Dependence of Na+-Ca2+ exchange and Ca2+-Ca2+ exchange on monovalent cations

Acute kidney injury

Acute kidney injury (AKI) is defined by a sudden loss of excretory kidney function. AKI is part of a range of conditions summarized as acute kidney diseases and disorders (AKD), in which slow deterioration of kidney function or persistent kidney dysfunction is associated with an irreversible loss of kidney cells and nephrons, which can lead to chronic kidney disease (CKD). New biomarkers to identify injury before function loss await clinical implementation. AKI and AKD are a global concern. In low-income and middle-income countries, infections and hypovolaemic shock are the predominant causes of AKI. In high-income countries, AKI mostly occurs in elderly patients who are in hospital, and is related to sepsis, drugs or invasive procedures. Infection and trauma-related AKI and AKD are frequent in all regions. The large spectrum of AKI implies diverse pathophysiological mechanisms. AKI management in critical care settings is challenging, including appropriate volume control, nephrotoxic drug management, and the timing and type of kidney support. Fluid and electrolyte management are essential. As AKI can be lethal, kidney replacement therapy is frequently required. AKI has a poor prognosis in critically ill patients. Long-term consequences of AKI and AKD include CKD and cardiovascular morbidity. Thus, prevention and early detection of AKI are essential.

The sodium-calcium exchange system

Sodium-calcium counter-transport represents one of a number of processes for transporting calcium ions across cellular membranes. The physiological importance of the exchanger is outlined and its underlying mechanism discussed in terms of a comparison of the partial reactions of Na+-Ca2+ exchange in intact cells with those of plasma membrane vesicles.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Na:Ca Stoichiometry and Cytosolic Ca-Dependent Activation of NCX in Intact Cardiomyocytes

We are studying both Na:Ca exchange stoichiometry and cytosolic [Ca] ([Ca]i)-dependent regulation of Na-Ca exchange (NCX) in intact rabbit ventricular myocytes. Analysis of NCX fluxes in subcellular systems strongly supports a dominant 3Na:1Ca exchange, and our measurements in intact cells confirm this. However, in intact native cells, local ion gradients and other factors complicate the process of inferring stoichiometry. From a functional viewpoint, NCX stoichiometry is near 3:1 but is affected by ion accumulation/depletion as well as non-NCX fluxes. We and others have viewed [Ca]i-dependent NCX regulation as a static process (dependent on instantaneous local [Ca]i). However, evidence from subcellular and expression systems shows the process to be dynamic, and our observations confirm this to be the case in intact cardiac cells as well.

Stoichiometry of the Cardiac Na+/Ca2+ exchanger NCX1.1 measured in transfected HEK cells

The stoichiometry with which the Na+/Ca2+ exchanger, NCX1, binds and transports Na+ and Ca2+ has dramatic consequences for ionic homeostasis and cellular function of heart mycocytes and brain neurons, where the exchanger is highly expressed. Previous studies have examined this question using native NCX1 in its endogenous environment. We describe here whole-cell voltage clamp studies using recombinant rat heart NCX1.1 expressed heterologously in HEK-293 cells. This system provides the advantages of a high level of NCX1 protein expression, very low background ion transport levels, and excellent control over clamped voltage and ionic composition. Using ionic conditions that allowed bi-directional currents, voltage ramps were employed to determine the reversal potential for NCX1.1-mediated currents. Analysis of the relation between reversal potential and external [Na+] or [Ca2+], under a variety of intracellular conditions, yielded coupling ratios for Na+ of 1.9-2.3 ions per net charge and for Ca2+ of 0.45 +/- 0.03 ions per net charge. These data are consistent with a stoichiometry for the NCX1.1 protein of 4 Na+ to 1 Ca2+ to 2 charges moved per transport cycle.

Effect of extracellular Mg(2+) on ROS and Ca(2+) accumulation during reoxygenation of rat cardiomyocytes

The effects of Mg(2+) on reactive oxygen species (ROS) and cell Ca(2+) during reoxygenation of hypoxic rat cardiomyocytes were studied. Oxidation of 2',7'-dichlorodihydrofluorescein (DCDHF) to dichlorofluorescein (DCF) and of dihydroethidium (DHE) to ethidium (ETH) within cells were used as markers for intracellular ROS levels and were determined by flow cytometry. DCDHF/DCF is sensitive to H(2)O(2) and nitric oxide (NO), and DHE/ETH is sensitive to the superoxide anion (O(2)(-).), respectively. Rapidly exchangeable cell Ca(2+) was determined by (45)Ca(2+) uptake. Cells were exposed to hypoxia for 1 h and reoxygenation for 2 h. ROS levels, determined as DCF fluorescence, were increased 100-130% during reoxygenation alone and further increased 60% by increasing extracellular Mg(2+) concentration to 5 mM at reoxygenation. ROS levels, measured as ETH fluorescence, were increased 16-24% during reoxygenation but were not affected by Mg(2+). Cell Ca(2+) increased three- to fourfold during reoxygenation. This increase was reduced 40% by 5 mM Mg(2+), 57% by 10 microM 3,4-dichlorobenzamil (DCB) (inhibitor of Na(+)/Ca(2+) exchange), and 75% by combining Mg(2+) and DCB. H(2)O(2) (25 and 500 microM) reduced Ca(2+) accumulation by 38 and 43%, respectively, whereas the NO donor S-nitroso-N-acetyl-penicillamine (1 mM) had no effect. Mg(2+) reduced hypoxia/reoxygenation-induced lactate dehydrogenase (LDH) release by 90%. In conclusion, elevation of extracellular Mg(2+) to 5 mM increased the fluorescence of the H(2)O(2)/NO-sensitive probe DCF without increasing that of the O(2)(-).-sensitive probe ETH, reduced Ca(2+) accumulation, and decreased LDH release during reoxygenation of hypoxic cardiomyocytes. The reduction in LDH release, reflecting the protective effect of Mg(2+), may be linked to the effect of Mg(2+) on Ca(2+) accumulation and/or ROS levels.

Bombesin-like peptides depolarize rat hippocampal interneurones through interaction with subtype 2 bombesin receptors

1. Whole-cell patch-clamp recordings were made from visually identified hippocampal interneurones in slices of rat brain tissue in vitro. Bath application of the bombesin-like neuropeptides gastrin-releasing peptide (GRP) or neuromedin B (NMB) produced a large membrane depolarization that was blocked by pre-incubation with the subtype 2 bombesin (BB2) receptor antagonist [D-Phe6, Des-Met14]bombesin-(6-14)ethyl amide. 2. The inward current elicited by NMB or GRP was unaffected by K+ channel blockade with external Ba2+ or by replacement of potassium gluconate in the electrode solution with caesium acetate. 3. Replacement of external NaCl with Tris-HCl significantly reduced the magnitude of the GRP-induced current at -60 mV. In contrast, replacement of external NaCl with LiCl had no effect on the magnitude of this current. 4. Photorelease of caged GTPgammaS inside neurones irreversibly potentiated the GRP-induced current at -60 mV. Similarly, bath application of the phospholipase C (PLC) inhibitor U-73122 significantly reduced the size of the inward current induced by GRP. 5. Reverse transcription followed by the polymerase chain reaction using cytoplasm from single hippocampal interneurones demonstrated the expression of BB2 receptor mRNA together with glutamate decarboxylase (GAD67). 6. Although bath application of GRP or NMB had little or no effect on the resting membrane properties of CA1 pyramidal cells per se, these neuropeptides produced a dramatic increase in the number and amplitude of miniature inhibitory postsynaptic currents in these cells in a TTX-sensitive manner.

Sodium/calcium exchange: its physiological implications

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.

Codependence of renal calcium and sodium transport

Calcium and sodium absorption by the kidney normally proceed in parallel. However, a number of physiological, pharmacological, pathological, and genetic conditions dissociate this relation. In each instance, the dissociation can be traced to the distal convoluted tubule, where calcium and sodium transport are inversely related. Based on the identification of the relevant sodium transporters in these cells and on analysis of the mechanism of calcium transport, an explanation for this inverse relation can be developed. Apical membrane calcium entry is mediated by voltage-sensitive calcium channels that are activated upon membrane hyperpolarization. Basolateral calcium efflux is effected primarily by Na+/Ca2+ exchange. According to the model, inhibition of sodium entry through either the Na-Cl cotransporter or the Na+ channel hyperpolarizes the cell, as does parathyroid hormone, thereby activating the calcium entry channel and increasing the driving force for diffusional entry. Membrane hyperpolarization also increases the driving force of calcium efflux through the Na+/Ca2+ exchanger. Thus sodium-dependent changes of calcium transport are indirect and occur secondarily through effects on membrane voltage.

Rapid charge translocation by the cardiac Na(+)-Ca2+ exchanger after a Ca2+ concentration jump

The kinetics of Na(+)-Ca2+ exchange current after a cytoplasmic Ca2+ concentration jump (achieved by photolysis of DM-nitrophen) was measured in excised giant membrane patches from guinea pig or rat heart. Increasing the cytoplasmic Ca2+ concentration from 0.5 microM in the presence of 100 mM extracellular Na+ elicits an inward current that rises with a time constant tau 1 < 50 microseconds and decays to a plateau with a time constant tau 2 = 0.65 +/- 0.18 ms (n = 101) at 21 degrees C. These current signals are suppressed by Ni2+ and dichlorobenzamil. No stationary current, but a transient inward current that rises with tau 1 < 50 microseconds and decays with tau 2 = 0.28 +/- 0.06 ms (n = 53, T = 21 degrees C) is observed if the Ca2+ concentration jump is performed under conditions that promote Ca(2+)-Ca2+ exchange (i.e., no extracellular Na+, 5 mM extracellular Ca2+). The transient and stationary inward current is not observed in the absence of extracellular Ca2+ and Na+. The application of alpha-chymotrypsin reveals the influence of the cytoplasmic regulatory Ca2+ binding site on Ca(2+)-Ca2+ and forward Na(+)-Ca2+ exchange and shows that this site regulates both the transient and stationary current. The temperature dependence of the stationary current exhibits an activation energy of 70 kj/mol for temperatures between 21 degrees C and 38 degrees C, and 138 kj/mol between 10 degrees C and 21 degrees C. For the decay time constant an activation energy of 70 kj/mol is observed in the Na(+)-Ca2+ and the Ca(2+)-Ca2+ exchange mode between 13 degrees C and 35 degrees C. The data indicate that partial reactions of the Na(+)-Ca2+ exchanger associated with Ca2+ binding and translocation are very fast at 35 degrees C, with relaxation time constants of about 6700 s-1 in the forward Na(+)-Ca2+ exchange and about 12,500 s-1 in the Ca(2+)-Ca2+ exchange mode and that net negative charge is moved during Ca2+ translocation. According to model calculations, the turnover number, however, has to be at least 2-4 times smaller than the decay rate of the transient current, and Na+ inward translocation appears to be slower than Ca2+ outward movement.

Current-voltage relations and steady-state characteristics of Na+-Ca2+ exchange: characterization of the eight-state consecutive transport model

An analytical expression for Na+-Ca2+ exchange currents in cardiac cells has been obtained for an eight-state model. The equation obtained has been used to derive theoretical expressions for current-voltage relationships, maximum Na+-Ca2+ exchange currents, and half-saturating concentrations for Na+ and Ca2+. These equations were analyzed over a wide range of cytoplasmic and extracellular Na+ and Ca2+ concentrations, under forward and reverse "zero-trans" conditions. Correspondence of theoretical results with those obtained from giant excised patch experiments are presented. Rate constants from published reports were used to evaluate turnover rates for Na+-Ca2+ exchange in the forward and reverse directions. A factor, epsilon, is introduced that permits prediction of the extent to which the Na+-Ca2+ exchange cycle is under voltage or diffusion control. This factor can be conveniently used for data interpretation and comparison. The derived equations also provide a foundation for continuing experimental evaluation of the fidelity of this model.

The Na(+)-Ca2+ exchange system in rat glial cells in culture: activation by external monovalent cations

Cultured rat glial cells display a Na(+)-Ca2+ exchange system located at the plasma membrane levels. This was evidenced by the Na+ (i)-dependency of a Na+ (o)-inhibitable influx of Ca2+, or reversal exchange mode. This antiporter has an external site where monovalent cations (K+, Li+, and Na+ were investigated) stimulate the exchange by a chemical action. The monovalent cation is not transported during the exchange cycle. The mechanism of that stimulation agrees with an increase in the apparent affinity of the carrier for Ca2+(o) without effect on the maximal translocation rate. Two models can equally well account for the data: i) the formation of ECa(o) is essential for the binding of the monovalent cation, or ii) the activating cation can bind even when the carrier is free of Ca2+(o). The cations K+ and Li+ produced only stimulation, although that of K+ seem to require actions other than the chemical effect. The response to Na+ was biphasic; this can be fully explained considering that at low concentrations, Na+(o) binds preferentially to the activating monovalent site while at high concentrations it displaces Ca2+ from its external transporting site. Pure type I astrocytes displayed the same Na(+)-Ca2+ exchange mechanism.

Intracellular free Ca2+, Na+, and H+ concentrations in the isolated perfused rat heart during the Ca2+ paradox

Enzyme release from the normothermic perfused rat heart was determined to evaluate myocardial cell damage during the perfusion sequence of the Ca2+ Ca2+ paradox. In addition sarcosolic free Ca2+ and monovalent cation concentrations were measured using ion-selective microelectrodes (ISMEs). A reduction of the extracellular Ca2+ concentration, [Ca2+]e, from 1.0 mmol/l to 0.3 or 0.1 mmol/l, respectively, during the re-perfusion period markedly decreased the rate of enzyme release. Slow Ca2+ channel blockers were less (verapamil) or not at all (nifedipine) effective in providing protection. The sarcosolic free Ca2+ and Na+ concentrations, [Ca2+]i and [Na+]i, were significantly (P < 0.01) elevated during Ca(2+)-free perfusion, [H+]i was not significantly changed, while the membrane potential became continuously more positive. Addition of verapamil to the perfusion medium increased [Na+]i, but did not further increase [Ca2+]i. The critical [Ca2+]i for cell damage was between 12 and 18 mumol/l. It could be demonstrated, that Ca2+ entry during re-perfusion via Na/Ca exchange is thermodynamically unlikely to elevate [Ca2+]i to critical values. It is concluded therefore that the predisposition of the rat heart for the Ca2+ paradox is brought about by membrane leaks, which form during Ca(2+)-free perfusion and that Ca2+ influx into the sarcosol proceeds mainly through these leaks.

Enhanced calcium influx by parathyroid hormone in identified Helisoma trivolvis snail neurons

The modulation of [Ca2+]i by parathyroid hormone (PTH) has been extensively studied in vertebrates. The present study examined the effects of PTH on [Ca2+]i in isolated invertebrate neurons B5 from buccal ganglia of the pond snail, Helisoma trivolvis, utilizing the Fura-2 fluorescence technique. Bovine PTH, bPTH-(1-84), induced a slow and sustained increase in [Ca2+]i in neurons B5. In contrast, the elevation of extracellular K+ concentration from 1.7 mM to 15 mM induced a rapid and transient increase in [Ca2+]i. Simultaneous application of 15 mM KCl and bPTH-(1-84), or application of 15 mM KCl in the presence of bPTH-(1-84) additively increased [Ca2+]i in neurons B5. An increase in [Ca2+]i in neurons B5 was also induced by a PTH agonist [bPTH-(1-34)], but not by a PTH antagonist [bPTH-(3-34)]. The absence of calcium, or the presence of lanthanum (2 mM) or omega-conotoxin (10 microM), in the bath solution abolished the effect of bPTH-(1-84) on [Ca2+]i. Furthermore, our results demonstrated that the effect of PTH on [Ca2+]i in neurons B5 was not due to the hormonal modulation of voltage-dependent Na+ or K+ channels or a Na+/Ca2+ exchanger. The results from this study show that PTH can modulate [Ca2+]i in an identified invertebrate neuron mainly by promoting extracellular calcium influx via the N-like voltage-dependent calcium channel.

ATP as a positive effector of the sodium efflux in single barnacle muscle fibers

A study has been made of the mechanism by which the injection of ATPNa2 stimulates the ouabain-insensitive Na efflux in fibers from the barnacle, Balanus nubilus. The results of this study are as follows: ATPNa2 is found to be a more potent effector of the Na efflux in unpoisoned fibers than ATPMg on an equimolar basis, but not more potent than ADPNa2. In ouabain-poisoned fibers ATPNa2 and ATPMg are equipotent but the former is more potent than ADPNa2. The magnitude of the response to ATPNa2 injection into ouabain-poisoned fibers depends on: (i) the ouabain concentration used; (ii) the concentration of ATPNa2 injected, and (iii) the external Ca2+ concentration. Ouabain is without effect when it is applied at the time of ATPNa2 injection. Responsiveness to ouabain, however, is found to return if the glycoside is applied after complete decay of the response to ATP. Under these conditions, the effect of ouabain in fibers injected with ATPNa2 is significantly less than in fibers injected with ATPMg. Preinjection of EGTA in high concentrations fails to reduce the size of the response to ATPNa2 injection. Injection of Mg2+ following peak stimulation by ATP almost completely reverses the response. The response to Mg2+ is concentration-dependent. Ryanodine but not neomycin reduces the response to ATP. ATP gamma S is not as effective as ATPNa2. Nor is AMP-PNP consistently as effective as ATPNa2. Collectively, these results support the hypothesis that the response of the Na efflux to ATPNa2 injection involves the operation of the putative Na(+)-Ca2+ exchanger in the reverse mode and that a raised Cai2+ is not an absolute requirement. They also strongly suggest that two other governing factors are the Na+ gradient across the sarcolemma and the myoplasmic pMg. Mg2+ seems to act as an inhibitor.

Translocation mechanism of cardiac Na-Ca exchange

In single cardiac ventricular cells of guinea pig, we have studied the ionic translocation mechanism of the electrogenic Na-Ca exchange, that is, whether Na and Ca ions countercross the membrane simultaneously or consecutively. The dose-response relations between the external Ca ([Ca]o) and the outward Na-Ca exchange current were measured at three different internal Na concentrations ([Na]i) in the absence of external Na. Hyperbolic regression curves and Hanes-Woolf linear plots of the dose-response relation revealed that apparent Km values for external Ca (K'mCao) decrease progressively as [Na]i decreases. The ratio of K'mCao to apparent Imax value (I'max) showed a slight increasing tendency as [Na]i decreased. We previously interpreted the data as consistent with the simultaneous mechanism but without statistical analysis. Here we performed careful statistical analysis, which indicated that the K'max/I'max values were not significantly different among the different [Na]i at most of the potentials. This result suggests that Na-Ca exchange is likely to be a consecutive mechanism.

Inhibition of the veratridine-induced increase in cytosolic Ca2+ and respiration by Ca2+ antagonists in isolated cardiac myocytes

1. We studied the effect of verapamil, nitrendipine, 3',4'-dichlorobenzamil (DCB) and Cd2+ on the increase in cytosolic free Ca2+ ([Ca2+]c) and the rate of O2-uptake induced by depolarization of isolated rat cardiac myocytes with veratridine. 2. The degree of inhibition by the several drugs tested on the increase in [Ca2+]c and respiration was dependent on extracellular Ca2+, pH and Na+. 3. Low verapamil and nitrendipine concentrations (2.5 microM) were fully effective in Ca2+ channel blockade, as indicated from experiments with isoproterenol and in a low-Na+ medium. 4. A complete inhibition of veratridine-induced increase in [Ca2+]c and O2-uptake was attained with higher Ca2+ blocker concentrations (25-30 microM), implying that these processes depend to a major extent on some other Ca2+ transport system, probably Na+/Ca2+ exchange.

The effects of quinidine on sodium-dependent calcium efflux in isolated rod photoreceptors of the salamander retina

The effect of quinidine on the membrane current generated by the Na:Ca, K exchange has been investigated in the outer segment of isolated rod photoreceptors from the retina of the larval tiger salamander. The inward exchange current associated with the efflux of Ca2+ was selectively recorded by introducing a Ca2+ load through the light-sensitive channels, and then shutting these channels with a bright light. Quinidine (20-1000 microM) reduced the magnitude of the exchange current and slowed its decay during the removal of a Ca2+ load. Quinidine did not alter the form of the relation between the exchange current and the total concentration of exchangeable calcium remaining within the outer segment. [Ca]T, showing that it does not change the affinity of the exchange mechanism for internal Ca2+. The relation between exchange current inhibition and the quinidine concentration could be described by a simple Michaelis relation with a Ki of 287 microM and a maximum inhibition of 50%. The incomplete block of the Na:Ca, K exchange current by quinidine shows that it does not act by simple competition with external Na+, and suggests that the inhibition of the exchange by quinidine may be non-specific.

Effects of buffer magnesium on positive inotropic agents in guinea pig cardiac muscle

Experiments examined effects of extracellular Mg2+ concentration (Mgo2+) on dose-dependent actions of strophanthidin, norepinephrine, Bay K-8644 and extracellular Ca2+ (Cao2+) in electrically stimulated atrial and ventricular muscle isolated from guinea pig heart. Mgo2+ itself elicited a concentration-dependent negative inotropic effect. Elevation of Mgo2+ between 0.6 and 12 mM increased the concentration of strophanthidin necessary to produce its toxic effects without affecting the maximum developed tension prior to toxicity. Similarly, Mgo2+ did not alter the maximum contractile force elicited by cumulative addition of norepinephrine, Bay K-8644 or Cao2+, but increased their ED50 values. These data suggest that interactions between Mgo2+ and the four positive inotropic agents were not mediated by effects on receptor binding or Na+,K+-ATPase, but rather by alterations at one or more steps involved in excitation-contraction coupling.

Measurement of reversal potential of Na+-Ca2+ exchange current in single guinea-pig ventricular cells

1. To identify the Na+- or Ca2+-induced current as Na+-Ca2+ exchange current and to determine the stoichiometry of the Na+-Ca2+ exchange, the reversal potential was measured in a wide range of external Na+ [( Na+]o) or Ca2+ [( Ca2+]o) concentrations. The Na+- or Ca2+-induced current was recorded in single ventricular cells enzymatically dispersed from guinea-pig hearts, using the technique of whole-cell voltage clamp combined with internal perfusion. 2. In the presence of 10-40 mM-Na+ and 55-803 nM-Ca2+ in the internal solution, an increase of [Ca2+]o from 0.1 to 0.5-20 mM or an increase of [Na+]o from 30 to 50-140 mM induced an extra current associated with an increase in membrane conductance. The reversal potential of these extra currents was determined from an intersection of the current-voltage (I-V) relations obtained in the absence and presence of a Na+-Ca2+ exchange blocker, Ni2+ (2 mM). 3. Ba2+ in the external solution failed to induce the extra current, but inhibited the background conductance having a reversal potential at around 0 mV. Thus, 1 mM-Ba2+ was added to all external solutions, so that a change in the background current was minimized during application of Ca2+ or Ni2+. 4. The relation between [Ca2+]o and amplitude of the Ca2+-induced current was examined in the presence and absence of Ni2+. Lineweaver-Burk analysis revealed that the action of Ni2+ on the extra current might be a mixed type of competitive and non-competitive inhibition. 5. During the application of Ca2+, the Ca2+-induced outward current decayed in a time-dependent manner, resulting in a shift of the I-V relations towards positive potentials. This current decay was inhibited by increasing the capacity of the internal Ca2+-buffer, using BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid) or higher concentrations of EGTA. The result indicates that [Ca2+]i, at least under the cell membrane, changes due to ion fluxes through the Na+-Ca2+ exchange and that control of the ion concentrations within the cell is prerequisite for measuring the reversal potential of the Na+-Ca2+ exchange. 6. The shift of both the holding current and the I-V relations during stimulation of the exchange was suppressed, when the membrane potential was clamped at the equilibrium potential of 3Na+:1Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Sodium dependence of NMDA's effects on cyclic GMP production in immature rat cerebellar slices

In immature rat cerebellar slices, the calcium-dependent increase in cyclic GMP levels provoked by N-methyl-D-aspartic acid (NMDA) (80 microM) displayed sodium dependence using bis-(2-hydroxyethyl)-dimethyl ammonium chloride, N-methyl-glucamine or Tris as sodium substitutes. The effects of NMDA (and also of veratrine, 100 microM) were attenuated by substitution of sodium chloride by lithium chloride. The response produced by depolarization with KCl (50 mM) was not affected by lithium substitution. As lithium is believed to permeate sodium-permeable channels but is not a substrate for sodium/calcium exchange, the data suggest that calcium entry mediated by the reverse mode of sodium/calcium exchange may play a contributory role to the calcium entry provoked by NMDA and veratrine.

Na+-Ca2+ exchange in sarcolemmal vesicles from bovine superior mesenteric artery

These studies were designed to determine whether a Na+-Ca2+ exchanger is present in sarcolemmal vesicles from bovine superior mesenteric artery and, if so, to determine whether this transport system is qualitatively similar to that found in other excitable tissues. Vesicles, preferentially enriched in sarcolemma, were prepared by a Mg2+ aggregation and differential centrifugation technique. An inwardly directed Ca2+ gradient stimulated 22Na+ efflux and an outwardly directed Ca2+ gradient stimulated 22Na+ uptake. Similarly, an inwardly directed Na+ gradient stimulated 45Ca2+ efflux, and an outwardly directed Na+ gradient stimulated 45Ca2+ uptake. Ca2+ gradient-stimulated Na+ transport and Na+ gradient-stimulated Ca2+ transport were not due to voltage coupling between the two ions. Hence, a Na+-Ca2+ exchanger is present in these vesicles. The Na+ gradient-dependent component of Ca2+ uptake (Na+-Ca2+ exchange) was stimulated by rendering the vesicles electropositive inside, and Na+-Ca2+ exchange activity was inhibited by amiloride and quinidine in a dose-dependent fashion. These data demonstrate similarities between this mesenteric arterial smooth muscle Na+-Ca2+ exchanger and that found in other excitable tissues. In the absence of added Ca2+, amiloride-sensitive 22Na+ uptake in the vesicles was stimulated by an outwardly directed proton gradient, and an inwardly directed Na+ gradient stimulated proton efflux. Thus these vesicles also contain a Na+-H+ exchanger, which has been found in the sarcolemma of other vascular smooth muscle cells. When Na+ uptake was stimulated via Na+-H+ exchange, the subsequent uptake of Ca2+ via Na+-Ca2+ exchange was tripled. In conclusion, these studies unequivocally demonstrate that sarcolemmal-enriched vesicles from bovine superior mesenteric artery contain a Na+-Ca2+ exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

Low-sodium contractures indicating sarcolemmal Na/Ca-exchange in the frog heart

1. In the frog heart, Ca2+ enters the cell by the slow inward current (Isi) and by an electrogenic, carrier-mediated, and passive Na-out/Ca-in-exchange. 2. The latter reverses to Na-in/Ca-out-exchange during depolarization and thereby controls relaxation. 3. The exchange ratio is 3 Na+ for 1 Ca2+. 4. The Na/Ca-exchange is not inhibited by organic Ca-antagonists in frog myocardium, indicating that the initiation of the heart beat may mainly depend on Isi. 5. This is not necessarily in contradiction with the Na-Ca-antagonism, since there also exists an antagonism between Na+ and Ca2+ in the slow channel. 6. However, the contractures caused by a decrease of NaO+ are mediated by the Na/Ca-exchange.

A constraint on possible stoichiometries of myocardial sodium-calcium exchange

We have examined the sodium-calcium exchange stoichiometry in Langendorff-perfused rabbit hearts using gamma-emitting tracers under conditions of sodium pump inhibition. Following a 60-min perfusion with 10(-5) acetylstrophanthidin, and extracellular concentrations [Na]o = 70 mM and [Ca]o = 300 microM, intracellular sodium rose to 59.2 mM. At this point an increase in extracellular calcium [Ca]o = 1.52 mM) caused a net efflux of sodium, but an increase in sodium [Na]o = 105 mM) caused no measurable change. When sodium and calcium were simultaneously increased according to the ratio [Na]o)n/[Ca]o = [Na]'o)n/[Ca]'o, a sodium efflux is observed when n = 4, but not when n = 3. These results are consistent with an exchange stoichiometry of 3 Na+ for each Ca2+ ion, but not values of 4 or more.

Electrogenic sodium-calcium exchange in cultured embryonic chick heart cells

1. The membrane potential (Em) of cultured chick embryonic heart cells depolarized to -36 mV after inhibition of the Na+-K+ pump by 0.1 mM-ouabain in a [K+]o of 24 mM: this was accompanied by a rise in Na+ content of approximately 65% in 3 min. Lowering [Na+]o to 27 mM then caused a fall in Na+ content, a rise in Ca2+ content and a small hyperpolarization of approximately 5 mV. The fall in Na+ content indicated a movement of Na+ which was in the opposite direction to the Na+ electrochemical gradient (a countergradient movement). 2. In the presence of 10 mM-Cs+ or 1 mM-Ba2+ the hyperpolarization was approximately 10 or approximately 30 mV, respectively. A 30 mV hyperpolarization took Em negative to the reversal potentials for K+, and Cl- as measured by ion-selective micro-electrodes. 3. The decay of the intracellular Na+ activity alpha iNa, in an [Na+]o of 27 mM followed a simple exponential time course (time constant, 36 s). The initial rate depended on the value to which [Na+]o was lowered in a manner suggesting a simple competitive inhibition of the exchange by external Na+. 4. The low-[Na+]o hyperpolarization was unaffected by amiloride (0.1 or 1 mM) or verapamil (20 microM). Both La3+ (1 mM) and Mn2+ (20 mM) blocked the hyperpolarization sufficiently to prevent Em hyperpolarizing negative to the reversal potentials for K+, Na+ and Cl-. 5. Re-establishing [Na+]o caused a rise in Na+ content and a countergradient drop in Ca2+ content. The effects of verapamil (20 microM), amiloride (0.1 and 1 mM), dichlorobenzamil (0.1 mM), quinidine (1 mM), Mn2+ (20 mM) and La3+ (1 mM) were tested on the movements of Na+ and Ca2+ both during exposure to an [Na+]o of 27 mM and on re-establishing [Na+]o. The only consistent and substantial effects were the attenuation by La3+ and Mn2+ and Ca2+ movements during exposure to an [Na+]o of 27 mM. However, neither La3+ nor Mn2+ affected the movements of Na+ and Ca2+ on re-establishing [Na+]o. 6. We conclude that cultured embryonic chick heart cells contain a Na+-Ca2+ exchange evidenced by the ability to cause movements of Na+ and Ca2+ which are counter to their respective electrochemical gradient and which are accompanied by downhill movements of the counter ion.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of sodium-calcium exchange current in single ventricular cells of guinea-pig

1. The Na-Ca exchange current was investigated in single ventricular cells from guinea-pig hearts by combining the techniques of whole-cell voltage clamp and intracellular perfusion. 2. The membrane conductance was minimized by blocking Ca and K channels as well as the Na-K pump. Under these conditions, when Ca2+ was loaded internally by a pipette solution containing 430 nM-Ca2+, changing the Li+-rich external solution to a Na+-rich one induced a significant inward current. Applying external Na+ in the absence of internal Ca2+ did not appreciably change the current. 3. In contrast, perfusing 1 mM-external Ca2+ in the presence of internal Na+ which was loaded by a 20 mM-Na+ pipette solution, induced a marked outward current. Ca2+ superfusion in the absence of internal Na+ caused only a small current change. 4. The current-voltage relation of external-Ca2+- and external-Na+-induced current showed almost exponential voltage dependence as given by the equation i = a exp (rEF/RT), where a is a scaling factor that determines the magnitude of the current and r is a partition parameter used in the rate theory and represents the position of the energy barrier in the electrical field, which indicates the steepness of the voltage dependence of the current. E, F, R and T have their usual meanings. The value of a was 1-2 microA/microF and r about 0.35 for the Ca2+-induced outward current. At very positive or negative potentials, the current magnitude became smaller than expected from an exponential relation. 5. The current was blocked by heavy metal cations, such as La3+, Cd2+, Mn2+ and Ni2+ and partially blocked by amiloride and D600. 6. The temperature coefficient (Q10) value of the Ca2+-induced outward current was 3.6 +/- 0.4 (n = 4) at 0 mV and 4.0 +/- 0.9 at 50 mV in the range between 21 and 36 degrees C. 7. The outward current magnitude showed a sigmoidal dependence upon the external Ca2+ concentration with a half-maximum concentration, K1/2 of 1.38 mM and a Hill coefficient of 0.9 +/- 0.2 (n = 5). 8. Sr2+ could replace Ca2+ with K1/2 of 7 mM. Mg2+ and Ba2+, however, did not replace Ca2+. 9. The inward current component also showed a sigmoidal external Na+ dependence with K1/2 of 87.5 +/- 10.7 mM and a Hill coefficient of 2.9 +/- 0.4 (n = 6). 10. The reversal potential of the current was obtained near the values expected for 3 Na+:1 Ca2+ exchange.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage dependence of sodium-calcium exchange: predictions from kinetic models

Voltage effects on the Na-Ca exchange system are analyzed on the basis of two kinetic models, a "consecutive" and a "simultaneous" reaction scheme. The voltage dependence of a given rate constant is directly related to the amount of charge which is translocated in the corresponding reaction step. Charge translocation may result from movement of an ion along the transport pathway, from displacement of charged ligand groups of the ion-binding site, or from reorientation of polar residues of the protein in the course of a conformational transition. The voltage dependence of ion fluxes is described by a set of coefficients reflecting the dielectric distances over which charge is translocated in the individual reaction steps. Depending on the charge of the ligand system and on the values of the dielectric coefficients, the flux-voltage curve can assume a variety of different shapes. When part of the transmembrane voltage drops between aqueous solution and binding site, the equilibrium constant of ion binding becomes a function of membrane potential. By studying the voltage dependence of ion fluxes in a wide range of sodium and calcium concentrations, detailed information on the microscopic properties of the transport system may be obtained.

Calcium transport mechanisms in dog red blood cells studied from measurements of initial flux rates

Ca2+ efflux from dog red blood cells loaded with Ca2+ using the A23187 ionophore could be separated into two main components: (1) Mg- and ATP-dependent (active transport) and (2) dependent on external Na (K1/2 around 15 mM); at 80 microM internal free Ca the relative magnitudes of these fluxes were 70% and 30% respectively. The Na-dependent Ca2+ efflux had the following additional properties: (i) it was partially inhibited by ATP depletion or preincubation with vanadate, but it was not affected by Mg2+ depletion; (ii) it failed to be stimulated by external monovalent cations other than Na: (iii) it was stimulated by reduction in the internal Na+ concentration. Both active and Na-dependent Ca2+ efflux remained unchanged in hypotonic solutions or in solutions with alkaline pH (8.5). In cells containing ATP and Mg2+, external Ca2+ inhibited Ca2+ efflux (K1/2 around 1 mM); on the other hand, in Mg-free dog red cells external Ca2+ stimulated Ca2+ efflux (K1/2 about 30 microM). In Mg-depleted red cells incubated in the absence of external Na2+, Ca2+ influx as a function of external Ca2+ followed a monotonically saturable function (K1/2 around 20 microM): addition of Na resulted in (i) inhibition of Ca2+ influx and (ii) a sigmoid relationship between flux and external Ca2+. Intracellular Ca2+ stimulated the external Na-dependent Ca2+ efflux along a sigmoid curve (K1/2 around 30 microM); on the other hand the Ca pump had a biphasic response to internal Ca2+: stimulation at low internal Ca2+ (K1/2 between 1 and 10 microM), followed by a decline at internal Ca2+ concentrations higher than 50 microM.

High pH-induced acrosome reaction and Ca2+ uptake in sea urchin sperm suspended in Na+-free seawater

The egg jelly-induced acrosome reaction of sea urchin sperm requires the presence of Ca2+ and Na+ in seawater at its normal pH 8. Sperm suspended in seawater at pH 9 undergo the acrosome reaction in the absence of jelly. We have attempted to understand the role of external Na+ in this reaction. Sperm were suspended in Na+-free seawater and the percentage of acrosome reaction and the amount of Ca2+ uptake were determined as a function of external pH. High pH (9.0) in Na+-free medium without jelly triggered a high percentage (above 65%) of sperm acrosome reactions and a two to fourfold increase in Ca2+ uptake. Both the percentage of acrosome reactions and the amount of Ca2+ uptake were similar to those induced by either jelly or pH 9 in Na+-containing seawater. On the other hand, the absence of Na+ in seawater inhibits jelly from inducing Ca2+ uptake and acrosome reactions at pH 8.0 and even at pH 8.5. These results indicate that the Na+ requirement for the acrosome reaction induced by jelly is lost when triggering is by high pH. In contrast, Ca2+ was strictly required since sperm did not react in Ca2+-free seawater at pH 9. We also found that like the jelly-induced acrosome reaction the high-pH-induced acrosome reaction and Ca2+ uptake in complete and Na+-free seawater were inhibited by D600. This finding suggests that the same transport system for Ca2+ uptake associated with the acrosome reaction operates at both triggering conditions, i.e., jelly or pH 9. Although D600 is not now considered a specific blocker, its effect has suggested the involvement of Ca2+ channels in the acrosome reaction. This proposal is supported by our results with nisoldipine, a highly specific inhibitor of calcium channels. The drug inhibited both the sperm acrosome reaction and Ca2+ uptake induced by jelly or pH 9 in complete seawater.

A possible action of nicardipine on the cardiac sarcolemmal Na+-Ca2+ exchange

The effects of nicardipine on sodium-calcium exchange activity of cardiac sarcolemma-enriched vesicles isolated from the rat heart were examined. Sodium-loaded, sarcolemma-enriched vesicles, when exposed to a medium containing 40 microM CaCl2, exhibited about 5 nmoles Ca2+/mg protein of the maximal calcium uptake; the initial rate was 21 nmoles Ca2+/mg protein/min. The calcium uptake was dependent on the extravesicular concentration of calcium ion. Nicardipine at concentrations of 0.1 to 10 microM depressed the rate of calcium uptake activity by 60-90%. The isolated membrane vesicles preloaded with Ca2+ showed a calcium efflux activity, when exposed to a medium containing sodium ion. The rate of calcium efflux was 2.5 nmoles Ca2+/mg protein/min, when measured in a medium containing 6.5 mM NaCl. The efflux rate was facilitated with increased concentrations of sodium ion in the medium. About 75% of the preloaded calcium in the vesicles was released within 3 min of incubation. The rate of calcium efflux was stimulated in the presence of 0.1 to 10 microM nicardipine (2.5- to 4-fold increase). The present results suggest a possible action of nicardipine on the sodium-calcium exchange mechanism at cardiac sarcolemmal sites.

Na+/Ca2+ countertransport in plasma membrane of rat pancreatic acinar cells

The presence of a coupled Na+/Ca2+ exchange system has been demonstrated in plasma membrane vesicles from rat pancreatic acinar cells. Na+/Ca2+ exchange was investigated by measuring 45Ca2+ uptake and 45Ca2+ efflux in the presence of sodium gradients and at different electrical potential differences across the membrane (= delta phi) in the presence of sodium. Plasma membranes were prepared by a MgCl2 precipitation method and characterized by marker enzyme distribution. When compared to the total homogenate, the typical marker for the plasma membrane, (Na+ + K+)-ATPase was enriched by 23-fold. Markers for the endoplasmic reticulum, such as RNA and NADPH cytochrome c reductase, as well as for mitochondria, the cytochrome c oxidase, were reduced by twofold, threefold and 10-fold, respectively. For the Na+/Ca2+ countertransport system, the Ca2+ uptake after 1 min of incubation was half-maximal at 0.62 mumol/liter Ca2+ and at 20 mmol/liter Na+ concentration and maximal at 10 mumol/liter Ca2+ and 150 mmol/liter Na+ concentration, respectively. When Na+ was replaced by Li+, maximal Ca2+ uptake was 75% as compared to that in the presence of Na+. Amiloride (10(-3) mol/liter) at 200 mmol/liter Na+ did not inhibit Na+/Ca2+ countertransport, whereas at low Na+ concentration (25 mmol/liter) amiloride exhibited dose-dependent inhibition to be 62% at 10(-2) mol/liter. CFCCP (10(-5) mol/liter) did not influence Na+/Ca2+ countertransport. Monensin inhibited dose dependently; at a concentration of 5 X 10(-6) mol/liter inhibition was 80%. A SCN- or K+ diffusion potential (= delta phi), being positive at the vesicle inside, stimulated calcium uptake in the presence of sodium suggesting that Na+/Ca2+ countertransport operates electrogenically, i.e. with a stoichiometry higher than 2 Na+ for 1 Ca2+. In the absence of Na+, delta phi did not promote Ca2+ uptake. We conclude that in addition to ATP-dependent Ca2+ outward transport as characterized previously (E. Bayerdörffer, L. Eckhardt, W. Haase & I. Schulz, 1985, J. Membrane Biol. 84:45-60) the Na+/Ca2+ countertransport system, as characterized in this study, represents a second transport system for the extrusion of calcium from the cell. Furthermore, the high affinity for calcium suggests that this system might participate in the regulation of the cytosolic free Ca2+ level.

A minimum mechanism for Na+-Ca++ exchange: net and unidirectional Ca++ fluxes as functions of ion composition and membrane potential

Both simultaneous and consecutive mechanisms for Na+-Ca++ exchange are formulated and the associated systems of steady-state equations are solved numerically, and the net and unidirectional Ca++ fluxes computed for a variety of ionic and electrical boundary conditions. A simultaneous mechanism is shown to be consistent with a broad range of experimental data from the squid giant axon, cardiac muscle and isolated sarcolemmal vesicles. In this mechanism, random binding of three Na+ ions and one Ca++ on apposing sides of a membrane are required before a conformational change can occur, translocating the binding sites to the opposite sides of the membranes. A similar (return) translocation step is also permitted if all the sites are empty. None of the other states of binding can undergo such translocating conformational changes. The resulting reaction scheme has 22 reaction steps involving 16 ion-binding intermediates. The voltage dependence of the equilibrium constant for the overall reaction, required by the 3:1 Na+: Ca++ stoichiometry was obtained by multiplying and dividing, respectively, the forward and reverse rate constants of one of the translocational steps by exp(-FV/2RT). With reasonable values for the membrane density of the enzyme (approximately 120 sites micron 2) and an upper limit for the rate constants of both translocational steps of 10(5) . sec-1, satisfactory behavior was obtainable with identical binding constants for Ca++ on the two sides of the membrane (10(6) M-1), similar symmetry also being assumed for the Na+ binding constant (12 to 60 M-1). Introduction of order into the ion-binding process eliminates behavior that is consistent with experimental findings.

Na-Ca exchange: stoichiometry and electrogenicity

This review discusses the evidence concerning the stoichiometry of Na-Ca exchange. In particular we consider whether the Na-Ca exchange has been shown to transport more than two Na+ ions per Ca2+ ion and therefore whether it generates an electric current. The first part of this review discusses both direct and indirect evidence concerning the stoichiometry of the exchange and its possible voltage dependence. We find that, although there is some evidence suggesting that more than two Na+ ions may exchange for each Ca2+ ion, most of the available evidence is equivocal and cannot fix the stoichiometry precisely. Furthermore, using a simple and explicit circulating carrier model for the Na-Ca exchange, we show that the effect of membrane potential on the Na-Ca exchange may be considerably more complicated than is generally believed. In particular we find that both electrogenic and electroneutral exchanges will be affected by membrane potential. We therefore conclude that the demonstration of the voltage dependence of the Na-Ca exchange does not necessarily imply that it is electrogenic. Additionally, this analysis shows that, apart from a restricted range near thermodynamic equilibrium, it is impossible to predict either the magnitude or the direction of the effects of membrane potential on the exchange. In the second part of the review we consider whether any known membrane currents may be attributed to Na-Ca exchange. We show, in contrast to previous suggestions, that the Na-Ca exchange can theoretically produce a current that appears to be activated by intracellular Ca and that has a reversal potential. However, the experimental demonstration that a given current is produced by Na-Ca exchange is hampered by the existence of other Ca- and Na-dependent currents. In conclusion, we feel that there is no evidence that allows any particular membrane current to be unambiguously identified with the Na-Ca exchange.

Changes in external Na induce a membrane current related to the Na-Ca exchange in cesium-loaded frog heart cells

The effects of transient alterations in Nao were investigated under voltage clamp conditions in frog heart cells previously loaded with Cs. Tetrodotoxin and Cs were used to inhibit Na and K currents. On applying a Na-poor solution (39.2 mM), an outward current was generated during both depolarizations and hyperpolarizations. The current amplitude described a U-shaped function of the membrane potential. On reapplying the standard solution after 15 min equilibration, an inward current was then induced that exhibited a bell-shaped function of the membrane potential. Current amplitude was sensitive to the external Ca concentration. Increasing pHi by 10 mM NH4Cl enhanced this current, while the internal acidification that occurred on switching back to the control solution greatly reduced it. Variations in the amplitude of this current during repetitive stimulations or long pauses are best explained by subsequent alterations in Nai and pHi; no evidence for a time dependence was found. This current was inhibited by La3+, Co2+, and D600, and was sensitive to adriamycin, quinidine, and disopyramide; lidocaine, another local anesthetic, and nifedipine had no effect. These observations extend previous work on intact heart cells and sarcolemmal vesicles. They suggest that the Na-Ca exchange may generate a current that is outward when Ca ions are moving into the cell.