Na+/Ca2+ exchange and cellular Ca2+ homeostasis

The investigation of the cellular electrophysiological and antiarrhythmic effects of a novel selective sodium-calcium exchanger inhibitor, GYKB-6635, in canine and guinea-pig hearts

The sodium-calcium exchanger (NCX) is considered as the major transmembrane transport mechanism that controls Ca²⁺ homeostasis. Its contribution to the cardiac repolarization has not yet been directly studied due to lack of specific inhibitors, so that an urgent need for more selective compounds. In this study, the electrophysiological effects of GYKB-6635, a novel NCX inhibitor, on the NCX, L-type calcium, and main repolarizing potassium currents as well as action potential (AP) parameters were investigated. Ion currents and AP recordings were investigated by applying the whole-cell patch clamp and standard microelectrode techniques in canine heart at 37 °C. Effects of GYKB-6635 were studied in ouabain-induced arrhythmias in isolated guinea-pig hearts. At a concentration of 1 μmol/L, GYKB significantly reduced both the inward and outward NCX currents (57% and 58%, respectively). Even at a high concentration (10 μmol/L), GYKB-6635 did not change the I_CaL, the maximum rate of depolarization (dV/dt_max), the main repolarizing K⁺ currents, and the main AP parameters. GYKB-6635 pre-treatment significantly delayed the time to the development of ventricular fibrillation (by about 18%). It is concluded that GYKB-6635 is a potent and highly selective inhibitor of the cardiac NCX and, in addition, it is suggested to also contribute to the prevention of DAD-based arrhythmias.

Sodium-calcium exchanger 1 regulates epithelial cell migration via calcium-dependent extracellular signal-regulated kinase signaling

Na(+)/Ca(2+) exchanger-1 (NCX1) is a major calcium extrusion mechanism in renal epithelial cells enabling the efflux of one Ca(2+) ion and the influx of three Na(+) ions. The gradient for this exchange activity is provided by Na,K-ATPase, a hetero-oligomer consisting of a catalytic α-subunit and a regulatory β-subunit (Na,K-β) that also functions as a motility and tumor suppressor. We showed earlier that mice with heart-specific ablation (KO) of Na,K-β had a specific reduction in NCX1 protein and were ouabain-insensitive. Here, we demonstrate that Na,K-β associates with NCX1 and regulates its localization to the cell surface. Madin-Darby canine kidney cells with Na,K-β knockdown have reduced NCX1 protein and function accompanied by 2.1-fold increase in free intracellular calcium and a corresponding increase in the rate of cell migration. Increased intracellular calcium up-regulated ERK1/2 via calmodulin-dependent activation of PI3K. Both myosin light chain kinase and Rho-associated kinase acted as mediators of ERK1/2-dependent migration. Restoring NCX1 expression in β-KD cells reduced migration rate and ERK1/2 activation, suggesting that NCX1 functions downstream of Na,K-β in regulating cell migration. In parallel, inhibition of NCX1 by KB-R7943 in Madin-Darby canine kidney cells, LLC-PK1, and human primary renal epithelial cells (HREpiC) increased ERK1/2 activation and cell migration. This increased migration was associated with high myosin light chain phosphorylation by PI3K/ERK-dependent mechanism in HREpiC cells. These data confirm the role of NCX1 activity in regulating renal epithelial cell migration.

Cardiac Na+-Ca2+ exchanger: dynamics of Ca2+-dependent activation and deactivation in intact myocytes

Cardiac Na(+)-Ca(2+) exchange (NCX) activity is regulated by [Ca(2+)]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca(2+) activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca(2+). We probed Ca(2+) activation without voltage clamp by applying Na(+)-free (0 Na(+)) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca(2+)]i transients due to Ca(2+) influx via NCX. In rested rabbit myocytes, Ca(2+) influx was undetectable even after 0 Na(+) applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca(2+) via L-type Ca(2+) channels, 0 Na(+) bouts activated Ca(2+) influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na(+)]i, either in rabbit cardiomyocytes as a result of inhibition of Na(+)-K(+) pumping, or in mouse cardiomyocytes where normal [Na(+)]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na(+) bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca(2+) that would otherwise cause activation. We modelled Ca(2+) activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca(2+) activation on a multibeat time scale, NCX might better maintain a stable long-term Ca(2+) balance while contributing to the ability of myocytes to produce Ca(2+) transients over a wide range of intensity.

NCX3 is a major functional isoform of the sodium-calcium exchanger in osteoblasts

The calcium phosphate-based skeleton of vertebrates serves as the major reservoir for metabolically available calcium ions. The skeleton is formed by osteoblasts which first secrete a proteinaceous matrix and then provide Ca++ for the calcification process. The two calcium efflux ports found in most cells are the plasma membrane Ca-ATPase (PMCA) and the sodium-calcium exchanger (NCX). In osteoblasts, PMCA and NCX are located on opposing sides of the cell with NCX facing the mineralizing bone surface. Two isoforms of NCX have been identified in osteoblasts NCX1, and NCX3. The purpose of this study was to determine the extent to which each of the two NCX isoforms support delivery of Ca++ into sites of calcification and to discern if one could compensate for the other. SiRNA technology was used to knockdown each isoform separately in MC3T3-E1 osteoblasts. Osteoblasts in which either NCX1 or NCX3 was impaired were tested for Ca++ efflux using the Ca++ specific fluorophore, fluo-4, in a sodium-dependent calcium uptake assay adapted for image analysis. NCX3 was found to serve as a major contributor of Ca++ translocation out of osteoblasts into calcifying bone matrix. NCX1 had little to no involvement.

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.

A structure-function analysis of ion transport in crustacean gills and excretory organs

Osmotic and ionic regulation in the Crustacea is mostly accomplished by the multifunctional gills, together with the excretory organs. In addition to their role in gas exchange, the gills constitute organs of active, transepithelial, ion transport, an activity of major importance that underlies many essential physiological functions like osmoregulation, calcium homeostasis, ammonium excretion and extracellular pH regulation. This review focuses on structure-function relationships in crustacean gills and excretory effectors, from the organ to molecular levels of organization. We address the diversity of structural architectures encountered in different crustacean gill types, and in constituent cell types, before examining the physiological mechanisms of Na(+), Cl(-), Ca(2+) and NH(4)(+) transport, and of acid-base equivalents, based on findings obtained over the last two decades employing advanced techniques. The antennal and maxillary glands constitute the principal crustacean excretory organs, which have received less attention in functional studies. We examine the diversity present in antennal and maxillary gland architecture, highlighting the structural similarities between both organ types, and we analyze the functions ascribed to each glandular segment. Emphasis is given to volume and osmoregulatory functions, capacity to produce dilute urine in freshwater crustaceans, and the effect of acclimation salinity on urine volume and composition. The microanatomy and diversity of function ascribed to gills and excretory organs are appraised from an evolutionary perspective, and suggestions made as to future avenues of investigation that may elucidate evolutionary and adaptive trends underpinning the invasion and exploitation of novel habitats.

The regulation of the Na/Ca exchanger and plasmalemmal Ca2+ ATPase by other proteins

Na/Ca exchanger (NCX) and plasma membrane Ca2+ ATPase are the Ca2+ efflux mechanisms known in mammalian cells. NCX is the main transporter to efflux intracellular Ca2+ in the heart. NCX protein contains nine putative transmembrane domains and a large intracellular loop joining two sets of the transmembrane domains. The intracellular loop regulates the activity of the NCX by interacting with other proteins and nonprotein factors, such as ions, PIP2. Several proteins that are associated with NCX have been identified recently. Similarly, plasmalemmal Ca2+ ATPase (PMCA) has 10 putative transmembrane domains, and the C-terminal intracellular region inhibits transporter activity. There are several proteins associated with PMCA, and the roles of the associated proteins of PMCA vary from specific localization to involving PMCA in signal transduction. Elucidation of structural and functional roles played by these associated proteins of NCX and PMCA will provide opportunities to develop drugs of potential therapeutic value.

Maximal Ca2+i stimulation of cardiac Na+/Ca2+ exchange requires simultaneous alkalinization and binding of PtdIns-4,5-P2 to the exchanger

Using bovine heart sarcolemma vesicles we studied the effects of protons and phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) on the affinity of the mammalian Na(+)/Ca(2+) exchanger (NCX1) for intracellular Ca(2+). By following the effects of extravesicular ligands in inside-out vesicles, their interactions with sites of NCX1 facing the intracellular medium were investigated. Two Na(+)-gradient-dependent fluxes were studied: Ca(2+) uptake and Ca(2+) release. PtdIns-4,5-P2 binding to NCX1 was investigated in parallel. Without MgATP (no 'de novo' synthesis of PtdIns-4,5-P2), alkalinization increased the affinity for Ca(2+) and the PtdIns-4,5-P2 bound to NCX1. Vesicles depleted of phosphoinositides were insensitive to alkalinization, but became responsive following addition of exogenous PtdIns-4,5-P2 or PtdIns plus MgATP. Acidification reduced the affinity for Ca(2+)(ev); this was only partially reversed by MgATP, despite the increase in bound PtdIns-4,5-P2 to levels observed with alkalinization. Inhibition of Ca(2+) uptake by increasing extravesicular [Na(+)] indicates that it is related to H(+)(i) and Na(+)(i) synergistic inhibition of the Ca(2+)(i) regulatory site. Therefore, the affinity of the NCX1 Ca(2+)(i) regulatory site for Ca(2+) was maximal when both intracellular alkalinization and an increase in PtdIns-4,5-P2 bound to NCX1 (not just of the total membrane PtdIns-4,5-P2) occurred simultaneously. In addition, protons influenced the distribution, or the exposure, of PtdIns-4,5-P2 molecules in the surroundings and/or on the exchanger protein.

Pharmacology of KB-R7943: A Na+-Ca2+ exchange inhibitor

The Na+-Ca2+ exchange (NCX) system plays a pivotal role in regulating intracellular Ca2+ concentration in cardiomyocytes, neuronal cells, kidney and a variety of other cells. It performs a particularly important function in regulating cardiac contractility and electrical activity. One of the leading NCX inhibitors is KB-R9743 (KBR) that appears to exhibit selectivity for Ca2+-influx-mode NCX activity (reverse mode of NCX). In this article we reviewed pharmacology of KBR and provide a brief summary of studies with other NCX inhibitors, such as SEA0400 (SEA) and SN-6 (SN). Potential clinical usefulness of KBR and other NCX inhibitors is still controversial but the reviewed findings may be helpful in designing more selective and clinically useful NCX inhibitors for the treatment of cardiac, neuronal and kidney diseases.

Functional differences of Na+/Ca2+ exchanger expression in Ca2+ transport system of smooth muscle of guinea pig stomach

The plasma membrane ATP-dependent Ca2+ pump and the Na+/Ca2+ exchanger (NCX) are the major means of Ca2+ extrusion in smooth muscle. However, little is known regarding distribution and function of the NCX in guinea pig gastric smooth muscle. The expression pattern and distribution of NCX isoforms suggest a role as a regulator of Ca2+ transport in cells. Na+ pump inhibition and the consequent to removal of K+ caused gradual contraction in fundus. In contrast, the response was significantly less in antrum. Western blotting analysis revealed that NCX1 and NCX2 are the predominant NCX isoforms expressed in stomach, the former was expressed strongly in antrum, whereas the latter displayed greater expression in fundus. Isolated plasma membrane fractions derived from gastric fundus smooth muscle were also investigated to clarify the relationship between NCX protein expression and function. Na+-dependent Ca2+ uptake increased directly with Ca2+ concentration. Ca2+ uptake in Na+-loaded vesicles was markedly elevated in comparison with K+-loaded vesicles. Additionally, Ca2+ uptake by the Na+- or K+-loaded vesicles was substantially higher in the presence of A23187 than in its absence. The result can be explained based on the assumption that Na+ gradients facilitate downhill movement of Ca2+. Na+-dependent Ca2+ uptake was abolished by the monovalent cationic ionophore, monensin. NaCl enhanced Ca2+ efflux from vesicles, and this efflux was significantly inhibited by gramicidin. Results documented evidence that NCX2 isoform functionally contributes to Ca2+ extrusion and maintenance of contraction-relaxation cycle in gastric fundus smooth muscle.

C6 cells express a sodium-calcium exchanger/GM1 complex in the nuclear envelope but have no exchanger in the plasma membrane: comparison to astrocytes

Previous work demonstrated the presence of an isoform of Na(+)/Ca(2+) exchanger in the nuclear envelope of neurons and NG108-15 cells that is tightly associated with GM1 ganglioside and potentiated by the latter. This contrasted with the Na(+)/Ca(2+) exchanger(s) in the plasma membrane, which were suggested to associate more loosely with GM1. To study these aspects of Na(+)/Ca(2+) exchanger expression in nonneuronal neural cells, we have examined nuclear and plasma membrane exchanger patterns in astrocytes and C6 cells, a glia-derived line. We find both cell types contain the tightly associated exchanger/GM1 complex in the nuclear envelope but, surprisingly, only astrocytes possess Na(+)/Ca(2+) exchanger activity in the plasma membrane. This is the first reported example of a cell (C6) with Na(+)/Ca(2+) exchangers in the nuclear envelope but not in the plasma membrane. RT-PCR established the presence of the NCX1 subtype in C6 cells and both NCX1 and NCX2 in astrocytes. Comparison was made with NG108-15 cells, which have Na(+)/Ca(2+) exchangers in both nuclear and plasma membranes, and Jurkat cells, which have no Na(+)/Ca(2+) exchanger in either membrane. Culturing of C6 cells in the presence dibutyryl-cAMP caused upregulation of a high molecular weight isoform of the exchanger together with GM1 in the nuclear envelope, resulting in significant elevation of Na(+)/Ca(2+) exchanger activity in the latter. Application of exogenous GM1 to nuclei from non-treated cells also potentiated exchanger activity, although to a lesser degree. The Na(+)/Ca(2+) exchanger/GM1 complex occurs in the inner membrane of the nuclear envelope, suggesting a functional role in transferring Ca(2+) between nucleoplasm and the envelope lumen.

Allosteric activation of sodium-calcium exchange activity by calcium: persistence at low calcium concentrations

The activity of the cardiac Na+/Ca2+ exchanger is stimulated allosterically by Ca2+, but estimates of the half-maximal activating concentration have varied over a wide range. In Chinese hamster ovary cells expressing the cardiac Na+/Ca2+ exchanger, the time course of exchange-mediated Ca2+ influx showed a pronounced lag period followed by an acceleration of Ca2+ uptake. Lag periods were absent in cells expressing an exchanger mutant that was not dependent on regulatory Ca2+ activation. We assumed that the rate of Ca2+ uptake during the acceleration phase reflected the degree of allosteric activation of the exchanger and determined the value of cytosolic Ca2+ ([Ca2+]i) at which the rate of Ca2+ influx was half-maximal (Kh). After correcting for the effects of mitochondrial Ca2+ uptake and fura-2 buffering, Kh values of approximately 300 nM were obtained. After an increase in [Ca2+]i, the activated state of the exchanger persisted following a subsequent reduction in [Ca2+]i to values <100 nM. Thus, within 30 s after termination of a transient increase in [Ca2+]i, exchange-mediated Ca2+ entry began without a lag period and displayed a linear rate of Ca2+ uptake in most cells; a sigmoidal time course of Ca2+ uptake returned 60-90 s after the transient increase in [Ca2+]i was terminated. Relaxation of the activated state was accelerated by the activity of the endoplasmic reticulum Ca2+ pump, suggesting that local Ca2+ gradients contribute to maintaining exchanger activation after the return of global [Ca2+]i to low values.

Lanthanum is transported by the sodium/calcium exchanger and regulates its activity

La3+ uptake was measured in fura 2-loaded Chinese hamster ovary cells expressing the bovine cardiac Na+/Ca2+ exchanger (NCX1.1). La3+ was taken up by the cells after an initial lag phase of 50-60 s and achieved a steady state within 5-6 min. Neonatal cardiac myocytes accumulated La3+ in a similar manner. La3+ uptake was due to the activity of the exchanger, because no uptake was seen in nontransfected cells or in transfected cells that had been treated with gramicidin to remove cytosolic Na+. The low rate of La3+ uptake during the lag period resulted from insufficient cytosolic Ca2+ to activate the exchanger at its regulatory sites, as shown by the following observations. La3+ uptake occurred without a lag period in cells expressing a mutant of NCX1.1 that does not exhibit regulatory activation by cytosolic Ca2+. The rate of La3+ uptake by wild-type cells was increased, and the lag phase was reduced or eliminated, when the cytosolic Ca2+ concentration was increased before initiating La3+ uptake. La3+ could substitute for Ca2+ at very low concentrations to activate exchange activity. Thus preloading cells expressing NCX1.1 with a small quantity of La3+ increased the rate of exchange-mediated Ca2+ influx by 20-fold; in contrast, cytosolic La3+ partially inhibited Ca2+ uptake by the regulation-deficient mutant. With an estimated KD of 30 pM for the binding of La3+ to fura 2, we conclude that cytosolic La3+ activates exchange activity at picomolar concentrations. We speculatively suggest that endogenous trace metals might activate exchange activity under physiological conditions.

Time course of acute-phase response induced by Tityus serrulatus venom and TsTX-I in mice

Animal venom can induce systemic alterations similar to those observed in acute-phase inflammatory response. In the present study, we report the systemic (circulatory) and local (peritoneal cavity) effects induced by Tityus serrulatus venom and its major toxin TsTX-I (Ts1) in mice over various time periods. Both the venom and TsTX-I elicited quite similar responses in most assays. Responses included reduction of albumin, increased C-reactive protein, IL-6, IL-1alpha and TNF-alpha. Local and systemic leucocytosis, with a predominance of polymorphonuclear cells, was also observed. These effects show that a systemic inflammation-like syndrome is triggered during the severe envenomation caused by the T. serrulatus sting. The initial increases of albumin and total protein were probably consequences of the dehydration that occurs at the beginning of envenomation. Time-course analysis of these effects shows that responses are most pronounced on the first day after poisoning. However, leucocytosis and changes in acute-phase protein concentrations can be observed up to 7 days after envenomation.

Abnormal contractile activity and calcium cycling in cardiac myocytes isolated from DMPK knockout mice

Dysfunction of the gene encoding DMPK (myotonic dystrophy protein kinase) has been implicated in the human neuromuscular disease myotonic dystrophy (DM1). The cardiac features of the disease include progressive conduction defects and ventricular arrhythmias. These defects have been observed in hearts of mice deficient for DMPK function. We have investigated the role of DMPK in the function of ventricular cardiomyocytes using dmpk knockout (KO) mice. A deficit in DMPK caused enhanced basal contractility of single cardiomyocytes and an associated increase in intracellular Ca(2+), measured using fura-2. Biochemical measurements indicated hyperphosphorylation of phospholamban (PLB) in KO mice. This suggests increased Ca(2+) uptake into the sarcoplasmic reticulum (SR) as the underlying cause of enhanced contractility. This conclusion was supported by the larger amplitude of caffeine-induced Ca(2+) release from the SR in KO cardiomyocytes. Concurrent with hyperphosphorylated PLB, the response to isoprenaline was reduced. These observations suggest dmpk has a modulatory role in the control of intracellular Ca(2+) concentration in mouse ventricular cardiomyocytes, loss of which may contribute to cardiac dysfunction in DM1.

Calcium and voltage dependent inactivation of sodium and calcium currents limits calcium influx in Helisoma neurons

The control of free intracellular calcium concentration ([Ca2+]i) is necessary for cell survival because of the ubiquitous and essential role this second messenger plays in regulating numerous intracellular processes. Calcium regulation in neurons is especially vigorous because of the large calcium influx that occurs through voltage-gated channels during membrane depolarization. In this study we examined changes in ionic currents that can limit calcium influx into neurons during electrical activity. We found that the [Ca2+]i in electrically stimulated Helisoma B4 neurons initially increased to a peak and then relaxed to lower concentrations in tandem with a decline in the action potential peak voltage. The decline in [Ca2+]i and the peak action potential voltage in this sodium and calcium driven neuron was found to be a dual manifestation of I(Na) and I(Ca) inactivation. I(Na) and I(Ca) both displayed voltage dependent inactivation. Additionally, I(Na) and I(Ca) progressively inactivated at [Ca2+]i above 200 nM, concentrations readily attained in electrically stimulated B4 neurons. Calcium and voltage dependent I(Na) and I(Ca) inactivation were found to reduce calcium influx during continuous electrical stimulation by decreasing both the magnitude of I(Ca) that could be activated and the percent of the available I(Ca) that would be activated due to the diminished peak action potential voltage. Calculations based on data herein suggest that the voltage and calcium dependent I(Na) and I(Ca) inactivation that occurs during continuous electrical stimulation dramatically reduces calcium influx in this sodium and calcium driven neuron and thus limits the increase in [Ca2+]i.

Cellular regulation of sodium-calcium exchange

Na(+)/Ca(2+) exchange activity was studied in transfected Chinese hamster ovary (CHO) cells expressing the wild-type cardiac exchanger (NCX1.1) or mutants created by site-directed mutagenesis. The activity of the wild-type exchanger, but not exchanger mutants deficient in Ca(2+)-dependent activation, was inhibited by sphingolipids such as ceramide and sphingosine. We propose that sphingolipids interfere with the regulatory activation of exchange activity by Ca(2+) and suggest that this interaction provides a means for monitoring and regulating diastolic Ca(2+) levels in beating cardiac myocytes. Exchange activity in CHO cells was also linked, through a poorly understood feedback mechanism, to Ca(2+) accumulation within internal stores such as the endoplasmic reticulum and the mitochondria. Finally, the F-actin cytoskeleton was shown to modulate exchange activity through interactions involving the exchanger's central hydrophilic domain. We conclude that regulation of exchange activity in intact cells involves multiple interactions with various lipid species, cytosolic Ca(2+), organellar Ca(2+) stores, and the cytoskeleton.

Effects of docosahexaenoic acid on calcium pathway in adult rat cardiomyocytes

In this study we examined the effect of polyunsaturated fatty acids (PUFAs), in particular of docosahexaenoic acid (DHA), on calcium homeostasis in isolated adult rat cardiomyocytes exposed to KCl, ET-1 and anoxia. Free [Ca(2+)](i) in rat cardiomyocytes was 135.7 +/- 0.5 nM. Exposure to 50 mM KCl or 100 nM ET-1 resulted in a rise in free [Ca(2+)](i) in freshly isolated cells (465.4 +/- 15.6 nM and 311.3 +/- 12.6 nM, respectively) and in cultured cells (450.8 +/- 14.8 nM and 323.5 +/- 14.8 nM respectively). An acute treatment (20 minutes) with 10 microM DHA significantly reduced the KCl- and ET-1-induced [Ca(2+)](i) increase (300.9 +/- 18.1 nM and 232.08 +/- 11.8 nM, respectively). This reduction was greater after chronic treatment with DHA (72 h; 257.7 +/- 13.08 nM and 192.18 +/- 9.8 nM, respectively). Rat cardiomyocytes exposed to a 20 minute superfusion with anoxic solution, obtained by replacing O(2) with N(2) in gas mixture, showed a massive increase in cytosolic calcium (1200.2 +/- 50.2 nM). Longer exposure to anoxia induced hypercontraction and later death of rat cardiomyocytes. Preincubation with DHA reduced the anoxic effect on [Ca(2+)](i) (498.4 +/- 7.3 nM in acute and 200.2 +/- 12.2 nM in chronic treatment). In anoxic conditions 50 mM KCl and 100 nM ET-1 produced extreme and unmeasurable increases of [Ca(2+)](i.) Preincubation for 20 minutes with DHA reduced this phenomenon (856.1 +/- 20.3 nM and 782.3 +/- 7.6 nM, respectively). This reduction is more evident after a chronic treatment with DHA (257.7 +/- 10.6 nM and 232.2 +/- 12.5 nM, respectively). We conclude that in rat cardiomyocytes KCl, ET-1 and anoxia interfered with intracellular calcium concentrations by either modifying calcium levels or impairing calcium homeostasis. Acute, and especially chronic, DHA administration markedly reduced the damage induced by calcium overload in those cells.

Potentiation of a sodium-calcium exchanger in the nuclear envelope by nuclear GM1 ganglioside

Calcium is recognized as an important intracellular messenger with a pivotal role in the regulation of many cytosolic and nuclear processes. Gangliosides of various types, especially GM1, are known to have a role in some aspects of Ca2+ regulation, operating through a variety of mechanisms that are gradually coming to light. The present study provides evidence for a sodium-calcium exchanger in the nuclear envelope of NG108-15 neuroblastoma cells that is potently and specifically activated by GM1. Immunoblot analysis revealed an unusually tight association of GM1 with the exchanger in the nuclear envelope but not with that in the plasma membrane. Exchanger and associated GM1 were located in the inner membrane of the nuclear envelope, suggesting this system could function to transfer Ca2+ between nucleoplasm and the envelope lumen. The GM1-enhanced exchange was blocked by cholera toxin B subunit while C2-ceramide, a recently discovered inhibitor of the exchanger, blocked all transfer. Exchanger activity was significantly elevated in nuclei isolated from cells that were induced to differentiate by KCl + dibutyryl-cAMP, a treatment previously shown to promote up-regulation of nuclear GM1 in conjunction with axonogenesis. Similar enhancement was achieved by addition of exogenous GM1 to nuclei from undifferentiated cells. These results suggest a prominent role for nuclear GM1 in regulation of nuclear Ca2+ homeostasis.

Expressing and purifying membrane transport proteins in high yield

Structural analysis of native or recombinant membrane transport proteins has been hampered by the lack of effective methodologies to purify sufficient quantities of active protein. We addressed this problem by expressing a polyhistidine tagged construct of the cardiac sodium-calcium exchanger (NCX1) in Trichoplusia ni larvae (caterpillars) from which membrane vesicles were prepared. Larvae vesicles containing recombinant NCX1-his protein supported NCX1 transport activity that was mechanistically not different from activity in native cardiac sarcolemmal vesicles although the specific activity was reduced. SDS-PAGE and Western blot analysis demonstrated the presence of both the 120 and 70 kDa forms of the NCX1 protein. Larvae vesicle proteins were solubilized in sodium cholate detergent and fractionated on a chelated Ni(2+) affinity chromatography column. After extensive washing, eluted fractions were mixed with soybean phospholipids and reconstituted. The resulting proteoliposomes contained NCX1 activity suggesting the protein retained native conformation. SDS-PAGE revealed two major bands at 120 and 70 kDa. Purification of large amounts of active NCX1 via this methodology should facilitate biophysical analysis of the protein. The larva expression system has broad-based application for membrane proteins where expression and purification of quantities required for physical analyses is problematic.

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.

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

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

Inhibition of sodium-calcium exchange by ceramide and sphingosine

Na(+)/Ca(2+) exchange activity in Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger was inhibited by the short chain ceramide analogs N-acetylsphingosine and N-hexanoylsphingosine (5-15 micrometer). The sphingolipids reduced exchange-mediated Ba(2+) influx by 50-70% and also inhibited the Ca(2+) efflux mode of exchange activity. The biologically inactive ceramide analog N-acetylsphinganine had only modest effects on exchange activity. Cells expressing the Delta(241-680) and Delta(680-685) deletion mutants of the Na(+)/Ca(2+) exchanger were not inhibited by ceramide; these mutants show defects in both Na(+)-dependent and Ca(2+)-dependent regulatory behavior. Another mutant, which was defective only in Na(+)-dependent regulation, was as sensitive to ceramide inhibition as the wild-type exchanger. Inhibition of exchange activity by ceramide was time-dependent and was accelerated by depletion of internal Ca(2+) stores. Sphingosine (2.5 micrometer) also inhibited the Ca(2+) influx and efflux modes of exchange activity in cells expressing the wild-type exchanger; sphingosine did not affect Ba(2+) influx in the Delta(241-680) mutant. The effects of the exogenous sphingolipids were reproduced by blocking cellular ceramide utilization pathways, suggesting that exchange activity is inhibited by increased levels of endogenous ceramide and/or sphingosine. We propose that sphingolipids impair Ca(2+)-dependent activation of the exchanger and that in cardiac myocytes, this process serves as a feedback mechanism that links exchange activity to the diastolic concentration of cytosolic Ca(2+).

A polymorphic GT repeat from the human cardiac Na+Ca2+ exchanger intron 2 activates splicing

The sequence analysis of the human intron 2 from the Na+/Ca2+ exchanger 1 (NCX1) gene has revealed a GT repeat of variable length (10-16). The 5' sequence of intron 2 exhibited significant homology (65-70%) with other minisatellite sequences. DNA segments at the 5' end of intron 2 were inserted in the NCX1 cDNA (3.7 kb) to reconstruct the exon 2/intron 2 junction. Transient expression of these constructs in HEK293 cells generated shortened mRNAs ( approximately 2.5 kb). RT-PCR and ribonuclease protection analysis of the 3' end of the short transcripts indicated a splicing event at the intron 2/exon 2 junction (5' site) and in the vector sequence downstream of the NCX1 insert (3' site). Molecular dissection of the 5'-intron 2 sequence showed that the GT repeat was required for splicing activation, whereas the remainder of the 5'-intron 2 segment was completely inactive. The results indicate that the GT repeat is a strong intronic splicing enhancer that could be involved in the regulation of NCX1 expression, possibly mediating tissue-specific alternative splicing of the mutually exclusive exons 3 and 4, and/or exon-2 circularization.

Sodium-calcium exchange: a molecular perspective

Plasma membrane Na(+)-Ca2+ exchange is an essential component of Ca2+ signaling pathways in several tissues. Activity is especially high in the heart where the exchanger is an important regulator of contractility. An expanding exchanger superfamily includes three mammalian Na(+)-Ca2+ exchanger genes and a number of alternative splicing products. New information indicates that the exchanger protein has nine transmembrane segments. The exchanger, which transports Na+ and Ca2+, is also regulated by these substrates. Some molecular information is available on regulation by Na+ and Ca2+ and by PIP2 and phosphorylation. Altered expression of the exchanger in pathophysiological states may contribute to various cardiac phenotypes. Use of transgenic approaches is beginning to improve our knowledge of exchanger function.

Feedback inhibition of sodium/calcium exchange by mitochondrial calcium accumulation

Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger were subjected to two periods of 5 and 3 min, respectively, during which the extracellular Na(+) concentration ([Na(+)](o)) was reduced to 20 mm; these intervals were separated by a 5-min recovery period at 140 mm Na(+)(o). The cytosolic Ca(2+) concentration ([Ca(2+)](i)) increased during both intervals due to Na(+)-dependent Ca(2+) influx by the exchanger. However, the peak rise in [Ca(2+)](i) during the second interval was only 26% of the first. The reduced rise in [Ca(2+)](i) was due to an inhibition of Na(+)/Ca(2+) exchange activity rather than increased Ca(2+) sequestration since the influx of Ba(2+), which is not sequestered by internal organelles, was also inhibited by a prior interval of Ca(2+) influx. Mitochondria accumulated Ca(2+) during the first interval of reduced [Na(+)](o), as determined by an increase in fluorescence of the Ca(2+)-indicating dye rhod-2, which preferentially labels mitochondria. Agents that blocked mitochondrial Ca(2+) accumulation (uncouplers, nocodazole) eliminated the observed inhibition of exchange activity during the second period of low [Na(+)](o). Conversely, diltiazem, an inhibitor of the mitochondrial Na(+)/Ca(2+) exchanger, increased mitochondrial Ca(2+) accumulation and also increased the inhibition of exchange activity. We conclude that Na(+)/Ca(2+) exchange activity is regulated by a feedback inhibition process linked to mitochondrial Ca(2+) accumulation.

Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules

The Na(+)/Ca(2+) exchanger participates in Ca(2+) homeostasis in a variety of cells and has a key role in cardiac muscle physiology. We studied in this work the exchanger of amphibian skeletal muscle, using both isolated inside-out transverse tubule vesicles and single muscle fibers. In vesicles, increasing extravesicular (intracellular) Na(+) concentration cooperatively stimulated Ca(2+) efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca(2+) concentration ([Ca(2+)]) inhibited this reverse activity with an IC(50) of 91 nM. Exchanger-mediated currents were measured at 15 degrees C in single fibers voltage clamped at -90 mV. Photolysis of a cytoplasmic caged Ca(2+) compound activated an inward current (forward mode) of 23 +/- 10 nA (n = 3), with an average current density of 0.6 muA/muF. External Na(+) withdrawal generated an outward current (reverse mode) with an average current density of 0.36 +/- 0.17 muA/muF (n = 6) but produced a minimal increase in cytosolic [Ca(2+)]. These results suggest that, in skeletal muscle, the main function of the exchanger is to remove Ca(2+) from the cells after stimulation.

Expression of the sodium/calcium exchanger in mammalian skeletal muscle cells in primary culture

Previous investigations have demonstrated molecular and functional expression, at early phases of development of skeletal muscle cells in primary culture, of cardiac isoforms of proteins involved in calcium transport and regulation, like the L-type calcium channel. Here the expression of the cardiac isoform of the Na(+)/Ca(2+) exchanger (NCX1) was studied in skeletal muscle cells developing in vitro, by using biochemical, immunological, and electrophysiological techniques. Northern and Western blot experiments revealed the presence of this cardiac exchanger and its increasing expression during the early phases of development. Confocal imaging of myotubes showed an NCX1 distribution that was predominantly sarcolemmal. The whole-cell patch-clamp technique allowed us to record ionic currents, the direction and the amplitude of which depended on extracellular sodium and calcium concentrations. The developmental changes of this functional expression could be correlated with the molecular NCX1 expression changes. Taken together these data demonstrate the presence of the NCX1 isoform of the Na(+)/Ca(2+) exchanger during in vitro myogenesis and reinforce the theory that significant levels of cardiac-type proteins are transiently expressed during the early phases of the skeletal muscle cell development.

Spatial distribution and characteristics of voltage-gated calcium signals within visual interneurons

Most of our knowledge about insect calcium currents is derived from studies on cultured or dissociated somata. So far, only little data on calcium currents are available for neurons including their dendritic and presynaptic structures. Here we combined the switched-electrode voltage-clamp technique with optical recording using calcium-sensitive dyes in identified fly visual interneurons in vivo to characterize the voltage dependence and dynamics of calcium currents quantitatively and in a spatially resolved way. For all three cell types considered, i.e., centrifugal horizontal (CH), horizontal system (HS), and vertical system (VS) cells, the activation curve is rather flat and covers a voltage range from -60 to -20 mV in dendritic as well as presynaptic areas of the cells. The calcium increase is fastest for CH cells with a time constant of approximately 70 ms. In HS and VS cells, the time constant amounts to 400-700 ms. The calcium dynamics as determined in different regions of the cells are similar except for a small segment between the axon and the dendrite in HS and VS cells, where the calcium increase is significantly faster. In summary, the results show the existence of a low-voltage-activated calcium current with little or no inactivation in dendritic as well as presynaptic regions of fly lobula plate tangential cells.

Functional role of ionic regulation of Na+/Ca2+ exchange assessed in transgenic mouse hearts

Na+/Ca2+ exchange is the primary mechanism mediating Ca2+ efflux from cardiac myocytes during diastole and, thus, can prominently influence contractile force. In addition to transporting Na+ and Ca2+, the exchanger is also regulated by these ions. Although structure-function studies have identified protein regions of the exchanger subserving these regulatory processes, their physiological importance is unknown. In this study, we examined the electrophysiological and mechanical consequences of cardiospecific overexpression of the canine cardiac exchanger NCX1.1 and a deletion mutant of NCX1.1 (Delta680-685), devoid of intracellular Na+ (Na+i)- and Ca2+ (Ca2+i)- dependent regulatory properties, in transgenic mice. Using the giant excised patch-clamp technique, normal ionic regulation was observed in membrane patches from cardiomyocytes isolated from control and transgenic mice overexpressing NCX1.1. In contrast, ionic regulation was nearly abolished in mice overexpressing Delta680-685, indicating that the native regulatory processes could be overwhelmed by expression of the transgene. To address the physiological consequences of ionic regulation of the Na+/Ca2+ exchanger, we examined postrest force development in papillary muscles from NCX1.1 and Delta680-685 transgenic mice. Postrest potentiation was found to be substantially greater in Delta680-685 than in NCX1.1 transgenic mice, supporting the notion that ionic regulation of Na+/Ca2+ exchange plays a significant functional role in cardiac contractile properties.

Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade

The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.

Direction-independent block of bi-directional Na+/Ca2+ exchange current by KB-R7943 in guinea-pig cardiac myocytes

1. We investigated the inhibitory effect of KB-R7943 on 'bi-directional' Na+/Ca2+ exchange current (iNCX) with the reversal potential of iNCX (ENCX) in the middle of the ramp voltage pulse employed. 2. Bi-directional iNCX was recorded with 'full' ramp pulses given every 10 s from the holding potential of -60 mV over the voltage range between 30 and -150 mV under the ionic conditions of 140 mM [Na]o, 20 mM [Na]i, 1 mM [Ca]o and 433 nM [Ca]i with calculated ENCX at -50 mV. 3. KB-R7943 (0.1 - 100 mirconM) concentration-dependently inhibited the current, which reversed near the calculated ENCX, indicating that the blocked current was iNCX. 4. The inhibition levels were not significantly different between outward and inward iNCX measured at 0 and -120 mV, respectively. IC50 of KB-R7943 was approximately 1 micronM for both directions of iNCX. 5. Under the bi-directional ionic conditions, only an outward or inward iNCX was induced by positive or negative 'half' ramp pulses, respectively, from the holding potential of -60 mV. KB-R7943 inhibited both direction of iNCX and the concentration-inhibition relations were superimposable to the ones obtained by 'full' ramp pulses. 6. These results indicate that KB-R7943 inhibits iNCX direction-independently under bi-directional conditions. This conclusion is different from that of our previous results obtained from iNCX under uni-directional ionic conditions, where KB-R7943 inhibited iNCX direction-dependently. The difference could be attributed to slow dissociation of the drug from the exchanger.