Na(+)-Ca2+ exchangers from rod outer segments and cardiac sarcolemma: comparison of properties

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.

Functional adult myocardium in the absence of Na+-Ca2+ exchange: cardiac-specific knockout of NCX1

The excitation-contraction coupling cycle in cardiac muscle is initiated by an influx of Ca2+ through voltage-dependent Ca2+ channels. Ca2+ influx induces a release of Ca2+ from the sarcoplasmic reticulum and myocyte contraction. To maintain Ca2+ homeostasis, Ca2+ entry is balanced by efflux mediated by the sarcolemmal Na+-Ca2+ exchanger. In the absence of Na+-Ca2+ exchange, it would be expected that cardiac myocytes would overload with Ca2+. Using Cre/loxP technology, we generated mice with a cardiac-specific knockout of the Na+-Ca2+ exchanger, NCX1. The exchanger is completely ablated in 80% to 90% of the cardiomyocytes as determined by immunoblot, immunofluorescence, and exchange function. Surprisingly, the NCX1 knockout mice live to adulthood with only modestly reduced cardiac function as assessed by echocardiography. At 7.5 weeks of age, measures of contractility are decreased by 20% to 30%. We detect no adaptation of the myocardium to the absence of the Na+-Ca2+ exchanger as measured by both immunoblots and microarray analysis. Ca2+ transients of isolated myocytes from knockout mice display normal magnitudes and relaxation kinetics and normal responses to isoproterenol. Under voltage clamp conditions, the current through L-type Ca2+ channels is reduced by 50%, although the number of channels is unchanged. An abbreviated action potential may further reduce Ca2+ influx. Rather than upregulate other Ca2+ efflux mechanisms, the myocardium appears to functionally adapt to the absence of the Na+-Ca2+ exchanger by limiting Ca2+ influx. The magnitude of Ca2+ transients appears to be maintained by an increased gain of sarcoplasmic reticular Ca2+ release. The myocardium of the NCX1 knockout mice undergoes a remarkable adaptation to maintain near normal cardiac function.

Mutual inhibition of the dimerized Na/Ca-K exchanger in rod photoreceptors

In the dark, rod photoreceptors sustain a continuous influx of Na and Ca ions through the cGMP-gated channels of the rod outer segments (ROS). Whereas Na ions are extruded in the inner segment by the Na-pump, Ca ions are extruded already in the ROS by Na/Ca-K exchange. Our previous findings indicate that in the ROS plasma membrane, exchanger and channel form a complex of two exchangers associated per channel. Here, we report evidence of a novel regulatory mechanism of the dimerized exchanger, based on the following findings: (1), thiol-specific cross-linking with dimaleimides resulted in an increase of the Na/Ca-K exchange activity which correlated with the size of the cross-linking reagent, i.e., with increasing separation of the monomers in a dimerized exchanger; (2), partial proteolysis of the exchanger also increased the exchange rate by about a factor of two; (3), disintegration of the channel-exchanger complex by solubilization of the ROS membranes and preparation of proteoliposomes resulted in a twofold enhancement of the exchange rate; however (4), partial proteolysis of proteoliposomes, in which the exchanger molecules exist as monomers, did not result in any enhancement of the exchange rate. These findings suggest an inhibitory protein domain at the contact site of the dimerized exchanger. The physiological implication of this inference will be discussed in terms of a potential allosteric regulation of the exchanger in the channel-exchanger complex.

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.

Renal arteriolar Na+/Ca2+ exchange in salt-sensitive hypertension

The present studies were performed to assess Na+/Ca2+ exchange activity in afferent and efferent arterioles from Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats. Renal arterioles were obtained by microdissection from S and R rats on either a low-salt (0.3% NaCl) or high-salt (8.0% NaCl) diet. On the high-salt diet, S rats become markedly hypertensive. Cytosolic calcium concentration ([Ca2+]i) was measured in fura 2-loaded arterioles bathed in a Ringer solution in which extracellular Na (Nae) was varied from 150 to 2 mM (Na was replaced with N-methyl-D-glucamine). Baseline [Ca2+]i was similar in afferent arterioles of R and S rats fed low- and high-salt diet. The change in [Ca2+]i (Delta[Ca2+]i) during reduction in Nae from 150 to 2 mM was 80 +/- 10 and 61 +/- 3 nM (not significant) in afferent arterioles from R rats fed the low- and high-salt diet, respectively. In afferent arterioles from S rats on a high-salt diet, Delta[Ca2+]i during reductions in Nae from 150 to 2 mM was attenuated (39 +/- 4 nM) relative to the Delta[Ca2+]i of 79 +/- 13 nM (P < 0.05) obtained in afferent arterioles from S rats on a low-salt diet. In efferent arterioles, baseline [Ca2+]i was similar in R and S rats fed low- and high-salt diets, and Delta[Ca2+]i in response to reduction in Nae was also not different in efferent arterioles from R and S rats fed low- or high-salt diets. Differences in regulation of the exchanger in afferent arterioles of S and R rats were assessed by determining the effects of protein kinase C (PKC) activation by phorbol 12-myristate 13-acetate (PMA, 100 nM) on Delta[Ca2+]i in response to reductions in Nae from 150 to 2 mM. PMA increased Delta[Ca2+]i in afferent arterioles from R rats but not from S rats. These results suggest that Na+/Ca2+ exchange activity is suppressed in afferent arterioles of S rats that are on a high-salt diet. In addition, there appears to be a defect in the PKC-Na+/Ca2+ exchange pathway that might contribute to altered [Ca2+]i regulation in this important renal vascular segment in salt-sensitive hypertension.

Direct evidence of Na+/Ca2+ exchange in squid rhabdomeric membranes

Na+/Ca2+ exchange has been investigated in squid (Loligo pealei) rhabdomeric membranes. Ca2+-containing vesicles have been prepared from purified rhabdomeric membranes by extrusion through polycarbonate filters of 1-micrometer pore size. After removal of external Ca2+, up to 90% of the entrapped Ca2+ could be specifically released by the addition of Na+; this finding indicates that most of the vesicles contained Na+/Ca2+ exchanger. The Na+-induced Ca2+ efflux had a half-maximum value (K1/2) of approximately 44 mM and a Hill coefficient of approximately 1.7. The maximal Na+-induced Ca2+ efflux was approximately 0.6 nmol Ca2+. s-1. mg protein-1. Similar Na+-induced Ca2+ effluxes were measured if K+ was replaced with Li+ or Cs+. Vesicles loaded with Ca2+ by Na+/Ca2+ exchange also released this Ca2+ by Na+/Ca2+ exchange, suggesting that Na+/Ca2+ exchange operated in both forward and reverse modes. Limited proteolysis by trypsin resulted in a rate of Ca2+ efflux enhanced by approximately fivefold when efflux was activated with 95 mM NaCl. For vesicles subjected to limited proteolysis by trypsin, Na+/Ca2+ exchange was characterized by a K1/2 of approximately 25 mM and a Hill coefficient of 1.6. For these vesicles, the maximal Na+-induced Ca2+ efflux was about twice as great as in control vesicles. We conclude that Na+/Ca2+ exchange proteins localized in rhabdomeric membranes mediate Ca2+ extrusion in squid photoreceptors.

Structure-function relationships and localization of the Na/Ca-K exchanger in rod photoreceptors

The structural and functional properties of the bovine rod photoreceptor Na/Ca-K exchanger and its distribution in vertebrate photoreceptor cells were studied using a panel of monoclonal antibodies. Antibodies that bind to distinct epitopes along the large hydrophilic N-terminal segment of the exchanger labeled the extracellular surface of the rod outer segment plasma membrane, whereas antibodies against a large hydrophilic loop between the two membrane domains labeled the intracellular side. Enzymatic deglycosylation studies indicated that the exchanger primarily contains O-linked sialo-oligosaccharides located within the N-terminal domain. Removal of the extracellular domain with trypsin or the large intracellular domain with kallikrein did not alter the Na+- or K+-dependent Ca2+ efflux activity of the exchanger when reconstituted into lipid vesicles. Anti-exchanger antibodies were also used to visualize the distribution of the exchanger in the retina by light and electron microscopy. The exchanger was localized to the plasma membrane of rod outer segments. No labeling was observed in the disk membranes, cone photoreceptor cells, or other retinal neurons, and only faint staining was seen in the rod inner segment. These results indicate that the O-linked glycosylated rod Na/Ca-K exchanger is specifically targeted to the plasma membrane of rod photoreceptors and has a topological organization similar to that reported for the cardiac Na/Ca exchanger. The large intracellular and extracellular domains do not directly function in the transport of ions across the rod outer segment plasma membrane, but instead may play a role in protein-protein interactions that maintain the spatial organization of the exchanger in the plasma membrane or possibly regulate transport activity of the exchanger.

The Na/Ca-K exchanger of rod photoreceptor exists as dimer in the plasma membrane

The oligomeric state of the Na/Ca-K exchanger in the plasma membrane of bovine photoreceptors was investigated using chemical cross-linking techniques. In the natural membrane, virtually all Na/Ca-K exchanger could be cross-linked mainly to a complex having an apparent molecular mass of 490 kDa by cupric phenanthroline catalyzed disulfide bonding as evidenced by Western blotting. Stable cross-links of the exchanger were also obtained with the thiol-specific reagent N,N'-p-phenylidenedimaleimide. Neuraminidase treatment reduced the apparent molecular mass of the highly glycosylated Na/Ca-K exchanger and of the 490 kDa cross-link product by 50 and 85 kDa, respectively. DL-1,4-Bismaleimido-2,3-butanediol (BMBD), a novel cleavable dimaleimide, was synthesized in order to produce cross-links that were stable to reductive conditions. Purification of the BMBD cross-linked exchanger followed by two-dimensional SDS polyacrylamide electrophoresis identified the cross-linked homodimers of the exchanger. There was no indication of higher oligomers, suggesting that the exchanger exists as a dimer in the plasma membrane. Hydrodynamic properties of the detergent-solubilized exchanger were determined by velocity sedimentation and gel filtration chromatography. The Triton X-100-solubilized exchanger ran as a single species having a Stokes radius of 10.0 nm, a sedimentation coefficient of 5.4 S, and a partial specific volume of 0.74 mL/g in Triton X-100. Similar results were obtained for the CHAPS-solubilized exchanger. A molecular mass of 236 and 205 kDa was calculated for the exchanger-detergent complex and the detergent-free protein, respectively. Neuraminidase treatment further reduced the molecular mass of the exchanger indicating that glycosylation contributes significantly to the mass of the exchanger. Cross-links of the exchanger were not detected if cross-linking was attempted after solubilization in 10 mM CHAPS. However, after reconstitution of the purified exchanger into soybean phosphatidylcholine vesicles, chemical cross-linking yielded again dimers. On the basis of these cross-linking and hydrodynamic studies, we conclude that the exchanger exists as a homodimer in the rod outer segment plasma membrane but dissociates into a monomer when solubilized in detergent.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.

Immunologic localization and kinetic characterization of a Na+/Ca2+ exchanger in neuronal and non-neuronal cells

The plasma membrane Na+/Ca2+ exchanger is believed to play a role in the regulation of Ca2+ fluxes in neurons, though the lack of specific inhibitors has limited the delineation of its precise contribution. We recently reported the development of antibodies against a 36-kDa brain synaptic membrane protein which immunoprecipitated exchanger activity from solubilized membranes. In the present study we examined the kinetics of the Na+/Ca2+ exchanger in primary neurons in culture, in a neuronal hybrid cell line (NCB-20), and in a fibroblast-like cell line (CV-1) to see whether the level of exchanger activity correlated with the degree of immunostaining produced by our antibodies. The Vmax was determined for each cell type and found to be highest in primary neurons. Exchanger activity increased in primary neurons between days 1 and 6 in culture, but no such time-dependent change occurred in either of the cell lines. Immunoblot analysis of the three cell types probed with the anti-36-kDa protein antibodies revealed significantly greater immunostaining in the primary neurons compared with the other two cell types. Intensity of staining of neurons also increased significantly between days 1 and 6 in culture. Immunocytochemistry showed significant labelling of the primary neurons on the neuritic processes and points of contact between cells. The NCB-20 and CV-1 cells showed considerably lower levels of immunoreactivity. The antibodies immunoextracted approximately 90% of the exchanger activity in the primary neurons and approximately 70 and 50% of the activity in NCB-20 and CV-1 cells respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Na(+)-Ca2+,K+ exchange in bovine retinal rod outer segments: quantitative characterization of normal and reversed mode

Ca2+ homeostasis of bovine retinal rod outer segments is maintained through Na(+)-Ca2+,K+ exchangers and cGMP-gated channels in the plasma membrane. It has recently been demonstrated that both proteins are associated. This novel finding allowed us to investigate quantitatively normal and reversed mode Na(+)-Ca2+,K+ exchange in rod outer segment membrane vesicles and reconstituted proteoliposomes both containing exchangers in rightside-out and inside-out orientations. Addition of Na+ activated both normal and reversed mode exchange; if, however, initially Ca2+ from vesicles containing inside-out oriented exchangers has been released by activation of the associated channels, only normal mode exchange was observed upon addition of Na+. Using this approach, the fractions of vesicles containing rightside-out and inside-out oriented exchangers were about similar in these vesicle preparations. Normal and reversed mode exchange had similar Na+ concentrations of about 70 mM for half maximal activation (in the presence of 115 mM K+) and cooperativity parameters, nHill, of about 2.0. Furthermore, both modes were electrogenic, and showed only little Na(+)-Ca2+,K+ exchange in the absence of K+. The two modes of exchange differed, however, in the maximal exchange rate, the normal mode being about twice as fast as the reversed mode.

Regional distribution in the rat central nervous system of a mRNA encoding a portion of the cardiac sodium/calcium exchanger isolated from cerebellar granule neurons

The cardiac Na+/Ca2+ exchanger is a bidirectional electrogenic ion transporter that exchanges three Na+ ions for each Ca2+ ion and plays a critical role in returning sarcolemma Ca2+ concentrations to their resting levels. Because of the importance that the Na+/Ca2+ exchanger may play in maintaining neuronal Ca2+ homeostasis in the central nervous system, we subcloned a 456 bp portion of the Na+/Ca2+ exchanger cDNA from RNA isolated from primary cultures of rat cerebellar granule neurons using the polymerase chain reaction (PCR). This cDNA fragment was sequenced and shown to share 91.4% sequence identity with the human and 88% sequence identity with the canine cardiac Na+/Ca2+ exchangers. The PCR amplification product was used to analyze the distribution of this portion of the Na+/Ca2+ exchanger mRNA in various regions of the CNS by both Northern blotting and in situ hybridization histochemistry. The Northern analysis showed that the rank order of abundance of this mRNA was: hippocampus > cortex > cerebellum > hypothalamus > midbrain > striatum. The in situ hybridization data indicated that the corresponding mRNA containing this portion of the exchanger was present in numerous brain regions including multiple cortical layers, the hippocampus, septal nuclei, various thalamic nuclei, cerebellum, hypothalamus, olfactory bulb, brainstem, in various regions of the thoracic spinal cord and to a lesser extent in the striatum. The differential distribution of the mRNA as revealed by the in situ hybridization pattern suggests that either additional molecular variants exist or that different Na+/Ca2+ exchange mechanisms may be operative in those cell types that contain low amounts of this fragment of the exchanger mRNA.

Sodium-calcium exchange

During the past year, significant advances have been made in the investigation of molecular, kinetic and electrophysiological aspects of Na(+)-Ca2+ exchange. The cardiac and retinal exchangers have been cloned and structure-function studies have begun.

Sarcolemmal calcium binding sites in heart: II. Mathematical model for diffusion of calcium released from the sarcoplasmic reticulum into the diadic region

We present a model for predicting the temporal and spatial dependence of [Ca] in the cardiac subsarcolemmal diadic region (cleft), following Ca release from the "feet" of the sarcoplasmic reticulum. This region is modeled as a disc 10 nm thick, 430 nm in radius, with or without Ca binding sites and open at its periphery to the cytosol. [Ca] is computed for three diffusion coefficients (100, 20 and 4% of aqueous diffusion), following release of a 20-msec square pulse sufficient to produce 50% maximal contractile force, or repetitive release (400/min) of such pulses. Numerical solutions are obtained for the general diffusion/binding problem and analytic solutions for the case of no binding sites. For the middle value of diffusion coefficient, and in the absence of binding sites, [Ca] rises to approximately 1.5 mM in 20-msec and then falls to approximately 0.1 microM in less than 3 msec. Adding binding sites reduces peak [Ca] to approximately 0.6 mM but prolongs its decline, requiring approximately 200 msec to reach 20 microM. For repetitive release [Ca] is greater than 100 microM for roughly half of each cycle. Two major implications of the predicted [Ca] are: (i) The effect of Ca binding sites on [Ca] will cause Ca efflux from the cleft via the Na-Ca exchanger (Km(Ca) approximately 20 microM) to continue at a significant level for greater than 200 msec. (ii) The time constant for inactivation of release from the "feet" must be much greater than for activation if Ca-induced Ca release is to continue for greater than 1-2 msec.

Role of Na+/Ca2+ exchange in transcellular Ca2+ transport across primary cultures of rabbit kidney collecting system

Cells from connecting tubule and cortical collecting duct of rabbit kidney were isolated by immunodissection with mAb R2G9 and cultured on permeable filters. Confluent monolayers developed an amiloride-sensitive transepithelial potential difference of -50 +/- 1 mV (lumen negative) and a transepithelial resistance of 507 +/- 18 omega cm2. Transepithelial Ca2+ transport increased dose-dependently with apical [Ca2+] and, in solutions containing 1 mM Ca2+, the active transcellular Ca2+ transport rate was 92 +/- 2 nmol h-1 cm-2. Transcellular Ca2+ transport was dependent on basolateral Na+ (Nab+). Isoosmotic substitution of Nab+ for N-methylglucamine resulted in a concentration-dependent decrease in Ca2+ absorption, with maximal inhibition of 67 +/- 5%. A Hill plot of the Na(+)-dependence yielded a coefficient of 1.9 +/- 0.4, indicating more than one Na+ site on a Na(+)-dependent Ca2+ transport system. In addition, the absence of Cab2+ resulted in a significant increase in Ca2+ transport both in the presence and absence of Nab+. Added basolaterally, ouabain (0.1 mM) inhibited Ca2+ transport to the same extent as did Na(+)-free solutions, while bepridil (0.1 mM), an inhibitor of Na+/Ca2+ exchange, reduced Ca2+ transport by 32 +/- 6%. Methoxyverapamil, felodipine, flunarizine and diltiazem (10 microM) were without effect. Depolarisation of the basolateral membrane, by raising [K+]b to 60 mM, significantly decreased transcellular Ca2+ transport, which is indicative of electrogenic Na+/Ca2+ exchange. In conclusion, active Ca2+ transport in the collecting system of rabbit kidney is largely driven by basolateral Na+/Ca2+ exchange. However, a residual Ca2+ absorption of about 30% was always observed, suggesting that other Ca2+ transport mechanisms, presumably a Ca(2+)-ATPase, participate as well.