Store-refilling involves both L-type calcium channels and reverse-mode sodium–calcium exchange in airway smooth muscle

S. Hirota, L. J. Janssen
European Respiratory Journal 2007 30: 269-278; DOI: 10.1183/09031936.00008507

Abstract

The aim of the present study was to examine the relative contributions to store-refilling of the two following voltage-regulated calcium ion influx pathways: 1) L-type-Ca2+ channels; and 2) the reverse-mode of the sodium–calcium exchanger (NCX).

Successive acetylcholine-induced contractions, triggered in bovine tracheal smooth muscle strips, were measured to determine the effect of intervention on contractions as an indication of the extent of store-refilling.

Pre-treating the tissues with cromakalim significantly reduced the magnitude of successive contractions. Zero-Ca2+ bathing media abolished the contractions, an effect that was completely reversed upon reintroduction of Ca2+. Inhibition of L-type Ca2+ channels, with nifedipine, significantly reduced the magnitude of the contractions. Similarly, inhibition of the reverse-mode of the NCX, with KB-R7943, significantly reduced the magnitude of the contractions. However, neither nifedipine nor KB-R7943 alone reduced contractions to the same extent as observed under zero-Ca2+ conditions. Concurrent treatment with nifedipine and KB-R7943 almost abolished successive contractions. Furthermore, concurrent treatment with nifedipine and zero-Na+ bathing media displayed a significantly greater effect than nifedipine alone. Probing the expression of NCX1 isoforms by Western blotting revealed the presence of three bands of 160, 120 and 110 kDa. The 120- and 110-kDa bands were identified as variably spliced NCX isoforms, NCX1.1 and NCX1.3, respectively.

Taken together, these data suggest that influx of calcium ions through both L-type calcium channels and the reverse-mode of the sodium–calcium exchanger is necessary for complete store-refilling in airway smooth muscle.

Contraction of airway smooth muscle (ASM) is mediated in large part through agonist-induced mobilisation of Ca2+ from the sarcoplasmic reticulum (SR). Following Ca2+-mobilisation, mechanisms must exist to reduce intracellular Ca2+ levels, leading to relaxation. Cytosolic Ca2+ can be sequestered into the SR through the active pumping of the sarco/endoplasmic reticulum calcium-adenosine triphosphatase (ATPase) for subsequent mobilisation or extruded to the extracellular domain by the plasma membrane calcium-ATPase and the forward-mode of the sodium–calcium exchanger (NCX). The Ca2+ deficit resulting from the extrusion of Ca2+ from the cell must be reversed in order for cells to store adequate Ca2+ for subsequent mobilisation and contraction.

Indeed, agonist stimulation can activate Ca2+ influx pathways that contribute to the overall intracellular Ca2+ pool. Mobilisation of Ca2+ from the SR can activate depolarising membrane chlorine ion currents that trigger voltage-dependent Ca2+ influx 1–3. L-type Ca2+ channels have been viewed as the primary conduit for voltage-dependent Ca2+ influx, and there are several reports that these appear to play a role in Ca2+ store-refilling 4–6, although inhibition of this pathway did not completely mimic the effect of complete removal of external Ca2+. Ca2+ influx via the reverse-mode of the NCX is also driven by membrane depolarisation, in conjunction with increased intracellular Na+ levels mediated by the gating of store-operated nonselective cation channels (NSCCs) [7–11, reviewed in 12]. An additional contribution of reverse-mode NCX activity to partial refilling of the internal store in ASM following agonist stimulation has recently been documented 13. Finally, a membrane conductance that is activated upon store depletion, and which might play a role in store refilling, has also been described 14. The relative contributions of the two voltage-dependent Ca2+-influx pathways (the reverse-mode of the NCX and L-type Ca2+-channels) and the voltage-independent receptor/store-operated cation channel (ROC/SOC) current to store refilling in ASM are still the subject of much investigation and debate.

The aim of the present study was to address this question by comparing the effects of selective blockers of the reverse-mode of the NCX (KB-R7943) and/or L-type Ca2+-channels (nifedipine) with those of suppressing voltage-dependent mechanisms in general using a potassium channel opener (cromakalim) and suppressing Ca2+ influx in general by omitting Ca2+ from the bathing medium.

MATERIALS AND METHODS

Animals

All experimental procedures were approved by the McMaster University Animal Care Committee (McMaster University, Hamilton, ON, Canada) and conformed to the guidelines set out by the Canadian Council on Animal Care (Ottawa, ON, Canada). Tracheae were obtained from cows (136–454 kg) euthanised at the local abattoir and transported to the laboratory in ice-cold modified Krebs buffer (116 mM NaCl, 4.6 mM KCl, 1.2 mM MgSO4, 2.5 mM CaCl2, 1.3 mM NaH2PO4, 23 mM NaHCO3, 11 mM d-glucose, 0.01 mM indomethacin, 0.0001 mM propranolol and 0.1 mM Nω-nitro-l-arginine (l-NNA), saturated with 95% oxygen/5% carbon dioxide to maintain pH at 7.4). Upon receipt of a trachea, the epithelium was removed and tracheal strips (∼2–3 mm wide and ∼10 mm long) were excised and used immediately or stored at 4°C for up to 48 h.

Study design

In order to examine the extent of store-refilling in smooth muscle, successive agonist-induced contractions of isolated ASM strips were measured using a standard organ bath preparation. Interventions aimed at inhibiting Ca2+ influx pathways (using Ca2+-free Krebs buffer, composed as modified Krebs buffer but with the omission of CaCl2 and addition of 1 mM ethyleneglycol bis-(2-aminoethylether)-N,N,N’N’-tetra-acetic acid (EGTA)), namely the reverse-mode of the NCX (using KB-R7943 (0.1 M in dimethylsulphoxide diluted in Krebs buffer as appropriate) and Zero-Na+ Krebs buffer, composed as modified Krebs buffer but with N-methyl-d-glucamine in place of NaCl and KH2PO4 and KHCO3 instead of NaH2PO4 and NaHCO3, with the pH adjusted to 7.4 using 5 N HCl and saturated with 95% oxygen/5% carbon dioxide to maintain this pH) and L-type Ca2+ channels (using 0.1 M nifedipine in ethanol diluted in Krebs buffer as appropriate), were employed after obtaining a control acetylcholine (Ach) response (using 0.1 M ACh in distilled deionised water diluted in Krebs buffer as appropriate), and the magnitude of each subsequent contractile response was normalised to this control response in order to determine what effect inhibition of various Ca2+ influx pathways might have on store-refilling, reflected by a reduction in successive contractile responses.

Methods

Organ bath studies

Contractile studies were performed as described previously by Hirota and co-workers 13, 15. Briefly, ASM strips were mounted in 4-mL organ baths, using silk thread to anchor one end of the strip and attach the other to a Grass FT.03 force transducer (Grass Technologies, Longueuil, QC, Canada). Isometric tension was recorded on-line using the DigiMed System Integrator program (MicroMed, Louisville, KY, USA). Tissues were bathed in modified Krebs buffer saturated with 95% oxygen/5% carbon dioxide, and heated to 37oC. During the 1-h equilibration, tissues were repeatedly washed with modified Krebs buffer and tested for viability using 60 mM KCl. The KCl was then washed out, and tissues were allowed to recover before experiments were conducted. Successive ACh-induced contractions were triggered using the concentration of ACh previously found to be half-maximal (3×10−7 M) 13, 15. Each tissue strip underwent only one intervention.

Western blot assay for NCX isoform 1

Tissues were homogenised in ice-cold magnesium lysis buffer (50 mM tris(hydroxymethyl)aminomethane (Tris)–HCl (pH 7.5), 0.1 mM EDTA, 0.1 mM EGTA, 750 mM NaCl, 5% Igepal CA-630 (Sigma–Aldrich Canada, Oakville, ON, Canada), 50 mM MgCl2, 10% glycerol, 10 μg·mL−1 aprotinin, 10 μg·mL−1 leupeptin, 1 mM phenylmethylsulphonyl fluoride, 1 mM 4-(2-aminoethyl)-benzenesulphonyl fluoride and 2 mM sodium orthovanadate), total protein content determined (Bradford method) and protein concentration adjusted to 100 μg·mL−1 using 2× Laemmli sample/loading buffer (62.5 mM Tris–HCl (pH 6.8), 2% sodium dodecylsulphate, 10% glycerol, 50 mM dithiothreitol, 0.1% 2-mercaptoethanol and 0.01% bromophenol blue). Samples were boiled for 5 min in this buffer, subjected to electrophoresis and then transferred to nitrocellulose membrane (blocked with 5% bovine serum albumin and 5% skimmed milk in Tris-buffered saline). NCX isoform 1 (NCX1) was visualised using a polyclonal rabbit primary antibody directed against NCX1 (Chemicon, Billerica, MA, USA) and a mouse anti-immunoglobulin G secondary antibody preparation (Upstate Biotechnology, Waltham, MA, USA).

Analysis

The effect of interventions targeting Ca2+ influx pathways is expressed as the percentage change from within-tissue control ACh responses (3×10−7 M). All responses are reported as mean±sem. Comparisons were performed using an unpaired t-test (for single pairwise comparisons) or two-way ANOVA (for multiple comparisons of means) followed by the appropriate post hoc test (Newman–Keuls). A p-value of <0.05 was considered significant.

RESULTS

In order to probe the relative involvement of Ca2+ influx pathways in store-refilling in ASM, a standard organ bath preparation was employed. Using this technique, successive agonist-induced contractions were triggered using ACh (3×10−7 M). Reproducible contractions could be triggered at 15-min intervals (fig. 1⇓). Removal of Ca2+ from the bathing solution led to a rapid and complete but reversible reduction in the magnitude of the agonist-induced contractions (fig. 2⇓).

Fig. 1—
Fig. 1—

a) Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (n = 10). …….: Magnitude of first contraction. b) Mean change in peak magnitude of the ACh-induced contractions. Data are presented as mean±sem.

Fig. 2—
Fig. 2—

a) Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) before, during and after removal of extracellular calcium ions (n = 5). At the end of the zero-Ca2+ period (- - - -), the ASM was washed with standard Ca2+-containing buffer. …….: Magnitude of first contraction. b) Mean change in peak magnitude of these ACh-induced contractions (□) compared with control responses (░; n = 10). Data are presented as mean±sem. *: p<0.05; #: p<0.005.

In order to investigate the pathways through which Ca2+ enters the cell on its way to the SR, a series of pharmacological interventions were employed. First, successive contractions were evoked in the presence of cromakalim (30 μM (0.01 M in distilled deionised water diluted in Krebs buffer as appropriate)), a compound that increases the open probability of adenosine triphosphate (ATP)-dependent potassium channels (KATP) [6, reviewed in 16]. Activation of KATP interferes with membrane depolarisation, thereby inhibiting voltage-dependent downstream events (e.g. the reverse-mode of the NCX and L-type Ca2+ channels). Cromakalim treatment led to a significant reduction in the magnitude of successive ACh-induced contractions (figs 3a⇓ and c). This effect was abolished by concurrent treatment with glibenclamide

Fig. 3—
Fig. 3—

Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) before, during and after exposure to 30 μM cromakalim: a) alone (n = 12) and b) in combination with 10 μM glibenclamide (n = 5). After exposure (- - - -), the ASM was washed with standard Ca2+-containing buffer. …….: Magnitude of first contraction. c, d) Mean change in peak magnitude of these ACh-induced contractions (□: cromakalim; ▒: cromakalim plus glibenclamide) compared with control responses (░; n = 10). Data are presented as mean±sem. *: p<0.05.

Since hyperpolarisation of the membrane reduced the magnitude of successive contractions, an approach that targeted the two downstream Ca2+ influx pathways that can be activated by membrane depolarisation was employed to determine their relative contribution to store-refilling. In order to examine the contribution of the NCX, tissues were treated with KB-R7943 (10 or 20 μM), a selective inhibitor of the reverse-mode (i.e. Ca2+ influx mode) of the NCX 17–21. Both 10 and 20 μM KB-R7943 significantly reduced successive ACh-induced contractions by ∼50% (fig. 4⇓). Similarly, successive responses from tissues treated with nifedipine (1 μM), a selective inhibitor of L-type Ca2+ channels, were significantly reduced by ∼50% (fig. 5⇓). Thus neither KB-R7943 nor nifedipine alone reduced successive contractions to the same extent as observed when tissues were exposed to zero-Ca2+ conditions (fig. 6⇓). However, concurrent treatment with KB-R7943 (10 or 20 μM) and nifedipine (1 μM) effectively abolished successive ACh-induced contractions (fig. 7⇓) to the extent observed during zero-Ca2+ treatment (fig. 8⇓).

Fig. 4—
Fig. 4—

Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) in the presence of KB-R7943 (- - - -). a) 10 μM (n = 10) and b) 20 μM (n = 7). …….: Magnitude of first contraction. c, d) Mean change in peak magnitude of these ACh-induced contractions (□: 10 μM KB-R7943; ▒: 20 μM KB-R7943) compared with control responses (░; n = 7). After exposure, the ASM was washed with standard Ca2+-containing buffer. Data are presented as mean±sem. *: p<0.05; #: p<0.005.

Fig. 5—
Fig. 5—

a) Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) in the presence of 1 μM nifedipine (- - - -; n = 6). …….: Magnitude of first contraction. b) Mean changes in peak magnitude of these ACh-induced contractions (□) compared with control responses (░; n = 10). After exposure, the ASM was washed with standard Ca2+-containing buffer. Data are presented as mean±sem. *: p<0.05.

Fig. 6—
Fig. 6—

Mean changes in peak magnitudes of 3×10−7 M acetylcholine (ACh)-induced contractions in the presence (░) of: a) 10 μM KB-R7943 (n = 10); b) 20 μM KB-R7943 (n = 7); and c) 1 μM nifedipine (n = 6) compared with responses in the absence of extracellular Ca2+ (□; n = 5). After exposure (contractions 2–4), the ASM was washed with zero-Ca2+ buffer (contractions 5 and 6). Data are presented as mean±sem. *: p<0.05.

Fig. 7—
Fig. 7—

Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) during concurrent treatment with 1 μM nifedipine and: a) 10 μM KB-R7943 (n = 4); and b) 20 μM KB-R7943 (n = 8). After exposure (- - - -), the ASM was washed with standard Ca2+-containing buffer. …….: Magnitude of first contraction. c, d) Mean change in peak magnitude of these ACh-induced contractions (□: 1 μM nifedipine plus 10 μM KB-R7943; ▒: 1 μM nifedipine plus 20 μM KB-R7943) compared with control responses (░; n = 10). Data are presented as mean±sem. *: p<0.05; #: p<0.005.

Fig. 8—
Fig. 8—

Mean changes in peak magnitudes of 3×10−7 M acetylcholine (ACh)-induced contractions in the presence (░) of 1 μM nifedipine plus: a) 10 μM KB-R7943 (n = 4) and b) 20 μM KB-R7943 (n = 8) compared with responses in the absence of extracellular Ca2+ (□; n = 5). After exposure (contractions 2–4), the ASM was washed with zero-Ca2+ buffer (contractions 5 and 6). Data are presented as mean±sem. #: p<0.005.

In addition to pharmacological inhibition of the NCX, an ionic substitution protocol was employed to generate Na+-free Krebs buffer. It has previously been shown that long-term bathing in zero-Na+ buffer reduces refilling through inhibition of the reverse-mode of the NCX 13. Treating tissues with nifedipine (1 μM) in the absence of extracellular Na+ significantly reduced successive ACh-induced contractions (figs 9a⇓ and b). Furthermore, removal of extracellular Na+ during nifedipine treatment led to a significantly greater reduction in responses than observed when tissues were treated with nifedipine alone (fig. 9c⇓). However, this treatment did not reduce contractions to the extent observed under zero-Ca2+ conditions (fig. 9d⇓).

Fig. 9—
Fig. 9—

a) Representative 3×10−7 M acetylcholine (ACh; horizontal bars)-induced contractions of bovine airway smooth muscle (ASM) during concurrent treatment with 1 μM nifedipine and zero-Na+ buffer (- - - -; n = 5). ……..: Magnitude of first contraction. b–d) Mean changes in peak magnitude of these ACh-induced contractions in the presence of 1 μM nifedipine and zero-Na+ buffer (□) compared with b) control responses (░; n = 10); c) nifedipine treatment alone (▒; n = 6); and d) zero-Ca2+ conditions (▓; n = 5). After exposure (contractions 2–4), the ASM was washed with fresh buffer (contractions 5 and 6). Data are presented as mean±sem. *: p<0.05; #: p<0.005.

In addition to examining the functional importance of the NCX in store-refilling, NCX1 expression was examined in protein isolates from bovine tracheal smooth muscle. In tracheal strips from six different animals, Western blot analysis revealed the existence of three predominant proteins of 160, 120 and 110 kDa (fig. 10⇓).

Fig. 10—
Fig. 10—

Western blot detection of sodium–calcium exchanger (NCX) isoform 1, performed in protein extracts isolated from bovine tracheal smooth muscle cells, indicating expression of NCX1.1 (∼120 kDa) and NCX1.3 (∼110 kDa). Each lane represents a protein isolate from a different animal.

DISCUSSION

The aim of the present study was to examine the relative roles of two Ca2+ influx pathways, both regulated by membrane potential, in the process of refilling the SR. A role for the NCX in store-refilling has recently been described, whereby its reverse-mode provides substantial Ca2+ influx for the refilling of the SR following agonist-induced Ca2+ mobilisation 13. In this earlier report, inhibition of the reverse-mode of the NCX, through removal of extracellular Na+ or treatment with KB-R7943, reduced the magnitude of successive agonist-induced contractions. This was associated with a reduction in agonist-induced Ca2+ mobilisation, measured via Ca2+ fluorescence imaging, in freshly isolated ASM cells. In addition to the reverse-mode of the NCX, others have reported that voltage-dependent Ca2+ influx, via activation of L-type Ca2+ channels, plays a significant role in store-refilling 4–6. Interestingly, these conclusions have often been based on experimental protocols designed to manipulate membrane potential in order to alter Ca2+ influx via L-type Ca2+ channels. However, the reverse-mode of the NCX is also driven by membrane depolarisation, in conjunction with local increases in intracellular Na+ concentration that can be triggered through gating of NSCCs 7–11, reviewed in 12. The previous studies examining voltage-dependent Ca2+ influx have overlooked the role of the reverse-mode of the NCX, attributing influx entirely to L-type Ca2+ channels.

Influx pathways activated during agonist-induced contraction must exist to provide the cell with an intracellular Ca2+ pool from which the SR can be appropriately refilled. The importance of Ca2+ influx in SR refilling is especially evident when tissues are repetitively stimulated in Ca2+-free bathing solutions. Under these conditions, successive agonist-induced contractions are almost abolished, but return to control levels following the reintroduction of extracellular Ca2+ (fig. 2⇑).

Agonist stimulation of ASM triggers membrane depolarisation, primarily through the activation of Ca2+-dependent Cl- channels. These responses have been characterised in various airway preparations 1–3, 22, 23. Thus it is plausible that agonist-induced membrane depolarisation activates the Ca2+ influx pathways required for appropriate store-refilling. In order to probe the role of membrane depolarisation in store-refilling, tissues were treated with cromakalim (30 μM), a compound that selectively activates KATP 16. Cromakalim is known to cause relaxation of many smooth muscle types, including ASM, and this effect is universally attributed to its action on glibenclamide-sensitive KATP 24–27. By triggering K+ efflux via the KATP, cromakalim hyperpolarises the membrane potential towards the potassium equilibrium potential (i.e. ∼80 mV) 6. Cromakalim treatment significantly reduced the magnitude of successive agonist-induced contractions (figs 3a⇑ and c), an effect that was abolished in the presence of glibenclamide (10 μM); a selective blocker of the KATP (figs 3b⇑ and d) 16.

The importance of this finding is three-fold. First, the effect of cromakalim was almost abolished in the presence of glibenclamide, a selective inhibitor of the KATP, supporting the selectivity of the intervention. Secondly, inhibition of membrane depolarisation through pre-treatment with cromakalim caused a graded reduction in the magnitude of successive contractions, suggesting a gradual reduction in SR refilling throughout the protocol, in response to inhibition of voltage-dependent Ca2+ influx pathways. If voltage-dependent Ca2+ influx were directly involved in excitation–contraction coupling, cromakalim’s effect would have been immediate. Instead the gradual and progressive reduction suggests that agonist-induced Ca2+ mobilisation is the primary mechanism driving Ca2+-dependent contraction, and that the SR is being refilled, in part, through voltage-dependent Ca2+-influx. Finally, these data call into question any major role of ROCs/SOCs in the store-refilling process in ASM. When examining the electrophysiological properties of NSCCs, the current–voltage relationship is such that the inward current is greatest under negative membrane potentials 14. Thus Ca2+ influx through ROCs/SOCs should be increased by membrane hyperpolarisation, or clamping in negative potentials, during cromakalim treatment. As such, any contribution of these channels to store-refilling should have been amplified by treatment with cromakalim. In contrast, the data suggest that store-refilling is dominated by voltage-dependent mechanisms, and that voltage-independent mechanisms, such as ROC/SOC currents, play only a minor role, if any.

Since it is apparent that voltage-dependent Ca2+ influx pathways play a role in store-refilling, successive agonist-induced contractions were examined in the presence of agents targeting the reverse-mode of the NCX and L-type Ca2+ channels, both voltage-regulated systems. KB-R7943 is a selective inhibitor of the NCX, and blocks the reverse-mode almost 60 times more strongly than the forward-mode 17–21. When examining the pharmacology of KB-R7943, Bradley et al. 18 reported that it had no effect on the release of Ca2+ from the SR, and nor did it alter the inward current mediated by Ca2+-dependent Cl- channels. Furthermore, Kawano et al. 28 reported that high concentrations of KB-R7943 (30–100 μM) reduced spontaneous Ca2+ oscillations in human mesenchymal stem cells, but had no effect on the lanthanum-sensitive component of these oscillations mediated by NSCCs. In intact tissue preparations, Dai et al. 7 reported that KB-R7943, at the concentration used in the present study, abolished agonist-induced asynchronous Ca2+ oscillations, an effect attributed to inhibition of Ca2+ influx via the reverse-mode of the NCX. More recently, Dai et al. 29 used KB-R7943 to identify a role for the reverse-mode of the NCX in the agonist-induced asynchronous Ca2+ oscillations observed in intact human ASM preparations.

In the present study, both KB-R7943 and nifedipine, a prototypical L-type Ca2+ channel blocker, significantly reduced the magnitude of successive contractions (figs 4⇑ and 5⇑). However, neither KB-R7943 nor nifedipine alone reduced contractions to the extent observed in the absence of extracellular Ca2+ (fig. 6⇑). Thus it seems that Ca2+ influx through both the NCX and L-type Ca2+ channels is necessary for complete refilling of the intracellular Ca2+ pool; concurrent treatment with both KB-R7943 and nifedipine displayed an additive effect in terms of reducing the magnitude of successive agonist-induced contractions (fig. 7⇑). Indeed, simultaneous inhibition of the reverse-mode of the NCX and L-type Ca2+ channels practically abolished successive agonist-induced contractions, similar to the effect observed when tissues were bathed in Ca2+-free buffering solution (fig. 8⇑).

In addition to pharmacological interventions, the removal of extracellular Na+ from buffering solutions can gradually inhibit the NCX. This approach was utilised to probe the role of the NCX in store-refilling following agonist-induced Ca2+ mobilisation 13. In the present study, removal of extracellular Na+ in the presence of nifedipine (to inhibit L-type Ca2+ channels) led to a significant reduction in the magnitude of successive responses, similar to that observed during treatment with KB-R7943 (figs 9a⇑ and b). However, the reduction in magnitude observed during concurrent zero Na+ and nifedipine treatment was not as great as that observed under zero Ca2+ conditions (fig. 9d⇑). This may reflect the nature of this intervention; its effect is reliant upon the gradual depletion of intracellular Na+ throughout the series of successive responses. Inhibition of the NCX occurs when the intracellular Na+ concentration is depleted, which is a gradual rather than immediate event. In contrast, pharmacological inhibition of the NCX is a more immediate event since it is dependent upon the direct interaction of KB-R7943 with the exchanger 19. Interestingly, Na+ depletion, in the presence of nifedipine, decreased responses beyond those observed during nifedipine treatment alone (fig. 9c⇑). This observation highlights the existence of a Na+-dependent Ca2+ influx pathway (i.e. the reverse-mode of the NCX), which plays a role in store-refilling following agonist-induced Ca2+ mobilisation.

In addition to the functional identification of a Ca2+-influx pathway mediated through the reverse-mode activity of the NCX, protein expression was probed in bovine tracheal smooth muscle samples. Three major isoforms of the NCX exist: NCX1, NCX2 and NCX3 12, 30. NCX2 and NCX3 expression are mainly limited to the brain and skeletal muscle 12, 31–33, whereas NCX1 displays a more profuse expression profile 12, 34. In addition to this major classification, the NCX1 isoform can be expressed as a variety of splice variants which determine variable properties in terms of ionic sensitivity and regulation by intracellular processes 35–39. When probing the present preparation for NCX1, three major bands corresponding to molecular masses of 160, 120 and 110 kDa were observed. The 160-kDa band has been reported in cases in which the reducing agents have insufficiently altered the protein structure prior to separation 39, 40. The 120-kDa band corresponds to the unspliced NCX1.1 isoform reported in a number of different systems 12, 30, 36, 39, 40. Alternative splicing of an intracellular loop of NCX1 can lead to a number of smaller isoforms 12, 30. Indeed a band of ∼110 kDa, which corresponds to the theoretical and reported size of the NCX1.3 isoform, was identified 12, 36, 41. Interestingly, the NCX1.3 isoform has been associated with the aberrant Ca2+ influx observed in salt-dependent hypertension 37, 38, 42.

Taken together, the present data suggest that previous reports examining the role of voltage-dependent calcium ion influx in store-refilling may have overlooked the contribution of the reverse-mode of the sodium–calcium exchanger. It is apparent that inhibition of L-type calcium channels alone does not completely abolish successive agonist-induced contractions of airway smooth muscle. Indeed, this approach has failed in the clinical setting, whereby dihydropyridine compounds were proven ineffective in the management of obstructive airway diseases, such as asthma 43–45. However, treatment aimed at inhibiting both L-type calcium channels and the reverse-mode of the sodium–calcium exchanger in airway smooth muscle may prove effective at reversing the increased contractile tone associated with airway obstruction. Interestingly, inhibition of the sodium–calcium exchanger is currently touted as a potential clinical strategy for treating hypertensive conditions associated with increased calcium ion influx through the reverse-mode of the sodium–calcium exchanger 37, 38, 42, 46, 47. It is apparent that further in vivo study is required in order to determine whether or not the sodium–calcium exchanger plays a role in obstructive airway disease. Furthermore, animal models of airway hyperresponsiveness may prove the ideal setting in which to determine what effect concurrent inhibition of the reverse-mode of the sodium–calcium exchanger and L-type calcium channels might have on the airway hyperresponsiveness and reduced airway calibre associated with asthma.

  • Received January 22, 2007.
  • Accepted March 29, 2007.

References

  1. ↵
    Janssen LJ, Sims SM. Emptying and refilling of Ca2+ store in tracheal myocytes as indicated by ACh-evoked currents and contraction. Am J Physiol 1993;265:C877–C886.
    OpenUrlPubMedWeb of Science
  2. Janssen LJ, Sims SM. Spontaneous transient inward currents and rhythmicity in canine and guinea-pig tracheal smooth muscle cells. Pflugers Arch 1994;427:473–480.
    OpenUrlCrossRefPubMedWeb of Science
  3. ↵
    Janssen LJ, Sims SM. Ca2+-dependent Cl- current in canine tracheal smooth muscle cells. Am J Physiol 1995;269:C163–C169.
    OpenUrlPubMedWeb of Science
  4. ↵
    Bourreau JP, Kwan CY, Daniel EE. Excitation–contraction coupling in canine tracheal smooth muscle cells. Adv Exp Med Biol 1992;311:375–377.
    OpenUrlPubMed
  5. Bourreau J, Abela A, Kwan C, Daniel E. Acetylcholine Ca2+ stores refilling directly involves a dihydropyrodine-sensitive channel in dog trachea. Am J Physiol 1991;261:C496–C505.
    OpenUrl
  6. ↵
    Bourreau J, Kwan C, Daniel E. Distinct pathways to refill ACh-sensitive internal Ca2+ stores in canine airway smooth muscle. Am J Physiol 1993;265:C28–C35.
    OpenUrlPubMedWeb of Science
  7. ↵
    Dai JM, Kuo KH, Leo JM, van Breemen C, Lee CH. Mechanism of ACh-induced asynchronous calcium waves and tonic contraction in porcine tracheal muscle bundle. Am J Physiol Lung Cell Mol Physiol 2006;290:L459–L469.
    OpenUrlAbstract/FREE Full Text
  8. Lemos VS, Poburko D, Liao CH, Cole WC, van Breemen C. Na+ entry via TRPC6 causes Ca2+ entry via NCX reversal in ATP stimulated smooth muscle cells. Biochem Biophys Res Commun 2007;352:130–134.
    OpenUrlCrossRefPubMedWeb of Science
  9. Poburko D, Kuo KH, Dai J, Lee CH, van Breemen C. Organellar junctions promote targeted Ca2+ signaling in smooth muscle: why two membranes are better than one. Trends Pharmacol Sci 2004;25:8–15.
    OpenUrlCrossRefPubMed
  10. Poburko D, Potter K, van Breemen E, et al. Mitochondria buffer NCX-mediated Ca2+-entry and limit its diffusion into vascular smooth muscle cells. Cell Calcium 2006;40:359–371.
    OpenUrlCrossRefPubMedWeb of Science
  11. ↵
    Wu C, Fry CH. Na+/Ca2+ exchange and its role in intracellular Ca2+ regulation in guinea pig detrusor smooth muscle. Am J Physiol Cell Physiol 2001;280:C1090–C1096.
    OpenUrlAbstract/FREE Full Text
  12. ↵
    Blaustein MP, Lederer WJ. Sodium/calcium exchange: its physiological implications. Physiol Rev 1999;79:763–854.
    OpenUrlAbstract/FREE Full Text
  13. ↵
    Hirota S, Pertens E, Janssen LJ. The reverse mode of the Na+/Ca2+ exchanger provides a source of Ca2+ for store refilling following agonist-induced Ca2+ mobilization. Am J Physiol Lung Cell Mol Physiol 2007;292:L438–L447.
    OpenUrlAbstract/FREE Full Text
  14. ↵
    Helli PB, Pertens E, Janssen LJ. Cyclopiazonic acid activates a Ca2+-permeable, nonselective cation conductance in porcine and bovine tracheal smooth muscle. J Appl Physiol 2005;99:1759–1768.
    OpenUrlAbstract/FREE Full Text
  15. ↵
    Hirota S, Trimble N, Pertens E, Janssen LJ. Intracellular Cl- fluxes play a novel role in Ca2+ handling in airway smooth muscle. Am J Physiol Lung Cell Mol Physiol 2006;290:L1146–L1153.
    OpenUrlAbstract/FREE Full Text
  16. ↵
    Quayle JM, Nelson MT, Standen NB. ATP-sensitive and inwardly rectifying potassium channels in smooth muscle. Physiol Rev 1997;77:1165–1232.
    OpenUrlAbstract/FREE Full Text
  17. ↵
    Birinyi P, Acsai K, Banyasz T, et al. Effects of SEA0400 and KB-R7943 on Na+/Ca2+ exchange current and L-type Ca2+ current in canine ventricular cardiomyocytes. Naunyn Schmiedebergs Arch Pharmacol 2005;372:63–70.
    OpenUrlCrossRefPubMedWeb of Science
  18. ↵
    Bradley E, Hollywood MA, Johnston L, et al. Contribution of reverse Na+/Ca2+ exchange to spontaneous activity in interstitial cells of Cajal in the rabbit urethra. J Physiol 2006;574:651–661.
    OpenUrlCrossRefPubMedWeb of Science
  19. ↵
    Iwamoto T, Kita S. Development and application of Na+/Ca2+ exchange inhibitors. Mol Cell Biochem 2004;259:157–161.
    OpenUrlCrossRefPubMedWeb of Science
  20. Takai N, Yamada A, Muraki K, Watanabe M, Imaizumi Y. KB-R7943 reveals possible involvement of Na+–Ca2+ exchanger in elevation of intracellular Ca2+ in rat carotid arterial myocytes. J Smooth Muscle Res 2004;40:35–42.
    OpenUrlCrossRefPubMed
  21. ↵
    Watano T, Kimura J, Morita T, Nakanishi H. A novel antagonist, No. 7943, of the Na+/Ca2+ exchange current in guinea-pig cardiac ventricular cells. Br J Pharmacol 1996;119:555–563.
    OpenUrlCrossRefPubMedWeb of Science
  22. ↵
    Janssen L. Acetylcholine and caffeine activate Cl− and suppress K+ conductances in human bronchial smooth muscle. Am J Physiol 1996;270:L772–L781.
    OpenUrlPubMedWeb of Science
  23. ↵
    Kotlikoff MI, Wang YX. Calcium release and calcium-activated chloride channels in airway smooth muscle cells. Am J Respir Crit Care Med 1998;158:S109–S114.
    OpenUrlCrossRefPubMedWeb of Science
  24. ↵
    Buckle DR, Arch JR, Bowring NE, et al. Relaxant effects of the potassium channel activators BRL 38227 and pinacidil on guinea-pig and human airway smooth muscle, and blockade of their effects by glibenclamide and BRL 31660. Pulm Pharmacol 1993;6:77–86.
    OpenUrlCrossRefPubMedWeb of Science
  25. Cortijo J, Sarria B, Pedros C, Perpina M, Paris F, Morcillo E. The relaxant effects of cromakalim (BRL 34915) on human isolated airway smooth muscle. Naunyn Schmiedebergs Arch Pharmacol 1992;346:462–468.
    OpenUrlPubMedWeb of Science
  26. Nielsen-Kudsk JE. Potassium channel modulation: a new drug principle for regulation of smooth muscle contractility. Studies on isolated airways and arteries. Dan Med Bull 1996;43:429–447.
    OpenUrlPubMedWeb of Science
  27. ↵
    Raeburn D, Brown TJ. RP 49356 and cromakalim relax airway smooth muscle in vitro by opening a sulphonylurea-sensitive K+ channel: a comparison with nifedipine. J Pharmacol Exp Ther 1991;256:480–485.
    OpenUrlAbstract/FREE Full Text
  28. ↵
    Kawano S, Otsu K, Shoji S, Yamagata K, Hiraoka M. Ca2+ oscillations regulated by Na+–Ca2+ exchanger and plasma membrane Ca2+ pump induce fluctuations of membrane currents and potentials in human mesenchymal stem cells. Cell Calcium 2003;34:145–156.
    OpenUrlCrossRefPubMedWeb of Science
  29. ↵
    Dai JM, Kuo KH, Leo JM, Pare PD, van Breemen C, Lee CH. Acetylcholine-induced asynchronous calcium waves in intact human bronchial muscle bundle. Am J Respir Cell Mol Biol 2007;36:600–608.
    OpenUrlCrossRefPubMedWeb of Science
  30. ↵
    Philipson KD, Nicoll DA. Sodium–calcium exchange: a molecular perspective. Annu Rev Physiol 2000;62:111–133.
    OpenUrlCrossRefPubMedWeb of Science
  31. ↵
    Li Z, Matsuoka S, Hryshko LV, et al. Cloning of the NCX2 isoform of the plasma membrane Na+–Ca2+ exchanger. J Biol Chem 1994;269:17434–17439.
    OpenUrlAbstract/FREE Full Text
  32. Linck B, Qiu Z, He Z, Tong Q, Hilgemann DW, Philipson KD. Functional comparison of the three isoforms of the Na+/Ca2+ exchanger (NCX1, NCX2, NCX3). Am J Physiol 1998;274:C415–C423.
    OpenUrlPubMedWeb of Science
  33. ↵
    Nicoll DA, Quednau BD, Qui Z, Xia YR, Lusis AJ, Philipson KD. Cloning of a third mammalian Na+–Ca2+ exchanger, NCX3. J Biol Chem 1996;271:24914–24921.
    OpenUrlAbstract/FREE Full Text
  34. ↵
    Quednau BD, Nicoll DA, Philipson KD. Tissue specificity and alternative splicing of the Na+/Ca2+ exchanger isoforms NCX1, NCX2, and NCX3 in rat. Am J Physiol 1997;272:C1250–C1261.
    OpenUrlPubMedWeb of Science
  35. ↵
    Ander BP, Hurtado C, Raposo CS, et al. Differential sensitivities of the NCX1.1 and NCX1.3 isoforms of the Na+–Ca2+ exchanger to α-linolenic acid. Cardiovasc Res 2007;73:395–403.
    OpenUrlAbstract/FREE Full Text
  36. ↵
    Hurtado C, Prociuk M, Maddaford TG, et al. Cells expressing unique Na+/Ca2+ exchange (NCX1) splice variants exhibit different susceptibilities to Ca2+ overload. Am J Physiol Heart Circ Physiol 2006;290:H2155–H2162.
    OpenUrlAbstract/FREE Full Text
  37. ↵
    Iwamoto T, Kita S, Katsuragi T. Salt-sensitive hypertension, Na+/Ca2+ exchanger, and vascular smooth muscle. Trends Cardiovasc Med 2005;15:273–277.
    OpenUrlCrossRefPubMedWeb of Science
  38. ↵
    Iwamoto T, Kita S, Zhang J, et al. Salt-sensitive hypertension is triggered by Ca2+ entry via Na+/Ca2+ exchanger type-1 in vascular smooth muscle. Nat Med 2004;10:1193–1199.
    OpenUrlCrossRefPubMedWeb of Science
  39. ↵
    Van Eylen F, Kamagate A, Herchuelz A. Characterization and functional activity of a truncated Na/Ca exchange isoform resulting from a new splicing pattern of NCX1. Ann N Y Acad Sci 2002;976:81–84.
    OpenUrlPubMedWeb of Science
  40. ↵
    Philipson KD, Longoni S, Ward R. Purification of the cardiac Na+–Ca2+ exchange protein. Biochim Biophys Acta 1988;945:298–306.
    OpenUrlPubMed
  41. ↵
    Nicoll DA, Longoni S, Philipson KD. Molecular cloning and functional expression of the cardiac sarcolemmal Na+–Ca2+ exchanger. Science 1990;250:562–565.
    OpenUrlAbstract/FREE Full Text
  42. ↵
    Iwamoto T. Vascular Na+/Ca2+ exchanger: implications for the pathogenesis and therapy of salt-dependent hypertension. Am J Physiol Regul Integr Comp Physiol 2006;290:R536–R545.
    OpenUrlAbstract/FREE Full Text
  43. ↵
    Fish J. Calcium channel antagonists in the treatment of asthma. J Asthma 1984;21:407–418.
    OpenUrlCrossRefPubMedWeb of Science
  44. Gordon EH, Wong SC, Klaustermeyer WB. Comparison of nifedipine with a new calcium channel blocker, flordipine, in exercise-induced asthma. J Asthma 1987;24:261–265.
    OpenUrlCrossRefPubMedWeb of Science
  45. ↵
    Hoppe M, Harman E, Hendeles L. The effect of inhaled gallopamil, a potent calcium channel blocker, on the late-phase response in subjects with allergic asthma. J Allergy Clin Immunol 1992;89:688–695.
    OpenUrlCrossRefPubMedWeb of Science
  46. ↵
    Yu Y, Fantozzi I, Remillard CV, et al. Enhanced expression of transient receptor potential channels in idiopathic pulmonary arterial hypertension. Proc Natl Acad Sci USA 2004;101:13861–13866.
    OpenUrlAbstract/FREE Full Text
  47. ↵
    Zhang S, Dong H, Rubin LJ, Yuan JX. Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery SMC from patients with IPAH. Am J Physiol Cell Physiol 2006; [Epub ahead of print PMID: 17192285]
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Store-refilling involves both L-type calcium channels and reverse-mode sodium–calcium exchange in airway smooth muscle
S. Hirota, L. J. Janssen
European Respiratory Journal Aug 2007, 30 (2) 269-278; DOI: 10.1183/09031936.00008507
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