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

Functional asymmetry of bidirectional Ca2+-movements in an archaeal sodium-calcium exchanger (NCX_Mj)

Dynamic features of Ca(2+) interactions with transport and regulatory sites control the Ca(2+)-fluxes in mammalian Na(+)/Ca(2+)(NCX) exchangers bearing the Ca(2+)-binding regulatory domains on the cytosolic 5L6 loop. The crystal structure of Methanococcus jannaschii NCX (NCX_Mj) may serve as a template for studying ion-transport mechanisms since NCX_Mj does not contain the regulatory domains. The turnover rate of Na(+)/Ca(2+) exchange (kcat=0.5±0.2 s(-1)) in WT-NCX_Mj is 10(3)-10(4) times slower than in mammalian NCX. In NCX_Mj, the intrinsic equilibrium (Kint) for bidirectional Ca(2+) movements (defined as the ratio between the cytosolic and extracellular Km of Ca(2+)/Ca(2+) exchange) is asymmetric, Kint=0.15±0.5. Therefore, the Ca(2+) movement from the cytosol to the extracellular side is ∼7-times faster than in the opposite direction, thereby representing a stabilization of outward-facing (extracellular) access. This intrinsic asymmetry accounts for observed differences in the cytosolic and extracellulr Km values having a physiological relevance. Bidirectional Ca(2+) movements are also asymmetric in mammalian NCX. Thus, the stabilization of the outward-facing access along the transport cycle is a common feature among NCX orthologs despite huge differences in the ion-transport kinetics. Elongation of the cytosolic 5L6 loop in NCX_Mj by 8 or 14 residues accelerates the ion transport rates (kcat) ∼10 fold, while increasing the Kint values 100-250-fold (Kint=15-35). Therefore, 5L6 controls both the intrinsic equilibrium and rates of bidirectional Ca(2+) movements in NCX proteins. Some additional structural elements may shape the kinetic variances among phylogenetically distant NCX variants, although the intrinsic asymmetry (Kint) of bidirectional Ca(2+) movements seems to be comparable among evolutionary diverged NCX variants.

Sodium-calcium exchangers (NCX): molecular hallmarks underlying the tissue-specific and systemic functions

NCX proteins explore the electrochemical gradient of Na(+) to mediate Ca(2+)-fluxes in exchange with Na(+) either in the Ca(2+)-efflux (forward) or Ca(2+)-influx (reverse) mode, whereas the directionality depends on ionic concentrations and membrane potential. Mammalian NCX variants (NCX1-3) and their splice variants are expressed in a tissue-specific manner to modulate the heartbeat rate and contractile force, the brain's long-term potentiation and learning, blood pressure, renal Ca(2+) reabsorption, the immune response, neurotransmitter and insulin secretion, apoptosis and proliferation, mitochondrial bioenergetics, etc. Although the forward mode of NCX represents a major physiological module, a transient reversal of NCX may contribute to EC-coupling, vascular constriction, and synaptic transmission. Notably, the reverse mode of NCX becomes predominant in pathological settings. Since the expression levels of NCX variants are disease-related, the selective pharmacological targeting of tissue-specific NCX variants could be beneficial, thereby representing a challenge. Recent structural and biophysical studies revealed a common module for decoding the Ca(2+)-induced allosteric signal in eukaryotic NCX variants, although the phenotype variances in response to regulatory Ca(2+) remain unclear. The breakthrough discovery of the archaebacterial NCX structure may serve as a template for eukaryotic NCX, although the turnover rates of the transport cycle may differ ~10(3)-fold among NCX variants to fulfill the physiological demands for the Ca(2+) flux rates. Further elucidation of ion-transport and regulatory mechanisms may lead to selective pharmacological targeting of NCX variants under disease conditions.

The SLC8 gene family of sodium-calcium exchangers (NCX) - structure, function, and regulation in health and disease

The SLC8 gene family encoding Na(+)/Ca(2+) exchangers (NCX) belongs to the CaCA (Ca(2+)/Cation Antiporter) superfamily. Three mammalian genes (SLC8A1, SLC8A2, and SLC8A3) and their splice variants are expressed in a tissue-specific manner to mediate Ca(2+)-fluxes across the cell-membrane and thus, significantly contribute to regulation of Ca(2+)-dependent events in many cell types. A long-wanted mitochondrial Na(+)/Ca(2+) exchanger has been recently identified as NCLX protein, representing a gene product of SLC8B1. Distinct NCX isoform/splice variants contribute to excitation-contraction coupling, long-term potentiation of the brain and learning, blood pressure regulation, immune response, neurotransmitter and insulin secretion, mitochondrial bioenergetics, etc. Altered expression and regulation of NCX proteins contribute to distorted Ca(2+)-homeostasis in heart failure, arrhythmia, cerebral ischemia, hypertension, diabetes, renal Ca(2+) reabsorption, muscle dystrophy, etc. Recently, high-resolution X-ray structures of Ca(2+)-binding regulatory domains of eukaryotic NCX and of full-size prokaryotic NCX have become available and the dynamic properties have been analyzed by advanced biophysical approaches. Molecular silencing/overexpression of NCX in cellular systems and organ-specific KO mouse models provided useful information on the contribution of distinct NCX variants to cellular and systemic functions under various pathophysiological conditions. Selective inhibition or activation of predefined NCX variants in specific diseases might have clinical relevance, although this breakthrough has not yet been realized. A better understanding of the underlying molecular mechanisms as well as the development of in vitro procedures for high-throughput screening of "drug-like" compounds may lead to selective pharmacological targeting of NCX variants.

Direct Loading of the purified endogenous inhibitor into the cytoplasm of patched cardiomyocytes blocks the ion currents and calcium transport through the NCX1 protein

The Na(+)-Ca(2+) exchanger in mammalian heart muscle (NCX1) is the central transporter protein that regulates extrusion of Ca(2+) from the heart cell. However, the functional biochemistry and physiology of NCX1 have been severely hampered by the absence of any specific high-affinity inhibitor. Here we describe advanced procedures for purifying a candidate inhibitor, previously called endogenous inhibitor factor (NCX(IF)), and demonstrate its direct actions on NCX1 activities in the single-cell system. A combination of advanced HILIC (hydrophilic interaction liquid chromatography) procedures with analytical tests suggests that the properties of NCX(IF) resemble those of a small (disaccharide size) polar molecule lacking any aromatic rings, conjugated bonds, or a primary amino group. The effects of NCX(IF) on the NCX1-mediated ion currents (I(NCX)) and cytosolic Ca(2+) extrusion were detected by a combination of patch-clamp and confocal microscopy under conditions in which the purified NCX(IF) was directly loaded into the cytoplasm of patched cardiomyocytes. It was demonstrated that cytosolic NCX(IF) blocks the Ca(2+)-activated NCX1 inward current and the accompanying extrusion of Ca(2+) from the cell with high efficacy. A constant fraction of NCX1 inhibition was observed under conditions in which the cytosolic [Ca(2+)](i) was varied at fixed doses of NCX(IF), suggesting that the degree of inhibition is controlled by NCX(IF) dose and not by cytosolic Ca(2+) concentration. NCX(IF) blocks equally well both the Ca(2+) extrusion and Ca(2+) entry modes of NCX1, consistent with thermodynamic principles expected for the functioning of a bidirectional "carrier-type" transport system. We concluded that NCX(IF) interacts with a putative regulatory domain from the cytosolic side and, thus, may play an important regulatory role in controlling Ca(2+) signaling in the heart. This may represent a new potential tool for developing novel treatments for cardiac Ca(2+) signaling dysfunction.

Effects of purified endogenous inhibitor of the Na+/Ca2+ exchanger on ouabain-induced arrhythmias in the atria and ventricle strips of guinea pig

Previous studies demonstrated that the purified endogenous inhibitor (NCX(IF)) of the cardiac Na(+)/Ca(2+) exchanger (NCX1) has the capacity to modulate cardiac muscle contractility. Here, we tested the effects of purified NCX(IF) on arrhythmias induced by ouabain in the atria and ventricle strips of guinea pig. For the sake of comparison NCX(IF) was compared to lidocaine and KB-R7943. In the ventricle strip, NCX(IF) ( approximately 10 U/ml) results in rapid, complete and stable inhibition of ouabain-induced arrhythmias (the inhibition of arrhythmia is not followed by revival of irregular contractions). Under similar experimental conditions the atria strips require somewhat higher doses of NCX(IF) (25-50 U/ml) for complete suppression of arrhythmia. In the atria strip, NCX(IF) (10-25 U/ml) increases the threshold dose (1 microM) of ouabain for arrhythmia onset 2.2+/-0.5-fold (n=5, p<0.05) as well as prolongs the lag-phase for arrhythmia appearance 4.0+/-0.5-fold (n=5, p<0.01). The lag period for arrhythmia onset was also lengthened (2.0+/-0.4-fold) by NCX(IF) in the ventricle strips (n=6, p<0.002). At low frequency of pacing (1 Hz), all three tested substances, lidocaine, KB-R7943, and NCX(IF) can effectively suppress the ouabain-induced arrhythmia. However, at higher frequency (2 Hz), lidocaine is ineffective in suppressing arrhythmia, whereas KB-R7943 becomes pro-arrhythmic. In contrast to reference drugs, NCX(IF) retains its anti-arrhythmic capacity at high frequencies, either in the atria (n=6, p<0.01) or ventricle (n=5, p<0.05) strips. In conclusion, NCX(IF) results in rapid, effective and stable suppression of arrhythmia both in the atria and ventricle preparations under conditions at which the reference drugs become ineffective.

Purified endogenous inhibitor of the Na/Ca exchanger can enhance the cardiomyocytes contractility and calcium transients

Previous studies have shown that the newly found endogenous inhibitor (NCX(IF)) of the cardiac Na/Ca exchanger (NCX1) is capable of regulating the muscle strip's contractility and relaxation. Here, the effects of purified NCX(IF) were tested on single cell shortening-lengthening (by using the IR CCD camera coupled with the two-edge video-detector) and [Ca]i-transients (by monitoring the changes in fluo-3 fluorescence). A perfusion of isolated cardiomyocytes (paced at 0.5-1.0 Hz) with NCX(IF) results in 4-6-fold enhancement in the amplitude of cell shortening-lengthening reaching the steady-state levels within 5-8 min (n=20, p<0.009). Simultaneous recordings of cell shortening-lengthening and [Ca]i-transients from the same cell show that the amplitude enhancement is associated with accelerated decay of both signals. Therefore, the NCX(IF)-dependent modulation of the single cell contractility is primarily governed by Ca-related mechanisms. The observed data are consistent with a proposal suggesting that the inhibition of NCX1 by NCX(IF) results in Ca-dependent activation of SERCA (SR Ca ATPase), yielding the accelerated decay of the [Ca]i-transients. The subsequent increase in the SR Ca content may result in enhanced Ca-release reflecting the manifested promotion of [Ca]i-transients. More systematic study is required for confirming this working hypothesis.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Advanced procedures for separation and analysis of low molecular weight inhibitor (NCXIF) of the cardiac sodium-calcium exchanger

A low molecular weight inhibitor (NCX(IF)) of the cardiac Na/Ca exchanger, isolated from the calf ventricle tissue, is capable of regulating the muscle strip's contractility and relaxation without involving the beta-activation pathway. The structural analysis of NCX(IF) requires highly purified preparations that fulfill the demanding requirements for mass spectra and NMR analyses. No such preparation is yet available. To this end, new HPLC procedures were developed by a combination of the reverse phase, normal phase, and HILIC (hydrophilic liquid chromatography) techniques. The specific activity of NCX(IF) is 10(5) times higher in the purified preparations (as compared to the crude extract) showing a 2-5% yield of total inhibitory activity and 20-100 microg content of final material. The purification yield reveals that 1 kg ventricle muscle contains 0.1-0.2 mg NCX(IF), meaning that the tissue concentrations of NCX(IF) may reach 10(-7)-10(-6) M. The diode-array scanning of purified preparations of NCX(IF) shows a homogeneous 3D peak with a maximal absorption at 202 nm. These spectral properties may represent a five-membered ring (e.g., proline, histidine) and/or simple chemical groups (like amine, carbonyl, ester, etc.), but not an aromatic ring or complex conjugates (alkyne, alkene, aldehyde, etc.). NCX(IF) does not respond to phenol/sulfur reagent, suggesting that it lacks reducing (aldo) sugar. NCX(IF) shows a faint response to fluorescamine, meaning that it may contain an amino group (or its derivative). It is believed that a combination of presently developed procedures with LC/MS and LC/MS/MS may provide a useful tool for structural analysis of NCX(IF).

Inotropic and lusitropic effects induced by the inhibitory factor of the Na/Ca exchanger are not mediated by the beta-adrenergic activation

Recently, an endogenous inhibitory factor (NCXIF) of the cardiac Na/Ca exchanger (NCX1) has been isolated, purified, and preliminary characterized. Here, we demonstrate that low doses of NCXIF (10(-7)10(-8) M) induce strong inotropic effects in the guinea and rat ventricle strips, while having no detectable effects in the atria even at 10(-5) M. The inotropic effects of NCXIF are species-specific; the rat ventricle muscle is 20 to 50 times more sensitive to varying doses of NCXIF than the guinea pig. On the other hand the extent of maximal inotropic response is more prominent in the guinea pig model (up to 6-fold enhancement) than in the rat (up to 2-fold enhancement). The NCXIF accelerates the single-twitch relaxation (lusitropic effect) in dose-dependent manner, reaching approximately 2-fold shortening of twitch width at saturating doses. The dose-dependence curves of lusitropic and inotropic effects exhibit a reciprocal relationship, meaning that these two effects might share common mechanisms. To test a possible involvement of catecholamines, the effects of NCXIF were examined in the presence or absence of beta-adrenergic blocker, deralin. The saturating doses of deralin (1- 3 microM) do not alter either the NCXIF-induced acceleration of relaxation or twitch enhancement, meaning that the NCXIF effects cannot be mediated by occasional release of endogenous catecholamines. The capacity of NCXIF to modulate the ventricle contractility unconnectedly to the beta-adrenergic activation may provide new rational clues for future pharmacological interventions.

The low molecular weight inhibitor of NCX1 interacts with a cytosolic domain that differs from the ion-transport site of the Na/Ca exchanger

The endogenous inhibitory factor (NCX(IF)) of the cardiac Na/Ca exchanger (NCX1) is a low molecular weight substance, which has a strong capacity to modulate the ventricle muscle contractility. Previously, we have shown that NCX(IF) can completely inhibit either the forward (Na(i)-dependent Ca-uptake) or reverse (Na(o)-dependent Ca-release) mode of Na/Ca exchange as well as its partial reaction, the Ca/Ca exchange. Although the preliminary studies have shown that NCX(IF) can rapidly (within few milliseconds) interact with a putative inhibitory site of the Na/Ca exchanger protein (or within its vicinity), it was not clear whether the NCX(IF) can directly interact with the ion transport sites of the exchanger protein or the interaction site of NCX(IF) is distinct from the ion-binding/transport site of NCX1. In order to segregate between these possibilities the NCX(IF) was tested for its capacity to compete with Ca at the cytosolic side by using the preparation of sarcolemma vesicles having predominantly the inside-out orientation. For this goal, the initial rates of Na(i)-dependent (45)Ca-uptake were measured in the presence of extravesicular (cytosolic) NCX(IF) under conditions in which the concentration of extravesicular Ca was varied (2-200 microM) and intravesicular Na was kept fixed at saturating concentration (160 mM). Under these conditions the NCX(IF) results in several fold decrease in V(max) values, while having no significant effect on the K(m). Taking into account the molecular weight of 350-550 Da (derived from the gel-filtration and mass-spectra data), the experimentally measured inhibitory potency of NCX(IF) can be estimated as the IC(50) = 0.3-0.6 microM. Therefore, it is concluded that the NCX(IF) is reasonably potent blocker, which interacts with cytosolic domain thereby preventing the ion-translocation (and not ion-binding) events.

Distinct subcellular location of the Ca2+-binding protein S100A1 differentially modulates Ca2+-cycling in ventricular rat cardiomyocytes

Calcium is a key regulator of cardiac function and is modulated through the Ca2+-sensor protein S100A1. S100 proteins are considered to exert both intracellular and extracellular functions on their target cells. Here we report the impact of an increased intracellular S100A1 protein level on Ca2+-homeostasis in neonatal ventricular cardiomyocytes in vitro. Specifically, we compare the effects of exogenously added recombinant S100A1 to those resulting from the overexpression of a transduced S100A1 gene. Extracellularly added S100A1 enhanced the Ca2+-transient amplitude in neonatal ventricular cardiomyocytes (NVCMs) through a marked decrease in intracellular diastolic Ca2+-concentrations ([Ca2+]i). The decrease in [Ca2+]i was independent of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) activity and was probably the result of an increased sarcolemmal Ca2+-extrusion through the sodium-calcium exchanger (NCX). At the same time the Ca2+-content of the sarcoplasmic reticulum (SR) decreased. These effects were dependent on the uptake of extracellularly added S100A1 protein and its subsequent routing to the endosomal compartment. Phospholipase C and protein kinase C, which are tightly associated with this subcellular compartment, were found to be activated by endocytosed S100A1.

By contrast, adenoviral-mediated intracellular S100A1 overexpression enhanced the Ca2+-transient amplitude in NVCMs mainly through an increase in systolic [Ca2+]i. The increased Ca2+-load in the SR was based on an enhanced SERCA2a activity while NCX function was unaltered. Overexpressed S100A1 colocalized with SERCA2a and other Ca2+-regulatory proteins at the SR, whereas recombinant S100A1 protein that had been endocytosed did not colocalize with SR proteins. This study provides the first evidence that intracellular S100A1, depending on its subcellular location, modulates cardiac Ca2+-turnover via different Ca2+-regulatory proteins.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Expression and localization of the multidrug resistance protein 5 (MRP5/ABCC5), a cellular export pump for cyclic nucleotides, in human heart

The multidrug resistance protein 5 (MRP5/ABCC5) has been recently identified as cellular export pump for cyclic nucleotides with 3',5'-cyclic GMP (cGMP) as a high-affinity substrate. In view of the important role of cGMP for cardiovascular function, expression of this transport protein in human heart is of relevance. We analyzed the expression and localization of MRP5 in human heart [21 auricular (AS) and 15 left ventricular samples (LV) including 5 samples of dilated and ischemic cardiomyopathy]. Quantitative real-time polymerase chain reaction normalized to beta-actin revealed expression of the MRP5 gene in all samples (LV, 38.5 +/- 12.9; AS, 12.7 +/- 5.6; P < 0.001). An MRP5-specific polyclonal antibody detected a glycoprotein of approximately 190 kd in crude cell membrane fractions from these samples. Immunohistochemistry with the affinity-purified antibody revealed localization of MRP5 in cardiomyocytes as well as in cardiovascular endothelial and smooth muscle cells. Furthermore, we could detect MRP5 and ATP-dependent transport of [(3)H]cGMP in sarcolemma vesicles of human heart. Quantitative analysis of the immunoblots indicated an interindividual variability with a higher expression of MRP5 in the ischemic (104 +/- 38% of recombinant MRP5 standard) compared to normal ventricular samples (53 +/- 36%, P < 0.05). In addition, we screened genomic DNA from our samples for 20 single-nucleotide polymorphisms in the MRP5 gene. These results indicate that MRP5 is localized in cardiac and cardiovascular myocytes as well as endothelial cells with increased expression in ischemic cardiomyopathy. Therefore, MRP5-mediated cellular export may represent a novel, disease-dependent pathway for cGMP removal from cardiac cells.

The endogenous inhibitor of NCX1 does not resemble the properties of digitalis compound

In our previous study, we ware successful in isolation and purification of an endogenous inhibitor of the Na/Ca exchanger (NCX1) from the calf ventricle extracts. The purified factor has characterized to have strong positive inotropic effect on isometric contractions of isolated ventricle strips of guinea pig. A possibility is that besides the NCX1 the endogenous factor may also interact with other ion-transport systems (e.g., Na,K-ATPase) involved in modulation of muscle contractility-relaxation. Therefore, a primary goal of the present study was to detect a possible effect of newly found NCX1 inhibitor on Na,K-ATPase and Ca-ATPase activities. The preparations of isolated sarcolemma vesicles were used for this goal. Although the crude extracts of calf ventricles can inhibit both the Na/Ca exchange and Na,K-ATPase, these two inhibitory activities can be separated on the Sephadex G-10 column, meaning that different molecular entities might be responsible for inhibition of Na/Ca exchange and Na,K-ATPase. Addition of 100 U of purified endogenous factor to the assay medium results in nearly complete inhibition of forward (Na(i)-dependent Ca-uptake) and reverse (Na(o)-dependent Ca-efflux) modes of Na/Ca exchange. On the other hand, no effect was detected on activities of Na,K-ATPase and Ca-ATPase even in the presence of 500 U of purified factor in the assay medium. In light of the present data, it is concluded that the endogenous inhibitor of NCX1 does not resemble the targeting properties of digitalis like compound. Obviously, more systematic studies are required in the future for resolving a possible interaction of the endogenous inhibitor of NCX1 with other ion-transport systems involved in calcium homeostasis and action potential.

Is timing everything? Therapeutic potential of modulators of cardiac Na(+) transporters

Sodium ion (Na(+)) transporters have roles in the modulation of cardiomyocyte pH and Na(+) and Ca(2+) handling. Activation of the cardiac Na(+)-H(+) exchanger 1 (NHE1) during ischaemia induces arrhythmias, myocardial stunning and irreversible cell injury. As the benefits of NHE1 inhibitors (e.g., amiloride, cariporide) in models of myocardial infarction are usually much greater when used as pretreatment, rather than during or after ischaemia, it is probably not surprising that clinical trials with cariporide in ischaemia have shown little shortterm benefit. NHE1 inhibitors have been shown to be beneficial in animal models of ventricular fibrillation and resuscitation, cardioplegia, hypertrophy and heart failure, and their therapeutic potential in these conditions should be further developed. The Na(+)-HCO(3)(-) cotransporter (NBC) is also stimulated by intracellular acidification, and part of the benefit of angiotensin-converting enzyme inhibitors after myocardial infarction may be due to inhibition of the NBC. Selective inhibitors of the NBC are required to determine the therapeutic potential of this mechanism. The Na(+)-Ca(2+) exchanger (NCX) has a major role in cardiac Na(+) and Ca(2+) homeostasis and influences cardiac electrical activity. The NCX also has a role in ischaemia/infarction, arrhythmias, hypertrophy and heart failure. NCX inhibitors may have beneficial effects in animal models of ischaemia and reperfusion injury and the therapeutic benefit of these should be further studied in animal models.

Mechanisms underlying delayed afterdepolarizations in hypertrophied left ventricular myocytes of rats

Cardiac hypertrophy was induced in rats by daily injection of isoproterenol (5 mg/kg ip) for 7 days. Membrane voltage and currents were recorded using the whole cell patch-clamp technique in left ventricular myocytes from control and hypertrophied hearts. Ryanodine-sensitive delayed afterdepolarizations (DADs) and transient inward current (I(ti)) appeared in hypertrophied cells more often and were of larger amplitude than in control cells. DADs and I(ti) are carried principally by Na/Ca exchange with smaller contributions from a nonselective cation channel and from a Cl- channel. The latter is expressed only in hypertrophied myocytes. In hypertrophy, the density of caffeine-induced Na/Ca exchange current (I(Na/Ca)) was increased by 26%, sarcoplasmic reticulum (SR) Ca2+ content as assessed from the integral of I(Na/Ca) was increased by 30%, the density of Na-pump current (I(pump)) was reduced by 40%, and the intracellular Na+ content, measured by Na+-selective microelectrodes was increased by 55%. The results indicate that DADs and I(ti) are generated by spontaneous Ca2+ release from an overloaded SR caused by a downregulated Na pump and an upregulated Na/Ca exchange. These findings may explain the propensity for arrhythmias seen in this model of hypertrophy.

Cardiac Na + -Ca 2+ Exchange

Abstract —The Na + -Ca 2+ exchanger (NCX) is one of the essential regulators of Ca 2+ homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca 2+ -influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.

An unknown endogenous inhibitor of Na/Ca exchange can enhance the cardiac muscle contractility

The cardiac sarcolemma Na/Ca exchanger is a key system for controlling the intracellular calcium levels during the excitation-contraction coupling. Here, we test the hypothesis that the heart tissue contains a putative endogenous factor having a capacity to modulate the Na/Ca exchanger and muscle contractility. The concentrated cardiac extracts inhibit the Na(i)- or Ca(i)-dependent (45)Ca uptakes in isolated cardiac sarcolemma vesicles as well as the Na(o)-dependent Ca efflux, monitored by extravesicular Ca probe fluo-3. The inhibitory activity has been purified approximately 2000-fold by normal and reversed-phase HPLC procedures. The inhibitory activity is eluted from the Sephadex G-10 in the range of 350-550 Da, suggesting that the inhibitory factor is a low-molecular-weight substance. The mass spectra analysis shows a number of signals within m/z 380-560; however, it is not clear at this moment whether these recordings represent the mass of putative inhibitory factor or irrelevant impurities. The endogenous inhibitory factor of Na/Ca exchange does not resemble the properties (HPLC retention time, mass spectra, amino acid analysis, etc.) of autoinhibitory XIP peptide. The addition of inhibitory factor to muscle strip of guinea pig ventricles induces 2- to 5-fold enhancement of isometric contractions, thereby exhibiting a strong positive inotropic effect. This effect is a dose-dependent phenomenon, which can be reversed by washing the inhibitory factor from the organ bath. Assuming a molecular weight of 350-550 Da, the effective concentrations of putative inhibitor must be <10(-6) M. Therefore, the present findings demonstrate that the mammalian heart contains a low-molecular-weight factor that can inhibit Na/Ca exchange and enhance the cardiac contractility.

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.

Time-resolved monitoring of electrogenic Na+-Ca2+ exchange in the isolated cardiac sarcolemma vesicles by using a rapid-response fluorescent probe

As a major Ca exit system in myocytes, the electrogenic Na+-Ca2+ exchange is exposed to rapid changes of regulatory factors (e.g., cytosolic Ca) during the excitation-contraction coupling. The dynamic aspects of the exchanger response to regulatory factors have not been resolved in the past due to technical limitations. Here, we describe stopped-flow protocols for monitoring the electrogenic activity of Na+-Ca2+ exchange in cardiac sarcolemma vesicles by using a rapid-response voltage-sensitive dye Merocyanine-540 (M540). The M540 signal of Nao-dependent Ca efflux is generated by mixing the Ca-loaded vesicles with Na buffer, yielding 160 mM extravesicular Na and 6 microM Cafree. This signal is inhibited by a cyclic peptide blocker (FRCRCFa), by a Ca ionophore (ionomycin), or by an electrogenic uncoupler (valinomycin or FCCP). The M540 signal of Nao-dependent Ca efflux shows a rapid pre-steady-state burst (210 s-1), followed by slow steady-state phase (</=5 s-1). Extravesicular (cytosolic) Ni inhibits both phases with an IC50 of 0.80 +/- 0.24 mM. At an extravesicular pH of 6.0, the Nao-dependent Ca efflux is able to generate the M540 signal, thereby supporting the idea that the stoichiometry of Na+-Ca2+ exchange is not altered at low pH [Khanashvili, D., et al. (1995) Biochemistry 34, 10290-10297]. The M540 signal of Nao-dependent Ca efflux is lost when the extravesicular Cafree concentration drops to 0.2 microM. This effect cannot be explained by a lack of Ca access to extravesicular (cytosolic) transport sites, because the reaction of Nao-dependent Ca efflux utilizes intravesicular Ca as a substrate. These data suggest that in sarcolemma vesicles a regulatory cytosolic Ca site controls the exchanger activity. The properties of this putative regulatory site do not resemble the properties of the "slow" Ca regulatory mode, observed in electrophysiological studies. Under saturating ionic conditions, the Nao-dependent Ca efflux generates the initial rates of 21 mV/ms in the vesicles with a diameter of 3000-5000 A. If a site density of 300-400 exchangers/micrometer2 and a vesicular surface of 0.5 micrometer2 are assumed, each vesicle may contain 150-200 exchanger molecules with a maximal turnover rate of 4000-5000 s-1. This upper limit for turnover (no matter what the site density is) may put considerable restrictions on the exchanger capacity to mediate Ca entry in the cell under physiologically related conditions.

Functional comparison of the three isoforms of the Na+/Ca2+ exchanger (NCX1, NCX2, NCX3)

Three distinct mammalian Na+/Ca2+ exchangers have been cloned: NCX1, NCX2, and NCX3. We have undertaken a detailed functional comparison of these three exchangers. Each exchanger was stably expressed at high levels in the plasma membranes of BHK cells. Na+/Ca2+ exchange activity was assessed using three different complementary techniques: Na+ gradient-dependent 45Ca2+ uptake into intact cells, Na+ gradient-dependent 45Ca2+ uptake into membrane vesicles isolated from the transfected cells, and exchange currents measured using giant patches of excised cell membrane. Apparent affinities for the transported ions Na+ and Ca2+ were markedly similar for the three exchangers at both membrane surfaces. Likewise, generally similar responses to changes in pH, chymotrypsin treatment, and application of various inhibitors were obtained. Depletion of cellular ATP inhibited NCX1 and NCX2 but did not affect the activity of NCX3. Exchange activities of NCX1 and NCX3 were modestly increased by agents that activate protein kinases A and C. All exchangers were regulated by intracellular Ca2+. NCX1-induced exchange currents were especially large in excised patches and, like the native myocardial exchanger, were stimulated by ATP. Results may be influenced by our choice of expression system and specific splice variants, but, overall, the three exchangers appear to have very similar properties.

Alzheimer's amyloid-beta peptide inhibits sodium/calcium exchange measured in rat and human brain plasma membrane vesicles

Na+/Ca2+ exchange activity was measured by monitoring vesicular Ca2+ content after incubation in buffers containing 45Ca2+. When Na+-loaded vesicles were placed into Na+-free buffer, vesicular Ca2+ content increased rapidly and reached a plateau after two to three minutes. Only preaggregated amyloid-beta1-40 (Abeta1-40) and Abeta25-35 reduced vesicular Ca2+ content. Both peptides produced a maximal reduction in Ca2+ content of approximately 50%. The peptides reduced Ca2+ content with similar potency and half maximal effects were seen at less than 10 microM for Abeta25-35. Calcium-loaded vesicles mediate a rapid Ca2+/Ca2+ exchange, which also was inhibited by aggregated Abeta25-35. Aggregated Abeta25-35 did not affect the passive Ca2+ permeability of the vesicles. Aggregated Abeta25-35 reduced Ca2+ content in plasma membrane vesicles isolated from normal and Alzheimer's disease frontal cortex with less potency but the same efficacy as seen in rat brain. Aggregated Abeta25-35 did not produce nonspecific effects on vesicle morphology such as clumping or loss of intact vesicles. When placed in the buffer used to measure Ca2+ content, Congo Red at molar ratios of less than one blocked the inhibitory effect of preaggregated Abeta25-35. When added in equimolar concentrations to freshly dissolved and unaggregated Abeta25-35, Congo Red also was effective at blocking the inhibitory effect on Ca2+ content. In contrast, vitamin E (antioxidant) and N-tert-butyl-alpha-phenylnitrone (spin trapping agent) failed to block the inhibitory action of aggregated Abeta25-35. The exact mechanisms of Abeta-induced neurotoxicity in cell culture has yet to be solved. Accumulation of free radicals play a necessary role, but disruptions of Ca2+ homeostasis are also important. The data presented here are consistent with a proposed mechanism where aggregated Abeta peptides directly interact with hydrophobic surfaces of the exchanger protein and/or lipid bilayer and interfere with plasma membrane Ca2+ transport.

Inhibition of the cardiac sarcolemma Na+/Ca2+ exchanger by conformationally constrained small cyclic peptides

Positively charged cyclic peptides (three to seven amino acids) have been tested for their inhibitory effects on Na+/Ca2+ exchange in the cardiac sarcolemma vesicles. The lead structure of Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) has been systematically modified for identification of important pharmacophores. In cyclic peptides (intramolecular S-S bond, the carboxyl terminal is locked with amide (CONH2), and positive charge is retained by one or two arginines, ornithines, or lysines. Thirty-five different cyclic peptides show IC50 values in the range of 2-800 microM, suggesting that some specific structure-activity relationships may determine the inhibitory effects. Shortening of the FRCRCFa length to four amino acids decreases the inhibitory potency by 10-80-fold. The substitution of Arg2 or Arg4 in FRCRCFa with lysine or ornithine decreases the inhibitory potency by 5-12-fold, suggesting that both arginines are beneficial for inhibition. The substitution of Phe1 in FRCRCFa by 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid produces a potent inhibitor (IC50 = 2-4 microM). The N-myristoylated FRCRCFa exhibits an inhibitory potency (IC50 = 8-10 microM) similar to that of the parent FRCRCFa peptide, thereby arousing a new possibility for the development of a cell-permeable blocker of the Na+/Ca2+ exchanger, D-Arg4 or D-Cys5 substitutions in FRCRCFa do not alter the inhibitory effect, whereas the L-to-D substitutions of other amino acids in FRCRCFa reduce the inhibitory potency by 4-5-fold. Thus, the L-to-D substitutions of Arg4 and/or Cys5 have a potential to increase the peptide stability to proteolytic degradation. The insertion of proline outside of the ring of FRCRCFa diminishes the inhibitory potency by 3-6-fold, whereas proline introduction into the ring decreases the inhibitory potency by 16-20-fold. The replacement of Cys3 and Cys5 in FRCRCFa with beta, beta-dimethylcystein has no significant effect on the inhibitory potency, suggesting that the S-S bond is not exposed to the interface of the peptide/receptor interaction. In conclusion, the current data support a proposal that the conformationally constrained Arg-Cys-Arg-Cys structure is obligatory for inhibition of Na+/Ca2+ exchange, whereas hydrophobic additions at the carboxyl and amino ends have limited effects in increasing the inhibitory potency.

Rapid interaction of FRCRCFa with the cytosolic side of the cardiac sarcolemma Na(+)-Ca2+ exchanger blocks the ion transport without preventing the binding of either sodium or calcium

Positively charged cyclic hexapeptide Phe-Arg-Cys-Arg-Cys-Phe-CONH2 (FRCRCFa) represents a group of conformationally constrained peptides that block the cardiac sarcolemma Na(+)-Ca2+ exchanger [Khananshvili et al. (1995b) J. Biol. Chem. 270, 16182-16188]. Here, we study the kinetic mechanisms of FRCRCFa-induced inhibition of Na(+)-Ca2+ exchange and its partial reaction, the Ca(2+)-Ca2+ exchange. The Nai-dependent 45Ca uptake and Cai-dependent 45Ca uptake were measured by adding the EGTA quench in the semirapid mixer. The reverse mode of Na(+)-Ca2+ exchange (Nao-dependent Ca efflux) was monitored (t = 10-5000 ms) in the stopped-flow machine by measuring extravesicular free calcium with a fluorescence probe fluo-3. Saturating concentrations of FRCRCFa inhibit completely the forward and reverse modes of exchange, suggesting that the inside-out vesicles contribute to most (if not all) of the exchange activity. A short time exposure (t = 10-20 ms) of FRCRCFa with the vesicles is enough to reach a rapid equilibrium between FRCRCFa and a putative inhibitory site at the extravesicular (cytosolic) side of the membrane. A lower limit for the second-order rate constant of FRCRCFa binding can be estimated as a kon of > 10(6) M-1 s-1. A possible competition between FRCRCFa and either Na+ or Ca2+ has been tested at the extravesicular (cytosolic) side of the membrane. At different extravesicular Cao concentrations of 13-250 microM, FRCRCFa inhibits the Na(+)-Ca2+ and Ca(2+)-Ca2+ exchanges with an IC50 of 11-16 microM, suggesting no competition between FRCRCFa and Ca2+. At different extravesicular Nao concentrations of 40-160 mM, FRCRCFa inhibits Nao-dependent Ca efflux with an IC50 of 12-18 microM, suggesting that FRCRCFa and Na+ do not compete for binding at the extravesicular side. A mild proteolytic treatment of vesicles activates the Nai-dependent 45Ca uptake, but has a little effect on the FRCRCFa-induced inhibition. Thus, the "inhibitory site" is still functional after the proteolytic treatment of the inside-out vesicles. In conclusion, a rapid (< 20 ms) interaction of extravesicular (cytosolic) FRCRCFa with the exchanger prevents the ion translocation through the exchanger, while the inhibitory peptide does not interact with the ion transport sites of the exchanger at the cytosolic side of the membrane.

Unitary cardiac Na+, Ca2+ exchange current magnitudes determined from channel-like noise and charge movements of ion transport

The cardiac Na+, Ca2+ exchanger (NCX1) is thought to achieve a high turnover rate, but all estimates to date are indirect. Two new strategies demonstrate that maximum unitary exchange currents are about 1 fA (6000 unitary charges per s) and that they fluctuate between on and off levels similar to ion channel currents. First, exchange current noise has been identified in small cardiac patches with properties expected for a gated transport process. Noise power density spectra correlate well with exchanger inactivation kinetics, and the noise has a predicted bell-shaped dependence on the activation states of the exchanger. From the magnitudes of exchange current noise, maximum unitary exchange currents are estimated to be 0.6-1.3 fA. Second, charge movements with rates of approximately 5000 s-1 have been isolated for the transport of both Na+ and Ca2+ in giant membrane patches using nonsaturating ion concentrations. The Na+ transport reactions are disabled or "immobilized" by exchanger inactivation reactions, thus confirming that inactivation generates fully inactive exchanger states.