The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

https://www.physiology.org/doi/pdf/10.1152/physrev.1999.79.3.763

Sodium/Calcium Exchange: Its Physiological Implications

ObjectivesIn view of the critical role of intracellular Ca2+-overload in the genesis of myocyte dysfunction and the ability of reactive oxygen species (ROS) to induce the intracellular Ca2+-overload, this article is concerned with analysis of the existing literature with respect to the role of oxida

Role of oxidative stress in cardiovascular diseases.

Molecular oxygen: friend and foe. The role of the oxygen free radical system in the calcium paradox, the oxygen paradox and ischemia/reperfusion injury

In studies of ischemia and reperfusion, a major experimental problem has been the inability to measure intracellular ionized calcium ([Ca2+]i) in the intact heart. We have developed a new approach in which the bioluminescent calcium indicator aequorin is used to measure [Ca2+]i in the isolated, coronary-perfused ferret heart. Aequorin is loaded into subepicardial myocytes of the left ventricle, and the signals are recorded simultaneously along with isovolumic left ventricular (LV) pressure at a constant pacing rate. This system shows 1) no attenuation or change of time course of LV pressure development or coronary perfusion pressure after aequorin loading; 2) consistent responses to physiological interventions and drugs; 3) individual aequorin and pressure signals that do not require signal averaging for analysis; and 4) [Ca2+]i levels comparable with those reported in tissue or isolated myocyte cell preparations. During 5 minutes of hypoxia, diastolic [Ca2+]i and LV diastolic pressure increased while the systolic values of both [Ca2+]i and pressure decreased. The peak-to-peak systolic [Ca2+]i versus LV isovolumic pressure relation remained close to the control curve. In contrast, during 3 minutes of global ischemia, LV systolic and diastolic pressures fell rapidly, while [Ca2+]i increased substantially. The [Ca2+]i versus pressure relations for both systole and diastole shifted to the right, indicating desensitization of the contractile apparatus to [Ca2+]i. These results provide evidence that different primary mechanisms determine the systolic and diastolic responses to acute hypoxia versus ischemia. During hypoxia, changes in [Ca2+]i handling probably play a major role, while during ischemia, changes in the Ca2+ sensitivity of the myofilaments appear to be of primary importance in the modulation of contractile dysfunction.

Direct measurement of changes in intracellular calcium transients during hypoxia, ischemia, and reperfusion of the intact mammalian heart.

Both reperfusion (3, 26, 61, 73, 83, 89) and reoxygenation (14, 27, 28, 57, 58) after prolonged ischemia and hypoxia are associated with a large increase in CaH content in severely damaged myocardium, however CaH overload is not seen in the myocardium that is not irreversibly injured (36). Increased uptake of CaH correlates with the depressed recovery of mechanical function of the myocardium (3, 83). Therefore excessive Ca2+ accumulation has been implicated as a primary event in irreversible myocardial injury and cell necrosis (20). Increased influx of Ca2+ during reperfusion in excess of the handling capacity of systems for Ca2+ extrusion from the cytosol results in elevation of cytosolic-free [Ca2+]. Nevertheless, the exact mechanisms responsible for increased Ca2+ entry upon reperfusion have not been defined. Perturbations of ion transport systems in the sarcolemmal membrane appear to be responsible for increased Ca2+ accumulation in oxygen deficient myocytes because Ca2+ overload occurred without apparent ultrastructural changes in the membrane (14). The mechanisms involved in maintenance of Ca2+ homeostasis are complicated and interrelated with the regulatory systems of other cations. In addition, loss of membrane integrity and an increase in permeability can result from the action of endogenous enzymes (i.e. Ca2+­ dependent proteases or lipases) that are activated secondarily by an initial

Mechanisms of Ca2+ Overload in Reperfused Ischemic Myocardium

The effect of "sodium loading," of verapamil and of nifedipine on the gain in calcium and sodium, and on the loss of myoglobin, during the calcium paradox in adult rat hearts was examined. Raising cell sodium from 56.5 +/- 2.6 to 129.5 +/- 10.2 mumol sodium/gram dry weight did not alter the degree or rate of calcium gain or myoglobin release during calcium repletion after long periods (greater than 2 minutes) of calcium-free perfusion; under these conditions, and in the presence of 10 micrograms/liter verapamil, calcium gain was enhanced. However, after shorter periods (0.5-1.5 minutes), of calcium-free perfusion, calcium gain was enhanced in "sodium-loaded" hearts, even in the absence of verapamil, and particularly during the early stages of repletion. The presence of 1 and 10 mumol/liter dl-verapamil and 1 mumol/liter nifedipine before, during, and after 10 minutes of calcium-free perfusion significantly (P less than 0.01) slowed the early (up to 1 minute for verapamil and 2 minutes for nifedipine) but not the late gain in calcium. When verapamil was present, the late gain in calcium was actually enhanced. These agents also abolished the early (45 seconds) but not the late (greater than 2 minutes) gain in sodium that occurs during repletion. We propose that the gain in calcium that occurs during calcium repletion after a period of calcium-free perfusion can be divided into at least two phases (early and late), and that the early phase contains a verapamil/nifedipine-sensitive component and a verapamil/nifedipine-insensitive component, the latter probably involving sodium-calcium exchange.

Calcium, sodium, and the calcium paradox.

The role of contracture in the manifestation of calcium paradox-induced damage was examined using 2,3-butanedione monoxime (BDM) to inhibit myofibrillar activity. Calcium and sodium gain, loss of intracellular components, and changes in structure were monitored. If 30 mM BDM was added at the time of calcium repletion after 10 minutes of calcium-free perfusion, some protection was afforded, particularly at the early stages of calcium repletion. However, much greater protection was obtained if BDM was present during the final 2 minutes of calcium-free perfusion and throughout repletion. Sodium gain and loss of intracellular components were markedly attenuated, as was the incidence of severely contracted cells. Calcium gain, although significantly reduced during the period of repletion, was not abolished. After 10 minutes of repletion, a calcium content of 11.44 +/- 1.57 mumol/g dry wt was observed. This suggests that other noncontracture related routes of calcium entry are involved. If BDM is removed after 5 minutes of calcium repletion and perfusion is continued with BDM-free perfusate, there is a rapid gain of sodium, further gain of calcium, loss of intracellular components and the cells contract severely, tearing away from neighboring cells. It is evident, therefore, that returning calcium to hearts after a period of calcium-free perfusion under conditions that significantly reduce the typical calcium paradox-associated damage does not necessarily repair the underlying defect. These results support the hypothesis that contracture-induced sarcolemmal disruption may be responsible for the terminal manifestation of the calcium paradox.

Contracture and the calcium paradox in the rat heart.

Isolated spontaneously beating rat hearts were perfused in the Langendorff mode and divided into four groups, i.e., control (aerobic), hypoxic (95% N2-5% CO2), ischemic, and ischemic reperfused. After a total of 90 min of perfusion, the sarcolemma was isolated and enzymatically characterized. Ouabain-sensitive Na+-K+-ATPase was inhibited in all three experimental groups, whereas K+-stimulated phosphatase activity was decreased only in the ischemic and reperfused groups compared with control. 5'-Nucleotidase activity was inhibited (P less than 0.05) only in the ischemic group. Mg2+-ATPase activity was not different from control. Passive Ca2+ uptake and Ca2+ efflux were not significantly altered by any of the interventions. Na+-Ca2+ exchange rate, but not capacity, was decreased (by 32-42%) in the ischemic group but this was partially reversed on reperfusion. These results suggest that changes secondary to lack of flow rather than O2 play a major role in the etiology of ischemic damage to the membrane and that a 15-min period of reperfusion after 60 min of ischemia does not exacerbate this damage.

Sarcolemmal enzymes and Na+ -Ca2+ exchange in hypoxic, ischemic, and reperfused rat hearts.

The Ca2+ paradox is characterized by a rapid and uncontrolled entry of ca2+ Whilst the consequence of the resultant gain in Ca2+ are relatively well defined, uncertainty exists concerning the route of entry. Possible routes include passive diffusion across damaged sarcolemma and intercalated discs, active transport in exchange for K+ or Na+ and entry through the voltage-activated, Ca2+ slow channels. The following experiments were designed to differentiate between these possibilities.

Isolated spontaneously beating Sprague-Dawley rat hearts were perfused at 37ΰC with Ca2+ perfusion buffer for >1 min before starting Ca2+ repletion. Adding 2–4 m mol l−1 Co2+ or Mn2+ before, during but not coincident with Ca2+ repletion protected against the paradox as indicated by an absence of myoglobin release and Ca2+ overload. Despite this protection marked distortion of the glycocalyx occurred, with splitting of the basement coat and blebbing. The intercalated discs, however, remained intact. It seems unlikely therefore that splitting of the basement coat necessarily results in an uncontrolled entry of Ca2+

Since Mn2+ and Co2+ block Ca2+ entry through the slow channels their failure to prevent the uncontrolled entry of Ca2+ when added at the time of Ca2+ repletion favours the view that Ca2+ entry during the paradox does not depend upon entry of Ca2+ through the slow channels.

In other experiments hearts were preloaded with Na2+ to alter intracellular Na2+ before Ca2+ repletion. This procedure did not markedly affect the gain in Ca2+

These findings will be discussed in terms of the route(s) and consequence(s) of the uncontrolled entry of Ca2+ that occurs when Ca2+ is reintroduced after a period of Ca2+-free perfusion.

The biochemistry of uncontrolled calcium entry

The effect of pH of the reperfusion buffer on postischemic changes in tissue Ca and Na was examined in isolated Langendorff-perfused Sprague-Dawley rat hearts. Reperfusion began after 15-, 25-, or 60-min ischemia at 37 degrees C. After 60-min ischemia, reperfusion at pH 6.4 or 6.6 attenuated the reperfusion-induced Ca gain so long as the acidotic conditions were maintained (3.08 +/- 0.22, 1.37 +/- 0.41, and 16.96 +/- 1.18 mumol Ca gain/g dry wt for pH 6.4, 6.6, and 7.4, respectively after 15-min reperfusion). Conversely, reperfusion under alkalotic conditions (pH 7.9) after 60-min ischemia exacerbated the gain (27.45 +/- 4.75 and 8.92 +/- 1.53 mumol Ca gain/g dry wt during 5-min reperfusion at pH 7.9 and 7.4, respectively). Similar, but less pronounced Ca gains occurred during reperfusion after 15- or 25-min ischemia. Sodium content during reperfusion, but not during aerobic perfusion, was also found to be pH sensitive with acidosis causing a reduction and alkalosis an increase. These results could not be explained in terms of an effect of pH on recovery of high-energy phosphates, percentage "reflow" during reperfusion, or reperfusion-induced increases in tissue water or resting tension. The results are in agreement with the hypothesis that the "inhibitory" effect of acidosis on postischemic Ca overload could involve an effect of pH on the Na(+)-H+ exchanger and intracellular Ca storage.

Effect of acidosis and alkalosis on postischemic Ca gain in isolated rat heart.

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