Published online before print March 8, 2007, doi: 10.1161/01.RES.0000262659.09293.56
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© 2007 American Heart Association, Inc.
Cellular Biology |
Interaction Between Na+/K+-Pump and Na+/Ca2+-Exchanger Modulates Intercellular Communication
From the the Water and Salt Research Center, Institute of Physiology and Biophysics (V.V.M., A.R., D.M.B.B., S.G., A.K.H., E.V.B., H.A.P., C.A., H.N.), University of Aarhus, Denmark; Department of Physiology (H.G.), University of Göteborg, Sweden; and the Laboratoire de Biologie Moléculaire et Cellulaire du Développement (S.G.), Université Pierre et Marie Curie, Paris, France.
Correspondence to V.V. Matchkov, Institute of Physiology and Biophysics, University of Aarhus, Ole Worms Alle 160, 8000 Aarhus C, Denmark. E-mail vvm{at}fi.au.dk
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Abstract |
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Ouabain, a specific inhibitor of the Na+/K+-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger leading to an increase in the intracellular calcium concentration ([Ca2+]i) in discrete areas near the plasma membrane.
[Ca2+]i in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance.
Smooth muscle cells were uncoupled when the Na+/K+-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca2+]i near the membrane, or by ATP depletion. Reduction of Na+/K+-pump activity by removal of extracellular potassium ([K+]o) also uncoupled cells, but only after inhibition of KATP channels. Inhibition of the Na+/Ca2+-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na+ and clamping of [Ca2+]i at low concentrations prevented the uncoupling.
The experiments suggest that the Na+/K+-pump may affect gap junction conductivity via localized changes in [Ca2+]i through modulation of Na+/Ca2+-exchanger activity.
Key Words: Na+/K+-pump • Na+/Ca2+-exchanger • intercellular communication • [Ca2+]i signaling • ATP-sensitive K+ channels
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Introduction |
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The Na+/K+-pump is a universally expressed protein indispensable for cell function (for review see1,2). It is an oligomer composed of several subunits, where the
-subunit is the ion-transporting subunit and responsible for ATP hydrolysis as well as for the binding of the specific inhibitor ouabain. The various isoforms of the -subunit show different affinities for ouabain. Rodents express an ouabain-resistant 1-subunit and ouabain-sensitive 2- and 3-isoforms.3 The 1-subunit is abundant in rat vascular smooth muscle cells (SMCs), whereas 2- and 3-isoforms can be detected in small amounts only.4,5 The Na+/K+-pump is known to affect cellular functions through changes in [K+]i and [Na+]i, which determine the driving force for a number of membrane channels and transporters. For example, a functional interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger is well-established.6 This interaction is important for heart function7,8 and has also been documented in SMCs.9–12 The Na+/Ca2+-exchanger and the ouabain-sensitive Na+/K+-pump are closely associated in microdomains of the plasma membrane.5,10,12 This association suggests that low concentrations of ouabain could increase [Na+]i in restricted areas and lead to less net Ca2+-extrusion through the Na+/Ca2+-exchanger, and ultimately elevate local [Ca2+]i.10,12,13
High concentrations of ouabain have been shown previously to block dye coupling (mediated via gap junctions) between cultured SMCs within one hour.14 Furthermore, we have previously shown that ouabain, even at low concentrations, inhibits rhythmic contractions (vasomotion) within seconds.15,16 Here we show that a low concentration of ouabain inhibits vasomotion as a result of uncoupling of SMCs in the vascular wall. We tested the hypothesis that this effect could be caused by an interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger, affecting gap junctional conductivity via modulation of local [Ca2+]i.
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Materials and Methods |
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An expanded Materials and Methods section containing details for electrophysiology, Ca2+ imaging, Western blotting, and analysis can be found in the online data supplement available at http://circres. ahajournals.org.
Isometric Force and [Ca2+]i Imaging of Rat Mesenteric Small Arteries
All procedures complied with Danish animal welfare regulations. Animal facilities were approved by the Danish Inspectorate for Experimental Animals and the Animal Welfare Officer of the Medical Faculty of the University of Aarhus. A third-order branch of the superior mesenteric artery from male Wistar rats was dissected and mounted in a isometric myograph (Danish Myo Technology, Denmark) for simultaneous force recording and intracellular Ca2+ imaging as described previously.17,18 An inverted confocal laser scanning microscope (Odyssey XL, Noran Instruments Inc, USA and LSM 5 Pascal Exciter, Carl Zeiss GmbH, Germany) was used for [Ca2+]i imaging. Arteries were loaded with Calcium Green-1/AM and changes of [Ca2+]i within the cells were measured as the mean intensity of dye in regions of interest (ROIs) defined either as a whole cell or as local areas near plasma membrane or in the center of SMC.
Electrophysiology
Groups of a few rat aortic SMCs (A7r5), paired or single cells were used for patch-clamp studies. Electrical coupling between cells was evaluated by recording the membrane capacitance as described previously.19 Capacitance measurements were based on the time constant of current decay following a voltage step. All recordings were made at room temperature (22 to 24°C) with an Axopatch 200B amplifier (Molecular Devices C., USA) in conventional whole-cell configuration. Drugs and solutions were applied to the bath locally over the patched cell or by perfusion.
[Ca2+]i Imaging of A7r5 Cells
Changes of [Ca2+]i in A7r5 cells were measured ratiometrically using an inverted confocal laser scanning microscope (LSM 5 Pascal Exciter, Carl Zeiss GmbH, Germany). A7r5 cells attached to a cover slip were simultaneously loaded with Calcium Green-1/AM and Fura Red/AM. Elevated [Ca2+]i results in increased fluorescence intensity of Calcium Green-1 and decreased fluorescence intensity of Fura Red. Local changes of [Ca2+]i were analyzed as the change in the fluorescence ratio along a line either at the cell perimeter or in the center of the cell. Fluorescence from calcein/AM loaded cells was used as a further control for movement artifacts.
Chemicals and Calculations
Calcium Green-1/AM, Fura-red/AM, calcein/AM, cremophor and pluronic F-127 were obtained from Molecular Probes (The Netherlands). SEA0400 was obtained from Calbiochem (EMD Biosciences Inc, USA). All other chemicals were obtained from Sigma-Aldrich (Denmark).
All data are shown as mean ± SEM. Differences between means were tested either with Paired Student’s t test or with one-way ANOVA followed by Bonferroni’s post-test, with P<0.05 being considered significant.
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Results |
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Submaximal norepinephrine (NE) concentrations evoked synchronized [Ca2+]i transients in SMCs, which led to vasomotion in mesenteric small arteries (Figure 1A). Ouabain (1 to 10 µmol/L) abolished vasomotion and desynchronized the [Ca2+]i transients in SMCs (Figure 1A, n=5) within 1 minute after application. The effect of micromolar concentrations of ouabain was reversible. The desynchronizing effect of ouabain was accompanied with a significant increase in mean global [Ca2+]i (Figure 1B) by 12.3±2.6% (n=5) and a slight increase in vascular tone, which did not obtain statistical significance (7.6±0.9 mN to 8.7±0.4 mN, without and with ouabain, respectively, n.s., n=5). Importantly [Ca2+]i in the center of the SMCs did not change significantly (increase by 8.1±4.1%, n.s., n=5) but a significant increase was observed near the plasma membrane (22.5±5.2%, P<0.05, n=5, Figure 1B).
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The frequency of near-membrane [Ca2+]i transients in SMCs in the nonstimulated arteries was also affected by low concentrations of ouabain. Ouabain (1 µmol/L) had no significant effect on the frequency (0.004±0.003 Hz (n=4) compared with 0.005±0.003 Hz (n=3)), whereas 10 µmol/L ouabain significantly increased the frequency to 0.02±0.007 Hz (P<0.05, n=4). Importantly, 1 mmol/L ouabain did not cause a further increase in frequency of the local [Ca2+]i events (0.02±0.008 Hz, n=4). This suggests that micromolar concentrations of ouabain are sufficient to increase [Ca2+]i near the membrane in SMCs.
It is possible that the increase in [Ca2+]i near the membrane induced by ouabain might be the result of altered Na+/Ca2+-exchanger activity. To test this we reduced the driving force for Ca2+ extrusion through the Na+/Ca2+-exchanger by reduction of [Na+]o. Reduction of [Na+]o below 100 mmol/L reversibly abolished vasomotion and desynchronized the [Ca2+]i transients in SMCs (Figure 1C). Although the reduction of [Na+]o to 50 mmol/L did not change global [Ca2+]i, [Ca2+]i near the membrane increased significantly (by 17.0±6.0%, n=9, P<0.05) (Figure 1D).
To study the suggested mechanism for uncoupling in more detail we used pairs of A7r5 cells, which are electrically coupled within 5 hours after resuspension. We evaluated the electrical communication in A7r5 pairs by measurements of membrane capacitance. The membrane capacitance was proportional to the number of coupled cells (Figure 2A). As expected the membrane capacitance of cell pairs was twice that of solitary cells (112.4±6.9 pF, n=10 versus 55.5±3.5 pF, n=11; P<0.001). Addition of 10 µmol/L ouabain did not change the membrane capacitance of solitary cells, but reduced the membrane capacitance of cell pairs to that of solitary cells (Figure 2A) in a concentration-dependent manner (Figure 2B). It is difficult to ascribe a quantitative significance to the concentration-dependence because of the nonlinear relation between intercellular resistance and measured capacitance.20 Nevertheless, all electrically coupled A7r5 cell pairs were uncoupled with 10 µmol/L ouabain and further increase of the ouabain concentration had no additional effect (Figure 2C). The ouabain sensitivity is consistent with the presence of an ouabain-sensitive isoform of the Na+/K+-pump as suggested by RT-PCR and Western blot analyses (supplemental Figure I in the online data supplement available at http://circres.ahajournals.org).
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Ouabain only uncoupled cells when applied extracellularly; 100 µmol/L ouabain dialyzed intracellularly did not affect the cell coupling and those cells still were uncoupled by extracellular ouabain application (supplemental Figure II available at http://circres.ahajournals.org). Additionally, 100 µmol/L ouabain did not affect the membrane conductance of A7r5 cells (data not shown; n=4).
To test the hypothesis that micromolar ouabain uncouples cells through an interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger we systematically examined experimental conditions that would affect this interaction. These interventions are shown in Figure 3. Omission of intracellular ATP should inhibit the Na+/K+-pump and uncouple the cells (Figure 3, intervention 2). Consistent with this, dialysis of the A7r5 cells with a solution where ATP was substituted by the nonhydrolysable ATP-analog AMP-PNP caused slow uncoupling of cells over 15 to 25 minutes (Figure 4A). After this treatment ouabain did not affect cell capacitance (Table).
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Superfusion with K+-free solution should also inhibit the Na+/K+-pump activity and thereby uncouple cell pairs (Figure 3, intervention 3). Surprisingly, K+-free solution did not uncouple cells and did not prevent the ouabain effect (Table 1; Figure 4B). The sensitivity to ouabain was the same under control conditions (logEC50=–6.3±0.2, n=4) and in K+-free solution (logEC50=–6.2±0.2, n=8). We therefore investigated whether a leak of [K+]i through neighboring K+-channels could provide the Na+/K+-pump with K+. We dialyzed cells with a Cs+-containing solution, and under these conditions K+-free solution did uncouple the cells (Table). To examine if KATP channels are involved we used glibenclamide to block these channels (Figure 3, intervention 5). In K+-free solution glibenclamide uncoupled cell pairs (Figure 4B), suggesting that the KATP channel supplies K+ for the Na+/K+-pump. Glibenclamide by itself did not uncouple cell pairs; the capacitance of cell pairs was 78.4±4.5 pF under control conditions and 77.8±5.2 pF (n=4) after addition of 10 µmol/L glibenclamide. Subsequent superfusion with K+-free solution significantly reduced the membrane capacitance to 39.7±5.6 pF (P<0.01, n=4). We tested whether this effect was specific for KATP channels by superfusing the coupled cells with K+-free solution and then applying a mixture of 5 µmol/L BaCl2, 500 nM apamin, 100 nM ChTX and 500 µmol/L TEA. These substances were without effect, whereas a subsequent application of 10 µmol/L glibenclamide uncoupled the cells in the K+-free solution. The measured capacitances were 103.7±7.7, 106.6±12.3, 93.9±9.8 and 47.9±1.8 pF (n=3) under control conditions, in the [K+]o-free solution, after addition of blocker mixture and in the presence of glibenclamide, respectively.
Although omission of [Na+]i should also inhibit Na+/K+-pump activity (Figure 3, intervention 6), it did not uncouple the cells. On the contrary, it prevented the ouabain-induced uncoupling (Table). This might suggest that the effect of ouabain is mediated through an increase in [Na+]i. Interestingly, a reduction of [Na+]o to 50 mmol/L, which should affect Ca2+ extrusion via Na+/Ca2+-exchanger (Figure 3, intervention 7), uncoupled cell pairs to the same extent as ouabain (Figure 4C; Table). This effect was observed at a holding potential of -60 mV. To examine if the Na+/Ca2+-exchanger is involved we took advantage of its electrogenic properties. By changing the holding potential it is possible to alter the driving force for the Na+/Ca2+-exchanger. Depolarization to +50 mV uncoupled cell pairs (Figure 5). Uncoupling was not observed at holding potentials less positive than +40 mV. Furthermore, when [Na+]o was reduced, which would move the reversal potential of the Na+/Ca2+-exchanger toward more negative values, the cells uncoupled at less positive potentials. After [Na+]o was reduced to 100 mmol/L the cell pairs uncoupled already at +10 mV (Figure 5A and B). The rate of the capacitance decay was voltage-dependent (Figure 5A): the more positive the holding potential, the more rapidly cells uncoupled (Figure 5C). Depolarization could also cause Ca2+ entry directly through voltage-gated Ca2+ channels. However, the depolarization-induced uncoupling was highly Na+-dependent and was not affected by the L-type Ca2+ channel inhibitor nifedipine (10 µmol/L), suggesting that the effect is mediated primarily via the Na+/Ca2+-exchanger (data not shown; n=3).
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The involvement of the Na+/Ca2+-exchanger in the regulation of intercellular communication was further supported by the effect of the specific blocker of the Na+/Ca2+-exchanger SEA0400 (Figure 3, intervention 8), which uncoupled cell pairs with a half-maximal concentration 
0.15 µmol/L (supplemental Figure III).
To obtain direct evidence for the role of a local increase in [Ca2+]i we measured [Ca2+]i (Figure 6). Ouabain caused an increase of peripheral [Ca2+]i without affecting the central [Ca2+]i (Figure 6A and 6B). This was because of an increased amplitude of [Ca2+]i transients at discrete spots along the periphery of the cell (Figure 6A and 6C). These [Ca2+]i transients occurred during the stimulation at constant sites along the perimeter of the cell (supplemental Figure IV available at http://circres.ahajournals.org). In contrast, vasopressin (100 nmol/L), which did not uncouple cells (data not shown; n=4), increased [Ca2+]i both peripherally and centrally (Figure 6B and 6C), and the peripheral [Ca2+]i transients occurred at different sites during the stimulation (supplemental Figure IV). To exclude that this result was confounded by movement artifacts, induced by solution changes or cell activation, identical experiments were made with calcein-loaded cells. The fluorescence from calcein-loaded cells did not spike and was unaffected by ouabain (data not shown; n=8).
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To further test whether the [Ca2+]i increase was a functional mediator of the ouabain-induced uncoupling, [Ca2+]i was clamped at a low concentration by high BAPTA concentration (Figure 3, intervention 9). Clamping of [Ca2+]i prevented the uncoupling of the cells by both ouabain and SEA0400 (Table), consistent with these compounds mediating their effects through an increase in [Ca2+]i. This also suggests that ouabain-induced uncoupling is a consequence of modulating Na+/Ca2+-exchanger activity and thereby [Ca2+]i.
Elevation of [Ca2+]i can either directly uncouple the cells or cause an activation of other Ca2+-sensitive messengers which close the gap junctions. Thus, [Ca2+]i can, for example, stimulate endogenous nitric oxide synthase (eNOS) and Ca2+-dependent NO formation which may result in the closure of gap junctions.21 We did not, however, see any effect of a nonselective inhibitor of eNOS, NG-nitro L-arginine (L-NAME), on the ouabain-induced uncoupling. In the presence of 100 µmol/L L-NAME 1 µmol/L ouabain still uncoupled the cells (capacity reduction from 95.7±9.7 to 41.1±5.1 pF, n=4). Although this result is consistent with an insensitivity of gap junctional conductance of A7r5 cells to the NO/cGMP pathway22 we cannot exclude involvement of other Ca2+-sensitive messengers.
Interventions that uncouple SMCs (Figure 3, interventions 3 with 5, 7, and 8) would also be expected to stop rhythmic contractions of the arterial wall. This was indeed the case. Blockade of Na+/Ca2+-exchanger with 1 to 3 µmol/L SEA0400 also inhibited vasomotion (n=4) (Figure 7A) and this effect was reversible. Omission of K+ from the bath solution did not stop vasomotion within 10 minutes (n=5), nor did 10 µmol/L glibenclamide (n=4). However, addition of 10 µmol/L glibenclamide under K+-free conditions abolished vasomotion within 2.3±0.9 minutes (n=5) (Figure 7B). This effect was reversible. Thus, the data obtained in intact arteries are consistent with our measurements of electrical coupling between A7r5 cells and the suggested hypothesis (Figure 3).
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Discussion |
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Electrical communication between SMCs is necessary for synchronized changes in [Ca2+]i in the vascular wall,23 and for conduction of vasoconstriction and vasodilatation.24 The intercellular communication is also important for generation of vasomotion.17,18,25 Blockade of gap junctions leads to desynchronization of [Ca2+]i transients in SMCs,18,26 inhibition of vascular conduction24 and vasomotion.19,25 It is therefore important to understand the regulation of gap junctional permeability in the vasculature. In this article we suggest that functional interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger is important for the regulation of the electrical communication between SMCs. Our results are consistent with [Ca2+]i as a local mediator.
Involvement of the Na+/K+-Pump
Low concentrations of ouabain have previously been shown to interrupt electrical communication between cardiac myocytes and to produce arrhythmias and decreased conduction velocity in the heart.27 Ouabain has also been shown to attenuate intercellular communication in smooth muscle14,28 as well as in other tissues.29–31 Ouabain in concentrations higher than 100 µmol/L had a dual effect: within an hour dye coupling was reduced and a few hours later connexin expression was suppressed.14 Here we observed an acute (within minutes) effect of ouabain (
100 µmol/L) on electrical coupling. Dye transfer is not always a good measure of intercellular communication because it depends on the physical and chemical properties of the dye and on the type of connexin involved.32 Furthermore, it has been shown previously that dye transfer does not correlate with electrical coupling.33,34 In addition, the previous study14 used ouabain concentrations that affect all isoforms of the Na+/K+-pump in rat tissue.3,35 In the present study we have investigated the effects of micromolar concentrations of ouabain. These concentrations block only the ouabain-sensitive 2- and 3-isoforms, which are expressed at low levels in the rat vasculature.4,5,11 This may explain the different results in the present and previous studies.
Vasomotion is inhibited by 1 to 10 µmol/L ouabain as shown previously by Gustafsson.15 As shown here, these concentrations inhibit synchronization of [Ca2+]i transients in SMCs. This is consistent with an effect on gap junctional conductance.19 To investigate the mechanisms underlying this effect we used cultured aortic smooth muscle cells. Here we obtained direct evidence that low concentrations of ouabain uncouple the cells. It has been controversial which of the ouabain-sensitive isoforms of the Na+/K+-pump is expressed in SMCs.5,36 In our A7r5 cells rt-PCR suggested the presence of mRNA for both the 2- and 3-isoform, whereas Western blot detected the 2-isoform but not the 3-isoform, which is similar to what we found for the mesenteric arteries (supplemental Figure I). The importance of the Na+/K+-pump for cell coupling was further supported by the uncoupling in ATP-depleted cells.
If the effect of ouabain is mediated through inhibition of the Na+/K+-pump, the omission of [K+]o should also uncouple the cell pairs (Figure 3, intervention 3). Surprisingly, the [K+]o-free solution did not by itself affect cell coupling. However, in the presence of glibenclamide [K+]o-free solution did uncouple cells. We have previously demonstrated a functional interaction between ouabain-sensitive Na+/K+-pumps and KATP channels.37 In that study we found that the activity of ouabain-sensitive Na+/K+-pumps could modify the activity of the KATP channels. The data in the present study suggest that KATP channels provide K+ for the Na+/K+-pump under [K+]o-free conditions. Thus, our present results support our previous finding of a functional interaction between ouabain-sensitive Na+/K+-pumps and KATP channels,37 and suggest that this interaction can be bidirectional. The uncoupling by the [K+]o-free solution is consistent with the effect of ouabain on cell coupling being mediated through inhibition of the Na+/K+-pump. These findings are supported by the effect of K+ omission and glibenclamide on vasomotion (Figure 7B).
Another way of inhibiting the Na+/K+-pump is to omit [Na+]i (Figure 3, intervention 6). This manipulation, however, did not uncouple the cells. On the contrary, the uncoupling effect of ouabain was lost under these conditions. This could indicate that the effect of ouabain is mediated through an increase in [Na+]i. Because we have previously shown that 10 µmol/L ouabain does not affect global [Na+]i38 this finding indicates that the effect is mediated through local increases in [Na+]i. The [Na+]i-free and [K+]o-free experiments further suggest that the acute uncoupling by ouabain is not mediated via signaling cascades, as suggested for other actions of ouabain.14,39
[Ca2+]i Mediates Uncoupling Induced by Na+/K+-Pump Inhibition
The Na+/K+-pump has been suggested to influence [Ca2+]i signaling40 via the close association with the Na+/Ca2+-exchanger.5,10,11 In fact, small fractions of the ouabain-sensitive Na+/K+-pump have been shown to colocalize with the Na+/Ca2+-exchanger in plasma membrane microdomains, which overlay the sarcoplasmic reticulum to form tiny cytosolic spaces where [Ca2+]i and [Na+]i can be controlled locally.10,13 In the current study our data of the local [Ca2+]i measurements in the intact artery and in the A7r5 cells provide direct novel evidence consistent with this. This is also consistent with a discrete distribution of the Na+/Ca2+-exchanger5,12,41 because the [Ca2+]i transients appeared at discrete sites along the perimeter of the cells (the resolution was only sufficient to see this in the A7r 5 cells). The difference between peripheral [Ca2+]i responses to ouabain and vasopressin (supplemental Figure IV) might explain the fact that although both these drugs elevate peripheral [Ca2+]i only ouabain uncouples the cells.
Our experiments with [Ca2+]i clamped at low concentrations support the involvement of [Ca2+]i in the interaction, because clamped [Ca2+]i prevented the ouabain-induced uncoupling (Table). Increase in [Ca2+]i could directly reduce the permeability of gap junctions.29,42 On the other hand it cannot be excluded that the [Ca2+]i transients cause activation of other Ca2+-sensitive messengers, which close the gap junctions either locally or at a distant site (this uncertainty is illustrated by the broken line between Ca2+ and gap junctions in Figure 3). We tested whether the NO/cGMP pathway could be such a messenger, and found this was not the case.
Evidence for a Role of the Na+/Ca2+-Exchanger
To test the suggested involvement of the Na+/Ca2+-exchanger we modified the electrochemical driving force for the exchanger both through modification of the Na+ gradient and through modification of the holding potential. These data consistently showed that a change of the electrochemical driving force favoring the Na+ efflux–Ca2+ influx mode caused uncoupling, strongly implicating the Na+/Ca2+-exchanger. A role for enhanced Ca2+ influx through L-type calcium channels consequent to the depolarization was excluded using nifedipine, which was without effect on the uncoupling. The involvement of the Na+/Ca2+-exchanger was further supported by inhibition of cell coupling by the Na+/Ca2+-exchanger inhibitor SEA0400.36 The SEA0400-sensitive Na+/Ca2+-exchanger-1 has been shown to be expressed in vascular SMCs.6,36 The findings are consistent with the inhibitory effect of low [Na+]o and by SEA0400 on vasomotion and synchronization of [Ca2+]i transients and confirms the role of the Na+/Ca2+-exchanger for coupling of SMCs in the intact arterial wall.
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Conclusion |
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In this study we have investigated the mechanism responsible for blockade of vasomotion with low ouabain concentrations. Our data strongly suggest that this mechanism involves a functional interaction between the Na+/K+-pump and the Na+/Ca2+-exchanger leading to near-membrane local [Ca2+]i transients that block gap junctions and consequently desynchronize SMC activity. The data thus suggest a potentially important mechanism for regulating intercellular communication in the vascular wall and suggest an important novel consequence of the previously demonstrated association of the Na+/K+-pump and the Na+/Ca2+-exchanger.
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Acknowledgments |
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Sources of Funding
The work was supported by the Danish Heart Foundation, the Novo Nordisk Foundation and the Faculty of Health Sciences, University of Aarhus. The Water and Salt Research Center at the University of Aarhus is established and supported by the Danish National Research Foundation (Danmarks Grundforskningsfond).
Disclosures
None.
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Footnotes |
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Original received August 4, 2005; resubmission received October 18, 2006; revised resubmission received February 5, 2007; accepted February 22, 2007.
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References |
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