Mechanism of cardiac Na(+)-Ca2+ exchange current stimulation by MgATP: possible involvement of aminophospholipid translocase

19F-NMR Probing of Ion-Induced Conformational Changes in Detergent-Solubilized and Nanodisc-Reconstituted NCX_Mj

Consecutive interactions of 3Na+ or 1Ca2+ with the Na+/Ca2+ exchanger (NCX) result in an alternative exposure (access) of the cytosolic and extracellular vestibules to opposite sides of the membrane, where ion-induced transitions between the outward-facing (OF) and inward-facing (IF) conformational states drive a transport cycle. Here, we investigate sub-state populations of apo and ion-bound species in the OF and IF states by analyzing detergent-solubilized and nanodisc-reconstituted preparations of NCX_Mj with 19F-NMR. The 19F probe was covalently attached to the cysteine residues at entry locations of the cytosolic and extracellular vestibules. Multiple sub-states of apo and ion-bound species were observed in nanodisc-reconstituted (but not in detergent-solubilized) NCX_Mj, meaning that the lipid-membrane environment preconditions multiple sub-state populations toward the OF/IF swapping. Most importantly, ion-induced sub-state redistributions occur within each major (OF or IF) state, where sub-state interconversions may precondition the OF/IF swapping. In contrast with large changes in population redistributions, the sum of sub-state populations within each inherent state (OF or IF) remains nearly unchanged upon ion addition. The present findings allow the further elucidation of structure-dynamic modules underlying ion-induced conformational changes that determine a functional asymmetry of ion access/translocation at opposite sides of the membrane and ion transport rates concurring physiological demands.

Structure-Based Function and Regulation of NCX Variants: Updates and Challenges

The plasma-membrane homeostasis Na⁺/Ca²⁺ exchangers (NCXs) mediate Ca²⁺ extrusion/entry to dynamically shape Ca²⁺ signaling/in biological systems ranging from bacteria to humans. The NCX gene orthologs, isoforms, and their splice variants are expressed in a tissue-specific manner and exhibit nearly 10⁴-fold differences in the transport rates and regulatory specificities to match the cell-specific requirements. Selective pharmacological targeting of NCX variants could benefit many clinical applications, although this intervention remains challenging, mainly because a full-size structure of eukaryotic NCX is unavailable. The crystal structure of the archaeal NCX_Mj, in conjunction with biophysical, computational, and functional analyses, provided a breakthrough in resolving the ion transport mechanisms. However, NCX_Mj (whose size is nearly three times smaller than that of mammalian NCXs) cannot serve as a structure-dynamic model for imitating high transport rates and regulatory modules possessed by eukaryotic NCXs. The crystal structures of isolated regulatory domains (obtained from eukaryotic NCXs) and their biophysical analyses by SAXS, NMR, FRET, and HDX-MS approaches revealed structure-based variances of regulatory modules. Despite these achievements, it remains unclear how multi-domain interactions can decode and integrate diverse allosteric signals, thereby yielding distinct regulatory outcomes in a given ortholog/isoform/splice variant. This article summarizes the relevant issues from the perspective of future developments.

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

Regulation of ion transport from within ion transit pathways

All cells must control the activities of their ion channels and transporters to maintain physiologically appropriate gradients of solutes and ions. The complexity of underlying regulatory mechanisms is staggering, as exemplified by insulin regulation of transporter trafficking. Simpler strategies occur in single-cell organisms, where subsets of transporters act as solute sensors to regulate expression of their active homologues. This Viewpoint highlights still simpler mechanisms by which Na transporters use their own transport sites as sensors for regulation. The underlying principle is inherent to Na/K pumps in which aspartate phosphorylation and dephosphorylation are controlled by occupation of transport sites for Na and K, respectively. By this same principle, Na binding to transport sites can control intrinsic inactivation reactions that are in turn modified by extrinsic signaling factors. Cardiac Na/Ca exchangers (NCX1s) and Na/K pumps are the best examples. Inactivation of NCX1 occurs when cytoplasmic Na sites are fully occupied and is regulated by lipid signaling. Inactivation of cardiac Na/K pumps occurs when cytoplasmic Na-binding sites are not fully occupied, and inactivation is in turn regulated by Ca signaling. Potentially, Na/H exchangers (NHEs) and epithelial Na channels (ENaCs) are regulated similarly. Extracellular protons and cytoplasmic Na ions oppose secondary activation of NHEs by cytoplasmic protons. ENaCs undergo inactivation as cytoplasmic Na rises, and small diffusible molecules of an unidentified nature are likely involved. Multiple other ion channels have recently been shown to be regulated by transiting ions, thereby underscoring that ion permeation and channel gating need not be independent processes.

Cracking the code of sodium/calcium exchanger (NCX) gating: Old and new complexities surfacing from the deep web of secondary regulations

Cell membranes spatially define gradients that drive the complexity of biological signals. To guarantee movements and exchanges of solutes between compartments, membrane transporters negotiate the passages of ions and other important molecules through lipid bilayers. The Na⁺/Ca²⁺ exchangers (NCXs) in particular play central roles in balancing Na⁺ and Ca²⁺ fluxes across diverse proteolipid borders in all eukaryotic cells, influencing cellular functions and fate by multiple means. To prevent progression from balance to disease, redundant regulatory mechanisms cooperate at multiple levels (transcriptional, translational, and post-translational) and guarantee that the activities of NCXs are finely-tuned to cell homeostatic requirements. When this regulatory network is disturbed by pathological forces, cells may approach the end of life. In this review, we will discuss the main findings, controversies and open questions about regulatory mechanisms that control NCX functions in health and disease.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling

The electrogenic Na/Ca exchanger (NCX) mediates bidirectional Ca movements that are highly sensitive to changes of Na gradients in many cells. NCX1 is implicated in the pathogenesis of heart failure and a number of cardiac arrhythmias. We measured NCX1 palmitoylation using resin-assisted capture, the subcellular location of yellow fluorescent protein–NCX1 fusion proteins, and NCX1 currents using whole-cell voltage clamping. Rat NCX1 is substantially palmitoylated in all tissues examined. Cysteine 739 in the NCX1 large intracellular loop is necessary and sufficient for NCX1 palmitoylation. Palmitoylation of NCX1 occurs in the Golgi and anchors the NCX1 large regulatory intracellular loop to membranes. Surprisingly, palmitoylation does not influence trafficking or localization of NCX1 to surface membranes, nor does it strongly affect the normal forward or reverse transport modes of NCX1. However, exchangers that cannot be palmitoylated do not inactivate normally (leading to substantial activity in conditions when wild-type exchangers are inactive) and do not promote cargo-dependent endocytosis that internalizes 50% of the cell surface following strong G-protein activation or large Ca transients. The palmitoylated cysteine in NCX1 is found in all vertebrate and some invertebrate NCX homologs. Thus, NCX palmitoylation ubiquitously modulates Ca homeostasis and membrane domain function in cells that express NCX proteins.—Reilly, L., Howie, J., Wypijewski, K., Ashford, M. L. J., Hilgemann, D. W., Fuller, W. Palmitoylation of the Na/Ca exchanger cytoplasmic loop controls its inactivation and internalization during stress signaling.

Human-induced pluripotent stem cell-derived cardiomyocytes for studies of cardiac ion transporters

Human-induced pluripotent stem cells (hiPSCs) can differentiate into functional cardiomyocytes (iCell Cardiomyocytes) with ion channel activities that are remarkably similar to adult cardiomyocytes. Here, we extend this characterization to cardiac ion transporters. Additionally, we document facile molecular biological manipulation of iCell Cardiomyocytes to overexpress and knockdown transporters and regulatory proteins. Na/Ca exchange (NCX1) and Na/K pump currents were recorded via patch clamp, and Na/H and Cl/OH exchanges were recorded via oscillating proton-selective microelectrodes during patch clamp. Flux densities of all transport systems are similar to those of nonrodent adult cardiomyocytes. NCX1 protein and NCX1 currents decline after NCX1 small interfering (si)RNA transfection with similar time courses (τ ≈ 2 days), and an NCX1-Halo fusion protein is internalized after its extracellular labeling by AlexaFluor488 Ligand with a similar time course. Loss of the cardiac regulatory protein phospholemman (PLM) occurs over a longer time course (τ ≈ 60 h) after PLM small interfering RNA transfection. Similar to multiple previous reports for adult cardiomyocytes, Na/K pump currents in iCell Cardiomyocytes are not enhanced by activating cAMP production with either maximal or submaximal cytoplasmic Na and using either forskolin or isoproterenol to activate adenylate cyclases. Finally, we describe Ca influx-dependent changes of iCell Cardiomyocyte capacitance (Cm). Large increases of Cm occur during Ca influx via NCX1, thereby documenting large internal membrane reserves that can fuse to the sarcolemma, and subsequent declines of Cm document active endocytic processes. Together, these results document a great potential of iCell Cardiomyocytes for both short- and long-term studies of cardiac ion transporters and their regulation.

Functional and structural properties of the NCKX2 Na(+)-Ca (2+)/K (+) exchanger: a comparison with the NCX1 Na (+)/Ca (2+) exchanger

Na(+)/Ca(2+)-K(+) exchangers (NCKX), alongside the more widely known Na(+)/Ca(2+) exchangers (NCX), are important players in the cellular Ca(2+) toolkit. But, unlike NCX, much less is known about the physiological roles of NCKX, while emergent evidence indicates that NCKX has highly specialized functions in cells and tissues where it is expressed. As their name implies, there are functional similarities in the properties of the two Ca(2+) exchanger families, but there are specific differences as well. Here, we compare and contrast their key functional properties of ionic dependence and affinities, as well as report on the effects of KB-R7943 - a compound that is widely used to differentiate the two exchangers. We also review structural similarities and differences between the two exchangers. The aim is to draw attention to key differences that will aid in differentiating the two exchangers in physiological contexts where both exist but perhaps play distinct roles.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Mechanistic analysis of massive endocytosis in relation to functionally defined surface membrane domains

A large fraction of endocytosis in eukaryotic cells occurs without adaptors or dynamins. Here, we present evidence for the involvement of lipid domains in massive endocytosis (MEND) activated by both large Ca transients and amphipathic compounds in baby hamster kidney and HEK293 cells. First, we demonstrate functional coupling of the two MEND types. Ca transients can strongly facilitate detergent-activated MEND. Conversely, an amphipath with dual alkyl chains, ditridecylphthalate, is without effect in the absence of Ca transients but induces MEND to occur within seconds during Ca transients. Ca transients, like amphipaths, enhance the extraction of lipids from cells by β-cyclodextrins. Second, we demonstrate that electrical and/or optical signals generated by selected membrane probes are nearly insensitive to MEND, suggesting that those probes segregate into membrane domains that are not taken up by MEND. Triphenylphosphoniums are increasingly excluded from domains that internalize as the carbon chain length increases from 4 to 12. The small cationic membrane dye, FM 4–64, binds well to domains that internalize, whereas a closely related dye with a larger hydrophobic moiety, di-4-ANEPPDHQ (ANEPPDHQ) is excluded. Multiple carrier-type ionophores and a small amphipathic anion, niflumic acid, are also excluded. Probes with modest MEND sensitivity include the hydrophobic anion, dipicrylamine, carbonyl cyanide m-chlorophenylhydrazone, and NBD-phosphatidylethanolamine. Third, we demonstrate that large Ca transients can strongly enhance the extracellular binding of several membrane probes, monitored electrically or optically, consistent with a more disordered membrane with more amphipath-binding sites. Fluorescence shifts of ANEPPDHQ report increased disorder of the extracellular monolayer after large Ca transients, consistent with an increased propensity of the membrane to phase separate and vesiculate. Collectively, the results indicate that >50% of the outer monolayer is ordered and can be selectively internalized during MEND responses initiated by two very different cell perturbations.

Sodium-dependent inactivation of sodium/calcium exchange in transfected Chinese hamster ovary cells

High concentrations of cytosolic Na(+) ions induce the time-dependent formation of an inactive state of the Na(+)/Ca(2+) exchanger (NCX), a process known as Na(+)-dependent inactivation. NCX activity was measured as Ca(2+) uptake in fura 2-loaded Chinese hamster ovary (CHO) cells expressing the wild-type (WT) NCX or mutants that are hypersensitive (F223E) or resistant (K229Q) to Na(+)-dependent inactivation. As expected, 1) Na(+)-dependent inactivation was promoted by high cytosolic Na(+) concentration, 2) the F223E mutant was more susceptible than the WT exchanger to inactivation, whereas the K229Q mutant was resistant, and 3) inactivation was enhanced by cytosolic acidification. However, in contrast to expectations from excised patch studies, 1) the WT exchanger was resistant to Na(+)-dependent inactivation unless cytosolic pH was reduced, 2) reducing cellular phosphatidylinositol-4,5-bisphosphate levels did not induce Na(+)-dependent inactivation in the WT exchanger, 3) Na(+)-dependent inactivation did not increase the half-maximal cytosolic Ca(2+) concentration for allosteric Ca(2+) activation, 4) Na(+)-dependent inactivation was not reversed by high cytosolic Ca(2+) concentrations, and 5) Na(+)-dependent inactivation was partially, but transiently, reversed by an increase in extracellular Ca(2+) concentration. Thus Na(+)-dependent inactivation of NCX expressed in CHO cells differs in several respects from the inactivation process measured in excised patches. The refractoriness of the WT exchanger to Na(+)-dependent inactivation suggests that this type of inactivation is unlikely to be a strong regulator of exchange activity under physiological conditions but would probably act to inhibit NCX-mediated Ca(2+) influx during ischemia.

Unrestricted diffusion of exogenous and endogenous PIP(2 )in baby hamster kidney and Chinese hamster ovary cell plasmalemma

We used two approaches to characterize the lateral mobility of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the plasmalemma of baby hamster kidney and Chinese hamster ovary fibroblasts. First, nitrobenzoxadiazole-labeled C6-phosphatidylcholine and C16-PIP(2) were incorporated into plasma membrane "lawns" ( approximately 20 x 30 microm) from these cells and into the outer monolayer of intact cells. Diffusion coefficients determined by fluorescence recovery after photobleaching were similar for the two lipids and were higher in lawns, approximately 0.3 microm(2)/s, than on the cell surface, approximately 0.1 microm(2)/s. For membrane lawns, the fractional recoveries (75-90%) were close to those expected from the fraction of total membrane bleached, and labeling by the probes was several times greater than for intact cells. Second, we analyzed cells expressing M1 muscarinic receptors and green fluorescent protein fused with PIP(2)-binding pleckstrin-homology domains, Tubby domains or diacylglycerol (DAG)-binding C1 domains. On-cell gigaseal patches were formed with pipette tips >5 microm in diameter. When the agonist carbachol (0.3 mM: ) was applied either within or outside of the pipette, lipid signals crossed the pipette barrier rapidly in both directions and membrane blebbing occurred on both membrane sides. Accurate simulations of lipid gradients required diffusion coefficients >1 microm(2)/s. Exogenous DAG also crossed the pipette barrier rapidly. In summary, we found no evidence for restricted diffusion of signaling lipids in these cells. The lower mobility and incorporation of phospholipid at the extracellular leaflet may reflect a more ordered and condensed extracellular monolayer, as expected from previous studies.

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.

Enhanced separation of membranes during free flow zonal electrophoresis in plants

Free flow zonal electrophoresis (FFZE) is a versatile technique that allows for the separation of cells, organelles, membranes, and proteins based on net surface charge during laminar flow through a thin aqueous layer. We have been optimizing the FFZE technique to enhance separation of plant vacuolar membranes (tonoplast) from other endomembranes to pursue a directed proteomics approach to identify novel tonoplast transporters. Addition of ATP to a mixture of endomembranes selectively enhanced electrophoretic mobility of acidic vesicular compartments during FFZE toward the positive electrode. This has been attributed to activation of the V-ATPase generating a more negative membrane potential outside the vesicles, resulting in enhanced migration of acidic vesicles, including tonoplast, to the anode (Morré, D. J.; Lawrence, J.; Safranski, K.; Hammond, T.; Morré, D. M. J. Chromatogr., A 1994, 668, 201-213). We confirm that ATP does induce a redistribution of membranes during FFZE of microsomal membranes isolated from several plant species, including Arabidopsis thaliana, Thellungiella halophila, Mesembryanthemum crystallinum, and Ananas comosus. However, we demonstrate, using V-ATPase-specific inhibitors, nonhydrolyzable ATP analogs, and ionophores to dissipate membrane potential, that the ATP-dependent migrational shift of membranes under FFZE is not due to activation of the V-ATPase. Addition of EDTA to chelate Mg2+, leading to the production of the tetravalent anionic form of ATP, resulted in a further enhancement of membrane migration toward the anode, and manipulation of cell surface charge by addition of polycations also influenced the ATP-dependent migration of membranes. We propose that ATP enhances the mobility of endomembranes by screening positive surface charges on the membrane surface.

Dual control of cardiac Na+ Ca2+ exchange by PIP(2): electrophysiological analysis of direct and indirect mechanisms

Cardiac Na(+)-Ca(2+) exchange (NCX1) inactivates in excised membrane patches when cytoplasmic Ca(2+) is removed or cytoplasmic Na(+) is increased. Exogenous phosphatidylinositol-4,5-bis-phosphate (PIP(2)) can ablate both inactivation mechanisms, while it has no effect on inward exchange current in the absence of cytoplasmic Na(+). To probe PIP(2) effects in intact cells, we manipulated PIP(2) metabolism by several means. First, we used cell lines with M1 (muscarinic) receptors that couple to phospholipase C's (PLCs). As expected, outward NCX1 current (i.e. Ca(2+) influx) can be strongly inhibited when M1 agonists induce PIP(2) depletion. However, inward currents (i.e. Ca(2+) extrusion) without cytoplasmic Na(+) can be increased markedly in parallel with an increase of cell capacitance (i.e. membrane area). Similar effects are incurred by cytoplasmic perfusion of GTPgammaS or the actin cytoskeleton disruptor latrunculin, even in the presence of non-hydrolysable ATP (AMP-PNP). Thus, G-protein signalling may increase NCX1 currents by destabilizing membrane cytoskeleton-PIP(2) interactions. Second, to increase PIP(2) we directly perfused PIP(2) into cells. Outward NCX1 currents increase as expected. But over minutes currents decline substantially, and cell capacitance usually decreases in parallel. Third, using BHK cells with stable NCX1 expression, we increased PIP(2) by transient expression of a phosphatidylinositol-4-phosphate-5-kinase (hPIP5KIbeta) and a PI4-kinase (PI4KIIalpha). NCX1 current densities were decreased by > 80 and 40%, respectively. Fourth, we generated transgenic mice with 10-fold cardiac-specific overexpression of PI4KIIalpha. This wortmannin-insensitive PI4KIIalpha was chosen because basal cardiac phosphoinositides are nearly insensitive to wortmannin, and surface membrane PI4-kinase activity, defined functionally in excised patches, is not blocked by wortmannin. Both phosphatidylinositol-4-phosphate (PIP) and PIP(2) were increased significantly, while NCX1 current densities were decreased by 78% with no loss of NCX1 expression. Most mice developed cardiac hypertrophy, and immunohistochemical analysis suggests that NCX1 is redistributed away from the outer sarcolemma. Cholera toxin uptake was increased 3-fold, suggesting that clathrin-independent endocytosis is enhanced. We conclude that direct effects of PIP(2) to activate NCX1 can be strongly modulated by opposing mechanisms in intact cells that probably involve membrane cytoskeleton remodelling and membrane trafficking.

On the physiological roles of PIP(2) at cardiac Na+ Ca2+ exchangers and K(ATP) channels: a long journey from membrane biophysics into cell biology

Over the last 10 years we have tried to understand the roles of PIP(2) in regulating cardiac Na(+)-Ca(2+) exchangers and K(ATP) K(+) channels, both of which are directly activated by PIP(2). Up to now, the idea that hormones might physiologically regulate these mechanisms by causing changes of PIP(2) concentrations in the cardiac sarcolemma, either locally or globally, is not well supported. In intact myocardium, but not excised patches, phosphatidylinositol 4-phosphate 5-kinase (PIP5K) activity appears to be Ca(2+) activated and dependent on cardiac activity. Potentially therefore the primary second messenger of the heart, cytoplasmic Ca(2+), may regulate PIP(2) and therewith numerous cardiac membrane processes. In general, however, PIP(2) may simply serve to strongly activate various cardiac channels and transporters when they are inserted in the sarcolemma, while a lack of PIP(2) on internal membranes maintains transporters and channels inactive during trafficking and processing. As in most, if not all, strong regulatory systems of cells, the activating effects of PIP(2) can apparently be countered by strong inactivation mechanisms. In this context, our recent work suggests that internalization of cardiac Na(+)-Ca(2+) exchangers is promoted by increased PIP(2) synthesis, especially in combination with other cell signals. Assuming that multiple adapter-PIP(2) interactions are necessary to initiate the budding of individual membrane vesicles, the dependence of endocytosis on PIP(2) in the surface membrane can potentially be a very steep function. Thus, a better understanding of the regulation of cardiac lipid kinases may be key to understanding when and how cardiac ion transporters and channels are internalized.

New Modes of Exchanger Regulation: Physiological Implications

Exchange activity is regulated principally by cytosolic Na+, Ca2+, and PIP2. However, the properties of these modes of regulation that have emerged from excised patch studies appear to be poorly suited to regulating exchange activity on a beat-to-beat basis. Here we summarize recent findings from our lab indicating that (a) allosteric activation by Ca2+ exhibits hysteresis, (b) elevated concentrations of cytosolic Na+ induce a mode of activity that no longer requires regulatory Ca2+ activation, and (c) the requirement for PIP2 is reduced or eliminated after allosteric Ca2+ activation. Our results suggest that exchange activity in cardiac myocytes may be regulated by the time-integral of Ca2+ transients occurring over multiple beats.

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.

Phospholipase C in living cells: activation, inhibition, Ca2+ requirement, and regulation of M current

We have further tested the hypothesis that receptor-mediated modulation of KCNQ channels involves depletion of phosphatidylinositol 4,5-bisphosphate (PIP2) by phosphoinositide-specific phospholipase C (PLC). We used four parallel assays to characterize the agonist-induced PLC response of cells (tsA or CHO cells) expressing M1 muscarinic receptors: translocation of two fluorescent probes for membrane lipids, release of calcium from intracellular stores, and chemical measurement of acidic lipids. Occupation of M1 receptors activates PLC and consumes cellular PIP2 in less than a minute and also partially depletes mono- and unphosphorylated phosphoinositides. KCNQ current is simultaneously suppressed. Two inhibitors of PLC, U73122 and edelfosine (ET-18-OCH3), can block the muscarinic actions completely, including suppression of KCNQ current. However, U73122 also had many side effects that were attributable to alkylation of various proteins. These were mimicked or occluded by prior reaction with the alkylating agent N-ethylmaleimide and included block of pertussis toxin-sensitive G proteins and effects that resembled a weak activation of PLC or an inhibition of lipid kinases. By our functional criteria, the putative PLC activator m-3M3FBS did stimulate PLC, but with a delay and an irregular time course. It also suppressed KCNQ current. The M1 receptor-mediated activation of PLC and suppression of KCNQ current were stopped by lowering intracellular calcium well below resting levels and were slowed by not allowing intracellular calcium to rise in response to PLC activation. Thus calcium release induced by PLC activation feeds back immediately on PLC, accelerating it during muscarinic stimulation in strong positive feedback. These experiments clarify important properties of receptor-coupled PLC responses and their inhibition in the context of the living cell. In each test, the suppression of KCNQ current closely paralleled the expected fall of PIP2. The results are described by a kinetic model.

Mechanisms of Na+-Ca2+ exchange inhibition by amphiphiles in cardiac myocytes: importance of transbilayer movement

The membrane lipid environment and lipid signaling pathways are potentially involved in the modulation of the activity of the cardiac Na(+)-Ca(2+) exchanger (NCX). In the present study biophysical mechanisms of interactions of amphiphiles with the NCX and the functional consequences were examined. For this purpose, intracellular Ca(2+) concentration jumps were generated by laser-flash photolysis of caged Ca(2+) in guinea-pig ventricular myocytes and Na(+)-Ca(2+) exchange currents ( I(Na/Ca)) were recorded in the whole-cell configuration of the patch-clamp technique. The inhibitory effect of amphiphiles increased with the length of the aliphatic chain between C(7) and C(10) and was more potent with cationic or anionic head groups than with uncharged head groups. Long-chain cationic amines (C(12)) exhibited a cut-off in their efficacy in I(Na/Ca) inhibition. Analysis of the time-course, comparison with the Ni(2+)-induced I(Na/Ca) block and confocal laser scanning microscopy experiments with fluorescent lipid analogs (C(6)- and C(12)-NBD-labeled analogs) suggested that amphiphiles need to be incorporated into the membrane. Furthermore, NCX block appears to require transbilayer movement of the amphiphile to the inner leaflet ("flip"). We conclude that both, hydrophobic and electrostatic interactions between the lipids and the NCX may be important factors for the modulation by lipids and could be relevant in cardiac diseases where the lipid metabolism is altered.

Sensitivity of oocyte-expressed epithelial Na+ channel to glibenclamide

The effect of glibenclamide on heterologously expressed amiloride-sensitive sodium channels (ENaCs) was investigated in Xenopus oocytes. The ENaC is a heteromer and consists of alpha-, beta- and gamma-subunits and the alpha- and beta-subunits have previously been shown to confer sensitivity to glibenclamide. We coexpressed either colonic rat alpha- (ralpha) or guinea-pig alpha-subunit (gpalpha) with Xenopus betagamma-subunits. The gpalphaxbetagamma was significantly stimulated by glibenclamide (100 microM) (184+/-15%), whereas the ralpha-combination was slightly down-regulated by the sulfonylurea (79+/-4%). The stimulating effect did not interfere with Na(+)-self-inhibition resulting from intracellular accumulation of Na(+)-ions. We exchanged cytosolic termini between both orthologs but the gpalpha-chimera with the termini from rat retained sensitivity to glibenclamide. The effect of glibenclamide on Xenopus ENaC (xENaC) was inhibited by ADP-beta-S but not by ATP-gamma-S, when applied intracellularly. Intracellular loading with Na(+)-ions after inhibition of Na(+)/K(+)-ATPases with ouabain prevented an up-regulation of ENaC activity by glibenclamide. Pretreatment of oocytes expressing xENaC with edelfosine (ET-18-OCH(3)) slightly reduced stimulation of I(ami) (118+/-12%; control: 132+/-9%) while phosphatidylinositol-4,5-biphosphate (PIP(2)) significantly reduced the effect of glibenclamide to 101+/-3%.

Tissue-specific modes of Na/Ca exchanger regulation

Ionic regulation of Na(+)/Ca(2+) exchange describes the secondary modulating effects exerted on exchange activity by the transport substrates Na(+) and Ca(2+). These effects have been extensively characterized for the cardiac Na(+)/Ca(2+) exchanger, NCX1.1, primarily by the giant excised patch-clamp technique. Moreover, several studies have provided functional evidence for ionic regulation of Na(+)/Ca(2+) exchange activity in intact cellular systems. Through structure-function analyses, important protein domains involved in these regulatory processes have been identified. However, despite major progress in characterizing ionic regulation at the functional and molecular levels, the physiological importance of these processes remains unknown. In this study, we have examined Na(+)/Ca(2+) exchange activity for three members of the NCX1 family, namely NCX1.1, NCX1.3, and NCX1.4. These exchangers were expressed in Xenopus laevis oocytes and were characterized using the giant excised patch-clamp technique. We show that these three splice variants exhibit considerable differences in the kinetic features of their ionic regulatory profiles. Information of this type is beginning to provide insight into the physiological basis for tissue-specific expression of alternatively spliced Na(+)/Ca(2+) exchangers.

The complex and intriguing lives of PIP2 with ion channels and transporters

Phosphatidylinositol-4,5-bisphosphate (PIP(2)), the precursor of several signaling molecules in eukayotic cells, is itself also used by cells to signal to membrane-associated proteins. PIP(2) anchors numerous signaling molecules and cytoskeleton at the cell membrane, and the metabolism of PIP(2) is closely connected to membrane trafficking. Recently, ion transporters and channels have been discovered to be regulated by PIP(2). Systems reported to be activated by PIP(2) include (i) plasmalemmal calcium pumps (PMCA), (ii) cardiac sodium-calcium exchangers (NCX1), (iii) sodium-proton exchangers (NHE1-4), (iv) a sodium-magnesium exchanger of unknown identity, (v) all inward rectifier potassium channels (KATP, IRK, GIRK, and ROMK channels), (vi) epithelial sodium channels (ENaC), and (vii) ryanodine-sensitive calcium release channels (RyR). Systems reported to be inhibited by PIP(2) include (i) cyclic nucleotide-gated channels of the rod (CNG), (ii) transient receptor potential-like (TRPL) Drosophila phototransduction channels, (iii) capsaicin-activated transient receptor potential (TRP) channels (VR1), and (iv) IP(3)-gated calcium release channels (IP3R). Systems that appear to be completely insensitive to PIP(2) include (i) voltage-gated sodium channels, (ii) most voltage-gated potassium channels, (iii) sodium-potassium pumps, (iv) several neurotransmitter transporters, and (v) cystic fibrosis transmembrane receptor (CFTR)-type chloride channels. Presumably, local changes of the concentration of PIP(2) in the plasma membrane represent cell signals to those mechanisms sensitive to PIP(2) changes. Unfortunately, our understanding of how local PIP(2) concentrations are regulated remains very limited. One important complexity is the probable existence of phospholipid microdomains, or lipid rafts. Such domains may serve to localize PIP(2) and thereby PIP(2) signaling, as well as to organize PIP(2) binding partners into signaling complexes. A related biological role of PIP(2) may be to control the activity of ion transporters and channels during biosynthesis or vesicle trafficking. Low PIP(2) concentrations in the secretory pathway would inactivate all of the systems that are stimulated by PIP(2). How, in detail, is PIP(2) used by cells to control ion channel and transporter activities? Further progress requires an improved understanding of lipid kinases and phosphatases, how they are regulated, where they are localized in cells, and with which ion channels and transporters they might localize.

Time course of myocardial sodium accumulation after burn trauma: a (31)P- and (23)Na-NMR study

In this study, (23)Na- and (31)P- nuclear magnetic resonance (NMR) spectra were examined in perfused rat hearts harvested 1, 2, 4, and 24 h after 40% total body surface area burn trauma and lactated Ringer resuscitation, 4 ml. kg(-1). %(-1) burn. (23)Na-NMR spectroscopy monitored myocardial intracellular Na+ using the paramagnetic shift reagent thulium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylenephosphonic acid). Left ventricular function, cardiac high-energy phosphates (ATP/PCr), and myocyte intracellular pH were studied by using (31)P NMR spectroscopy to examine the hypothesis that burn-mediated acidification of cardiomyocytes contributes to subsequent Na+ accumulation by this cell population. Intracellular Na+ accumulation was confirmed by sodium-binding benzofuran isophthalate loading and fluorescence spectroscopy in cardiomyocytes isolated 1, 2, 4, 8, 12, 18, and 24 h postburn. This myocyte Na+ accumulation as early as 2 h postburn occurred despite no changes in cardiac ATP/PCr and intracellular pH. Left ventricular function progressively decreased after burn trauma. Cardiomyocyte Na+ accumulation paralleled cardiac contractile dysfunction, suggesting that myocardial Na+ overload contributes, in part, to the progressive postburn decrease in ventricular performance.

The importance of glycolytically-derived ATP for the Na+/H+ exchange activity in guinea pig ventricular myocytes

The cardiac subtype of Na+/H+ exchanger (NHE-1) plays an important role in the regulation of intracellular pH (pHi) and also can be a major route for Na+ influx. Although intracellular ATP is required for the optimal function of NHE-1, the regulation of the exchanger by ATP is less well characterized. This study was designed to investigate which intracellular ATP generated by oxidative phosphorylation or by glycolysis is dominant for the activation of NHE-1 in intact cardiac myocytes. Isolated guinea pig ventricular myocytes were loaded with the pHi-sensitive fluorescent indicator, 2'-7'-bis(carboxyl)-5',6'-carboxy fluorescein (BCECF), and exposed to 20 mM 2-deoxyglucose (2-DG) or 2 mM sodium cyanide (CN) to inhibit glycolysis or oxidative phosphorylation, respectively. The activity of NHE-1 was estimated with pHi recovery following transient application of 15 mM NH4Cl (NH4Cl prepulse). After the NH4Cl prepulse, pHi decreased from 7.00 +/- 0.03 (mean +/- S.E.) to 6.60 +/- 0.06 and recovered to 6.94 +/- 0.13 at 10 min (n = 7). The pHi recovery was suppressed in the presence of 2-DG (6.67 +/- 0.05, p < 0.01, n = 7), but was not changed in the presence of CN (6.88 +/- 0.18, n = 6). Since there was no difference in the intrinsic H+ buffering power, the estimation of the net acid efflux demonstrated that the activity of NHE-1 was significantly depressed in 2-DG. The inhibitory effect of 2-DG was not due to more extensive depletion of global intracellular ATP or secondary to the change in either intracellular Na+ or Ca2+ concentration. We concluded that ATP generated by glycolysis rather than by oxidative phosphorylation is essential to activate NHE-1 in ventricular myocytes.

Paradoxical block of the Na+-Ca2+ exchanger by extracellular protons in guinea-pig ventricular myocytes

1. The Na+-Ca2+ exchange is a major pathway for removal of cytosolic Ca2+ in cardiac myocytes. It is known to be inhibited by changes of intracellular pH that may occur, for example, during ischaemia. In the present study, we examined whether extracellular protons (pHo) can also affect the cardiac exchange. 2. Na+-Ca2+ exchange currents (INa-Ca) were recorded from single adult guinea-pig ventricular myocytes in the whole-cell voltage-clamp configuration while [Ca2+]i was simultaneously imaged with fluo-3 and a laser-scanning confocal microscope. To activate INa-Ca, intracellular Ca2+ concentration jumps were generated by laser flash photolysis of caged Ca2+ (DM-nitrophen). 3. Exposure of the cell to moderately and extremely acidic conditions (pHo 6 and 4) was accompanied by a decrease of the peak INa-Ca to 70 % and less than 10 %, respectively. The peak INa-Ca was also inhibited to about 45 % of its initial value by increasing pHo to 10. The largest INa-Ca was found at pHo approximately 7.6. 4. Simultaneous measurements of [Ca2+]i and INa-Ca during partial proton block of the Na+-Ca2+ exchanger revealed that the exchange current was more inhibited by acidic pHo than the rate of Ca2+ transport. This observation is consistent with a change in the electrogenicity of the Na+-Ca2+ exchange cycle after protonation of the transporter. 5. We conclude that both extracellular alkalinization and acidification affect the Na+-Ca2+ exchanger during changes of pHo that may be present under pathophysiological conditions. During both extreme acidification or alkalinization the Na+-Ca2+ exchanger is strongly inhibited, suggesting that extracellular protons may interact with the Na+-Ca2+ exchanger at multiple sites. In addition, the electrogenicity and stoichiometry of the Na+-Ca2+ exchange may be modified by extracellular protons.

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.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

ATP dependence is not an intrinsic property of Na+/H+ exchanger NHE1: requirement for an ancillary factor

Na+/H+ exchange is a passive process not requiring expenditure of metabolic energy. Nevertheless, depletion of cellular ATP produces a marked inhibition of the antiport. No evidence has been found for direct binding of nucleotide to exchangers or alteration in their state of phosphorylation, suggesting ancillary factors may be involved. This possibility was tested by comparing the activity of dog red blood cells (RBC) and their resealed ghosts. Immunoblotting experiments using isoform-specific polyclonal and monoclonal antibodies indicated RBC membranes express Na+/H+ exchanger isoform 1 (NHE1). In intact RBC, uptake of Na+ was greatly stimulated when the cytosol was acidified. The stimulated uptake was largely eliminated by amiloride and by submicromolar concentrations of the benzoyl guanidinium compound HOE-694, consistent with mediation by NHE1. Although exchange activity could also be elicited by acidification in resealed ghosts containing ATP, the absolute rate of transport was markedly diminished at comparable pH. Dissipation of the pH gradient was ruled out as the cause of diminished transport rate in ghosts. This was accomplished by a "pH clamping" procedure based on continued export of base equivalents by the endogenous anion exchanger. These observations suggest a critical factor required to maintain optimal Na+/H+ exchange activity is lost or inactivated during preparation of ghosts. Depletion of ATP, achieved by incubation with 2-deoxy-D-glucose, inhibited Na+/H+ exchange in intact RBC, as reported for nucleated cells. In contrast, the rate of exchange was similar in control and ATP-depleted resealed ghosts. Interestingly, the residual rate of Na+/H+ exchange in ATP-depleted but otherwise intact cells was similar to the transport rate of ghosts. Therefore, we tentatively conclude that full activation of NHE1 requires both ATP and an additional regulatory factor, which may mediate the action of the nucleotide. Ancillary phosphoproteins or phospholipids or the kinases that mediate their phosphorylation are likely candidates for the regulatory factor(s) that is inactivated or missing in ghosts.

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.

Cloning, expression, and characterization of the squid Na+-Ca2+ exchanger (NCX-SQ1)

We have cloned the squid neuronal Na+-Ca2+ exchanger, NCX-SQ1, expressed it in Xenopus oocytes, and characterized its regulatory and ion transport properties in giant excised membrane patches. The squid exchanger shows 58% identity with the canine Na+-Ca2+ exchanger (NCX1.1). Regions determined to be of functional importance in NCX1 are well conserved. Unique among exchanger sequences to date, NCX-SQ1 has a potential protein kinase C phosphorylation site (threonine 184) between transmembrane segments 3 and 4 and a tyrosine kinase site in the Ca2+ binding region (tyrosine 462). There is a deletion of 47 amino acids in the large intracellular loop of NCX-SQ1 in comparison with NCX1. Similar to NCX1, expression of NCX-SQ1 in Xenopus oocytes induced cytoplasmic Na+-dependent 45Ca2+ uptake; the uptake was inhibited by injection of Ca2+ chelators. In giant excised membrane patches, the NCX-SQ1 outward exchange current showed Na+-dependent inactivation, secondary activation by cytoplasmic Ca2+, and activation by chymotrypsin. The NCX-SQ1 exchange current was strongly stimulated by both ATP and the ATP-thioester, ATP gamma S, in the presence of F- (0.2 mM) and vanadate (50 microM), and both effects reversed on application of a phosphatidylinositol-4',5'-bisphosphate antibody. NCX1 current was stimulated by ATP, but not by ATP gamma S. Like NCX1 current, NCX-SQ1 current was strongly stimulated by phosphatidylinositol-4',5'-bisphosphate liposomes. In contrast to results in squid axon, NCX-SQ1 was not stimulated by phosphoarginine (5-10 mM). After chymotrypsin treatment, both the outward and inward NCX-SQ1 exchange currents were more strongly voltage dependent than NCX1 currents. Ion concentration jump experiments were performed to estimate the relative electrogenicity of Na+ and Ca2+ transport reactions. Outward current transients associated with Na+ extrusion were much smaller for NCX-SQ1 than NCX1, and inward current transients associated with Ca2+ extrusion were much larger. For NCX-SQ1, charge movements of Ca2+ transport could be defined in voltage jump experiments with a low cytoplasmic Ca2+ (2 microM) in the presence of high extracellular Ca2+ (4 mM). The rates of charge movements showed "U"-shaped dependence on voltage, and the slopes of both charge-voltage and rate-voltage relations (1,600 s-1 at 0 mV) indicated an apparent valency of -0.6 charges for the underlying reaction. Evidently, more negative charge moves into the membrane field in NCX-SQ1 than in NCX1 when ions are occluded into binding sites.

Na+/Ca2+ exchange and cellular Ca2+ homeostasis

The Na+/Ca2+ exchange system is the primary Ca2+ efflux mechanism in cardiac myocytes, and plays an important role in controlling the force of cardiac contraction. The exchanger protein contains 11 transmembrane segments plus a large hydrophilic domain between the 5th and 6th transmembrane segments; the transmembrane regions are responsible for mediating ion translocation while the hydrophilic domain is responsible for regulation of activity. Exchange activity is regulated in vitro by interconversions between an active state and either of two inactive states. High concentrations of cytosolic Na+ or the absence of cytosolic Ca2+ promote the formation of the inactive states; phosphatidylinositol-(4,5)bisphosphate (or other negatively charged phospholipids) and cytosolic Ca2+ counteract the inactivation process. The importance of these mechanisms in regulating exchange activity under normal physiological conditions is uncertain. Exchanger function is also dependent upon cytoskeletal interactions, and the exchanger's location with respect to intracellular Ca2+-sequestering organelles. An understanding of the exchanger's function in normal cell physiology will require more detailed information on the proximity of the exchanger and other Ca2+-transporting proteins, their interactions with the cytoskeleton, and local concentrations of anionic phospholipids and transported ions.

Sprint training attenuates myocyte hypertrophy and improves Ca2+ homeostasis in postinfarction myocytes

Myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) had decreased Na+/Ca2+ exchange currents (I Na/Ca; 3 Na+ out:1 Ca2+ in) and sarcoplasmic reticulum (SR)-releasable Ca2+ contents. These defects in Ca2+ regulation may contribute to abnormal contractility in MI myocytes. Because exercise training elicits positive adaptations in cardiac contractile function and myocardial Ca2+ regulation, the present study examined whether 6-8 wk of high-intensity sprint training (HIST) would ameliorate some of the cellular maladaptations observed in post-MI rats with limited exercise activity (Sed). In MI rats, HIST did not affect citrate synthase activities of plantaris muscles but significantly increased the percentage of cardiac alpha-myosin heavy chain (MHC) isoforms (57.2 +/- 1.9 vs. 49.3 +/- 3.5 in MI-HIST vs. MI-Sed, respectively; P < or = 0.05). At the single myocyte level, HIST attenuated cellular hypertrophy observed post-MI, as evidenced by reductions in cell lengths (112 +/- 4 vs. 130 +/- 5 micrograms in MI-HIST vs. MI-Sed, respectively; P < or = 0.005) and cell capacitances (212 +/- 8 vs. 242 +/- 9 pF in MI-HIST vs. MI-Sed, respectively; P < or = 0.015). Reverse I Na/Ca was significantly lower (P < or = 0.0001) in myocytes from MI-Sed rats compared with those from rats that were sham operated and sedentary. HIST significantly increased reverse I Na/Ca (P < or = 0.05) without affecting the amount of Na+/Ca2+ exchangers (detected by immunoblotting) in MI myocytes. SR-releasable Ca2+ content, as estimated by integrating forward I Na/Ca during caffeine-induced SR Ca2+ release, was also significantly increased (P < or = 0.02) by HIST in MI myocytes. We conclude that the enhanced cardiac output and stroke volume in post-MI rats subjected to HIST are mediated, at least in part, by reversal of cellular maladaptations post-MI.

The selective activation of the cardiac sarcolemmal sodium-calcium exchanger by plasmalogenic phosphatidic acid produced by phospholipase D

Since plasmalogens are the predominant phospholipid of cardiac sarcolemma, the activation of the sodium-calcium exchanger by either plasmenylethanolamine or plasmalogenic phosphatidic acid generated by phospholipase D was explored. Sodium-calcium exchange activity was 7-fold greater in proteoliposomes comprised of plasmenylethanolamine compared to proteoliposomes comprised of only plasmenylcholine. Phospholipase D treatment of proteoliposomes resulted in 1 mol % conversion of plasmenylcholine or phosphatidylcholine to their respective phosphatidic acid molecular species with a concomitant 8-fold or 2-fold activation of sodium-calcium exchange activity, respectfully. Thus, phospholipase D-mediated hydrolysis of plasmalogens to phosphatidic acid may be an important mechanism for the regulation of the sodium-calcium exchanger.

ATP dependence of Na+/H+ exchange. Nucleotide specificity and assessment of the role of phospholipids

We studied the ATP dependence of NHE-1, the ubiquitous isoform of the Na+/H+ antiporter, using the whole-cell configuration of the patch-clamp technique to apply nucleotides intracellularly while measuring cytosolic pH (pHi) by microfluorimetry. Na+/H+ exchange activity was measured as the Na(+)-driven pHi recovery from an acid load, which was imposed via the patch pipette. In Chinese hamster ovary (CHO) fibroblasts stably transfected with NHE-1, omission of ATP from the pipette solution inhibited Na+/H+ exchange. Conversely, ATP perfusion restored exchange activity in cells that had been metabolically depleted by 2-deoxy-D-glucose and oligomycin. In cells dialyzed in the presence of ATP, no "run-down" was observed even after extended periods, suggesting that the nucleotide is the only diffusible factor required for optimal NHE-1 activity. Half-maximal activation of the antiporter was obtained at approximately 5 mM Mg-ATP. Submillimolar concentrations failed to sustain Na+/H+ exchange even when an ATP regenerating system was included in the pipette solution. High ATP concentrations are also known to be required for the optimal function of other cation exchangers. In the case of the Na/Ca2+ exchanger, this requirement has been attributed to an aminophospholipid translocase, or "flippase.". The involvement of this enzyme in Na+/H+ exchange was examined using fluorescent phosphatidylserine, which is actively translocated by the flippase. ATP depletion decreased the transmembrane uptake of NBD-labeled phosphatidylserine (NBD-PS), indicating that the flippase was inhibited. Diamide, an agent reported to block the flippase, was as potent as ATP depletion in reducing NBD-PS uptake. However, diamide had no effect on Na+/H+ exchange, implying that the effect of ATP is not mediated by changes in lipid distribution across the plasma membrane. K-ATP and ATP gamma S were as efficient as Mg-ATP in sustaining NHE-1 activity, while AMP-PNP and AMP-PCP only partially substituted for ATP. In contrast, GTP gamma S was ineffective. We conclude that ATP is the only soluble factor necessary for optimal activity of the NHE-1 isoform of the antiporter. Mg2+ does not appear to be essential for the stimulatory effect of ATP. We propose that two mechanisms mediate the activation of the antiporter by ATP: one requires hydrolysis and is likely an energy-dependent event. The second process does not involve hydrolysis of the gamma-phosphate, excluding mediation by protein or lipid kinases. We suggest that this effect is due to binding of ATP to an as yet unidentified, nondiffusible effector that activates the antiporter.

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.

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

Regulation of cardiac Na+,Ca2+ exchange and KATP potassium channels by PIP2

Cardiac Na+,Ca2+ exchange is activated by a mechanism that requires hydrolysis of adenosine triphosphate (ATP) but is not mediated by protein kinases. In giant cardiac membrane patches, ATP acted to generate phosphatidylinositol-4,5-bisphosphate (PIP2) from phosphatidylinositol (PI). The action of ATP was abolished by a PI-specific phospholipase C (PLC) and recovered after addition of exogenous PI; it was reversed by a PIP2-specific PLC; and it was mimicked by exogenous PIP2. High concentrations of free Ca2+ (5 to 20 microM) accelerated reversal of the ATP effect, and PLC activity in myocyte membranes was activated with a similar Ca2+ dependence. Aluminum reversed the ATP effect by binding with high affinity to PIP2. ATP-inhibited potassium channels (KATP) were also sensitive to PIP2, whereas Na+,K+ pumps and Na+ channels were not. Thus, PIP2 may be an important regulator of both ion transporters and channels.

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.

Mutation of amino acid residues in the putative transmembrane segments of the cardiac sarcolemmal Na+-Ca2+ exchanger

We have examined the role of conserved regions and acidic or basic residues located in the putative transmembrane segments of the cardiac sarcolemmal Na+-Ca2+ exchanger by site-directed mutagenesis. The alpha-1 and alpha-2 repeats are transmembrane regions of internal similarity, which are highly conserved among Na+-Ca2+ exchangers. We find that Na+-Ca2+ exchange activity is highly sensitive to mutagenesis in the alpha-repeats. Mutation at residues Ser-109, Ser-110, Glu-113, Ser-139, Asn-143, Thr-810, Ser-811, Asp-814, Ser-818, or Ser-838 resulted in loss of exchanger activity. Mutation at residues Thr-103, Gly-108, Pro-112, Glu-120, Gly-138, Gly-809, Gly-837, and Asn-842 resulted in reduced exchanger activity, and altered current-voltage relationships were observed with mutations at residues Gly-138 and Gly-837. Only mutation at residue Ser-117 appeared to leave exchanger activity unaffected. Thus, the alpha-repeats appear to be important components for ion binding and translocation. Another region implicated in exchanger function is a region of similarity to the Na+,K+ pump (Nicoll, D. A., Longoni, S., Philipson, K. D. (1990) Science 250, 562-565). Mutations at two residues in the pump-like region, Glu-199 and Thr-203, resulted in nonfunctional exchangers, while mutation at two other residues, Glu-196 and Gly-200, had no effect. The role of acidic and basic residues in the transmembrane segments was also examined. Mutation of several basic residues (Arg-42, His-744, Lys-751, Lys-797, and His-858) did not affect exchange activity. Of the acidic residues located outside of the alpha-repeat and pump-like regions (Asp-740, Asp-785, and Asp-798), only mutation at Asp-785 resulted in reduction of exchanger activity.

Regulation of cardiac sodium-calcium exchanger by beta-adrenergic agonists

Na+-Ca2+ exchanger and Ca2+ channel are two major sarcolemmal Ca2+-transporting proteins of cardiac myocytes. Although the Ca2+ channel is effectively regulated by protein kinase A-dependent phosphorylation, no enzymatic regulation of the exchanger protein has been identified as yet. Here we report that in frog ventricular myocytes, isoproterenol down-regulates the Na+-Ca2+ exchanger, independent of intracellular Ca2+ and membrane potential, by activation of the beta-receptor/adenylate-cyclase/cAMP-dependent cascade, resulting in suppression of transmembrane Ca2+ transport via the exchanger and providing for the well-documented contracture-suppressant effect of the hormone on frog heart. The beta-blocker propranolol blocks the isoproterenol effect, whereas forskolin, cAMP, and theophylline mimic it. In the frog heart where contractile Ca2+ is transported primarily by the Na+-Ca2+ exchanger, the beta-agonists' simultaneous enhancement of Ca2+ current, ICa, and suppression of Na+-Ca2+ exchanger current, INa-Ca would enable the myocyte to develop force rapidly at the onset of depolarization (enhancement of ICa) and to decrease Ca2+ influx (suppression of INa-Ca) later in the action potential. This unique adrenergically induced shift in the Ca2+ influx pathways may have evolved in response to paucity of the sarcoplasmic reticulum Ca2+-ATPase/phospholamban complex and absence of significant intracellular Ca2+ release pools in the frog heart.

Inner sarcolemmal leaflet Ca(2+) binding: its role in cardiac Na/Ca exchange

A recently completed model of Ca concentration and movements in the cardiac cell diadic cleft space predicts that removal or neutralization of inner sarcolemmal (SL) leaflet anionic Ca-binding sites at the sarcolemmal border of this space will greatly diminish Na/Ca exchange-mediated Ca efflux. The present study tests this prediction using the local anesthetic dibucaine as a probe. It is shown, in isolated SL, that dibucaine competitively displaces Ca specifically from anionic phospholipid headgroups. Dibucaine also displaces Ca from the SL when applied to intact cells. It does not affect the content or release of Ca from sarcoplasmic reticulum (SR) in these cells. This eliminates a primary effect on SR Ca as a contributing factor to dibucaine's effect on Na/Ca exchange-mediated Ca efflux. Measurement of this efflux from whole cells shows a highly significant reduction of 58% (p < 0.001) by 0.5 mM dibucaine. The inhibiting effect of dibucaine on Na/Ca exchange-mediated Ca efflux can be significantly reversed by augmentation of Ca release from SR by caffeine at the time of activation of Na/Ca exchange. This supports the contention that the dibucaine-SL interaction is a competitive one vis-a-vis Ca. The results are supportive of the model in which inner SL leaflet Ca-binding sites account for the delay of Ca diffusion from the diadic cleft, thereby prolonging the time for which [Ca] remains elevated in the cleft. The prolonged increased [Ca] significantly enhances the ability of Na/Ca exchange to remove Ca from the cell during the excitation-contraction cycle.