Trafficking of Na,K-ATPase fused to enhanced green fluorescent protein is mediated by protein kinase A or C

Adverse effect of FTY720P on colonic Na+ /K+ ATPase is mediated via ERK, p38MAPK, PKC, and PI3K

FTY720P, an analogue of sphingosine 1-phosphate, has emerged lately as a potential causative agent of inflammatory bowel disease, in which electrolytes movements driven by the sodium gradient established by the Na⁺ /K⁺ ATPase are altered. We showed previously in Caco-2 cells, a 50% FTY720P-induced decrease in the ATPase activity, mediated via S1PR2 and PGE2. This work aims at delineating the mechanism underlying PGE2 release and at investigating if the ATPase inhibition is due to changes in its abundance. The activity of the ATPase and the localization of a GFP-tagged Na⁺ /K⁺ -ATPase α₁ -subunit were assessed in cells treated with 7.5 nM FTY720P. The involvement of ERK, p38 MAPK, PKC, and PI3K was studied in cells treated with 7.5 nM FTY720P or 1 nM PGE2 in presence of their inhibitors, or by determining changes in the protein expression of their activated phosphorylated forms. Imaging data showed ∼30% reduction in the GFP-tagged Na⁺ /K⁺ ATPase at the plasma membrane. Both FTY720P and PGE2 showed, respectively, 50% and 60% reduction in ATPase activity that disappeared when p38 MAPK, PKC, and PI3K were inhibited individually but not with ERK inhibition. The effect of FTY720P was imitated by PMA, an activator of PKC. Western blotting revealed inhibition of ERK by FTY720P. It was concluded that FTY720P, through activation of S1PR2, downregulates the Na⁺ /K⁺ ATPase by inhibiting ERK, which in turn activates p38 MAPK leading to the sequential activation of PKC and PI3K, PGE2 release, and a decrease in the Na⁺ /K⁺ ATPase activity and membrane abundance.

Chemical genetic identification of GAK substrates reveals its role in regulating Na+/K+-ATPase

Novel GAK phosphorylation targets are identified using chemical genetic methods. One of the substrates is the α subunit of the Na⁺/K⁺-ATPase, phosphorylation of which is necessary for its surface trafficking from endosomes. Conserved functions of NAK family kinases are described.

Cyclin G–associated kinase (GAK) is a ubiquitous serine/threonine kinase that facilitates clathrin uncoating during vesicle trafficking. GAK phosphorylates a coat adaptor component, AP2M1, to help achieve this function. GAK is also implicated in Parkinson's disease through genome-wide association studies. However, GAK's role in mammalian neurons remains unclear, and insight may come from identification of further substrates. Employing a chemical genetics method, we show here that the sodium potassium pump (Na⁺/K⁺-ATPase) α-subunit Atp1a3 is a GAK target and that GAK regulates Na⁺/K⁺-ATPase trafficking to the plasma membrane. Whole-cell patch clamp recordings from CA1 pyramidal neurons in GAK conditional knockout mice show a larger change in resting membrane potential when exposed to the Na⁺/K⁺-ATPase blocker ouabain, indicating compromised Na⁺/K⁺-ATPase function in GAK knockouts. Our results suggest a modulatory role for GAK via phosphoregulation of substrates such as Atp1a3 during cargo trafficking.

MLC1 protein: a likely link between leukodystrophies and brain channelopathies

Megalencephalic leukoencephalopathy with subcortical cysts (MLCs) disease is a rare inherited, autosomal recessive form of childhood-onset spongiform leukodystrophy characterized by macrocephaly, deterioration of motor functions, epileptic seizures and mental decline. Brain edema, subcortical fluid cysts, myelin and astrocyte vacuolation are the histopathological hallmarks of MLC. Mutations in either the MLC1 gene (>75% of patients) or the GlialCAM gene (<20% of patients) are responsible for the disease. Recently, the GlialCAM adhesion protein was found essential for the membrane expression and function of the chloride channel ClC-2 indicating MLC disease caused by mutation in GlialCAM as the first channelopathy among leukodystrophies. On the contrary, the function of MLC1 protein, which binds GlialCAM, its functional relationship with ClC-2 and the molecular mechanisms underlying MLC1 mutation-induced functional defects are not fully understood yet. The human MLC1 gene encodes a 377-amino acid membrane protein with eight predicted transmembrane domains which shows very low homology with voltage-dependent potassium (K⁺) channel subunits. The high expression of MLC1 in brain astrocytes contacting blood vessels and meninges and brain alterations observed in MLC patients have led to hypothesize a role for MLC1 in the regulation of ion and water homeostasis. Recent studies have shown that MLC1 establishes structural and/or functional interactions with several ion/water channels and transporters and ion channel accessory proteins, and that these interactions are affected by MLC1 mutations causing MLC. Here, we review data on MLC1 functional properties obtained in in vitro and in vivo models and discuss evidence linking the effects of MLC1 mutations to brain channelopathies.

Loss of vps54 function leads to vesicle traffic impairment, protein mis-sorting and embryonic lethality

The identification of the mutation causing the phenotype of the amyotrophic lateral sclerosis (ALS) model mouse, wobbler, has linked motor neuron degeneration with retrograde vesicle traffic. The wobbler mutation affects protein stability of Vps54, a ubiquitously expressed vesicle-tethering factor and leads to partial loss of Vps54 function. Moreover, the Vps54 null mutation causes embryonic lethality, which is associated with extensive membrane blebbing in the neural tube and is most likely a consequence of impaired vesicle transport. Investigation of cells derived from wobbler and Vps54 null mutant embryos demonstrates impaired retrograde transport of the Cholera-toxin B subunit to the trans-Golgi network and mis-sorting of mannose-6-phosphate receptors and cargo proteins dependent on retrograde vesicle transport. Endocytosis assays demonstrate no difference between wobbler and wild type cells, indicating that the retrograde vesicle traffic to the trans-Golgi network, but not endocytosis, is affected in Vps54 mutant cells. The results obtained on wobbler cells were extended to test the use of cultured skin fibroblasts from human ALS patients to investigate the retrograde vesicle traffic. Analysis of skin fibroblasts of ALS patients will support the investigation of the critical role of the retrograde vesicle transport in ALS pathogenesis and might yield a diagnostic prospect.

Nuclear Na+/K+-ATPase plays an active role in nucleoplasmic Ca2+ homeostasis

Na(+)/K(+)-ATPase, an integral membrane protein, has been studied for over a half century with respect to its transporter function in the plasma membrane, where it expels three Na(+) ions from the cell in exchange for two K(+) ions. In this study, we demonstrate a functioning Na(+)/K(+)-ATPase within HEK293 cell nuclei. This subcellular localization was confirmed by western blotting, ouabain-sensitive ATPase activity of the nuclear membrane fraction, immunocytochemistry and delivery of fluorescently tagged Na(+)/K(+)-ATPase α- and β-subunits. In addition, we observed an overlap between nuclear Na(+)/K(+)-ATPase and Na/Ca-exchanger (NCX) when nuclei were immunostained with commercially available Na(+)/K(+)-ATPase and NCX antibodies, suggesting a concerted physiological coupling between these transporters. In keeping with this, we observed an ATP-dependent, strophanthidin-sensitive Na(+) flux into the nuclear envelope (NE) lumen loaded with the Na-sensitive dye, CoroNa-Green. Analogous experiments using Fluo-5N, a low affinity Ca(2+) indicator, demonstrated a similar ATP-dependent and strophanthidin-sensitive Ca(2+) flux into the NE lumen. Our results reveal an intracellular physiological role for the coordinated efforts of the Na(+)/K(+)-ATPase and NCX to actively remove Ca(2+) from the nucleoplasm into the NE lumen (i.e. the nucleoplasmic reticulum).

Protein kinase A (PKA) phosphorylation of Na+/K+-ATPase opens intracellular C-terminal water pathway leading to third Na+-binding site in molecular dynamics simulations

Phosphorylation is one of the major mechanisms for posttranscriptional modification of proteins. The addition of a compact, negatively charged moiety to a protein can significantly change its function and localization by affecting its structure and interaction network. We have used all-atom Molecular Dynamics simulations to investigate the structural consequences of phosphorylating the Na(+)/K(+)-ATPase (NKA) residue Ser(936), which is the best characterized phosphorylation site in NKA, targeted in vivo by protein kinase A (PKA). The Molecular Dynamics simulations suggest that Ser(936) phosphorylation opens a C-terminal hydrated pathway leading to Asp(926), a transmembrane residue proposed to form part of the third sodium ion-binding site. Simulations of a S936E mutant form, for which only subtle effects are observed when expressed in Xenopus oocytes and studied with electrophysiology, does not mimic the effects of Ser(936) phosphorylation. The results establish a structural association of Ser(936) with the C terminus of NKA and indicate that phosphorylation of Ser(936) can modulate pumping activity by changing the accessibility to the ion-binding site.

Effects of PKA phosphorylation on the conformation of the Na,K-ATPase regulatory protein FXYD1

FXYD1 (phospholemman) is a member of an evolutionarily conserved family of membrane proteins that regulate the function of the Na,K-ATPase enzyme complex in specific tissues and specific physiological states. In heart and skeletal muscle sarcolemma, FXYD1 is also the principal substrate of hormone-regulated phosphorylation by c-AMP dependent protein kinase A and by protein kinase C, which phosphorylate the protein at conserved Ser residues in its cytoplasmic domain, altering its Na,K-ATPase regulatory activity. FXYD1 adopts an L-shaped alpha-helical structure with the transmembrane helix loosely connected to a cytoplasmic amphipathic helix that rests on the membrane surface. In this paper we describe NMR experiments showing that neither PKA phosphorylation at Ser68 nor the physiologically relevant phosphorylation mimicking mutation Ser68Asp induces major changes in the protein conformation. The results, viewed in light of a model of FXYD1 associated with the Na,K-ATPase alpha and beta subunits, indicate that the effects of phosphorylation on the Na,K-ATPase regulatory activity of FXYD1 could be due primarily to changes in electrostatic potential near the membrane surface and near the Na(+)/K(+) ion binding site of the Na,K-ATPase alpha subunit.

Na⁺,K⁺-ATPase as the Target Enzyme for Organic and Inorganic Compounds

This paper gives an overview of the literature data concerning specific and non specific inhibitors of Na⁺,K⁺-ATPase receptor. The immobilization approaches developed to improve the rather low time and temperature stability of Na⁺,K⁺-ATPase, as well to preserve the enzyme properties were overviewed. The functional immobilization of Na⁺,K⁺-ATPase receptor as the target, with preservation of the full functional protein activity and access of various substances to an optimum number of binding sites under controlled conditions in the combination with high sensitive technology for the detection of enzyme activity is the basis for application of this enzyme in medical, pharmaceutical and environmental research.

Membrane trafficking of the human organic anion-transporting polypeptide C (hOATPC)

INTRODUCTION

The human organic anion transporting polypeptide C (OATPC) is one of the major transport proteins involved in the enterohepatic circulation of bile salts and plays an important role in vectorial transport of organic anions and drugs across hepatocytes.

MATERIALS AND METHODS

In this study, the effects of biological reagents on the membrane localization of OATPC were investigated by confocal microscopy and estrone-3-sulfate transport.

RESULTS

Our results demonstrated that the functional membrane expression of fluorescent chimera OATPC-GFP was achieved in non-polarized (COS7 and HEK293) and polarized (MDCK) cells. Both brefeldin A (a Golgi complex disruptor) and bafilomycin A1 (an inhibitor of vacuolar H+-ATPase) treatment significantly decreased the polarized membrane trafficking and markedly reduced the uptake of estrone-3-sulfate ( approximately 40-90%) in OATPC-GFP transfected cells, suggesting that membrane sorting of hOATPC-GFP was mediated by Golgi complex and vacuolar H+-ATPase-related vesicle transport pathways. Treatment with 8-Br-cAMP (a cAMP analog) stimulated OATPC-GFP membrane localization and enhanced estrone-3-sulfate uptake by approximately 20%. The protein kinase A (PKA) inhibitors (H89 and KT5720), but not a PKG inhibitor, blocked the polarized membrane expression of OATPC-GFP and reduced estrone-3-sulfate transport activity. The simultaneous treatment of cells with PKA activator/inhibitor and bafilomycin A1 demonstrated that bafilomycin A1 did not change the effects of 8-Br-cAMP and H89 on the membrane localization of OATPC-GFP compared with the use of 8-Br-cAMP and H89 alone.

DISCUSSION

These data suggest that a cAMP-PKA sensitive membrane sorting pathway for OATPC-GFP is independent of the vacuolar H+-ATPase associated (bafilomycin A1 sensitive) vesicle mediated membrane sorting pathway. In contrast, with combined treatment with brefeldin A, neither the PKA-activator (8-Br-cAMP) nor the inhibitor (H89) further altered the plasma membrane expression and transport activity of OATPC-GFP compared with brefeldin A treatment alone. These data suggest that the cAMP-PKA regulation of OATPC membrane expression involves the Golgi complex. When the Golgi apparatus was disrupted by brefeldin A treatment, the effects of cAMP-PKA on the Golgi-to-basolateral surface sorting process of OATPC was also diminished. In summary, the plasma membrane localization of human OATPC is mediated by Golgi complex and vacuolar H+-ATPase vesicle mediated membrane sorting pathways. cAMP-PKA regulates sorting process through the Golgi complex but not the vacuolar H+-ATPase associated vesicular pathway.

Purinergic agonists stimulate lens Na-K-ATPase-mediated transport via a Src tyrosine kinase-dependent pathway

The Na-K-ATPase is vital for maintenance of lens transparency. Past studies using intact lens suggested the involvement of tyrosine kinases in short-term regulation of Na-K-ATPase. Furthermore, in vitro phosphorylation of a lens epithelial membrane preparation by Src family kinases (SFKs), a family of nonreceptor tyrosine kinases, resulted in modification of Na-K-ATPase activity. Here, the effect of purinergic agonists, ATP and UTP, on Na-K-ATPase function and SFK activation was examined in the rabbit lens. Na-K-ATPase function was examined using two different approaches, measurement of ouabain-sensitive potassium ((86)Rb) uptake by the intact lens, and Na-K-ATPase activity in lens epithelial homogenates. ATP and UTP caused a significant increase in ouabain-sensitive potassium ((86)Rb) uptake. Na-K-ATPase activity was increased in the epithelium of lenses pretreated with ATP. Lenses treated with ATP or UTP displayed activation of SFKs as evidenced by increased Western blot band density of active SFK (phosphorylated at the active loop Y416) and decreased band density of inactive SFKs (phosphorylated at the COOH terminal). A single PY416-Src immunoreactive band at approximately 60 kDa was observed, suggesting not all Src family members are activated. Immunoprecipitation studies showed that band density of active Src, and to a lesser extent active Fyn, was significantly increased, while active Yes did not change. Preincubation of the lenses with SFK inhibitor PP2 abolished the ATP-induced increase in ouabain-sensitive potassium ((86)Rb) uptake. The results suggest selective activation of Src and/or Fyn is part of a signaling mechanism initiated by purinergic agonists that increases Na-K-ATPase-mediated transport in the organ-cultured lens.

Modulation of the (Na(+)+K+)ATPase activity by Angiotensin-(1-7) in MDCK cells

In the present paper the effect of Ang-(1-7) on the distal tubule (Na(+)+K+)ATPase activity was evaluated by using MDCK cells as a model. Confluent cell monolayers were incubated with increasing concentrations of Ang-(1-7) for 30 min. Thereafter, the (Na(+)+K+)ATPase activity was evaluated and a dose-dependent (from 10(-12) to 10(-7) M) inhibition was observed. The maximal inhibitory effect (54%) was reached at the concentration of 10(-8) M. The inhibitory effect of Ang-(1-7) was not affected by the AT2 receptor selective antagonist PD123319 (from 10(-10) to 10(-7) M) but was blocked in a dose-dependent manner by the AT1 receptor selective antagonists losartan (10(-10) M), candesartan (10(-17) M), irbesartan (2 x 10(-12) M) and telmisartan (2 x 10(-16) M). The signaling pathway triggered by stimulation of the AT(1) receptor was also investigated. The PI-phospholipase C (PI-PLC) inhibitor U73122 (5 x 10(-8) M) blocked the inhibitory effect elicited by Ang-(1-7). Involvement of the protein kinase C (PKC) was evidenced by the sensitivity of the inhibitory effect of Ang-(1-7) to calphostin C (6.32 x 10(-7) M) and the lack of additive effects when the cells were co-incubated with Ang-(1-7) and 3.2 x 10(-8) M PMA. Altogether, these results demonstrate that Ang-(1-7) inhibits the (Na(+)+K+)ATPase activity of the prototypic distal tubule cell MDCK through the AT1 receptor-mediated stimulation of PI-PLC/PKC signaling pathway.

Development of an HTS Assay for Na+, K+-ATPase Using Nonradioactive Rubidium Ion Uptake

A high-throughput screening (HTS) assay was developed for the Na(+),K(+)-ATPase channel in order to study rubidium uptake as a measure of the functional activity and modulation of this exchanger. The assay uses elemental rubidium as a tracer for K(+) ions. Three cell lines were used to study the exchanger, and the assay was performed in a 96-well microtiter plate format. Rb(+) uptake was carried by the CHO-K1 cells at 37 degrees C; the maximum ion influx was at 80 min of incubation of the cell line in the medium containing 5.4 mM RbCl. The cells were incubated in Rb(+) uptake buffer (5.4 mM) and with the pump blocker ouabain for 1, 2, and 3 h, respectively. A complete block of the Rb(+) uptake was observed with a 5 mM concentration of ouabain for all the three time intervals. The ouabain 50% inhibitory concentration (IC(50)) value for CHO-K1 cell line ATPase was observed to be 298 microM after 3 h of incubation. In addition, IC(50) values of 94 and 89 microM were observed at 30 min of incubation, indicating that the protocol shows reproducible results. A Z' factor higher than 0.7 was observed in the assays. These studies extend the profile of Na(+),K(+)-ATPases and demonstrate the feasibility of this HTS assay system to screen for compounds that pharmacologically modulate the function of Na(+),K(+)-ATPase.

Cellular mechanisms in sympatho-modulation of the heart

Cardiovascular function relies on complex servo-controlled regulation mechanisms that involve both fast-acting feedback responses and long-lasting adaptations affecting the gene expression. The adrenergic system, with its specific receptor subtypes and intracellular signalling cascades provides the major regulatory system, while the parasympathetic system plays a minor role. At the molecular level, Ca(2+) acts as the general signal trigger for the majority of cell activities including contraction, metabolism and growth. During recent years, important new results have emerged allowing an integrated view of how the multifarious Ca(2+)-signalling mechanisms transmit adrenergic impulses to intracellular target sites. These insights into cellular and molecular mechanisms are pivotal in improving pharmacological control of the sympathetic responses to surgical trauma and perioperative stress. They are examined in detail in this review, with particular emphasis being given to the differences in intracellular signalling between cardiomyocytes and vascular smooth muscle cells.

The carboxy terminus of the colonic H+,K+-ATPase α-subunit is required for stable β subunit assembly and function

BACKGROUND

The present experiments were designed to study the importance of the carboxy-terminus of colonic H(+), K(+)-ATPase alpha-subunit (HKalpha(2)), for both function as well as integrity of assembly with beta1-Na(+), K(+)-ATPase.

METHODS

For this purpose, a mutation of 84 amino acids in the carboxy-terminus was created (DeltaHKalpha(2)) and HEK-293 cells were used as expression systems for functional studies using (86)Rb(+)-uptake, coimmunoprecipitation using specific antibodies and fluorescence microscopy using green fluorescent protein.

RESULTS

The results demonstrate that comparable levels of expression of HKalpha(2) and DeltaHKalpha(2) mRNA were observed when cells were cotransfected with beta1 subunit. However, the abundance of expression of full length HKalpha(2) protein exceeded that of the truncated protein DeltaHKalpha(2). Ouabain-sensitive (86)Rb(+)-uptake was present only in cells cotransfected with HKalpha(2)/beta(1), indicating that the mutation was incapable of sustaining functionality. Coimmunoprecipitation experiments demonstrated that HKalpha(2) protein was immunoprecipitated more abundantly than DeltaHKalpha(2) when coexpressed with beta1. The use of sucrose gradients and green fluorescence protein immunofluorescence demonstrated that while the DeltaHKalpha(2)/beta(1) complex was confined to the endoplasmic reticulum, the HKalpha(2)/beta(1) complex translocated to the plasma membrane.

CONCLUSION

Taken together, our results are consistent with the view that the carboxy-terminus of HKalpha(2) facilitates the proper folding of the HKalpha(2)/beta(1) complex allowing translocation of the heterodimer to the plasma membrane where potassium uptake occurs. Otherwise, the alpha/beta complex is destined for degradation.

Coordinated PKA and PKC phosphorylation suppresses RXR-mediated ER retention and regulates the surface delivery of NMDA receptors

Endoplasmic reticulum (ER) retention mediated by the RXR (Arg-X-Arg) motif is an important quality control mechanism used by G-protein coupled receptors and ion channels, including N-methyl-D-aspartate (NMDA) receptors, to ensure the proper assembly and trafficking of multimeric complexes. During assembly, RXR motifs are masked by intersubunit interactions thereby allowing ER release. Here, we find that PKA and PKC phosphorylation sites flanking the RXR motif of the NMDA receptor NR1 subunit suppress ER retention and regulate receptor forward trafficking. These sites are differentially phosphorylated during the trafficking of NR1 subunits in vivo, and phosphorylation at these sites occurs in early secretory compartments. In addition, residues near the RXR motif not involved in phosphorylation are also required for ER retention. These results indicate that ER retention of NMDA receptors is tightly regulated, and suggest that coordinated phosphorylation by PKA and PKC mediates release of receptors from the ER for subsequent traffic to synapses. Phosphorylation-induced ER export of RXR-containing channels and receptors may serve as a novel quality control mechanism for creating a readily releasable pool of receptors sensitive to the activation of intracellular signaling pathways.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin (Cx) protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of Cx40 and Cx43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.

Ouabain exerts biphasic effects on connexin functionality and expression in vascular smooth muscle cells

1. We have compared the effects of ouabain on the maintenance of gap junctional communication in rat aortic A7r5 smooth muscle cells, monkey COS-1 fibroblasts and human HeLa epithelial cells. 2. Ouabain (1 mM) interrupted dye coupling between confluent A7r5 cells within approximately 1 h, and high concentrations of ouabain were similarly required to reduce coupling between COS-1 cells selected to express the rat alpha1 Na+/K+-ATPase subunit, which is ouabain resistant. By contrast, low concentrations of ouabain (1-10 microM) attenuated dye transfer in wild-type COS-1 and HeLa cells, whose endogenous alpha1 subunits possess relatively high affinity for the glycoside (Ki approximately 0.3 vs approximately 100 microM) Ouabain-induced reductions in dye transfer therefore correlated with the ability of the glycoside to bind to the Na+/K+-ATPase isoenzymes expressed in these different cell lines. 3. No consistent relationship between inhibition of intercellular dye transfer and secondary changes in [Ca2+]i or pHi could be identified following incubation with ouabain. 4. In separate experiments, the effects of ouabain on real-time trafficking of connexin protein were monitored by time-lapse microscopy of A7r5 cells transfected to express a fluorescent Cx43-green fluorescent protein (GFP) and the ability of the glycoside to modulate endogenous expression of connexins (Cx) 40 and 43 evaluated in A7r5 cells by immunochemical and Western blot analysis. 5. Ouabain (1 mM) depressed vesicular trafficking of Cx43-GFP after approximately 1 h, and caused a time-dependent loss of endogenous Cx40 and Cx43 protein that was first evident at 2 h and almost complete after 4 h. These effects of ouabain on Cx expression were reversed approximately 90 min following washout of the glycoside. 6. We conclude that ouabain exerts biphasic effects on the intercellular communication that involve an initial decrease in gap junctional permeability followed by a global reduction in the expression of Cx protein. Further studies are necessary to establish to what extent these actions of ouabain reflect inversion of the normal [Na+]i/[K+]i ratio and/or conversion of the Na+/K+-ATPase into a general signal transducer that regulates downstream protein synthesis.