Molecular operations of the sodium-calcium exchanger revealed by conformation currents

Structural dynamics of Na+ and Ca2+ interactions with full-size mammalian NCX

Cytosolic Ca2+ and Na+ allosterically regulate Na+/Ca2+ exchanger (NCX) proteins to vary the NCX-mediated Ca2+ entry/exit rates in diverse cell types. To resolve the structure-based dynamic mechanisms underlying the ion-dependent allosteric regulation in mammalian NCXs, we analyze the apo, Ca2+, and Na+-bound species of the brain NCX1.4 variant using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and molecular dynamics (MD) simulations. Ca2+ binding to the cytosolic regulatory domains (CBD1 and CBD2) rigidifies the intracellular regulatory loop (5L6) and promotes its interaction with the membrane domains. Either Na+ or Ca2+ stabilizes the intracellular portions of transmembrane helices TM3, TM4, TM9, TM10, and their connecting loops (3L4 and 9L10), thereby exposing previously unappreciated regulatory sites. Ca2+ or Na+ also rigidifies the palmitoylation domain (TMH2), and neighboring TM1/TM6 bundle, thereby uncovering a structural entity for modulating the ion transport rates. The present analysis provides new structure-dynamic clues underlying the regulatory diversity among tissue-specific NCX variants.

Conformational free-energy landscapes of a Na+/Ca2+ exchanger explain its alternating-access mechanism and functional specificity

Secondary-active transporters catalyze the movement of myriad substances across all cellular membranes, typically against opposing concentration gradients, and without consuming any ATP. To do so, these proteins employ an intriguing structural mechanism evolved to be activated only upon recognition or release of the transported species. We examine this self-regulated mechanism using a homolog of the cardiac Na+/Ca2+ exchanger as a model system. Using advanced computer simulations, we map out the complete functional cycle of this transporter, including unknown conformations that we validate against existing experimental data. Calculated free-energy landscapes reveal why this transporter functions as an antiporter rather than a symporter, why it specifically exchanges Na+ and Ca2+, and why the stoichiometry of this exchange is exactly 3:1. We also rationalize why the protein does not exchange H+ for either Ca2+ or Na+, despite being able to bind H+ and its high similarity with H+/Ca2+ exchangers. Interestingly, the nature of this transporter is not explained by its primary structural states, known as inward- and outward-open conformations; instead, the defining factor is the feasibility of conformational intermediates between those states, wherein access pathways leading to the substrate binding sites become simultaneously occluded from both sides of the membrane. This analysis offers a physically coherent, broadly transferable route to understand the emergence of function from structure among secondary-active membrane transporters.

Neuronal and astrocyte NCX isoform/splice variants: How do they participate in Na+ and Ca2+ signalling?

NCX1, NCX2, and NCX3 gene isoforms and their splice variants are characteristically expressed in different regions of the brain. The tissue-specific splice variants of NCX1-3 isoforms show specific expression profiles in neurons and astrocytes, whereas the relevant NCX isoform/splice variants exhibit diverse allosteric modes of Na+- and Ca2+-dependent regulation. In general, overexpression of NCX1-3 genes leads to neuroprotective effects, whereas their ablation gains the opposite results. At this end, the partial contributions of NCX isoform/splice variants to neuroprotective effects remain unresolved. The glutamate-dependent Na+ entry generates Na+ transients (in response to neuronal cell activities), whereas the Na+-driven Ca2+ entry (through the reverse NCX mode) raises Ca2+ transients. This special mode of signal coupling translates Na+ transients into the Ca2+ signals while being a part of synaptic neurotransmission. This mechanism is of general interest since disease-related conditions (ischemia, metabolic stress, and stroke among many others) trigger Na+ and Ca2+ overload with deadly outcomes of downstream apoptosis and excitotoxicity. The recently discovered mechanisms of NCX allosteric regulation indicate that some NCX variants might play a critical role in the dynamic coupling of Na+-driven Ca2+ entry. In contrast, the others are less important or even could be dangerous under altered conditions (e.g., metabolic stress). This working hypothesis can be tested by applying advanced experimental approaches and highly focused computational simulations. This may allow the development of structure-based blockers/activators that can selectively modulate predefined NCX variants to lessen the life-threatening outcomes of excitotoxicity, ischemia, apoptosis, metabolic deprivation, brain injury, and stroke.

The role of the Na+/Ca2+-exchanger (NCX) in cancer-associated fibroblasts

Cancer is characterized by uncontrolled growth, invasion, and metastasis. In addition to solid cancer cells, cancer-associated fibroblasts (CAFs) play important roles in cancer pathophysiology. They arise from "healthy" cells but get manipulated by solid cancer cells to supply them and develop a tumor microenvironment (TME) that protects the cancer cells from the immune defense. A wide variety of cell types can differentiate into CAFs, including fibroblasts, endothelial cells, and epithelial cells. Precise Ca²⁺ regulation is essential for each cell including CAFs. The electrogenic Na⁺/Ca²⁺ exchanger (NCX) is one of the ubiquitously expressed regulatory Ca²⁺ transport proteins that rapidly responds to changes of the intracellular ion concentrations. Its transport function is also influenced by the membrane potential and thereby indirectly by the activity of ion channels. NCX transports Ca²⁺ out of the cell (forward mode) or allows its influx (reverse mode), always in exchange for 3 Na⁺ ions that are moved into the opposite direction. In this review, we discuss the functional roles NCX has in CAFs and how these depend on the properties of the TME. NCX activity modifies migration and leads to a reduced proliferation and apoptosis. The effect of the NCX in fibrosis is still largely unknown.

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.

Developmental differences in myocardial transmembrane Na+ transport: Implications for excitability and Na+ handling

KEY POINTS

Previous studies showed that myocardial preparations from immature rats are less sensitive to electrical field stimulation than adult preparations. Freshly-isolated ventricular myocytes from neonatal rats showed lower excitability than adult cells, e.g., less negative threshold membrane potential and greater membrane depolarization required for action potential triggering. In addition to differences in mRNA levels for Na⁺ channels isoforms and greater Na⁺ current (I_Na ) density, Na⁺ channel voltage-dependence was shifted to the right in immature myocytes, which seems to be sufficient to decrease excitability, according to computer simulations. Only in neonatal myocytes did cyclic activity promote marked cytosolic Na⁺ accumulation, which was prevented by abolition of systolic Ca²⁺ transients by blockade of Ca²⁺ currents. Developmental changes in I_Na may account for the difference in action potential initiation parameters, but not for cytosolic Na⁺ accumulation, which seems to be due mainly to Na⁺ /Ca²⁺ exchanger-mediated Na⁺ influx.

ABSTRACT

Little is currently known about possible developmental changes in myocardial Na⁺ handling, which may have impact on cell excitability and Ca²⁺ content. Resting intracellular Na⁺ concentration ([Na⁺ ]_i ), measured in freshly-isolated rat ventricular myocytes with CoroNa-green, was not significantly different in neonates (3-5 days old) and adults, but electrical stimulation caused marked [Na⁺ ]_i rise only in neonates. Inhibition of L-type Ca²⁺ current by CdCl₂ abolished not only systolic Ca²⁺ transients, but also activity-dependent intracellular Na⁺ accumulation in immature cells. This indicates that the main Na⁺ influx pathway during activity is the Na⁺ /Ca²⁺ exchanger, rather than voltage-dependent Na⁺ current (I_Na ), which was not affected by CdCl₂ . In immature myocytes, I_Na density was 2-fold greater, inactivation was faster, and the current peak occurred at less negative transmembrane potential (E_m ) than in adults. Na⁺ channel steady-state activation and inactivation curves in neonates showed a rightward shift, which should increase channel availability at diastolic E_m , but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulations. Ventricular mRNA levels of Na_v 1.1, Na_v 1.4 and Na_v 1.5 pore-forming isoforms were greater in neonate ventricles, while decrease was seen for the β1 subunit. Both molecular and biophysical changes in the channel profile may contribute to the differences in I_Na density and voltage-dependence, and also to the less negative threshold E_m in neonates, compared to adults. The apparently lower excitability in immature ventricle may confer protection against the development of spontaneous activity in this tissue. Abstract figure legend Little is currently known about possible developmental changes in myocardial Na⁺ transport, which may have impact on cell excitability and other physiological aspects. At the mRNA level, neonatal rat ventricle expresses a greater variety of Na⁺ channel isoforms than in adults. In immature ventricular cardiomyocytes, Na⁺ current (I_Na ) density was greater, but voltage-dependence is shifted to less negative potentials than in adults. This should increase channel availability at diastolic membrane potential, but also require greater depolarization for excitation, which was observed experimentally and reproduced in computer simulation. We also observed that electrical stimulation caused marked intracellular Na⁺ accumulation only in neonates, which was abolished when Ca²⁺ transients and the Na⁺ /Ca²⁺ exchanger (NCX) were inhibited by Cd²⁺ + Ni²⁺ . Thus, it seems that the main Na⁺ influx pathway during activity in neonates is the NCX, rather than voltage-dependent I_Na , which was not affected by these blockers. This article is protected by copyright. All rights reserved.

Characteristic attributes limiting the transport rates in NCX orthologs

The Na^+/Ca²⁺ exchangers (NCXs) modulate the Ca2+ signaling and homeostasis in health and disease. The transport cycle turnover rates (kcat) and the kcat/Km values of eukaryotic NCXs are ~10⁴-times higher than those of prokaryotic NCXs. Three ion-coordinating residues (out of twelve) differ between eukaryotic NCXs and NCX_Mj. The replacement of three ion-coordinating residues in NCX_Mj does not increase kcat, probably due to the structural rigidity of NCX_Mj. Phospholipids and cholesterol increase (up to 10-fold) the transport rates in the cardiac NCX1.1, but not in NCX_Mj. A lipid environment can partially contribute to the huge kinetic variances among NCXs.

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.

Thermal modulation of epicardial Ca2+ dynamics uncovers molecular mechanisms of Ca2+ alternans

Ca²⁺ alternans (Ca-Alts) are alternating beat-to-beat changes in the amplitude of Ca²⁺ transients that frequently occur during tachycardia, ischemia, or hypothermia that can lead to sudden cardiac death. Ca-Alts appear to result from a variation in the amount of Ca²⁺ released from the sarcoplasmic reticulum (SR) between two consecutive heartbeats. This variable Ca²⁺ release has been attributed to the alternation of the action potential duration, delay in the recovery from inactivation of RYR Ca²⁺ release channel (RYR2), or an incomplete Ca²⁺ refilling of the SR. In all three cases, the RYR2 mobilizes less Ca²⁺ from the SR in an alternating manner, thereby generating an alternating profile of the Ca²⁺ transients. We used a new experimental approach, fluorescence local field optical mapping (FLOM), to record at the epicardial layer of an intact heart with subcellular resolution. In conjunction with a local cold finger, a series of images were recorded within an area where the local cooling induced a temperature gradient. Ca-Alts were larger in colder regions and occurred without changes in action potential duration. Analysis of the change in the enthalpy and Q₁₀ of several kinetic processes defining intracellular Ca²⁺ dynamics indicated that the effects of temperature change on the relaxation of intracellular Ca²⁺ transients involved both passive and active mechanisms. The steep temperature dependency of Ca-Alts during tachycardia suggests Ca-Alts are generated by insufficient SERCA-mediated Ca²⁺ uptake into the SR. We found that Ca-Alts are heavily dependent on intra-SR Ca²⁺ and can be promoted through partial pharmacologic inhibition of SERCA2a. Finally, the FLOM experimental approach has the potential to help us understand how arrhythmogenesis correlates with the spatial distribution of metabolically impaired myocytes along the myocardium.

Proton-modulated interactions of ions with transport sites of prokaryotic and eukaryotic NCX prototypes

The cytosolic pH decline from 7.2 to 6.9 results in 90% inactivation of mammalian Na⁺/Ca²⁺ exchangers (NCXs) due to protons interactions with regulatory and transport domains ("proton block"). Remarkably, the pH titration curves of mammalian and prokaryotic NCXs significantly differ, even after excluding the allosteric effects through regulatory domains. This is fascinating since "only" three (out of twelve) ion-coordinating residues (T50S, E213D, and D240N) differ between the archaeal NCX_Mj and mammalian NCXs although they contain either three or two carboxylates, respectively. To resolve the underlying mechanisms of pH-dependent regulation, the ion-coordinating residues of NCX_Mj were mutated to imitate the ion ligation arrays of mammalian NCXs; the mutational effects were tested on the ion binding/transport by using ion-flux assays and two-dimensional infrared (2D IR) spectroscopy. Our analyses revealed that two deprotonated carboxylates ligate 3Na⁺ or 1Ca²⁺ in NCX prototypes with three or two carboxylates. The Na⁺/Ca²⁺ exchange rates of NCX_Mj reach saturation at pH 5.0, whereas the Na⁺/Ca²⁺ exchange rates of the cardiac NCX1.1 gradually increase even at alkaline pHs. The T50S replacement in NCX_Mj "recapitulates" the pH titration curves of mammalian NCX by instigating an alkaline shift. Proteolytic shaving of regulatory CBD domains activates NCX1.1, although the normalized pH-titration curves are comparable in trypsin treated and untreated NCX1.1. Thus, the T50S-dependent alkaline shift sets a dynamic range for "proton block" function at physiological pH, whereas the CBDs (and other regulatory modes) modulate incremental changes in the transport rates rather than affect the shape of pH dependent curves.

The Archaeal Na+/Ca2+ Exchanger (NCX_Mj) as a Model of Ion Transport for the Superfamily of Ca2+/CA Antiporters

The superfamily of Calcium/Cation (Ca²⁺/CA) antiporters extrude Ca²⁺ from the cytosol or subcellular compartments in exchange with Na⁺, K⁺, H⁺, Li⁺, or Mg²⁺ and thereby provide a key mechanism for Ca²⁺ signaling and ion homeostasis in biological systems ranging from bacteria to humans. The structure-dynamic determinants of ion selectivity and transport rates remain unclear, although this is of primary physiological significance. Despite wide variances in the ion selectivity and transport rates, the Ca²⁺/CA proteins share structural motifs, although it remains unclear how the ion recognition/binding is coupled to the ion translocation events. Here, the archaeal Na⁺/Ca²⁺ exchanger (NCX_Mj) is considered as a structure-based model that can help to resolve the ion transport mechanisms by using X-ray, HDX-MS, ATR-FTIR, and computational approaches in conjunction with functional analyses of mutants. Accumulating data reveal that the local backbone dynamics at ion-coordinating residues is characteristically constrained in apo NCX_Mj, which may predefine the affinity and stability of ion-bound species in the ground and transition states. The 3Na⁺ or 1Ca²⁺ binding to respective sites of NCX_Mj rigidify the backbone dynamics at specific segments, where the ion-dependent compression of the ion-permeating four-helix bundle (TM2, TM3, TM7, and TM8) induces the sliding of the two-helix cluster (TM1/TM6) on the protein surface to switch the OF (outward-facing) and IF (inward-facing) conformations. Taking into account the common structural elements shared by Ca²⁺/CAs, NCX_Mj may serve as a model for studying the structure-dynamic and functional determinants of ion-coupled alternating access, transport catalysis, and ion selectivity in Ca²⁺/CA proteins.

Mitochondrial calcium at the synapse

Mitochondria are dynamic organelles, which serve various purposes, including but not limited to the production of ATP and various metabolites, buffering ions, acting as a signaling hub, etc. In recent years, mitochondria are being seen as the central regulators of cellular growth, development, and death. Since neurons are highly specialized cells with a heavy metabolic demand, it is not surprising that neurons are one of the most mitochondria-rich cells in an animal. At synapses, mitochondrial function and dynamics is tightly regulated by synaptic calcium. Calcium influx during synaptic activity causes increased mitochondrial calcium influx leading to an increased ATP production as well as buffering of synaptic calcium. While increased ATP production is required during synaptic transmission, calcium buffering by mitochondria is crucial to prevent faulty neurotransmission and excitotoxicity. Interestingly, mitochondrial calcium also regulates the mobility of mitochondria within synapses causing mitochondria to halt at the synapse during synaptic transmission. In this review, we summarize the various roles of mitochondrial calcium at the synapse.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Structure-affinity insights into the Na+ and Ca2+ interactions with multiple sites of a sodium-calcium exchanger

Selective recognition and transport of Na⁺ and Ca²⁺ ions by sodium-calcium exchanger (NCX) proteins is a primary prerequisite for Ca²⁺ signaling and homeostasis. Twelve ion-coordinating residues are highly conserved among NCXs, and distinct NCX orthologs contain two or three carboxylates, while sharing a common ion-exchange stoichiometry (3Na⁺ :1Ca²⁺ ). How these structural differences affect the ion-binding affinity, selectivity, and transport rates remains unclear. Here, the mutational effects of three carboxylates (E54, E213, and D240) were analyzed on the ion-exchange rates in the archaeal NCX from Methanococcus jannaschii and ion-induced structure-affinity changes were monitored by attenuated total reflection-Fourier-transform infrared spectroscopy (ATR-FTIR). The D240N mutation elevated the ion-transport rates by twofold to threefold, meaning that the deprotonation of D240 is not essential for transport catalysis. In contrast, mutating E54 or E213 to A, D, N, or Q dramatically decreased the ion-transport rates. ATR-FTIR revealed high- and low-affinity binding of Na⁺ or Ca²⁺ with E54 and E213, but not with D240. These findings reveal distinct structure-affinity states at specific ion-binding sites in the inward-facing (IF) and outward-facing orientation. Collectively, two multidentate carboxylate counterparts (E54 and E213) play a critical role in determining the ion coordination/transport in prokaryotic and eukaryotic NCXs, whereas the ortholog substitutions in prokaryotes (aspartate) and eukaryotes (asparagine) at the 240 position affect the ion-transport rates differently (k_cat ), probably due to the structural differences in the transition state.

Multipurpose Na+ ions mediate excitation and cellular homeostasis: Evolution of the concept of Na+ pumps and Na+/Ca2+ exchangers

Ionic signalling is the most ancient form of regulation of cellular functions in response to environmental challenges. Signals, mediated by Na⁺ fluxes and spatio-temporal fluctuations of Na⁺ concentration in cellular organelles and cellular compartments contribute to the most fundamental cellular processes such as membrane excitability and energy production. At the very core of ionic signalling lies the Na⁺-K⁺ ATP-driven pump (or NKA) which creates trans-plasmalemmal ion gradients that sustain ionic fluxes through ion channels and numerous Na⁺-dependent transporters that maintain cellular and tissue homeostasis. Here we present a brief account of the history of research into NKA, Na⁺ -dependent transporters and Na⁺ signalling.

Dynamic distinctions in the Na+/Ca2+ exchanger adopting the inward- and outward-facing conformational states

Na⁺/Ca²⁺ exchanger (NCX) proteins operate through the alternating access mechanism, where the ion-binding pocket is exposed in succession either to the extracellular or the intracellular face of the membrane. The archaeal NCX_Mj (Methanococcus jannaschii NCX) system was used to resolve the backbone dynamics in the inward-facing (IF) and outward-facing (OF) states by analyzing purified preparations of apo- and ion-bound forms of NCX_Mj-WT and its mutant, NCX_Mj-5L6-8. First, the exposure of extracellular and cytosolic vestibules to the bulk phase was evaluated as the reactivity of single cysteine mutants to a fluorescent probe, verifying that NCX_Mj-WT and NCX_Mj-5L6-8 preferentially adopt the OF and IF states, respectively. Next, hydrogen-deuterium exchange-mass spectrometry (HDX-MS) was employed to analyze the backbone dynamics profiles in proteins, preferentially adopting the OF (WT) and IF (5L6-8) states either in the presence or absence of ions. Characteristic differences in the backbone dynamics were identified between apo NCX_Mj-WT and NCX_Mj-5L6-8, thereby underscoring specific conformational patterns owned by the OF and IF states. Saturating concentrations of Na⁺ or Ca²⁺ specifically modify HDX patterns, revealing that the ion-bound/occluded states are much more stable (rigid) in the OF than in the IF state. Conformational differences observed in the ion-occluded OF and IF states can account for diversifying the ion-release dynamics and apparent affinity (K_m ) at opposite sides of the membrane, where specific structure-dynamic elements can effectively match the rates of bidirectional ion movements at physiological ion concentrations.

Structure-Functional Basis of Ion Transport in Sodium-Calcium Exchanger (NCX) Proteins

The membrane-bound sodium-calcium exchanger (NCX) proteins shape Ca2+ homeostasis in many cell types, thus participating in a wide range of physiological and pathological processes. Determination of the crystal structure of an archaeal NCX (NCX_Mj) paved the way for a thorough and systematic investigation of ion transport mechanisms in NCX proteins. Here, we review the data gathered from the X-ray crystallography, molecular dynamics simulations, hydrogen-deuterium exchange mass-spectrometry (HDX-MS), and ion-flux analyses of mutants. Strikingly, the apo NCX_Mj protein exhibits characteristic patterns in the local backbone dynamics at particular helix segments, thereby possessing characteristic HDX profiles, suggesting structure-dynamic preorganization (geometric arrangements of catalytic residues before the transition state) of conserved α₁ and α₂ repeats at ion-coordinating residues involved in transport activities. Moreover, dynamic preorganization of local structural entities in the apo protein predefines the status of ion-occlusion and transition states, even though Na⁺ or Ca2+ binding modifies the preceding backbone dynamics nearby functionally important residues. Future challenges include resolving the structural-dynamic determinants governing the ion selectivity, functional asymmetry and ion-induced alternating access. Taking into account the structural similarities of NCX_Mj with the other proteins belonging to the Ca2+/cation exchanger superfamily, the recent findings can significantly improve our understanding of ion transport mechanisms in NCX and similar proteins.

Modeling Na+-Ca2+ exchange in the heart: Allosteric activation, spatial localization, sparks and excitation-contraction coupling

The cardiac sodium (Na⁺)/calcium (Ca²⁺) exchanger (NCX1) is an electrogenic membrane transporter that regulates Ca²⁺ homeostasis in cardiomyocytes, serving mainly to extrude Ca²⁺ during diastole. The direction of Ca²⁺ transport reverses at membrane potentials near that of the action potential plateau, generating an influx of Ca²⁺ into the cell. Therefore, there has been great interest in the possible roles of NCX1 in cardiac Ca²⁺-induced Ca²⁺ release (CICR). Interest has been reinvigorated by a recent super-resolution optical imaging study suggesting that ~18% of NCX1 co-localize with ryanodine receptor (RyR2) clusters, and ~30% of additional NCX1 are localized to within ~120nm of the nearest RyR2. NCX1 may therefore occupy a privileged position in which to modulate CICR. To examine this question, we have developed a mechanistic biophysically-detailed model of NCX1 that describes both NCX1 transport kinetics and Ca²⁺-dependent allosteric regulation. This NCX1 model was incorporated into a previously developed super-resolution model of the Ca²⁺ spark as well as a computational model of the cardiac ventricular myocyte that includes a detailed description of CICR with stochastic gating of L-type Ca²⁺ channels and RyR2s, and that accounts for local Ca²⁺ gradients near the dyad via inclusion of a peri-dyadic (PD) compartment. Both models predict that increasing the fraction of NCX1 in the dyad and PD decreases spark frequency, fidelity, and diastolic Ca²⁺ levels. Spark amplitude and duration are less sensitive to NCX1 spatial redistribution. On the other hand, NCX1 plays an important role in promoting Ca²⁺ entry into the dyad, and hence contributing to the trigger for RyR2 release at depolarized membrane potentials and in the presence of elevated local Na⁺ concentration. Whole-cell simulation of NCX1 tail currents are consistent with the finding that a relatively high fraction of NCX1 (~45%) resides in the dyadic and PD spaces, with a dyad-to-PD ratio of roughly 1:2. Allosteric Ca²⁺ activation of NCX1 helps to "functionally localize" exchanger activity to the dyad and PD by reducing exchanger activity in the cytosol thereby protecting the cell from excessive loss of Ca²⁺ during diastole.

Asymmetric Preorganization of Inverted Pair Residues in the Sodium-Calcium Exchanger

In analogy with many other proteins, Na⁺/Ca²⁺ exchangers (NCX) adapt an inverted twofold symmetry of repeated structural elements, while exhibiting a functional asymmetry by stabilizing an outward-facing conformation. Here, structure-based mutant analyses of the Methanococcus jannaschii Na⁺/Ca²⁺ exchanger (NCX_Mj) were performed in conjunction with HDX-MS (hydrogen/deuterium exchange mass spectrometry) to identify the structure-dynamic determinants of functional asymmetry. HDX-MS identified hallmark differences in backbone dynamics at ion-coordinating residues of apo-NCX_Mj, whereas Na⁺or Ca²⁺ binding to the respective sites induced relatively small, but specific, changes in backbone dynamics. Mutant analysis identified ion-coordinating residues affecting the catalytic capacity (k_cat/K_m), but not the stability of the outward-facing conformation. In contrast, distinct “noncatalytic” residues (adjacent to the ion-coordinating residues) control the stability of the outward-facing conformation, but not the catalytic capacity. The helix-breaking signature sequences (GTSLPE) on the α₁ and α₂ repeats (at the ion-binding core) differ in their folding/unfolding dynamics, while providing asymmetric contributions to transport activities. The present data strongly support the idea that asymmetric preorganization of the ligand-free ion-pocket predefines catalytic reorganization of ion-bound residues, where secondary interactions with adjacent residues couple the alternating access. These findings provide a structure-dynamic basis for ion-coupled alternating access in NCX and similar proteins.

Structural Features of Ion Transport and Allosteric Regulation in Sodium-Calcium Exchanger (NCX) Proteins

Na(+)/Ca(2+) exchanger (NCX) proteins extrude Ca(2+) from the cell to maintain cellular homeostasis. Since NCX proteins contribute to numerous physiological and pathophysiological events, their pharmacological targeting has been desired for a long time. This intervention remains challenging owing to our poor understanding of the underlying structure-dynamic mechanisms. Recent structural studies have shed light on the structure-function relationships underlying the ion-transport and allosteric regulation of NCX. The crystal structure of an archaeal NCX (NCX_Mj) along with molecular dynamics simulations and ion flux analyses, have assigned the ion binding sites for 3Na(+) and 1Ca(2+), which are being transported in separate steps. In contrast with NCX_Mj, eukaryotic NCXs contain the regulatory Ca(2+)-binding domains, CBD1 and CBD2, which affect the membrane embedded ion-transport domains over a distance of ~80 Å. The Ca(2+)-dependent regulation is ortholog, isoform, and splice-variant dependent to meet physiological requirements, exhibiting either a positive, negative, or no response to regulatory Ca(2+). The crystal structures of the two-domain (CBD12) tandem have revealed a common mechanism involving a Ca(2+)-driven tethering of CBDs in diverse NCX variants. However, dissociation kinetics of occluded Ca(2+) (entrapped at the two-domain interface) depends on the alternative-splicing segment (at CBD2), thereby representing splicing-dependent dynamic coupling of CBDs. The HDX-MS, SAXS, NMR, FRET, equilibrium (45)Ca(2+) binding and stopped-flow techniques provided insights into the dynamic mechanisms of CBDs. Ca(2+) binding to CBD1 results in a population shift, where more constraint conformational states become highly populated without global conformational changes in the alignment of CBDs. This mechanism is common among NCXs. Recent HDX-MS studies have demonstrated that the apo CBD1 and CBD2 are stabilized by interacting with each other, while Ca(2+) binding to CBD1 rigidifies local backbone segments of CBD2, but not of CBD1. The extent and strength of Ca(2+)-dependent rigidification at CBD2 is splice-variant dependent, showing clear correlations with phenotypes of matching NCX variants. Therefore, diverse NCX variants share a common mechanism for the initial decoding of the regulatory signal upon Ca(2+) binding at the interface of CBDs, whereas the allosteric message is shaped by CBD2, the dynamic features of which are dictated by the splicing segment.

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.

NCLX: the mitochondrial sodium calcium exchanger

The free Ca(2+) concentration within the mitochondrial matrix ([Ca(2+)]m) regulates the rate of ATP production and other [Ca(2+)]m sensitive processes. It is set by the balance between total Ca(2+) influx (through the mitochondrial Ca(2+) uniporter (MCU) and any other influx pathways) and the total Ca(2+) efflux (by the mitochondrial Na(+)/Ca(2+) exchanger and any other efflux pathways). Here we review and analyze the experimental evidence reported over the past 40years which suggest that in the heart and many other mammalian tissues a putative Na(+)/Ca(2+) exchanger is the major pathway for Ca(2+) efflux from the mitochondrial matrix. We discuss those reports with respect to a recent discovery that the protein product of the human FLJ22233 gene mediates such Na(+)/Ca(2+) exchange across the mitochondrial inner membrane. Among its many functional similarities to other Na(+)/Ca(2+) exchanger proteins is a unique feature: it efficiently mediates Li(+)/Ca(2+) exchange (as well as Na(+)/Ca(2+) exchange) and was therefore named NCLX. The discovery of NCLX provides both the identity of a novel protein and new molecular means of studying various unresolved quantitative aspects of mitochondrial Ca(2+) movement out of the matrix. Quantitative and qualitative features of NCLX are discussed as is the controversy regarding the stoichiometry of the NCLX Na(+)/Ca(2+) exchange, the electrogenicity of NCLX, the [Na(+)]i dependency of NCLX and the magnitude of NCLX Ca(2+) efflux. Metabolic features attributable to NCLX and the physiological implication of the Ca(2+) efflux rate via NCLX during systole and diastole are also briefly discussed.

Ca2+ regulation of ion transport in the Na+/Ca2+ exchanger

The binding of Ca(2+) to two adjacent Ca(2+)-binding domains, CBD1 and CBD2, regulates ion transport in the Na(+)/Ca(2+) exchanger. As sensors for intracellular Ca(2+), the CBDs form electrostatic switches that induce the conformational changes required to initiate and sustain Na(+)/Ca(2+) exchange. Depending on the presence of a few key residues in the Ca(2+)-binding sites, zero to four Ca(2+) ions can bind with affinities between 0.1 to 20 μm. Importantly, variability in CBD2 as a consequence of alternative splicing modulates not only the number and affinities of the Ca(2+)-binding sites in CBD2 but also the Ca(2+) affinities in CBD1.

Pre-steady-state kinetics of Ba-Ca exchange reveals a second electrogenic step involved in Ca2+ translocation by the Na-Ca exchanger

Kinetic properties of the Na-Ca exchanger (guinea pig NCX1) expressed in Xenopus oocytes were investigated with excised membrane patches in the inside-out configuration and photolytic Ca(2+) concentration jumps with either 5 mM extracellular Sr(2+) or Ba(2+). After a Ca(2+) concentration jump on the cytoplasmic side, the exchanger performed Sr-Ca or Ba-Ca exchange. In the Sr-Ca mode, currents are transient and decay in a monoexponential manner similar to that of currents in the Ca-Ca exchange mode described before. Currents recorded in the Ba-Ca mode are also transient, but the decay is biphasic. In the Sr-Ca mode the amount of charge translocated increases at negative potentials in agreement with experiments performed in the Ca-Ca mode. In the Ba-Ca mode the total amount of charge translocated after a Ca(2+) concentration jump is approximately 4 to 5 times that in Ca-Ca or Sr-Ca mode. In the Ba-Ca mode the voltage dependence of charge translocation depends on the Ca(2+) concentration on the cytosolic side before the Ca(2+) concentration jump. At low initial Ca(2+) levels (approximately 0.5 microM), charge translocation is voltage independent. At a higher initial concentration (1 microM Ca(2+)), the amount of charge translocated increases at positive potentials. Biphasic relaxation of the current was also observed in the Ca-Ca mode if the external Ca(2+) concentration was reduced to < or =0.5 mM. The results reported here and in previous publications can be described by using a 6-state model with two voltage-dependent conformational transitions.

The Na+/Ca2+ exchanger inhibitor, KB-R7943, blocks a nonselective cation channel implicated in chemosensory transduction

The mechanism(s) of olfactory transduction in invertebrates remains to be fully understood. In lobster olfactory receptor neurons (ORNs), a nonselective sodium-gated cation (SGC) channel, a presumptive transient receptor potential (TRP)C channel homolog, plays a crucial role in olfactory transduction, at least in part by amplifying the primary transduction current. To better determine the functional role of the channel, it is important to selectively block the channel independently of other elements of the transduction cascade, causing us to search for specific pharmacological blockers of the SGC channel. Given evidence that the Na(+)/Ca(2+) exchange inhibitor, KB-R7943, blocks mammalian TRPC channels, we studied this probe as a potential blocker of the lobster SGC channel. KB-R7943 reversibly blocked the SGC current in both inside- and outside-out patch recordings in a dose- and voltage-dependent manner. KB-R7943 decreased the channel open probability without changing single channel amplitude. KB-R7943 also reversibly and in a dose-dependent manner inhibited both the odorant-evoked discharge of lobster ORNs and the odorant-evoked whole cell current. Our findings strongly imply that KB-R7943 potently blocks the lobster SGC channel and likely does so directly and not through its ability to block the Na(+)/Ca(2+) exchanger.

Gene structure evolution of the Na+-Ca2+ exchanger (NCX) family

Background

The Na⁺-Ca²⁺ exchanger (NCX) is an important regulator of cytosolic Ca²⁺ levels. Many of its structural features are highly conserved across a wide range of species. Invertebrates have a single NCX gene, whereas vertebrate species have multiple NCX genes as a result of at least two duplication events. To examine the molecular evolution of NCX genes and understand the role of duplicated genes in the evolution of the vertebrate NCX gene family, we carried out phylogenetic analyses of NCX genes and compared NCX gene structures from sequenced genomes and individual clones.

Results

A single NCX in invertebrates and the protochordate Ciona, and the presence of at least four NCX genes in the genomes of teleosts, an amphibian, and a reptile suggest that a four member gene family arose in a basal vertebrate. Extensive examination of mammalian and avian genomes and synteny analysis argue that NCX4 may be lost in these lineages. Duplicates for NCX1, NCX2, and NCX4 were found in all sequenced teleost genomes. The presence of seven genes encoding NCX homologs may provide teleosts with the functional specialization analogous to the alternate splicing strategy seen with the three NCX mammalian homologs.

Conclusion

We have demonstrated that NCX4 is present in teleost, amphibian and reptilian species but has been secondarily and independently lost in mammals and birds. Comparative studies on conserved vertebrate homologs have provided a possible evolutionary route taken by gene duplicates subfunctionalization by minimizing homolog number.

Cytoplasmic Na+-dependent modulation of mitochondrial Ca2+ via electrogenic mitochondrial Na+-Ca2+ exchange

To clarify the role of mitochondrial Na(+)-Ca(2+) exchange (NCX(mito)) in regulating mitochondrial Ca(2+) (Ca(2+)(mito)) concentration at intact and depolarized mitochondrial membrane potential (DeltaPsi(mito)), we measured Ca(2+)(mito) and DeltaPsi(mito) using fluorescence probes Rhod-2 and TMRE, respectively, in the permeabilized rat ventricular cells. Applying 300 nm cytoplasmic Ca(2+) (Ca(2+)(c)) increased Ca(2+)(mito) and this increase was attenuated by cytoplasmic Na(+) (Na(+)(c)) with an IC(50) of 2.4 mm. To the contrary, when DeltaPsi(mito) was depolarized by FCCP, a mitochondrial uncoupler, Na(+)(c) enhanced the Ca(2+)(c)-induced increase in Ca(2+)(mito) with an EC(50) of about 4 mm. This increase was not significantly affected by ruthenium red or cyclosporin A. The inhibition of NCX(mito) by CGP-37157 further increased Ca(2+)(mito) when DeltaPsi(mito) was intact, while it suppressed the Ca(2+)(mito) increase when DeltaPsi(mito) was depolarized, suggesting that DeltaPsi(mito) depolarization changed the exchange mode from forward to reverse. Furthermore, DeltaPsi(mito) depolarization significantly reduced the Ca(2+)(mito) decrease via forward mode, and augmented the Ca(2+)(mito) increase via reverse mode. When the respiratory chain was attenuated, the induction of the reverse mode of NCX(mito) hyperpolarized DeltaPsi(mito), while DeltaPsi(mito) depolarized upon inducing the forward mode of NCX(mito). Both changes in DeltaPsi(mito) were remarkably inhibited by CGP-37157. The above experimental data indicated that NCX(mito) is voltage dependent and electrogenic. This notion was supported theoretically by computer simulation studies with an NCX(mito) model constructed based on present and previous studies, presuming a consecutive and electrogenic Na(+)-Ca(2+) exchange and a depolarization-induced increase in Na(+) flux. It is concluded that Ca(2+)(mito) concentration is dynamically modulated by Na(+)(c) and DeltaPsi(mito) via electrogenic NCX(mito).

Cell culture alters Ca2+ entry pathways activated by store-depletion or hypoxia in canine pulmonary arterial smooth muscle cells

Previous studies have shown that, in acutely dispersed canine pulmonary artery smooth muscle cells (PASMCs), depletion of both functionally independent inositol 1,4,5-trisphosphate (IP(3))- and ryanodine-sensitive Ca(2+) stores activates capacitative Ca(2+) entry (CCE). The present study aimed to determine if cell culture modifies intracellular Ca(2+) stores and alters Ca(2+) entry pathways caused by store depletion and hypoxia in canine PASMCs. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was measured in fura 2-loaded cells. Mn(2+) quench of fura 2 signal was performed to study divalent cation entry, and the effects of hypoxia were examined under oxygen tension of 15-18 mmHg. In acutely isolated PASMCs, depletion of IP(3)-sensitive Ca(2+) stores with cyclopiazonic acid (CPA) did not affect initial caffeine-induced intracellular Ca(2+) transients but abolished 5-HT-induced Ca(2+) transients. In contrast, CPA significantly reduced caffeine- and 5-HT-induced Ca(2+) transients in cultured PASMCs. In cultured PASMCs, store depletion or hypoxia caused a transient followed by a sustained rise in [Ca(2+)](i). The transient rise in [Ca(2+)](i) was partially inhibited by nifedipine, whereas the nifedipine-insensitive transient rise in [Ca(2+)](i) was inhibited by KB-R7943, a selective inhibitor of reverse mode Na(+)/Ca(2+) exchanger (NCX). The nifedipine-insensitive sustained rise in [Ca(2+)](i) was inhibited by SKF-96365, Ni(2+), La(3+), and Gd(3+). In addition, store depletion or hypoxia increased the rate of Mn(2+) quench of fura 2 fluorescence that was also inhibited by these blockers, exhibiting pharmacological properties characteristic of CCE. We conclude that cell culture of canine PASMCs reorganizes IP(3) and ryanodine receptors into a common intracellular Ca(2+) compartment, and depletion of this store or hypoxia activates voltage-operated Ca(2+) entry, reverse mode NCX, and CCE.

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.

Time resolved kinetics of the guinea pig Na–Ca exchanger (NCX1) expressed in Xenopus oocytes: voltage and Ca2+ dependence of pre-steady-state current investigated by photolytic Ca2+concentration jumps

Kinetic properties of the Na-Ca exchanger (guinea pig NCX1) expressed in Xenopus oocytes were investigated by patch clamp techniques and photolytic Ca(2+) concentration jumps. Current measured in oocyte membranes expressing NCX1 is almost indistinguishable from current measured in patches derived from cardiac myocytes. In the Ca-Ca exchange mode, a transient inward current is observed, whereas in the Na-Ca exchange mode, current either rises to a plateau, or at higher Ca(2+) concentration jumps, an initial transient is followed by a plateau. No comparable current was observed in membrane patches not expressing NCX1, indicating that photolytic Ca(2+) concentrations jumps activate Na-Ca exchange current. Electrical currents generated by NCX1 expressed in Xenopus oocytes are about four times larger than those obtained from cardiac myocyte membranes enabling current recording with smaller concentration jumps and/or higher time resolution. The apparent affinity for Ca(2+) of nonstationary exchange currents (0.1 mM) is much lower than that of stationary currents (6 muM). Measurement of the Ca(2+) dependence of the rising phase provides direct evidence that the association rate constant for Ca(2+) is about 5 x 10(8) M(-1) s(-1) and voltage independent. In both transport modes, the transient current decays with a voltage independent but Ca(2+)-dependent rate constant, which is about 9,000 s(-1) at saturating Ca(2+) concentrations. The voltage independence of this relaxation is maintained for Ca(2+) concentrations far below saturation. In the Ca-Ca exchange mode, the amount of charge translocated after a concentration jump is independent of the magnitude of the jump but voltage dependent, increasing at negative voltages. The slope of the charge-voltage relation is independent of the Ca(2+) concentration. Major conclusions are: (1) Photolytic Ca(2+) concentration jumps generate current related to NCX1. (2) The dissociation constant for Ca(2+) at the cytoplasmic transport binding site is about 0.1 mM. (3) The association rate constant of Ca(2+) at the cytoplasmic transport sites is high (5 x 10(-8) M(-1)s(-1)) and voltage independent. (4) The minimal five-state model (voltage independent binding reactions, one voltage independent conformational transition and one very fast voltage dependent conformational transition) used before to describe Ca(2+) translocation at saturating Ca(2+) concentrations is valid for Ca(2+) concentrations far below saturation.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.

Establishment of cell-free electrophysiology for ion transporters: application for pharmacological profiling

Ion transporters are emerging targets of increasing importance to the pharmaceutical industry because of their relevance to a wide range of numerous indications of cardiovascular, metabolic, and inflammatory diseases. However, traditional ion transporter assay technologies using radioactive or fluorescent ligands and substrates or manual patch clamping suffer from several problems: limited sensitivity and robustness, significant numbers of false positives and false negatives, and cost. The authors describe a novel method for the measurement of ion transporters using cell-free electrophysiology based on the SURFE (2) R (surface electrogenic event reader) technology platform. The main advantages of the method described here are high sensitivity and simple handling. Material for assays is mainly a simple membrane preparation, which can be stored over weeks and months. Thus, the application of the method does not depend on a permanently running cell-culture lab. The application of the technology itself uses a bench-top system and chips loaded with membrane fragments. The SURFE (2) R technology was used to establish an Na+/Ca2+-exchanger assay. The assay performance, as judged by the Z' value of 0.73 and the signal-to-background ratio of 7.6, suggests that this is a reliable and robust assay. The authors compared the technology with patch-clamp experiments: The measurement of activity of 17 different inhibitors and the determination of an IC (50)value indicated a good correlation between SURFE (2) R technology and patch clamp results. Using the SURFE (2) R technology, results were obtained with 20 times higher throughput and required less-qualified personnel compared with manual patch clamping.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

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.

Multiple transport modes of the cardiac Na+/Ca2+ exchanger

The cardiac Na+/Ca2+ exchanger (NCX1; ref. 2) is a bi-directional Ca2+ transporter that contributes to the electrical activity of the heart. When, and if, Ca2+ is exported or imported depends on the Na+/Ca2+ exchange ratio. Whereas a ratio of 3:1 (Na+:Ca2+) has been indicated by Ca2+ flux equilibrium studies, a ratio closer to 4:1 has been indicated by exchange current reversal potentials. Here we show, using an ion-selective electrode technique to quantify ion fluxes in giant patches, that ion flux ratios are approximately 3.2 for maximal transport in either direction. With Na+ and Ca2+ on both sides of the membrane, net current and Ca2+ flux can reverse at different membrane potentials, and inward current can be generated in the absence of cytoplasmic Ca2+, but not Na+. We propose that NCX1 can transport not only 1 Ca2+ or 3 Na+ ions, but also 1 Ca2+ with 1 Na+ ion at a low rate. Therefore, in addition to the major 3:1 transport mode, import of 1 Na+ with 1 Ca2+ defines a Na+-conducting mode that exports 1 Ca2+, and an electroneutral Ca2+ influx mode that exports 3 Na+. The two minor transport modes can potentially determine resting free Ca2+ and background inward current in heart.

On the mechanisms of post-rest adaptation in the isolated electrically driven left atria of rats

We studied the role of the resting period (1, 2, 4, 8, 16 min; n = 6-7), external Ca2+ (0.2, 0.4, 0.6 g/l; n + 5-6), stimulation frequency (1, 2, 3 Hz; n = 6), 4-aminopyridine (4-AP, 2 mM; n = 5); theophylline (1 mM; n = 6), ouabain (5 microM; n = 6), and verapamil (1 microM; n = 6) on post-rest adaptation in the isolated left atria of rats driven electrically by a 2x threshold intensity for 2 ms. Resting periods resulted in three-phasic adaptive changes in contractility during the post-rest stimulation before normalization: P1, hypercontractile phase, an initial twitch potentiation; P2, post-rest hypocontractile decay reached after 8 to 12 single twitches; and P3, a late reactive hypercontractile phase marked less than that of P1 and gradually declining to the pre-resting level. P1 and P2 were augmented along with increasing the resting period from 1 min to 16 min, whereas t1 (time between P1 and P2) shortened and P2 and t2 (time between P2 and P3) were not affected. P1 and P3 to become more apparent after shifting the stimulation frequency from 1 Hz to 3 Hz, accompanied by a shortening of t1 and t2 (p < 0.05) and an insignificant reversal of P2. An increase in Ca2+ concentration by 2- or 3-fold at 2 Hz reduced P1 was and antagonized P2, while leaving other parameters almost unaffected. The reduction of P1 by Ca2+ became more prominent at 3 Hz. Exposure to 4-AP depressed P1 and P3 at 1 Hz, which was reversed by increasing the stimulation frequency--P3 tended to diminish, whereas t1 and t2 were shortened. Theophylline reduced P1 antagonized P2, and shortened t1 and t2 significantly, and a combination of theophylline and 4-AP augmented the effects. Ouabain increased P1 and P2 in a frequency-dependent manner; prolonged t2 at 1 Hz, but shortened t2 at higher frequencies. Verapamil inhibition of Ca2+ channels augmented P1 and t1 and reduced P2 and P3, and the effects on all three parameters were augmented by combined 4-AP/verapamil. We concluded that the post-rest adaptive changes in contractility are a consequence of phasic changes in sarcoplasmic Ca2+ concentration and that such changes reflect an imbalance between the release from and uptake into the sarcoplasmic reticulum of Ca2+ and transsarcolemmal Ca2+ loss.

The interleukin-2-deficient mouse model

Interleukin-2-deficient (IL-2(-/-)) mice develop colitis with striking clinical and morphological similarities to ulcerative colitis. Since transport and barrier properties are impaired in ulcerative colitis, we studied transport and barrier functions in IL-2(-/-) mice in order to gain insight for the first time into the general pathomechanisms of disturbed transport and barrier function of the intestine during inflammation. Alternating current impedance analysis was used to determine tissue conductance in the inflamed proximal colon of IL-2(-/-) mice and to discriminate between pure epithelial and subepithelial conductance. Surprisingly, epithelial conductance was not increased but diminished in IL-2(-/-) mice compared to controls (20.2 +/- 1.3 versus 28.8 +/- 2.8 mS/cm(2)). Concomitantly, conductance of the subepithelial tissue layers was decreased in IL-2(-/-) mice as a result of edema and infiltration with inflammatory cells. In the distal colon, electrogenic Na(+) transport (J(Na)) mediated by the epithelial Na(+) channel (ENaC) was measured 8 h after stimulation with 3.10(-9) M aldosterone in vitro as the drop in I(SC) (short circuit current) after addition of 10(-4) M amiloride. In controls, J(Na) was 6.9 +/- 0.9 micromol x h(-1) x cm(-2), whereas it was abolished in IL-2(-/-) mice. In conclusion, the inflamed colon of IL-2(-/-) mice exhibits a severe disturbance in Na(+) uptake via the ENaC in the absence of a barrier defect. Thus, reduced expression of active absorptive transport and not a barrier defect is responsible for the diarrhea in this model of intestinal inflammation. This makes this model suitable for studying the general pathomechanisms of the inflammatory downregulation of intestinal transport proteins.

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

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

Stoichiometry of the Na+/Ca2+ exchanger: models and implications

We reevaluated the exchange stoichiometry of the Na(+)/Ca(2+) exchange current by measuring its reversal potential. The exchange current was measured from the inside-out macropatch excised from intact sarcolemma of guinea pig ventricular myocytes. This method provides more accurate control of extracellular and cytoplasmic ion concentrations and of membrane potential than is possible with a whole-cell clamped preparation. The exchange current was isolated as exchanger inhibitory peptide (XIP)-sensitive current or as cytoplasmic Na(+)- and Ca(2+)-induced current. The reversal potential of the Na(+)/Ca(2+) exchange current was, for the most part, close to the equilibrium potential of the 4Na(+):1Ca(2+) exchange, although it tended to get closer to that of 3Na(+): 1Ca(2+) exchange at lower Na(+) concentrations. We concluded that the stoichiometry is 4 or that it may vary depending on the cytoplasmic Na(+). The 4Na(+):1Ca(2+) exchange was further studied with computer modeling. A consecutive 4Na(+):1Ca(2+) exchange model with two active states and two inactive states (E2 model) could not well reconstruct the current-voltage relation of the exchanger. However, a consecutive 4Na(+):1Ca(2+) exchange model with 10 active states and 2 inactive states (E10 model), which included voltage-dependent Na(+) and Ca(2+) occlusions, well simulated the current-voltage relation. Implications of 4Na(+):1Ca(2+) exchange is also discussed.

Increased exchange current but normal Ca2+ transport via Na+-Ca2+ exchange during cardiac hypertrophy after myocardial infarction

Hypertrophied and failing cardiac myocytes generally show alterations in intracellular Ca2+ handling associated with changes in the contractile function and arrhythmogenicity. The cardiac Na+-Ca2+ exchange (NCX) is an important mechanism for Ca2+ extrusion and cell relaxation. Its possible involvement in changes of excitation-contraction coupling (EC-coupling) with disease remains uncertain. We analyzed the NCX function in rat ventricular myocytes 5 to 6 months after experimental myocardial infarction (PMI) produced by left coronary artery ligation and from sham-operated (SO) hearts. Caged Ca2+ was dialyzed into the cytoplasm via a patch-clamp pipette and Ca2+ was released by flash photolysis to activate NCX and measure the associated currents (I(NaCa)), whereas [Ca2+]i changes were simultaneously recorded with a confocal microscope. I(NaCa) density normalized to the [Ca2+]i jumps was 2.6-fold higher in myocytes from PMI rats. The level of total NCX protein expression in PMI myocytes was also increased. Interestingly, although the I(NaCa) density in PMI cells was larger, PMI and SO myocytes presented virtually identical Ca2+ transport via the NCX. This discrepancy was explained by a reduced surface/volume ratio (34.8%) observed in PMI cells. We conclude that the increase in NCX density may be a mechanism to maintain the required Ca2+ extrusion from a larger cell to allow adequate relaxation.

Amiloride and KB-R7943 in outward Na+ /Ca2+ exchange current in guinea pig ventricular myocytes

The reverse mode of Na+/Ca2+ exchange represents an important pathway in inducing Ca2+ overload during ischemia and reperfusion. The inhibitory effects of amiloride and KB-R7943 on Na+/Ca2+ exchange current (INa/Ca) were investigated in guinea pig ventricular myocytes. Whole-cell patch clamp techniques were used under bidirectional ionic conditions and 25 mM of Na+ in pipette solution. At +50 mV, amiloride 10, 30, and 100 microM inhibited the outward INa/Ca by 15, 23, and 41%, respectively; at -80 mV, it inhibited inward INa/Ca by 6, 15, and 23%, respectively. Its inhibitory effect on outward INa/Ca was greater than that on inward INa/Ca. At +50 mV, KB-R7943 1 and 10 microM inhibited the outward INa/Ca by 29 and 61%, respectively; at -80 mV, it inhibited inward INa/Ca by 22 and 57%, respectively. KB-R7943 inhibited both directions of the exchange current with an equal potency. The data suggest that KB-R7943 is not a selective inhibitor on reverse mode of Na+/Ca2+ exchange.

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

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

Voltage and Ca(2+) dependence of pre-steady-state currents of the Na-Ca exchanger generated by Ca(2+) concentration jumps

The Ca(2+) concentration and voltage dependence of the relaxation kinetics of the Na-Ca exchanger after a Ca(2+) concentration jump was measured in excised giant membrane patches from guinea pig heart. Ca(2+) concentration jumps on the cytoplasmic side were achieved by laser flash-induced photolysis of DM-nitrophen. In the Ca-Ca exchange mode a transient inward current is generated. The amplitude and the decay rate of the current saturate at concentrations >10 microM. The integrated current signal, i.e., the charge moved is fairly independent of the amount of Ca(2+) released. The amount of charge translocated increases at negative membrane potentials, whereas the decay rate constant shows no voltage dependence. It is suggested that Ca(2+) translocation occurs in at least four steps: intra- and extracellular Ca(2+) binding and two intramolecular transport steps. Saturation of the amplitude and of the relaxation of the current can be explained if the charge translocating reaction step is preceded by two nonelectrogenic steps: Ca(2+) binding and one conformational transition. Charge translocation in this mode is assigned to one additional conformational change which determines the equilibrium distribution of states. In the Na-Ca exchange mode, the stationary inward current depends on the cytoplasmic Ca(2+) concentration and voltage. The K(m) for Ca(2+) is 4 microM for guinea pig and 10 microM for rat myocytes. The amplitude of the pre-steady-state current and its relaxation saturate with increasing Ca(2+) concentrations. In this mode the relaxation is voltage dependent.

Temperature dependence of cloned mammalian and salmonid cardiac Na(+)/Ca(2+) exchanger isoforms

The cardiac Na(+)/Ca(2+) exchanger (NCX), an important regulator of cytosolic Ca(2+) concentration in contraction and relaxation, has been shown in trout heart sarcolemmal vesicles to have high activity at 7 degrees C relative to its mammalian isoform. This unique property is likely due to differences in protein structure. In this study, outward NCX currents (I(NCX)) of the wild-type trout (NCX-TR1.0) and canine (NCX 1.1) exchangers expressed in oocytes were measured to explore the potential contributions of regulatory vs. transport mechanisms to this observation. cRNA was transcribed in vitro from both wild-type cDNA and was injected into Xenopus oocytes. I(NCX) of NCX-TR1.0 and NCX1.1 were measured after 3-4 days over a temperature range of 7-30 degrees C using the giant excised patch technique. The I(NCX) for both isoforms exhibited Na(+)-dependent inactivation and Ca(2+)-dependent positive regulation. The I(NCX) of NCX1.1 exhibited typical mammalian temperature sensitivities with Q(10) values of 2.4 and 2.6 for peak and steady-state currents, respectively. However, the I(NCX) of NCX-TR1.0 was relatively temperature insensitive with Q(10) values of 1.2 and 1.1 for peak and steady-state currents, respectively. I(NCX) current decay was fit with a single exponential, and the resultant rate constant of inactivation (lambda) was determined as a function of temperature. As expected, lambda decreased monotonically with temperature for both isoforms. Although lambda was significantly greater in NCX1.1 compared with NCX-TR1.0 at all temperatures, the effect of temperature on lambda was not different between the two isoforms. These data suggest that the disparities in I(NCX) temperature dependence between these two exchanger isoforms are unlikely due to differences in their inactivation kinetics. In addition, similar differences in temperature dependence were observed in both isoforms after alpha-chymotrypsin treatment that renders the exchanger in a deregulated state. These data suggest that the differences in I(NCX) temperature dependence between the two isoforms are not due to potential disparities in either the I(NCX) regulatory mechanisms or structural differences in the cytoplasmic loop but are likely predicated on differences within the transmembrane segments.

Inhibition of Na+/Ca2+ exchange by KB-R7943: transport mode selectivity and antiarrhythmic consequences

The Na+/Ca2+ exchanger plays a prominent role in regulating intracellular Ca2+ levels in cardiac myocytes and can serve as both a Ca2+ influx and efflux pathway. A novel inhibitor, KB-R7943, has been reported to selectively inhibit the reverse mode (i.e., Ca2+ entry) of Na+/Ca2+ exchange transport, although many aspects of its inhibitory properties remain controversial. We evaluated the inhibitory effects of KB-R7943 on Na+/Ca2+ exchange currents using the giant excised patch-clamp technique. Membrane patches were obtained from Xenopus laevis oocytes expressing the cloned cardiac Na+/Ca2+ exchanger NCX1.1, and outward, inward, and combined inward-outward currents were studied. KB-R7943 preferentially inhibited outward (i.e., reverse) Na+/Ca2+ exchange currents. The inhibitory mechanism consists of direct effects on the transport machinery of the exchanger, with additional influences on ionic regulatory properties. Competitive interactions between KB-R7943 and the transported ions were not observed. The antiarrhythmic effects of KB-R7943 were then evaluated in an ischemia-reperfusion model of cardiac injury in Langendorff-perfused whole rabbit hearts using electrocardiography and measurements of left ventricular pressure. When 3 microM KB-R7943 was applied for 10 min before a 30-min global ischemic period, ventricular arrhythmias (tachycardia and fibrillation) associated with both ischemia and reperfusion were almost completely suppressed. The observed electrophysiological profile of KB-R7943 and its protective effects on ischemia-reperfusion-induced ventricular arrhythmias support the notion of a prominent role of Ca2+ entry via reverse Na+/Ca2+ exchange in this process.