Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.

Cardiac excitation–contraction coupling

Mitochondria play an important role in cell death and cardioprotection. During ischemia, when ATP is progressively depleted, ion pumps cannot function resulting in a rise in calcium (Ca2+), which f...

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Mechanisms Underlying Acute Protection From Cardiac Ischemia-Reperfusion Injury

Chesler, Mitchell. Regulation and Modulation of pH in the Brain. Physiol Rev 83: 1183-1221, 2003; 10.1152/physrev.00010.2003.—The regulation of pH is a vital homeostatic function shared by all tiss...

https://www.physiology.org/doi/pdf/10.1152/physrev.00010.2003

Regulation and Modulation of pH in the Brain

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrica...

Genesis and Regulation of the Heart Automaticity

▪ Abstract Plasma membrane Na+-Ca2+ exchange is an essential component of Ca2+ signaling pathways in several tissues. Activity is especially high in the heart where the exchanger is an important regulator of contractility. An expanding exchanger superfamily includes three mammalian Na+-Ca2+ exchanger genes and a number of alternative splicing products. New information indicates that the exchanger protein has nine transmembrane segments. The exchanger, which transports Na+ and Ca2+, is also regulated by these substrates. Some molecular information is available on regulation by Na+ and Ca2+ and by PIP2 and phosphorylation. Altered expression of the exchanger in pathophysiological states may contribute to various cardiac phenotypes. Use of transgenic approaches is beginning to improve our knowledge of exchanger function.

Sodium-calcium exchange: a molecular perspective.

The control of intracellular calcium is central to regulation of contractile force in cardiac muscle. This review illustrates how analysis of the control of calcium requires an integrated approach in which several systems are considered. Thus, the calcium content of the sarcoplasmic reticulum (SR) is a major determinant of the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient. The amplitude of the transient, in turn, controls Ca(2+) fluxes across the sarcolemma and thence SR content. This control of SR content influences the response to maneuvers that modify, for example, the properties of the SR Ca(2+) release channel or ryanodine receptor. Specifically, modulation of the open probability of the ryanodine receptor produces only transient effects on the Ca(2+) transient as a result of changes of SR content. These interactions between various Ca(2+) fluxes are modified by the Ca(2+) buffering properties of the cell. Finally, we predict that, under some conditions, the above interactions can result in instability (such as alternans) rather than ordered control of contractility.

Integrative Analysis of Calcium Cycling in Cardiac Muscle

The aim of this work was to investigate the role of sarcolemmal Ca2+-ATPase in rat ventricular myocytes. We have measured intracellular Ca2+ concentration ([Ca2+]i) using indo-1. The actions of the ATPase inhibitor carboxyeosin were studied.


Carboxyeosin increased resting [Ca2+]i and the magnitude of the systolic Ca2+ transient and slowed the rate of its relaxation by 5%.


Carboxyeosin increased the magnitude of the caffeine-evoked increase in [Ca2+]i and slowed its relaxation by 20%. Removal of extracellular Na+ slowed the rate constant by 80%. When Na+ was removed in a carboxyeosin-treated cell, the caffeine-evoked increase in [Ca2+]i did not decay.


Carboxyeosin increased the integral of the Na+-Ca2+ exchange current activated by caffeine. This is, in part, due to an increase in sarcoplasmic reticulum Ca2+ content.


When extracellular Na+ was removed, there was a transient increase in [Ca2+]i which then decayed. The rate of this decay was slowed by carboxyeosin by a factor of about four. The residual decay could be abolished with caffeine.


In the absence of extracellular Na+, increasing extracellular Ca2+ concentration ([Ca2+]o) elevated [Ca2+]i. In carboxyeosin-treated cells, [Ca2+]i was much more sensitive to [Ca2+]o.


These results demonstrate the role of a carboxyeosin-sensitive Ca2+-ATPase in the control of resting [Ca2+]i and the reduction in [Ca2+]i following an increase in [Ca2+]i.







In cardiac cells, as in many other cells, there is a large electrochemical gradient favouring Ca2+ entry into the cell from the extracellular fluid. Two sarcolemmal mechanisms are known to extrude Ca2+ from the cytoplasm: sarcolemmal Ca2+-ATPase and Na+-Ca2+ exchange. The role of Na+-Ca2+ exchange in Ca2+ homeostasis has been extensively characterized in cardiac muscle (Chapman & Tunstall, 1981; Allen et al. 1984; Crespo et al. 1990). In contrast, much less is known about Ca2+-ATPase. A sarcolemmal Ca2+-ATPase has been demonstrated in the heart (Caroni & Carafoli, 1981). It has been suggested that Ca2+-ATPase has a higher affinity and lower maximum velocity than Na+-Ca2+ exchange and may therefore be more important for control of resting [Ca2+]i than for removing larger Ca2+ loads produced by stimulation (DiPolo & Beauge, 1979) although recent work has questioned this functional distinction (Lamont & Eisner, 1996).

Most previous work studying the role of Ca2+-ATPase in cardiac muscle has been performed under conditions that inhibit Na+-Ca2+ exchange to reveal any potential contribution of ATPase. Specifically, the following observations have led to the suggestion that something other than Na+-Ca2+ exchange, perhaps sarcolemmal Ca2+-ATPase, may have a significant functional role in Ca2+ homeostasis, at least under experimental conditions. (1) When caffeine is applied to release Ca2+ from the sarcoplasmic reticulum (SR), inhibition of Na+-Ca2+ exchange slows but does not abolish the decay of [Ca2+]i (O'Neill et al. 1991; Bassani et al. 1992; Negretti et al. 1993; Bassani et al. 1994). It has been suggested that the Na+-Ca2+ exchange-independent component of the decay of [Ca2+]i is due to a combination of sarcolemmal Ca2+-ATPase and mitochondrial sequestration. Separation of mitochondrial and sarcolemmal Ca2+-ATPase components has been achieved by inhibiting the former with uncouplers or the latter by elevating extracellular Ca2+ (Bassani et al. 1992; Negretti et al. 1993). However, these approaches are both rather non-specific. (2) Removal of extracellular Na+ produces an increase in [Ca2+]i as Ca2+ enters the cell via Na+-Ca2+ exchange. However, [Ca2+]i then falls to levels which are only slightly elevated above the control. This secondary relaxation of [Ca2+]i may reflect the activity of a sarcolemmal Ca2+-ATPase. (3) In the absence of extracellular Na+, [Ca2+]i can still be regulated and respond to changes in extracellular Ca2+ concentration (Lamont & Eisner, 1996) and, again, this may reflect the activity of a sarcolemmal Ca2+-ATPase.

Evidence for the role of sarcolemmal Ca2+-ATPase would be much stronger if there was a specific inhibitor which could be applied to dissect out its contribution. It has been shown that ATPase is specifically inhibited by carboxyeosin (Gatto & Milanik, 1993). In rabbit and ferret ventricular myocytes, carboxyeosin has been used to show that Ca2+-ATPase makes an appreciable contribution to the decay of [Ca2+]i during a caffeine-evoked response (Bassani et al. 1995).

Using carboxyeosin, in the present paper we show that sarcolemmal Ca2+-ATPase has significant effects on cell function even under physiological conditions. Furthermore it provides the major component of the secondary decay of [Ca2+]i following Na+ removal and is essential for [Ca2+]i homeostasis in Na+-free solutions.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-7793.1999.109ad.x

The role of sarcolemmal Ca2+‐ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

We have investigated the mechanisms responsible for the changes of systolic Ca2+ that occur in voltage-clamped rat ventricular myocytes during acidosis produced by application of the weak acid butyrate (30 mM). Intracellular pH regulation was inhibited with dimethylamiloride (bicarbonate-free solution). The application of butyrate produced an intracellular acidification of 0.33 pH units. This was accompanied by a decrease in systolic Ca2+ to about 50% of control. However, within 2 min, systolic Ca2+ returned to control levels. The decrease in systolic Ca2+ was accompanied by a decrease in the Na+-Ca2+ exchange current observed on repolarisation so that the calculated Ca2+ efflux on Na+-Ca2+ exchange was less than the entry on the L-type Ca2+ current. The magnitude of the Na+-Ca2+ exchange current recovered along with systolic Ca2+ until it equalled the Ca2+ entry on the L-type Ca2+ current. From the measurement of Ca2+ fluxes, it was calculated that, during acidosis, the cell gains 121.6+/-16.2 micromol l(-1) of Ca2+. This is equal to the measured increase of sarcoplasmic reticulum (SR) calcium content obtained by applying caffeine (20 mM) and integrating the resulting Na+-Ca2+ exchange current. We conclude that the recovery of the amplitude of the systolic Ca2+ transient is due to decreased SR calcium release, resulting in reduced Ca2+ efflux from the cell leading to increased SR calcium content.

The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes

The effects of carboxyeosin, an inhibitor of the sarcolemmal Ca-ATPase, were studied on intracellular Ca and membrane currents in isolated rat ventricular myocytes. In the absence of carboxyeosin, 150-ms-duration depolarizing pulses from –40 to 0 mV resulted in an L-type Ca current on depolarization and a Na-Ca exchange ”tail” current on repolarization. The calculated entry of Ca on the L-type current was 1.3 times greater than the efflux via the Na-Ca exchange. The addition of carboxyeosin (20 µM) resulted in either an increase of the Na-Ca exchange current or a decrease of the L-type Ca current such that the Ca entry and efflux were exactly equal. These results suggest that, under control conditions, a carboxyeosin-sensitive Ca-ATPase contributes about 24% of the total Ca efflux from the cell and, therefore, that the sarcolemmal Ca-ATPase has a significant role in regulation of sarcolemmal Ca fluxes.

The effects of inhibition of the sarcolemmal Ca-ATPase on systolic calcium fluxes and intracellular calcium concentration in rat ventricular myocytes.

The aim of this work was to obtain the first quantitative measurements of Ca2+ influx and efflux in quiescent cardiac cells. The relationship between free and total Ca2+ was obtained during a caffeine application. This buffering curve was then used to calculate changes of total Ca2+ from measurements of free cytosolic [Ca2+] ([Ca2+]i) made with Indo-1. The rate of Ca2+ removal from the cytoplasm was calculated by differentiating total Ca2+ with respect to time. The dependence of d(total Ca2+)/dt on [Ca2+]i was hyperbolic. Inhibition of either Na+-Ca2+ exchange (by addition of 10 mmol l(-1) NiCl2 or removal of external Na+) or the sarcolemmal Ca2+-activated adenosine triphosphatase (Ca2+-ATPase) (with carboxyeosin) decreased the calculated efflux. In both cases, the main effect was on the apparent maximum rate (Vmax) with little effect on the Michaelis-Menten constant (Km). These results suggest that the Na+-Ca2+ exchange and Ca2+-ATPase have very similar affinities for [Ca2+]i and that their fractional contributions do not change over the systolic range of [Ca2+]i. Ca2+ influx was quantified in two ways. The first method was to extrapolate the curve relating Ca2+ efflux to [Ca2+]i to zero [Ca2+]i. This gave a value of 4.49+/-0.54 micromol l(-1) s(-1) which was reduced to zero by either removal of external Ca2+ or addition of Ni2+. In other experiments external Ca2+ was removed and the maximum rate of fall of total Ca2+ calculated as 2.53+/-0.93 micromol l(-1) s(-1). This approach can be used to provide a quantitative analysis of the control of resting [Ca2+]i.

Ho Sook Choi

Papers

Integrative Analysis of Calcium Cycling in Cardiac Muscle

The role of sarcolemmal Ca2+‐ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes

The effects of inhibition of the sarcolemmal Ca-ATPase on systolic calcium fluxes and intracellular calcium concentration in rat ventricular myocytes.

Measurement of calcium entry and exit in quiescent rat ventricular myocytes.