Sodium-calcium exchange

Simultaneous Increases in Intracellular Sodium and Tonicity Boost Antimicrobial Activity of Macrophages

Inflamed and infected tissues can display increased local sodium (Na+) levels, which can have various effects on immune cells. In macrophages, high salt (HS) leads to a Na+/Ca2+-exchanger 1 (NCX1)-dependent increase in intracellular Na+ levels. This results in augmented osmoprotective signaling and enhanced proinflammatory activation, such as enhanced expression of type 2 nitric oxide synthase and antimicrobial function. In this study, the role of elevated intracellular Na+ levels in macrophages was investigated. Therefore, the Na+/K+-ATPase (NKA) was pharmacologically inhibited with two cardiac glycosides (CGs), ouabain (OUA) and digoxin (DIG), to raise intracellular Na+ without increasing extracellular Na+ levels. Exposure to HS conditions and treatment with both inhibitors resulted in intracellular Na+ accumulation and subsequent phosphorylation of p38/MAPK. The CGs had different effects on intracellular Ca2+ and K+ compared to HS stimulation. Moreover, the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5) was not upregulated on RNA and protein levels upon OUA and DIG treatment. Accordingly, OUA and DIG did not boost nitric oxide (NO) production and showed heterogeneous effects toward eliminating intracellular bacteria. While HS environments cause hypertonic stress and ionic perturbations, cardiac glycosides only induce the latter. Cotreatment of macrophages with OUA and non-ionic osmolyte mannitol (MAN) partially mimicked the HS-boosted antimicrobial macrophage activity. These findings suggest that intracellular Na+ accumulation and hypertonic stress are required but not sufficient to mimic boosted macrophage function induced by increased extracellular sodium availability.

Role of annexin A6 isoforms in catecholamine secretion by PC12 cells: distinct influence on calcium response

Noradrenaline and adrenaline are secreted by adrenal medulla chromaffin cells via exocytosis. Exocytosis of catecholamines occurs after cell stimulation with various endogenous activators such as nicotine or after depolarization of the plasma membrane and is regulated by calcium ions. Cytosolic [Ca(2+)] increases in response to cell excitation and triggers a signal-initiated secretion. Annexins are known to participate in the regulation of membrane dynamics and are also considered to be involved in vesicular trafficking. Some experimental evidence suggests that annexins may participate in Ca(2+)-regulated catecholamine secretion. In this report the effect of annexin A6 (AnxA6) isoforms 1 and 2 on catecholamine secretion has been described. Overexpression of AnxA6 isoforms and AnxA6 knock-down in PC12 cells were accompanied by almost complete inhibition or a 20% enhancement of dopamine secretion, respectively. AnxA6-1 and AnxA6-2 overexpression reduced Delta[Ca(2+)](c) upon depolarization by 32% and 58%, respectively, while AnxA6 knock-down increased Delta[Ca(2+)](c) by 44%. The mechanism of AnxA6 action on Ca(2+) signalling is not well understood. Experimental evidence suggests that two AnxA6 isoforms interact with different targets engaged in regulation of calcium homeostasis in PC12 cells.

The co-presence of Na+/Ca2+-K+ exchanger and Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

We have previously shown that there is high Na(+)/Ca(2+) exchange (NCX) activity in bovine adrenal chromaffin cells. In this study, by monitoring the [Ca(2+)](i) change in single cells and in a population of chromaffin cells, when the reverse mode of exchanger activity has been initiated, we have shown that the NCX activity is enhanced by K(+). The K(+)-enhanced activity accounted for a significant proportion of the Na(+)-dependent Ca(2+) uptake activity in the chromaffin cells. The results support the hypothesis that both NCX and Na(+)/Ca(2+)-K(+) exchanger (NCKX) are co-present in chromaffin cells. The expression of NCKX in chromaffin cells was further confirmed using PCR and northern blotting. In addition to the plasma membrane, the exchanger activity, measured by Na(+)-dependent (45)Ca(2+) uptake, was also present in membrane isolated from the chromaffin granules enriched fraction and the mitochondria enriched fraction. The results support that both NCX and NCKX are present in bovine chromaffin cells and that the regulation of [Ca(2+)](i) is probably more efficient with the participation of NCKX.

Exporting calcium from cells

All eukaryotic cells import Ca2+ through a number of variously gated plasma membrane channels. Once inside cells, Ca2+ transmits information to a large number of (enzyme) targets. Eventually, it must be exported again, to prevent the overloading of the cytosol with Ca2+. Two systems export Ca2+ from cells: a high affinity, low capacity Ca2+-ATPase, and a lower affinity, but much larger capacity, Na+/Ca2+ exchanger. The ATPase (commonly called the Ca2+ pump) is the fine-tuner of cell Ca2+, as it functions well even if the concentration of the ion drops below the microM level. It is a large enzyme, with 10 transmembrane domains and a C-terminal cytosolic tail that contains regulatory sites, including a calmodulin-binding domain. Four distinct gene products plus a large number of splice variants have been described. Some are tissue specific, the isoform 2 being specifically expressed in the sensorial cells of the Corti organ in the inner-ear. Its genetic absence causes deafness in mice. Two different families of the Na+/Ca2+ exchanger exist, one of which, originally described in photoreceptors, transports K+ and Ca2+ in exchange for Na+. The exchanger is particularly active in excitable cells, e.g., heart, where the necessity cyclically arises to rapidly eject large amounts of Ca2+. In addition to heart, the exchanger is particularly important to neurons: the cleavage of the most important neuronal isoform (NCX3) by calpains activated by excitotoxic treatments generates Ca2+ overload and eventually cell death.

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.

The Na+/Ca2+ exchanger is active and working in the reverse mode in human umbilical artery smooth muscle cells

The data presented in this work suggest that in human umbilical artery (HUA) smooth muscle cells, the Na(+)/Ca(2+) exchanger (NCX) is active and working in the reverse mode. This supposition is based on the following results: (i) microfluorimetry in HUA smooth muscle cells in situ showed that a Ca(2+)-free extracellular solution diminished intracellular Ca(2+) ([Ca(2+)](i)), and KB-R7943 (5microM), a specific inhibitor of the Ca(2+) entry mode of the exchanger, also decreased [Ca(2+)](i) (40.6+/-4.5% of Ca(2+)-free effect); (ii) KB-R7943 produced the relaxation of HUA rings (-24.7+/-7.3gF/gW, n=8, p<0.05); (iii) stimulation of the NCX by lowering extracellular Na(+) increases basal [Ca(2+)](i) proportionally to Na(+) reduction (Delta fluorescence ratio=0.593+/-0.141 for Na(+)-free solution, n=8) and HUA rings' contraction (peak force=181.5+/-39.7 for 130mM reduction, n=8), both inhibited by KB-R7943 and a Ca(2+)-free extracellular solution. In conclusion, the NCX represents an important Ca(2+) entry route in HUA smooth muscle cells.

Impaired neuromuscular transmission and skeletal muscle fiber necrosis in mice lacking Na/Ca exchanger 3

We produced and analyzed mice deficient for Na/Ca exchanger 3 (NCX3), a protein that mediates cellular Ca(2+) efflux (forward mode) or Ca(2+) influx (reverse mode) and thus controls intracellular Ca(2+) concentration. NCX3-deficient mice (Ncx3(-/-)) present a skeletal muscle fiber necrosis and a defective neuromuscular transmission, reflecting the absence of NCX3 in the sarcolemma of the muscle fibers and at the neuromuscular junction. The defective neuromuscular transmission is characterized by the presence of electromyographic abnormalities, including low compound muscle action potential amplitude, a decremental response at low-frequency nerve stimulation, an incremental response, and a prominent postexercise facilitation at high-frequency nerve stimulation, as well as neuromuscular blocks. The analysis of quantal transmitter release in Ncx3(-/-) neuromuscular junctions revealed an important facilitation superimposed on the depression of synaptic responses and an elevated delayed release during high-frequency nerve stimulation. It is suggested that Ca(2+) entering nerve terminals is cleared relatively slowly in the absence of NCX3, thereby enhancing residual Ca(2+) and evoked and delayed quantal transmitter release during repetitive nerve stimulation. Our findings indicate that NCX3 plays an important role in vivo in the control of Ca(2+) concentrations in the skeletal muscle fibers and at the neuromuscular junction.

Expression, purification, and characterization of isoform 1 of the plasma membrane Ca2+ pump: focus on calpain sensitivity

The plasma membrane Ca2+ ATPase isoform 1(PMCA1) is ubiquitously distributed in tissues and cells, but only scarce information is available on its properties. The isoform was overexpressed in Sf9 cells, purified on calmodulin columns, and characterized functionally. The level of expression was very low, but sufficient amounts of the protein could be isolated for biochemical characterization. The affinity of PMCA1 for calmodulin was similar to that of PMCA4, the other ubiquitous PMCA isoform. The affinity of PMCA1 for ATP, evaluated by the formation of the phosphorylated intermediate, was higher than that of the PMCA4 pump. The recombinant PMCA1 pump was a much better substrate for the cAMP-dependent protein kinase than the PMCA2 and PMCA4 isoforms. Pulse and chase experiments on Sf9 cells overexpressing the PMCA pumps showed that PMCA1 was much less stable than the PMCA4 and PMCA2 isoforms, i.e. PMCA1 had a much higher sensitivity to degradation by calpain. The effect of calpain was not the result of a general higher susceptibility of the PMCA1 to proteolytic degradation, because the pattern of degradation by trypsin was the same in the three isoforms.

Co-ordinated expression of 5-HT2C receptors with the NCX1 Na+/Ca2+ exchanger in histaminergic neurones

The different roles of Na+/Ca2+ (NCX) exchangers and Na+/Ca2+/K+ (NCKX) exchangers in regulation of the ionic homeostasis in neurones are poorly understood. We have previously shown that serotonin excites histaminergic tuberomamillary (TM) neurones by activation of 5-HT2C-receptors and Na+/Ca2+ exchange. With the help of single-cell RT-PCR (sc-RT-PCR) we have now determined the coexpression pattern of different subtypes of NCX and NCKX with serotonin receptors. The majority of TM neurones express NCX1, NCX2 and NCKX3. Serotonin 2C receptor-mRNA was detected in 70% while 5-HT2A mRNA was found in only 10% of TM neurones. In all neurones expressing the 5-HT2C receptor NCX1-mRNA was present. Double immunostaining revealed the presence of the NCX1 protein in histidine decarboxylase-positive neurones. In the majority of TM neurones one or two out of five isoforms, NCX1.4, NCX1.5, NCX1.7, NCX1.14, NCX1.15, were detected by cDNA sequencing and/or by restriction analysis. The alternative splicing region is important for the Ca2+ sensitivity and presumably for the modulation of NCX1 function by second messengers. We conclude that several exchanger-subtypes can be coexpressed in single neurones and that TM cells are heterogeneous with respect to their calcium homeostasis regulation.

Na(+)/Ca(2+) exchanger in porcine oocytes

The presence of the Na(+)/Ca(2+) exchange mechanism was investigated in porcine oocytes. Immature and in vitro-matured oocytes were loaded with the Ca(2+)-sensitive fluorescent dye fura 2 and changes in the intracellular free Ca(2+) concentration ([Ca(2+)](i)) were monitored after altering the Na(+) concentration gradient across the plasma membrane. Decreasing the extracellular Na(+) concentration induced an increase in [Ca(2+)](i) possibly by a Ca(2+) influx via the Na(+)/Ca(2+) exchanger. A similar Ca(2+) influx could also be triggered after increasing the intracellular Na(+) concentration by incubation in the presence of ouabain (0.4 mM), a Na(+)/K(+)-ATPase inhibitor. The increase in the [Ca(2+)](i) was due to Ca(2+) influx since it was abolished in the absence of extracellular Ca(2+), and the increase was mediated by the Na(+)/Ca(2+) exchanger since it was blocked by the application of amiloride or bepridil, inhibitors of Na(+)/Ca(2+) exchange. Verapamil (50 micro M) and pimozide (50 micro M), inhibitors of L- and T-type voltage-gated Ca(2+) channels, respectively, could not block the Ca(2+) influx. The Ca(2+) entry via the Na(+)/Ca(2+) exchanger could not induce the release of cortical granules and did not stimulate the resumption of meiosis. This was unexpected because Ca(2+) is thought to be a universal trigger for activation. Using antibodies raised against the exchanger, it was demonstrated that the Na(+)/Ca(2+) exchanger was localized predominantly in the plasma membrane. Reverse transcription-polymerase chain reaction revealed that porcine oocytes contain a transcript that shows 98.1% homology to the NACA3 isoform of the porcine Na(+)/Ca(2+) exchanger.

Distinct effects of different calcium-mobilizing agents on cell death in NG108-15 neuroblastoma X glioma cells

The effects of different calcium-mobilizing agents on cell death were characterized in NG108-15 neuroblastoma x glioma hybrid cells. Carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) increased the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and caused cell death. Thapsigargin (TG) not only increased the [Ca(2+)](i) and caused cell death but also induced neurite outgrowth via activation of phospholipase A(2) and cytochrome P450 epoxygenase. In contrast, bradykinin increased the [Ca(2+)](i), but had no effect on cell morphology or cell death. Cell death occurred by two different mechanisms, one of which was caspase-3-dependent and the other caspase-3-independent. Caspase-3 activation was Ca(2+)-dependent, whereas neurite outgrowth was Ca(2+)-independent. TG- or FCCP-induced caspase-3 activation occurred at the same time, but the cell death induced by TG was delayed. TG treatment did not enhance the generation of nitric oxide or cAMP or secretion of glial-derived neurotrophic factor or neurotrophin-3, but activated sphingosine kinase. Furthermore, inhibition of sphingosine kinase accelerated TG-induced cell death, and exogenous sphingosine 1-phosphate (S1P) protected cells from FCCP-induced cell death by about 60%. These results indicate that, in these cells, depletion of intracellular nonmitochondrial or mitochondrial Ca(2+) stores causes cell death, that TG activates phospholipase A(2) and sphingosine kinase, and that arachidonic acid induces neurite outgrowth, whereas S1P delays cell death.

Na(+)/Ca(2+) exchange and its role in intracellular Ca(2+) regulation in guinea pig detrusor smooth muscle

The role of Na(+)/Ca(2+) exchange in regulating intracellular Ca(2+) concentration ([Ca(2+)](i)) in isolated smooth muscle cells from the guinea pig urinary bladder was investigated. Incremental reduction of extracellular Na(+) concentration resulted in a graded rise of [Ca(2+)](i); 50-100 microM strophanthidin also increased [Ca(2+)](i). A small outward current accompanied the rise of [Ca(2+)](i) in low-Na(+) solutions (17.1 +/- 1.8 pA in 29.4 mM Na(+)). The quantity of Ca(2+) influx through the exchanger was estimated from the charge carried by the outward current and was approximately 30 times that which is necessary to account for the rise of [Ca(2+)](i), after correction was made for intracellular Ca(2+) buffering. Ca(2+) influx through the exchanger was able to load intracellular Ca(2+) stores. It is concluded that the level of resting [Ca(2+)](i) is not determined by the exchanger, and under resting conditions (membrane potential -50 to -60 mV), there is little net flux through the exchanger. However, a small rise of intracellular Na(+) concentration would be sufficient to generate significant net Ca(2+) influx.

Na(+)-Ca(2+) exchanger isoforms in rat neuronal preparations: different changes in their expression during postnatal development

We examined the relative amounts of Na(+)-Ca(2+) exchanger (NCX) isoform mRNAs in cultured neurons, astrocytes and developmental rat brain. NCX1 transcript was predominant in neurons and astrocytes, but NCX2 transcript was about four-fold higher than NCX1 or NCX3 transcript in adult rat cortex. NCX2 transcript in the cortex increased markedly during postnatal development, whereas NCX1 and NCX3 transcripts decreased. Na(+)-dependent 45Ca(2+) uptake in the cortical homogenate increased significantly during postnatal development.

Calcineurin controls the transcription of Na+/Ca2+ exchanger isoforms in developing cerebellar neurons

The Na(+)/Ca(2+) exchanger (NCX) and the plasma membrane Ca(2+)-ATPase export Ca(2+) from the cytosol to the extracellular space. Three NCX genes (NCX1, NCX2, and NCX3), encoding proteins with very similar properties, are expressed at different levels in tissues. Essentially, no information is available on the mechanisms that regulate their expression. Specific antibodies have been prepared and used to explore the expression of NCX1 and NCX2 in rat cerebellum. The expression of NCX2 became strongly up-regulated during development, whereas comparatively minor effects were seen for NCX1. This was also observed in cultured granule cells induced to mature in physiological concentrations of potassium. By contrast, higher K(+) concentrations, which induce partial depolarization of the plasma membrane and promote the influx of Ca(2+), caused the complete disappearance of NCX2. Reverse transcription-polymerase chain reaction analysis showed that the process occurred at the transcriptional level and depended on the activation of the Ca(2+) calmodulin-dependent protein phosphatase, calcineurin. The NCX1 and NCX3 genes were also affected by the depolarizing treatment: the transcription of the latter became up-regulated, and the pattern of expression of the splice variants of the former changed. The effects on the NCX1 and NCX3 genes were calcineurin-independent.

The N-terminal portion of the main cytosolic loop mediates K+ sensitivity in the retinal rod Na+/Ca2+-K+-exchanger

Two types of Na+/Ca2+-exchangers have been characterized in the literature: The first is the cardiac, skeletal muscle and brain type, which exchanges 1 Ca2+ for 3 Na+, the second, found in retinal photosensor cells, transports 1 Ca2+ and 1 K+ in exchange for 4 Na+. The present work describes the properties of chimeric constructs of the two exchanger types. Ca2+ gel overlay experiments have identified a high affinity (Kd in the 1 microM range) Ca2+-binding domain between Glu601 and Asp733 in the main cytosolic loop of the retinal protein, just after transmembrane domain 5. Insertion of the retinal Ca2+-binding domain in the cytosolic loop of the cardiac exchanger conferred K+-dependence to the Ca2+ uptake activity of the chimeric constructs expressed in HeLa cells. The apparent Km of the K+ effect was about 1 mM. Experiments with C-terminally truncated versions of the retinal insert indicated that the sequence between Leu643 and Asp733 was critical in mediating K+ sensitivity of the recombinant chimeras. Thus, the high affinity Ca2+-binding domain in the main cytosolic loop of the retinal exchanger may regulate the activity of the retinal protein by binding Ca2+, and by conferring to it K+ sensitivity.

The retinal rod Na(+)/Ca(2+),K(+) exchanger contains a noncleaved signal sequence required for translocation of the N terminus

The retinal rod Na(+)/Ca(2+),K(+) exchanger (RodX) is a polytopic membrane protein found in photoreceptor outer segments where it is the principal extruder of Ca(2+) ions during light adaptation. We have examined the role of the N-terminal 65 amino acids in targeting, translocation, and integration of the RodX using an in vitro translation/translocation system. cDNAs encoding human RodX and bovine RodX through the first transmembrane domain were correctly targeted and integrated into microsomal membranes; deletion of the N-terminal 65 amino acids (aa) resulted in a translation product that was not targeted or integrated. Deletion of the first 65 aa had no effect on membrane targeting of full-length RodX, but the N-terminal hydrophilic domain no longer translocated. Chimeric constructs encoding the first 65 aa of bovine RodX fused to globin were translocated across microsomal membranes, demonstrating that the sequence could function heterologously. Studies of fresh bovine retinal extracts demonstrated that the first 65 aa are present in the native protein. These data demonstrate that the first 65 aa of RodX constitute an uncleaved signal sequence required for the efficient membrane targeting and proper membrane integration of RodX.

Reduced myocardial sarcoplasmic reticulum Ca(2+)-ATPase protein expression in compensated primary and secondary human cardiac hypertrophy

Pathological intracellular calcium handling has been proposed to underlie the alterations of contractile behavior in hypertrophied myocardium. However, the myocardial protein expression of intracellular calcium transport proteins in compensated human left ventricular hypertrophy has not yet been studied. We investigated septal myocardial specimens of patients suffering from hypertrophic obstructive cardiomyopathy (n=14) or from acquired aortic valve stenosis (n=11) undergoing myectomy or aortic valve replacement, respectively. For comparison, we studied non-hypertrophied myocardium of six non-failing hearts which could not be transplanted for technical reasons. The myocardial density of the calcium release channel of the sarcoplasmic reticulum (SR) was determined by(3)H-ryanodine binding. Myocardial contents of SR Ca(2+)-ATPase, phospholamban, calsequestrin and Na(+)/Ca(2+)-exchanger were analysed by Western blot analysis. The myocardial SR calcium release channel density was not significantly different in hypertrophied and non-failing human myocardium. In both hypertrophic obstructive cardiomyopathy and in aortic valve stenosis, SR Ca(2+)-ATPase expression was reduced by about 30% compared to non-failing myocardium (P<0.05), whereas the expression of phospholamban, calsequestrin, and the Na(+)/Ca(2+)-exchanger was unchanged. The decrease of SR Ca(2+)-ATPase expression was still observable when related to its regulatory protein phospholamban or to the myosin content of the homogenates (P<0.05). Furthermore, the SR Ca(2+)-ATPase expression was inversely correlated to the septum thickness assessed by echocardiography, but not to age, cardiac index or outflow tract gradient. In primary as well as in secondary hypertrophied human myocardium, the expression of SR Ca(2+)-ATPase is reduced and inversely related to the degree of the hypertrophy. The diminished SR Ca(2+)-ATPase expression might result in reduced Ca(2+)reuptake into the SR and might contribute to altered contractile behavior in hypertrophied human myocardium.

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.

Molecular study of the Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

To identify the Na+/Ca2+ exchanger expressed in bovine chromaffin cells, the ncx gene was cloned from a bovine chromaffin cell cDNA library. Five partial clones were obtained and their nucleotide sequences showed that there were at least three isoforms containing different intracellular loops. The 3'-untranslated region was the same in all the clones. To examine the Na+/Ca2+ exchange activity of the clones, full-length ncx1 genes were constructed by replacing the corresponding region of bovine cardiac ncx1 clone p17 with the different regions from two bovine chromaffin cell clones; these were designated p17c and p17h. p17h, but not p17c, showed Na+/Ca2+ exchange activity when expressed in Chinese hamster ovary cells and Xenopus oocytes. The expressed exchange activity of p17 was inhibited by 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cAMP) but was not affected by PMA. However, the activity of p17h was inhibited by PMA but enhanced by 8-Br-cAMP. The agents that changed the activity of protein kinase C and cAMP-dependent protein kinase modulated the endogenous Na+/Ca2+ exchange current of chromaffin cells in a manner similar to that of p17h. Our results suggest that the p17h clone is the major isoform of the exchanger in chromaffin cells and is similar to the major ncx1 isoform in kidney. The exchange activity could be regulated by phosphorylation, and the variable region in the intracellular loop is important for the different effects of phosphorylation on the different isoforms.

Intracellular calcium clearance in Purkinje cell somata from rat cerebellar slices

1. The mechanisms governing the return of intracellular calcium (Cai2+) to baseline levels following depolarization-evoked [Ca2+]i rises were investigated in Purkinje cell somata using tight-seal whole-cell recordings and fura-2 microfluorometry, for peak [Ca2+]i ranging from 50 nm to 2 microM. 2. Cai2+ decay was well fitted by a double exponential with time constants of O.6 and 3 s. Both time constants were independent of peak [Ca2+]i but the contribution of the faster component increased with [Ca2+]i. 3. Thapsigargin (10 microM) and cyclopiazonic acid (50 microM) prolonged Cai2+ decay indicating that sarco-endoplasmic reticulum Ca2+ (SERCA) pumps contribute to Purkinje cell Cai2+ clearance. 4. A modest participation in clearance was found for the plasma membrane Ca2+ (PMCA) pumps using 5,6-succinimidyl carboxyeosin (40 microM). 5. The Na(+)-Ca2+ exchanger also contributed to the clearance process, since replacement of extracellular Na+ by Li+ slowed Cai2+ decay. 6. Carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 microM) and rotenone (10 microM) increased [Ca2+]i and elicited large inward currents at -60 mV. Both effects were also obtained with CCCP in the absence of external Ca2+, suggesting that mitochondrial Ca2+ uptake uncouplers release Ca2+ from intracellular stores and may alter the membrane permeability to Ca2+. These effects were irreversible and impeded tests on the role of mitochondria in Cai2+ clearance. 7. The relative contribution of the clearance systems characterized in this study varied as a function of [Ca2+]i. At 0.5 microM Cai2+, SERCA pumps and the Na(+)-Ca2+ exchanger contribute equally to removal and account for 78% of the process. Only 45% of the removal at 2 microM Cai2+ can be explained by these systems. In this high [Ca2+]i range the major contribution is that of SERCA pumps (21%) and of the Na(+)-Ca2+ exchanger (18%), whereas the contribution of PMCA pumps is only 6%.

The sodium-calcium ion membrane exchanger: physiologic significance and pharmacologic implications

The Na(+)-Ca2+ exchanger is a non-ATP-dependent protein that, under steady-state conditions, extrudes Ca2+ from the interior of the cell into the extracellular space via facilitated transport. The activity of the exchanger seems to be reduced in myocardial ischemia, leading to increased intracellular Ca2+ in the ischemic heart, which can result in arrhythmia, myocardial stunning, and necrosis. In contrast, congestive heart failure and myocardial hypertrophy are associated with increased exchanger activity and a decreased inotropic state. Pharmacologic agents are being developed to modulate sodium ion levels in the cell, which could enhance or reduce sodium-calcium exchange as needed in various pathophysiologic states. At this time there are no available drugs that act specifically on the Na(+)-Ca2+ exchanger itself. The exchanger has been cloned, and inhibitory peptides of the exchanger may soon be available for possible use in treatment of congestive heart failure.

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.

Interplay between sodium and calcium dynamics in granule cell presynaptic terminals

Fluorescent indicators were used to detect stimulus-evoked changes in presynaptic levels of intracellular sodium (Na(i)) and calcium (Ca(i)) in granule cell parallel fibers in brain slices from rat cerebellum. Ca(i) increased during stimulation, and three exponentials were needed to approximate its return to prestimulus levels. Ca(i) decayed to approximately 10% of peak levels with tau approximately 100 ms, to approximately 1% of peak values with tau approximately 6 s, and then returned to prestimulus levels with tau approximately 1-2 min. After stimulation, Na(i) accumulated in two phases; one rapid, the other continuing for several hundred milliseconds. The return of Na(i) to prestimulus levels was well approximated by a double exponential decay with time constants of 6-17 s and 2-3 min. Manipulations that prevented calcium entry eliminated both the slow component of sodium entry and the rapid component of Na(i) decay. Reductions of extracellular sodium slowed the rapid phase of Ca(i) decay. These Ca(i) and Na(i) transients were well described by a model in which the plasma membrane of presynaptic boutons contained both a sodium/calcium exchanger and a calcium ATPase (Ca-ATPase). According to this model, immediately after stimulation the sodium/calcium exchanger removes calcium from the terminal more rapidly than does the Ca-ATPase. Eventually, the large concomitant sodium influx brings the exchanger into steady-state, leaving only the Ca-ATPase to remove calcium. This perturbs the equilibrium of the sodium/calcium exchanger, which opposes the Ca-ATPase, leading to a slow return of Ca(i) and Na(i) to resting levels.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Molecular dissection of Ca2+ efflux in immortalized proximal tubule cells

Plasma membrane Ca(2+)-ATPase (PMCA) and the Na+/Ca2+ exchanger participate in regulating cell function by maintaining proper intracellular Ca2+ concentrations ([Ca2+]i). In renal epithelial cells these proteins have been additionally implicated in cellular calcium absorption. The purpose of the present studies was to determine the Ca2+ extrusion mechanisms in cells derived from the proximal tubule. Homology-based RT-PCR was used to amplify PMCA transcripts from RNA isolated from mouse cell lines originating from the S1, S2, and S3 proximal tubule segments. S1, S2, and S3 cells exhibited only PMCA1 and PMCA4 products. PCR product identity was confirmed by sequence analysis. Northern analysis of proximal tubule cell RNAs revealed appropriate transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4, but were negative for PMCA2 and PMCA3. Western analysis with a monoclonal antibody to PMCA showed that all proximal cell lines expressed a reacting plasma membrane protein of 140 kD, the reported PMCA molecular mas. Na+/Ca2+ exchanger (NCX1) mRNA expression, analyzed by RT-PCR, protein expression by Western analysis, and functional exchange activity were uniformly absent from all proximal tubule cell lines. These observations support the idea that immortalized cells derived from the proximal tubule express PMCA1 and PMCA4, which may serve as the primary mechanism of cellular Ca2+ efflux.

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.

Alteration of loosely bound calcium in the guinea pig organ of Corti after treatment with diltiazem as calcium channel blocker

After oral administration of the organic calcium channel blocker diltiazem to guinea pigs for 7 days, calcium ions were precipitated with potassium antimonate in the cochleae. The spatial distribution of the precipitates was studied by energy-filtering transmission electron microscopy and the amount of the ultrastructural reaction products formed was determined semiquantitatively by an image processing system. Compared with untreated control ears, the number of the formed precipitates was reduced drastically in the inner hair cells after diltiazem treatment. In addition, electron microscopic analysis revealed that the number of calcium precipitates attached at the basolateral membrane of the outer hair cells was clearly reduced when compared with untreated control specimens. A large number of histochemical reaction products could be identified in the basilar membrane and were also observed in the untreated control specimens. The spatial distribution of the calcium precipitates in the lamina reticularis was not affected by diltiazem treatment and calcium precipitates could be identified within different cell membranes. The techniques used was considered to be helpful for identifying calcium channels ultrastructurally in intact undissected tissues and to support light microscopic analyses and patch-clamp electrophysiological measurements.

Na(+)-Ca2+ exchange in rat dorsal root ganglion neurons

The role of the Na(+)-Ca2+ exchanger was examined in isolated rat dorsal root ganglion (DRG) neurons. Neurons were dialyzed with the Ca2+ indicator Indo-1. Ca2+ transients were elicited by depolarizing the cells from -80 to 0 mV for 100 ms under voltage clamp conditions. In most cells (45 of 67), the decay of intracellular Ca2+ concentration ([Ca2+]i) could be fitted with a single exponential with a time constant of 2.43 s. In the remaining 22 cells, the decay of [Ca2+]i could be described with a double exponential with time constants of 0.76 and 11.84 s. In cells that displayed a biphasic [Ca2+]i relaxation, Na(+)-free medium caused resting [Ca2+]i to increase from 116 to 186 nM; the slow component of recovery to basal [Ca2+]i was nearly abolished in Na(+)-free medium or by application of 5 mM Ni2+. In 35 of 45 cells displaying a monophasic [Ca2+]i decay, omitting external Na+ increased the time constant of [Ca2+]i decay from 2.02 to 3.63 s. In the remaining 10 cells, Na(+)-free solution did not affect Ca2+ handling. The time constant of [Ca2+]i relaxation was voltage dependent. These findings demonstrate the important role of the Na(+)-Ca2+ exchanger in DRG neurons. Its presence was further confirmed both at the mRNA and the protein level.

Functional expression of Na-Ca exchanger clones measured with the fluorescent Ca(2+)-indicating dye fluo-3

The process of Ca2+ homeostasis is of prime importance to all cells because of the ubiquitous role of cytoplasmic Ca2+ as an intracellular messenger and the cytotoxicity of sustained elevated cytosolic Ca2+ concentrations. Two classes of plasma membrane proteins are responsible for maintaining cytosolic free Ca2+ in the submicromolar range against a very large electrochemical Ca2+ gradient across the plasma membrane, the ATP-driven Ca2+ pump and Na-Ca exchangers. Two types of Na-Ca exchangers are known, the 3Na:1Ca exchangers found in heart, brain, kidney, and most other tissues and the 4Na:1Ca+ 1K exchanger found in retinal rod and cone photoreceptors. Functional expression of Na-Ca(/K) exchangers is most often measured as 45Ca uptake in Na(+)-loaded cells or as Na-Ca exchange currents with the giant excised patch technique. In this study, two functional assays used to detect expression of the bovine heart Na-Ca exchanger in CHO cells are described. Both assays are based on measurements of cytosolic free Ca2+ with the fluorescent Ca(2+)-indicating dye fluo-3 and should be equally applicable in the study of functional expression of both Na-Ca and Na-Ca/K exchanger clones.

Involvement of Na+-Ca2+ exchanger in reperfusion-induced delayed cell death of cultured rat astrocytes

In some cells, Ca2+ depletion induces an increase in intracellular Ca2+ ([Ca2+]i) after reperfusion with Ca2+-containing solution, but the mechanism for the reperfusion injury is not fully elucidated. Using an antisense strategy we studied the role of the Na+-Ca2+ exchanger in reperfusion injury in cultured rat astrocytes. When astrocytes were perfused in Ca2+-free medium for 15-60 min, a persistent increase in [Ca2+]i was observed immediately after reperfusion with Ca2+-containing medium, and the number of surviving cells decreased 3-5 days later. The increase in [Ca2+]i was enhanced by low extracellular Na+ ([Na+]0) during reperfusion and blocked by the inhibitors of the Na+-Ca2+ exchanger amiloride and 3, 4-dichlorobenzamil, but not by the Ca2+ channel antagonists nifedipine, Ca2+ and Ni2+. Treatment of astrocytes with antisense, but not sense, oligodeoxynucleotide to the Na+-Ca2+ exchanger decreased Na+-Ca2+ exchanger protein level and exchange activity. The antisense oligomer attenuated reperfusion-induced increase in [Ca2+]i and cell toxicity. The Na+-Ca2+ exchange inhibitors 3, 4-dichlorobenzamil and ascorbic acid protected astrocytes from reperfusion injury partially, while the stimulators sodium nitroprusside and 8-bromo-cyclic GMP and low [Na+]0 exacerbated the injury. Pretreatment of astrocytes with ouabain and monensin caused similar delayed glial cell death. These findings suggest that Ca2+ entry via the Na+-Ca2+ exchanger plays an important role in reperfusion-induced delayed glial cell death.

Calcineurin inhibits VCX1-dependent H+/Ca2+ exchange and induces Ca2+ ATPases in Saccharomyces cerevisiae

The PMC1 gene in Saccharomyces cerevisiae encodes a vacuolar Ca2+ ATPase required for growth in high-Ca2+ conditions. Previous work showed that Ca2+ tolerance can be restored to pmc1 mutants by inactivation of calcineurin, a Ca2+/calmodulin-dependent protein phosphatase sensitive to the immunosuppressive drug FK506. We now report that calcineurin decreases Ca2+ tolerance of pmc1 mutants by inhibiting the function of VCX1, which encodes a vacuolar H+/Ca2+ exchanger related to vertebrate Na+/Ca2+ exchangers. The contribution of VCX1 in Ca2+ tolerance is low in strains with a functional calcineurin and is high in strains which lack calcineurin activity. In contrast, the contribution of PMC1 to Ca2+ tolerance is augmented by calcineurin activation. Consistent with these positive and negative roles of calcineurin, expression of a vcx1::lacZ reporter was slightly diminished and a pmc1::lacZ reporter was induced up to 500-fold by processes dependent on calcineurin, calmodulin, and Ca2+. It is likely that calcineurin inhibits VCX1 function mainly by posttranslational mechanisms. Activities of VCX1 and PMC1 help to control cytosolic free Ca2+ concentrations because their function can decrease pmc1::lacZ induction by calcineurin. Additional studies with reporter genes and mutants indicate that PMR1 and PMR2A, encoding P-type ion pumps required for Mn2+ and Na+ tolerance, may also be induced physiologically in response to high-Mn2+ and -Na+ conditions through calcineurin-dependent mechanisms. In these situations, inhibition of VCX1 function may be important for the production of Ca2+ signals. We propose that elevated cytosolic free Ca2+ concentrations, calmodulin, and calcineurin regulate at least four ion transporters in S. cerevisiae in response to several environmental conditions.

Expression of a functionally active human renal sodium-calcium exchanger lacking a signal sequence

The Na+-Ca2+ exchanger is an unusual membrane transport protein as it contains an NH2-terminal signal sequence which is co-translationally removed in the endoplasmic reticulum during synthesis. To determine if the signal sequence was essential for biosynthesis, mutations were introduced in the NH2 terminus of the cDNA coding for the human renal Na+-Ca2+ exchanger in order to alter processing of the protein. To prevent cleavage of the signal sequence during biosynthesis, the last residue of the consensus signal sequence, Ala-1, was changed to Phe. Deletion mutants were also constructed to encode for exchangers which lacked the signal sequence, the signal sequence and the first extracellular loop, or all of the NH2 terminus including the first transmembrane segment of the mature protein. These mutants were expressed in HEK 293 cells and assayed for Na+-Ca2+ exchange activity. Mutants lacking either a signal sequence or containing a noncleavable signal sequence were still targeted to the plasma membrane, where they exhibited Na+-Ca2+ exchange activity. By contrast, the mutants which had more than the signal sequence deleted did not demonstrate any exchange activity. These mutants were, however, still integrated into the membrane and were resistant to alkali extraction. These results show that the signal sequence is not essential for biogenesis of the Na+-Ca2+ exchanger and suggests that the molecule contains one or more internal signal sequences for insertion into the membrane during biosynthesis.

Evidence for a Na+/Ca2+ exchanger in neuroblastoma x glioma hybrid NG108-15 cells

To determine whether NG108-15 cells contain a functional Na+/Ca2+ exchanger, we isotonically replaced extracellular Na+ with N-methyl-D-glucamine (NMG) and measured the effect on cytosolic Ca2+ concentration ([Ca2+]i) using the fluorescent Ca2+ indicator fura 2. Replacement with NMG alone had no effect on basal [Ca2+]i or the rise in [Ca2+]i evoked by 80 mM K+ or 10 microM bradykinin, but caused a larger [Ca2+]i increase when thapsigargin and carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) were added to the cells; this enhanced [Ca2+]i increase could be reversed by adding Na+ back to the bathing buffer. The elevation in [Ca2+]i induced by thapsigargin and FCCP was inversely proportional to extracellular Na+ concentration. Furthermore, the exchanger operated in the reverse mode, as measured by either [Ca2+]i change or 45Ca2+ uptake. An 810 bp cDNA fragment of the exchanger was amplified by PCR; it differed by a single amino acid residue from the corresponding segment of the rat brain Na+/Ca2+ exchanger. These data suggest that a functioning Na+/Ca2+ exchanger exists in NG108-15 cells.

Positively charged cyclic hexapeptides, novel blockers for the cardiac sarcolemma Na(+)-Ca2+ exchanger

Positively charged cyclic hexapeptides have been synthesized and tested for their effects on the cardiac sarcolemma Na(+)-Ca2+ exchange activities with a goal to identify a potent blocker. The cyclic hexapeptides, having the different amino acid sequence, contain two arginines (to retain a positive charge), two phenylalanines (to control hydrophobicity), and two cysteines (to form an intramolecular S-S bond). The effect of cyclic hexapeptides were tested on Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange, by measuring the 45Ca fluxes in the semi-rapid mixer or monitoring the calcium-sensitive dye Arsenazo III and voltage-sensitive dyes (Oxanol-V or Merocyanine-540). Seven cyclic hexapeptides inhibit Na(+)-Ca2+ exchange with a different potency (IC50 = 2-300 microM). Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) inhibits the Na+i-dependent 45Ca uptake (Na(+)-Ca2+ exchange) and Ca2+i-dependent 45Ca uptake (Ca(2+)-Ca2+ exchange) in the isolated cardiac sarcolemma vesicles with IC50 = 10 +/- 2 microM and IC50 = 7 +/- 3 microM, respectively. Interaction of FRCRCFa with a putative inhibitory site does not involve a "slow" binding (a maximal inhibitory effect is already observed after t = 1 s of mixing). The inside positive potential, generated by Na+o-dependent Ca2+ efflux, was monitored by Oxanol-V (A635-A612) or Merocyanine-540 (A570-A500). In both assay systems, FRCRCFa inhibits the Na(+)-Ca2+ exchange with IC50 = 2-3 microM, while a complete inhibition occurs at 20 microM FRCRCFa. The forward (Na+i-dependent Ca2+ influx) and reverse (Na+o-dependent Ca2+ efflux) modes of Na(+)-Ca2+ exchange, monitored by Arsenazo III (A600-A785), are also inhibited by FRCRCFa. The L-Arg4-->D-Arg4 substitution in FRCRCFa does not alter the IC50, meaning that this structural change may increase a proteolytic resistance without a loss of inhibitory potency. At fixed [Na+]i (160 mM) or [Ca2+]i (250 microM) and varying 45Cao (2-200 microM), FRCRCFa decreases Vmax without altering the Km. Therefore, FRCRCFa is a noncompetitive inhibitor in regard to extravesicular Ca2+ either for Na(+)-Ca2+ or Ca(2+)-Ca2+ exchange. It is suggested that FRCRCFa prevents the ion movements through the exchanger rather than the ion binding.

Regulation of the cardiac Na(+)-Ca2+ exchanger by Ca2+. Mutational analysis of the Ca(2+)-binding domain

The sarcolemmal Na(+)-Ca2+ exchanger is regulated by intracellular Ca2+ at a high affinity Ca2+ binding site separate from the Ca2+ transport site. Previous data have suggested that the Ca2+ regulatory site is located on the large intracellular loop of the Na(+)-Ca2+ exchange protein, and we have identified a high-affinity 45Ca2+ binding domain on this loop (Levitsky, D. O., D. A. Nicoll, and K. D. Philipson. 1994. Journal of Biological Chemistry. 269:22847-22852). We now use electrophysiological and mutational analyses to further define the Ca2+ regulatory site. Wild-type and mutant exchangers were expressed in Xenopus oocytes, and the exchange current was measured using the inside-out giant membrane patch technique. Ca2+ regulation was measured as the stimulation of reverse Na(+)-Ca2+ exchange (intracellular Na+ exchanging for extracellular Ca2+) by intracellular Ca2+. Single-site mutations within two acidic clusters of the Ca2+ binding domain lowered the apparent Ca2+ affinity at the regulatory site from 0.4 to 1.1-1.8 microM. Mutations had parallel effects on the affinity of the exchanger loop for 45Ca2+ binding (Levitsky et al., 1994) and for functional Ca2+ regulation. We conclude that we have identified the functionally important Ca2+ binding domain. All mutant exchangers with decreased apparent affinities at the regulatory Ca2+ binding site also have a complex pattern of altered kinetic properties. The outward current of the wild-type Na(+)-Ca2+ exchanger declines with a half time (th) of 10.8 +/- 3.2 s upon Ca2+ removal, whereas the exchange currents of several mutants decline with th values of 0.7-4.3 s. Likewise, Ca2+ regulation mutants respond more rapidly to Ca2+ application. Study of Ca2+ regulation has previously been possible only with the exchanger operating in the reverse mode as the regulatory Ca2+ and the transported Ca2+ are then on opposite sides of the membrane. The use of exchange mutants with low affinity for Ca2+ at regulatory sites also allows demonstration of secondary Ca2+ regulation with the exchanger in the forward or Ca2+ efflux mode. In addition, we find that the affinity of wild-type and mutant Na(+)-Ca2+ exchangers for intracellular Na+ decreases at low regulatory Ca2+. This suggests that Ca2+ regulation modifies transport properties and does not only control the fraction of exchangers in an active state.

Cultured rat astrocytes possess Na(+)-Ca2+ exchanger

Na(+)-Ca2+ exchange activity in its reverse mode was demonstrated in cultured rat astrocytes. Combination of ouabain (1 mM) and monensin (20 microM) caused a marked increase in 45Ca2+ uptake in astrocytes. 45Ca2+ uptake was also stimulated by lowering the external Na+ concentration. Ouabain plus monensin-stimulated 45Ca2+ uptake was blocked by 3,4-dichlorobenzamil (IC50, 16 microM), an inhibitor of Na(+)-Ca2+ exchanger, but not by nifedipine (0.1 microM). The stimulated-45Ca2+ uptake was observed even in K(+)-free medium, and external K+ at 5-10 mM caused a 2.2-fold increase in the uptake. Microspectrofluorimetry using the Ca(2+)-sensitive dye fura-2 showed that ouabain plus monensin increased intracellular Ca2+ concentration in single astrocytes. The Ca2+ signal was dependent on external Ca2+ (EC50, 1.4 mM), and blocked by 20 microM 3,4-dichlorobenzamil, but not by Ca2+ channel blockers (Cd2+, 20 microM; Ni2+, 100 microM). Antiserum of cardiac Na(+)-Ca2+ exchanger recognized 160 and 120-135 kDa proteins on SDS-polyacrylamide gel electrophoresis of astrocyte homogenate. Northern blot analysis revealed the presence of mRNA for the exchanger protein in astrocytes. These findings indicate that Na(+)-Ca2+ exchanger which is modulated by K+ is present in cultured rat astrocytes.

Asparagine-linked oligosaccharides are localized to single extracytosolic segments in multi-span membrane glycoproteins

A comprehensive survey of mammalian multi-span (polytopic) membrane proteins showed that asparagine(N)-linked oligosaccharides are localized to single extracytosolic segments. In most membrane proteins this is because potential consensus sites for N-glycosylation (Asn-Xaa-Ser/Thr, X not equal to Pro) are not found in multiple extracytosolic segments. In functional proteins where consensus N-glycosylation sites are contained within more than one extracytosolic segment, only the first segment contains N-linked carbohydrate. An exception is the alpha-subunit of the Na+ channel, which consists of a duplicated structure containing two glycosylated segments. The average size of established N-glycosylated loops connecting two transmembrane segments is 62 residues, with the smallest glycosylated loop being 33 residues in size. N-glycosylated sites are more highly conserved than non-glycosylated (primarily cytosolic) sites and are more common toward the N-terminus of the membrane domain of multi-span membrane proteins. The optimal conditions for glycosylation of consensus sites within an extracytosolic domain of a multi-span membrane protein are (i) the acceptor site is well-spaced (greater than 10 residues) from the transmembrane domain, (ii) the loop is greater than 30 residues in size and (iii) the segment is the first in the protein to contain a suitable extracytosolic consensus site. The localization of N-linked oligosaccharide chains to a single protein segment suggests either glycosylation of multiple loops may compromise protein folding or function, or only a single polypeptide domain can be optimally glycosylated during biosynthesis in vivo.

Mapping of the cardiac sodium-calcium exchanger with monoclonal antibodies

We used a panel of monoclonal antibodies raised against the canine cardiac Na(+)-Ca2+ exchanger expressed in Sf9 insect cells to analyze the immunoreactive domains and the topological organization of this membrane protein. Antibodies, which reacted strongly on Western blots of the recombinant protein, were used to screen an expression sublibrary composed of exchanger cDNA fragments. Positive clones thus indicated the expression of antibody binding sites. Linear epitopes, 16-155 amino acids in length, could be identified for four antibodies. One antibody recognized two neighboring, but nonoverlapping, sequences. All epitopes were localized to the large hydrophilic region of the exchanger connecting the putative transmembrane segments 5 and 6. The immunodominant region of the protein is a highly charged domain in the carboxy-terminal half of the hydrophilic region. Binding studies with the 3H-labeled high-affinity antibody R3F1 establish that the immunodominant region is located on the intracellular surface of the membrane. The same antibody was used to directly determine the membrane concentration of the exchanger in different cell types. Newborn rat heart cells contain approximately 6 x 10(5) exchanger molecules per cell. Exchanger densities in different cells seem to correlate with the Na(+)-dependent Ca2+ transport activity in the corresponding membrane vesicles.