Na+, K+ and Cl- transport in isolated small intestinal cells from guinea pig. Evidences for the existence of a second Na+ pump

Cell and tissue polarity in the intestinal tract during tumourigenesis: cells still know the right way up, but tissue organization is lost

Cell and tissue polarity are tightly coupled and are vital for normal tissue homeostasis. Changes in cellular and tissue organization are common to even early stages of disease, particularly cancer. The digestive tract is the site of the second most common cause of cancer deaths in the developed world. Tumours in this tissue arise in an epithelium that has a number of axes of cell and tissue polarity. Changes in cell and tissue polarity in response to genetic changes that are known to underpin disease progression provide clues about the link between molecular-, cellular- and tissue-based mechanisms that accompany cancer. Mutations in adenomatous polyposis coli (APC) are common to most colorectal cancers in humans and are sufficient to cause tumours in mouse intestine. Tissue organoids mimic many features of whole tissue and permit identifying changes at different times after inactivation of APC. Using gut organoids, we show that tissue polarity is lost very early during cancer progression, whereas cell polarity, at least apical-basal polarity, is maintained and changes only at later stages. These observations reflect the situation in tumours and validate tissue organoids as a useful system to investigate the relationship between cell polarity and tissue organization.

Is the second sodium pump electrogenic?

Transepithelial sodium transport is a process that involves active Na⁺ transport at the basolateral membrane of the epithelial cell. This process is mediated by the Na⁺/K⁺ pump, which exchanges 3 internal Na⁺ by 2 external K⁺ inducing a net charge movement and the second Na⁺ pump, which transports Na⁺ accompanied by Cl⁻ and water. It has been suggested that this pump could also be electrogenic. Herein, we evaluated, in MDCK cells, the short-circuit current (I_sc) generated by these Na⁺ pumps at the basolateral membrane of the epithelial cells, using amphotericin B as an apical permeabilizing agent. In Cl⁻-containing media, I_sc induced by amphotericin B is totally inhibited by ouabain, indicating that only the electrogenic Na⁺/K⁺ pump is detectable in the presence of Cl⁻. Electrogenicity of the second Na⁺ pump can be demonstrated in Cl⁻-free media. The existence of a furosemide-sensitive component of I_sc, in addition to an ouabain-sensitive one, was identified in absence of chloride. Passive Cl⁻ movement associated with the function of the second Na⁺ pump seems to be regulated by the pump itself. These results demonstrate that the second Na⁺ pump is an electroneutral mechanism result from the stoichiometric movement of Na⁺ and Cl⁻ across the basolateral plasma membrane of the epithelial cell.

The second sodium pump: from the function to the gene

Transepithelial Na(+) transport is mediated by passive Na(+) entry across the luminal membrane and exit through the basolateral membrane by two active mechanisms: the Na(+)/K(+) pump and the second sodium pump. These processes are associated with the ouabain-sensitive Na(+)/K(+)-ATPase and the ouabain-insensitive, furosemide-inhibitable Na(+)-ATPase, respectively. Over the last 40 years, the second sodium pump has not been successfully associated with any particular membrane protein. Recently, however, purification and cloning of intestinal α-subunit of the Na(+)-ATPase from guinea pig allowed us to define it as a unique biochemical and molecular entity. The Na(+)- and Na(+)/K(+)-ATPase genes are at the same locus, atp1a1, but have independent promoters and some different exons. Herein, we spotlight the functional characteristics of the second sodium pump, and the associated Na(+)-ATPase, in the context of its role in transepithelial transport and its response to a variety of physiological and pathophysiological conditions. Identification of the Na(+)-ATPase gene (atna) allowed us, using a bioinformatics approach, to explore the tertiary structure of the protein in relation to other P-type ATPases and to predict regulatory sites in the promoter region. Potential regulatory sites linked to inflammation and cellular stress were identified in the atna gene. In addition, a human atna ortholog was recognized. Finally, experimental data obtained using spontaneously hypertensive rats suggest that the Na(+)-ATPase could play a role in the pathogenesis of essential hypertension. Thus, the participation of the second sodium pump in transepithelial Na(+) transport and cellular Na(+) homeostasis leads us to reconsider its role in health and disease.

Isolation and cloning of the K+-independent, ouabain-insensitive Na+-ATPase

Primary Na+ transport has been essentially attributed to Na+/K+ pump. However, there are functional and biochemical evidences that suggest the existence of a K+-independent, ouabain-insensitive Na+ pump, associated to a Na+-ATPase with similar characteristics, located at basolateral plasma membrane of epithelial cells. Herein, membrane protein complex associated with this Na+-ATPase was identified. Basolateral membranes from guinea-pig enterocytes were solubilized with polyoxyethylene-9-lauryl ether and Na+-ATPase was purified by concanavalin A affinity and ion exchange chromatographies. Purified enzyme preserves its native biochemical characteristics: Mg2+ dependence, specific Na+ stimulation, K+ independence, ouabain insensitivity and inhibition by furosemide (IC50: 0.5 mM) and vanadate (IC50: 9.1 μM). IgY antibodies against purified Na+-ATPase did not recognize Na+/K+-ATPase and vice versa. Analysis of purified Na+-ATPase by SDS-PAGE and 2D-electrophoresis showed that is constituted by two subunits: 90 (α) and 50 (β) kDa. Tandem mass spectrometry of α-subunit identified three peptides, also present in most Na+/K+-ATPase isoforms, which were used to design primers for cloning both ATPases by PCR from guinea-pig intestinal epithelial cells. A cDNA fragment of 1148 bp (atna) was cloned, in addition to Na+/K+-ATPase α1-isoform cDNA (1283 bp). In MDCK cells, which constitutively express Na+-ATPase, silencing of atna mRNA specifically suppressed Na+-ATPase α-subunit and ouabain-insensitive Na+-ATPase activity, demonstrating that atna transcript is linked to this enzyme. Guinea-pig atna mRNA sequence (2787 bp) was completed using RLM-RACE. It encodes a protein of 811 amino acids (88.9 kDa) with the nine structural motifs of P-type ATPases. It has 64% identity and 72% homology with guinea-pig Na+/K+-ATPase α1-isoform. These structural and biochemical evidences identify the K+-independent, ouabain-insensitive Na+-ATPase as a unique P-type ATPase.

Mechanisms of carbacholine and GABA action on resting membrane potential and Na+/K+-ATPase of Lumbricus terrestris body wall muscles

This work was aimed to identify the action of several ion channel and pump inhibitors as well as nicotinic, GABAergic, purinergic and serotoninergic drugs on the resting membrane potential (RMP) and assess the role of cholinergic and GABAergic sensitivity in earthworm muscle electrogenesis. The nicotinic agonists acetylcholine (ACh), carbacholine (CCh) and nicotine depolarize the RMP at concentrations of 5 μM and higher. The nicotinic antagonists (+)tubocurarine, α-bungarotoxin, muscarinic antagonists atropine and hexamethonium do not remove or prevent the CCh-induced depolarization. Verapamil, tetrodotoxin, removal of Cl(-) and Ca(2+) from the solution also cannot prevent the depolarization by CCh. In a Na(+)-free medium, however, CCh lost this depolarization ability and this indicates that the drug opens the sodium permeable pathway. Serotonin, glutamate, glycine, adenosine triphosphate (ATP) and cis-4-aminocrotonic acid (GABA(C) receptor antagonist) had no effect on the RMP. On the other hand, isoguvacin, γ-aminobutyric acid (GABA) and baclofen (GABA(B) receptor agonist) hyperpolarized the RMP. Ouabain, bicucullin (GABA(A) antagonist) and phaclofen (GABA(B) antagonist), as well as the removal of Cl(-), suppressed the effect of GABA and baclofen. CCh did not enhance the depolarization generated by ouabain but, on the other hand, hindered the hyperpolarizing activity of baclofen both in the absence and presence of atropine and (+)tubocurarine. The long-term application of CCh depolarizes the RMP primarily by inhibiting the Na(+)/K(+)-ATPase. The muscle membrane also contains A and B type GABA binding sites, the activation of which increases the RMP at the expense of increasing the action of ouabain- and Cl(-) -sensitive electrogenic pumps.

Backdoor phosphorylation of basolateral plasma membranes of small intestinal epithelial cells: characterization of a furosemide-induced phosphoprotein related to the second sodium pump

Enterocyte has two different Na+-stimulated ATPases, the ouabain-sensitive Na+/K+ ATPase and a furosemide-inhibitable Na+ ATPase. To identify the polypeptide associated with the Na+-ATPase, 32Pi phosphorylation into basolateral membranes of enterocyte was investigated. Both, ouabain and furosemide induced Mg2+-dependent, vanadate-sensitive 32Pi incorporation into a 100kDa polypeptide. K(m) for Pi was 17.7+/-1.82 microM and 16.8+/-0.69 microM for ouabain-induced and furosemide-induced phosphorylation, respectively. K(m) for furosemide was 1.3+/-0.21 mM. Furosemide-induced 32Pi incorporation was sensitive to alkaline pH and hydroxylamine suggesting an acyl-phosphate bond. Na+ and K+ inhibited 32Pi incorporation induced by ouabain. In contrast, Na+ stimulated furosemide-induced phosphorylation with a K(m) of 16.5+/-5.59 mM while K+ had no effect. Purified Na+/K+ ATPase only presented ouabain-induced phosphoprotein, indicating that furosemide-induced phosphorylation is not related to this enzyme and appears to correspond to a new member of P-type ATPases associated with the second Na+ pump.

Effect of zinc on aminopeptidase N activity and L-threonine transport in rabbit jejunum

Zinc is a nutritionally essential trace element required for many biological functions to be successfully carried out. The aim of the present work was to study the influence of zinc on the intestinal absorption of L-threonine and on the aminopeptidase N activity in rabbit jejunum, after in vitro addition and/or oral administration of ZnCl2 in drinking water. Results obtained show that zinc decreases L-threonine absorption in the jejunal tissue. This effect would appear to be owing to an action mainly located in active amino acid transport, because zinc does not seem to modify the amino acid diffusion across the intestinal epithelium, of the mucosal border of the intestinal epithelium. Zinc has also been shown to inhibit the (Na(+)-K+)-ATPase activity of the enterocyte, which might explain the inhibition of the L-threonine Na(+)-dependent transport. Nevertheless, a direct action of the zinc on carriers of active transport cannot be rejected. However, zinc did not significantly modify the aminopeptidase N activity in rabbit jejunum.

Action of zinc on enzymatic digestion and intestinal transport of sugar in the rabbit

The aim of the present work was to study whether zinc chloride added to the drinking water of rabbits affected the intestinal absorption of D-galactose and the activity of sucrase in the jejunum. The results showed that zinc decreased D-galactose absorption in the jejunal tissue. The effect appeared to be due mainly to an action on the active transport of the sugar by the mucosal border of the intestinal epithelium, because the zinc seemed not to affect its diffusion across the intestinal epithelium. Zinc was also shown to inhibit the (Na(+)-Ka+)-ATPase activity of the enterocyte, which might explain the inhibition of the Na(+)-dependent transport of D-galactose. Nevertheless, a possible direct action of the zinc ion on the Na(+)-dependent carrier cannot be discounted. Zinc did not alter the activity of sucrase in the jejunum of the rabbit.

Effect of cadmium on enzymatic digestion and sugar transport in the small intestine of rabbit

Cadmium compounds are found widely in our environment: for example, in food, water, soil, and ambient air. The most important exposure route of animals to cadmium in the general environment is via oral exposure. In oral cadmium intoxication, the immediate target organ is the gastrointestinal tract. The aim of the present work was to determine how cadmium acts on the intestinal absorption of sugars and on the sucrase activity through rabbit jejunum, after in vitro administration and/or oral administration of CdCl2 in drinking water. Results obtained show that cadmium decreases D-galactose accumulation in the jejunum tissue. This effect seems to be the results of an action mainly located on Na(+)-dependent sugar transport of the mucosal border of the intestinal epithelium, because cadmium seems not to modify the sugar diffusion across the intestinal epithelium. Cadmium has also been shown to inhibit the (Na(+)-K+)-ATPase activity of the enterocyte, which might explain the inhibition of the D-galactose Na(+)-dependent transport. Nevertheless, a direct action of the cadmium molecule on the Na(+)-dependent carrier cannot be discarded. Cadmium altered the sucrose activity when it was administered in the drinking water for 4 d.

Na,K-ATPase and the development of Na+ transport in rat distal colon

Na,K-ATPase function was studied in order to evaluate the mechanism of increased colonic Na+ transport during early postnatal development. The maximum Na(+)-pumping activity that was represented by the equivalent short-circuit current after addition of nystatin (ISCN) did not change during postnatal life or after adrenalectomy performed in 16-day-old rats. ISCN was entirely inhibited by ouabain; the inhibitory constant was 0.1 mM in 10-day-old (young) and 0.4 mM in 90-day-old (adult) rats. The affinity of the Na,K pump for Na+ was higher in young (11 mM) than in adult animals (19 mM). The Na,K-ATPase activity (measured after unmasking of latent activity by treatment with sodium dodecylsulfate) increased during development and was also not influenced by adrenalectomy of 16-day-old rats. The inhibitory constant for ouabain (KI) was not changed during development (0.1-0.3 mM). Specific [3H]ouabain binding to isolated colonocytes increased during development (19 and 82 pmol/mg protein), the dissociation constant (KD) was 8 and 21 microM in young and adult rats, respectively. The Na+ turnover rate per single Na,K pump, which was calculated from ISCN and estimated density of binding sites per cm2 of tissue was 500 in adult and 6400 Na+/min.site in young rats. These data indicate that they very high Na+ transport during early postnatal life reflects an elevated turnover rate and increased affinity for Na+ of a single isoform of the Na,K pump. The development of Na+ extrusion across the basolateral membrane is not directly regulated by corticosteroids.

K+ and Cl- currents in enterocytes isolated from guinea-pig small intestinal villi

1. The whole-cell configuration of the patch-clamp technique has been used to investigate the conductance properties of villus enterocytes isolated from guinea-pig small intestinal epithelium. 2. With near physiological ionic gradients inward and outward rectification was observed in the hyperpolarizing and depolarizing voltage domains respectively. 3. Replacement of intra- and extracellular K+ with N-methyl-D-glucamine (NMG) eliminated inward rectification but did not alter outward currents. In symmetrical low Cl- solutions outward currents were reduced but inward rectification was not affected. Under these conditions increases in extracellular K+ shifted both the current-voltage relation and the extrapolated reversal potential as expected for a K(+)-selective current. 4. The inwardly rectifying nature of the K+ current observed here remained unaltered after chelation of internal Mg2+ with ATP or EDTA. 5. Extracellular application of 5 mM-Ba2+ or 50 micrograms ml-1 of the venom of the scorpion Leiurus quinquestriatus abolished the inward K+ current, while 5 mM-extracellular tetraethylammonium (TEA) had little effect. 6. The current remaining in the presence of symmetrical Cl- solutions and in the complete absence of K+ rectified outwardly and reversed at 0 mV. The anionic nature of this current was confirmed by replacing Cl- with different anions. SCN- and Br- carried more current than Cl-, while F- and gluconate were less permeant. 7. Anionic currents of villus guinea-pig enterocytes were not stimulated by cyclic AMP and were strongly and reversibly inhibited by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 10(-5) M). 8. The inwardly rectifying K+ current described here shares some, but not all, characteristics with others previously described. It is postulated that this conductance might function to couple K+ permeability and the Na(+)-K+ pump rate in enterocytes. Absorption of chloride may be mediated by the Cl- channels.

Ouabain-insensitive Na-ATPase activity in the basolateral membrane from rat jejunum

1. In the basolateral membrane preparation of the rat enterocyte (jejunal tract) there is not only the well-known (Na,K)-ATPase activity, but also a ouabain-insensitive Na-ATPase. 2. The Na-ATPase is not activated by anions or other monovalent cations. As a substrate, ATP cannot be replaced by other nucleotides. 3. The Na-ATPase is insensitive to ouabain and bumetanide, inhibited partially by furosemide and totally by ethacrynate. 4. The activation of Na-ATPase at different Na concentrations shows an hyperbolic curve (Km = 15.7 +/- 2.3 mM and Vmax = 204 +/- 19 nmoles Pi/mg protein per min) different from the sigmoidal curve (Km = 9.8 +/- 1.2 mM and Vmax = 640 +/- 15 nmoles Pi/mg protein per min) shown by (Na,K)-ATPase. 5. These results are compared with the corresponding ones found in other animals and tissues in which the Na-ATPase was found. 6. The Na-ATPase activity can be interpreted as the enzymatic correspondent of a ouabain-insensitive Na pump, present in the basolateral membrane of the enterocyte different in behaviour with respect to the known Na pump.