Use of La3+ to distinguish activity of the plasmalemmal Ca2+ pump from Na+/Ca2+ exchange in arterial myocytes

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

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

Life-On-Hold: Lanthanoids Rapidly Induce a Reversible Ametabolic State in Mammalian Cells

Simple Summary

We found that incubation with a solution containing ~50 mM neodymium (one of the rare-earth elements, REE) induces a rapid of active metabolism in mammalian cells. We have named this state REEbernation and found that the process involves a rapid replacement of calcium with neodymium in membranes and organelles of a cell, allowing it to maintain its shape and membrane integrity under extreme conditions, including vacuum. Furthermore, phosphate exchange is blocked because of non-dissolvable neodymium salts formation, which “discharged” the cell. We also showed that REEbernation is characterized by instant shutting down RNA transcriptional activity in the cells, providing an intriguing opportunity to study a snapshot of gene expression at a given time point. Finally, we found that the REEbernation state is reversible, and we could restore the metabolism and proliferation capacity of the cells. The REEbernation provides a new method to reversibly place a cell into “on-hold” mode, opening opportunities to develop protocols for biological samples fixation with a minimum effect on the omics profile for biomedical needs.

Abstract

Until now, the ability to reversibly halt cellular processes has been limited to cryopreservation and several forms of anabiosis observed in living organisms. In this paper we show that incubation of living cells with a solution containing ~50 mM neodymium induces a rapid shutdown of intracellular organelle movement and all other evidence of active metabolism. We have named this state REEbernation (derived from the terms REE (rare earth elements) and hibernation) and found that the process involves a rapid replacement of calcium with neodymium in membranes and organelles of a cell, allowing it to maintain its shape and membrane integrity under extreme conditions, such as low pressure. Furthermore, phosphate exchange is blocked as a result of non-dissolvable neodymium salts formation, which “discharged” the cell. We further showed that REEbernation is characterized by an immediate cessation of transcriptional activity in observed cells, providing an intriguing opportunity to study a snapshot of gene expression at a given time point. Finally, we found that the REEbernation state is reversible, and we could restore the metabolism and proliferation capacity of the cells. The REEbernation, in addition to being an attractive model to further investigate the basic mechanisms of cell metabolism control, also provides a new method to reversibly place a cell into “on-hold” mode, opening opportunities to develop protocols for biological samples fixation with a minimum effect on the omics profile for biomedical needs.

Astrocytes respond to a neurotoxic Aβ fragment with state-dependent Ca2+ alteration and multiphasic transmitter release

Excessive amounts of amyloid β (Aβ) peptide have been suggested to dysregulate synaptic transmission in Alzheimer's disease (AD). As a major type of glial cell in the mammalian brain, astrocytes regulate neuronal function and undergo activity alterations upon Aβ exposure. Yet the mechanistic steps underlying astrocytic responses to Aβ peptide remain to be elucidated. Here by fluorescence imaging of signaling pathways, we dissected astrocytic responses to Aβ25-35 peptide, a neurotoxic Aβ fragment present in AD patients. In native health astrocytes, Aβ25-35 evoked Ca²⁺ elevations via purinergic receptors, being also dependent on the opening of connexin (CX) hemichannels. Aβ25-35, however, induced a Ca²⁺ diminution in Aβ-preconditioned astrocytes as a result of the potentiation of the plasma membrane Ca²⁺ ATPase (PMCA). The PMCA and CX protein expression was observed with immunostaining in the brain tissue of hAPPJ20 AD mouse model. We also observed both Ca²⁺-independent and Ca²⁺-dependent glutamate release upon astrocytic Aβ exposure, with the former mediated by CX hemichannel and the latter by both anion channels and lysosome exocytosis. Our results suggest that Aβ peptide causes state-dependent responses in astrocytes, in association with a multiphasic release of signaling molecules. This study therefore helps to understand astrocyte engagement in AD-related amyloidopathy.

The Ever-Growing Puzzle of Asynchronous Release

Invasion of an action potential (AP) to presynaptic terminals triggers calcium dependent vesicle fusion in a relatively short time window, about a millisecond, after the onset of the AP. This allows fast and precise information transfer from neuron to neuron by means of synaptic transmission and phasic mediator release. However, at some synapses a single AP or a short burst of APs can generate delayed or asynchronous synaptic release lasting for tens or hundreds of milliseconds. Understanding the mechanisms underlying asynchronous release (AR) is important, since AR can better recruit extrasynaptic metabotropic receptors and maintain a high level of neurotransmitter in the extracellular space for a substantially longer period of time after presynaptic activity. Over the last decade substantial work has been done to identify the presynaptic calcium sensor that may be involved in AR. Several models have been suggested which may explain the long lasting presynaptic calcium elevation a prerequisite for prolonged delayed release. However, the presynaptic mechanisms underlying asynchronous vesicle release are still not well understood. In this review article, we provide an overview of the current state of knowledge on the molecular components involved in delayed vesicle fusion and in the maintenance of sufficient calcium concentration to trigger AR. In addition, we discuss possible alternative models that may explain intraterminal calcium dynamics underlying AR.

Na+/Ca2+ exchanger overexpression in smooth muscle augments cytosolic Ca2+ in femoral arteries of living mice

Plasma membrane Na+/Ca2+ exchanger-1 (NCX1) helps regulate the cytosolic Ca2+ concentration ([Ca2+]CYT) in arterial myocytes. NCX1 mediates both Ca2+ entry and exit and tends to promote net Ca2+ entry in partially constricted arteries. Mean blood pressure (telemetry) is elevated by ≈10 mmHg in transgenic (TG) mice that overexpress NCX1 specifically in smooth muscle. We tested the hypothesis that NCX1 overexpression mediates Ca2+ gain and elevated [Ca2+]CYT in exposed femoral arteries that also express the Ca2+ biosensor exogenous myosin light chain kinase. [Ca2+]CYT and the NCX1-dependent (SEA0400-sensitive) component, ≈15% of total basal constriction in controls, were increased in TG arteries, but constrictions to phenylephrine and ANG II were comparable in TG and control arteries. Normalized phenylephrine dose-response curves and constriction to 30 and 300 ng/kg iv ANG II were virtually identical in control and TG arteries. ANG II-evoked constrictions, superimposed on elevated basal tone, accounted for the larger blood pressure responses to ANG II in TG arteries. TG and control mouse arteries fit the same pCa-constriction relationship over a wide range of pCa (≈125-500 nM). Vasodilation to acetylcholine, normalized to passive diameter, was also comparable in TG and control arteries, implying normal endothelial function. TG artery Na+ nitroprusside (nitric oxide donor)-induced dilations were, however, shifted to lower Na+ nitroprusside concentrations, indicating that TG myocyte vasodilator mechanisms were augmented. Maximum arterial dilation was comparable in TG and control mice, although passive diameter was ≈6-7% smaller in TG mice. The changes in TG arteries were apparently largely functional rather than structural, despite the congenital hypertension. NEW & NOTEWORTHY Smooth muscle Na+/Ca2+ exchanger-1 transgene overexpression (TG mice) increases femoral artery basal cytosolic Ca2+ concentration ([Ca2+]CYT) and tone in vivo and raises blood pressure. Arterial constriction to phenylephrine and angiotensin II are normal but superimposed on the augmented basal [Ca2+]CYT and tone (constriction) in TG mouse arteries. Similar effects in resistance arteries would explain the elevated blood pressure. Acetylcholine-induced vasodilation is unimpaired, implying a normal endothelium, but TG arteries are hypersensitive to sodium nitroprusside.

Na+/Ca2+ Exchanger 1 in Airway Smooth Muscle of Allergic Inflammation Mouse Model

Cytosolic free Ca²⁺ ([Ca²⁺]_cyt) is essential for airway contraction, secretion and remodeling. [Ca²⁺]_cyt homeostasis is controlled by several critical molecules, one of which is the Na⁺/Ca²⁺ exchanger 1 (NCX1) in the plasma membrane. Since little is currently known about NCX1 in the airway smooth muscle and its involvement in airway diseases, the present study was designed to investigate the expression and function of NCX1 in normal airway smooth muscle and its relevance to airway inflammation. Western blot analysis, tracheal smooth muscle contraction, and [Ca²⁺]_cyt measurements were performed in mouse tracheal smooth muscle tissues and primary airway smooth muscle cell cultures. Additional studies were performed in a mouse model of allergic airway inflammation. Our data showed that NCX1 proteins were expressed in the human bronchial smooth muscle cells (HBSMCs), murine airway and whole lung. Carbachol raised [Ca²⁺]_cyt in mouse tracheal smooth muscle cells and induced murine tracheal contraction, all of which were significantly attenuated by KB-R7943, a selective NCX inhibitor. Removal of extracellular Na⁺ increased [Ca²⁺]_cyt in HBSMCs and mouse tracheal SMCs, which was dependent on extracellular Ca²⁺ and sensitive to KB-R7943. TNF-α treatment of HBSMCs significantly upregulated mRNA and protein expression of NCX1 and enhanced NCX activity. Finally, KB-R7943 abolished the airway hyperresponsiveness to methacholine in an ovalbumin-induced mouse model of allergic airway inflammation. Together, these findings indicate that NCX1 in airway smooth muscle may play an important role in the development of airway hyperresponsiveness, and downregulation or inhibition of NCX1 may serve as a potential therapeutic approach for asthma.

Role of Na⁺/Ca²⁺ exchanger in Ca²⁺ homeostasis in rat suprachiasmatic nucleus neurons

Intracellular Ca(2+) is critical to the central clock of the suprachiasmatic nucleus (SCN). However, the role of Na(+)/Ca(2+) exchanger (NCX) in intracellular Ca(2+) concentration ([Ca(2+)]i) homeostasis in the SCN is unknown. Here we show that NCX is an important mechanism for somatic Ca(2+) clearance in SCN neurons. In control conditions Na(+)-free solution lowered [Ca(2+)]i by inhibiting TTX-sensitive as well as nimodipine-sensitive Ca(2+) influx. With use of the Na(+) ionophore monensin to raise intracellular Na(+) concentration ([Na(+)]i), Na(+)-free solution provoked rapid Ca(2+) uptake via reverse NCX. The peak amplitude of 0 Na(+)-induced [Ca(2+)]i increase was larger during the day than at night, with no difference between dorsal and ventral SCN neurons. Ca(2+) extrusion via forward NCX was studied by determining the effect of Na(+) removal on Ca(2+) clearance after high-K(+)-induced Ca(2+) loads. The clearance of Ca(2+) proceeded with two exponential decay phases, with the fast decay having total signal amplitude of ∼85% and a time constant of ∼7 s. Na(+)-free solution slowed the fast decay rate threefold, whereas mitochondrial protonophore prolonged mostly the slow decay. In contrast, blockade of plasmalemmal and sarco(endo)plasmic reticulum Ca(2+) pumps had little effect on the kinetics of Ca(2+) clearance. RT-PCR indicated the expression of NCX1 and NCX2 mRNAs. Immunohistochemical staining showed the presence of NCX1 immunoreactivity in the whole SCN but restricted distribution of NCX2 immunoreactivity in the ventrolateral SCN. Together our results demonstrate an important role of NCX, most likely NCX1, as well as mitochondrial Ca(2+) uptake in clearing somatic Ca(2+) after depolarization-induced Ca(2+) influx in SCN neurons.

The vascular barrier-protecting hawthorn extract WS® 1442 raises endothelial calcium levels by inhibition of SERCA and activation of the IP3 pathway

WS® 1442 has been proven as an effective and safe therapeutical to treat mild forms of congestive heart failure. Beyond this action, we have recently shown that WS® 1442 protects against thrombin-induced vascular barrier dysfunction and the subsequent edema formation by affecting endothelial calcium signaling. The aim of the study was to analyze the influence of WS® 1442 on intracellular calcium concentrations [Ca(2+)](i) in the human endothelium and to investigate the underlying mechanisms. Using ratiometric calcium measurements and a FRET sensor, we found that WS® 1442 concentration-dependently increased basal [Ca(2+)](i) by depletion of the endoplasmic reticulum (ER) and inhibited a subsequent histamine-triggered rise of [Ca(2+)](i). Interestingly, the augmented [Ca(2+)](i) did neither trigger an activation of the contractile machinery nor led to a barrier breakdown (macromolecular permeability). It also did not impair endothelial cell viability. As assessed by patch clamp recordings, WS® 1442 did only slightly affect endothelial Na(+)/K(+)-ATPase, but increased [Ca(2+)](i) by inhibiting the sarcoplasmic/endoplasmic reticulum Ca(2+) ATPase (SERCA) and by activating the inositol 1,4,5-trisphosphate (IP(3)) pathway. Most importantly, WS® 1442 did not induce store-operated calcium entry (SOCE), but even irreversibly prevented histamine-induced SOCE. Taken together, WS® 1442 prevented the deleterious hyperpermeability-associated rise of [Ca(2+)](i) by a preceding, non-toxic release of Ca(2+) from the ER. WS® 1442 interfered with SERCA and the IP(3) pathway without inducing SOCE. The elucidation of this intriguing mechanism helps to understand the complex pharmacology of the cardiovascular drug WS® 1442.

A novel role for Bcl-2 in regulation of cellular calcium extrusion

The antiapoptotic protein Bcl-2 [1, 2] plays important roles in Ca²⁺ signaling [3] by influencing inositol triphosphate receptors and regulating Ca²⁺-induced Ca²⁺ release [4–6]. Here we investigated whether Bcl-2 affects Ca²⁺ extrusion in pancreatic acinar cells. We specifically blocked the Ca²⁺ pumps in the endoplasmic reticulum and assessed the rate at which the cells reduced an elevated cytosolic Ca²⁺ concentration after a period of enhanced Ca²⁺ entry. Because external Ca²⁺ was removed and endoplasmic reticulum Ca²⁺ pumps were blocked, Ca²⁺ extrusion was the only process responsible for recovery. Cells lacking Bcl-2 restored the basal cytosolic Ca²⁺ level much faster than control cells. The enhanced Ca²⁺ extrusion in cells from Bcl-2 knockout (Bcl-2 KO) mice was not due to increased Na⁺/Ca²⁺ exchange activity, because removal of external Na⁺ did not influence the Ca²⁺ extrusion rate. Overexpression of Bcl-2 in the pancreatic acinar cell line AR42J decreased Ca²⁺ extrusion, whereas silencing Bcl-2 expression (siRNA) had the opposite effect. Loss of Bcl-2, while increasing Ca²⁺ extrusion, dramatically decreased necrosis and promoted apoptosis induced by oxidative stress, whereas specific inhibition of Ca²⁺ pumps in the plasma membrane (PMCA) with caloxin 3A1 reduced Ca²⁺ extrusion and increased necrosis. Bcl-2 regulates PMCA function in pancreatic acinar cells and thereby influences cell fate.

Ca2+ sensitivity of the maxi chloride channel in interstitial cells of Cajal

BACKGROUND

Interstitial cells of Cajal (ICC) associated with the myenteric plexus of the small intestine express maxi chloride channels. Our aim was to investigate whether or not these channels would be activated by increases in intracellular Ca(2+) , as that would strengthen evidence for their potential role in ICC pacemaking. A further aim was to examine whether inwardly and outwardly rectifying maxi chloride currents signify different channels.

METHODS

We used Fluo-4 AM Ca(2+) imaging and patch clamp electrophysiology (cell-attached and inside-out) on isolated ICC in short term culture.

KEY RESULTS

Increasing intracellular Ca(2+) by three functionally distinct mechanisms (blocking sarcoplasmic reticulum Ca(2+) refilling, creating membrane Ca(2+) pores and a solution designed to block plasmalemmal Ca(2+) extrusion) was followed by inwardly rectifying maxi chloride channel activation assessed in the cell-attached configuration. Furthermore, in the inside-out configuration, increased outwardly rectifying maxi-chloride channel activity followed an increase in Ca(2+) to 2 mmol L(-1) at the cytoplasmic face of the channel.

CONCLUSIONS & INFERENCES

Increase in intracellular Ca(2+) will activate the maxi chloride channels. Maxi chloride currents are inwardly rectifying in the cell-attached patch clamp configuration under physiological conditions and are outwardly rectifying in the inside-out configuration. The same channel is responsible for both currents. Ca(2+) does not appear to regulate the rectification.

Roles of cytosolic Ca2+ concentration and myofilament Ca2+ sensitization in age-dependent cerebrovascular myogenic tone

In light of evidence that immature arteries contain a higher proportion of noncontractile smooth muscle cells than found in fully differentiated mature arteries, the present study explored the hypothesis that age-related differences in the smooth muscle phenotype contribute to age-related differences in contractility. Because Ca(2+) handling differs markedly between contractile and noncontractile smooth muscle, the present study specifically tested the hypothesis that the relative contributions of Ca(2+) influx and myofilament sensitization to myogenic tone are upregulated, whereas Ca(2+) release is downregulated, in immature [14 days postnatal (P14)] compared with mature (6 mo old) rat middle cerebral arteries (MCAs). Myofilament Ca(2+) sensitivity measured in β-escin-permeabilized arteries increased with pressure in P14 but not adult MCAs. Cyclopiazonic acid (an inhibitor of Ca(2+) release from the sarcoplasmic reticulum) increased diameter and reduced Ca(2+) in adult MCAs but increased diameter with no apparent change in Ca(2+) in P14 MCAs. La(3+) (Ca(2+) influx inhibitor) increased diameter and decreased Ca(2+) in adult MCAs, but in P14 MCAs, La(3+) increased diameter with no apparent change in Ca(2+). After treatment with both La(3+) and CPA, diameters were passive in both adult and P14 MCAs, but Ca(2+) was decreased only in adult MCAs. To quantify the fraction of smooth muscle cells in the fully differentiated contractile phenotype, extents of colocalization between smooth muscle α-actin and SM2 myosin heavy chain were determined and found to be at least twofold greater in adult than pup MCAs. These data suggest that compared with adult MCAs, pup MCAs contain a greater proportion of noncontractile smooth muscle and, as a consequence, rely more on myofilament Ca(2+) sensitization and Ca(2+) influx to maintain myogenic reactivity. The inability of La(3+) to reduce cytosolic Ca(2+) in the pup MCA appears due to La(3+)-insensitive noncontractile smooth muscle cells, which contribute to the spatially averaged measurements of Ca(2+) but not contraction.

Calcium clearance and its energy requirements in cerebellar neurons

Quick cytosolic calcium clearance is essential for the effective modulation of various cellular functions. An excess of cytosolic calcium after influx is largely removed via ATP-dependent mechanisms located in the plasma membrane and the endoplasmic reticulum. Therefore, calcium clearance depends critically on the adequate supply of ATP, which may come from either glycolysis or mitochondria, or both. However, it presently remains unknown the degree to which individual ATP generating pathways - glycolysis and mitochondria power ATP-dependent calcium as well as other vital ion clearance mechanisms in neurons. In this study, we explored the relationship between the energy generating pathways and ion clearance mechanisms in neurons by characterizing the effects of glycolytic and mitochondrial inhibitors of ATP synthesis on calcium clearance kinetics in the soma, dendrites and spines. Stimulation of cultured cerebellar granule cells by brief pulses of 60mM potassium ACSF, and electrical stimulation of purkinje cells in acutely prepared slices led to a transient calcium influx, whose clearance was largely mediated by the plasma membrane Ca(2+)-ATPase pump. Inhibition of glycolysis by deoxyglucose or iodoacetic acid resulted in a marked slowing in calcium clearance in the soma, dendrites, and spines with the spines affected the most. However, inhibition of the mitochondrial citric acid cycle with fluoroacetate and arsenite, or mitochondrial ATP synthase with oligomycin did not produce any immediate effects on calcium clearance kinetics in any of those neuronal regions. Although cytosolic calcium clearance was not affected by the inhibition of mitochondria, the magnitude of the calcium clearance delay induced by glycolytic inhibitors in different neuronal compartments was related to their mitochondrial density. Conversely, the endoplasmic reticulum Ca(2+)-ATPase pump activity is fuelled by both glycolytic and mitochondrial ATP, as evidenced by a minimal change in the endoplasmic reticulum calcium contents in cells treated with either deoxyglucose supplemented with lactate or arsenite. Taken together, these data suggest that calcium clearance in cerebellar granule and purkinje cells relies on the plasma membrane Ca(2+)-ATPase, and is powered by glycolysis.

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.

Molecular substrates mediating lanthanide-evoked neurotransmitter release in central synapses

Noncanonical secretagogues such as hypertonicity or alpha-latrotoxin have been extremely informative in studying neurotransmission. Lanthanum and lanthanides can also trigger neurotransmitter release through an unknown mechanism. Here, we studied the effect of lanthanides on neurotransmission in hippocampal cultures. Application of 2 mM La3+ caused rapid and robust neurotransmitter release within seconds. In addition, transient application of La3+ uncovered a sustained facilitation of miniature neurotransmission. The response to La3+ was detectable at 2 microM and increased in a concentration-dependent manner<or=2 mM. Rapid effect of La3+ was independent of extracellular and intracellular Ca2+ and did not require La3+ entry into cells or activation of phospholipaseCbeta. Synapses deficient in synaptobrevin-2, the major synaptic vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein in the brain, did not display any rapid release in response to La3+, whereas the slow facilitation of release detected after La3+ removal remained intact. In contrast, preincubation with intracellular Ca2+ chelators selectively attenuated the delayed release triggered by La3+. Moreover, synapses deficient in synaptotagmin-1 maintained a rapid response to La3+, suggesting that La3+-triggered neurotransmitter release does not require synaptotagmin-1 as a sensor. Therefore La3+ has two separate effects on synaptic transmission. For its rapid action, La3+ interacts with a target on the surface membrane, and unlike other forms of release, it triggers strictly synaptobrevin-2-dependent fusion, implying that in central synapses synaptobrevin-2 function is secretagogue specific. For the delayed action, La3+ may act intracellularly after its entry or through intracellular Ca2+ via a mechanism that does not require synaptobrevin-2.

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.

The role of the Na(+)/Ca(2+) exchanger (NCX) in neurons following ischaemia

The Na(+)/Ca(2+) exchanger (NCX) is a bi-directional membrane ion transporter. Under normal conditions, the exchanger transports one calcium ion out of the cell and three sodium ions into the cell. This is known as the calcium exit, or "forward" mode. Under certain conditions, however, the exchanger can reverse and transport calcium ions into the cell (calcium entry mode). Because dysregulation of sodium and calcium homeostasis is an integral feature of ischaemic brain injury, the role of the NCX in neurons following ischaemia has been investigated using a number of in vitro and in vivo models. Studies using in vitro ischaemia-related models (hypoxia, glutamate) have produced conflicting results, with some showing that NCX activity is neuroprotective while others indicate that it is neurodamaging. The majority of in vivo studies using the focal cerebral ischaemia model indicate that blocking NCX activity is neurodamaging while increasing NCX activity is neuroprotective. We have reviewed the major in vitro and in vivo neuronal ischaemia-related NCX studies in an attempt to clarify the reason for the conflicting findings. The use of different ischaemia models and doubts as to the specificity of pharmacological NCX inhibitors and stimulators has contributed to the confusion over the role of the NCX in ischaemic brain injury. The development of NCX transgenic animals may help our understanding of the role of this ion exchanger in neurons following ischaemia and aid the development of an effective stroke treatment.

Ouabain modulation of endothelial calcium signaling in descending vasa recta

Using fura 2-loaded vessels, we tested whether ouabain modulates endothelial cytoplasmic calcium concentration ([Ca(2+)](CYT)) in rat descending vasa recta (DVR). Over a broad range between 10(-10) and 10(-4) M, ouabain elicited biphasic peak and plateau [Ca(2+)](CYT) elevations. Blockade of voltage-gated Ca(2+) entry with nifedipine did not affect the response to ouabain mitigating against a role for myo-endothelial gap junctions. Reduction of extracellular Na(+) concentration ([Na(+)](o)) or Na(+)/Ca(2+) exchanger (NCX) inhibition with SEA-0400 (10(-6) M) elevated [Ca(2+)](CYT), supporting a role for NCX in the setting of basal [Ca(2+)](CYT). SEA-0400 abolished the [Ca(2+)](CYT) response to ouabain implicating NCX as a mediator. The transient peak phase of [Ca(2+)](CYT) elevation that followed either ouabain or reduction of [Na(+)](o) was abolished by 2-aminoethoxydiphenyl borate (5 x 10(-5) M). Cation channel blockade with La(3+) (10 muM) or SKF-96365 (10 muM) also attenuated the ouabain-induced [Ca(2+)](CYT) response. Ouabain pretreatment increased the [Ca(2+)](CYT) elevation elicited by bradykinin (10(-7) M). We conclude that inhibition of ouabain-sensitive Na(+)-K(+)-ATPase enhances DVR endothelial Ca(2+) store loading and modulates [Ca(2+)](CYT) signaling through mechanisms that involve NCX, Ca(2+) release, and cation channel activation.

Effects of aging on Ca2+ signaling in murine mesenteric arterial smooth muscle cells

Pathophysiological changes in arterial smooth muscle structure and function occur with aging and there are a number of reports illustrating reductions in vascular responsiveness with aging. While much is known about arterial remodeling and functional adaptations with aging, very little is known about the biophysical adaptations in individual arterial myocytes. Cytosolic Ca2+ signaling, involving activation of L-type Ca2+ channels on the plasma membrane as well as InsP3 and ryanodine receptors on the sarcoplasmic reticulum, is integral to vascular tone and reactivity. Thus, we tested the hypothesis that aging results in reductions in the functional expression of L-type channels and temporal aspects of ryanodine receptor and InsP3 receptor Ca2+ signaling, in mesenteric arterial smooth muscle cells isolated from 6 and 30 months old C57Bl/6 mice. Comparisons of L-type current activity were made using dialyzed, whole-cell voltage-clamp techniques and Ba2+ as charge carrier. Ca2+ signaling was measured using fura-2 fluorescence microscopy techniques. Cell morphological changes were also investigated using electrophysiological and immunocytochemical approaches. The amplitudes of L-type Ca2+ currents were increased in older mice, but this was associated with membrane surface area increases of approximately 50%, due to increases in cell length not cell width. Consequently, L-type Ca2+ current densities were preserved with age, indicating functional channel expression was unchanged. In contrast, aging was associated with decrements in Ca2+ signaling in response to either ryanodine receptor stimulation by caffeine or InsP3 receptor activation with phenylephrine. These changes with aging may be related to the previously reported depression in myogenic reactivity.

The endoplasmic reticulum gateway to apoptosis by Bcl-X(L) modulation of the InsP3R

Members of the Bcl-2 protein family modulate outer mitochondrial membrane permeability to control apoptosis. However, these proteins also localize to the endoplasmic reticulum (ER), the functional significance of which is controversial. Here we provide evidence that anti-apoptotic Bcl-2 proteins regulate the inositol 1,4,5-trisphosphate receptor (InsP(3)R) ER Ca(2+) release channel resulting in increased cellular apoptotic resistance and enhanced mitochondrial bioenergetics. Anti-apoptotic Bcl-X(L) interacts with the carboxyl terminus of the InsP(3)R and sensitizes single InsP(3)R channels in ER membranes to low [InsP(3)], enhancing Ca(2+) and InsP(3)-dependent regulation of channel activity in vitro and in vivo, reducing ER Ca(2+) content and stimulating mitochondrial energetics. The pro-apoptotic proteins Bax and tBid antagonize this effect by blocking the biochemical interaction of Bcl-X(L) with the InsP(3)R. These data support a novel model in which Bcl-X(L) is a direct effector of the InsP(3)R, increasing its sensitivity to InsP(3) and enabling ER Ca(2+) release to be more sensitively coupled to extracellular signals. As a consequence, cells are protected against apoptosis by a more sensitive and dynamic coupling of ER to mitochondria through Ca(2+)-dependent signal transduction that enhances cellular bioenergetics and preserves survival.

Antisense-mediated Inhibition of the Plasma Membrane Calcium-ATPase Suppresses Proliferation of MCF-7 Cells

Alterations in Ca2+ signaling may contribute to tumorigenesis and the mechanism of action of some anti-cancer drugs. The plasma membrane calcium-ATPase (PMCA) is a crucial controller of intracellular Ca2+ signaling. Altered PMCA expression occurs in the mammary gland during lactation and in breast cancer cell lines. Despite this, the consequences of PMCA inhibition in breast cancer cell lines have not been investigated. In this work, we used Tet-off PMCA antisense-expressing MCF-7 cells to assess the effects of PMCA inhibition in a human breast cancer cell line. At a level of PMCA inhibition that did not completely prevent PMCA-mediated Ca2+ efflux and did not induce cell death, a dramatic inhibition of cellular proliferation was observed. Fluorescence-activated cell sorting analysis indicated that PMCA antisense involves changes in cell cycle kinetics but not cell cycle arrest. We concluded that modulation of PMCA has important effects in regulating the proliferation of human breast cancer MCF-7 cells.

Acute hypoxia increases intracellular [Ca2+] in pulmonary arterial smooth muscle by enhancing capacitative Ca2+ entry

Hypoxic pulmonary vasoconstriction (HPV) requires influx of extracellular Ca2+ in pulmonary arterial smooth muscle cells (PASMCs). To determine whether capacitative Ca2+ entry (CCE) through store-operated Ca2+ channels (SOCCs) contributes to this influx, we used fluorescent microscopy and the Ca2+-sensitive dye fura-2 to measure effects of 4% O2 on intracellular [Ca2+] ([Ca2+]i) and CCE in primary cultures of PASMCs from rat distal pulmonary arteries. In PASMCs perfused with Ca2+-free Krebs Ringer bicarbonate solution (KRBS) containing cyclopiazonic acid to deplete Ca2+ stores in sarcoplasmic reticulum and nifedipine to prevent Ca2+ entry through L-type voltage-operated Ca2+ channels (VOCCs), hypoxia markedly enhanced both the increase in [Ca2+]i caused by restoration of extracellular [Ca2+] and the rate at which extracellular Mn2+ quenched fura-2 fluorescence. These effects, as well as the increased [Ca2+]i caused by hypoxia in PASMCs perfused with normal salt solutions, were blocked by the SOCC antagonists SKF-96365, NiCl2, and LaCl3 at concentrations that inhibited CCE >80% but did not alter [Ca2+]i responses to 60 mM KCl. In contrast, the VOCC antagonist nifedipine inhibited [Ca2+]i responses to hypoxia by only 50% at concentrations that completely blocked responses to KCl. The increased [Ca2+]i caused by hypoxia was completely reversed by perfusion with Ca2+-free KRBS. LaCl3 increased basal [Ca2+]i during normoxia, indicating effects other than inhibition of SOCCs. Our results suggest that acute hypoxia enhances CCE through SOCCs in distal PASMCs, leading to depolarization, secondary activation of VOCCs, and increased [Ca2+]i. SOCCs and CCE may play important roles in HPV.

Plant Cell Growth and Ion Flux Responses to the Streptomycete Phytotoxin Thaxtomin A: Calcium and Hydrogen Flux Patterns Revealed by the Non-invasive MIFE Technique

Thaxtomin A, a key phytotoxin produced by plant pathogenic Streptomyces sp., is implicit in common scab disease expression in potato. Primary targets and modes of action of thaxtomin A toxicity in plant cells are not well understood. In this work, early signalling events associated with thaxtomin A toxicity were studied using the ion-selective microelectrode ion flux estimation (MIFE) technique. Thaxtomin A-induced changes in net ion fluxes were measured across the plasma membrane (PM) of root and pollen tube tissue in Arabidopsis thaliana and tomato. Within a minute after toxin application, a rapid and short-lived Ca2+ influx was observed. Well ahead of the marked inhibition of root growth, a significant shift towards net H+ efflux across the PM occurred in all tissues. Similar to root tissues, thaxtomin A significantly modified ion flux profiles from growing pollen tubes. Thaxtomin A was more effective in young, physiologically active tissues (root elongation zone or pollen tube apex), suggesting a higher density of thaxtomin A-binding sites in these regions. Overall, our data provide the first evidence that thaxtomin A triggers an early signalling cascade, which may be crucial in plant-pathogen interactions. It also suggests a possible interaction between thaxtomin A and PM auxin receptors, as revealed from experiments on the auxin-sensitive ucu2-2/gi2 A. thaliana mutant.

Inhibition of hypoxic pulmonary vasoconstriction by antagonists of store-operated Ca2+ and nonselective cation channels

Previous studies indicated that acute hypoxia increased intracellular Ca(2+) concentration ([Ca(2+)](i)), Ca(2+) influx, and capacitative Ca(2+) entry (CCE) through store-operated Ca(2+) channels (SOCC) in smooth muscle cells from distal pulmonary arteries (PASMC), which are thought to be a major locus of hypoxic pulmonary vasoconstriction (HPV). Moreover, these effects were blocked by Ca(2+)-free conditions and antagonists of SOCC and nonselective cation channels (NSCC). To test the hypothesis that in vivo HPV requires CCE, we measured the effects of SOCC/NSCC antagonists (SKF-96365, NiCl(2), and LaCl(3)) on pulmonary arterial pressor responses to 2% O(2) and high-KCl concentrations in isolated rat lungs. At concentrations that blocked CCE and [Ca(2+)](i) responses to hypoxia in PASMC, SKF-96365 and NiCl(2) prevented and reversed HPV but did not alter pressor responses to KCl. At 10 microM, LaCl(3) had similar effects, but higher concentrations (30 and 100 microM) caused vasoconstriction during normoxia and potentiated HPV, indicating actions other than SOCC blockade. Ca(2+)-free perfusate and the voltage-operated Ca(2+) channel (VOCC) antagonist nifedipine were potent inhibitors of pressor responses to both hypoxia and KCl. We conclude that HPV required influx of Ca(2+) through both SOCC and VOCC. This dual requirement and virtual abolition of HPV by either SOCC or VOCC antagonists suggests that neither channel provided enough Ca(2+) on its own to trigger PASMC contraction and/or that during hypoxia, SOCC-dependent depolarization caused secondary activation of VOCC.

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.

Role of Na+/Ca2+ exchange in regulating cytosolic Ca2+ in cultured human pulmonary artery smooth muscle cells

A rise in cytosolic Ca2+ concentration ([Ca2+]cyt) in pulmonary artery smooth muscle cells (PASMC) is an important stimulus for cell contraction, migration, and proliferation. Depletion of intracellular Ca2+ stores opens store-operated Ca2+ channels (SOC) and causes Ca2+ entry. Transient receptor potential (TRP) cation channels that are permeable to Na+ and Ca2+ are believed to form functional SOC. Because sarcolemmal Na+/Ca2+ exchanger has also been implicated in regulating [Ca2+]cyt, this study was designed to test the hypothesis that the Na+/Ca2+ exchanger (NCX) in cultured human PASMC is functionally involved in regulating [Ca2+]cyt by contributing to store depletion-mediated Ca2+ entry. RT-PCR and Western blot analyses revealed mRNA and protein expression for NCX1 and NCKX3 in cultured human PASMC. Removal of extracellular Na+, which switches the Na+/Ca2+ exchanger from the forward (Ca2+ exit) to reverse (Ca2+ entry) mode, significantly increased [Ca2+]cyt, whereas inhibition of the Na+/Ca2+ exchanger with KB-R7943 (10 microM) markedly attenuated the increase in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Store depletion also induced a rise in [Ca2+]cyt via the reverse mode of Na+/Ca2+ exchange. Removal of extracellular Na+ or inhibition of the Na+/Ca2+ exchanger with KB-R7943 attenuated the store depletion-mediated Ca2+ entry. Furthermore, treatment of human PASMC with KB-R7943 also inhibited cell proliferation in the presence of serum and growth factors. These results suggest that NCX is functionally expressed in cultured human PASMC, that Ca2+ entry via the reverse mode of Na+/Ca2+ exchange contributes to store depletion-mediated increase in [Ca2+]cyt, and that blockade of the Na+/Ca2+ exchanger in its reverse mode may serve as a potential therapeutic approach for treatment of pulmonary hypertension.

Control of calcium oscillations by membrane fluxes

It is known that Ca(2+) influx plays an important role in the modulation of inositol trisphosphate-generated Ca(2+) oscillations, but controversy over the mechanisms underlying these effects exists. In addition, the effects of blocking membrane transport or reducing Ca(2+) entry vary from one cell type to another; in some cell types oscillations persist in the absence of Ca(2+) entry (although their frequency is affected), whereas in other cell types oscillations depend on Ca(2+) entry. We present theoretical and experimental evidence that membrane transport can control oscillations by controlling the total amount of Ca(2+) in the cell (the Ca(2+) load). Our model predicts that the cell can be balanced at a point where small changes in the Ca(2+) load can move the cell into or out of oscillatory regions, resulting in the appearance or disappearance of oscillations. Our theoretical predictions are verified by experimental results from HEK293 cells. We predict that the role of Ca(2+) influx during an oscillation is to replenish the Ca(2+) load of the cell. Despite this prediction, even during the peak of an oscillation the cell or the endoplasmic reticulum may not be measurably depleted of Ca(2+).

Intracellular Ca2+ homeostatic regulation and 4-hydroxynonenal-induced aortic endothelial dysfunction

The aldehydic lipid peroxidation product 4-hydroxynonenal (HNE) is known to compromise erythrocyte passive Ca2+ permeability and to irreversibly inhibit the plasma membrane (Ca2+ + Mg2+)-ATPase and Ca2+-transport. To measure the effects of HNE on passive and active Ca2+ transport in endothelial cells, we first characterized 45Ca2+ uptake and efflux in cultured porcine aortic endothelial cells (PAEC). PAEC exchanged 45Ca2+ to a cumulative near-isotopic equilibrium of about 4.5 pmole 45Ca2+/10(6) cells in 120 min at 37 degrees C. This Ca2+ pool was diminished by thapsigargin, cyclopiazonic acid, oligomycin B, and sodium azide. In contrast, ouabain enhanced Ca2+ uptake capacity from 5.17 to 5.77 pmole/10(6) cells. Accumulated 45Ca2+ was extruded at rate of 8.7 fmole 45Ca2+/10(6) cells/min or shunted rapidly by the ionophore A23187. HNE increased total 45Ca2+ accumulation in a time- and concentration-dependent manner by as much as 562% with an EC50 of 64.0 wM. Concomitant morphological analysis of PAEC revealed vacuolization, nuclear swelling, cell shrinking, and cell detachment. Initial structural changes, such as vacuolization, began well before any changes in Ca2+ accumulation were observed. These functional and morphological changes indicate that HNE significantly increases intracellular Ca2+ accumulation in vascular endothelium, which may explain the cytotoxic effects associated with HNE exposure and provide further evidence that atherogenic effects of HNE may, in part, be caused by disturbances in Ca2+ homeostasis.

Dynamics of calcium clearance in mouse pancreatic beta-cells

Pancreatic beta-cells maintain glucose homeostasis by their regulated Ca(2+)-dependent secretion of insulin. Several cellular mechanisms control intracellular Ca(2+) levels, but their relative significance in mouse beta-cells is not fully known. We used photometry to measure the dynamics of cytosolic Ca(2+) ([Ca(2+)](i)) clearance after brief, depolarization-induced Ca(2+) entry. Treatment with thapsigargin or cyclopiazonic acid, inhibitors of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pumps, nearly doubled the peak and slowed the decay of the depolarization-induced Ca(2+) transients. The remaining thapsigargin-insensitive decay was slowed further by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) and plasma membrane Na(+)/Ca(2+) exchanger (NCX) via alkalization of the bath solution, by adding lanthanum, or by substitution of Na(+) with Li(+). Mitochondrial Ca(2+) uptake contributed little to clearance in thapsigargin-pretreated cells. Together, the SERCA, PMCA, and NCX transport mechanisms accounted for 89 to 97% of clearance in normal solutions. We developed a quantitative model for the dynamic role of removal mechanisms over a wide range of [Ca(2+)](i). According to our model, 50 to 64% of initial Ca(2+) removal is via the SERCA pump, whereas the NCX contributes 21-30% of the extrusion at high [Ca(2+)](i), and the PMCA contributes 21-27% at low [Ca(2+)](i).

Calcium clearance mechanisms of mouse sperm

The spermatozoon is specialized for a single vital role in fertilization. Past studies show that Ca2+ signals produced by the opening of plasma membrane entry channels initiate several events required for the sperm to reach and enter the egg but reveal little about how resting [Ca2+]i is maintained or restored after elevation. We examined these homeostatic mechanisms by monitoring the kinetics of recovery from depolarizing stimuli under conditions intended to inhibit candidate mechanisms for sequestration or extrusion of Ca2+ from the cytosol. We found that the Ca2+-ATPase pump of the plasma membrane performs the major task of Ca2+ clearance. It is essential in the final stages of recovery to achieve a low resting [Ca2+]i. With immunomethods we found a approximately 130-kD plasma membrane Ca2+-ATPase protein on Western blots of whole sperm extracts and showed immunolocalization to the proximal principal piece of the flagellum. The plasma membrane Na+-Ca2+ exchanger also exports Ca2+ when [Ca2+]i is elevated. Simultaneous inhibition of both mechanisms of extrusion revealed an additional contribution to clearance from a CCCP-sensitive component, presumably sequestration by the mitochondria. Involvement of SERCA pumps was not clearly detected. Many aspects of the kinetics of Ca2+ clearance observed in the presence and absence of inhibitors were reproduced in a mathematical model based on known and assumed kinetic parameters. The model predicts that when cytosolic [Ca2+] is at 1 microM, the rates of removal by the Ca2+-ATPase, Na+-Ca2+-exchanger, mitochondrial uniporter, and SERCA pump are approximately 1.0, 0.35, 0.33, and 0 micromole l(-1) s(-1), rates substantially slower than those reported for other cells studied by similar methods. According to the model, the Na+-Ca2+ exchanger is poised so that it may run in reverse at resting [Ca2+]i levels. We conclude that the essential functions of sperm do not require the ability to recover rapidly from globally elevated cytosolic [Ca2+].

Ca2+ clearance at growth cones produced by crayfish motor axons in an explant culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 microM) and CCCP (10 microM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Ca2+ Clearance at Growth Cones Produced by Crayfish Motor Axons in an Explant Culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 μM) and CCCP (10 μM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Removal of Ca(2+) following depolarization-evoked cytoplasmic Ca(2+) transients in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus

Cytoplasmic [Ca(2+)] ([Ca(2+)](i)) was measured using Fura-2 in pyramidal neurones isolated from the rat dorsal cochlear nucleus (DCN). The kinetic properties of Ca(2+) removal following K(+) depolarization-induced Ca(2+) transients were characterized by fitting exponential functions to the decay phase. The removal after small transients (<82 nM peak [Ca(2+)](i)) had monophasic time course (time constant of 6.43 +/- 0.48 s). In the cases of higher Ca(2+) transients biphasic decay was found. The early time constant decreased (from 3.09 +/- 0.26 to 1.46 +/- 0.11 s) as the peak intracellular [Ca(2+)] increased. The value of the late time constant was 18.15 +/- 1.60 s at the smallest transients, and showed less dependence on [Ca(2+)](i). Blockers of Ca(2+) uptake into intracellular stores (thapsigargin and cyclopiazonic acid) decreased the amplitude of the Ca(2+) transients and slowed their decay. La(3+) (3 mM) applied extracellularly during the declining phase dramatically changed the time course of the Ca(2+) transients as a plateau developed and persisted until the La(3+) was present. When the other Ca(2+) removal mechanisms were available, reduction of the external [Na(+)] to inhibit the Na(+)/Ca(2+) exchange resulted in a moderate increase of the time constants. It is concluded that in the isolated pyramidal neurones of the DCN the removal of Ca(2+) depends mainly on the activity of Ca(2+) pump mechanisms.

Autocrine action and its underlying mechanism of nitric oxide on intracellular Ca2+ homeostasis in vascular endothelial cells

The rise in cytosolic Ca(2+) concentration (Ca(2+)(i)) in vascular endothelial cells (ECs) activates the production and release of nitric oxide (NO). NO modifies Ca(2+)(i) homeostasis in many types of nonendothelial cells. However, its effect on endothelial Ca(2+)(i) homeostasis at basal and excited states remains unclear. In the present study, to elucidate the effect of NO on basal Ca(2+)(i), inositol 1,4,5-trisphosphate-induced Ca(2+)(i) release (IICR) was blocked by expressing an antisense against type-1 inositol 1,4,5-trisphosphate receptors or by microinjecting heparin to individual ECs, and the effects of NO that was released by and diffused from adjacent IICR-intact ECs were recorded. After ATP or bradykinin stimulation, IICR-inhibited ECs showed a marked reduction of basal Ca(2+)(i), which was abolished by N(G)-monomethyl-l-arginine monoacetate pretreatment. The reduction disappeared in sparsely seeded ECs. Exogenous NO gas mimicked the effect of ATP or bradykinin to reduce basal Ca(2+)(i). Blocking plasma membrane Ca(2+)-ATPase (PMCA), but not Na(+)-Ca(2+) exchange or sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, suppressed the reduction, indicating that the reduction resulted from a NO-dependent potentiation of PMCA. To elucidate the effect of NO on elevated Ca(2+)(i), ATP-, bradykinin-, or thapsigargin-evoked Ca(2+)(i) response in the presence and absence of NO production was compared in adjacent IICR-intact ECs. NO was found to potentiate PMCA, which, in turn, greatly attenuated agonist-evoked Ca(2+)(i) elevation. NO also potentiated Ca(2+) influx, which markedly increased the sustained phase of Ca(2+)(i) elevation and possibly NO production. NO did not affect other Ca(2+)(i)-elevating and Ca(2+)(i)-sequestrating components. Thus, NO-dependent potentiation of PMCA is crucial for Ca(2+)(i) homeostasis over a wide Ca(2+)(i) range.

Mobilizing store Ca(2+) in the presence of La(3+) evokes exocytosis in bovine chromaffin cells

The effect on exocytosis of La(3+), a known inhibitor of plasma membrane Ca(2+)-ATPases and Na(+)/Ca(2+) exchangers, was studied using cultured bovine adrenal chromaffin cells. At high concentrations (0.3-3 mM), La(3+) substantially increased histamine-induced catecholamine secretion. This action was mimicked by other lanthanide ions (Nd(3+), Eu(3+), Gd(3+), and Tb(3+)), but not several divalent cations. In the presence of La(3+), the secretory response to histamine became independent of extracellular Ca(2+). La(3+) enhanced secretion evoked by other agents that mobilize intracellular Ca(2+) stores (angiotensin II, bradykinin, caffeine, and thapsigargin), but not that due to passive depolarization with 20 mM K(+). La(3+) still enhanced histamine-induced secretion in the presence of the nonselective inhibitors of Ca(2+)-permeant channels SKF96365 and Cd(2+), but the enhancement was abolished by prior depletion of intracellular Ca(2+) stores with thapsigargin. La(3+) inhibited (45)Ca(2+) efflux from preloaded chromaffin cells in the presence or absence of Na(+). It also enhanced and prolonged the rise in cytosolic [Ca(2+)] measured with fura-2 during mobilization of intracellular Ca(2+) stores with histamine in Ca(2+)-free buffer. The results suggest that the efficacy of intracellular Ca(2+) stores in evoking exocytosis is enhanced dramatically by inhibiting Ca(2+) efflux from the cell.

Ouabain augments Ca(2+) transients in arterial smooth muscle without raising cytosolic Na(+)

Ouabain and other cardiotonic steroids (CTS) inhibit Na(+) pumps and are widely believed to exert their cardiovascular effects by raising the cytosolic Na(+) concentration ([Na(+)](cyt)) and Ca(2+). This view has not been rigorously reexamined despite evidence that low-dose CTS may act without elevating [Na(+)](cyt); also, it does not explain the presence of multiple, functionally distinct isoforms of the Na(+) pump in many cells. We investigated the effects of Na(+) pump inhibition on [Na(+)](cyt) (with Na(+) binding benzofuran isophthalate) and Ca(2+) transients (with fura 2) in primary cultured arterial myocytes. Low concentrations of ouabain (3-100 nM) or human ouabain-like compound or reduced extracellular K(+) augmented hormone-evoked mobilization of stored Ca(2+) but did not increase bulk [Na(+)](cyt). Augmentation depended directly on external Na(+), but not external Ca(2+), and was inhibited by 10 mM Mg(2+) or 10 microM La(3+). Evoked Ca(2+) transients in pressurized small resistance arteries were also augmented by nanomolar ouabain and inhibited by Mg(2+). These results suggest that Na(+) enters a tiny cytosolic space between the plasmalemma (PL) and the adjacent sarcoplasmic reticulum (SR) via an Mg(2+)- and La(3+)-blockable mechanism that is activated by SR store depletion. The Na(+) and Ca(2+) concentrations within this space may be controlled by clusters of high ouabain affinity (alpha3) Na(+) pumps and Na/Ca exchangers located in PL microdomains overlying the SR. Inhibition of the alpha3 pumps by low-dose ouabain should raise the local concentrations of Na(+) and Ca(2+) and augment hormone-evoked release of Ca(2+) from SR stores. Thus the clustering of small numbers of specific PL ion transporters adjacent to the SR can regulate global Ca(2+) signaling. This mechanism may affect vascular tone and blood flow and may also influence Ca(2+) signaling in many other types of cells.

Gadolinium inhibits Na(+)-Ca(2+) exchanger current in guinea-pig isolated ventricular myocytes

The trivalent cation, gadolinium (Gd(3+)) is commonly used to inhibit stretch-activated channels. In this report, we show that Gd(3+) also inhibits ionic current (I(NaCa)), carried by the Na(+)-Ca(2+) exchanger protein. Under selective recording conditions, Gd(3+) inhibited both outward and inward I(NaCa) from guinea-pig isolated ventricular myocytes in a dose-dependent manner, with half-maximal inhibition concentrations (IC(50)) of 30.0+/-4.0 microM at +60 mV (Hill-coefficient, h=1.04+/-0.13) and 20.0+/-2.7 microM at -100 mV (h=1.13+/-0.16), respectively (P>0.05, n=5 - 9). Thus, inhibition was not voltage-dependent. The time from Gd(3+) application to steady-state effect was slow compared to the divalent blocker Ni(2+). The slow time course appeared to reflect gradual Gd(3+) accumulation at its binding site on the exchanger, rather than a use-dependent blocking mechanism. This study indicates that for experiments in which Gd(3+) is used, its inhibitory effect on I(NaCa) should be taken into account.

Pharmacological and immunohistochemical characterization of calmodulin-stimulated (Ca(2+)+Mg(2+))-ATPase in cultured porcine aortic endothelial cells

Plasma membrane (Ca(2+)+Mg(2+))-ATPase and Ca(2+) transport activities, best characterized in human erythrocytes, are stimulated by calmodulin and thought to play a crucial role in the termination of cellular Ca(2+) signaling in all cells. In plasma membranes isolated from cultured porcine aortic endothelial cells, the (Ca(2+)+Mg(2+))-ATPase was not readily measured. This is in part because of an overabundance of nonspecific Ca(2+)- and/or Mg(2+)-activated ecto-5'-nucleotide phosphohydrolases. Moreover, addition of exogenous calmodulin (10(-9) to 10(-6) mol/L) produced no measurable stimulation of ATPase activities, suggesting a permanently activated state or, alternatively, a complete lack thereof. To establish and verify the presence of a calmodulin-regulated (Ca(2+)+Mg(2+))-ATPase activity in these endothelial cells, immunohistochemical localization using a monoclonal mouse anti-(Ca(2+)+Mg(2+))-ATPase antibody (clone 5F10) was applied to intact pig aorta endothelium, cultured endothelial monolayers, and isolated endothelial plasma membrane fractions. This approach clearly demonstrated Ca(2+) pump immunoreactivity in each of these preparations. To confirm functional calmodulin stimulation of the (Ca(2+)+Mg(2+))-ATPase, 10(-5) mol/L calmidazolium (R24571) was added to the isolated plasma membrane preparation, which lowered the (Ca(2+)+Mg(2+))-ATPase activity from 143.0 to 78.15 nmol P(i)/mg protein x min(-1). This calmidazolium-reduced activity could then be stimulated 113.1+/-0.8% in a concentration-dependent manner by the addition of exogenous calmodulin (10(-7) to 2 x 10(-6) mol/L) with an EC(50) of 3.45+/-0.04 x 10(-7) mol/L (n=4). This represents a competitive lowering of the apparent calmodulin affinity by approximately 100 compared with other unopposed calmodulin-stimulated processes. Together, these findings support evidence for the presence of a calmodulin-stimulated plasma membrane (Ca(2+)+Mg(2+))-ATPase activity in cultured porcine aortic endothelial cells.

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.

Multiple protein kinase pathways are involved in gastrin-releasing peptide receptor-regulated secretion

Gastrin-releasing peptide (GRP) and its amphibian homolog, bombesin, are potent secretogogues in mammals. We determined the roles of intracellular free Ca(2+) ([Ca(2+)](i)), protein kinase C (PKC), and mitogen-activated protein kinases (MAPK) in GRP receptor (GRP-R)-regulated secretion. Bombesin induced either [Ca(2+)](i) oscillations or a biphasic elevation in [Ca(2+)](i). The biphasic response was associated with peptide secretion. Receptor-activated secretion was blocked by removal of extracellular Ca(2+), by chelation of [Ca(2+)](i), and by treatment with inhibitors of phospholipase C, conventional PKC isozymes, and MAPK kinase (MEK). Agonist-induced increases in [Ca(2+)](i) were also inhibited by dominant negative MEK-1 and the MEK inhibitor, PD89059, but not by an inhibitor of PKC. Direct activation of PKC by a phorbol ester activated MAPK and stimulated peptide secretion without a concomitant increase in [Ca(2+)](i). Inhibition of MEK blocked both bombesin- and phorbol 12-myristate 13-acetate-induced secretion. GRP-R-regulated secretion is initiated by an increase in [Ca(2+)](i); however, elevated [Ca(2+)](i) is insufficient to stimulate secretion in the absence of activation of PKC and the downstream MEK/MAPK pathways. We demonstrated that the activity of MEK is important for maintaining elevated [Ca(2+)](i) levels induced by GRP-R activation, suggesting that MEK may affect receptor-regulated secretion by modulating the activity of Ca(2+)-sensitive PKC.

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.

Heterogeneity of mitochondrial matrix free ca2+: resolution of Ca2+ dynamics in individual mitochondria in situ

The role of mitochondria in Ca2+ homeostasis is controversial. We employed the Ca2+-sensitive dye rhod 2 with novel, high temporal and spatial resolution imaging to evaluate changes in the matrix free Ca2+ concentration of individual mitochondria ([Ca2+]m) in agonist-stimulated, primary cultured aortic myocytes. Stimulation with 10 microM serotonin (5-HT) evoked modest cytosolic Ca2+ transients [cytosolic free Ca2+ concentration ([Ca2+]cyt) <500 nM; measured with fura 2] and triggered contractions in short-term cultured myocytes. However, 5-HT triggered a large mitochondrial rhod 2 signal (indicating pronounced elevation of [Ca2+]m) in only 4% of cells. This revealed heterogeneity in the responses of individual mitochondria, all of which stained with MitoTracker Green FM. In contrast, stimulation with 100 microM ATP evoked large cytosolic Ca2+ transients (>1,000 nM) and induced pronounced, reversible elevation of [Ca2+]m (measured as rhod 2 fluorescence) in 60% of cells. This mitochondrial Ca2+ uptake usually lagged behind the cytosolic Ca2+ transient peak by 3-5 s, and [Ca2+]m declined more slowly than did bulk [Ca2+]cyt. The uptake delay may prevent mitochondria from interfering with rapid signaling events while enhancing the mitochondrial response to large, long-duration elevations of [Ca2+]cyt. The responses of arterial myocytes to modest physiological stimulation do not, however, depend on such marked changes in [Ca2+]m.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

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

Antisense oligodeoxynucleotides (AS-oligos) targeted to the Na+/Ca2+ exchanger (NCX) inhibit NCX-mediated Ca2+ influx in mesenteric artery (MA) myocytes [Am. J. Physiol. 269 (Cell Physiol. 38): C1340-C1345, 1995]. Here, we show AS-oligo knockdown of NCX-mediated Ca2+ efflux. In initial experiments, the cytosolic free Ca2+ concentration ([Ca2+]cyt) was raised, and sarcoplasmic reticulum (SR) Ca2+ sequestration was blocked with caffeine and cyclopiazonic acid; the extracellular Na+-dependent (NCX) component of Ca2+ efflux was then selectively inhibited in AS-oligo-treated cells but not in controls (no oligos or nonsense oligos). In contrast, the La3+-sensitive (plasmalemma Ca2+ pump) component of Ca2+ efflux was unaffected in AS-oligo-treated cells. Knockdown of NCX activity was reversed by incubating AS-oligo-treated cells in normal media for 5 days. Transient [Ca2+]cyt elevations evoked by serotonin (5-HT) at 15-min intervals in AS-oligo-treated cells were indistinguishable from those in controls. When cells were stimulated every 3 min, however, the peak amplitudes of the second and third responses were larger, and [Ca2+]cyt returned to baseline more slowly, in AS-oligo-treated cells than in controls. Peak 5-HT-evoked responses in the controls, but not AS-oligo-treated cells, were augmented more than twofold in Na+-free media. This implies that NCX is involved in Na+ gradient modulation of SR Ca2+ stores and cell responsiveness. The repetitive stimulation data suggest that the NCX may be important during tonic activation of arterial myocytes.