Is potassium co-transported by the cardiac Na-Ca exchange?

The prokaryotic Na+/Ca2+ exchanger NCX_Mj transports Na+ and Ca2+ in a 3:1 stoichiometry

Sodium–calcium exchangers contribute to the generation of intracellular Ca²⁺ signals in numerous physiological processes. Shlosman et al. determine the ion stoichiometry of the only sodium–calcium exchanger of known atomic structure, revealing its functional similarity to mammalian exchangers.

Intracellular Ca²⁺ signals control a wide array of cellular processes. These signals require spatial and temporal regulation of the intracellular Ca²⁺ concentration, which is achieved in part by a class of ubiquitous membrane proteins known as sodium–calcium exchangers (NCXs). NCXs are secondary-active antiporters that power the translocation of Ca²⁺ across the cell membrane by coupling it to the flux of Na⁺ in the opposite direction, down an electrochemical gradient. Na⁺ and Ca²⁺ are translocated in separate steps of the antiport cycle, each of which is thought to entail a mechanism whereby ion-binding sites within the protein become alternately exposed to either side of the membrane. The prokaryotic exchanger NCX_Mj, the only member of this family with known structure, has been proposed to be a good functional and structural model of mammalian NCXs; yet our understanding of the functional properties of this protein remains incomplete. Here, we study purified NCX_Mj reconstituted into liposomes under well-controlled experimental conditions and demonstrate that this homologue indeed shares key functional features of the NCX family. Transport assays and reversal-potential measurements enable us to delineate the essential characteristics of this antiporter and establish that its ion-exchange stoichiometry is 3Na⁺:1Ca²⁺. Together with previous studies, this work confirms that NCX_Mj is a valid model system to investigate the mechanism of ion recognition and membrane transport in sodium–calcium exchangers.

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.

Forefront of Na+/Ca2+ exchanger studies: stoichiometry of cardiac Na+/Ca2+ exchanger; 3:1 or 4:1?

The stoichiometry of the Na+/Ca2+ exchanger (NCX) had been generally accepted as 3 Na+:1 Ca2+. However, recently a challenging stoichiometry of 4:1 was proposed. Therefore, using guinea pig ventricular cells, we re-examined the stoichiometry by measuring the reversal potential of the NCX current and intracellular Ca2+ concentrations under the whole-cell voltage clamp. We confirmed that the stoichiometry of NCX is 3:1 not 4:1. In addition, we explored the possible reasons for obtaining erroneous results of a 4:1 stoichiometry.

Stoichiometry of Na+-Ca2+ exchange is 3:1 in guinea-pig ventricular myocytes

In single guinea-pig ventricular myocytes, we examined the stoichiometry of Na(+)-Ca(2+) exchange (NCX) by measuring the reversal potential (E(NCX)) of NCX current (I(NCX)) and intracellular Ca(2+) concentration ([Ca(2+)](i)) with the whole-cell voltage-clamp technique and confocal microscopy, respectively. With given ionic concentrations in the external and pipette solutions, the predicted E(NCX) were -73 and -11 mV at 3:1 and 4:1 stoichiometries, respectively. E(NCX) measured were -69 +/- 2 mV (n = 11), -47 +/- 1 mV (n = 14) and -15 +/- 1 mV (n = 15) at holding potentials (HP) of -73, -42 and -11 mV, respectively. Thus, E(NCX) almost coincided with HP, indicating that [Ca(2+)](i) and/or [Na(+)](i) changed due to I(NCX) flow. Shifts of E(NCX) (deltaE(NCX)) were measured by changing [Ca(2+)](o) or [Na(+)](o). The measured values of deltaE(NCX) were almost always smaller than those expected theoretically at a stoichiometry of either 3:1 or 4:1. Using indo-1 fluorescence, [Ca(2+)](i) measured under the whole-cell voltage-clamp supported a 3:1 but not 4:1 stoichiometry. To prevent Ca(2+) accumulation, we inhibited I(NCX) with Ni(2+) and re-examined E(NCX) during washing out Ni(2+). With HP at predicted E(NCX) at a 3:1 stoichiometry, E(NCX) developed was close to predicted E(NCX) and did not change with time. However, with HP at predicted E(NCX) for a 4:1 stoichiometry, E(NCX) developed initially near a predicted E(NCX) for a 3:1 stoichiometry and shifted toward E(NCX) for a 4:1 stoichiometry with time. We conclude that the stoichiometry of cardiac NCX is 3:1.

Inhibitory effect of 2,3-butanedione monoxime (BDM) on Na(+)/Ca(2+) exchange current in guinea-pig cardiac ventricular myocytes

1. The effect of 2,3-butanedione monoxime (BDM), a 'chemical phosphatase', on Na(+)/Ca(2+) exchange current (I(NCX)) was investigated using the whole-cell voltage-clamp technique in single guinea-pig cardiac ventricular myocytes and in CCL39 fibroblast cells expressing canine NCX1. 2. I(NCX) was identified as a current sensitive to KB-R7943, a relatively selective NCX inhibitor, at 140 mM Na(+) and 2 mM Ca(2+) in the external solution and 20 mM Na(+) and 433 nM free Ca(2+) in the pipette solution. 3. In guinea-pig ventricular cells, BDM inhibited I(NCX) in a concentration-dependent manner. The IC(50) value was 2.4 mM with a Hill coefficients of 1. The average time for 50% inhibition by 10 mM BDM was 124+/-31 s (n=5). 4. The effect of BDM was not affected by 1 microM okadaic acid in the pipette solution, indicating that the inhibition was not via activation of okadaic acid-sensitive protein phosphatases. 5. Intracellular trypsin treatment via the pipette solution significantly suppressed the inhibitory effect of BDM, implicating an intracellular site of action of BDM. 6. PAM (pralidoxime), another oxime compound, also inhibited I(NCX) in a manner similar to BDM. 7. Isoprenaline at 50 microM and phorbol 12-myristate 13-acetate (PMA) at 8 microM did not reverse the inhibition of I(NCX) by BDM. 8. BDM inhibited I(NCX) in CCL39 cells expressing NCX1 and in its mutant in which its three major phosphorylatable serine residues were replaced with alanines. 9. We conclude that BDM inhibits I(NCX) but the mechanism of inhibition is not by dephosphorylation of the Na(+)/Ca(2+) exchanger as a 'chemical phosphatase'.

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.

Interaction of the Na+-K+ pump and Na+-Ca2+ exchange via [Na+]i in a restricted space of guinea-pig ventricular cells

1. The whole-cell Na+-K+ pump current (INa-K) and Na+-Ca2+ exchange current (INa-Ca) were recorded in guinea-pig ventricular myocytes to study the interaction between the two Na+ transport mechanisms. 2. INa-K was isolated as an external K+-induced current, and INa-Ca as an external Ca2+- induced or Ni2+-sensitive current. The experimental protocol used for one ion carrier did not affect the other. 3. The amplitude of INa-K decreased to 54 +/- 17 % of the initial peak during continuous application of K+ with 20 mM Na+ in the pipette. The outward INa-Ca, which was intermittently activated by brief applications of Ca2+, decreased during activation of INa-K, and recovered after cessation of INa-K activation. These findings revealed a dynamic interaction between INa-K and INa-Ca via a depletion of Na+ under the sarcolemma. 4. To estimate changes in Na+ concentration ([Na+]i) under the sarcolemma, the reversal potential (Vrev) of INa-Ca was measured. Unexpectedly, Vrev hardly changed during activation of INa-K. However, when INa-Ca was blocked by Ni2+ at the same time that INa-K was activated, Vrev changed markedly, maximally by +100 mV, immediately after the removal of Ni2+ and K+. 5. Subsarcolemmal [Na+]i was calculated from the Vrev of INa-Ca on the assumption that the subsarcolemmal Ca2+ concentration ([Ca2+]i) was fixed with EGTA, and mean [Na+]i was calculated from both the time integral of INa-K and the cell volume. The subsarcolemmal [Na+]i was about seven times greater than the mean [Na+]i. 6. The interaction between the Na+-K+ pump and Na+-Ca2+ exchange was well simulated by a diffusion model, in which Na+ diffusion was restricted to one-seventh (14 %) of the total cell volume.

A quantitative estimate of flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange in blowfly Calliphora photoreceptors

The flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange activity in blowfly Calliphora photoreceptors were investigated. The change in membrane potential, induced by a bright flash, was intracellularly measured in vivo. Based on a biophysical photoreceptor model, the Na(+)- and Ca(2+)-currents and concentration changes were determined from the first transient depolarization phase of the photoreceptor response. The activity of Na+/Ca(2+)-exchange was determined from the after depolarization phase. It appeared that the Na(+)-influx by Na+/Ca(2+)-exchange is about twice that through light-activated channels, suggesting a substantial contribution of Na+/Ca(2+)-exchange to Na(+)-regulation.

Calcium homeostasis in guinea pig type-I vestibular hair cell: possible involvement of an Na(+)-Ca2+ exchanger

In type-I vestibular hair cells (VHCs), the mechanisms involved in intracellular calcium homeostasis have not yet been established. In order to investigate the involvement of an Na(+)-dependent ionic exchanger in the regulation of cytosolic free calcium concentration, we analyzed the effect of the removal of external sodium on the cytosolic concentration of calcium ions ([Ca2+]i), sodium ions ([Na+]i), and protons (pHi). These concentrations were measured in type-I VHCs isolated from guinea pig labyrinth, using Fura-2, sodium benzofuran isophtalate (SBFI), and 1,4 diacetoxy-2,3 dicyanobenzol (ADB) respectively. Complete replacement of Na+ in the superfusion solution with N-methyl-D-glucamine (NMDG+), reversibly increased [Ca2+]i by 276 +/- 89% (n = 46) and decreased [Na+]i by 23 +/- 6% (n = 14). Both responses were prevented by removing external Ca2+ or chelating internal Ca2+. This suggests the presence of coupled Ca2+ and Na+ transport. The [Ca2+]i increase evoked by Na(+)-free solution was reduced by about 55% with the application of amiloride derivatives and was totally abolished in the presence of high [Mg2+]o. No pHi variation was detected during [Na+]o reduction. In the absence of external K+, the Na(+)-free solution failed to induce [Ca2+]i increase; the readmission of external K+ restored the [Ca2+]i response. These results are consistent with a Na(+)-Ca2+ exchanger operating in reverse mode. An K+ dependence of this exchange is also suggested.

Kinetic properties of the sodium-calcium exchanger in rat brain synaptosomes

1. The kinetic properties of the internal Na+ (Na+i)- dependent 45Ca2+ influx and external Na+ (Na+o)-dependent 45Ca2+ efflux were determined in isolated rat brain nerve terminals (synaptosomes) under conditions which the concentrations of internal Na+ ([Na+]i), external Na+ ([Na+]o), external Ca2+ (Ca2+]o), and external K+ ([K+]o) were varied. Both fluxes are manifestations of Na(+)-Ca2+ exchange. 2. Ca2+ uptake was augmented by raising [Na+]i and / or lowering [Na+]o. The increase in Ca2+ uptake induced by removing external Na+ was, in most instances, quantitatively equal to the Na+i-dependent Ca2+ uptake. 3. The Na+i-dependent Ca2+ uptake (measured at 1 s) was activated with an apparent half-maximal [Ca2+]o (KCa(o)) of about 0.23 mM. External Na+ inhibited the uptake in a non- competitive manner: increasing [Na+]o from 4.7 to 96 mM reduced the maximal Na+(i)-dependent Ca2+ uptake but did not affect KCa(o). 4. The inhibition of Ca2+ uptake by Na+o was proportional to ([Na+]o)2, and had a Hill coefficient (nH) of approximately 2.0. The mean apparent half-maximal [Na+]o for inhibition (KI(Na)) was about 60mM, and was independent of [Ca2+]o between 0.1 and 1.2mM; this, too, is indicative of non-competitive inhibition. 5. Low concentrations of alkali metal ions (M+) in the medium, including Na+, stimulated the Na+i-dependent uptake. The external Na+ and K+ concentrations required for apparent half-maximal activation (KM(Na) and KM(K), respectively) were 0.12 and 0.10mM. Thus, the relationship between Ca2+ uptake and [Na+]o was biphasic: uptake was stimulated by [Na+]o < or = 10 mM, and inhibited by higher [Na+]o. 6. The calculated maximal Na+i-dependent Ca2+ uptake (Jmax) was about 1530 pmol (mg protein) -1s-1 at 30 degrees C saturating [Ca2+]o and external M+ concentration ([M+]o), and with negligible inhibition by external Na+. 7. Internal Na+ activated the Ca2+ uptake with an apparent half-maximal concentration (KNa(i)) of about 20 mM and a Hill coefficient, nH, of approximately 3.0. 8. The Jmax for the Na+o-dependent efflux of Ca2+ from 45Ca(2+)-loaded synaptosomes treated with carbonyl cyanide p-trifluormethoxy-phenylhydrazone (FCCP) and caffeine (to release stored Ca2+ and raise the internal Ca2+ concentration ([Ca2+]i) was about 1800-2000 pmol (mg protein -1s-1 at 37 degrees C. 9. When the membrane potential (Vm) was reduced (depolarized) by increasing [K+]o, the Na+i-dependent Ca2+ influx increased, and the Na+o-dependent Ca2+ efflux declined. Both fluxes changed about 2-fold per 60 mV change in Vm. This voltage sensitivity corresponds to the movement of one elementary charge through about 60% of the membrane electric field. The symmetry suggests that the voltage-sensitive step is reversible. 10. The Jmax values for both Ca2P influx and efflux correspond to a Na+-Ca2+ exchange-mediated flux of about 425-575 jumol Ca2P (1 cell water)-' s-' or a turnover of about one quarter of the total synaptosome Ca2P in 1 s. We conclude that the Na+-Ca2P exchanger may contribute to Ca2P entry during nerve terminal depolarization; it is likely to be a major mechanism mediating Ca2P extrusion during subsequent repolarization and recovery.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Regulation of Na+/Ca2+ exchange in the rat pancreatic B cell

Na+/Ca2+ exchange in the B cell was recently characterized by measuring intracellular-Na(+)-dependent 45Ca2+ uptake in isolated rat pancreatic islet cells. The aim of the present study was to investigate the regulation of this process. Extracellular pH (pHo) and intracellular pH (pHi) markedly affected Na+/Ca2+ exchange. A fall of 0.04 unit in pHi decreased the exchange by 45%, whereas a rise of 0.13 unit increased the uptake by 70%. Mitochondrial poisons (oligomycin, antimycin A and 2,4-dinitrophenol) inhibited reverse Na+/Ca2+ exchange by about 25-50%. The exchanger displayed a low Q10 (temperature coefficient), indicating that it is only indirectly dependent on metabolic energy. The phorbol ester phorbol 12-myristate 13-acetate did not affect Na+/Ca2+ exchange. Likewise, lowering the extracellular K+ concentration did not inhibit 45Ca2+ uptake. In conclusion, the pHi and the metabolic state of the cell may represent important modulatory signals by which insulin secretagogues such as glucose could regulate reverse Na+/Ca2+ exchange in the B cell. The process does not appear to co-transport K+ nor to be influenced by protein kinase C.

Modeling the dynamic features of the electrogenic Na,K pump of cardiac cells

The purpose of this paper is to examine the dynamic features of the electrogenic Na,K pump of cardiac cells, based on a comparative analysis of a mechanistic model and an ad hoc mathematical description of the Na,K pump. Both representations are incorporated into a modified version of the Beeler-Reuter model for the ventricular membrane, and the resulting action potential models are studied under conditions of repetitive stimulation at steady rates between 0 and 3 Hz. The two Na,K pump representations have nearly identical steady-state characteristics of sensitivity to internal Na+ concentration, external K+ concentration, and membrane potential. Rapid voltage-dependent transient pump currents are present in the mechanistic model, while they are absent in the ad hoc mathematical description we used. The stimulation results show that a sizable peak of pump current caused by the action potential upstroke in the mechanistic model affects phase 1 repolarization, and that this effect is relatively independent of the stimulation rate. The pump current generated by our ad hoc mathematical description is constant during the action potential and does not affect directly the repolarization time course. While the two Na,K pump models show similar pumping efficiency at low stimulation rates, the mechanistic pump is more efficient at high rates of activity. In essence, the distinctive features of the mechanistic model are due to an energy barrier expressing the voltage dependence of the translocation step of the mechanism, and to the redistribution of the intermediates of the biochemical reactions during activity. In comparison, the ad hoc mathematical description exhibits a fixed dependence of the pump current on voltage and ionic concentrations.

The stoichiometry of Na-Ca+K exchange in rod outer segments isolated from bovine retinas

Ca2+ extrusion in the outer segments of retinal rods (ROS) is mediated by a protein that couples both the inward Na+ gradient and the outward K+ gradient to Ca2+ extrusion. Na(+)-stimulated Ca2+ release from ROS requires internal K+ and is accompanied by release of internal K+, whereas a slow component of Na(+)-stimulated Ca2+ release does not require K+. In this paper we discuss our observations on the K+ transport via Na-Ca+ K exchange in bovine ROS, on the electrogenicity and stoichiometry of the ROS Na-Ca+ K exchanger, and on the mechanism on coupling Ca2+ to K+ via this protein. Finally, we discuss briefly the physiological implications of Na-Ca+ K exchange.

The exchange in intact squid axons

The exchange in intact axons displays a number of features in common with other systems, but a number of interesting points remain to be examined. Both forward (Nao-Cai) and reverse (Cao-Nai) exchange are sensitive to changes in membrane potential, but potassium depolarization can also stimulate Cao-Nai exchange by chemical activation at a monovalent cation-binding site. By monitoring lithium uptakes into intact axons, activating cations do not appear to be transported on the exchange, but this deserves further examination under more stringent conditions. Cao-Nai exchange in intact axons appears activated by monovalent cations to a greater extent compared to dialyzed axons that exhibit little, if any, shift in the Km for Cao. The catalytic effect of Cai on Cao-Nai exchange seen in dialyzed axons proves elusive to study in intact axons, with or without introduction of Ca chelators. Experiments using ruthenium red suggest that free calcium can be dissociated from Cao-Nai exchange fluxes; this finding is also important to those studies monitoring exchange activity using Ca indicators. The possibility that Ca chelators may effect changes in the kinetics of Na-Ca exchange is a subject that needs further investigation.

Ca2+ movement in smooth muscle cells studied with one- and two-dimensional diffusion models

Although many of the processes involved in the regulation of Ca2+ in smooth muscle have been studied separately, it is still not well known how they are integrated into an overall regulatory system. To examine this question and to study the time course and spatial distribution of Ca2+ in cells after activation, one- and two-dimensional diffusion models of the cell that included the major processes thought to be involved in Ca regulation were developed. The models included terms describing Ca influx, buffering, plasma membrane extrusion, and release and reuptake by the sarcoplasmic reticulum. When possible these processes were described with known parameters. Simulations with the models indicated that the sarcoplasmic reticulum Ca pump is probably primarily responsible for the removal of cytoplasmic Ca2+ after cell activation. The plasma membrane Ca-ATPase and Na/Ca exchange appeared more likely to be involved in the long term regulation of Ca2+. Pumping processes in general had little influence on the rate of rise of Ca transients. The models also showed that spatial inhomogeneities in Ca2+ probably occur in cells during the spread of the Ca signal following activation and during the subsequent return of Ca2+ to its resting level.

Cytosolic free Ca2+ during operation of sodium-calcium exchange in guinea-pig heart cells

1. Membrane current generated by the Na(+)-Ca2+ exchange mechanism was recorded in single guinea-pig ventricular myocytes using the whole-cell voltage-clamp technique and the intracellular free calcium concentration ([Ca2+]i) was monitored using the fluorescent probe Indo-1, applied intracellularly through a perfused patch pipette. The reversal potential of the exchanger (ENa, Ca) was measured from records of the 2 mM-Ni(2+)-sensitive current and used in an attempt to clamp [Ca2+]i at a level determined by the ionic compositions of the external and pipette solutions. 2. Measurements of ENa, Ca indicated that [Ca2+]i was close to that in the pipette solution when the holding potential was set at the ENa, Ca expected for a 3Na+:1Ca2+ exchanger. The measured value of ENa, Ca was more positive than the theoretical value when the membrane potential was held positive to ENa, Ca and the opposite was true when the holding potential was more negative than the expected ENa, Ca. 3. As Indo-1 diffused into the cell from the whole-cell clamp electrode, the intensities of the fluorescent signals measured at 405 and 480 nm increased with time, with no obvious saturation over a 10-45 min recording period. However, the ratio of these two signals reached a steady level within 5 min after rupture of the patch membrane, when the holding potential was set at the expected ENa, Ca of the exchanger. The intensity ratios measured using pipette solutions containing 600 and 803 nM [Ca2+] were almost equal to the ratios obtained extracellularly from internal solutions of identical compositions, but in experiments using pipette solutions having lower [Ca2+] the intensity ratios measured in myocytes were higher than those obtained extracellularly. 4. If the membrane was depolarized or hyperpolarized, the fluorescence ratio either increased or decreased, respectively. These changes in the fluorescence ratio were virtually blocked by the extracellular application of 2 mM-Ni2+. 5. When the concentration of bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the recording pipette was reduced from 30 to 1 mM, an increase in [Ca2+]i was observed during a depolarizing ramp pulse. The Ca2+ influx estimated by integrating the 2 mM-Ni(2+)-sensitive current during the pulse correlated with the increase in [Ca2+]i estimated from Indo-1 using the extracellular calibration curve, but the values of the influx determined directly from Indo-1 fluorescence were always larger than those calculated from the exchanger current.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium regulation in neurons: transport processes

The ionic stoichiometry of the major Ca2+ transport mechanisms in neurons is still a matter for debate. The past year has seen some particularly interesting developments in this field, not least the finding that the neuronal Na(+)-Ca2+ exchange may be able to transport K+.

The sodium-calcium exchanger of bovine rod photoreceptors: K(+)-dependence of the purified and reconstituted protein

The K(+)-dependence of the rod photoreceptor sodium-calcium exchanger was investigated using the Ca2(+)-sensitive dye arsenazo III after reconstitution of the purified protein into proteoliposomes. The uptake of Ca2+ by Na(+)-loaded liposomes was found to be greatly enhanced by the presence of external K+ (EC50 approximately 1 mM) in a Michaelis-Menten manner, suggesting that one K+ ion is involved in the transport of one Ca2+ ion. We also found a minimal degree of Ca2+ uptake in the total absence of K+. Other alkali cations, notably Rb+ and, to a lesser extent, Cs+, were also able to stimulate Na(+)-Ca2+ exchange. We also investigated the K(+)-dependence of the photoreceptor Na(+)-Ca2+ exchanger by determining the effects of electrochemical K+ gradients on the Na(+)-activated Ca2+ efflux from proteoliposomes. We found that, under conditions of membrane voltage clamp with FCCP, inwardly directed electrochemical K+ gradients (i.e., K0+ greater than Ki+) inhibited, whereas an outwardly directed electrochemical K+ gradient (i.e., Ki+ greater than K0+) enhanced, Na(+)-dependent Ca2+ efflux, consistent with the notion that K+ is cotransported in the same direction as Ca2+. The investigation of the reconstituted exchanger at physiological (i.e. Ki+ = 110 mM, K0+ = 2.5 mM) potassium concentrations revealed that the Na(+)-dependence of Ca2(+)-efflux was highly cooperative (n = 3.01 from Hill plots), indicating that at least three, but possibly four, Na+ ions are exchanged for one Ca2+ ion. Under these conditions the reconstituted exchanger showed a Km for Na+ of 26.1 mM, and a turnover number of 115 Ca2+.s-1 per exchanger molecule. Our results with the purified and reconstituted sodium-calcium exchanger from rod photoreceptors are therefore consistent with previous reports (Cervetto, L., Lagnado, L., Perry, R.J., Robinson, D.W. and McNaughton, P.A. (1989) Nature 337, 740-743; Schnetkamp, P.P.M., Basu, D.K. and Szerencsei, R.T. (1989) Am. J. Physiol. 257, C153-C157) that the sodium-calcium exchanger of rod photoreceptors cotransports K+ under physiological conditions with a stoichiometry of 4 Na+:1 Ca2+, 1K+.

Sodium-calcium exchange in cultured bovine pulmonary artery endothelial cells

1. Intracellular free calcium ([Ca2+]i) was measured in cultured bovine pulmonary artery endothelial cell monolayers loaded with the fluorescent calcium indicator Fura-2. 2. Resting [Ca2+]i was 112 +/- 10 nM. Application of ouabain (20 microM) was without effect on [Ca2+]i for periods of up to 1 h. Monensin (10 microM) resting [Ca2+]i to 145 +/- 32 nM over approximately 2 min. In the presence of ouabain (20 microM), 10 microM-monensin increased [Ca2+]i to 146 +/- 15 nM. 3. Removal of extracellular sodium was without effect in resting cells or cells exposed to ouabain alone. However, in the presence of monensin, replacement of extracellular Na+ with Li+ resulted in a prompt increase in [Ca2+]i to a peak of 280 +/- 37 nM, which then returned towards resting levels. When Na+ was removed in the presence of both ouabain and monensin, [Ca2+]i reached a peak of 585 +/- 53 nM. 4. When extracellular Na+ was replaced with K+, to achieve simultaneous Na+ removal and depolarization, [Ca2+]i reached a peak of 568 +/- 63 nM, compared with a peak of 462 +/- 38 nM when Li+ was used as a Na+ substitute in paired experiments. The transient increase in [Ca2+]i evoked by sodium removal peaked earlier when K+ was used as the sodium substitute, showing that depolarization increased the rate of calcium influx into the cell when sodium was removed from the bathing medium. 5. Removal of extracellular K+ had no effect on the low-Na(+)-evoked increase in [Ca2+]i. 6. Returning extracellular Na+ during the increase in [Ca2+]i resulting from Na+ removal increased the rate of return of [Ca2+]i towards basal levels. In the absence of Na+, [Ca2+]i took 41 +/- 5 s to decline from 400 to 200 nM, and this was reduced to 26 +/- 6 s (n = 4, S.E.M.) when Na+ was returned to the bathing solution. 7. These results indicate endothelial cells possess a voltage-dependent Na(+) -Ca2+ exchange mechanism in the surface membrane. However, this mechanism does not appear to be of primary importance in the maintenance of resting [Ca2+]i since cells were able to restore a low [Ca2+]i in the absence of extracellular Na+. The evidence for the existence of a Na(+) -Ca2+ exchanger in the surface membrane of endothelial cells and the possibility that this mechanism may contribute to calcium entry and/or extrusion during agonist-evoked responses is discussed.

Ca2+ extrusion across plasma membrane and Ca2+ uptake by intracellular stores

The aim of this review is to summarize the various systems that remove Ca2+ from the cytoplasm. We will initially focus on the Ca2+ pump and the Na(+)-Ca2+ exchanger of the plasma membrane. We will review the functional regulation of these systems and the recent progress obtained with molecular-biology techniques, which pointed to the existence of different isoforms of the Ca2+ pump. The Ca2+ pumps of the sarco(endo)plasmic reticulum will be discussed next, by summarizing the discoveries obtained with molecular-biology techniques, and by reviewing the physiological regulation of these proteins. We will finally briefly review the mitochondrial Ca(2+)-uptake mechanism.

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.