KATP-channel-induced vasodilation is modulated by the Na,K-pump activity in rabbit coronary small arteries

The microtubule network enables Src kinase interaction with the Na,K-ATPase to generate Ca2+ flashes in smooth muscle cells

Background: Several local Ca²⁺ events are characterized in smooth muscle cells. We have previously shown that an inhibitor of the Na,K-ATPase, ouabain induces spatially restricted intracellular Ca²⁺ transients near the plasma membrane, and suggested the importance of this signaling for regulation of intercellular coupling and smooth muscle cell contraction. The mechanism behind these Na,K-ATPase-dependent “Ca²⁺ flashes” remains to be elucidated. In addition to its conventional ion transport function, the Na,K-ATPase is proposed to contribute to intracellular pathways, including Src kinase activation. The microtubule network is important for intracellular signaling, but its role in the Na,K-ATPase-Src kinase interaction is not known. We hypothesized the microtubule network was responsible for maintaining the Na,K-ATPase-Src kinase interaction, which enables Ca²⁺ flashes.

Methods: We characterized Ca²⁺ flashes in cultured smooth muscle cells, A7r5, and freshly isolated smooth muscle cells from rat mesenteric artery. Cells were loaded with Ca²⁺-sensitive fluorescent dyes, Calcium Green-1/AM and Fura Red/AM, for ratiometric measurements of intracellular Ca²⁺. The Na,K-ATPase α2 isoform was knocked down with siRNA and the microtubule network was disrupted with nocodazole. An involvement of the Src signaling was tested pharmacologically and with Western blot. Protein interactions were validated with proximity ligation assays.

Results: The Ca²⁺ flashes were induced by micromolar concentrations of ouabain. Knockdown of the α2 isoform Na,K-ATPase abolished Ca²⁺ flashes, as did inhibition of tyrosine phosphorylation with genistein and PP2, and the inhibitor of the Na,K-ATPase-dependent Src activation, pNaKtide. Ouabain-induced Ca²⁺ flashes were associated with Src kinase activation by phosphorylation. The α2 isoform Na,K-ATPase and Src kinase colocalized in the cells. Disruption of microtubule with nocodazole inhibited Ca²⁺ flashes, reduced Na,K-ATPase/Src interaction and Src activation.

Conclusion: We demonstrate that the Na,K-ATPase-dependent Ca²⁺ flashes in smooth muscle cells require an interaction between the α2 isoform Na, K-ATPase and Src kinase, which is maintained by the microtubule network.

The Na,K-ATPase in vascular smooth muscle cells

The Na,K-ATPase is an enzyme essential for ion homeostasis in all cells. Over the last decades, it has been well-established that in addition to the transport of Na⁺/K⁺ over the cell membrane, the Na,K-ATPase acts as a receptor transducing humoral signals intracellularly. It has been suggested that ouabain-like compounds serve as endogenous modulators of this Na,K-ATPase signal transduction. The molecular mechanisms underlying Na,K-ATPase signaling are complicated and suggest the confluence of divergent biological pathways. This review discusses recent updates on the Na,K-ATPase signaling pathways characterized or suggested in vascular smooth muscle cells. The conventional view on this signaling is based on a microdomain structure where the Na,K-ATPase controls the Na,Ca-exchanger activity via modulation of intracellular Na⁺ in the spatially restricted submembrane space. This, in turn, affects intracellular Ca²⁺ and Ca²⁺ load in the sarcoplasmic reticulum leading to modulation of contractility as well as gene expression. An ion-transport-independent signal transduction from the Na,K-ATPase is based on molecular interactions. This was primarily characterized in other cell types but recently also demonstrated in vascular smooth muscles. The downstream signaling from the Na,K-ATPase includes Src and phosphatidylinositol-4,5-bisphosphate 3 kinase signaling pathways and generation of reactive oxygen species. Moreover, in vascular smooth muscle cells the interaction between the Na,K-ATPase and proteins responsible for Ca²⁺ homeostasis, e.g., phospholipase C and inositol triphosphate receptors, contributes to an integration of the signaling pathways. Recent update on the Na,K-ATPase dependent intracellular signaling and the significance for physiological functions and pathophysiological changes are discussed in this review.

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc kinase-dependent connexin43 phosphorylation

Communication between vascular smooth muscle cells (VSMCs) is dependent on gap junctions and is regulated by the Na-K-ATPase. The Na-K-ATPase is therefore important for synchronized VSMC oscillatory activity, i.e., vasomotion. The signaling between the Na-K-ATPase and gap junctions is unknown. We tested here the hypothesis that this signaling involves cSrc kinase. Intercellular communication was assessed by membrane capacitance measurements of electrically coupled VSMCs. Vasomotion in isometric myograph, input resistance, and synchronized [Ca2+]i transients were used as readout for intercellular coupling in rat mesenteric small arteries in vitro. Phosphorylation of cSrc kinase and connexin43 (Cx43) were semiquantified by Western blotting. Micromole concentration of ouabain reduced the amplitude of norepinephrine-induced vasomotion and desynchronized Ca2+ transients in VSMC in the arterial wall. Ouabain also increased input resistance in the arterial wall. These effects of ouabain were antagonized by inhibition of tyrosine phosphorylation with genistein, PP2, and by an inhibitor of the Na-K-ATPase-dependent cSrc activation, pNaKtide. Moreover, inhibition of cSrc phosphorylation increased vasomotion amplitude and decreased the resistance between cells in the vascular wall. Ouabain inhibited the electrical coupling between A7r5 cells, but pNaKtide restored the electrical coupling. Ouabain increased cSrc autophosphorylation of tyrosine 418 (Y418) required for full catalytic activity whereas pNaKtide antagonized it. This cSrc activation was associated with Cx43 phosphorylation of tyrosine 265 (Y265). Our findings demonstrate that Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc-dependent Cx43 tyrosine phosphorylation.

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Extracellular Calcium-Dependent Modulation of Endothelium Relaxation in Rat Mesenteric Small Artery: The Role of Potassium Signaling

The nature of NO- and COX-independent endothelial hyperpolarization (EDH) is not fully understood but activation of small- and intermittent-conductance Ca²⁺-activated K⁺ channels (SK_Ca and IK_Ca) is important. Previous studies have suggested that the significance of IK_Ca depends on [Ca²⁺]_out. Also it has been suggested that K⁺ is important through localized [K⁺]_out signaling causing activation of the Na⁺,K⁺-ATPase and inward-rectifying K⁺ channels (K_ir). Here we tested the hypothesis that the modulating effect of [Ca²⁺]_out on the EDH-like response depends on [K⁺]_out. We addressed this possibility using isometric myography of rat mesenteric small arteries. When [K⁺]_out was 4.2 mM, relaxation to acetylcholine (ACh) was stronger at 2.5 mM [Ca²⁺]_out than at 1 mM [Ca²⁺]_out. Inhibition of IK_Ca with TRAM34 suppressed the relaxations but did not change the relation between the relaxations at the low and high [Ca²⁺]_out. This [Ca²⁺]_out-dependence disappeared at 5.9 mM [K⁺]_out and in the presence of ouabain or BaCl₂. Our results suggest that IK_Ca are involved in the localized [K⁺]_out signaling which acts through the Na⁺,K⁺-ATPase and K_ir channels and that the significance of this endothelium-dependent pathway is modulated by [Ca²⁺]_out.

Arsenic exposure decreases rhythmic contractions of vascular tone through sodium transporters and K+ channels

Arsenic-contaminated drinking water is a public health problem in countries such as Taiwan, Bangladesh, United States, Mexico, Argentina, and Chile. The chronic ingestion of arsenic-contaminated drinking water increases the risk for ischemic heart disease, cerebrovascular disease, and prevalence of hypertension. Although toxic arsenic effects are controversial, there is evidence that a high concentration of arsenic may induce hypertension through increase in vascular tone and resistance. Vascular tone is regulated by the rhythmic contractions of the blood vessels, generated by calcium oscillations in the cytosol of vascular smooth muscle cells. To regulate the cytosolic calcium oscillations, the membrane oscillator model involves the participation of Ca²⁺ channels, calcium-activated K⁺ channels, Na⁺/Ca²⁺ exchange, plasma membrane Ca²⁺-ATPase, and the Na⁺/K⁺-ATPase. However, little is known about the role of K⁺ uptake by sodium transporters [Na⁺/K⁺-ATPase or Na⁺-K⁺-2Cl^- (NKCC1)] on the rhythmic contractions. Vascular rhythmic contractions, or vasomotion are a local mechanism to regulate vascular resistance and blood flow. Since vascular rhythmic contractions of blood vessels are involved in modulating the vascular resistance, the blood flow, and the systemic pressure, we suggest a model explaining the participation of the sodium pump and NKCC1 co-transporter in low dose arsenic exposure effects on vasomotion and vascular dysfunction.

Endothelium-dependent nitric oxide and hyperpolarization-mediated venous relaxation pathways in rat inferior vena cava

INTRODUCTION

The vascular endothelium plays a major role in the control of arterial tone; however, its role in venous tissues is less clear. The purpose of this study was to determine the role of endothelium in the control of venous function and the relaxation pathways involved.

METHODS

Circular segments of inferior vena cava (IVC) from male Sprague-Dawley rats were suspended between two wires and isometric contraction to phenylephrine (Phe; 10(-5)M) and 96 mM KCl was measured. Acetylcholine (Ach; 10(-10) to 10(-5)M) was added and the percentage of venous relaxation was measured. To determine the role of nitric oxide (NO) and prostacyclin (PGI(2)), vein relaxation was measured in the presence of the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester (L-NAME; 3 × 10(-4) M) and the cyclooxygenase inhibitor indomethacin (10(-5) M). To measure the role of hyperpolarization, vein relaxation was measured in the presence of K(+) channel activator cromakalim (10(-11) to 10(-6) M), and the nonselective K(+) channel blocker tetraethylammonium (TEA; 10(-3) M). To test for the contribution of a specific K(+) channel, the effects of K(+) channel blockers: glibenclamide (adenosine triphosphate [ATP]-sensitive K(ATP), 10(-5) M), 4-aminopyridine (4-AP; voltage-dependent K(v), 10(-3) M), apamin (small conductance Ca(2+)-dependent SK(Ca), 10(-7) M), and iberiotoxin (large conductance Ca(2+)-dependent BK(Ca), 10(-8) M) on Ach-induced relaxation were tested.

RESULTS

Ach caused concentration-dependent relaxation of Phe contraction (maximum 49.9 ± 4.9%). Removal of endothelium abolished Ach-induced relaxation. IVC treatment with L-NAME partially reduced Ach relaxation (32.8 ± 4.9%). In IVC treated with L-NAME plus indomethacin, significant Ach-induced relaxation (33.6 ± 3.2%) could still be observed, suggesting a role of endothelium-derived hyperpolarizing factor (EDHF). In IVC treated with L-NAME, indomethacin and TEA, Ach relaxation was abolished, supporting a role of EDHF. In veins stimulated with high KCl, Ach caused relaxation (maximum 59.5 ± 3.5%) that was abolished in the presence of L-NAME and indomethacin suggesting that any Ach-induced EDHF is blocked in the presence of high KCl depolarizing solution, which does not favor outward movement of K(+) ion and membrane hyperpolarization. Cromakalim, an activator of K(ATP), caused significant IVC relaxation when applied alone or on top of maximal Ach-induced relaxation, suggesting that the Ach response may not involve K(ATP). Ach-induced relaxation was not inhibited by glibenclamide, 4-AP, or apamin, suggesting little role of K(ATP), K(v) or SK(Ca), respectively. In contrast, iberiotoxin significantly inhibited Ach-induced relaxation, suggesting a role of BK(Ca).

CONCLUSIONS

Thus, endothelium-dependent venous relaxation plays a major role in the control of venous function. In addition to NO, an EDHF pathway involving BK(Ca) may play a role in endothelium-dependent venous relaxation.

Contributions of A2A and A2B adenosine receptors in coronary flow responses in relation to the KATP channel using A2B and A2A/2B double-knockout mice

Adenosine plays a role in physiological and pathological conditions, and A(2) adenosine receptor (AR) expression is modified in many cardiovascular disorders. In this study, we elucidated the role of the A(2B)AR and its relationship to the A(2A)AR in coronary flow (CF) changes using A(2B) single-knockout (KO) and A(2A/2B) double-KO (DKO) mice in a Langendorff setup. We used two approaches: 1) selective and nonselective AR agonists and antagonists and 2) A(2A)KO and A(2B)KO and A(2A/2B)DKO mice. BAY 60-6583 (a selective A(2B) agonist) had no effect on CF in A(2B)KO mice, whereas it significantly increased CF in wild-type (WT) mice (maximum of 23.3 ± 9 ml·min(-1)·g(-1)). 5'-N-ethylcarboxamido adenosine (NECA; a nonselective AR agonist) increased CF in A(2B)KO mice (maximum of 34.6 ± 4.7 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Also, CGS-21680 (a selective A(2A) agonist) increased CF in A(2B)KO mice (maximum of 29 ± 1.9 ml·min(-1)·g(-1)) to a significantly higher degree compared with WT mice (maximum of 25.1 ± 2.3 ml·min(-1)·g(-1)). SCH-58261 (an A(2A)-selective antagonist) inhibited the NECA-induced increase in CF to a significantly higher degree in A(2B)KO mice (19.3 ± 1.6 vs. 0.5 ± 0.4 ml·min(-1)·g(-1)) compared with WT mice (19 ± 3.5 vs. 3.6 ± 0.5 ml·min(-1)·g(-1)). NECA did not induce any increase in CF in A(2A/2B)DKO mice, whereas a significant increase was observed in WT mice (maximum of 23.1 ± 2.1 ml·min(-1)·g(-1)). Furthermore, the mitochondrial ATP-sensitive K(+) (K(ATP)) channel blocker 5-hydroxydecanoate had no effect on the NECA-induced increase in CF in WT mice, whereas the NECA-induced increase in CF in WT (17.6 ± 2 ml·min(-1)·g(-1)), A(2A)KO (12.5 ± 2.3 ml·min(-1)·g(-1)), and A(2B)KO (16.2 ± 0.8 ml·min(-1)·g(-1)) mice was significantly blunted by the K(ATP) channel blocker glibenclamide (to 0.7 ± 0.7, 2.3 ± 1.1, and 0.9 ± 0.4 ml·min(-1)·g(-1), respectively). Also, the CGS-21680-induced (22 ± 2.3 ml·min(-1)·g(-1)) and BAY 60-6583-induced (16.4 ± 1.60 ml·min(-1)·g(-1)) increase in CF in WT mice was significantly blunted by glibenclamide (to 1.2 ± 0.4 and 1.8 ± 1.2 ml·min(-1)·g(-1), respectively). In conclusion, this is the first evidence supporting the compensatory upregulation of A(2A)ARs in A(2B)KO mice and demonstrates that both A(2A)ARs and A(2B)ARs induce CF changes through K(ATP) channels. These results identify AR-mediated CF responses that may lead to better therapeutic approaches for the treatment of cardiovascular disorders.

Ankyrin-B regulates Kir6.2 membrane expression and function in heart

Ankyrin polypeptides are critical for normal membrane protein expression in diverse cell types, including neurons, myocytes, epithelia, and erythrocytes. Ankyrin dysfunction results in defects in membrane expression of ankyrin-binding partners (including ion channels, transporters, and cell adhesion molecules), resulting in aberrant cellular function and disease. Here, we identify a new role for ankyrin-B in cardiac cell biology. We demonstrate that cardiac sarcolemmal K(ATP) channels directly associate with ankyrin-B in heart via the K(ATP) channel alpha-subunit Kir6.2. We demonstrate that primary myocytes lacking ankyrin-B display defects in Kir6.2 protein expression, membrane expression, and function. Moreover, we demonstrate a secondary role for ankyrin-B in regulating K(ATP) channel gating. Finally, we demonstrate that ankyrin-B forms a membrane complex with K(ATP) channels and the cardiac Na/K-ATPase, a second key membrane transporter involved in the cardiac ischemia response. Collectively, our new findings define a new role for cardiac ankyrin polypeptides in regulation of ion channel membrane expression in heart.

Adenosine receptors and the heart: role in regulation of coronary blood flow and cardiac electrophysiology

Adenosine is an autacoid that plays a critical role in regulating cardiac function, including heart rate, contractility, and coronary flow. In this chapter, current knowledge of the functions and mechanisms of action of coronary flow regulation and electrophysiology will be discussed. Currently, there are four known adenosine receptor (AR) subtypes, namely A(1), A(2A), A(2B), and A(3). All four subtypes are known to regulate coronary flow. In general, A(2A)AR is the predominant receptor subtype responsible for coronary blood flow regulation, which dilates coronary arteries in both an endothelial-dependent and -independent manner. The roles of other ARs and their mechanisms of action will also be discussed. The increasing popularity of gene-modified models with targeted deletion or overexpression of a single AR subtype has helped to elucidate the roles of each receptor subtype. Combining pharmacologic tools with targeted gene deletion of individual AR subtypes has proven invaluable for discriminating the vascular effects unique to the activation of each AR subtype. Adenosine exerts its cardiac electrophysiologic effects mainly through the activation of A(1)AR. This receptor mediates direct as well as indirect effects of adenosine (i.e., anti-beta-adrenergic effects). In supraventricular tissues (atrial myocytes, sinuatrial node and atriovetricular node), adenosine exerts both direct and indirect effects, while it exerts only indirect effects in the ventricle. Adenosine exerts a negative chronotropic effect by suppressing the automaticity of cardiac pacemakers, and a negative dromotropic effect through inhibition of AV-nodal conduction. These effects of adenosine constitute the rationale for its use as a diagnostic and therapeutic agent. In recent years, efforts have been made to develop A(1)R-selective agonists as drug candidates that do not induce vasodilation, which is considered an undesirable effect in the clinical setting.

Time-dependent alteration in cromakalim-induced relaxation of corpus cavernosum from streptozocin-induced diabetic rats

The purpose of the present study was to investigate the relaxant responses to the ATP-sensitive potassium (K(ATP)) channel opener cromakalim in corpus cavernosum strips from 1-, 2-, 4-, 6-, and 8-week streptozocin-induced diabetic rats. Cromakalim (1 nM-0.1 mM) produced concentration-dependent relaxation in phenylephrine (7.5 microM)-precontracted isolated rat corporal strips. Compared with age-matched control animals, a significant enhancement in cromakalim-induced relaxation of corpus cavernosum was observed in 2-week diabetic animals, whereas the relaxant responses to cromakalim were decreased in 6-and 8-week diabetic animals. However, the cromakalim-induced relaxation was not altered in either 1-week or 4-week rat corporal strips in comparison with corresponding age-matched non-diabetic groups. Preincubation with the K(ATP) channel blocker glibenclamide (10 microM) significantly inhibited the cromakalim-induced relaxation in both non-diabetic and diabetic rat corpus cavernosum, but neither the voltage-dependent K(+) channel (K(V)) antagonist 4-aminopyridine (1 mM) nor the calcium-activated K(+) channel (K(Ca)) antagonist charybdotoxin (0.1 microM) had significant effect on cromakalim-induced relaxation in both control and diabetic rat corporal strips. Relaxation responses to the nitric oxide donor sodium nitroprusside (1 nM-0.1 mM) in diabetic rat corpus cavernosum were similar to that of age-matched controls. These data demonstrated that the relaxant responses to cromakalim were altered in diabetic cavernosal strips in a time dependent manner, suggesting that the period of diabetes mellitus may play a key role in the K(ATP) channels function in rat corpus cavernosum.

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

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(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](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 [Ca(2+)](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 K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-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 [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.