Physiological effects of Na+/Ca2+ exchanger knockdown by antisense oligodeoxynucleotides in arterial myocytes

Association of Disease Recurrence With Survival Outcomes in Patients With Cutaneous Squamous Cell Carcinoma of the Head and Neck Treated With Multimodality Therapy

Importance

It has previously been demonstrated that immunosuppressed patients with cutaneous squamous cell cancer of the head and neck (cSCC-HN) treated with surgery and postoperative radiotherapy have significantly inferior disease-related outcomes compared with immunocompetent patients, but data on outcomes after disease recurrence are limited.

Objectives

To report survival outcomes in patients with cSCC-HN after disease recurrence after surgery and postoperative radiotherapy and to investigate the association of immune status with disease-related outcomes.

Design, Setting, and Participants

A multi-institutional study of 205 patients treated at the Cleveland Clinic, Washington University in St Louis, and the University of California, San Francisco, in which patients who underwent surgical resection and postoperative radiotherapy for primary or recurrent stage I to IV (nonmetastatic) cSCC-HN between January 1, 1995, and December 31, 2014, were identified. Patients with any disease recurrence, defined as local, regional, and/or distant failure, were included. Patients were categorized as immunosuppressed if they received a diagnosis of chronic hematologic malignant neoplasm or HIV or AIDS, or were treated with immunosuppressive therapy for organ transplantation 6 months or more before diagnosis. Statistical analysis was conducted from January 1, 1995, to December 31, 2015.

Main Outcomes and Measures

Overall survival calculated using the Kaplan-Meier method and compared using the log-rank test.

Results

Of the 205 patients in the original cohort, 72 patients (63 men and 9 women; median age, 71 years [range, 43-91 years]) developed disease recurrence after surgery and postoperative radiotherapy. Forty patients (55.6%) were immunosuppressed, and 32 patients (44.4%) were immunocompetent. Locoregional recurrence was the most common first pattern of failure for both groups (31 immunosuppressed patients [77.5%]; 21 immunocompetent patients [65.6%]). After any recurrence, 1-year overall survival was 43.2% (95% CI, 30.9%-55.4%), and median survival was 8.4 months. For patients for whom information on salvage treatment was available (n = 45), those not amenable to surgical salvage had significantly poorer median cumulative incidence of survival compared with those who were amenable to surgical salvage (4.7 months; 95% CI, 3.7-7.0 months vs 26.1 months; 95% CI, 6.6 months to not reached; P = .01), regardless of their immune status.

Conclusions and Relevance

Results of this study suggest that patients with cSCC-HN who experience disease recurrence after definitive treatment with surgery and postoperative radiotherapy have poor survival, irrespective of immune status. Survival rates are low for patients with recurrent disease that is not amenable to surgical salvage. The low rate of successful salvage underscores the importance of intensifying upfront treatment to prevent recurrence.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Overactive bladder mediated by accelerated Ca2+ influx mode of Na+/Ca2+ exchanger in smooth muscle

The Na(+)/Ca(2+) exchanger (NCX) is thought to be a key molecule in the regulation of cytosolic Ca(2+) dynamics. The relative importance of the two Ca(2+) transport modes of NCX activity leading to Ca(2+) efflux (forward) and influx (reverse) in smooth muscle, however, remains unclear. Unexpectedly, spontaneous contractions of urinary bladder smooth muscle (UBSM) were enhanced in transgenic mice overexpressing NCX1.3 (NCX1.3(tg/tg)). The enhanced activity was attenuated by KB-R7943 or SN-6. Whole cell outward NCX current sensitive to KB-R7943 or Ni(2+) was readily detected in UBSM cells from NCX1.3(tg/tg) but not wild-type mice. Spontaneous Ca(2+) transients in myocytes of NCX1.3(tg/tg) were larger and frequently resulted in propagating events and global elevations in cytosolic Ca(2+) concentration. Significantly, NCX1.3(tg/tg) mice exhibited a pattern of more frequent urination of smaller volumes and this phenotype was reversed by oral administration of KB-R7943. On the other hand, KB-R7943 did not improve it in KB-R7943-insensitive (G833C-)NCX1.3(tg/tg) mice. We conclude that NCX1.3 overexpression is associated with abnormal urination owing to enhanced Ca(2+) influx via reverse mode NCX leading to prolonged, propagating spontaneous Ca(2+) release events and a potentiation of spontaneous UBSM contraction. These findings suggest the possibility that NCX is a candidate molecular target for overactive bladder therapy.

New insights into the contribution of arterial NCX to the regulation of myogenic tone and blood pressure

Plasma membrane protein Na(+)/Ca(2+) exchanger (NCX) in vascular smooth muscle (VSM) cells plays an important role in intracellular Ca(2+) homeostasis, Ca(2+) signaling, and arterial contractility. Recent evidence in intact animals reveals that VSM NCX type 1 (NCX1) is importantly involved in the control of arterial blood pressure (BP) in the normal state and in hypertension. Increased expression of vascular NCX1 has been implicated in human primary pulmonary hypertension and several salt-dependent hypertensive animal models. Our aim is to determine the molecular and physiological mechanisms by which vascular NCX influences vasoconstriction and BP normally and in salt-dependent hypertension. Here, we describe the relative contribution of VSM NCX1 to Ca(2+) signaling and arterial contraction, including recent data from transgenic mice (NCX1(smTg/Tg), overexpressors; NCX1(sm-/-), knockouts) that has begun to elucidate the specific contributions of NCX to BP regulation. Arterial contraction and BP correlate with the level of NCX1 expression in smooth muscle: NCX1(sm-/-) mice have decreased arterial myogenic tone (MT), vasoconstriction, and low BP. NCX1(smTg/Tg) mice have high BP and are more sensitive to salt; their arteries exhibit upregulated transient receptor potential canonical channel 6 (TRPC6) protein, increased MT, and vasoconstriction. These observations suggest that NCX is a key component of certain distinct signaling pathways that activate VSM contraction in response to stretch (i.e., myogenic response) and to activation of certain G-protein-coupled receptors. Arterial NCX expression and mechanisms that control the local (sub-plasma membrane) Na(+) gradient, including cation-selective receptor-operated channels containing TRPC6, regulate arterial Ca(2+) and constriction, and thus BP.

Long-term maternal hypoxia: the role of extracellular Ca2+ entry during serotonin-mediated contractility in fetal ovine pulmonary arteries

Antenatal maternal long-term hypoxia (LTH) can alter serotonin (5-HT) and calcium (Ca(2+)) signaling in fetal pulmonary arteries (PAs) and is associated with persistent pulmonary hypertension of the newborn (PPHN). In humans, the antenatal maternal hypoxia can be secondary to smoking, anemia, and chronic obstructive pulmonary disorders. However, the mechanisms of antenatal maternal hypoxia-related PPHN are unresolved. Because both LTH and 5-HT are associated with PPHN, we tested the hypothesis that antenatal maternal LTH can increase 5-HT-mediated PA contraction and associated extracellular Ca(2+) influx through L-type Ca(2+) channels (Ca(L)), nonselective cation channels (NSCCs), and reverse-mode sodium-calcium exchanger (NCX) in the near-term fetus. We performed wire myography and confocal-Ca(2+) imaging approaches on fetal lamb PA (∼ 140 days of gestation) from normoxic ewes or those acclimatized to high-altitude LTH (3801 m) for ∼110 days. Long-term hypoxia reduced the potency but not the efficacy of 5-HT-induced PA contraction. Ketanserin (100 nmol/L), a 5-HT(2A) antagonist, shifted 5-HT potency irrespective of LTH, while GR-55562 (1 µmol/L), a 5-HT(1B/D) inhibitor, antagonized 5-HT-induced contraction in normoxic fetuses only. Various inhibitors for Ca(L), NSCC, and reverse-mode NCX were used in contraction studies. Contraction was reliant on extracellular Ca(2+) regardless of maternal hypoxia, NSCC was more important to contraction than Ca(L), and reverse-mode NCX had little or no role in contraction. Long-term hypoxia also attenuated the effects of 2-APB and flufenamic acid and reduced Ca(2+) responses observed by imaging studies. Overall, LTH reduced 5HT(1B/D) function and increased NSCC-related Ca(2+)-dependent contraction in ovine fetuses, which may compromise pulmonary vascular function in the newborn.

Sodium-calcium exchanger and lipid rafts in pig coronary artery smooth muscle

Pig coronary artery smooth muscle expresses, among many other proteins, Na+-Ca²+-exchanger NCX1 and sarcoplasmic reticulum Ca²+ pump SERCA2. NCX1 has been proposed to play a role in refilling the sarcoplasmic reticulum Ca²+ pool suggesting a functional linkage between the two proteins. We hypothesized that this functional linkage may require close apposition of SERCA2 and NCX1 involving regions of plasma membrane like lipid rafts. Lipid rafts are specialized membrane microdomains that appear as platforms to co-localize proteins. To determine the distribution of NCX1, SERCA2 and lipid rafts, we isolated microsomes from the smooth muscle tissue, treated them with non-ionic detergent and obtained fractions of different densities by sucrose density gradient centrifugal flotation. We examined the distribution of NCX1; SERCA2; non-lipid raft plasma membrane marker transferrin receptor protein; lipid raft markers caveolin-1, flotillin-2, prion protein, GM1-gangliosides and cholesterol; and cytoskeletal markers clathrin, actin and myosin. Distribution of markers identified two subsets of lipid rafts that differ in their components. One subset is rich in caveolin-1 and flotillin-2 and the other in GM1-gangliosides, prion protein and cholesterol. NCX1 distribution correlated strongly with SERCA2, caveolin-1 and flotillin-2, less strongly with the other membrane markers and negatively with the cytoskeletal markers. These experiments were repeated with a non-detergent method of treating microsomes with sonication at high pH and similar results were obtained. These observations are consistent with the observed functional linkage between NCX1 and SERCA2 and suggest a role for NCX1 in supplying Ca²+ for refilling the sarcoplasmic reticulum.

Knockout of Na+/Ca2+ exchanger in smooth muscle attenuates vasoconstriction and L-type Ca2+ channel current and lowers blood pressure

Mice with smooth muscle (SM)-specific knockout of Na(+)/Ca(2+) exchanger type-1 (NCX1(SM-/-)) and the NCX inhibitor, SEA0400, were used to study the physiological role of NCX1 in mouse mesenteric arteries. NCX1 protein expression was greatly reduced in arteries from NCX1(SM-/-) mice generated with Cre recombinase. Mean blood pressure (BP) was 6-10 mmHg lower in NCX1(SM-/-) mice than in wild-type (WT) controls. Vasoconstriction was studied in isolated, pressurized mesenteric small arteries from WT and NCX1(SM-/-) mice and in heterozygotes with a global null mutation (NCX1(Fx/-)). Reduced NCX1 activity was manifested by a marked attenuation of responses to low extracellular Na(+) concentration, nanomolar ouabain, and SEA0400. Myogenic tone (MT, 70 mmHg) was reduced by approximately 15% in NCX1(SM-/-) arteries and, to a similar extent, by SEA0400 in WT arteries. MT was normal in arteries from NCX1(Fx/-) mice, which had normal BP. Vasoconstrictions to phenylephrine and elevated extracellular K(+) concentration were significantly reduced in NCX1(SM-/-) arteries. Because a high extracellular K(+) concentration-induced vasoconstriction involves the activation of L-type voltage-gated Ca(2+) channels (LVGCs), we measured LVGC-mediated currents and Ca(2+) sparklets in isolated mesenteric artery myocytes. Both the currents and the sparklets were significantly reduced in NCX1(SM-/-) (vs. WT or NCX1(Fx/-)) myocytes, but the voltage-dependent inactivation of LVGCs was not augmented. An acute application of SEA0400 in WT myocytes had no effect on LVGC current. The LVGC agonist, Bay K 8644, eliminated the differences in LVGC currents and Ca(2+) sparklets between NCX1(SM-/-) and control myocytes, suggesting that LVGC expression was normal in NCX1(SM-/-) myocytes. Bay K 8644 did not, however, eliminate the difference in myogenic constriction between WT and NCX1(SM-/-) arteries. We conclude that, under physiological conditions, NCX1-mediated Ca(2+) entry contributes significantly to the maintenance of MT. In NCX1(SM-/-) mouse artery myocytes, the reduced Ca(2+) entry via NCX1 may lower cytosolic Ca(2+) concentration and thereby reduce MT and BP. The reduced LVGC activity may be the consequence of a low cytosolic Ca(2+) concentration.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Functional linkage of Na+-Ca2+-exchanger to sarco/endoplasmic reticulum Ca2+ pump in coronary artery: comparison of smooth muscle and endothelial cells

An increase in cytosolic Ca(2+) concentration in coronary artery smooth muscle causes a contraction but in endothelium it causes relaxation. Na(+)-Ca(2+)-exchanger (NCX) may play a role in Ca(2+) dynamics in both the cell types. Here, the NCX-mediated (45)Ca(2+) uptake was compared in Na(+)-loaded pig coronary artery smooth muscle and endothelial cells. In both the cell types, this uptake was inhibited by KB-R7943, SEA 0400 and by monensin, but not by cariporide. Prior loading of the cells with the Ca(2+) chelator BAPTA increased the NCX-mediated (45)Ca(2+) uptake in smooth muscle but not in endothelial cells. In the presence or absence of BAPTA loading, the Na(+)-mediated (45)Ca(2+) uptake was greater in endothelial than in smooth muscle cells. In smooth muscle cells without BAPTA loading, thapsigargin diminished the NCX-mediated (45)Ca(2+) entry. This effect was not observed in endothelial cells or in either cell type after BAPTA loading. The results in the smooth muscle cells are consistent with a limited diffusional space model in which the NCX-mediated (45)Ca(2+) uptake was enhanced by chelation of cytosolic Ca(2+) or by its sequestration by the sarco/endoplasmic reticulum Ca(2+) pump (SERCA). They suggest a functional linkage between NCX and SERCA in the smooth muscle but not in the endothelial cells. The concept of a linkage between NCX and SERCA in smooth muscle was also confirmed by similar distribution of NCX and SERCA2 proteins when detergent-treated microsomes were fractionated by flotation on sucrose density gradients. Thus, the coronary artery smooth muscle and endothelial cells differ not only in the relative activities of NCX but also in its functional linkage to SERCA.

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.

Involvement of Na+-Ca2+ exchanger on metabotropic glutamate receptor 1-mediated [Ca2+]i transients in rat cerebellar Purkinje neurons

Cerebellar Purkinje neurons have intracellular regulatory systems including Ca2+-binding proteins, intracellular Ca2+ stores, Ca2+-ATPase and Na+-Ca2+ exchanger (NCX) that keep intracellular Ca2+ concentration ([Ca2+]i) in physiological range. Among these, NCX interacts with AMPA receptors, activation of which induces cerebellar synaptic plasticity. And the activation of metabotropic glutamate receptor 1 (mGluR1) is also involved in the induction of cerebellar long-term depression. The interaction of NCX with mGluR1 is not known yet. Thus, in this study, the functional relationship between NCX and mGluR1 in modulating the [Ca2+]i in rat Purkinje neurons was investigated. The interaction between NCX and mGluR1 in Purkinje neurons was studied by measuring intracellular Ca2+ transients induced by an agonist of group I mGluRs, 3,5-dihydroxyphenylglycine (DHPG). The DHPG-induced Ca2+ transient was significantly reduced by treatments of NCX inhibitors, bepridil and KB-R7943. When cells were pretreated with antisense oligodeoxynucleotides of NCX, the DHPG-induced Ca2+ transient was also inhibited. These results suggest that NCX modulates the activity of mGluR1 in cerebellar Purkinje neurons. Therefore, NCX appears to play an important role in the physiological function of cerebellar Purkinje neurons such as synaptic plasticity.

Sodium-Calcium Exchanger in Pulmonary Artery Smooth Muscle Cells

The expression and function of the Na+/Ca2+ exchanger (NCX) in the regulation of intracellular Ca2+ homeostasis have been well studied in cardiac, skeletal, and systemic vascular myocytes, but not in pulmonary artery smooth muscle cells (SMCs). We have recently demonstrated that the NCX current is present in freshly isolated pulmonary artery SMCs using the patch-clamp technique. The current has a mean amplitude of 13 pA under near physiological resting conditions. The NCX may function in the forward mode to make a significant contribution to the decay of intracellular Ca2+ following Ca2+ release and/or depolarization. Hypoxic stimulation inhibits the NCX current, reduces the removal of intracellular Ca2+, and enhances Ca2+ release from the sarcoplasmic reticulum. Using RT-PCR, subcloning and sequence analysis, we have shown that three NCX1 splice variants: NCX1.2 (containing exons B, C, and D), NCX1.3 (exons B and D), and NCX1.7 (exons B, D, and F) are expressed in pulmonary artery smooth muscle. Each of these splice variants expressed in HEK293 cells it likely to show a distinct activity in the removal of intracellular Ca2+. Taken together, we provide clear evidence that NCX1 is functionally and molecularly expressed and plays a physiological role in pulmonary artery SMCs.

Involvement of Na+-Ca2+ exchanger in cAMP-mediated relaxation in mice aorta: evaluation using transgenic mice

BACKGROUND AND PURPOSE

Although vascular smooth muscle cells are known to express the Na+-Ca2+ exchanger (NCX), its functional role has remained unclear, mainly because of its relatively low expression. We thus investigated the involvement of NCX in the mechanism for the forskolin-induced vaso-relaxation, using wild type (WT) and transgenic (TG) mice that specifically over-express NCX1.3 in smooth muscle.

EXPERIMENTAL APPROACH

We examined the relaxing effect of forskolin during the pre-contraction induced by 100 nM U46619, a thromboxane A2 analogue in the mouse isolated thoracic aorta. We also measured the intracellular Ca2+ concentration ([Ca2+]i) in fura-PE3-loaded aortic strips.

KEY RESULTS

The forskolin-induced decreases in [Ca2+]i and tension were much greater in aortas from TG mice than in those from WT mice. In a low Na+ solution, forskolin-induced decreases in [Ca2+]i and tension were greatly inhibited in both groups of aortas. In WT aortas, the presence of 100 nM SEA0400, an NCX inhibitor, had only a little effect on the forskolin-induced decreases in [Ca2+]i, but inhibited the forskolin-induced relaxation. However, in TG aortas, the presence of SEA0400 greatly inhibited the forskolin-induced decreases in [Ca2+]i and tension.

CONCLUSIONS AND IMPLICATIONS

The NCX was involved in the forskolin-induced reduction of [Ca2+]i and tension in the mouse thoracic aorta. Measurement of [Ca2+]i and tension in aortas of the TG mouse is thus considered to be a useful tool for evaluating the role of NCX in vascular tissue.

Topics on the Na+/Ca2+ exchanger: involvement of Na+/Ca2+ exchanger in the vasodilator-induced vasorelaxation

Many kinds of vasodilators induce relaxation of the vascular smooth muscle cells (VSMCs) through the production of cyclic AMP (cAMP) or cyclic GMP (cGMP). The relaxant effects mediated by these second messengers are thought to be mainly due to the decrease in intracellular Ca(2+) concentration ([Ca(2+)](i)), as well as the decrease in Ca(2+) sensitivity of the contractile apparatus of VSMCs. To explain the cAMP- or cGMP-mediated decrease in [Ca(2+)](i), several mechanisms have been proposed, including the inhibition of Ca(2+) influx due to a hyperpolarization, a stimulation of Ca(2+) uptake into the intracellular store, and an increase in Ca(2+) extrusion from VSMCs by stimulation of sarcolemmal Ca(2+)-pump. VSMCs have two major systems for Ca(2+) extrusion, namely, sarcolemmal Ca(2+)-pump and Na(+)/Ca(2+) exchanger (NCX). However, the involvement of NCX in the vasodilator-induced relaxation of VSMCs has not been well established. In this article, the possible involvement of NCX in the vasodilator-induced relaxation of VSMCs will be reviewed.

Hypertension, Na+/Ca2+ exchanger, and Na+, K+-ATPase

Hypertension is the most prevalent risk factor for stroke, myocardial infarction, or end-stage renal failure. The critical importance of excess salt intake in the pathogenesis of hypertension is widely recognized, but the mechanisms whereby salt intake elevates blood pressure have puzzled researchers. Recent studies using Na+/Ca2+ exchange inhibitors and genetically engineered mice provide evidence that vascular Na+/Ca2+ exchanger type 1 (NCX1) is involved in the development of salt-dependent hypertension. Endogenous cardiac glycosides, which may contribute to salt-dependent hypertension, seem to be necessary for NCX1-mediated hypertension. Intriguingly, studies using knock-in mice with modified cardiac glycoside binding affinity of Na+,K+-ATPases provide a clear demonstration that this cardiac glycoside-binding site plays an important role in blood pressure regulation. Taken all together: (1) endogenous cardiac glycosides are secreted after high salt intake; (2) these cardiac glycosides inhibit Na+,K+-ATPase in vascular smooth muscle cells; (3) this inhibition results in the elevation of local Na+ on the submembrane area; and (4) this elevation of local Na+ facilitates Ca2+ entry through NCX1, resulting in vasoconstriction. This proposed pathway may have enabled us to explain how to link dietary salt to hypertension.

Vascular Na+/Ca2+exchanger: implications for the pathogenesis and therapy of salt-dependent hypertension

The Na+/Ca2+exchanger is an ion transporter that exchanges Na+and Ca2+in either Ca2+efflux or Ca2+influx mode, depending on membrane potential and transmembrane ion gradients. In arterial smooth muscle cells, the Na+/Ca2+exchanger is thought to participate in the maintenance of vascular tone by regulating cytosolic Ca2+concentration. Recent pharmacological and genetic engineering studies have revealed that the Ca2+influx mode of vascular Na+/Ca2+exchanger type-1 (NCX1) is involved in the pathogenesis of salt-dependent hypertension. SEA0400, a specific Na+/Ca2+exchange inhibitor that preferentially blocks the Ca2+influx mode, lowers arterial blood pressure in salt-dependent hypertensive models, but not in normotensive rats or other types of hypertensive rats. Furthermore, heterozygous mice with reduced expression of NCX1 are resistant to development of salt-dependent hypertension, whereas transgenic mice with vascular smooth muscle-specific overexpression of NCX1 readily develop hypertension after high-salt loading. SEA0400 reverses the cytosolic Ca2+elevation and vasoconstriction induced by nanomolar ouabain, as well as humoral factors in salt-loaded animals. One possibility is that circulating endogenous cardiotonic steroids may be necessary for NCX1-mediated hypertension. These findings help to explain how arterial smooth muscle cells in blood vessels contribute to salt-elicited blood pressure elevation and suggest that NCX1 inhibitors might be therapeutically useful for salt-dependent hypertension.

Salt-sensitive hypertension, Na+/Ca2+ exchanger, and vascular smooth muscle

Hypertension is the most common chronic disease and is the leading risk factor for death caused by stroke, myocardial infarction, and end-stage renal failure. The critical importance of excess salt intake in the pathogenesis of hypertension is widely recognized. However, the molecular mechanisms underlying salt-sensitive hypertension remain obscure. Recent studies using selective Na(+)/Ca(2+) exchanger (NCX) inhibitors and genetically engineered mice provide compelling evidence that salt-sensitive hypertension is triggered by Ca(2+) entry through NCX type 1 (NCX1) in arterial smooth muscle. Cardiotonic steroids, such as endogenous ouabain, which may contribute to the pathogenesis of salt-sensitive hypertension, seem to be necessary for NCX1-mediated hypertension. These findings have enabled us to explain how high salt intake leads to hypertension and further to describe the potential of vascular NCX1 as a new therapeutic or diagnostic target for salt-sensitive hypertension.

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

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

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Rapid turnover of the "functional" Na(+)-Ca2+ exchanger in cardiac myocytes revealed by an antisense oligodeoxynucleotide approach

Antisense oligodeoxynucleotides (AS-ODNs) were used in combination with transient functional expression of the cardiac Na(+)-Ca2+ exchanger (NCX1) to correlate suppression of the Na(+)-Ca2+ exchange function with down-regulation of NCX1 protein expression. In a de-novo expression system (Sf9 cells), a decrease in both, NCX1 mRNA and protein after AS-ODN application was paralleled by diminished NCX1 activity, a typical hallmark of a true "antisense effect". Although AS-ODN uptake was also efficient in rat neonatal cardiac myocytes, in whole-cell extracts of these cells treated with AS-ODNs, the amount of NCX1 protein determined in a quantitative binding assay remained almost unchanged, despite a prompt loss of NCX1 function. Immunocytochemical staining of myocytes revealed that most of the immunoreactivity was not localized in the plasma membrane, but in intracellular compartments and was barely affected by AS-ODN treatment. These results indicate that the "functional half-life" of the NCX1 protein in the plasma membrane of neonatal cardiac myocytes is surprisingly short, much shorter than reported half-lifes of about 30 h for other membrane proteins.

Effects of Na+-Ca2+ exchanger activity on the alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-induced Ca2+ influx in cerebellar Purkinje neurons

Variations in intracellular calcium activity ([Ca2+]i) play crucial roles in information processing in Purkinje neurons such as synaptic plasticity. Although Na+-Ca2+ exchanger (NCX) has been shown to participate in the regulation of homeostasis and secretion in neuronal cells, the physiological role of NCX in Purkinje neurons, such as a role in cerebellar synaptic plasticity, is not well understood. NCX in acutely dissociated rat Purkinje neurons was identified by double staining with anti-calbindin D-28k antibody and anti-NCX antibody. The physiological activity of NCX was examined by measuring transient intracellular Ca2+ changes resulting from the Ca2+ influx via reverse mode of NCX (with 0 mM Na+/2.5 mM Ca2+ solutions) and the efflux via the forward mode of NCX (with 140 mM Na+/0 mM Ca2+ solutions). This transient increase in Ca2+ concentration was not elicited in the cells pretreated with NCX antisense oligodeoxynucleotides. And the Ca2+ influx resulting from the reverse mode of NCX was significantly reduced by 2-[2-[4-(4-nitrobenyloxy) phenyl] ethyl] isothiourea methanesulfonate, while the Ca2+ efflux via forward mode was inhibited by bepridil. The physiological role of NCX in synaptic function was studied by measuring Ca2+ transients induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate (AMPA) receptor activation. This AMPA-evoked response was decreased with the inhibition of NCX forward mode and also, to less degree, with the inhibition of reverse mode. In antisense oligodeoxynucleotides pretreated cells, the AMPA-evoked response was also reduced, as was the case in NCX-inhibitor treated cells. The inhibition of NCX activity had depressant effects on Ca2+ transients induced by AMPA receptor activation. These results suggest that NCX plays a physiological role in modulating the activity of cerebellar Purkinje neurons, such as synaptic plasticity, via interaction with AMPA receptors in Purkinje neurons.

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

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

Salt-sensitive hypertension is triggered by Ca2+ entry via Na+/Ca2+ exchanger type-1 in vascular smooth muscle

Excessive salt intake is a major risk factor for hypertension. Here we identify the role of Na(+)/Ca(2+) exchanger type 1 (NCX1) in salt-sensitive hypertension using SEA0400, a specific inhibitor of Ca(2+) entry through NCX1, and genetically engineered mice. SEA0400 lowers arterial blood pressure in salt-dependent hypertensive rat models, but not in other types of hypertensive rats or in normotensive rats. Infusion of SEA0400 into the femoral artery in salt-dependent hypertensive rats increases arterial blood flow, indicating peripheral vasodilation. SEA0400 reverses ouabain-induced cytosolic Ca(2+) elevation and vasoconstriction in arteries. Furthermore, heterozygous NCX1-deficient mice have low salt sensitivity, whereas transgenic mice that specifically express NCX1.3 in smooth muscle are hypersensitive to salt. SEA0400 lowers the blood pressure in salt-dependent hypertensive mice expressing NCX1.3, but not in SEA0400-insensitive NCX1.3 mutants. These findings indicate that salt-sensitive hypertension is triggered by Ca(2+) entry through NCX1 in arterial smooth muscle and suggest that NCX1 inhibitors might be useful therapeutically.

KB-R7943 reveals possible involvement of Na+-Ca2+ exchanger in elevation of intracellular Ca2+ in rat carotid arterial myocytes

A Na(+)/Ca(2+) exchanger (NCX) is one of the major regulators of intracellular Ca(2+) concentration ([Ca(2+)](i)) in cardiac muscle cells. Although vascular smooth muscle myocytes also express NCX proteins, their functional role has not been clear, mainly due to the lack of specific inhibitors of NCX and relatively low levels of expression of NCX. In the present study, we have examined the involvement of NCX in the Na(+) deficient (0 Na(+)) elevation of [Ca(2+)](i) in rat carotid arterial myocytes using KB-R7943, an inhibitor of NCX. Perfusion with a Na(+)-free bathing solution, prepared by replacement of Na(+) with N-methyl-D-glucamine, induced an elevation of [Ca(2+)](i), which was effectively inhibited by KB-R7943 (IC(50)=3.5 microM). This inhibition was reversed by washout of KB-R7943. In contrast, D600, a blocker of voltage dependent L-type Ca(2+) channels (VDCC), did not affect the 0 Na(+)-induced elevation of [Ca(2+)](i). Treatment of myocytes with ryanodine abolished the elevation of [Ca(2+)](i) caused by caffeine but not that caused by 0 Na(+). Application of Cd(2+), which is known to block NCX as well as VDCC, also significantly inhibited the 0 Na(+) induced elevation. These results suggest that KB-R7943 inhibits the extracellular Na(+) dependent ([Na(+)](o)) change in [Ca(2+)](i) in rat carotid arterial myocytes, which is presumably activated by the reverse mode of NCX.

An important role for the Na+-Ca2+ exchanger in the decrease in cytosolic Ca2+ concentration induced by isoprenaline in the porcine coronary artery

The role of the Na+-Ca2+ exchanger (NCX) in the mechanism of the isoprenaline (Iso)-induced vasorelaxation was investigated by simultaneously monitoring the intracellular Ca2+ concentration ([Ca2+]i) and tension of fura-2-loaded medial strips of porcine coronary arteries. Normal physiological salt solution (PSS) contained 137.3 mM Na+ and 5.9 mM K+. During the sustained phase of contraction, Iso induced only a transient decrease in [Ca2+]i when contraction was induced by depolarization with 118 mM K+ solution containing 25.2 mM Na+. When contraction was induced with 30 mM K+ in PSS containing 113.2 mM Na+, Iso induced a sustained decrease in [Ca2+]i, whereas in contractions induced by 30 mM K+ in a low Na+ (25.2 mM Na+) PSS, Iso transiently decreased [Ca2+]i. Replacement of Ca2+ with Ba2+ (which cannot be extruded by the Ca2+ pumps but can be extruded through the NCX) resulted in decreased [Ba2+]i induced by Iso in normal but not in low Na+ PSS. On the other hand, Iso induced a sustained decrease in [Ca2+]i when strips were pre-contracted by U46619, a thromboxane A2 analogue, in PSS. Various types of K+ channel blockers (iberiotoxin, 4-aminopyridine, apamin or glibenclamide) or combinations of these blockers failed to completely inhibit the Iso-induced decreases in [Ca2+]i and tension. However, Iso-induced sustained decreases in [Ca2+]i during the contraction induced by U46619 were greatly inhibited in a low Na+ PSS. The Iso-induced decrease in tension during contraction by U46619 was greatly inhibited by 2',4'-dichlorobenzamil, a forward- and reverse-mode NCX inhibitor, but not by ouabain, a selective inhibitor of Na+,K+-ATPase. These results indicate that the NCX is involved in the Iso-induced reduction of [Ca2+]i and tension of the porcine coronary arterial smooth muscle.

Impaired ability of the Na+/Ca2+ exchanger from the Dahl/Rapp salt-sensitive rat to regulate cytosolic calcium

We previously cloned Na(+)/Ca(2+) exchanger (NCX1) from mesangial cells of salt-sensitive (SNCX = NCX1.7) and salt-resistant (RNCX = NCX1.3) Dahl/Rapp rats. The abilities of these isoforms to regulate cytosolic Ca(2+) concentration ([Ca(2+)](i)) were assessed in fura 2-loaded OK cells expressing the vector (VOK), RNCX (ROK), and SNCX (SOK). Baseline [Ca(2+)](i) was 98 +/- 20 nM (n = 12) in VOK and was significantly lower in ROK (44 +/- 5 nM; n = 12) and SOK (47 +/- 13 nM; n = 12) cells. ATP at 100 microM increased [Ca(2+)](i) by 189 +/- 55 nM (n = 12), 21 +/- 9 nM (n = 12), and 69 +/- 18 nM (n = 12) in VOK, ROK, and SOK cells, respectively. ATP (1 mM) or bradykinin (0.1 mM) caused large increases in [Ca(2+)](i) and ROK but not SOK cells were much more efficient in reducing [Ca(2+)](i) back to baseline levels. Parental Sprague-Dawley rat mesangial cells express both RNCX (SDRNCX) and SNCX (SDSNCX). SDRNCX and RNCX are identical at every amino acid residue, but SDSNCX and SNCX differ at amino acid 218 where it is isoleucine in SDSNCX and not phenylalanine. OK cells expressing SDSNCX (SDSOK) reduced ATP (1 mM)-induced [Ca(2+)](i) increase back to baseline at a rate equivalent to that for ROK cells. PKC downregulation significantly attenuated the rate at which ROK and SDSOK cells reduced ATP-induced [Ca(2+)](i) increase but had no effect in SOK cells. The reduced efficiency of SNCX to regulate [Ca(2+)](i) is attributed, in part, to the isoleucine-to-phenylalanine mutation at amino acid 218.

Na+ pump alpha 2-subunit expression modulates Ca2+ signaling

The role of the Na(+) pump alpha(2)-subunit in Ca(2+) signaling was examined in primary cultured astrocytes from wild-type (alpha(2)+/+ = WT) mouse fetuses and those with a null mutation in one [alpha(2)+/- = heterozygote (Het)] or both [alpha(2)-/- = knockout (KO)] alpha(2) genes. Na(+) pump catalytic (alpha) subunit expression was measured by immunoblot; cytosol [Na(+)] ([Na(+)](cyt)) and [Ca(2+)] ([Ca(2+)](cyt)) were measured with sodium-binding benzofuran isophthalate and fura 2 by using digital imaging. Astrocytes express Na(+) pumps with both alpha(1)- ( approximately 80% of total alpha) and alpha(2)- ( approximately 20% of total alpha) subunits. Het astrocytes express approximately 50% of normal alpha(2); those from KO express none. Expression of alpha(1) is normal in both Het and KO cells. Resting [Na(+)](cyt) = 6.5 mM in WT, 6.8 mM in Het (P > 0.05 vs. WT), and 8.0 mM in KO cells (P < 0.001); 500 nM ouabain (inhibits only alpha(2)) equalized [Na(+)](cyt) at 8 mM in all three cell types. Resting [Ca(2+)](cyt) = 132 nM in WT, 162 nM in Het, and 196 nM in KO cells (both P < 0.001 vs. WT). Cyclopiazonic acid (CPA), which inhibits endoplasmic reticulum (ER) Ca(2+) pumps and unloads the ER, induces transient (in Ca(2+)-free media) or sustained (in Ca(2+)-replete media) elevation of [Ca(2+)](cyt). These Ca(2+) responses to 10 microM CPA were augmented in Het as well as KO cells. When CPA was applied in Ca(2+)-free media, the reintroduction of Ca(2+) induced significantly larger transient rises in [Ca(2+)](cyt) (due to Ca(2+) entry through store-operated channels) in Het and KO cells than in WT cells. These results correlate with published evidence that alpha(2) Na(+) pumps and Na(+)/Ca(2+) exchangers are confined to plasma membrane microdomains that overlie the ER. The data suggest that selective reduction of alpha(2) Na(+) pump activity can elevate local [Na(+)] and, via Na(+)/Ca(2+) exchange, [Ca(2+)] in the tiny volume of cytosol between the plasma membrane and ER. This, in turn, augments adjacent ER Ca(2+) stores and thereby amplifies Ca(2+) signaling without elevating bulk [Na(+)](cyt).

Na/Ca exchanger and PMCA localization in neurons and astrocytes: functional implications

Immunocytochemistry reveals that the Na/Ca exchanger (NCX) in neuronal somata and astrocytes is confined to plasma membrane (PM) microdomains that overlie sub-PM (junctional) endoplasmic reticulum (jER). By contrast, the PM Ca(2+) pump (PMCA) is more uniformly distributed in the PM. At presynaptic nerve terminals, the NCX distribution is consistent with that observed in the neuronal somata, but the PMCA is clustered at the active zones. Thus, the PMCA, with high affinity for Ca(2+) (K(d) congruent with 100 nM), may keep active zone Ca(2+) very low and thereby "reprime" the vesicular release mechanism following activity. NCX, with lower affinity for Ca(2+) (K(d) congruent with 1,000 nM), on the other hand, may extrude Ca(2+) that has diffused away from the active zones and been temporarily sequestered in the endoplasmic reticulum. The PL microdomains that contain the NCX also contain Na(+) pump high ouabain affinity alpha2 (astrocytes) or alpha 3 (neurons) subunit isoforms (IC(50) congruent with 5-50 nM ouabain). In contrast, the alpha1 isoform (low ouabain affinity in rodents; IC(50) >10,000 nM), like the PMCA, is more uniformly distributed in these cells. The sub-PM endoplasmic reticulum in neurons (and probably glia and other cell types as well) and the adjacent PM form junctions that resemble cardiac muscle dyads. We suggest that the PM microdomains containing NCX and alpha 2/alpha 3 Na(+) pumps, the underlying jER, and the intervening tiny volume of cytosol (<10(-18) l) form functional units (PLasmERosomes); diffusion of Na(+) and Ca(2+) between these cytosolic compartments and "bulk" cytosol may be markedly restricted. The activity of the Na(+) pumps with alpha 2/alpha 3 subunits may thus regulate NCX activity and jER Ca(2+) content. This view is supported by studies in mice with genetically reduced (by congruent with 50%) alpha 2 Na(+) pumps: evoked Ca(2+) transients were augmented in these cells despite normal cytosolic Na(+) and resting Ca(2+) concentrations ([Na(+)](CYT) and [Ca(2+)](CYT)). We conclude that alpha 2/alpha 3 Na(+) pumps control PLasmERosome (local) [Na(+)](CYT). This, in turn, via NCX, modulates local [Ca(2+)](CYT), jER Ca(2+) storage, Ca(2+) signaling, and cell responses.

Antisense-inhibition of plasma membrane Ca2+ pump induces apoptosis in vascular smooth muscle cells

The effect of antisense oligodeoxynucleotides (ODNs) of plasma membrane Ca(2+)-pumping ATPase (PMCA) on rat aortic vascular smooth muscle cells (VSMCs) in primary culture was examined. More than 80% of the PMCA expressed in cultured VSMCs was the PMCA-1B subtype. Exposed to antisense ODNs against PMCA-1, not only the expression of the PMCA protein but also mRNA of PMCA-1B was diminished in a concentration-dependent manner. Extracellular Na(+)-independent (45)Ca(2+) efflux catalyzed via PMCA was inhibited with antisense ODNs. Both the resting and ionomycin- or ATP-stimulated levels of intracellular Ca(2+) were increased by antisense ODNs. Furthermore, prolonged treatment with antisense ODNs caused apoptosis in VSMCs. The occurrence of apoptosis was inhibited by FK506, a potent immunosuppressant. These results demonstrate that the PMCA was specifically inhibited by antisense ODNs and suggest that PMCA plays an important role in regulation of intracellular Ca(2+) concentrations, especially at the resting condition to prevent an occurrence of apoptosis that may be induced through the activation of calcineurin.

Antisense oligonucleotide suppression of Na(+)/Ca(2+) exchanger activity in primary neurons from rat brain

An antisense (AS) oligodeoxynucleotide based on a conserved sequence in the three isoforms of the Na(+)/Ca(2+) exchanger (NCX) was used to decrease expression of this Ca(2+) transporter in primary neuronal cultures. Two AS oligo applications decreased NCX activity by approximately 40% within 12-24 h, and neither sense (S) or missense (MS) oligos altered NCX activity. The reduced NCX expression was confirmed by immunoblots and enzyme-linked immunosorbent assays (ELISAs). Resting [Ca(2+)](i) levels were 20% higher in AS-treated neurons and showed a slower return to baseline levels following activation of Ca(2+) influx by N-methyl-D-aspartate (NMDA). These results suggest that NCX plays a significant role in maintaining neuronal Ca(2+) homeostasis and in restoring baseline Ca(2+) levels following depolarization.

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.

Hypoxia inhibits the Na(+)/Ca(2+) exchanger in pulmonary artery smooth muscle cells

The cellular mechanisms underlying hypoxic pulmonary vasoconstriction are not fully understood. We examined the effect of hypoxia on Ca(2+) efflux from the cytosol in single Fura-2-loaded pulmonary artery myocytes. During mild hypoxia (pO(2)=50-60 Torr), peak [Ca(2+)](i) was increased and the rate of Ca(2+) removal from the cytosol was markedly slowed after stimuli that elevated [Ca(2+)](i). Removal of extracellular Na(+) potentiated the peak [Ca(2+)](i) rise and slowed the Ca(2+) decay rate in cells recorded under normoxic conditions; it did not further slow the Ca(2+) decay rate or potentiate the [Ca(2+)](i) increase in hypoxic cells. An Na(+)/Ca(2+) exchange current was recorded in isolated pulmonary artery myocytes. Switching from Li(+) to Na(+) (130 mM) revealed an inward current with reversal potential consistent with the Na(+)/Ca(2+) exchange current in cells in which [Ca(2+)](i) was clamped at 1 microM similar currents, although smaller, were observed with normal resting [Ca(2+)](i) using the perforated patch clamp technique. The Na(+)/Ca(2+) exchange current was markedly inhibited in myocytes exposed to mild hypoxia. RT-PCR revealed the expression of specific alternatively spliced RNAs of NCX1 in rat pulmonary arteries. These findings provide an enhanced understanding of the molecular mechanisms underlying hypoxic sensing in pulmonary arteries.

Na(+) entry via store-operated channels modulates Ca(2+) signaling in arterial myocytes

In many nonexcitable cells, hormones and neurotransmitters activate Na(+) influx and mobilize Ca(2+) from intracellular stores. The stores are replenished by Ca(2+) influx via "store-operated" Ca(2+) channels (SOC). The main routes of Na(+) entry in these cells are unresolved, and no role for Na(+) in signaling has been recognized. We demonstrate that the SOC are a major Na(+) entry route in arterial myocytes. Unloading of the Ca(2+) stores with cyclopiazonic acid (a sarcoplasmic reticulum Ca(2+) pump inhibitor) and caffeine induces a large external Na(+)-dependent rise in the cytosolic Na(+) concentration. One component of this rise in cytosolic Na(+) concentration is likely due to Na(+)/Ca(2+) exchange; it depends on elevation of cytosolic Ca(2+) and is insensitive to 10 mM Mg(2+) and 10 microM La(3+). Another component is inhibited by Mg(2+) and La(3+), blockers of SOC; this component persists in cells preloaded with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid to buffer Ca(2+) transients and prevent Na(+)/Ca(2+) exchange-mediated Na(+) entry. This Na(+) entry apparently is mediated by SOC. The Na(+) entry influences Na(+) pump activity and Na(+)/Ca(2+) exchange and has unexpectedly large effects on cell-wide Ca(2+) signaling. The SOC pathway may be a general mechanism by which Na(+) participates in signaling in many types of cells.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.