Repolarization of Na+-K+ pumps during establishment of epithelial monolayers

The Apical Localization of Na+, K+-ATPase in Cultured Human Retinal Pigment Epithelial Cells Depends on Expression of the β2 Subunit

Na⁺, K⁺-ATPase, or the Na⁺ pump, is a key component in the maintenance of the epithelial phenotype. In most epithelia, the pump is located in the basolateral domain. Studies from our laboratory have shown that the β₁ subunit of Na⁺, K⁺-ATPase plays an important role in this mechanism because homotypic β₁-β₁ interactions between neighboring cells stabilize the pump in the lateral membrane. However, in the retinal pigment epithelium (RPE), the Na⁺ pump is located in the apical domain. The mechanism of polarization in this epithelium is unclear. We hypothesized that the apical polarization of the pump in RPE cells depends on the expression of its β₂ subunit. ARPE-19 cells cultured for up to 8 weeks on inserts did not polarize, and Na⁺, K⁺-ATPase was expressed in the basolateral membrane. In the presence of insulin, transferrin and selenic acid (ITS), ARPE-19 cells cultured for 4 weeks acquired an RPE phenotype, and the Na⁺ pump was visible in the apical domain. Under these conditions, Western blot analysis was employed to detect the β₂ isoform and immunofluorescence analysis revealed an apparent apical distribution of the β₂ subunit. qPCR results showed a time-dependent increase in the level of β₂ isoform mRNA, suggesting regulation at the transcriptional level. Moreover, silencing the expression of the β₂ isoform in ARPE-19 cells resulted in a decrease in the apical localization of the pump, as assessed by the mislocalization of the α₂ subunit in that domain. Our results demonstrate that the apical polarization of Na⁺, K⁺-ATPase in RPE cells depends on the expression of the β₂ subunit.

21-Benzylidene digoxin: a proapoptotic cardenolide of cancer cells that up-regulates Na,K-ATPase and epithelial tight junctions

Cardiotonic steroids are used to treat heart failure and arrhythmia and have promising anticancer effects. The prototypic cardiotonic steroid ouabain may also be a hormone that modulates epithelial cell adhesion. Cardiotonic steroids consist of a steroid nucleus and a lactone ring, and their biological effects depend on the binding to their receptor, Na,K-ATPase, through which, they inhibit Na+ and K+ ion transport and activate of several intracellular signaling pathways. In this study, we added a styrene group to the lactone ring of the cardiotonic steroid digoxin, to obtain 21-benzylidene digoxin (21-BD), and investigated the effects of this synthetic cardiotonic steroid in different cell models. Molecular modeling indicates that 21-BD binds to its target Na,K-ATPase with low affinity, adopting a different pharmacophoric conformation when bound to its receptor than digoxin. Accordingly, 21-DB, at relatively high µM amounts inhibits the activity of Na,K-ATPase α1, but not α2 and α3 isoforms. In addition, 21-BD targets other proteins outside the Na,K-ATPase, inhibiting the multidrug exporter Pdr5p. When used on whole cells at low µM concentrations, 21-BD produces several effects, including: 1) up-regulation of Na,K-ATPase expression and activity in HeLa and RKO cancer cells, which is not found for digoxin, 2) cell specific changes in cell viability, reducing it in HeLa and RKO cancer cells, but increasing it in normal epithelial MDCK cells, which is different from the response to digoxin, and 3) changes in cell-cell interaction, altering the molecular composition of tight junctions and elevating transepithelial electrical resistance of MDCK monolayers, an effect previously found for ouabain. These results indicate that modification of the lactone ring of digoxin provides new properties to the compound, and shows that the structural change introduced could be used for the design of cardiotonic steroid with novel functions.

Expression of transcripts related to intestinal ion and nutrient absorption in pregnant and lactating rats as determined by custom-designed cDNA microarray

In pregnancy and lactation, maternal adaptation for the enhancement of intestinal ion and nutrient absorption is of paramount importance for fetal development and lactogenesis. This nutrient hyperabsorption has been reported to result from upregulation of transporter gene expression, in part, under control of lactogenic hormone prolactin (PRL). Since a number of gene families are responsible for ion and nutrient transport in the rat small intestine, we herein developed a custom-designed cDNA microarray (CalGeneArray) to determine the transcriptome responses of duodenal epithelial cells during these reproductive periods, which was subsequently validated by quantitative real-time PCR. We thus designed 277 oligonucleotide probes to detect 113 transcripts related to ion/nutrient transport, bone/calcium metabolism, paracrine regulator, and cell metabolism. Pregnancy was found to upregulate the expressions of several duodenal transporters, e.g., Trpm6, Trpm7, Glut5, and Trpv6. Pregnant rats subjected to 7-day injection of bromocriptine, an inhibitor of PRL release, showed the increased levels of some other transcripts, e.g., insulin-2 and Cyp27b1, compared to untreated pregnant rats. Bromocriptine also increased the mRNA levels of insulin-2, glucose transporter-1 (Sglt1), and Cyp27b1, while decreasing those of Fgfr2c, Atp1b2, and Cldn19 in early lactation. During late lactation, the levels of eight studied transcripts (i.e., NaPi-IIb, Cyp27b1, Cldn18, Casr, Atp1b2, Xpnpep, Pept1, and Trpm7) were altered. In conclusion, the CalGeneArray was powerful to help reveal that pregnancy and lactation modulated the expression of genes related to duodenal nutrient transport and cell metabolism. Our findings supported the physiological significance of PRL in regulating nutrient absorption during pregnancy and lactation.

Atelectrauma disrupts pulmonary epithelial barrier integrity and alters the distribution of tight junction proteins ZO-1 and claudin 4

Mechanical ventilation inevitably exposes the delicate tissues of the airways and alveoli to abnormal mechanical stresses that can induce pulmonary edema and exacerbate conditions such as acute respiratory distress syndrome. The goal of our research is to characterize the cellular trauma caused by the transient abnormal fluid mechanical stresses that arise when air is forced into a liquid-occluded airway (i.e., atelectrauma). Using a fluid-filled, parallel-plate flow chamber to model the "airway reopening" process, our in vitro study examined consequent increases in pulmonary epithelial plasma membrane rupture, paracellular permeability, and disruption of the tight junction (TJ) proteins zonula occludens-1 and claudin-4. Computational analysis predicts the normal and tangential surface stresses that develop between the basolateral epithelial membrane and underlying substrate due to the interfacial stresses acting on the apical cell membrane. These simulations demonstrate that decreasing the velocity of reopening causes a significant increase in basolateral surface stresses, particularly in the region between neighboring cells where TJs concentrate. Likewise, pulmonary epithelial wounding, paracellular permeability, and TJ protein disruption were significantly greater following slower reopening. This study thus demonstrates that maintaining a higher velocity of reopening, which reduces the damaging fluid stresses acting on the airway wall, decreases the mechanical stresses on the basolateral cell surface while protecting cells from plasma membrane rupture and promoting barrier integrity.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

The polarized distribution of Na+,K+-ATPase: role of the interaction between {beta} subunits

Na⁺,K⁺-ATPase polarity depends on the interaction between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells. In the present work, we use energy transfer methods (FRET), in vivo to demonstrate that these β subunits interact directly at the intercellular space of epithelial cells.

The very existence of higher metazoans depends on the vectorial transport of substances across epithelia. A crucial element of this transport is the membrane enzyme Na⁺,K⁺-ATPase. Not only is this enzyme distributed in a polarized manner in a restricted domain of the plasma membrane but also it creates the ionic gradients that drive the net movement of glucose, amino acids, and ions across the entire epithelium. In a previous work, we have shown that Na⁺,K⁺-ATPase polarity depends on interactions between the β subunits of Na⁺,K⁺-ATPases located on neighboring cells and that these interactions anchor the entire enzyme at the borders of the intercellular space. In the present study, we used fluorescence resonance energy transfer and coprecipitation methods to demonstrate that these β subunits have sufficient proximity and affinity to permit a direct interaction, without requiring any additional extracellular molecules to span the distance.

Potassium Channels Lost During Harvesting of Epithelial Cells are Restored with a Kinetics that Depends on Channel Species

The polarized distribution of K(+) channels in MDCK cells is lost upon harvesting and restored upon re-seeding. Using semi-quantitative PCR, in the present work we find that (i) Cells do not "wait" for the normal recycling of membrane proteins to restore their lost channels, but trigger their replacement, suggesting that the membrane has a way of engaging the nucleus. (ii) Replacement channels do not come from an internal reservoir, as it is the case with Na(+), K(+)-ATPase, but requires a de novo synthesis. (iii) Replacement is not an all-or-none response, since mRNA for MaxiK channels increases by 8-fold after re-seeding, but those for Kv1.6 and Kv1.7 are not affected by harvesting/re-seeding. (iv) TEA, charybdotoxin and iberiotoxin fail to trigger the replacement response in mature monolayers, suggesting that replacement is not due to suppression of channel function. (v) MDCK cells have a typical transporting epithelial phenotype (TEP) consisting of tight junctions (TJs) plus polarity. Although the polarized distribution of K-channels is a prominent attribute of TEP, blocking their function does not perturb the development of TEP, as gauged through the development of TJs, nor level of expression (Western blot) and distribution (confocal microscopy) of occludin, and claudins 1, 3 and 7.

New diseases derived or associated with the tight junction

The space between neighboring epithelial cells is sealed by the tight junction (TJ). When this seal is leaky, such as in the proximal tubule of the kidney or the gallbladder, substances may cross the epithelium between the cells (paracellular pathway). Yet, when TJs are really hermetic, as is the case in the epithelium of the urinary bladder or the colon, substances can mainly cross the epithelium through the transcellular pathway. The structure of the TJ involves (so far) some 50-odd protein species. Failure of any of these components causes a variety of diseases, some of them so serious that fetuses are not viable. A fast-growing number of diseases are recognized to depend or involve alterations in the TJ. These include autoimmune diseases, in which intestinal TJs allow the passage of antigens from the intestinal flora, challenging the immune system to produce antibodies that may cross react with proteins in the brain, thyroid gland or pancreas. TJs are also involved in cancer development, infections, allergies, etc. The present article does not catalogue all TJ diseases known so far, but describes one of each type as illustration. It also depicts the efforts being made to find pharmaceutical agents that would seal faulty TJs or release their grip to allow for the passage of large molecules through the upper respiratory and digestive tracts, such as insulin, thyroid, appetite-regulatory peptide, etc.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Prolactin is an important regulator of intestinal calcium transport

Prolactin has been shown to stimulate intestinal calcium absorption, increase bone turnover, and reduce renal calcium excretion. The small intestine, which is the sole organ supplying new calcium to the body, intensely expresses mRNAs and proteins of prolactin receptors, especially in the duodenum and jejunum, indicating the intestine as a target tissue of prolactin. A number of investigations show that prolactin is able to stimulate the intestinal calcium transport both in vitro and in vivo, whereas bromocriptine, which inhibits pituitary prolactin secretion, antagonizes its actions. In female rats, acute and long-term exposure to high prolactin levels significantly enhances the (i) transcellular active, (ii) solvent drag-induced, and (iii) passive calcium transport occurring in the small intestine. These effects are seen not only in pregnant and lactating animals, but are also observed in non-pregnant and non-lactating animals. Interestingly, young animals are more responsive to prolactin than adults. Prolactin-enhanced calcium absorption gradually diminishes with age, thus suggesting it has an age-dependent mode of action. Although prolactin's effects on calcium absorption are not directly vitamin D-dependent; a certain level of circulating vitamin D may be required for the basal expression of genes related to calcium transport. The aforementioned body of evidence supports the hypothesis that prolactin acts as a regulator of calcium homeostasis by controlling the intestinal calcium absorption. Cellular and molecular signal transductions of prolactin in the enterocytes are largely unknown, however, and still require investigation.

Contacts and cooperation between cells depend on the hormone ouabain

Cell adhesion is a crucial step in proliferation, differentiation, migration, apoptosis, and metastasis. In previous works we have shown that cell adhesion is modulated by ouabain, a highly specific inhibitor of Na+,K+-ATPase, recently found to be a hormone. In the present work we pursue the investigation of the effect of ouabain on a special type of cell-cell interaction: the rescue of ouabain-sensitive MDCK cells (W) by ouabain-resistant cells (R). In cultured monolayers of pure W cells, ouabain triggers the "P-->A mechanism" (from pump/adhesion) consisting of a cascade of phosphorylations that retrieves adhesion-associated molecules occludin and beta-catenin and results in detachment of the cell. When W cells are instead cocultured with R cells, the P-->A reaction is blocked, and W cells are rescued. Furthermore, in these R/W cocultures ouabain promotes cell-cell communication by means of gap junctions by specifically enhancing the expression of connexin 32 and addressing this molecule to the plasma membrane. Ouabain also promotes the internalization of the beta-subunit of the Na+,K+-ATPase. These observations open the possibility that the crucial processes mentioned at the beginning would be under the control of the hormone ouabain.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

Effect of Endoscopic Transthoracic Sympathicotomy on Plasma Natriuretic Peptide Concentrations in Humans

BACKGROUND

The effects of endoscopic transthoracic sympathicotomy (ETS) on plasma natriuretic peptides concentrations in humans were examined in order to elucidate the role of the sympathetic nervous system in their regulation.

METHODS AND RESULTS

Thirty-seven patients with palmar hyperhidrosis underwent ETS. Cardiac functional indices were assessed by echocardiography, and plasma atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) concentrations were measured before and after ETS. ETS caused decreases in heart rate, mean arterial pressure, systemic vascular resistance, and increases in left ventricular (LV) end-diastolic volume, stroke index, ejection fraction, and left atrial diameter. LV end-systolic volume and cardiac index remained unchanged. Following ETS, ANP increased from 10.7+/-5.9 to 24.7+/-16.8 pg/ml (p < 0.01), and BNP increased from 5.1+/-4.2 to 19.7+/-21.5 pg/ml (p < 0.01). From the multivariate regression analysis, ETS, age and gender were determined to be significant predictors of changes in the ANP and BNP concentrations. None of the hemodynamic parameters were useful as independent predictors.

CONCLUSION

The plasma concentrations of ANP and BNP increased after ETS, independent of hemodynamic changes, and apparently because of the release of the inhibitory effects of the cardiac sympathetic nerves on natriuretic peptide secretion.

The polarized expression of Na+,K+-ATPase in epithelia depends on the association between beta-subunits located in neighboring cells

The polarized distribution of Na+,K+-ATPase plays a paramount physiological role, because either directly or through coupling with co- and countertransporters, it is responsible for the net movement of, for example, glucose, amino acids, Ca2+, K+, Cl-, and CO3H- across the whole epithelium. We report here that the beta-subunit is a key factor in the polarized distribution of this enzyme. 1) Madin-Darby canine kidney (MDCK) cells (epithelial from dog kidney) express the Na+,K+-ATPase over the lateral side, but not on the basal and apical domains, as if the contact with a neighboring cell were crucial for the specific membrane location of this enzyme. 2) MDCK cells cocultured with other epithelial types (derived from human, cat, dog, pig, monkey, rabbit, mouse, hamster, and rat) express the enzyme in all (100%) homotypic MDCK/MDCK borders but rarely in heterotypic ones. 3) Although MDCK cells never express Na+,K+-ATPase at contacts with Chinese hamster ovary (CHO) cells, they do when CHO cells are transfected with beta1-subunit from the dog kidney (CHO-beta). 4) This may be attributed to the adhesive property of the beta1-subunit, because an aggregation assay using CHO (mock-transfected) and CHO-beta cells shows that the expression of dog beta1-subunit in the plasma membrane does increase adhesiveness. 5) This adhesiveness does not involve adherens or tight junctions. 6) Transfection of beta1-subunit forces CHO-beta cells to coexpress endogenous alpha-subunit. Together, our results indicate that MDCK cells express Na+,K+-ATPase at a given border provided the contacting cell expresses the dog beta1-subunit. The cell-cell interaction thus established would suffice to account for the polarized expression and positioning of Na+,K+-ATPase in epithelial cells.

Cell Adhesion, Polarity, and Epithelia in the Dawn of Metazoans

Transporting epithelia posed formidable conundrums right from the moment that Du Bois Raymond discovered their asymmetric behavior, a century and a half ago. It took a century and a half to start unraveling the mechanisms of occluding junctions and polarity, but we now face another puzzle: lest its cells died in minutes, the first high metazoa (i.e., higher than a sponge) needed a transporting epithelium, but a transporting epithelium is an incredibly improbable combination of occluding junctions and cell polarity. How could these coincide in the same individual organism and within minutes? We review occluding junctions (tight and septate) as well as the polarized distribution of Na+-K+-ATPase both at the molecular and the cell level. Junctions and polarity depend on hosts of molecular species and cellular processes, which are briefly reviewed whenever they are suspected to have played a role in the dawn of epithelia and metazoan. We come to the conclusion that most of the molecules needed were already present in early protozoan and discuss a few plausible alternatives to solve the riddle described above.

Prolactin directly stimulated the solvent drag-induced calcium transport in the duodenum of female rats

Prolactin has been reported to stimulate the calcium absorption of the duodenum where three components of the active calcium transport, namely transcellular active, voltage-dependent and solvent drag-induced calcium transport, were identified. It was known that the transcellular active, but not the voltage-dependent, duodenal calcium transport was directly stimulated by prolactin. The present study thus aimed to evaluate the direct action of prolactin on the solvent drag-induced duodenal calcium transport by using the Ussing chamber technique. The jejunum was used as a reference for the existence of solvent drag and the widening of tight junction induced by cytochalasin E. Results showed that the solvent drag-induced calcium transport existed in both intestinal segments, but the magnitude was significantly greater in the duodenum (29.27+/-2.27 vs. 17.31+/-1.65 nmol h(-1) cm(-2), P<0.001). We further demonstrated that 200, 600 and 800, but not 1000 ng/ml, prolactin significantly promoted the solvent drag-induced duodenal calcium transport in a dose-response manner, i.e. from the control value of (nmol h(-1) cm(-2)) 24.31+/-2.36 to 45.42+/-3.47 (P<0.01), 63.82+/-5.28 (P<0.001) and 53.93+/-5.41 (P<0.01), respectively. However, prolactin did not manifest any effect on the jejunum. Because the paracellular transport was suggested to be size-selective as well as charge-selective, further experiments were designed to evaluate the mechanism by which prolactin stimulated the solvent drag-induced calcium transport. The duodenum was exposed to 20 microM cytochalasin E, 600 ng/ml prolactin or the combination of both in the presence of a paracellular marker 3H-mannitol, while the jejunum was a positive reference. The results showed that, in the jejunum, cytochalasin E alone and cytochalasin E plus prolactin significantly increased the mannitol fluxes from (micromol h(-1) cm(-2)) 0.29+/-0.04 to 0.49+/-0.03 (P<0.05) and 0.48+/-0.05 (P<0.05), respectively, while having no effect on the calcium fluxes. Prolactin alone had no effect on the jejunal calcium flux. In the duodenum, neither mannitol nor calcium fluxes were enhanced by cytochalasin E, however, prolactin still increased the solvent drag-induced calcium flux from 27.74+/-2.41 to 51.03+/-4.35 nmol h(-1) cm(-2) (P<0.001). It was concluded that prolactin directly stimulated the solvent drag-induced duodenal calcium transport in a dose-response and biphasic manner without the widening of tight junction.

Ouabain Binding to Na+,K+-ATPase Relaxes Cell Attachment and Sends a SpecificSignal (NACos) to the Nucleus

Abstract. In previous work we described a "P-->A mechanism" that transduces occupancy of the pump ( P) by ouabain into changes in phosphorylation, stimulation of mitogen-activated protein kinase (MAPK), and endocytosis of cell-cell- and cell-substrate-attaching molecules ( A), thereby causing a release of the cell from the monolayer. In the present work we try to understand the mechanism of this effect; whether, in order to trigger the P-->A mechanism, ouabain should block the pumping activity of Na(+),K(+)-ATPase as pump, or whether it would suffice that the drug occupies this enzyme as a receptor. We assay a series of drugs known to act on the pump, such as ouabain, digoxin, digitoxin, palytoxin, oligomycin, strophanthidin, neothyoside-A, proscillaridin-A, etc. We gauge their ability to block the pump by measuring the K(+) content in the cells, and their ability to detach the cells from the monolayer by determining the amount of protein remaining in the culturing well. None of the drugs tested was able to cause detachment without stopping the pump. Ouabain also enhances phosphorylation, yet pump inhibition and signal transduction do not seem to be intimately associated in a causal chain, but to occur simultaneously. To investigate the response of the site of cell attachment, we analyze the position of beta-catenin by fluorescence confocal microscopy, and find that this adherent junction-associated molecule is sent to the nucleus, where it is known to act as a transcriptional cofactor.

Increased Plasma Concentration of Natriuretic Peptides by Selective β1-Blocker Bisoprolol

Beta-blocker therapy has been shown to be associated with an increase in the plasma concentration of A-type natriuretic peptide (ANP). Whether the plasma concentration of B-type natriuretic peptide (BNP), which is mainly derived from ventricular tissue, is also increased and whether this increase is caused by increased production or decreased metabolism by down-regulation of the natriuretic peptide-clearance receptor remains to be established. In a double-blind crossover study effects of 8 weeks' treatment of bisoprolol,10 mg once daily, and losartan, 50 mg once daily, on plasma concentrations of ANP, BNP, and N-terminal (Nt)-ANP and ambulatory blood pressure (ABP) were measured in 24 hypertensive patients. With bisoprolol plasma concentrations of ANP and BNP increased (P < 0.001) by 93 +/- 88% (mean +/- SD) and 148 +/- 117%, whereas these parameters did not change with losartan. Nt-ANP, which is not metabolized by the NP clearance receptor, increased by 83 +/- 45%, and increments in ANP and Nt-ANP were related (r =0.77, P < 0.001). The decrease in ABP was greater with bisoprolol than with losartan. Monotherapy with bisoprolol, but not with losartan, is associated with substantial increments in plasma concentrations of ANP, Nt-ANP, and BNP. As the magnitude of the increase in ANP and Nt-ANP was comparable, the beta-blocker-induced increase in NPs is not likely to be explained by a decrease in NP clearance receptor density.

Melatonin induced cyclic modulation of vectorial water transport in kidney-derived MDCK cells

BACKGROUND

Melatonin, newly synthesized by the pineal gland, is rapidly released to general circulation reaching a nanomolar concentration. Cyclic production of melatonin synchronizes body rhythms with the photoperiod. Moreover, changes in urine production and osmolarity have been observed in the kidney during the night. However, the precise mechanisms by which plasma-circulating melatonin modifies renal physiology are not clearly understood.

METHODS

Madin-Darby canine kidney (MDCK) cell monolayers transport water vectorially from the apical to the basolateral side forming blisters or domes. Transport in epithelial cells is regulated by tight junction sealing, ion pumps and channels, and cytoskeleton organization, among other processes. MDCK cells were used to study vectorial water transport to determine the role of microfilament organization and protein kinase C (PKC) in dome formation in culture conditions that mimic the cyclic pattern of melatonin circulation in plasma.

RESULTS

Melatonin cyclically increased dome formation by 50% and caused enlargement and thickening of stress fibers in cells surrounding the domes. Optimal increase in dome formation was observed at nanomolar concentrations of melatonin after 6 hours, concomitantly with a 28% decrease in the transepithelial electrical resistance, which remained low for up to 12 hours, without apparent change in fluorescein isothiocyanate (FITC)-dextran flux. A blockage in dome formation elicited by melatonin was observed in monolayers preincubated with the Na+-K+-ATPase or PKC inhibitors.

CONCLUSION

The results obtained indicate that melatonin cyclically modifies the transepithelial permeability in kidney-derived cells through PKC activation and microfilament reorganization, and supports the hypothesis that melatonin may synchronize daily body rhythms through cyclic cytoskeletal rearrangements.

Molecular physiology and pathophysiology of tight junctions. I. Biogenesis of tight junctions and epithelial polarity

The tight junction (TJ) was first noticed through its ability to control permeation across the paracellular route, but the homologies of its molecular components with peptides that participate in tumor suppression, nuclear addressing, and cell proliferation indicate that it may be involved in many other fundamental functions. TJs are formed by a dozen molecular species that assemble through PDZ and other protein-protein clustering promoting sequences, in response to the activation of E-cadherin. The TJ occupies a highly specific position between the apical and the basolateral domains. Its first molecular components seem to be delivered to such a position by addressing signals in their molecule and, once anchored, serve as a clustering nucleus for further TJ-associated molecules. Although in mature epithelial cells TJs and E-cadherin do not colocalize, a complex chain of reactions goes from one to the other that involves alpha-, beta-, and gamma-catenins, two different G proteins, phospholipase C, protein kinase C, calmodulin, mitogen-activated protein kinase, and molecules pertaining to the cytoskeleton, which keep the TJ sensitive to physiological requirements and local conditions (notably to Ca(2+)-dependent cell-cell contacts) throughout the life of the epithelium.

Nitric oxide activates PKCalpha and inhibits Na+-K+-ATPase in opossum kidney cells

Nitric oxide (NO) reduces the molecular activity of Na+-K+-ATPase in opossum kidney (OK) cells, a proximal tubule cell line. In the present study, we investigated the cellular mechanisms for the inhibitory effect of NO on Na+-K+-ATPase. Sodium nitroprusside (SNP), a NO donor, inhibited Na+-K+-ATPase in OK cells, but not in LLC-PK1 cells, another proximal tubule cell line. Similarly, phorbol 12-myristate 13-acetate, a protein kinase C (PKC) activator, inhibited Na+-K+-ATPase in OK, but not in LLC-PK1, cells. PKC inhibitors staurosporine or calphostin C, but not the protein kinase G inhibitor KT-5823, abolished the inhibitory effect of NO on Na+-K+-ATPase in OK cells. Immunoblotting demonstrated that treatment with NO donors caused significant translocation of PKCalpha from cytosolic to particulate fractions in OK, but not in LLC-PK1, cells. Furthermore, the translocation of PKCalpha in OK cells was attenuated by either the phospholipase C inhibitor U-73122 or the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one. U-73122 also blunted the inhibitory effect of SNP on Na+-K+-ATPase in OK cells. The phospholipase A2 inhibitor AACOCF3 did not blunt the inhibitory effect of SNP on Na+-K+-ATPase in OK cells. AACOCF3 alone, however, also decreased Na+-K+-ATPase activity in OK cells. In conclusion, our results demonstrate that NO activates PKCalpha in OK, but not in LLC-PK1, cells. The activation of PKCalpha in OK cells by NO is associated with inhibition of Na+-K+-ATPase.

Relationship between Na(+),K(+)-ATPase and cell attachment

A prolonged ouabain blockade of the Na(+),K(+)-ATPase detaches cells from each other and from the substrate. This suggests the existence of a link between pump (P) and attachment (A). In the present work, we report that MDCK-W cells treated with ouabain increase tyrosine phosphorylation and content of active MAP kinase, redistribute molecules involved in cell attachment (occludin, ZO-1, desmoplakin, cytokeratin, alpha-actinin, vinculin and actin), and detach. Genistein and UO126, inhibitors of protein tyrosine kinase and of MAP kinase kinase, respectively, block this detachment. The content of P190(Rho-GAP), a GTPase activating protein of the Rho small G-protein subfamily, is increased by ouabain, suggesting that both the Rho/Rac and MAPK pathways are involved. Another clone of MDCK cells whose Na(+),K(+)-ATPase has a negligible affinity for the drug, show none of the effects described for MDCK-W and remain attached. Ma104 cells, a line that has a high affinity for ouabain and stops pumping, fail to modify phosphorylation, as well as the pattern of distribution of attaching molecules, and remain in the monolayer. Taken together, these results suggest that there is a mechanism (P--&gt;A) that transduces a blockade of the pump in a detachment of the cell from neighbors and substrate, in which Ma104 cells are faulty.

Nitric oxide reduces the molecular activity of Na+,K+-ATPase in opossum kidney cells

BACKGROUND

Nitric oxide (NO) directly inhibits fluid and solute reabsorption in the proximal tubule. In the present study, we investigated the effect of NO on the Na+, K+-ATPase of opossum kidney (OK) cells, a proximal tubule cell line, and its mechanisms.

METHODS

Na+,K+-ATPase activity in the membrane fraction of OK cells was measured as the ouabain-sensitive ATP hydrolytic activity. The enzyme unit number on intact cells was measured by ouabain-binding assay.

RESULTS

Incubation with 0.5 mM sodium nitroprusside (SNP), a NO donor, for two hours inhibited the catalytic activity of the membrane-associated Na+,K+-ATPase in OK cells to 65.5 +/- 9.7% of control (N = 6, P < 0.05 vs. control). This effect of SNP was concentration- and time-dependent. The NO scavenger hemoglobin blunted, while another NO donor spermine NONOate (5 microM) mimicked this effect of SNP. At all concentrations and time points tested, SNP did not alter the molecular number of Na+,K+-ATPase on intact OK cells, indicating that NO inhibited the molecular activity of Na+,K+-ATPase. The soluble guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo-[4, 3-a]quinoxalin-1-one (ODQ), blunted the inhibitory effect of SNP on the Na+,K+-ATPase activity. An exogenous cGMP analog similarly inhibited the Na+,K+-ATPase activity. Neither lipid soluble antioxidants vitamin E/probucol or thiol group compound DL-dithiothreitol (DTT) altered the inhibitory effect of SNP on the Na+,K+-ATPase activity.

CONCLUSIONS

NO inhibited the molecular activity of the Na+,K+-ATPase of the OK proximal tubule cell line probably via cGMP-dependent mechanisms.

The cytoplasmic domains of a beta1 integrin mediate polarization in Madin-Darby canine kidney cells by selective basolateral stabilization

In Madin-Darby canine kidney cells, newly synthesized apical and basolateral membrane proteins are generally transported directly to their respective cell surface domain due to targeting determinants that mediate sorting in the Golgi complex. In several basolateral membrane proteins, these targeting determinants reside in the cytoplasmic domains. We show here that basolateral expression of the human alpha5beta1 integrin in stably transfected Madin-Darby canine kidney cells is also mediated by the cytoplasmic domains. Distinct regions in both cytoplasmic domains were found to be sufficient to mediate basolateral expression independently from one another. Unexpectedly, newly synthesized wild-type alpha5beta1 and basolaterally expressed chimeras containing the cytoplasmic domain of either alpha5 or beta1 were integrated into both cell surface domains, preferentially apically, during biosynthesis. The apical pools of wild-type integrin and chimeric subunits were found to become quickly degraded, whereas the basolateral pools were stabilized. Thus, the cytoplasmic domains of the alpha5beta1 integrin are independently sufficient to mediate sorting by selective basolateral stabilization.

Role of tight junctions in establishing and maintaining cell polarity

The tight junction (TJ) is not randomly located on the cell membrane, but occupies a precise position at the outermost edge of the intercellular space and, therefore, is itself considered a polarized structure. This article reviews the most common experimental approaches for studying this relationship. We then discuss three main topics. (a) The mechanisms of polarization that operate regardless of the presence of TJs: We explore a variety of polarization mechanisms that operate at stages of the cell cycle in which TJs may be already established. (b) TJs and polarity as partners in highly dynamic processes: Polarity and TJs are steady state situations that may be drastically changed by a variety of signaling events. (c) Polarized distribution of membrane molecules that depend on TJs: This refers to molecules (mainly lipids) whose polarized distribution, although not the direct result of TJs, depends on these structures to maintain such distribution.

Cellular mechanism of angiotensin II-induced atrial natriuretic peptide release in rat right atrial tissue

This study presents an investigation of the mechanism of angiotensin II (Ang II)-induced atrial natriuretic peptide (ANP) release in superfused sliced right atria of rats. Ang II (0.1 microM) enhanced ANP release by 49%. This phenomenon was significantly blocked by (Sara1-Ileu 8) Ang II (1 microM) and losartan (0.1 microM). The use of neomycin (100 microM), a phospholipase-C inhibitor completely suppressed the effect of Ang II on ANP increase. To elucidate the intracellular mechanism of ANP released by Ang II, the role of protein kinase C (PKC) was determined by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and phorbol ester : 4-beta-phorbol 12-myristate-13-acetate (PMA). We observed that PMA (0.1 microM) stimulated ANP release whereas H-7 (10 microM), an inhibitor of PKC in the presence of Ang II, prevented ANP increase. The role of calcium was also evaluated with 8-(N-N-diethylamino)-ocytyl-3,4,5, trimethoxy-benzoate (TMB-8) (10 microM) and N-(6-aminohexyl)-5-chloro-1-naphtalene sulfonamide (W-7) (10 microM), which completely inhibited ANP release by Ang II. Pre-treatment with diltiazem (10 microM), an antagonist of the Ca++ channel, did not prevent ANP increase due to Ang II, but A23187 (5 microM) enhanced ANP release by Ang II. These results suggest that PKC and intracellular calcium play an important role in ANP release under the influence of Ang II in rat atrial tissue.

Properties and regulation of ion channels in MDCK cells

The MDCK cell has proven to be a useful model cell line for the study of properties and regulation of renal epithelial ion channels. Patch clamp studies disclosed the existence of several K+ channels and of a Cl- channel, and their regulation by hormones, cell volume, trace elements and drugs. Most hormones affect K+ channels at least in part by increasing cytosolic Ca2+. However, indirect evidence points to additional mechanisms contributing to K+ channel activation. Cell swelling activates both K+ channels and unselective anion channels. ICln, a protein cloned from MDCK cells, is either a Cl- channel or a regulator of thereof. ICln is up-regulated by cellular acidification and is crucial for rapid regulatory cell volume decrease.

Effects of angiotensin II on plasma atrial natriuretic factor in nonpregnant and pregnant ewes

The release of atrial natriuretic factor (ANF) is primarily determined by atrial stretch, but may also be modulated by circulating angiotensin II (AngII). During pregnancy, the circulating concentrations of both ANF and AngII are increased. To further define possible effects of AngII on ANF release, four doses of AngII (0.5, 5, 20, 40 ng.kg-1.min-1) were intravenously infused into five nonpregnant and five pregnant (105-140 days of gestation) ewes alone and during the simultaneous infusion of sodium nitroprusside at doses sufficient to abolish the pressor effects of AngII. Mean arterial pressure (MAP) was increased from 80 +/- 2 to a maximum of 121 +/- 5 mmHg (1 mmHg = 133.3 Pa) in nonpregnant ewes (p < 0.01) and from 79 +/- 2 to 116 +/- 4 mmHg in pregnant ewes (p < 0.01) over the range of AngII infusion. MAP was unaltered during AngII plus nitroprusside infusion, averaging 78 +/- 3 mmHg in nonpregnant ewes and 80 +/- 2 mmHg in pregnant ewes. Basal ANF was higher (p < 0.01) in pregnant sheep than in nonpregnant sheep. With AngII infusion alone, plasma ANF was increased from 13 +/- 2 to 42 +/- 4 fmol/microL in nonpregnant ewes (p < 0.01) and from 23 +/- 5 to 72 +/- 16 fmol/microL in pregnant ewes (p < 0.01). However, during AngII plus nitroprusside infusion, the increases in plasma ANF observed were completely abolished in both nonpregnant and pregnant ewes.(ABSTRACT TRUNCATED AT 250 WORDS)

Epithelial cell polarity in early Xenopus development

The Xenopus blastula consists of two morphologically distinct cell types. Polarized epithelial cells build up the embryonic surface and fence off an inner non-polarized cell population. We examined the establishment of this early functional cell diversification in the embryo by single cell analysis, in vitro cell culture, and transplantation experiments. Single blastomeres from a 64-cell embryo (1/64 cells) exhibit several features of polarized cells. The plasma membrane of 1/64 cells consists of an apical domain, which is inherited from the original egg membrane, and a basolateral domain derived from newly formed membrane during cleavage. These are inherent, cell-autonomous properties of the blastomeres, as they form and are maintained in blastomeres raised in the absence of any cell interactions in calcium free medium. Upon in vitro culture a single 1/64 cell gives rise to an aggregate of two different cell types. Cells carrying a part of the former egg membrane domain differentiate into polarized epithelial cells, whereas cells lacking this membrane domain are not polarized. These results demonstrate that the inclusion of the egg membrane, rather than external signals related to the position of a cell in the intact embryo, is required for the apical/basolateral differentiation of the surface epithelium. This view is supported by cell transplantation studies. A single 1/64 cell was implanted into the blastocoel of a stage 8 blastula embryo. The progeny of the implanted cell proliferate within the host embryo and split into two morphologically distinct populations with different cell behaviours. Cells incorporating a part of the egg membrane form coherent patches of polarized epithelial cell sheets in the interior of the host embryo. In contrast, cells lacking egg membrane do not exhibit any characteristics of polarized cells and eventually spread into different regions of the host embryo. Our results show that the egg membrane and/or components of the submembrane cortex play a determinative role in the formation of the blastula epithelium.

Polarized distribution and delivery of plasma membrane proteins in thyroid follicular epithelial cells

Thyroid follicular cells coordinate several oppositely located surface enzyme activities. Recent studies have raised questions about the basic mechanisms used to achieve thyroid surface polarity. We investigated these mechanisms in primary thyroid epithelial monolayers cultured on porous filters. In the steady state, most Na+/K(+)-ATpase and aminopeptidase N were available for surface biotinylation, and these proteins exhibited physiological distributions (basolateral and apical, respectively). Glycosylphosphatidylinositol-anchored proteins were also apically distributed. By pulse-chase, newly synthesized transmembrane proteins exhibited polarized surface delivery that was oriented similarly to that observed at steady state. Little time elapsed between acquisition of Golgi-specific processing and cell surface arrival. Interestingly, when either newly synthesized or steady state-labeled thyroid peroxidase was similarly analyzed, only approximately 30% of the enzyme was ever detected at the cell surface. Of this, the majority was localized apically. The data suggest that most thyroid peroxidase remains intracellular in these monolayers, consistent with the possibility of intracellular iodination activity in addition to apical extracellular iodination. Nevertheless, in filter-polarized thyrocytes, most newly synthesized plasma membrane proteins appear to be sorted in the Golgi complex for direct delivery to apical and basolateral domains.

Different effects of protein kinase inhibitors on the localization of junctional proteins at cell-cell contact sites

The protein kinase inhibitor H-7 prevents the assembly of tight junctions in cultured Madin Darby Canine Kidney (MDCK) epithelial cells (Balda et al. (1991) J. Membr. Biol. 122, 193–202; Nigam et al. (1991) Biochem. Biophys. Res. Commun. 181, 548–553); however, its mechanism of action is unknown. To understand the basis of the activity of H-7 and other inhibitors we compared the effect of H-7 on the localization of proteins belonging to tight junctions and adherens-type junctions (zonula adhaerens and desmosome), and on the organization of actin microfilaments. Junction assembly was induced in MDCK cells either by the ‘Ca2+ switch’ procedure or by incubating trypsinized cells at normal extracellular Ca2+, and the cells were then immunofluorescently labeled with antibodies against cingulin, ZO-1, E-cadherin and desmoplakin, and with FITC-phalloidin. Here we show by measuring the transepithelial resistance that, in addition to H-7, H-8 and staurosporine can also significantly block the assembly of tight junctions, whereas HA1004 is poorly active. H-7 inhibited the accumulation of cingulin and ZO-1 in junctional areas most effectively when added during assembly at normal extracellular Ca2+. On the other hand, H-7 did not have major effects on the accumulation of E-cadherin and desmoplakin in the regions of cell-cell contact using either assembly protocol. Electron microscopy confirmed that H-7 does not abolish the formation of adherens-type junctions, suggesting that phosphorylation plays a different role in the assembly of tight junctions versus adherens-type junctions. Finally, in both protocols of junction assembly H-7 caused a major disorganization of actin microfilaments, suggesting that H-7 may prevent TJ assembly through its effect on the cytoskeleton.

Atrial natriuretic peptide in plasma after prolonged physical strain, energy deficiency and sleep deprivation

Plasma concentrations of atrial natriuretic peptide (ANP) were investigated daily in 16 male cadets during a 6-day military training course with continuous heavy physical activities, sleep and energy deficiency (course I). At the end of another similar course (course II) 15 cadets were studied during 30-min cycle exercise at 50% maximal oxygen uptake with and without glucose infusion. A small, but not significant increase was found in the plasma concentrations of ANP during course I from 9.6 (SEM 1.1) pmol.l-1 in the control experiment to 11.1 (SEM 0.5) pmol.l-1 on day 5. During course II a small but significant increase was found from 7.8 (SEM 0.5) pmol.l-1 in the control experiment to 9.1 (SEM 0.5) pmol.l-1 at the end of the course. Plasma osmolality and chloride concentration decreased during the course. During the exercise test a significant increase was seen in ANP concentration from 8.2 (SEM 0.8) to 13.1 (SEM 2.0) pmol.l-1 in the control experiment and from 9.4 (SEM 0.7) to 13.5 (SEM 1.2) pmol.l-1 during the course. This response was attenuated by glucose infusion, an effect which may have been due to an exercise induced increase in plasma chloride concentration being abolished. In contrast, the potassium concentration response to exercise was increased during the course but unaffected by glucose infusion. In conclusion, the large increases in endogenous plasma catecholamine concentration shown to take place during previous courses were not reflected in the plasma concentrations of ANP, indicating only a moderate cardiac stress or no cardiac work overload during such courses.

Water handling in Caco-2 cells: effects of acidification of the medium

Caco-2 cells were cultured on permeable supports. At confluence the minute-by-minute net water movement (Jw) was automatically recorded. Simultaneously, unidirectional [14C]mannitol, 22Na+, and/or 36Cl- fluxes and transepithelial resistances were measured. The water and mannitol permeabilities went progressively down between 9 and 16 days after seeding and then stabilized. In this last condition the hydrostatic permeability coefficient (Phydr) was 2.67 +/- 0.31 cm s-1 while the osmotic permeability coefficient (Posm) was 0.0017 +/- 0.0004 cm s-1. Phydr but not Posm was dependent on the temperature and on the presence of Na+ in the medium. A net secretory Jw was observed 16 days after seeding, in the absence of any osmotic, hydrostatic or chemical gradient. This secretory Jw was associated with net Cl- (1.43 +/- 0.43 muequiv h-1 cm-2) and Na+ (1.05 +/- 0.35 muequiv h-1 cm-2) secretions. Amiloride reduced, in open-circuit conditions, both Na+ and Cl- apical to basal fluxes, thus enhancing the net Na+ and Cl- exit. Acidification of the medium (pH 6.2) reversibly increased water and mannitol permeabilities in 10-day-old cultures. In 16-day-old cultures the same shift in medium pH did not change mannitol permeability, while stimulating water secretion. These results, obtained in the absence of supracellular structures (villae, crypts) and subepithelial components (muscular, vascular and conjunctive tissues) indicate that paracellular and transport-associated water pathways are sensitive to changes in the pH of the medium in Caco-2 cell layers.

Release of atrial natriuretic factor

The cardiac atria synthesize and store a hormone termed atrial natriuretic factor (ANF). ANF is released into the systemic circulation, and the circulating 28 amino acid peptide can be measured by radioimmunoassay. The hormone participates in body fluid homeostasis through its effect on renal sodium excretion and by inducing a shift of circulating fluid to the interstitial space. Release of ANF is mainly regulated by mechanical changes in the left and right atrial wall. It has been demonstrated that ANF release is related to changes in atrial wall tension occurring during each atrial cycle, and therefore, release of ANF will increase with increasing heart rate. Not only the increase in wall tension during passive atrial distension (v wave), but also the increase in tension during atrial systole (a wave) are determinants of ANF release. The mechanochemical transducer is most likely located in the atrial myocytes, but its nature is unknown. There is no evidence to suggest that efferent cardiac nerves are essential in the regulation of ANF release. Humoral factors have been suggested as regulators of ANF release, particularly catecholamines and angiotensin II. A receptor-mediated direct stimulatory effect of alpha-adrenergic stimulation and an inhibitory effect of beta-adrenergic stimulation have been demonstrated, but these direct effects are small compared to the effect of changes in atrial wall tension. Circulating catecholamines and angiotensin II stimulate ANF release mainly through their haemodynamic effects.

Basolateral distribution of fibronectin matrix assembly sites on vascular endothelial monolayers is regulated by substratum fibronectin

Endothelial cells exhibit binding sites for the amino terminus of fibronectin that participate in subendothelial fibronectin matrix assembly. These binding sites, termed matrix assembly sites, are localized on the basolateral surface of confluent endothelial monolayers (Kowalczyk et al. Blood, 75:2335, 1990). The present study investigates the role of cell-cell and cell-substratum interactions in the localization of matrix assembly sites to the basal surface of endothelial cells. Cells were cultured in Transwell culture inserts and matrix assembly sites were detected by binding assays using an iodinated 70 Kd amino-terminal fibronectin fragment. Integrity of intercellular junctions was monitored by measuring protein flux across Transwell filters. Time course experiments demonstrated that matrix assembly site expression on the basolateral cell surface preceded intercellular junction formation. Transfer of confluent monolayers to calcium-free medium resulted in the loss of junctions and in an increase in 125I-70 kD binding from the apical medium. The increased 125I-70 kD binding resulted from increased access of 125I-70 kD to basolateral matrix assembly sites and not from the relocation of binding sites to the apical membrane. To determine the effect of matrix composition on matrix assembly site expression and localization, cells were seeded onto vitronectin- or fibronectin-coated substrates. Fibronectin increased the expression of matrix assembly sites on the apical surface within 24 hours. By 48 hours, matrix assembly sites were located only on the basolateral surface. Vitronectin had no effect on the expression or localization of matrix assembly sites. These results indicate that the expression and localization of matrix assembly sites on the surface of vascular endothelial cells can be regulated by substratum fibronectin.

Regulation of the Na,K-ATPase activity of Madin-Darby canine kidney cells in defined medium by prostaglandin E1 and 8-bromocyclic AMP

The role of PGE1 in regulating the activity of the Na+, K(+)-ATPase in Madin Darby Canine Kidney (MDCK) cells has been examined. PGE1 increased the initial rate of ouabain-sensitive Rb+ uptake by MDCK cells, a process that continued to occur over a 5-day period. The increase in the initial rate of ouabain-sensitive Rb+ uptake in MDCK cells treated with PGE1 could be explained by a 1.6-fold increase in the Vmax for ouabain-sensitive Rb+ uptake. The increase in the Vmax for ouabain-sensitive Rb+ uptake observed in MDCK cells under these conditions can be explained either by an increase in the number of active Na+ pumps, or by an increase in the efficiency of the Na+ pumps. Consistent with the former possibility is the observed increase in the number of ouabain binding sites, as well as the increase in Na+, K(+)-ATPase activity in cell lysates obtained from MDCK monolayers treated with PGE1. The involvement of cyclic AMP in mediating these effects of PGE1 on the Na+, K(+)-ATPase in MDCK cells is supported by: (1) the observation of similar effects in 8-bromocyclic AMP treated MDCK monolayers, and (2) a dramatic reduction of the stimulatory effects of PGE1 and 8-bromocyclic AMP on the Vmax for ouabain-sensitive Rb+ uptake, and on the number of ouabain binding sites in dibutyryl cyclic AMP resistant clone 3 (DBr3) (which is defective in cyclic AMP dependent protein kinase activity). PGE1 independent MDCK monolayers exhibit both an increase in the Vmax for ouabain-sensitive Rb+ uptake and an increase in the number of ouabain binding sites in response to 8-bromocyclic AMP. Apparently, the cyclic AMP phosphodiesterase defect in these PGE1 independent cells did not cause cellular cyclic AMP levels to be elevated to a sufficient extent to maximally increase the Na+, K(+)-ATPase activity in these variant cells.

Mechanism for regulating cell surface distribution of Na+,K(+)-ATPase in polarized epithelial cells

Restriction of sodium, potassium adenosine triphosphatase (Na+,K(+)-ATPase) to either the apical or basal-lateral membrane domain of polarized epithelial cells is fundamental to vectorial ion and solute transport in many tissues and organs. A restricted membrane distribution of Na+,K(+)-ATPase in Madin-Darby canine kidney (MDCK) epithelial cells was found experimentally to be generated by preferential retention of active enzyme in the basal-lateral membrane domain and selective inactivation and loss from the apical membrane domain, rather than by vectorial targeting of newly synthesized protein from the Golgi complex to the basal-lateral membrane domain. These results show how different distributions of the same subunits of Na+,K(+)-ATPase may be generated in normal polarized epithelial and in disease states.