Barium influx mediated by the cardiac sodium-calcium exchanger in transfected Chinese hamster ovary cells

Critical contributions of pre-S1 shoulder and distal TRP box in DAG-activated TRPC6 channel by PIP2 regulation

Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ or PIP₂) regulates the activities of numerous membrane proteins, including diacylglycerol(DAG)-activated TRPC3/6/7 channels. Although PIP₂ binding is known to support DAG-activated TRP channel activity, its binding site remains unknown. We screened for PIP₂ binding sites within TRPC6 channels through extensive mutagenesis. Using voltage-sensitive phosphatase (DrVSP), we found that Arg437 and Lys442, located in the channel’s pre-S1 domain/shoulder, are crucial for interaction with PIP₂. To gain structural insights, we conducted computer protein–ligand docking simulations with the pre-S1 domain/shoulder of TRPC6 channels. Further, the functional significance of PIP₂ binding to the pre-S1 shoulder was assessed for receptor-operated channel functions, cross-reactivity to DAG activation, and the kinetic model simulation. These results revealed that basic residues in the pre-S1 domain/shoulder play a central role in the regulation of PIP₂-dependent gating. In addition, neutralizing mutation of K771 in the distal TRP box reversed the effect of PIP₂ depletion from inhibiting to potentiating channel activity. A similar effect was seen in TRPV1 channels, which suggests that TRPC6 possesses a common but robust polarity switch mediating the PIP₂-dependent effect. Overall, these mutagenesis studies reveal functional and structural insights for how basic residues and channel segments in TRP channels are controlled through phosphoinositides recognition.

Probing Nanoelectroporation and Resealing of the Cell Membrane by the Entry of Ca2+ and Ba2+ Ions

The principal bioeffect of the nanosecond pulsed electric field (nsPEF) is a lasting cell membrane permeabilization, which is often attributed to the formation of nanometer-sized pores. Such pores may be too small for detection by the uptake of fluorescent dyes. We tested if Ca²⁺, Cd²⁺, Zn²⁺, and Ba²⁺ ions can be used as nanoporation markers. Time-lapse imaging was performed in CHO, BPAE, and HEK cells loaded with Fluo-4, Calbryte, or Fluo-8 dyes. Ca²⁺ and Ba²⁺ did not change fluorescence in intact cells, whereas their entry after nsPEF increased fluorescence within <1 ms. The threshold for one 300-ns pulse was at 1.5–2 kV/cm, much lower than >7 kV/cm for the formation of larger pores that admitted YO-PRO-1, TO-PRO-3, or propidium dye into the cells. Ba²⁺ entry caused a gradual emission rise, which reached a stable level in 2 min or, with more intense nsPEF, kept rising steadily for at least 30 min. Ca²⁺ entry could elicit calcium-induced calcium release (CICR) followed by Ca²⁺ removal from the cytosol, which markedly affected the time course, polarity, amplitude, and the dose-dependence of fluorescence change. Both Ca²⁺ and Ba²⁺ proved as sensitive nanoporation markers, with Ba²⁺ being more reliable for monitoring membrane damage and resealing.

Regenerative Adaptation to Electrochemical Perturbation in Planaria: A Molecular Analysis of Physiological Plasticity

Anatomical homeostasis results from dynamic interactions between gene expression, physiology, and the external environment. Owing to its complexity, this cellular and organism-level phenotypic plasticity is still poorly understood. We establish planarian regeneration as a model for acquired tolerance to environments that alter endogenous physiology. Exposure to barium chloride (BaCl₂) results in a rapid degeneration of anterior tissue in Dugesia japonica. Remarkably, continued exposure to fresh solution of BaCl₂ results in regeneration of heads that are insensitive to BaCl₂. RNA-seq revealed transcriptional changes in BaCl₂-adapted heads that suggests a model of adaptation to excitotoxicity. Loss-of-function experiments confirmed several predictions: blockage of chloride and calcium channels allowed heads to survive initial BaCl₂ exposure, inducing adaptation without prior exposure, whereas blockade of TRPM channels reversed adaptation. Such highly adaptive plasticity may represent an attractive target for biomedical strategies in a wide range of applications beyond its immediate relevance to excitotoxicity preconditioning.

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Exposure to BaCl₂ causes the heads of Dugesia japonica to degenerate

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Prolonged exposure to BaCl₂ results in regeneration of a BaCl₂-insensitive head

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Ion channel expression is altered in the head to compensate for excitotoxic stress

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TRPMa is upregulated in BaCl₂-treated animals; blocking TRPM prevents adaptation

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

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

Electrophysiological characterization of store-operated and agonist-induced Ca2+ entry pathways in endothelial cells

In endothelial cells, agonist-induced Ca(2+) entry takes place via the store-operated Ca(2+) entry pathway and/or via channel(s) gated by second messengers. As cell stimulation leads to both a partial Ca(2+) store depletion as well as the production of second messengers, these two pathways are problematic to distinguish. We showed that passive endoplasmic reticulum (ER) depletion by thapsigargin or cell stimulation by histamine activated a similar Ca(2+)-release-activated Ca(2+) current (CRAC)-like current when 10 mM Ba(2+)/2 mM Ca(2+) was present in the extracellular solution. Importantly, during voltage clamp recordings, histamine stimulation largely depleted the ER Ca(2+) store, explaining the activation of a CRAC-like current (due to store depletion) upon histamine in Ba(2+) medium. On the contrary, in the presence of 10 mM Ca(2+), the ER Ca(2+) content remained elevated, and histamine induced an outward rectifying current that was inhibited by Ni(2+) and KB-R7943, two blockers of the Na(+)/Ca(2+) exchanger (NCX). Both blockers also reduced histamine-induced cytosolic Ca(2+) elevation. In addition, removing extracellular Na(+) increased the current amplitude which is in line with a current supported by the NCX. These data are consistent with the involvement of the NCX working in reverse mode (Na(+) out/Ca(2+) in) during agonist-induced Ca(2+) entry in endothelial cells.

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

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

The sodium-calcium exchanger is a mechanosensitive transporter

This report describes the influence of fluid flow and osmotically induced volume changes on Na(+)-Ca(2+) exchange (NCX) activity in transfected CHO cells. Exchange activity was measured as Na(+)-dependent Ca(2+) or Ba(2+) fluxes using the fluorescent probe fura-2. When exchange activity was initiated by superfusing Ba(2+)-containing solutions over the cells for a 20 s interval, a high rate of Ba(2+) uptake was observed while the solution was being applied but the rate of Ba(2+) uptake declined > 10-fold when the solution flow ceased. Ba(2+) efflux in exchange for extracellular Na(+) or Ca(2+) (Ba(2+)-Ca(2+) exchange) was similarly biphasic. During NCX-mediated Ca(2+) uptake, a rapid increase in cytosolic [Ca(2+)] to a peak value occurred, followed by a decline in [Ca(2+)](i) to a lower steady-state value after solution flow ceased. When NCX activity was initiated by an alternate procedure that minimized the duration of solution flow, the rapid phase of Ba(2+) influx was greatly reduced in magnitude and Ca(2+) uptake became nearly monophasic. Solution superfusion did not produce any obvious changes in cell shape or volume. NCX-mediated Ba(2+) and Ca(2+) influx were also sensitive to osmotically induced changes in cell volume. NCX activity was stimulated in hypotonic media and inhibited in hypertonic media; the osmotically induced changes in activity occurred within seconds and were rapidly reversible. We conclude that NCX activity is modulated by both solution flow and osmotically induced volume changes.

Muscarinic acetylcholine receptors activate TRPC6 channels in PC12D cells via Ca2+ store-independent mechanisms

In this paper we report that stimulation of mAChRs in PC12D cells activates Ca2+ channels that are regulated independently of intracellular Ca2+ stores. In nominally Ca2+-free medium, exposure of PC12D cells to carbachol stimulates a robust influx of Ba2+, a Ca2+ substitute. This influx is blocked by atropine, but not by inhibitors of the nicotinic acetylcholine receptor or L-, N-, or T-type voltage-regulated Ca2+ channels. By contrast, depletion of intracellular Ca2+ stores with thapsigargin only weakly stimulates Ba2+ influx. Unlike store-operated Ca2+ channels (SOCCs), which close only after intracellular Ca2+ stores refill, channels mediating carbachol-stimulated Ba2+ influx rapidly close following the inactivation of mAChRs with atropine. Ba2+ influx is inhibited by extracellular Ca2+, by the Ca2+ channel blocker SKF-96365, and by activation of protein kinase C (PKC). Exogenous expression of antisense RNA encoding the rat canonical-transient receptor potential Ca2+ channel subtype 6 (TRPC6) or the N-terminal domain of TRPC6 blocks carbachol-stimulated Ba2+ influx in PC12D cells. Expression of TRPC6 antisense RNA or the TRPC6 N-terminal domain also blocks Ba2+ influx stimulated by 1-oleoyl-2-acetyl-sn-glycerol (OAG), a diacylglycerol analog previously shown to activate exogenously expressed TRPC6 channels. These data show that mAChRs in PC12D cells activate endogenous Ca2+ channels that are regulated independently of Ca2+ stores and require the expression of TRPC6.

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.

Sodium-calcium exchange does not require allosteric calcium activation at high cytosolic sodium concentrations

The activity of the cardiac Na(+)-Ca(2+) exchanger (NCX1.1) is allosterically regulated by Ca(2+), which binds to two acidic regions in the cytosolically disposed central hydrophilic domain of the NCX protein. A mutation in one of the regulatory Ca(2+) binding regions (D447V) increases the half-activation constant (K(h)) for allosteric Ca(2+) activation from approximately 0.3 to > 1.8 microm. Chinese hamster ovary cells expressing the D447V exchanger showed little or no activity under physiological ionic conditions unless cytosolic [Ca(2+)] was elevated to > 1 microm. However, when cytosolic [Na(+)] was increased to 20 mm or more (using ouabain-induced inhibition of the Na(+),K(+)-ATPase or the ionophore gramicidin), cells expressing the D447V mutant rapidly accumulated Ca(2+) or Ba(2+) when the reverse (Ca(2+) influx) mode of NCX activity was initiated, although initial cytosolic [Ca(2+)] was < 100 nm. Importantly, the time course of Ca(2+) uptake did not display the lag phase that reflects allosteric Ca(2+) activation of NCX activity in the wild-type NCX1.1; indeed, at elevated [Na(+)], the D447V mutant behaved similarly to the constitutively active deletion mutant Delta(241-680), which lacks the regulatory Ca(2+) binding sites. In cells expressing wild-type NCX1.1, increasing concentrations of cytosolic Na(+) led to a progressive shortening of the lag phase for Ca(2+) uptake. The effects of elevated [Na(+)] developed rapidly and were fully reversible. The activity of the D447V mutant was markedly inhibited when phosphatidylinositol 4,5-bisphosphate (PIP2) levels were reduced. We conclude that when PIP2 levels are high, elevated cytosolic [Na(+)] induces a mode of exchange activity that does not require allosteric Ca(2+) activation.

Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.

Mitochondria play a critical role in shaping the exocytotic response of rat pancreatic acinar cells

We have previously demonstrated [M. Campos-Toimil, T. Bagrij, J.M. Edwardson, P. Thomas, Two modes of secretion in pancreatic acinar cells: involvement of phosphatidylinositol 3-kinase and regulation by capacitative Ca(2+) entry, Curr. Biol. 12 (2002) 211-215] that in rat pancreatic acinar cells, Gd(3+)-sensitive Ca(2+) entry is instrumental in governing which second messenger pathways control secretory activity. However, in those studies, we were unable to demonstrate a significant increase in cytoplasmic [Ca(2+)] during agonist application as a result of this entry pathway. In the present study, we combined pharmacology with ratiometric imaging of fura-2 fluorescence to resolve this issue. We found that 2 microM Gd(3+) significantly inhibits store-mediated Ca(2+) entry. Furthermore, both the protonophore, CCCP (5 microM) and the mitochondrial Ca(2+)-uptake blocker, RU360 (10 microM), led to an enhancement of the plateau phase of the biphasic Ca(2+) response induced by acetylcholine (1 microM). This enhancement was completely abolished by Gd(3+); and as has been previously shown for Gd(3+), RU360 led to a switch to a wortmannin-sensitive form of exocytosis. Using MitoTracker Red staining we found a close association of mitochondria with the lateral plasma membrane. We propose that in rat pancreatic acinar cells, capacitative Ca(2+) entry is targeted directly to mitochondria; and that as a result of Ca(2+) uptake, these mitochondria release "third" messengers which both enhance exocytosis and suppress phosphatidylinositol 3-kinase-dependent secretion.

Allosteric activation of sodium-calcium exchange by picomolar concentrations of cadmium

Chinese hamster ovary cells expressing the bovine cardiac Na+-Ca2+ exchanger (NCX1.1) accumulated Cd2+ after a lag period of several tens of seconds. The lag period reflects the progressive allosteric activation of exchange activity by Cd2+ as it accumulates within the cytosol. The lag period was greatly reduced in cells expressing a mutant exchanger, Delta(241-680), that does not require allosteric activation by Ca2+ for activity. Non-transfected cells did not show Cd2+ uptake under the same conditions. In cells expressing NCX1.1, the lag period was nearly abolished following an elevation of the cytosolic Ca2+ concentration. Cytosolic Cd2+ concentrations estimated at 0.5-2 pm markedly stimulated the subsequent uptake of Ca2+ by Na+-Ca2+ exchange. Outward exchange currents in membrane patches from Xenopus oocytes expressing the canine NCX1.1 were rapidly and reversibly stimulated by 3 pm Cd2+ applied at the cytosolic membrane surface. Exchange currents activated by 3 pm Cd2+ were 40% smaller than currents activated by 1 mum cytosolic Ca2+. Current amplitudes declined by 30% and the rate of current development fell sharply upon repetitive applications of Na+ in the presence of 3 pm Cd2+. Cd2+ mimicked the anomalous inhibitory effects of Ca2+ on outward exchange currents generated by the Drosophila exchanger CALX1.1. We conclude that the regulatory sites responsible for allosteric Ca2+ activation bind Cd2+ with high affinity and that Cd2+ mimics the regulatory effects of Ca2+ at concentrations 5 orders of magnitude lower than Ca2+.

Calcium-dependent regulation of calcium efflux by the cardiac sodium/calcium exchanger

Allosteric regulation by cytosolic Ca2+of Na+/Ca2+exchange activity in the Ca2+efflux mode has received little attention because it has been technically difficult to distinguish between the roles of Ca2+as allosteric activator and transport substrate. In this study, we used transfected Chinese hamster ovary cells to compare the Ca2+efflux activities in nontransfected cells and in cells expressing either the wild-type exchanger or a mutant, Δ(241–680), that operates constitutively; i.e., its activity does not require allosteric Ca2+activation. Expression of the wild-type exchanger did not significantly lower the cytosolic Ca2+concentration ([Ca2+]i) compared with nontransfected cells. During Ca2+entry through store-operated Ca2+channels, Ca2+efflux by the wild-type exchanger became evident only after [Ca2+]iapproached 100–200 nM. A subsequent decline in [Ca2+]iwas observed, suggesting that the activation process was time dependent. In contrast, Ca2+efflux activity was evident under all experimental conditions in cells expressing the constitutive exchanger mutant. After transient exposure to elevated [Ca2+]i, the wild-type exchanger behaved similarly to the constitutive mutant for tens of seconds after [Ca2+]ihad returned to resting levels. We conclude that Ca2+efflux activity by the wild-type exchanger is allosterically activated by Ca2+, perhaps in a time-dependent manner, and that the activated state is briefly retained after the return of [Ca2+]ito resting levels.

Lithium-calcium exchange is mediated by a distinct potassium-independent sodium-calcium exchanger

Sodium-calcium exchangers have long been considered inert with respect to monovalent cations such as lithium, choline, and N-methyl-d-glucamine. A key question that has remained unsolved is how despite this, Li(+) catalyzes calcium exchange in mammalian tissues. Here we report that a Na(+)/Ca(2+) exchanger, NCLX cloned from human cells (known as FLJ22233), is distinct from both known forms of the exchanger, NCX and NCKX in structure and kinetics. Surprisingly, NCLX catalyzes active Li(+)/Ca(2+) exchange, thereby explaining the exchange of these ions in mammalian tissues. The NCLX protein, detected as both 70- and 55-KDa polypeptides, is highly expressed in rat pancreas, skeletal muscle, and stomach. We demonstrate, moreover, that NCLX is a K(+)-independent exchanger that catalyzes Ca(2+) flux at a rate comparable with NCX1 but without promoting Na(+)/Ba(2+) exchange. The activity of NCLX is strongly inhibited by zinc, although it does not transport this cation. NCLX activity is only partially inhibited by the NCX inhibitor, KB-R7943. Our results provide a cogent explanation for a fundamental question. How can Li(+) promote Ca(2+) exchange whereas the known exchangers are inert to Li(+) ions? Identification of this novel member of the Na(+)/Ca(2+) superfamily, with distinct characteristics, including the ability to transport Li(+), may provide an explanation for this phenomenon.

Activation of human TRPC6 channels by receptor stimulation

The human TRPC6 channel was expressed in human embryonic kidney (HEK) cells, and activity was monitored using the giga-seal technique. Whole cell membrane currents with distinctive inward and outward rectification were activated by carbachol (CCh) in TRPC6-expressing cells, but not in lacZ-transfected controls. The effect of CCh was steeply dose-dependent with a K(0.5) of approximately 10 microm and a Hill coefficient of 3-4. A steep concentration-response relationship was also observed when TRPC6 activity was measured using a fluorescence-based imaging plate reader (FLIPR) assay for membrane depolarization. Ionomycin, thapsigargin, and dialysis of the cell with inositol 1,4,5-trisphosphate via the patch pipette had no effect on TRPC6 currents, but exogenous application of 1-oleoyl acetyl-sn-glycerol (OAG, 30-300 microm) produced a slow increase in channel activity. The PKC activator, phorbol 12-myristate 13-acetate (PMA, 0.5 microm) had no significant acute effect on TRPC6, or on the subsequent response to OAG. In contrast, the response to CCh was blocked >90% by PMA pretreatment. To further explore the role of DAG in receptor stimulation, TRPC6 currents were monitored following the sequential addition of CCh and OAG. Surprisingly, concentrations of CCh that produced little or no response in the absence of OAG, produced increases in TRPC6 currents in the presence of OAG that were larger than the sum of either agent alone. Likewise, the response to OAG was superadditive following prior stimulation of the cells with near threshold concentrations of CCh. Overall, these results suggest that generation of DAG alone may not fully account for activation of TRPC6, and that other receptor-mediated events act synergistically with DAG to stimulate channel activity. This synergy may explain, at least in part, the steep dose-response relationship observed for CCh-induced TRPC6 currents expressed in HEK cells.

Cytoskeletal modulation of electrical and mechanical activity in cardiac myocytes

The cardiac myocyte has an intracellular scaffold, the cytoskeleton, which has been implicated in several cardiac pathologies including hypertrophy and failure. In this review we describe the role that the cytoskeleton plays in modulating both the electrical activity (through ion channels and exchangers) and mechanical (or contractile) activity of the adult heart. We focus on the 3 components of the cytoskeleton, actin microfilaments, microtubules, and desmin filaments. The limited visual data available suggest that the subsarcolemmal actin cytoskeleton is sparse in the adult myocyte. Selective disruption of cytoskeletal actin by pharmacological tools has yet to be verified in the adult cell, yet evidence exists for modulation of several ionic currents, including I(CaL), I(Na), I(KATP), I(SAC) by actin microfilaments. Microtubules exist as a dense network throughout the adult cardiac cell, and their structure, architecture, kinetics and pharmacological manipulation are well described. Both polymerised and free tubulin are functionally significant. Microtubule proliferation reduces contraction by impeding sarcomeric motion; modulation of sarcoplasmic reticulum Ca(2+) release may also be involved in this effect. The lack of effect of microtubule disruption on cardiac contractility in adult myocytes, and the concentration-dependent modulation of the rate of contraction by the disruptor nocodazole in neonatal myocytes, support the existence of functionally distinct microtubule populations. We address the controversy regarding the stimulation of the beta-adrenergic signalling pathway by free tubulin. Work with mice lacking desmin has demonstrated the importance of intermediate filaments to normal cardiac function, but the precise role that desmin plays in the electrical and mechanical activity of cardiac muscle has yet to be determined.

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

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

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

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

Recombinant expression of the plasma membrane Na(+)/Ca(2+) exchanger affects local and global Ca(2+) homeostasis in Chinese hamster ovary cells

The cardiac type Na(+)/Ca(2+) exchanger (NCX1) has been transiently expressed in Chinese hamster ovary cells, which do not contain an endogenous exchanger, together with aequorin chimeras that are targeted to different intracellular compartments to investigate intracellular Ca(2+) homeostasis. The expression of NCX decreased the endoplasmic reticulum Ca(2+) concentration, [Ca(2+)](er), in resting cells, showing that the exchanger was operative under these conditions. It induced a greater reduction in the height of the mitochondrial and cytosolic Ca(2+) transients in agonist-stimulated cells than would have been expected from the [Ca(2+)](er) decrease. It also had a major effect on the sub-plasma membrane Ca(2+) concentration, [Ca(2+)](pm): after a transient [Ca(2+)](pm) rise induced by the activation of capacitative Ca(2+) influx, [Ca(2+)](pm) settled to a value about 3-fold higher than in controls. The sustained [Ca(2+)](pm) increase after the transient was due to the operation of the exchanger, either directly by operating in the Ca(2+) entry mode, or indirectly by removing the Ca(2+) inhibition on the capacitative Ca(2+) influx channels.

Characterization of cholera toxin B subunit-induced Ca(2+) influx in neuroblastoma cells: evidence for a voltage-independent GM1 ganglioside-associated Ca(2+) channel

The role of endogenous GM1 ganglioside in neurite outgrowth has been studied in N18 and NG108-15 neuroblastoma cells with the GM1-specific ligand cholera toxin B subunit (Ctx B), which stimulates Ca(2+) influx together with neuritogenesis. Our primary goal has been to identify the nature of the calcium channel that is modulated by GM1. An L-type voltage-operated Ca(2+) channel (VOCC) was previously proposed as the mediator of this phenomenon. This investigation, employing fura-2 fluorescent measurements and specific channel blockers and other agents, revealed that GM1 modulates a hitherto unidentified Ca(2+) channel not of the L type. It was opened by Ctx B; was permeable to Ca(2+) and Ba(2+) but not Mn(2+); and was blocked by Ni(2+), Cd(2+), and La(3+). Although most dihydropyridines inhibited Ctx B-induced Ca(2+) influx as well as neurite outgrowth at higher concentrations, they and other VOCC blockers at normally employed concentrations failed to do so, suggesting uninvolvement of VOCC. In addition, Ca(2+) influx induced by Ctx B was not mediated by cGMP-dependent or G-protein-coupled nonselective cation channels, as demonstrated by the cGMP antagonist Rp-cGMPS or the G-protein/receptor uncoupling agent suramin, respectively. Finally, Ca(2+) influx was unlikely to be due to inhibition or reversal of Na(+)-Ca(2+) exchanger via Ctx B induction of Na(+) uptake, insofar as no effect was seen on blocking Na(+) channels, inhibiting Na(+)-K(+)-ATPase, or eliminating extracellular Na(+). The results suggest that this novel channel is gated by interaction with GM1, which, when associated with the channel and bound by appropriate ligand, promotes Ca(2+) influx. This in turn induces signaling for the onset of neuritogenesis.

Anthrax edema toxin requires influx of calcium for inducing cyclic AMP toxicity in target cells

The anthrax edema toxin comprises two proteins: protective antigen and edema factor. Anthrax protective antigen binds to the receptors on the surface of target cells and facilitates the entry of edema factor into these target cells. Edema factor (EF) is an adenylate cyclase that catalyzes the synthesis of cyclic AMP (cAMP) in the cytosol of the host cells. In this study, we examined the requirement of extracellular calcium for anthrax edema toxin-induced toxicity in host cells. The cAMP response generated by edema toxin was analyzed in a variety of cells, including CHO, macrophage-like RAW264.7, human neutrophils, and human lymphocytes. Our investigations reveal that after EF reaches the cell cytosol, a rapid influx of calcium is triggered in the host cell that has a pivotal role in determining the cAMP response of the affected cells. Although the cAMP response generated by edema toxin in different cell types varied in intensity and in the time of initiation, the influx of calcium invariably preceded cAMP accumulation. Agents that blocked the uptake of calcium also inhibited edema toxin-induced accumulation of cAMP in the host cells. This is the first report that demonstrates that edema toxin induces accumulation of cAMP in lymphocytes. By accumulating cAMP, a potent inhibitor of immune cell function, edema toxin may actually be poisoning the immune system and thus facilitating the survival of the bacteria in the host.

Sarcoplasmic reticulum Ca(2+) uptake is impaired in coronary smooth muscle distal to coronary occlusion

After chronic occlusion, collateral-dependent coronary arteries exhibit alterations in both vasomotor reactivity and associated myoplasmic free Ca(2+) levels that are prevented by chronic exercise training. We tested the hypotheses that coronary occlusion diminishes Ca(2+) uptake by the sarcoplasmic reticulum (SR) and that exercise training would prevent impaired SR Ca(2+) uptake. Ameroid constrictors were surgically placed around the proximal left circumflex (LCx) artery of female swine 8 wk before initiating 16-wk sedentary (pen confined) or exercise-training (treadmill run) protocols. Twenty-four weeks after Ameroid placement, smooth muscles cells were enzymatically dissociated from both the LCx and nonoccluded left anterior descending (LAD) arteries of sedentary and exercise-trained pigs, and myoplasmic free Ca(2+) was studied using fura 2 microfluorometry. After the SR Ca(2+) store was partially depleted with caffeine (5 mM), KCl-induced membrane depolarization produced a significant decrease in the time to half-maximal (t(1/2)) myoplasmic free Ca(2+) accumulation in LCx versus LAD cells of sedentary pigs. Furthermore, inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA; 10 microM cyclopiazonic acid) significantly reduced t(1/2) in cells isolated from the LAD but not from the LCx. Exercise training did not prevent the differences in t(1/2) myoplasmic free Ca(2+) accumulation observed between LCx and LAD cells. Occlusion or exercise training did not alter SERCA protein levels. These results support our hypothesis of impaired SR Ca(2+) uptake in coronary smooth muscle cells isolated distal to chronic occlusion. Impaired SR Ca(2+) uptake was independent of SERCA protein levels and was not prevented by exercise training.

Mechanisms of amyloid beta protein-induced modification in ion transport systems: implications for neurodegenerative diseases

1. Alzheimer's disease (AD) is a neurodegenerative disorder that affects the cognitive function of the brain. Pathological changes in AD are characterized by the formation of amyloid plaques and neurofibrillary tangles as well as extensive neuronal loss. Abnormal proteolytic processing of amyloid precursor protein (APP) is the central step that leads to formation of amyloid plaque, neurofibrillary tangles, and neuronal loss. 2. The plaques, which accumulate extracellularly in the brain, are composed of aggregates and cause direct neurotoxic effects and/or increase neuronal vulnerability to excitotoxic insults. The aggregates consist of soluble pathologic amyloid beta peptides AbetaP[1-42] and AbetaP[1-43] and soluble nonpathologic AbetaP[1-40]. Both APP and AbetaP interact with ion transport systems. AbetaP induces a wide range of effects as the result of activating a cascade of mechanisms. 3. The major mechanisms proposed for AbetaP-induced cytotoxicity involve the loss of Ca2+ homeostasis and the generation of reactive oxygen species (ROS). The changes in Ca2+ homeostasis could be the result of (1) changes in endogenous ion transport systems, e.g. Ca2+ and K+ channels and Na+/K+-ATPase, and membrane receptor proteins, such as ligand-driven ion channels and G-protein-driven releases of second messengers, and (2) formation of heterogeneous ion channels. 4. The consequences of changes in Ca2+-homeostasis-induced generation of ROS are (a) direct modification of intrinsic ion transport systems and their regulatory mechanisms, and (b) indirect effects on ion transport systems via peroxidation of phospholipids in the membrane, inhibition of phosphorylation, and reduction of ATP levels and cytoplasmic pH. 5. We propose that in AD, AbetaP with its different conformations alters cell regulation by modifying several ion transport systems and also by forming heterogeneous ion channels. The changes in membrane transport systems are proposed as early steps in impairing neuronal function preceding plaque formation. We conclude that these changes damage the membrane by compromising its integrity and increasing its ion permeability. This mechanism of membrane damage is not only central for AD but also may explain other malfunctioned protein-processing-related pathologies.

Inhibition of sodium-calcium exchange by ceramide and sphingosine

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

Enhanced L-type Ca2+ channel current density in coronary smooth muscle of exercise-trained pigs is compensated to limit myoplasmic free Ca2+ accumulation

We hypothesized that enhanced voltage-gated Ca2+ channel current (VGCC) density in coronary smooth muscle cells of exercise-trained miniature Yucatan pigs is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation. Whole-cell voltage clamp experiments demonstrated enhanced VGCC density in smooth muscle cells freshly dispersed from coronary arteries of exercise-trained vs. sedentary animals. In separate experiments using fura-2 microfluorometry, we measured depolarization-induced (80 mM KCl) accumulation of myoplasmic free Ba2+ and free Ca2+. Both maximal rate and net accumulation of free Ba2+ in response to membrane depolarization were increased in smooth muscle cells isolated from exercise-trained pigs, consistent with an increased VGCC density. Depolarization also produced an enhanced maximal rate of free Ca2+ accumulation in cells of exercise-trained pigs; however, net accumulation of free Ca2+ was not significantly increased suggesting enhanced Ca2+ influx was compensated to limit net free Ca2+ accumulation. Inhibition of sarco-endoplasmic reticulum Ca2+-transporting ATPase (SERCA; 10 microM cyclopiazonic acid) and/or sarcolemmal Na+-Ca2+ exchange (low extracellular Na+) suggested neither mechanism compensated the enhanced VGCC in cells of exercise-trained animals. Local Ca2+-dependent inactivation of VGCC, assessed by buffering myoplasmic Ca2+ with EGTA in the pipette and using Ca2+ and Ba2+ as charge carriers, was not different between cells of sedentary and exercise-trained animals. Our findings indicate that increased VGCC density is compensated by other cellular Ca2+ regulatory mechanisms to limit net myoplasmic free Ca2+ accumulation in smooth muscle cells of exercise-trained animals. Further, SERCA, Na+-Ca2+ exchange and local Ca2+-dependent inactivation of VGCC do not appear to function as compensatory mechanisms. Additional potential compensatory mechanisms include Ca2+ extrusion via plasma membrane Ca2+-ATPase, mitochondrial uptake, myoplasmic Ca2+-binding proteins and other sources of VGCC inactivation.

Role of gap junctions in endothelium-derived hyperpolarizing factor responses and mechanisms of K(+)-relaxation

We have examined the effects of ouabain (1 mM), the gap junction inhibitors, 18 alpha-glycyrrhetinic acid (100 microM), N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A; 10 microM) and palmitoleic acid (50 microM), and clotrimazole (10 microM) against endothelium-derived hyperpolarizing factor (EDHF)-mediated and K(+)-induced vasorelaxations in the rat mesentery. In the presence of indomethacin (10 microM) and 300-microM N(G)nitro-L-arginine methyl ester (L-NAME), carbachol caused EDHF-mediated relaxations (R(max)=85.3+/-4.0%). In the presence of ouabain, these responses were substantially reduced (R(max)=11.0+/-2.3%). 18 alpha-glycyrrhetinic acid, SR141716A, palmitoleic acid and clotrimazole also significantly inhibited these EDHF-mediated responses. K(+) caused vasorelaxation of preparations perfused with K(+)-free buffer (R(max)=73.7+/-2.4%), which were reduced by 10-microM indomethacin (R(max)=56.4+/-6.2%). K(+) vasorelaxation was essentially abolished by endothelial denudation. Both ouabain and 18 alpha-glycyrrhetinic acid opposed K(+) relaxations, however, neither SR141716A, clotrimazole nor palmitoleic acid had any effect. Direct cell-cell coupling via gap junctions was attenuated by ouabain, clotrimazole and palmitoleic acid. We conclude that: (i) that gap junctional communication plays a major role in EDHF-mediated relaxations, (ii) that K(+)-vasorelaxation is endothelium-dependent (thus, K(+) is unlikely to represent an EDHF), and (iii) that the inhibitory actions of ouabain and clotrimazole on gap junctions might contribute towards their effects against EDHF.

Feedback inhibition of sodium/calcium exchange by mitochondrial calcium accumulation

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

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

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

Ionic regulatory properties of brain and kidney splice variants of the NCX1 Na(+)-Ca(2+) exchanger

Ion transport and regulation of Na(+)-Ca(2+) exchange were examined for two alternatively spliced isoforms of the canine cardiac Na(+)-Ca(2+) exchanger, NCX1.1, to assess the role(s) of the mutually exclusive A and B exons. The exchangers examined, NCX1.3 and NCX1.4, are commonly referred to as the kidney and brain splice variants and differ only in the expression of the BD or AD exons, respectively. Outward Na(+)-Ca(2+) exchange activity was assessed in giant, excised membrane patches from Xenopus laevis oocytes expressing the cloned exchangers, and the characteristics of Na(+)(i)- (i.e., I(1)) and Ca(2+)(i)- (i.e., I(2)) dependent regulation of exchange currents were examined using a variety of experimental protocols. No remarkable differences were observed in the current-voltage relationships of NCX1.3 and NCX1.4, whereas these isoforms differed appreciably in terms of their I(1) and I(2) regulatory properties. Sodium-dependent inactivation of NCX1.3 was considerably more pronounced than that of NCX1.4 and resulted in nearly complete inhibition of steady state currents. This novel feature could be abolished by proteolysis with alpha-chymotrypsin. It appears that expression of the B exon in NCX1.3 imparts a substantially more stable I(1) inactive state of the exchanger than does the A exon of NCX1.4. With respect to I(2) regulation, significant differences were also found between NCX1.3 and NCX1.4. While both exchangers were stimulated by low concentrations of regulatory Ca(2+)(i), NCX1.3 showed a prominent decrease at higher concentrations (>1 microM). This does not appear to be due solely to competition between Ca(2+)(i) and Na(+)(i) at the transport site, as the Ca(2+)(i) affinities of inward currents were nearly identical between the two exchangers. Furthermore, regulatory Ca(2+)(i) had only modest effects on Na(+)(i)-dependent inactivation of NCX1.3, whereas I(1) inactivation of NCX1.4 could be completely eliminated by Ca(2+)(i). Our results establish an important role for the mutually exclusive A and B exons of NCX1 in modulating the characteristics of ionic regulation and provide insight into how alternative splicing tailors the regulatory properties of Na(+)-Ca(2+) exchange to fulfill tissue-specific requirements of Ca(2+) homeostasis.

Physiological and molecular characterization of the Na+/Ca2+ exchanger in human platelets

In this report, we have demonstrated that Na+/Ca2+ exchanger activity in a human megakaryocytic cell line (CHRF-288 cells) is K+ dependent, similar to the properties previously described for Na+/Ca2+ exchange activity in human platelets. With the use of RT-PCR techniques and mRNA, the exchanger expressed in CHRF-288 cells was found to be identical to that expressed in human retinal rods. Northern blot analysis of the mRNA for the human retinal rod exchanger in CHRF-288 cells revealed a major transcript at 5.8 kb with two minor bands at 4.9 and 6.8 kb. mRNA for the retinal rod exchanger was also identified in human platelets. Using Ba2+ influx as a measure of Na+/Ca2+ exchange activity in human platelets, we have demonstrated that exchange activity is driven by the transmembrane gradient for K+ as well as that for Na+. We propose that the K+ dependence of the platelet Na+/Ca2+ exchanger could make platelets especially sensitive to daily fluctuations in salt intake.

Energy expenditure by Ba(2+) contracture in rat ventricular slices derives from cross-bridge cycling

To clarify the energy-expenditure mechanism during Ba(2+) contracture of mechanically unloaded rat left ventricular (LV) slices, we measured myocardial O(2) consumption (VO(2)) of quiescent slices in Ca(2+)-free Tyrode solution and VO(2) during Ba(2+) contracture by substituting Ca(2+) with Ba(2+). We then investigated the effects of cyclopiazonic acid (CPA) and 2,3-butanedione monoxime (BDM) on the Ba(2+) contracture VO(2). The Ca(2+)-free VO(2) corresponds to that of basal metabolism (2.32 +/- 0.53 ml O(2). min(-1). 100 g LV(-1)). Ba(2+) increased the VO(2) in a dose-dependent manner (from 0.3 to 3.0 mmol/l) from 110 to 150% of basal metabolic VO(2). Blockade of the sarcoplasmic reticulum (SR) Ca(2+) pump by CPA (10 micromol/l) did not at all decrease the Ba(2+)-activated VO(2). BDM (5 mmol/l), which specifically inhibits cross-bridge cycling, reduced the Ba(2+)activated VO(2) almost to basal metabolic VO(2). These energetic results revealed that the Ba(2+)-activated VO(2) was used for the cross-bridge cycling but not for the Ca(2+) handling by the SR Ca(2+) pump.

Na+-Ca2+ exchange and Ca2+ efflux in transfected Chinese hamster ovary cells

The objective of this study was to assess the contribution of Na+-Ca2+ exchange activity to Ca2+ efflux at various cytosolic Ca2+ concentrations ([Ca2+]i) in transfected Chinese hamster cells expressing the bovine cardiac Na+-Ca2+ exchanger. Ionomycin was added to fura-2 loaded cells and the resulting [Ca2+]i transient was monitored in Ca2+-free media with or without extracellular Na+. The presence of Na+ reduced both the amplitude and duration of the [Ca2+]i transient. Na+ had similar effects when the peak of the [Ca2+]i transient was buffered to 100 nM by cytosolic EGTA, or when Ca2+ was slowly released from internal stores with thapsigargin. Ca2+ efflux following ionomycin addition was directly measured with extracellular fura-2 and followed a biphasic time course (t(1/2) approximately = 10 s and 90s). The proportion of total efflux owing to the rapid phase was increased by Na+ and reduced by EGTA-loading. Na+ accelerated the initial rate of Ca2+ efflux by 65% in unloaded cells but only by 16% in EGTA-loaded cells. In both cases, the stimulation by Na+ was less than expected, given the pronounced effects of Na+ on the [Ca2+]i transient. We conclude that the exchanger contributes importantly to Ca2+ efflux activity at all [Ca2+]i values above 40 nM. We also suggest that Ca2+ efflux pathways may involve non-cytosolic or local routes of Ca2+ traffic.

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.

Regulation of Na+/Ca2+ exchange activity by cytosolic Ca2+ in transfected Chinese hamster ovary cells

Transfected Chinese hamster ovary cells stably expressing the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells) were used to determine the range of cytosolic Ca2+ concentrations ([Ca2+]i) that activate Na+/Ca2+ exchange activity. Ba2+ influx was measured in fura 2-loaded, ionomycin-treated cells under conditions in which the intracellular Na+ concentration was clamped with gramicidin at approximately 20 mM. [Ca2+]i was varied by preincubating ionomycin-treated cells with either the acetoxymethyl ester of EGTA or medium containing 0-1 mM added CaCl2. The rate of Ba2+ influx increased in a saturable manner with [Ca2+]i, with the half-maximal activation value of 44 nM and a Hill coefficient of 1.6. When identical experiments were carried out with cells expressing a Ca2+-insensitive mutant of the exchanger, Ba2+ influx did not vary with [Ca2+]i. The concentration for activation of exchange activity was similar to that reported for whole cardiac myocytes but approximately an order of magnitude lower than that reported for excised, giant patches. The reason for the difference in Ca2+ regulation between whole cells and membrane patches is unknown.

Interaction of reactive oxygen species with ion transport mechanisms

The use of electrophysiological and molecular biology techniques has shed light on reactive oxygen species (ROS)-induced impairment of surface and internal membranes that control cellular signaling. These deleterious effects of ROS are due to their interaction with various ion transport proteins underlying the transmembrane signal transduction, namely, 1) ion channels, such as Ca2+ channels (including voltage-sensitive L-type Ca2+ currents, dihydropyridine receptor voltage sensors, ryanodine receptor Ca2+-release channels, and D-myo-inositol 1,4,5-trisphosphate receptor Ca2+-release channels), K+ channels (such as Ca2+-activated K+ channels, inward and outward K+ currents, and ATP-sensitive K+ channels), Na+ channels, and Cl- channels; 2) ion pumps, such as sarcoplasmic reticulum and sarcolemmal Ca2+ pumps, Na+-K+-ATPase (Na+ pump), and H+-ATPase (H+ pump); 3) ion exchangers such as the Na+/Ca2+ exchanger and Na+/H+ exchanger; and 4) ion cotransporters such as K+-Cl-, Na+-K+-Cl-, and Pi-Na+ cotransporters. The mechanism of ROS-induced modifications in ion transport pathways involves 1) oxidation of sulfhydryl groups located on the ion transport proteins, 2) peroxidation of membrane phospholipids, and 3) inhibition of membrane-bound regulatory enzymes and modification of the oxidative phosphorylation and ATP levels. Alterations in the ion transport mechanisms lead to changes in a second messenger system, primarily Ca2+ homeostasis, which further augment the abnormal electrical activity and distortion of signal transduction, causing cell dysfunction, which underlies pathological conditions.

Properties of a novel pH-dependent Ca2+ permeation pathway present in male germ cells with possible roles in spermatogenesis and mature sperm function

Rises of intracellular Ca2+ ([Ca2+]i) are key signals for cell division, differentiation, and maturation. Similarly, they are likely to be important for the unique processes of meiosis and spermatogenesis, carried out exclusively by male germ cells. In addition, elevations of [Ca2+]i and intracellular pH (pHi) in mature sperm trigger at least two events obligatory for fertilization: capacitation and acrosome reaction. Evidence implicates the activity of Ca2+ channels modulated by pHi in the origin of these Ca2+ elevations, but their nature remains unexplored, in part because work in individual spermatozoa are hampered by formidable experimental difficulties. Recently, late spermatogenic cells have emerged as a model system for studying aspects relevant for sperm physiology, such as plasmalemmal ion fluxes. Here we describe the first study on the influence of controlled intracellular alkalinization on [Ca2+]i on identified spermatogenic cells from mouse adult testes. In BCECF [(2',7')-bis(carboxymethyl)- (5, 6)-carboxyfluorescein]-AM-loaded spermatogenic cells, a brief (30-60 s) application of 25 mM NH4Cl increased pHi by approximately 1.3 U from a resting pHi approximately 6.65. A steady pHi plateau was maintained during NH4Cl application, with little or no rebound acidification. In fura-2-AM-loaded cells, alkalinization induced a biphasic response composed of an initial [Ca2+]i drop followed by a two- to threefold rise. Maneuvers that inhibit either Ca2+ influx or intracellular Ca2+ release demonstrated that the majority of the Ca2+ rise results from plasma membrane Ca2+ influx, although a small component likely to result from intracellular Ca2+ release was occasionally observed. Ca2+ transients potentiated with repeated NH4Cl applications, gradually obliterating the initial [Ca2+]i drop. The pH-sensitive Ca2+ permeation pathway allows the passage of other divalents (Sr2+, Ba2+, and Mn2+) and is blocked by inorganic Ca2+ channel blockers (Ni2+ and Cd2+), but not by the organic blocker nifedipine. The magnitude of these Ca2+ transients increased as maturation advanced, with the largest responses being recorded in testicular sperm. By extrapolation, these findings suggest that the pH-dependent Ca2+ influx pathway could play significant roles in mature sperm physiology. Its pharmacology and ion selectivity suggests that it corresponds to an ion channel different from the voltage-gated T-type Ca2+ channel also present in spermatogenic cells. We postulate that the Ca2+ permeation pathway regulated by pHi, if present in mature sperm, may be responsible for the dihydropyridine-insensitive Ca2+ influx required for initiating the acrosome reaction and perhaps other important sperm functions.

Na+/Ca2+ exchange and cellular Ca2+ homeostasis

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

Transport and regulation of the cardiac Na(+)-Ca2+ exchanger, NCX1. Comparison between Ca2+ and Ba2+

Cardiac muscle fails to relax upon replacement of extracellular Ca2+ with Ba2+. Among the manifold consequences of this intervention, one major possibility is that Na(+)-Ba2+ exchange is inadequate to support normal relaxation. This could occur due to reduced transport rates of Na(+)-Ba2+ exchange and/or by failure of Ba2+ to activate the exchanger molecule at the high affinity regulatory Ca2+ binding site. In this study, we examined transport and regulatory properties for Na(+)-Ca2+ and Na(+)-Ba2+ exchange. Inward and outward Na(+)-Ca2+ or Na(+)-Ba2+ exchange currents were examined at 30 degrees C in giant membrane patches excised from Xenopus oocytes expressing the cloned cardiac Na(+)-Ca2+ exchanger, NCX1. When excised patches were exposed to either cytoplasmic Ca2+ or Ba2+, robust inward Na(+)-Ca2+ exchange currents were observed, whereas Na(+)-Ba2+ currents were absent or barely detectable. Similarly, outward currents were greatly reduced when pipette solutions contained Ba2+ rather than Ca2+. However, when solution temperature was elevated from 30 degrees C to 37 degrees C, a substantial increase in outward Na(+)-Ba2+ exchange currents was observed, but not so for inward currents. We also compared the relative abilities of Ca2+ and Ba2+ to activate outward Na(+)-Ca2+ exchange currents at the high affinity regulatory Ca2+ binding site. While Ba2+ was capable of activating the exchanger, it did so with a much lower affinity (KD approximately 10 microM) compared with Ca2+ (KD approximately 0.3 microM). Moreover, the efficiency of Ba2+ regulation of Na(+)-Ca2+ exchange is also diminished relative to Ca2+, supporting approximately 60% of maximal currents obtainable with Ca2+. Ba2+ is also much less effective at alleviating Na+i-induced inactivation of NCX1. These results indicate that the reduced ability of NCX1 to adequately exchange Na+ and Ba2+ contributes to failure of the relaxation process in the cardiac muscle.

Acceleration of sodium-calcium exchange activity during ATP-induced calcium release in transfected Chinese hamster ovary cells

The P2U purinergic agonist ATP (0.3 mM) elicited an increase in [Ca2+]i due to Ca2+ release from intracellular stores in transfected Chinese hamster ovary cells that express the bovine cardiac Na+/Ca2+ exchanger (CK1.4 cells). The following observations indicate that ATP-evoked Ca2+ release was accompanied by a Ca(2+)-dependent regulatory activation of Na+/Ca2+ exchange activity: Addition of extracellular Ca2+ (0.7 mM) 0-1 min after ATP evoked a dramatic rise in [Ca2+]i in Na(+)-free media (Li+ substitution) compared to Na(+)-containing media; no differences between Na(+)- and Li(+)-based media were observed with vector-transfected cells. In the presence of physiological concentrations of extracellular Na+ and Ca2+, the ATP-evoked rise in [Ca2+]i declined more rapidly in CK1.4 cells compared to control cells, but then attained a long-lived plateau of elevated [Ca2+]i which eventually came to exceed the declining [Ca2+]i values in control cells. ATP elicited a transient acceleration of exchange-mediated Ba2+ influx, consistent with regulatory activation of the Na+/Ca2+ exchanger. The acceleration of Ba2+ influx was not observed in vector-transfected control cells, or in CK1.4 cells in the absence of intracellular Na+ or when the Ca2+ content of the intracellular stores had been reduced by prior treatment with ionomycin. The protein kinase C activator phorbol 12-myristate 13-acetate attenuated the exchange-mediated rise in [Ca2+]i under Na(+)-free conditions, but did not inhibit the ATP-evoked stimulation of Ba2+ influx. The effects of PMA are therefore not due to inhibition of exchange activity, but probably reflect the influence of protein kinase C on other Ca2+ homeostatic mechanisms. We conclude that exchange activity is accelerated during ATP-evoked Ca2+ release from intracellular stores through regulatory activation by increased [Ca2+]i. In the absence of extracellular Ca2+, the stimulation of exchange activity is short-lived and follows the time course of the [Ca2+]i transient; in the presence of extracellular Ca2+, we suggest that the exchanger remains activated for a longer period of time, thereby stabilizing and prolonging the plateau phase of store-dependent Ca2+ entry.