak of magnitude 1.0 results in a resting [Ca2+]i of 0.1 µM. Note that [Ca2+]i is more sensitive to a given change of leak for the low than the high Kd case.
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The above discussion is concerned only with the steady-state level of Ca that is reached. However, in this review we also consider the stability of Ca regulation. It is easy to show that the systems described above are always stable. This can be seen as follows. Consider the case if [Ca2+]i is greater than the steady-state level. This will mean that the pumped efflux will be greater than the influx and, as a consequence, [Ca2+]i will decrease. This will then decrease the efflux and this process will continue until a point is reached at which [Ca2+]i is reduced to the level at which influx is equal to efflux. Note that this system is perfectly stable because there are no delays. A change of [Ca2+]i is immediately sensed by the pump and appropriate changes in the flux are produced. One could envisage another method of control that is less stable. For example, a change of [Ca2+]i might activate a slow enzymic reaction, perhaps a phosphorylation that would then affect Ca pumping. This sort of delay could lead to instability if the pump rate remained high for a period when [Ca2+]i had been reduced to low levels. It has been reported that, in a reconstituted system, activation of the surface membrane Ca pump (PMCA) by calmodulin is slow (Caride et al. 1999) and, if a similar phenomenon occurs in intact cells, this could be a potential source of instability.
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The control of calcium concentration in endo- and sarcoplasmic reticulum
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From first principles, the control of store Ca concentration requires (1) a mechanism for sensing the store Ca content and (2) some way to control the net uptake into the SR. As we will see, the store Ca content can be measured either directly by a sensor in the SR or, alternatively, indirectly by an effect on cytoplasmic Ca concentration. The net uptake can be controlled either by directly regulating the activity of the Ca uptake mechanism (SERCA) or by changing the surface membrane fluxes to increase [Ca2+]i and thereby activating SERCA by elevating [Ca2+]i. An increase of SR or ER Ca content will increase the gradient against which SERCA pumps and thereby decrease its rate (Inesi & De Meis, 1989) and this may be an important mechanism in controlling ER content (Mogami et al. 1998). In addition, one report has proposed the existence of an SR luminal site that regulates SERCA activity such that depletion of SR Ca activates SERCA and thereby restores SR Ca content (Bhogal & Colyer, 1998). However, most work reports that depletion of SR leads to changes of sarcolemmal Ca fluxes. We will consider two examples: (1) store-operated Ca entry into the cell and (2) modulation of other sarcolemmal Ca pathways.
Store-operated channels
Much evidence, particularly in non-excitable cells, shows that depletion of the SR Ca content leads to an increase of Ca entry into the cell via so-called store-operated channels. This phenomenon is also known as capacitative Ca entry (Takemura & Putney, 1989). It is best demonstrated experimentally by removing external Ca and then depleting the store with an agonist. Subsequent readmission of Ca produces an increase of [Ca2+]i due to Ca entry through store-operated channels. For further information on this subject, the reader is referred to recent reviews (Elliott, 2001). If a second messenger is involved in linking the store Ca content to the Ca influx then, if this signal lags behind the content, the resulting delay could, in principle, result in instability. Whether instability results will depend on the gain of the feedback
Control of SR Ca in cardiac muscle
The amplitude of the systolic Ca transient is a steep function of SR Ca content (Bassani et al. 1995; Trafford et al. 1997, 2000). This means that changes of SR Ca content are a useful means to control the strength of cardiac contraction. The ability to maintain a constant force of contraction requires precise control of SR Ca content. Additionally, if the SR Ca content becomes too high, a condition known as Ca overload results in which spontaneous release of Ca occurs from the SR (Orchard et al. 1983; Wier et al. 1983). Such spontaneous Ca release can occur during diastole and activates inward sarcolemmal currents such as NCX, thereby producing arrhythmogenic afterdepolarizations (Ferrier et al. 1973; Lederer & Tsien, 1976; Mechmann & Pott, 1986; Rosen et al. 1973). It is therefore important to regulate SR Ca. With the exception of a report in neonatal cells (Uehara et al. 2002) there have been no reports of store-operated Ca entry in the heart and the question therefore arises as to how SR content is controlled.
The aim of the experiment illustrated in Fig. 2 was to investigate what factors determine the extent and rate of refilling of the SR. A qualitative measure of SR Ca content was obtained from the amplitude of the increase of cytoplasmic Ca concentration produced by adding 10 mM caffeine to release Ca from the SR. A control measurement is shown in response to the first caffeine application. After removing caffeine, the cell was left unstimulated for 20 s and then caffeine reapplied. This produces a much smaller increase of [Ca2+]i than in the control, indicating that 20 s is not sufficient for the SR to refill in the absence of stimulation. However, the last caffeine application was obtained after the cell had been stimulated five times and the response is the same size as for controls. This experiment shows that stimulation is required for the SR to refill fully.
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Figure 2. Stimulation increases SR refilling in ventricular myocytes
A, Indo-1 ratio (an index of [Ca2+]i). The cell was electrically stimulated three times. Caffeine (10 mM) was added for the periods indicated by the horizontal bars. The resulting increases of [Ca2+]i give a qualitative measure of SR content. B, current records (top) and their integrals (below) in response to the caffeine applications above. The integrals are calibrated in terms of the calculated SR content. Taken from an experiment on a ferret myocyte (Trafford et al. 1997).
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An obvious explanation of the stimulation-induced refilling of the SR is that stimulation activates Ca entry into the cell via the L-type Ca current. However, the explanation is somewhat more complicated, as shown in Fig. 3. Again, caffeine had been applied to empty the SR and stimulation recommenced after caffeine had been removed. The gradual increase of the amplitude of systolic [Ca2+]i is presumably due to recovery of SR Ca content. Figure 3B compares the Ca transients and associated currents for the first (1) and steady-state (21) responses. The steady-state responses show that the amount of Ca that enters via the L-type Ca current is equal to that which is pumped out of the cell on repolarization (see integrals). This is, of course, what would be expected in the steady state when Ca entry and exit should be balanced. The first response (1) shows two differences. First, the total amount of Ca that enters on the pulse is larger than in the steady state and the efflux is less. Consequently, there is a net influx of Ca on this first pulse. The second panel of Fig. 3A shows similar calculations for Ca entry and efflux on each pulse. At first, while the SR is empty, Ca influx is larger than efflux. However, as the SR refills, influx decreases and efflux increases. Previous work has shown that the decrease of influx is due to increased Ca-dependent inactivation of the L-type Ca current (Eckert & Chad, 1984; Kass & Sanguinetti, 1984; Yue et al. 1990) whereas the increase of efflux is as a result of the larger Ca transients producing increased activation of NCX. The lower panels of Fig. 3A show the net gain of Ca per pulse and (bottom) the calculated cumulative gain of Ca. It is important to note that the predicted gain of about 80 µmol l–1 is entirely due to changes in the L-type Ca current and NCX.
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Figure 3. Control of SR content is due to effects of the Ca transient on sarcolemmal currents
A, timecourse of original data. The top trace shows measurements of [Ca2+]i. The cell was held at –40 mV and 100 ms duration depolarizing pulses were applied to 0 mV at 0.5 Hz. Caffeine (10 mM) had been applied until 10 s before the record began in order to empty the SR. The next panel shows the Ca influx (via the L-type Ca current) and efflux on each pulse (measured as shown for the specimen records of B). The third panel shows the net Ca entry (influx–efflux) and the bottom panel the calculated cumulative gain of Ca. B, specimen records showing (from top to bottom): [Ca2+]i; membrane current; calculated net sarcolemmal Ca movement. The records were taken from the first and 21st pulses in A. The Ca movements were calculated by integrating the currents after making allowance for the stoichiometry (charges per Ca2+ transported) and allowing for the fact that some of the Ca efflux is produced by the electroneutral sarcolemmal Ca-ATPase. C, expanded tail currents for the two pulses. Data obtained from a ferret myocyte (Trafford et al. 1997).
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The data reviewed above constitute a simple yet powerful negative feedback system to regulate SR Ca content. As shown in Fig. 4A, an increase of SR content will increase the amplitude of the Ca transient. This, in turn, will decrease Ca entry and increase Ca efflux. As a result the SR Ca content will gradually decrease towards the initial value. In some ways, this feedback system has a similar effect to that produced by store-operated channels. The major differences are: (1) the sarcolemmal flux is produced by a combination of NCX and the L-type Ca channel rather than a store-operated channel and (2) the Ca content of the SR signals via changes of the amplitude of the systolic Ca transient rather than via some, as yet unidentified, cytosolic messenger. An interesting, if teleological, question is why the cardiac cell does not make use of a store-operated channel? It may simply be that the cardiac cell ‘needs’ a Ca influx through the L-type channel both to maintain the plateau of the action potential and to trigger calcium-induced calcium realease (CICR) and therefore this must be balanced by Ca efflux from the cell. It may be simpler to regulate these pathways than to make use of an additional, store-operated mechanism. It should also be noted that the heart must reach Ca balance on every beat and, even in humans, this can occur every 500 ms. This is a much faster timescale than that over which store-operated pathways appear to operate.
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Figure 4. Homeostasis of SR Ca
Ca entry through the L-type Ca channel (1) triggers release of Ca from the SR via the RyR (2), resulting in a systolic Ca transient (3). Some of this Ca is then pumped out of the cell on the Na–Ca exchange (4) and also feeds back to inactivate Ca entry (5). A, stable conditions. An increase of SR content increases the Ca transient (3), resulting in more Ca efflux from the cell (4) and less Ca entry (5). As a result there is a compensatory decrease of SR content towards normal levels (6). B, unstable conditions when the feedback gain is too high. The same increase of SR content produces a larger increase of the Ca transient (3) and larger changes of sarcolemmal fluxes such that SR content falls below the normal level (6). On the next beat the low SR content will result in less efflux and more influx such that SR content increases back to the elevated level (data not shown).
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Cardiac alternans as an example of instability of control
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We have reviewed above some of the mechanisms that serve to regulate [Ca2+]i. In the remainder of this article we discuss the condition of cardiac alternans in which [Ca2+]i and contraction are not properly regulated and attempt to explain its origins in terms of Ca regulation. Pulsus alternans (Fig. 5A) was first reported as a beat to beat alternation in the arterial pressure pulse (Traube, 1872; Wenckebach & Winterberg, 1927; for reviews see Euler, 1999; Lab & Seed, 1993). This is often referred to as mechanical alternans to distinguish it from the associated phenomenenon of electrical alternans, which is characterized by alternation of action potential duration. Clinically it is associated with heart failure. A recent study of ventricular pressure in patients with moderate or severe heart failure found that about 20% showed pulsus alternans at rest and 40% showed pulsus alternans during a dobutamine challenge designed to mimic the effects of exercise (Kodama et al. 2004).
It has been suggested that alternans may result from differences in the diastolic length of the myocardium (Mitchell et al. 1963). However, a role for alternation of the underlying systolic Ca transient is supported by much work showing that in a variety of experimental models (Fig. 5B) the amplitude of the systolic [Ca2+]i transient alternates in phase with that of contraction (Lab & Lee, 1990). Several explanations have been suggested as to the origin of alternans.
- Properties of action potential restitution. If the action potential is prolonged, the diastolic interval before the next beat will be reduced. Therefore, time-dependent membrane currents, including the calcium current will not have recovered fully. This will decrease the duration of the next action potential. The next diastolic interval will be longer and alternans may result (Qu et al. 2000).
- Increased delays in Ca cycling. In this model, alternans is seen as an instability resulting from delays in the processes controlling Ca handling. When Ca is taken up into the SR by SERCA there may be a delay before it can be released from the SR. This delay could represent the time it takes either for Ca to move back to the release sites or, alternatively, for recovery from inactivation of the Ryanodine Receptor (RyR). In these models, following a first large release, little Ca will be released on the second stimulus. Consequently, there will have been time for Ca to be returned to the release sites and/or for the RyR to recover from inactivation and the third Ca transient will be large.
- Increased feedback gain. In this hypothesis, alternans is a consequence of too high a gain in the feedback loop (Adler et al. 1985; Eisner et al. 2000). As discussed by Díaz et al. (2004) the overall gain can be represented as:
The overall feedback gain then depends on two factors: (1) the relationship between SR Ca content and Ca release – the steeper this is the higher the gain; and (2) the change of net sarcolemmal flux produced by a given release of Ca from the SR. This will include the effect of released Ca on (a) the L-type Ca current and (b) the efflux on NCX. Figure 4B shows how increasing the dependence of Ca release on SR content can result in alternans.
In the remainder of this article we will discuss evidence suggesting that increased feedback gain can be an important factor in the origin of alternans and also the origin of this increased gain.
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Experimental conditions that produce alternans
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As mentioned above, alternans is associated with heart failure and ischaemia and this correlation is seen in animal studies (Dumitrescu et al. 2002; Hata et al. 1997; Qian et al. 2001; Wu & Clusin, 1997) as well as in humans (Hata et al. 1997; Kodama et al. 2001a,b; Ryan et al. 1955; Salerno et al. 1986). It is not, however, obvious as to which of the many changes of Ca handling that occurs in failure is the cause of alternans, and we will return to this point below. It is therefore useful to review those conditions that produce alternans in otherwise normal cardiac preparations. One general observation is that an increase of stimulation frequency predisposes to alternans. This would be expected from many of the models of alternans mentioned above. For example, the effects of any delay will be more obvious at higher rates: there will be less time for the RyR to recover from inactivation and for Ca to be made available at the release site. Furthermore, electrical restitution effects may be more prominent. Acidosis also produces alternans (Orchard et al. 1991) and this may be relevant to the situation in poorly perfused regions of the myocardium. We have studied the effects of intracellular acidosis in isolated myocytes and found that different regions of the cell alternated from beat to beat. On any given beat, different regions could either alternate in or out of phase with each other (Díaz et al. 2002). The entirety of the cell showed an initial small systolic Ca transient. This was then followed by a secondary increase of [Ca2+]i in some regions of the cell. Within such regions, the rise of [Ca2+]i began in one place and then propagated as a wave through the rest of the region. Similar effects to those of acidosis were produced by the local anaesthetic tetracaine (Fig. 6), an agent that decreases the open probability of the RyR (Györke et al. 1997; Xu et al. 1993). Because acidosis also decreases RyR open probability (Xu et al. 1996), it is likely that the induction of alternans by either acidosis or tetracaine is due to a decrease of RyR open probability.
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Figure 6. Decreasing RyR open probability produces subcellular alternans
The panels show linescans. The cell was loaded with fluo-3 and stimulated with depolarizing pulses from –40 to 0 mV. Panels A and B were obtained under control conditions and show uniform systolic increases of [Ca2+]i. The other panels were in the presence of tetracaine (50 µM). Note that (1) on each pulse some regions release Ca but others do not and (2) if a region releases on one pulse it does not on the next (Díaz et al. 2002).
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Calcium waves in alternans
Previous work has shown that Ca waves in the heart occur only above a threshold SR Ca content (Díaz et al. 1997). Our explanation for the occurrence of alternans and waves in tetracaine or acidosis is as follows (see also Fig. 9 below). Under control conditions, opening of L-type Ca channels results in the opening of adjacent RyRs. Therefore, the opening of a large proportion of the L-type Ca channels results in the opening of a correspondingly large fraction of RyRs and thence to a uniform Ca transient. In the presence of tetracaine or acidosis the open probability of the RyR is decreased. Therefore, not all the RyRs will be activated by the adjacent L-type Ca channel. However, if the Ca content of the SR is above the threshold for wave propagation then Ca released from the RyRs that do open can spread to regions where the L-type channel has not opened and a large transient will result. As a result of this wave some Ca will be lost from the cell. On the next Ca transient the SR content will be below the threshold for wave propagation and the transient will be small. In other words, the threshold dependence of wave propagation produces exactly the sort of high gain suggested, above, to produce alternans.
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Figure 9. Diagrammatic representation of the role of Ca waves in the origin of alternans
In each panel the upper part shows 3 L-type Ca channels in the sarcolemma (SL). The lower red part shows the SR with the arrows denoting Ca efflux through the RyRs. In each condition the results of three consecutive pulses are shown from top to bottom. From left to right the conditions are as follows. (1) Control. On all three pulses all three L-type channels open and trigger release of Ca from the SR. (2) Tetracaine. Here all three L-type channels open but the open probability of the RyR is decreased such that only one opens. On the first pulse the SR content is sufficient for waves but, the resulting depletion of the SR means that there is no wave on pulse 2. (3) Low voltage. Here only one L-type channel opens. This triggers release of Ca from the adjacent RyR. On pulse 1 the SR Ca content is above the threshold for wave propagation and this Ca release sets up a wave that propagates to the neighbouring RyRs where it triggers further release. This decreases SR content such that on pulse 2 the wave cannot occur. The SR then refills and, on pulse 3, waves are again seen.
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What happens to SR Ca content during alternans?
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The hypotheses reviewed above make different predictions for the SR Ca content during alternans. If alternans results from the delay for Ca to be returned to the release sites or slow recovery from inactivation of the RyR then overall SR Ca will be the same before the large and small Ca transients. By contrast, if alternans is due to a high feedback gain resulting in Ca efflux from the cell then the alternans of the systolic Ca transient will be accompanied by alternation of SR Ca content. Determining whether SR Ca alternates should shed light on the mechanism of alternans. Early attempts to investigate this issue gave equivocal results. In cat atrial cells the alternans of the Ca transient was not accompanied by any change of SR content, at least as measured by the amplitude of the caffeine-evoked increase of [Ca2+]i (Hüser et al. 2000). In experiments in which tetracaine was used to produce alternans, we calculated that any change of SR content would only be a few per cent (Díaz et al. 2002). The question therefore arose as to whether any change of SR content would be sufficient to account for the alternans. One problem with using tetracaine or acidosis to study this question is that, because different regions of the cell alternate out of phase with each other, one might not expect any great change of total cell SR Ca. In subsequent work we have turned to a different method to produce alternans.
The method used is to stimulate with small depolarizing pulses. As shown in Fig. 7, these activate a very small L-type Ca current and a marked alternans. As was the case for acidosis and tetracaine, the larger Ca transients are characterized by waves of Ca release spreading through the cell. These waves result in activation of most of the cell and the alternans is much more uniform than is the case with tetracaine. With this protocol it is possible to measure the SR content by adding caffeine at the time when either a small or a large Ca transient would have been expected. As shown in Fig. 8, the SR Ca content accompanying the large transient is about 20% greater than that associated with the small transient. This experiment shows that the alternans of the systolic Ca transient is, indeed, accompanied by alternans of SR content.
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Figure 7. Depolarization with small pulses produces Ca alternans
A, linescans showing changes of [Ca2+]i produced by depolarizing from –40 to 0 mV. Three consecutive pulses are shown. The first and third produce large Ca release whereas the second much less. Note that the large releases are not uniform and spread as several waves through the cell. B, Ca transients obtained by measuring the mean cellular fluorescence from 10 consecutive transients. C, averaged membrane current records corresponding to large (black) and small (red) Ca transients. Note that the L-type Ca current is the first downward deflection on the current trace. NCX current develops after repolarization (Díaz et al. 2004).
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Figure 8. SR Ca content changes during alternans
A, stimulation was stopped after either a large (left) or small (right) transient. Caffeine (10 mM) was applied at the time when the next stimulus would have occurred. The caffeine-evoked increase of [Ca2+]i is greater in the right-hand panel when a large transient would have been expected. B, caffeine-evoked NCX currents (top) and their integrals (bottom). C, mean data: SR Ca content measured from the integrated NCX current is greater before a large than a small transient. D, histogram showing the mean amplitudes of small and large transients during alternans (Díaz et al. 2004).
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Figure 8 shows that a 20% change of SR Ca content is associated with a roughly five-fold alternation of the amplitude of the systolic Ca transient. This steep dependence of Ca release on SR content is, again, due to the threshold dependence of wave propagation (see the right-hand panel in Fig. 9). It is also associated with a large change of Ca efflux from the cell (Fig. 7). Again, alternans is seen to be associated with a high feedback gain (Díaz et al. 2004).
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Relation to pulsus alternans
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An unaswered question is the extent to which the mechanisms described above involving wave propagation apply to clinical pulsus alternans. It is likely that regions of ischaemic hearts may be acidotic thereby decreasing RyR opening and producing effects similar to those shown in Fig. 6. Furthermore, metabolic inhibition decreases the calcium current (Lederer et al. 1989) and may lead to effects such as those shown in Fig. 7. An outstanding issue, however, is whether the effects seen at the level of single cells are coordinated to produce alternans of the whole ventricle.
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Summary
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In this review we have described some of the factors that determine the stability of the control of intracellular calcium. With particular reference to cardiac muscle we describe a simple feedback mechanism for controlling both SR and cytoplasmic Ca and show that instabilities in this control can result in beat to beat alternation of the amplitude of the Ca transient. As mentioned above, there are other explanations for such alternans and future work needs to assess the relative contributions of the various mechanisms.
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Abstract
Introduction
