Transport and pharmacological properties of nine different human Na, K-ATPase isozymes

Na,K-ATPase reconstituted in liposomes: effects of lipid composition on hydrolytic activity and enzyme orientation

In this paper, the reconstitution of Na,K-ATPase in liposomes (formed by single or mixed phospholipids and cholesterol) was investigated and the enzyme orientation was determined on kinetic basis using only specific inhibitors of ATP hydrolysis. A condition of foremost importance for enzyme reconstitution is the achievement of complete solubilization of the lipid in the initial stage of the cosolubilization process for the subsequent formation of the liposomes and/or proteoliposomes. PC-liposomes showed that increasing the fatty acid chain length increases the percentage of Na,K-ATPase incorporated. The average diameter of the proteoliposomes also increases in proportion, reaching a maximum with phospholipids with 16 carbon chains, resulting in 75.1% protein reconstitution and 319.4 nm diameter size, respectively. Binary lipid systems with PC and PE were efficient for incorporation of Na,K-ATPase, depending on the lipid:protein ratio used, varying from 15 to 80% recovery of total ATPase activity. The best results for Na,K-ATPase reconstitution using PC and PE mixture were obtained using a lipid:lipid ratio 1:1 (w/w) and lipid:protein 1:3 (w/w). Integrity studies using calcein release mediated by detergent or alamethicin, in association with inhibition of ATPase activity (ouabain and vanadate) showed that the enzyme is oriented inside-out in DPPC:DPPE proteoliposomes. In these vesicular systems, the enzyme is reconstituted with about 78.9% ATPase activity recovery and 89% protein incorporation, with an average diameter of 140 nm. Systems constituted by DPPC:DPPE, DPPC:DLOPE or DLOPC:DLOPE showed approximately 80, 71 and 70% of recovery of total ATPase activity, but no homogeneity in the distribution of Na,K-ATPase orientation. Reconstitution of Na,K-ATPase in DPPC:DPPE:cholesterol or DPPC:DLOPE:cholesterol systems (55% of cholesterol) showed recovery of about 86 and 82%, respectively, of its total ATPase activity. The results point to an important effect of the lipid acyl chain length and lipid-protein ratio in relation to the composition of the lipid matrix to finely tune the structural asymmetry and the amount of enzyme that can be incorporated a lipid bilayer vesicle while preserving membrane permeability.

Subunit composition and role of Na+,K+-ATPases in adrenal chromaffin cells

Adrenal medullary (AM) cells are exposed to high concentrations of cortical hormones, one of which is a ouabain-like substance. Thus, the effects of ouabain on catecholamine secretion and distribution of Na+,K+-ATPase alpha and beta subunits in rat and guinea-pig AM cells were examined using amperometry and immunological techniques. While exposure to 1 microm ouabain did not have a marked effect on resting secretion, it induced an increase in secretion due to mobilization of Ca2+ ions that were stored during a 4 min interval between muscarine applications. Immunocytochemistry revealed that Na+,K+-ATPase alpha1 subunit-like and beta3 subunit-like immunoreactive (IR) materials were distributed ubiquitously at the cell periphery, whereas alpha2- and beta2-like IR materials were present in restricted parts of the cell periphery. The alpha1 and alpha2 subunits were mainly immunoprecipitated from AM preparations by anti-beta3 and anti-beta2 antisera, respectively. Peripheral BODIPY-FL-InsP3 binding sites were localized below membrane domains with alpha2- and beta2-like IR materials. The results indicate that in AM cells, alpha1beta3 isozymes of Na+,K+-ATPase were present ubiquitously in the plasma membrane, while alpha2beta2 isozymes were in the membrane domain closely associated with peripheral Ca2+ store sites. This close association of the alpha2beta2 isozyme with peripheral Ca2+ store sites may account for the facilitation of mobilization-dependent secretion in the presence of 1 microm ouabain.

Regeneration influences expression of the Na+, K+-atpase subunit isoforms in the rat peripheral nervous system

Neural injury triggers changes in the expression of a large number of gene families. Particularly interesting are those encoding proteins involved in the generation, propagation or restoration of electric potentials. The expression of the Na+, K+-ATPase subunit isoforms (alpha, beta and gamma) was studied in dorsal root ganglion (DRG) and sciatic nerve of the rat in normal conditions, after axotomy and during regeneration. In normal DRG, alpha1 and alpha2 are expressed in the plasma membrane of all cell types, while there is no detectable signal for alpha3 in most DRG cells. After axotomy, alpha1 and alpha2 expression decreases evenly in all cells, while there is a remarkable onset in alpha3 expression, with a peak about day 3, which gradually disappears throughout regeneration (day 7). beta1 Is restricted to the nuclear envelope and plasma membrane of neurons and satellite cells. Immediately after injury, beta1 shows a homogeneous distribution in the soma of neurons. No beta2 expression was found. Beta3 Specific immunofluorescence appears in all neurons, although it is brightest in the smallest, diminishing progressively after injury until day 3 and, thereafter, increasing in intensity, until it reaches normal levels. FXYD7 is expressed weakly in a few DRG neurons (less than 2%) and Schwann cells. It increases intensely in satellite cells immediately after axotomy, and in all cell types at day 3. Transient switching of members of the Na+, K+-ATPase isoform family elicited by axotomy suggests variations in the sodium pump isozymes with different affinities for Na+, K+ and ATP from those in intact nerve. This adaptation may be important for regeneration.

FXYD3 (Mat-8), a new regulator of Na,K-ATPase

Four of the seven members of the FXYD protein family have been identified as specific regulators of Na,K-ATPase. In this study, we show that FXYD3, also known as Mat-8, is able to associate with and to modify the transport properties of Na,K-ATPase. In addition to this shared function, FXYD3 displays some uncommon characteristics. First, in contrast to other FXYD proteins, which were shown to be type I membrane proteins, FXYD3 may have a second transmembrane-like domain because of the presence of a noncleavable signal peptide. Second, FXYD3 can associate with Na,K- as well as H,K-ATPases when expressed in Xenopus oocytes. However, in situ (stomach), FXYD3 is associated only with Na,K-ATPase because its expression is restricted to mucous cells in which H,K-ATPase is absent. Coexpressed in Xenopus oocytes, FXYD3 modulates the glycosylation processing of the beta subunit of X,K-ATPase dependent on the presence of the signal peptide. Finally, FXYD3 decreases both the apparent affinity for Na+ and K+ of Na,K-ATPase.

Membrane excitability, weakness, and fatigue

A failure in membrane excitability, defined as an inability of the sarcolemma and T-tubule to translate the neural discharge command into repetitive action potentials, represents an inviting cause of mechanical disfunction in both health and disease. A failure at this level would precipitate a disturbance in signal transmission between the T-tubule and the calcium release channels of the sarcoplasmic reticulum, resulting in reduced release of Ca2+, lower cytosolic free Ca2+ levels, and depressed myofibrillar activation and force generation. The ability of the sarcolemma and T-tubules to conduct repetitive action potentials is intimately dependent on active transport of Na+ and K+ following an action potential. The active transport of these cations is mediated by the Na+-K+-ATPase, an integral membrane protein that uses the energy from the hydrolysis of 1 ATP to transport 3 Na+ out of the cell and 2 K+ into the cell. A failure to recruit sufficient Na+-K+-ATPase activity during contractile activity could result in a rundown of the transmembrane gradients for Na+ and K+, leading to a loss of membrane excitability. The Na+-K+-ATPase activity depends on the amount and isoform composition of the protein, substrate availability, and acute regulatory factors. Each of these factors is examined as a potential cause of altered activation of the Na+-K+-ATPase activity and loss of membrane excitability in fatigue. Regular exercise represents a potent stimulus for upregulating Na+-K+-ATPase levels and for increasing the ability for cation transport across the sarcolemma and T-tubule membrane. As such, training may be a valuable tool in the management of fatigue in health and disease.

Deletion of the Na/K-ATPase alpha1-subunit gene (Atp1a1) does not prevent cavitation of the preimplantation mouse embryo

Increases in Na/K-ATPase activity occur concurrently with the onset of cavitation and are associated with increases in Na(+)-pump subunit mRNA and protein expression. We have hypothesized that the alpha1-isozyme of the Na/K-ATPase is required to mediate blastocyst formation. We have tested this hypothesis by characterizing preimplantation development in mice with a targeted disruption of the Na/K-ATPase alpha1-subunit (Atp1a1) using embryos acquired from matings between Atp1a1 heterozygous mice. Mouse embryos homozygous for a null mutation in the Na/K-ATPase alpha1-subunit gene are able to undergo compaction and cavitation. These findings demonstrate that trophectoderm transport mechanisms are maintained in the absence of the predominant isozyme of the Na(+)-pump that has previously been localized to the basolateral membranes of mammalian trophectoderm cells. The presence of multiple isoforms of Na/K-ATPase alpha- and beta-subunits at the time of cavitation suggests that there may be a degree of genetic redundancy amongst isoforms of the catalytic alpha-subunit that allows blastocyst formation to progress in the absence of the alpha1-subunit.

The human Na,K-ATPase alpha4 isoform is a ouabain-sensitive alpha isoform that is expressed in sperm

The Na,K-ATPase generates electrochemical gradients across the plasma membrane that are responsible for numerous cellular and physiological processes. The active Na,K-ATPase is minimally composed of an alpha and a beta subunit and families of isoforms for both subunits exist. Recent studies have identified a physiological role for the rat Na,K-ATPase alpha4 isoform in sperm motility. However, very little is known about the human Na,K-ATPase alpha4 isoform other than its genomic sequence and structure and its mRNA expression pattern. Here, the human alpha4 isoform of the Na,K-ATPase is cloned, expressed, and characterized. Full length cDNAs encoding the putative human alpha4 isoform of the Na,K-ATPase were identified from a number of ESTs and a protein product corresponding to this isoform was shown to be expressed from these cDNAs. The human Na,K-ATPase alpha4 isoform protein was found to be expressed in mature sperm in human testes sections and it is localized specifically to the principle piece of human sperm. In addition, the presence of the Na,K-ATPase alpha4 isoform is absent in immature testes however its expression appears coincident with sexual maturity. And finally, the human Na,K-ATPase alpha4 isoform was shown to be as sensitive to cardiac glycoside inhibition as the human Na,K-ATPase alpha1 isoform. Considering the important role of the rat Na,K-ATPase alpha4 isoform in rat sperm motility, the demonstration that the human alpha4 isoform is a sperm-specific protein localized to the flagellum suggests a role for the human Na,K-ATPase alpha4 isoform in human sperm physiology.

Effects of warm acclimation on Na+,K+-ATPase α-subunit expression in chloride cells of Antarctic fish

The teleosts Trematomus bernacchii thrive in southern oceanic waters with temperatures below 0 degrees C. These fish have serum osmolalities almost double those found in fish of temperate waters, thereby lowering their serum's freezing point and the energy needed for ionic homeostasis. Upon warm acclimation to 4 degrees C, T. bernacchii decrease their serum osmolality and increase the Na+,K+-ATPase activity in their gills. Na+,K+-ATPase alpha1-, alpha2-, and alpha3-subunit isoforms are expressed in the gills of T. bernacchii and it is thought that Na+,K+-ATPase subunit composition in chloride cells changes with warm acclimation. Using immunohistochemistry, we compared the number of chloride cells expressing various alpha-isoforms of the Na+,K+-ATPase in the gills of cold- and warm-acclimated T. bernacchii. We found no change in the number of alpha2- or alpha3-immunopositive cells in warm-acclimated fish gills or in the number of cells immunopositive for the Na+,K+,2Cl- cotransporter. However, the number of pan-alpha-immunopositive (recognizing all three alpha-isoforms) and alpha1-immunopositive cells both increased in warm-acclimated fish. This suggests that changes in the number of alpha1-isoform-expressing chloride cells could contribute to the increased Na+,K+-ATPase activity that occurs with warm-acclimation.

Molecular regulation of blastocyst formation

Preimplantation development encompasses the interval from insemination until embryo implantation and thus includes the 'freeliving' period of oviduct and uterine development. Formation of the blastocyst is required for implantation and establishment of pregnancy, and is a principal determinant of embryo quality prior to embryo transfer. Development through this period is regulated by the expression of specific gene families that encode for cell polarity, cell junctional, cytoskeletal, ion transporter, and water channel gene products that direct the acquisition of cell polarity and differentiation of the outer cells of the early embryo. This results in the formation of the trophectoderm, which is the first epithelium of development. This review considers the roles of each of these gene families in trophectoderm differentiation and blastocyst formation. The principal hypothesis under investigation is that blastocyst formation is regulated by a Na/K-ATPase-generated trans-trophectoderm ion gradient that promotes the accumulation of water across the epithelium. This, combined with the formation of the tight junction seal controlling paracellular movement of water between adjacent trophectoderm cells, results in the formation of a fluid-filled blastocyst cavity and the expansion of the blastocyst. Results from recent experiments, however, have cast some doubt on the role of Na/K-ATPase in mediating these events and have defined water channels or Aquaporins (AQPs) as physiological mediators of fluid movement across the trophectoderm. In addition, studies have now implicated mitogen-activated protein kinase (MAPK) signaling as an important mediator of development to the blastocyst stage. Such studies define the physiology of blastocyst formation and serve to support the application of assisted reproductive technologies (ART) to both human and animal species.

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.

Kinetic Alterations due to a Missense Mutation in the Na,K-ATPase α2 Subunit Cause Familial Hemiplegic Migraine Type 2

A number of missense mutations in the ATP1A2 gene, which encodes the Na,K-ATPase alpha2 subunit, have been identified in familial hemiplegic migraine with aura. Loss of function and haploinsufficiency have been the suggested mechanisms in mutants for which functional analysis has been reported. This paper describes a kinetic analysis of mutant T345A, recently identified in a detailed genetic analysis of a large Finnish family (Kaunisto, M. A., Harno, H., Vanmolkot, K. R., Gargus, J. J., Sun, G., Hamalainen, E., Liukkonen, E., Kallela, M., van den Maagdenberg, A. M., Frants, R. R., Farkkila, M., Palotie, A., and Wessman, M. (2004) Neurogenetics 5, 141-146). Introducing T345A into the conserved rat alpha2 enzyme does not alter cell growth or catalytic turnover but causes a substantial decrease in apparent K+ affinity (2-fold increase in K0.5(K+)). In view of the location of Thr-345 in the cytoplasmic stalk domain adjacent to transmembrane segment 4, the 2-fold increase in K0.5(K+) is probably due to T345A replacement altering K+ occlusion/deocclusion. Faster K+ deocclusion of the mutant via the E2(K) + ATP --> E1.ATP + K+ partial reaction is evidenced in (i) a marked increase (300%) in K+ stimulation of Na-ATPase at micromolar ATP, (ii) a 4-fold decrease in KATP, and (iii) only a modest increase (approximately 3-fold) in I50 for vanadate, which was used as a probe of the steady state E1/E2 conformational equilibrium. We suggest that the decreased apparent K+ affinity is the basis for a reduced rate of extracellular K+ removal, which delays the recovery phase of nerve impulse transmission in the central nervous system and, thereby, the clinical picture of migraine with aura. This is the first demonstration of a mutation that leads to a disease associated with a kinetically altered but fully functional Na,K-ATPase, refining the molecular mechanism of pathogenesis in familial hemiplegic migraine.

The role of loop 6/7 in folding and functional performance of Na,K-ATPase

Alanine substitutions were made for 15 amino acids in the cytoplasmic loop between transmembrane helices 6 and 7 (L6/7) of the human alpha(1)-subunit of Na,K-ATPase. Most mutations reduced Na,K-ATPase activity by less than 50%; however, the mutations R834A, R837A, and R848A reduced Na,K-ATPase activity by 75, 89, and 66%, respectively. Steady-state phosphoenzyme formation from ATP was reduced in mutants R834A, R837A, and R848A, and R837A also had a faster E(2)P --> E(2) dephosphorylation rate compared with the wild-type enzyme. Effects of L6/7 mutations on the phosphorylation domain of the protein were also demonstrated by (18)O exchange, which showed that intrinsic rate constants for P(i) binding and/or reaction with the protein were altered. Although most L6/7 mutations had no effect on the interaction of Na(+) or K(+) with Na,K-ATPase, the E825A, E828A, R834A, and R837A mutations reduced the apparent affinity of the enzyme for both Na(+) and K(+) by 1.5-3-fold. 1-Bromo-2,4,6-tris(methylisothiouronium)benzene (Br-TITU(3+)), a competitive antagonist of Rb(+) and Na(+) occlusion, was used to test whether charged residues in L6/7 are involved in binding monovalent cations and cation antagonists. Br-TITU(3+) inhibited ouabain binding to wild type Na,K-ATPase with an IC(50) of 30 microM. Ouabain binding to the E825A, E828A, R834A, or R837A mutants was still inhibited by Br-TITU(3+), indicating that Br-TITU(3+) does not bind to charged residues in L6/7. This observation makes it unlikely that L6/7 functions as a cytoplasmic cation binding site in Na,K-ATPase, and together with the effects of L6/7 mutations on phosphate interactions with the enzyme suggests that L6/7 is important in stabilizing the phosphorylation domain and its relationship to the ion binding sites of the protein.

Functional genomic dissection of multimeric protein families in zebrafish

The study of multimeric protein function in the postgenomicera has become complicated by the discovery of multiple isoforms for each subunit of those proteins. A correspondingly large number of potential isoform combinations offer the multicellular organism a constellation of protein assemblies from which to generate a variety of functions across different cells, tissues, and organs. At the same time, the multiplicity of potential subunit isoform combinations presents a significant challenge when attempting to dissect the functions of particular isoform combinations. Biochemical and cell culture methods have brought us to a significant state of understanding of multimeric proteins but are unable to answer questions of function within the context of the many tissues and developmental stages of the multicellular organism. Answering those questions can be greatly facilitated in model systems in which expression can be determined over time, in the context of the whole organism, and in which hypomorphic function of each subunit can be studied individually and in combination. Fortunately, the potential for high-throughput in situ hybridization studies and antisense-based reverse genetic knockdowns in zebrafish offers exciting opportunities to meet this challenge. Some of these opportunities, along with cautions of interpretation and gaps in the existing technologies, are discussed in the context of ongoing investigations of the dimeric Na,K-ATPases and heterotrimeric G proteins.

Structural and functional interaction sites between Na,K-ATPase and FXYD proteins

Several members of the FXYD protein family are tissue-specific regulators of Na,K-ATPase that produce distinct effects on its apparent K(+) and Na(+) affinity. Little is known about the interaction sites between the Na,K-ATPase alpha subunit and FXYD proteins that mediate the efficient association and/or the functional effects of FXYD proteins. In this study, we have analyzed the role of the transmembrane segment TM9 of the Na,K-ATPase alpha subunit in the structural and functional interaction with FXYD2, FXYD4, and FXYD7. Mutational analysis combined with expression in Xenopus oocytes reveals that Phe(956), Glu(960), Leu(964), and Phe(967) in TM9 of the Na,K-ATPase alpha subunit represent one face interacting with the three FXYD proteins. Leu(964) and Phe(967) contribute to the efficient association of FXYD proteins with the Na,K-ATPase alpha subunit, whereas Phe(956) and Glu(960) are essential for the transmission of the functional effect of FXYD proteins on the apparent K(+) affinity of Na,K-ATPase. The relative contribution of Phe(956) and Glu(960) to the K(+) effect differs for different FXYD proteins, probably reflecting the intrinsic differences of FXYD proteins on the apparent K(+) affinity of Na,K-ATPase. In contrast to the effect on the apparent K(+) affinity, Phe(956) and Glu(960) are not involved in the effect of FXYD2 and FXYD4 on the apparent Na(+) affinity of Na,K-ATPase. The mutational analysis is in good agreement with a docking model of the Na,K-ATPase/FXYD7 complex, which also predicts the importance of Phe(956), Glu(960), Leu(964), and Phe(967) in subunit interaction. In conclusion, by using mutational analysis and modeling, we show that TM9 of the Na,K-ATPase alpha subunit exposes one face of the helix that interacts with FXYD proteins and contributes to the stable interaction with FXYD proteins, as well as mediating the effect of FXYD proteins on the apparent K(+) affinity of Na,K-ATPase.

Malleability of human skeletal muscle Na+-K+-ATPase pump with short-term training

To investigate the hypothesis that short-term submaximal training would result in changes in Na+-K+-ATPase content, activity, and isoform distribution in skeletal muscle, seven healthy, untrained men [peak aerobic power (peak oxygen consumption; V̇o2 peak) = 45.6 ml·kg−1·min−1 (SE 5.4)] cycled for 2 h/day at 60–65% V̇o2 peak for 6 days. Muscle tissue, sampled from the vastus lateralis before training (0 days) and after 3 and 6 days of training and analyzed for Na+-K+-ATPase content, as assessed by the vanadate facilitated [3H]ouabain-binding technique, was increased ( P < 0.05) at 3 days (294 ± 8.6 pmol/g wet wt) and 6 days (308 ± 15 pmol/g wet wt) of training compared with 0 days (272 ± 9.7 pmol/g wet wt). Maximal Na+-K+-ATPase activity as evaluated by the 3- O-methylfluorescein phosphatase assay was increased ( P < 0.05) by 6 days (53.4 ± 5.9 nmol·h−1·mg protein−1) but not by 3 days (35.9 ± 4.5 nmol·h−1·mg protein−1) compared with 0 days (37.8 ± 3.7 nmol·h−1·mg protein−1) of training. Relative isoform distribution, measured by Western blot techniques, indicated increases ( P < 0.05) in α2-content by 3 days and β1-content by 6 days of training. These results indicate that prolonged aerobic exercise represents a potent stimulus for the rapid adaptation of Na+-K+-ATPase content, isoform, and activity characteristics.

FXYD7, Mapping of Functional Sites Involved in Endoplasmic Reticulum Export, Association With and Regulation of Na,K-ATPase

The brain-specific FXYD7 is a member of the recently defined FXYD family that associates with the alpha1-beta1 Na,K-ATPase isozyme and induces an about 2-fold decrease in its apparent K+ affinity. By using the Xenopus oocyte as an expression system, we have investigated the role of conserved and FXYD7-specific amino acids in the cellular routing of FXYD7 and in its association with and regulation of Na,K-ATPase. In contrast to FXYD2 and FXYD4, the studies on FXYD7 show that the conserved FXYD motif in the extracytoplasmic domain is not involved in the efficient association of FXYD7 with Na,K-ATPase. On the other hand, the conserved Gly40 and Gly29, located on the same face of the transmembrane helix, were found to be implicated both in the association with and the regulation of Na,K-ATPase. Mutational analysis of FXYD7-specific regions revealed the presence of an ER export signal at the end of the cytoplasmic tail. Deletion of a C-terminal valine residue in FXYD7 significantly delayed and decreased its O-glycosylation processing and retarded the rate of its cell surface expression. This result indicates that the C-terminal valine residue is involved in the rapid and selective ER export of FXYD7, which could explain the observed post-translational association of FXYD7 with Na,K-ATPase. In conclusion, our study on FXYD7 provides new information on structural determinants of general importance for FXYD protein action. Moreover, FXYD7 is identified as a new member of proteins with a regulated ER export, which suggests that, among FXYD proteins, FXYD7 has a particular regulatory function in brain.

Identification of the β-subunit for nongastric H-K-ATPase in rat anterior prostate

The structural organization of nongastric H-K-ATPase, unlike that of closely related Na-K-ATPase and gastric H-K-ATPase, is not well characterized. Recently, we demonstrated that nongastric H-K-ATPase α-subunit (αng) is expressed in apical membranes of rodent prostate. Its highest level, as well as relative abundance, with respect to α1-isoform of Na-K-ATPase, was observed in anterior lobe. Here, we aimed to determine the subunit composition of nongastric H-K-ATPase through the detailed analysis of the expression of all known X-K-ATPase β-subunits in rat anterior prostate (AP). RT-PCR detects transcripts of β-subunits of Na-K-ATPase only. Measurement of absolute protein content of these three β-subunit isoforms, with the use of quantitative Western blotting of AP membrane proteins, indicates that the abundance order is β1> β3≫ β2. Immunohistochemical experiments demonstrate that β1is present predominantly in apical membranes, coinciding with αng, whereas β3is localized in the basolateral compartment, coinciding with α1. This is the first direct demonstration of the αng-β1colocalization in situ indicating that, in rat AP, αngassociates only with β1. The existence of αng-β1complex has been confirmed by immunoprecipitation experiments. These results indicate that β1-isoform functions as the authentic subunit of Na-K-ATPase and nongastric H-K-ATPase. Putatively, the intracellular polarization of X-K-ATPase isoforms depends on interaction with other proteins.

Up-regulation of Na(+),K(+)-ATPase alpha 3-isoform and down-regulation of the alpha1-isoform in human colorectal cancer

We investigated expression levels of Na(+),K(+)-ATPase alpha-isoforms and their ATPase activities in human colorectal cancer tissue and the accompanying normal mucosa. A decrease in expression of the alpha1-isoform protein was observed in all sampled cancer tissues compared with the normal mucosae. The level of ouabain (5 microM)-sensitive Na(+),K(+)-ATPase activity in carcinomas was 81+/-5% that of in the normal mucosae. The mRNA expression of alpha2- and alpha 4-isoforms was decreased in almost all the carcinoma samples. Interestingly, the expression level of the alpha 3-isoform protein in the cancer tissue was higher than that of the normal mucosa. These results indicate that a decrease in the alpha1-isoform expression and an increase in the alpha 3-isoform expression may be associated with colorectal cancer.

Accumulation of βm, a structural member of X,K-ATPase β-subunit family, in nuclear envelopes of perinatal myocytes

Recently discovered muscle-specific βmprotein is structurally closely related to the X,K-ATPase β-subunits. However, it has a number of unique properties such as predominant localization in intracellular stores and lack of association with known X,K-ATPase α-subunits on heterologous coexpression. In this study, the primary structure of mouse βmwas determined and developmental regulation of the gene (ATP1B4) was analyzed. The expression is first detected at day 14 of gestation, is sharply increased at day 16, and reaches its maximum at day 18. After birth, the expression quickly decreases and is hardly detectable in adult mice. A more detailed subcellular localization study was undertaken, and its results indicate that βmnot only is located in sarcoplasmic reticulum but is concentrated in nuclear envelopes of both prenatal and postnatal skeletal muscles. Immunohistochemical studies show that βmis specific to myocytes and, at the subcellular level, many nuclear envelopes are intensively labeled in both fetal and newborn skeletal muscles. Accordingly, βmis detected by immunoblotting in purified nuclei and nuclear membranes from neonatal skeletal muscles. On transfection of human rhabdomyosarcoma cell line RD, green fluorescent protein-tagged βmresides intracellularly with significant enrichment in nuclear envelopes, whereas βmwith transmembrane domain deleted localizes in both cytoplasm and nucleoplasm. Nuclear βmapparently is not in association with Na,K-ATPase because we never detected its α-subunit in myonuclear membranes. These results indicate that βmhas a specialized function in mammalian perinatal myocytes, different from functions of other X,K-ATPase β-subunits. The unique temporospatial distribution of βmprotein expression suggests its important role in development of growing skeletal muscle.

Angiotensin II regulation of the Na+ pump involves the phosphatidylinositol-3 kinase and p42/44 mitogen-activated protein kinase signaling pathways in vascular smooth muscle cells

This investigation used primary cultured rat vascular smooth muscle cells to examine angiotensin II (Ang II) regulation of Na(+), K(+)-ATPase (Na(+) pump) activity, and Na(+) pump alpha(1)- and beta(1)-subunit gene transcription. This regulation was mediated through both phosphatidylinositol-3 kinase (PI3K) and p42/44 mitogen-activated protein kinase (p42/44(MAPK)) signaling pathways. Both acute (10 min) and prolonged (24 h) treatment with Ang II stimulated Na(+) pump activity. Also, prolonged exposure to Ang II (24 h) increased promoter transcription of the Na(+) pump alpha(1)- and beta(1)-subunits. Furthermore, PI3K activities because well because p42/44(MAPK) phosphorylation were increased within 10 min after Ang II treatment. To determine whether these stimulatory activities of Ang II are acting through Ang II receptors 1 and/or 2 (AT(1), AT(2)), cells were pretreated with either AT(1) receptor blocker losartan or the AT(2) receptor blocker PD 123,319. Indeed, these treatments prevented the stimulatory effect of Ang II on Na(+) pump activity at both acute and 24-h time points. Furthermore, the Ang II-stimulated alpha(1)-subunit promoter transcription was inhibited by losartan but not by the AT(2) receptor blocker. These results indicate that Ang II acts through both the AT(1) and AT(2) receptor to up-regulate Na(+) pump activity; however, Ang II regulates alpha(1)-gene transcription through AT(1) but not AT(2) receptors. It was also observed that the Ang II-stimulated beta(1)-subunit gene transcription is not mediated through either AT(1) or AT(2) receptors. To examine whether the Na(+)/H(+) exchanger is involved in Ang II-stimulated Na(+) pump activity, cells were pretreated with amiloride, a specific inhibitor of the Na(+)/H(+) exchanger. This pretreatment prevented 24 h, but not acute, Ang II-stimulated Na(+) pump activity. The 24-h Ang II-stimulated alpha(1)-subunit promoter transcription was also inhibited by amiloride. This suggests that the prolonged effect of Ang II on Na(+) pump activity is dependent on increased Na(+)/H(+) exchange. Because Ang II treatment for 10 min increased PI3K activity because well because p42/44(MAPK) phosphorylation, studies were performed to determine the involvement of PI3K and p42/44(MAPK) signaling pathways in both Ang II-stimulated Na(+) pump activity and alpha(1)- and beta(1)-gene transcription. Cells were pretreated with either the PI3K inhibitor wortmannin or the p42/44(MAPK) inhibitor PD 98059. Ang II-stimulated PI3K or p42/44(MAPK) activity was inhibited by these pretreatments. Furthermore, pretreatment of cells with the PI3K inhibitors wortmannin and LY29404 or the MAPK inhibitors U0126 and PD 98059 were all observed to inhibit Ang II-stimulated Na(+) pump activity. To more specifically determine the role of PI3K in Ang II-regulation of alpha(1)-and beta(1)-gene transcription, cells were cotransfected with a dominant-negative p85 construct. Cotransfection with dominant-negative p85 reduced Ang II-stimulated alpha(1)-but not beta(1)-gene transcription in vascular smooth muscle cells. These results indicate that Ang II acts through PI3K/p42/44(MAPK) signaling pathways to up-regulate Na(+) pump activity and alpha(1)-gene transcription and that Ang II-regulated beta(1)-gene transcription is not mediated through either PI3K or p42/44 (MAPK) signaling pathways.

Isoform specificity of human Na(+), K(+)-ATPase localization and aldosterone regulation in mouse kidney cells

Short-term aldosterone coordinately regulates the cell-surface expression of luminal epithelial sodium channels (ENaC) and of basolateral Na(+) pumps (Na(+), K(+)-ATPase alpha1-beta1) in aldosterone-sensitive distal nephron (ASDN) cells. To address the question of whether the subcellular localization of the Na(+), K(+)-ATPase and its regulation by aldosterone depend on subunit isoform-specific structures, we expressed the cardiotonic steroid-sensitive human alpha isoforms 1-3 by retroviral transduction in mouse collecting duct mpkCCD(c14) cells. Each of the three exogenous human isoforms could be detected by Western blotting. Immunofluorescence indicated that the exogenous alpha1 subunit to a large extent localizes to the basolateral membrane or close to it, whereas much of the alpha2 subunit remains intracellular. An ouabain-sensitive current carried by exogenous pumps could be detected in apically amphotericin B-permeabilized epithelia expressing human alpha1 and alpha2 subunits, but not the alpha3 subunit. This current displayed a higher apparent Na(+) affinity in pumps containing human alpha2 subunits (10 mM) than in pumps containing human alpha1 (33.2 mM) or endogenous (cardiotonic steroid-resistant) mouse alpha1 subunits (mean: 16.3 mM). A very low mRNA level of the Na(+), K(+)-ATPase gamma subunit (FXYD2) in mpkCCD(c14) cells suggested that this ancillary gene product is not responsible for the relatively low apparent Na(+) affinity measured for a1 subunit-containing pumps. Aldosterone increased the pump current carried by endogenous pumps and by pumps containing the human alpha1 subunit. In contrast, the current carried by pumps with a human alpha2 subunit was not stimulated by the same treatment. In summary, quantitative basolateral localization of the Na(+), K(+)-ATPase and its responsiveness to aldosterone require alpha1 subunit-specific sequences that differentiate this isoform from the alpha2 and alpha3 subunit isoforms.

The signal transduction pathway that mediates the effect of interleukin-1 beta on the Na+-K+-ATPase in LLC-PK1 cells

IL-1beta reduces the activity and protein expression of Na(+)-K(+)-ATPase in rat kidney cells. The aim of the present study was to elucidate the signalling pathway involved, using the LLC-PK(1) cell line. In these cells IL-1beta caused a time and concentration-dependent decrease in the protein expression of the Na(+)-K(+)-ATPase. Inhibition of extracellular signal-regulated kinase (ERK), nuclear factor-kappaB (NF-kappaB) and cyclooxygenase (COX), but not p38 mitogen-activated kinase (MAPK), abolished the effect of the cytokine on the pump. The activation of NF-kappaB by IL-1beta was maximal at 20 min and declined thereafter. Inhibition of the transcription factor by pyrrolidinediethyldithiocarbamate (PDTC) down-regulated the ATPase. The effects of IL-1beta on the pump and NF-kappaB were prevented by the COX inhibitor indomethacin. Exogenous PGE(2) reduced protein expression of the ATPase within 15 min, even in presence of an ERK inhibitor. It is concluded that IL-1beta stimulates the mitogen and extracellular signal regulated protein kinase kinase/extracellular signal regulated protein kinase (MEK/ERK) pathway. This activates NF-kappaB, thus leading to increased COX-2 expression and PGE(2) release. PGE(2) in turn inhibits NF-kappaB and reduces the protein expression of Na(+)-K(+)-ATPase.

New molecular determinants controlling the accessibility of ouabain to its binding site in human Na,K-ATPase alpha isoforms

Inhibition of Na,K-ATPase alpha2 isoforms in the human heart is supposed to be involved in the inotropic effect of cardiac glycosides, whereas inhibition of alpha1 isoforms may be responsible for their toxic effects. Human Na,K-ATPase alpha1 and alpha2 isoforms exhibit a high ouabain affinity but significantly differ in the ouabain association and dissociation rates. To identify the structural determinants that are involved in these differences, we have prepared chimeras between human alpha1 and alpha2 isoforms and alpha2 mutants in which nonconserved amino acids were exchanged with those of the alpha1 isoform, expressed these constructs in Xenopus laevis oocytes, and measured their ouabain binding kinetics. Our results show that replacement of Met119 and Ser124 in the M1-M2 extracellular loop of the alpha2 isoform by the corresponding Thr119 and Gln124 of the alpha1 isoform shifts both the fast ouabain association and dissociation rates of the alpha2 isoform to the slow ouabain binding kinetics of the alpha1 isoform. The amino acids at position 119 and 124 cooperate with the M7-M8 hairpin and are also responsible for the small differences in the ouabain affinity of the ouabain-sensitive alpha1 and alpha2 isoforms. Thus, we have identified new structural determinants in the Na,K-ATPase alpha-subunit that are involved in ouabain binding and probably control, in an alpha isoform-specific way, the access and release of ouabain to and from its binding site.

Quantification of α-subunit isoforms of Na,K-ATPase in rat resistance vessels

AIM

Rat mesenteric resistance vessels (RV) were characterized with respect to concentration of individual alpha-subunit isoforms of Na,K-ATPase.

METHODS

Total vessel homogenates were used to avoid any loss or subfractionation of membranes. They were applied to sodium dodecyl sulphate gels and, for calibration, in parallel lanes were run purified rat Na,K-ATPase preparations with known isoform distribution and content. The capacity per mg protein for Na+-dependent 32P-phosphorylation of Na,K-ATPase isolated from rat kidney was used for alpha1 calibration and that for high-affinity (3H)ouabain binding of Na,K-ATPase isolated from rat brain was used for (alpha2 + alpha3) calibration. Western blots containing homogenate proteins and reference enzyme were incubated with isoform-specific antibodies and radiolabelled secondary antibodies. The signals from adjacent alpha spots were used for qualitative and quantitative characterization of rat vessels.

RESULTS

A concentration of 100.7 +/- 14.4 pmol (n = 11) per g wet weight of the alpha1-isoform containing Na,K-ATPase was found in RV from 12-14-week rats. A much lower and more unreliable content of alpha2- and alpha3-isoforms was found. These ouabain-sensitive isoforms seem to represent a maximum of 5-10% each compared with the ouabain-insensitive rat alpha1-isoform.

CONCLUSIONS

The isoform pattern in RV, in which the isoform with high/intermediate Na+-affinity is the absolutely dominating one representing nearly all sodium pumps in this tissue, is very different from that seen in rat skeletal muscles. Due to the high content of the ouabain-insensitive alpha1-isoform in rat RV this species would seem a less relevant model in studies addressing a role of cardiac glycosides and putative endogenous ouabain-like factors in hypertension.

Na+-K+-ATPase in rat skeletal muscle: content, isoform, and activity characteristics

The purpose of this study was to investigate the hypothesis that muscle Na+-K+-ATPase activity is directly related to Na+-K+-ATPase content and the content of the alpha2-catalytic isoform in muscles of different fiber-type composition. To investigate this hypothesis, tissue was sampled from soleus (Sol), red gastrocnemius (RG), white gastrocnemius (WG), and extensor digitorum longus (EDL) muscles at rest from 38 male Wistar rats weighing 413 +/- 6.0 g (mean +/- SE). Na+-K+-ATPase activity was determined in homogenates (Hom) and isolated crude membranes (CM) by the regenerating ouabain-inhibitable hydrolytic activity assay (ATPase) and the 3-O-methylfluorescein K+-stimulated phosphatase (3-O-MFPase) assay in vitro. In addition, Na+-K+-ATPase content (Bmax) and the distribution of alpha1-, alpha2-, beta1-, and beta2-isoforms were determined by [3H]ouabain binding and Western blot, respectively. For the ATPase assay, differences (P < 0.05) in enzyme activity between muscles were observed in Hom (EDL > WG) and in CM (Sol > EDL = WG). For the 3-O-MFPase assay, differences (P < 0.05) were also found for Hom (Sol > RG = EDL > WG) and CM (Sol = WG > RG). For Bmax, differences in the order of RG = EDL > Sol = WG (P < 0.05) were observed. Isoform distribution was similar between Hom and CM and indicated in CM, a greater density (P < 0.05) of alpha1 in Sol than WG and EDL (P < 0.05), but more equal distribution of alpha2 between muscles. The beta1 was greater (P < 0.05) in Sol and RG, and the beta2 was greater in EDL and WG (P < 0.05). Over all muscles, the correlation (r) between Hom 3-O-MFPase and Bmax was 0.45 (P < 0.05) and between Hom alpha2 and Bmax, 0.59 (P < 0.05). The alpha1 distribution correlated to Hom 3-O-MFPase (r = 0.79, P < 0.05) CM ATPase (r = 0.69, P < 0.005) and CM 3-O-MFPase activity (r = 0.32, P < 0.05). The alpha2 distribution was not correlated with any of the Na+-K+-ATPase activity measurements. The results indicate generally poor relationships between activity and total pump content and alpha2 isoform content of the Na+-K+-ATPase. Several factors, including the type of preparation and the type of assay, appear important in this regard.

2-Methoxy-3,8,9-trihydroxy coumestan: a new synthetic inhibitor of Na+,K+-ATPase with an original mechanism of action

The aim of the present work was to analyse the interaction between Na(+),K(+)-ATPase and one of our recent synthesized coumestans, namely the original molecule 2-methoxy-3,8,9-trihydroxy coumestan (PCALC36). Rat brain (mainly alpha 2 and alpha 3 Na(+),K(+)-ATPase isoforms) and kidney (alpha 1 isoform) fractions enriched with Na(+),K(+)-ATPase were utilized to compare the inhibition promoted by PCALC36 with that of classical inhibitors like ouabain and vanadate. Analysis of inhibition curves revealed that unlike ouabain, which was about a thousand times more potent to inhibit brain isoforms than kidney isoform, PCALC36 had a similar affinity for brain (IC(50)=4.33+/-0.90 microM) and kidney (IC(50)=11.04+/-0.86 microM) isoforms. The inhibitory effect of PCALC36 was not antagonized by 1-10 mM K(+), as observed with ouabain. Whereas vanadate was more potent in ionic conditions promoting the E2 conformation of the enzyme, the inhibitory effect of PCALC36 was equal in ionic conditions favouring either the E1 or E2 conformations. Equilibrium binding assays with [3H]ouabain revealed that the addition of 2-10 microM PCALC36 did not change the K(d) of ouabain but decreased its maximal binding (B(max)) in a concentration-dependent manner (from 76.6 to 44.0 pmol/mg protein). This inhibitory effect of PCALC36 was not reverted after an extensive washing procedure indicating that it forms a very stable complex with Na(+),K(+)-ATPase. We conclude that PCALC36, a new molecule with a non-steroidal skeleton, inhibits the Na(+),K(+)-ATPase by a mechanism of action different from the cardiac glycosides and could thus serve as a structural paradigm to develop new inotropic drugs.

Effects of ouabain on human bronchial muscle in vitro

The effects of ouabain, an inhibitor of the plasmalemmal Na(+)/K(+)-ATPase activity, were examined in human isolated bronchus. Ouabain produced concentration-dependent contraction with -logEC(50)=7.16+/-0.11 and maximal effect of 67+/-4% of the response to acetylcholine (1 mM). Ouabain (10 microM)-induced contraction was epithelium-independent and was not depressed by inhibitors of cyclooxygenase and lipoxygenase, antagonists of muscarinic, histamine H(1)-receptors and alpha-adrenoceptors, or neuronal Na(+) channel blockade. The inhibition of ouabain contraction in tissues bathed in K(+)-free medium, and the inhibition by ouabain of the K(+)-induced relaxation confirm that the contractile action of ouabain is mediated by inhibition of Na(+)/K(+)-ATPase. Furthermore, depolarization (16.4+/-0.9 mV) was observed in human isolated bronchus by intracellular microelectrode recording. Ouabain (10 microM)-induced contractions were abolished by a Ca(2+)-free solution but not by blockers of L-type Ca(2+) channels. In human cultured bronchial smooth muscle cells, ouabain (10 microM) produced a sustained increase in [Ca(2+)](i) (116+/-26 nM) abolished in Ca(2+)-free medium. Incubation with a Na(+)-free medium or amiloride (0.1 mM) markedly inhibited the spasmogenic effect of ouabain thus suggesting the role of Na(+)/Ca(2+) exchange in ouabain contraction while selective inhibitors of Na(+)/H(+)-antiport, Na(+)/K(+)/Cl(-)-antiport, or protein kinase C had no effect. Ouabain (10 microM) failed to increase inositol phosphate accumulation in human bronchus. Ouabain (10 microM) did not alter bronchial responsiveness to acetylcholine or histamine but inhibited the relaxant effects of isoprenaline, forskolin, levcromakalim, or sodium nitroprusside. These results indicate that ouabain acts directly to produce contraction of human airway smooth muscle that depends on extracellular Ca(2+) entry unrelated to L-type channels and involving the Na(+)/Ca(2+)-antiporter.

Exercise training differentially modifies age-associated alteration in expression of Na+-K+-ATPase subunit isoforms in rat skeletal muscles

The present study tests the hypothesis that endurance exercise training (ETr) reverses age-associated alterations in expression of Na+-K+-ATPase subunit isoforms in rat skeletal muscles. Expression of the isoforms was examined in 16-mo-old sedentary middle-aged, 29-mo-old sedentary senescent, and 29-mo-old treadmill exercise-trained senescent Fischer 344 x Brown Norway rats. Levels of the alpha1-isoform increased with age in red gastrocnemius (GR), white gastrocnemius (GW), and extensor digitorum longus (EDL) muscles, and ETr further increased its levels. Levels of the alpha2-isoform were unchanged in GR, had a strong trend for a decrease in GW, and decreased significantly in EDL. ETr increased expression of the alpha2-isoform in all three muscle groups. There was no increase in expression of the beta1-isoform in GR, GW, or EDL with age, whereas ETr markedly increased its levels in the muscles. There was a marked decrease with age in expression of the beta2-isoform in the muscle groups that was not reversed by ETr. By contrast, beta3-isoform levels increased with age in GR and GW, and ETr was able to reverse this increase. Na+-K+-ATPase enzyme activity was unchanged with age in GR and GW but increased in EDL. ETr increased enzyme activity in GR and GW and did not change in EDL. Myosin heavy chain isoforms in the muscle groups did not change significantly with age; ETr caused a general shift toward more oxidative fibers. Thus ETr differentially modifies age-associated alterations in expression of Na+-K+-ATPase subunit isoforms, and a mechanism(s) other than physical inactivity appears to play significant role in some of the age-associated changes.

AMOG/beta2 and glioma invasion: does loss of AMOG make tumour cells run amok?

The beta2 subunit of Na,K-ATPase, initially described as adhesion molecule on glia (AMOG), has been shown to mediate neurone-astrocyte adhesion as well as neural cell migration in vitro. We have investigated the expression of AMOG/beta2 in human gliomas and its effect on glioma cell adhesion and migration. Compared to normal astrocytes of human brain, AMOG/beta2 expression levels of neoplastic astrocytes were down-regulated in biopsy specimens and inversely related to the grade of malignancy. One rat and four human glioma cell lines showed complete loss of AMOG. To investigate the function of AMOG/beta2, its expression was re-established by transfecting an expression plasmid into AMOG/beta2-negative C6 rat glioma cells. In vitro assays revealed increased adhesion and decreased migration on matrigel of AMOG/beta2-positive cells as compared to their AMOG/beta2-negative counterparts. We conclude that increasing loss of AMOG/beta2 during malignant progression parallels and may underlie the extensive invasion pattern of malignant gliomas.

Kinetic characterization of Na,K-ATPase from rabbit outer renal medulla: properties of the (alpha beta)(2) dimer

We describe and compare the main kinetic characteristics of the (alpha beta)(2) form of rabbit kidney Na,K-ATPase. The dependence of ATPase activity on ATP concentration revealed high (K(0.5)=4 microM) and low (K(0.5)=1.4 mM) affinity sites for ATP, exhibiting negative cooperativity and a specific activity of approximately 700 U/mg. For p-nitrophenylphosphate (PNPP) as substrate, a single saturation curve was found, with a smaller apparent affinity of the enzyme for this substrate (K(0.5)=0.5 mM) and a lower hydrolysis rate (V(M)=42 U/mg). Stimulation of ATPase activity by K(+) (K(0.5)=0.63 mM), Na(+) (K(0.5)=11 mM) and Mg(2+) (K(0.5)=0.60 mM) all showed V(M)'s of approximately 600 U/mg and negative cooperativity. K(+) (K(0.5)=0.69 mM) and Mg(2+) (K(0.5)=0.57 mM) also stimulated PNPPase activity of the (alpha beta)(2) form. Ouabain (K(0.5)=0.01 microM and K(0.5)=0.1 mM) and orthovanadate (K(0.5)=0.06 microM) completely inhibited the ATPase activity of the (alpha beta)(2) form. The kinetic characteristics obtained constitute reference values for diprotomeric (alpha beta)(2)-units of Na,K-ATPase, thus contributing to a better understanding of the biochemical mechanisms of the enzyme.

Differential protein expression in lens epithelial whole-mounts and lens epithelial cell cultures

PURPOSE

Lens fibergenesis is a problem in several types of cataract and in the posterior capsular opacification following cataract surgery. To correct improper fiber differentiation or to prevent unwanted growth on the posterior capsule following cataract surgery requires a thorough understanding of normal and abnormal fiber formation. To this end, studies were initiated to characterize fiber differentiation in the bovine lens and in lens epithelial cell cultures.

METHODS

Indirect immunofluorescence and immunoblot analysis were employed to study the expression of vimentin, beta-crystallin, gamma-crystallin, filensin, aquaporin 0 and the Na, K-ATPase catalytic subunit isoforms (alpha1, alpha2, alpha3) in bovine lens epithelium whole-mounts as well as lens epithelial cell cultures propagated in medium containing 10% bovine serum or in medium supplemented with bovine serum concentrations < or =4%.

RESULTS

Three distinct cell types were observed in the bovine lens epithelium. The cells of the central zone were identified by a polarized distribution of two distinct Na, K-ATPase catalytic subunit isoforms, alpha1 to the apical (fiber side) and alpha3 to the basal (aqueous humor side) membranes. Lateral to the polarized central zone, was the germinative zone of cells, best characterized by perinuclear vimentin basket-like structures and the loss of polarized Na, K-ATPase catalytic subunit isoforms. Lateral to the germinative zone were the cells of the transition zone (meridinal rows) where expression of the lens specific proteins beta-crystallin, gamma-crystallin, filensin and aquaporin 0 as well as the lens fiber-, adipocyte- and brain glia-specific Na, K-ATPase catalytic subunit, alpha2 are expressed. The cultured cells propagated in medium supplemented with 10% serum bore no resemblance to any of the cells of the bovine lens epithelium whole-mounts. The cells propagated in the medium supplemented with the lower bovine serum levels resembled the differentiating fibers of the transition zone of the bovine lens epithelium whole-mounts as well as superficial cortical fibers.

CONCLUSIONS

Since the low-serum lens epithelial cell cultures bear a remarkable resemblance to early differentiating fibers, they are reasonable models for the study of early fiber differentiation or prevention of differentiation. The culture conditions employed do not yield the polarized cells of the central zone. Nor has the function of these polarized cells in lens fluid, nutrient and ion homeostasis been determined.

Training-induced changes in skeletal muscle Na+-K+ pump number and isoform expression in rats with chronic heart failure

The mechanisms responsible for the decrements in exercise performance in chronic heart failure (CHF) remain poorly understood, but it has been suggested that sarcolemmal alterations could contribute to the early onset of muscular fatigue. Previously, our laboratory demonstrated that the maximal number of ouabain binding sites (B(max)) is reduced in the skeletal muscle of rats with CHF (Musch TI, Wolfram S, Hageman KS, and Pickar JG. J Appl Physiol 92: 2326-2334, 2002). These reductions may coincide with changes in the Na(+)-K(+)-ATPase isoform (alpha and beta) expression. In the present study, we tested the hypothesis that reductions in B(max) would coincide with alterations in the alpha- and beta-subunit expression of the sarcolemmal Na(+)-K(+)-ATPase of rats with CHF. Moreover, we tested the hypothesis that exercise training would increase B(max) along with producing significant changes in alpha- and beta-subunit expression. Rats underwent a sham operation (sham; n = 10) or a surgically induced myocardial infarction followed by random assignment to either a control (MI; n = 16) or exercise training group (MI-T; n = 16). The MI-T rats performed exercise training (ET) for 6-8 wk. Hemodynamic indexes demonstrated that MI and MI-T rats suffered from severe left ventricular dysfunction and congestive CHF. Maximal oxygen uptake (Vo(2 max)) and endurance capacity (run time to fatigue) were reduced in MI rats compared with sham. B(max) in the soleus and plantaris muscles and the expression of the alpha(2)-isoform of the Na(+)-K(+)-ATPase in the red portion of the gastrocnemius (gastrocnemius(red)) muscle were reduced in MI rats. After ET, Vo(2 max) and run time to fatigue were increased in the MI-T group of rats. This coincided with increases in soleus and plantaris B(max) and the expression of the alpha(2)-isoform in the gastrocnemius(red) muscle. In addition, the expression of the beta(2)-isoform of the gastrocnemius(red) muscle was increased in the MI-T rats compared with their sedentary counterparts. This study demonstrates that CHF-induced alterations in skeletal muscle Na(+)-K(+)-ATPase, including B(max) and isoform expression, can be partially reversed by ET.

Apoptotic insults impair Na+, K+-ATPase activity as a mechanism of neuronal death mediated by concurrent ATP deficiency and oxidant stress

The Na+, K+-ATPase (Na+, K+-pump) plays critical roles in maintaining ion homeostasis. Blocking the Na+, K+-pump may lead to apoptosis. By contrast, whether an apoptotic insult may affect the Na+, K+-pump activity is largely undefined. In cultured cortical neurons, the Na+, K+-pump activity measured as a membrane current Ipump was time-dependently suppressed by apoptotic insults including serum deprivation, staurosporine, and C2-ceramide, concomitant with depletion of intracellular ATP and production of reactive oxygen species. Signifying a putative relationship among these events, Ipump was highly sensitive to changes in ATP and reactive oxygen species levels. Moreover, the apoptosis-associated Na+, K+-pump failure and serum deprivation-induced neuronal death were antagonized by pyruvate and succinate in ATP- and reactive-oxygen-species-dependent manners. We suggest that failure of the Na+, K+-pump as a result of a combination of energy deficiency and production of reactive oxygen species is a common event in the apoptotic cascade; preserving the pump activity provides a neuroprotective strategy in certain pathological conditions.

Stretch receptor-associated expression of alpha 3 isoform of the Na+, K+-ATPase in rat peripheral nervous system

Expression of the neuronal alpha(3) isoform of the Na(+),K(+)-ATPase (alpha(3) Na(+),K(+)-ATPase) was studied in the rat peripheral nervous system using histological and immunohistochemical techniques. Non-uniform expression of the alpha(3) Na(+),K(+)-ATPase was observed in L5 ventral and dorsal roots, dorsal root ganglion, sciatic nerve and its branches into skeletal muscle. The alpha(3) Na(+),K(+)-ATPase was not detected in nerve fibers in skin, saphenous and sural nerves. In dorsal root ganglion 12+/-2% of neurons were immunopositive for alpha(3) Na(+),K(+)-ATPase and all these neurons were large primary afferents that were not labeled by Griffonia simplicifolia isolectin B4 (marker of small primary sensory neurons). In dorsal and ventral roots 27+/-3% and 40+/-3%, respectively, of myelinated axons displayed immunoreactivity for alpha(3) Na(+),K(+)-ATPase. In contrast to the dorsal roots, strong immunoreactivity in ventral roots was observed only in myelinated axons of small caliber, presumably gamma-efferents. In the mixed sciatic nerve alpha(3) Na(+),K(+)-ATPase was detected in 26+/-5% of myelinated axons (both small and large caliber). In extensor hallicus proprius and lumbricales hind limb muscles alpha(3) Na(+),K(+)-ATPase was detected in some intramuscular axons and axonal terminals on intrafusal muscle fibers in the spindle equatorial and polar regions (regions of afferent and efferent innervation of the muscle stretch receptor, respectively). No alpha(3) Na(+),K(+)-ATPase was found in association with innervation of extrafusal muscle fibers or in tendon-muscle fusion regions. These data demonstrate non-uniform expression of the alpha(3) isoform of the Na(+),K(+)-ATPase in rat peripheral nervous system and suggest that alpha(3) Na(+),K(+)-ATPase is specifically expressed in afferent and efferent axons innervating skeletal muscle stretch receptors.

No evidence for a role in signal-transduction of Na+/K+-ATPase interaction with putative endogenous ouabain

A cascade of events (signal-transduction), mainly seen in rat cardiac myocytes and renal cells, is thought to occur after ouabain interaction with a minor fraction of Na+/K+-ATPase. A higher intracellular Na+ concentration followed sodium pump inhibition by ouabain with a subsequent gradual increase or oscillations in intracellular Ca2+ concentration. Whether this increase in intracellular Ca2+ concentration is part of the cascade, a result of the cascade or a totally independent phenomenon are conflicting interpretations that are discussed. At best, however, the cascade is initiated by ouabain concentrations several orders of magnitude higher than the measured plasma concentrations of putative endogenous ouabain. The experimentally high ouabain concentration may be critical for another reason. Most tissues contain various isoforms of the catalytic alpha-peptide of Na+/K+-ATPase with an individual sublocalization and, in rats, with different ouabain-sensitivity. The almost ouabain-insensitive alpha1-isoform of Na+/K+-ATPase is essentially unaffected by the high ouabain concentration, whereas ouabain-sensitive alpha-isoforms, possibly confined to membrane structures near cytosolic microdomains and Na+/Ca2+ exchangers, may be totally blocked. Classifying endogenous ouabain as a physiological inducer of the signaling system on this background seems hazardous.

Properties of the Na+/K+ pump current in small neurons from adult rat dorsal root ganglia

1 The present investigation was undertaken to characterize the Na(+)/K(+) pump current in small ( Collapse

Key Words

• na⁺/k⁺ pump current
• dorsal root ganglion
• neuron
• ouabain
• α1 isoform
• α3 isoform

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MESH Headings

• Animals
• Cell Size/drug effects
• Cell Size/physiology
• Cells, Cultured
• Dose-Response Relationship, Drug
• Ganglia, Spinal/cytology
• Ganglia, Spinal/drug effects
• Ganglia, Spinal/physiology
• Male
• Membrane Potentials/drug effects
• Membrane Potentials/physiology
• Neurons/cytology
• Neurons/drug effects
• Neurons/physiology
• Ouabain/pharmacology
• Rats
• Rats, Sprague-Dawley
• Sodium-Potassium-Exchanging ATPase/physiology

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Affiliation(s)

Kanako Hamada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Hiroshi Matsuura Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mitsuru Sanada Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Futoshi Toyoda Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Mariko Omatsu-Kanbe Department of Physiology, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan
Atsunori Kashiwagi Division of Endocrinology and Metabolism, Department of Medicine, Otsu, Shiga University of Medical Science, Shiga 520-2192, Japan
Hitoshi Yasuda Division of Neurology, Department of Medicine, Shiga University of Medical Science, Otsu, Shiga 520-2192, Japan

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FXYD proteins: new tissue- and isoform-specific regulators of Na,K-ATPase

The recently defined FXYD protein family contains seven members that are small, single-span membrane proteins characterized by a signature sequence containing an FXYD motif and three other conserved amino acid residues. Until recently, the functional role of FXYD proteins was largely unknown, with the exception of the gamma subunit of Na,K-ATPase, which was shown to be a specific regulator of renal alpha1-beta1 isozymes. We have investigated whether other members of the FXYD family may have a similar role as the gamma subunit and have found that CHIF (corticosteroid hormone-induced factor, FXYD4), FXYD7, as well as phospholemman (FXYD1) specifically associate with Na,K-ATPase and preferentially with alpha1-beta isozymes in native tissues, and produce distinct effects on the transport properties of Na,K-ATPase that are adapted to the physiological demands of the tissues in which they are expressed. These results provide evidence for a unique and novel mode of regulation of Na,K-ATPase by FXYD proteins that involves a tissue-specific expression of an auxiliary subunit of distinct Na,K-ATPase isozymes.

The Na,K-ATPase alpha 2 isoform is expressed in neurons, and its absence disrupts neuronal activity in newborn mice

Na,K-ATPase is an ion transporter that impacts neural and glial physiology by direct electrogenic activity and the modulation of ion gradients. Its three isoforms in brain have cell-type and development-specific expression patterns. Interestingly, our studies demonstrate that in late gestation, the alpha2 isoform is widely expressed in neurons, unlike in the adult brain, in which alpha2 has been shown to be expressed primarily in astrocytes. This unexpected distribution of alpha2 isoform expression in neurons is interesting in light of our examination of mice lacking the alpha2 isoform which fail to survive after birth. These animals showed no movement; however, defects in gross brain development, muscle contractility, neuromuscular transmission, and lung development were ruled out. Akinesia suggests a primary neuronal defect and electrophysiological recordings in the pre-Bötzinger complex, the brainstem breathing center, showed reduction of respiratory rhythm activity, with less regular and smaller population bursts. These data demonstrate that the Na,K-ATPase alpha2 isoform could be important in the modulation of neuronal activity in the neonate.

Global loss of Na,K-ATPase and its nitric oxide-mediated regulation in a transgenic mouse model of amyotrophic lateral sclerosis

Na,K-ATPase plays a critical role in energy metabolism and ion fluxes. Its loss was investigated in the G93A mouse model of amyotrophic lateral sclerosis (ALS) in which the mutation of Cu/Zn superoxide dismutase (SOD1) is thought to lead to aberrant oxidative damage. Observed losses in spinal cord Na,K-ATPase activity exceeded all expectations. All three catalytic subunit isoforms (alpha1, alpha2, alpha3) were reduced, and the global alpha subunit loss affected not just neurons, glia, and myelinated axon tracts but even ependymal and pial membranes. Decreases in Na,K-ATPase activity were greater than losses of protein, and there were losses of Na,K-ATPase alpha, but not beta, subunits. Together, these observations are consistent with selective degradation of the alpha subunit after damage. Overexpression of normal SOD1 does not cause ALS-like symptoms, but it has other known pathological effects. In transgenic mice overexpressed normal human SOD1 had a smaller but still considerable effect on Na,K-ATPase. Furthermore, the nitric oxide-mediated regulatory pathway for Na,K-ATPase inhibition was undetectable in spinal cord tissue slices from mice overexpressing either mutant or normal human SOD1. Na,K-ATPase activity did not respond to nitric oxide donors, and the free radical-dependent step of the pathway could not be bypassed by the addition of the downstream protein kinase G activator, 8-Br-cGMP. The data demonstrate that Na,K-ATPase is vulnerable to aberrant SOD1 activity, making it a potential contributing factor in disease pathology. Moreover, the global cellular distribution of Na,K-ATPase loss indicates that SOD1 overexpression is far-reaching in its pathological effects.

FXYD proteins: new tissue-specific regulators of the ubiquitous Na,K-ATPase

Maintenance of the Na+ and K+ gradients between the intracellular and extracellular milieus of animal cells is a prerequisite for basic cellular homeostasis and for functions of specialized tissues. The Na,K-ATPase, an oligomeric P-type adenosine triphosphatase (ATPase), is composed of a catalytic alpha subunit and a regulatory beta subunit and is the main player that fulfils these tasks. A variety of regulatory mechanisms are necessary to guarantee appropriate Na,K-ATPase expression and activity adapted to changing physiological demands. Recently, a regulatory mechanism was defined that is mediated by interaction of Na,K-ATPase with small proteins of the FXYD family, which possess a single transmembrane domain and so far have been considered as channels or regulators of ion channels. The mammalian FXYD proteins FXYD1 through FXYD7 exhibit tissue-specific distribution. Phospholemman (FXYD1) in heart and skeletal muscle, the gamma subunit of Na,K-ATPase (FXYD2) and corticosteroid hormone-induced factor (FXYD4, also known as CHIF) in the kidney, and FXYD7 in the brain associate preferentially with the widely expressed Na,K-ATPase alpha1-beta1 isozyme and modulate its transport activity in a way that conforms to tissue-specific requirements. Thus, tissue- and isozyme-specific interaction of Na,K-ATPase with FXYD proteins contributes to proper handling of Na+ and K+ by the Na,K-ATPase, and ensures correct function in such processes as renal Na+-reabsorption, muscle contraction, and neuronal excitability.

Na pump isoforms in human erythroid progenitor cells and mature erythrocytes

This study is aimed at identifying the Na pump isoform composition of human erythroid precursor cells and mature human erythrocytes. We used purified and synchronously growing human erythroid progenitor cells cultured for 7-14 days. RNA was extracted from the progenitor cells on different days and analyzed by RT-PCR. The results showed that only the alpha1, alpha3, beta2, and beta3 subunit isoforms and the gamma modulator were present. Northern analysis of the erythroid progenitor cells again showed that beta2 but not beta1 or alpha2 isoforms were present. The erythroid cells display a unique beta subunit expression profile (called beta-profiling) in that they contain the message for the beta2 isoform but not beta1, whereas leukocytes and platelets are known to have the message for the beta1 but not for the beta2 isoform. This finding is taken to indicate that our preparations are essentially purely erythroid and free from white cell contamination. Western analysis of these cultured progenitor cells confirmed the presence of alpha1, alpha3, (no alpha2), beta2, beta3, and gamma together now with clear evidence that beta1 protein was also present at all stages. Western analysis of the Na pump from mature human erythrocyte ghosts, purified by ouabain column chromatography, has also shown that alpha1, alpha3, beta1, beta2, beta3, and gamma are present. Thus, the Na pump isoform composition of human erythroid precursor cells and mature erythrocytes contains the alpha1 and alpha3 isoforms of the alpha subunit, the beta1, beta2, and beta3 isoforms of the beta subunit, and the gamma modulator.

Interleukin-1beta increases urine flow rate and inhibits protein expression of Na(+)/K(+)-ATPase in the rat jejunum and kidney

The effect of interleukin-1beta (IL-1beta) on urine flow rates and Na(+)/K(+)-ATPase activity and expression was studied in rat intestinal and renal epithelia. The cytokine produced a significant diuretic effect and increased urine flow rate by around 10-fold compared with the control. This effect was considered to be secondary to the well-documented natriuretic effect of the cytokine described in the literature. On the other hand, we have shown previously that IL-1beta inhibits glucose absorption from the jejunum. As sodium excretion and glucose absorption are both dependent on Na(+)/K(+)-ATPase activity, the effect of the cytokine on the renal and intestinal pump was investigated. IL-1beta inhibited, in a dose-dependent manner, the activity of Na(+)/K(+)-ATPase in villus and crypt jejunal cells and in medullary and cortical kidney cells. Western blot analysis revealed a decrease in the protein expression of the enzyme, which was confirmed by the radiolabeled ouabain binding assay. The results suggest that the diuretic and natriuretic effect of IL-1beta and its inhibitory effect on glucose absorption are all due to downregulation of the Na(+)/K(+) pump in the kidney and jejunum.

Thyroidal regulation of different isoforms of NaKATPase in the primary cultures of neurons derived from fetal rat brain

The developmental profile of the different isoforms of NaKATPase have been investigated using primary cultures of isolated neurons initiated from 17 day old fetal rat brain. Northern blot analysis showed that the expression of three alpha isoforms (alpha(1), alpha(2) and alpha(3)) and two beta isoforms (beta(1) and beta(2)) increased progressively and reached a peak between 12 to 16 days of culture. Comparison of the mRNA levels of these isoforms in the cells maintained in thyroid hormone deficient (TH def) and thyroid hormone supplemented (TH sup) media for 6-12 days, revealed for the first time that in the neurons three alpha and two beta isoforms of NaKATPase are sensitive to TH. Furthermore immunocytochemical staining of these cells with isoform specific NaKATPase antibodies showed that the uniform distribution of alpha(2), alpha(3) and beta(2) isoforms in the neuronal processes require the presence of TH. These results establish neurons as the target cells for the regulation of NaKATPase by TH in the developing brain.

Phospholemman (FXYD1) associates with Na,K-ATPase and regulates its transport properties

A family of small, single-span membrane proteins (the FXYD family) has recently been defined based on their sequence and structural homology. Some members of this family have already been identified as tissue-specific regulators of Na,K-ATPase (NKA). In the present study, we demonstrate that phospholemman (PLM) (FXYD1), so far considered to be a heart- and muscle-specific channel or channel-regulating protein, associates specifically and stably with six different alpha-beta isozymes of NKA after coexpression in Xenopus oocytes, and with alpha1-beta, and less efficiently with alpha2-beta isozymes, in native cardiac and skeletal muscles. Stoichiometric association of PLM with NKA occurs posttranslationally either in the Golgi or the plasma membrane. Interaction of PLM with NKA induces a small decrease in the external K+ affinity of alpha1-beta1 and alpha2-beta1 isozymes and a nearly 2-fold decrease in the internal Na+ affinity. In conclusion, this study demonstrates that PLM is a tissue-specific regulator of NKA that may play an essential role in muscle contractility.

K+-induced hyperpolarization in rat mesenteric artery: identification, localization and role of Na+/K+-ATPases

1. Mechanisms underlying K(+)-induced hyperpolarizations in the presence and absence of phenylephrine were investigated in endothelium-denuded rat mesenteric arteries (for all mean values, n=4). 2. Myocyte resting membrane potential (m.p.) was -58.8+/-0.8 mV. Application of 5 mM KCl produced similar hyperpolarizations in the absence (17.6+/-0.7 mV) or presence (15.8+/-1.0 mV) of 500 nM ouabain. In the presence of ouabain +30 microM barium, hyperpolarization to 5 mM KCl was essentially abolished. 3. In the presence of 10 microM phenylephrine (m.p. -33.7+/-3 mV), repolarization to 5 mM KCl did not occur in the presence or absence of 4-aminopyridine but was restored (-26.9+/-1.8 mV) on addition of iberiotoxin (100 nM). Under these conditions the K+-induced repolarization was insensitive to barium (30 microM) but abolished by 500 nM ouabain alone. 4. In the presence of phenylephrine + iberiotoxin the hyperpolarization to 5 mM K(+) was inhibited in the additional presence of 300 nM levcromakalim, an action which was reversed by 10 microM glibenclamide. 5. RT-PCR, Western blotting and immunohistochemical techniques collectively showed the presence of alpha(1)-, alpha(2)- and alpha(3)-subunits of Na(+)/K(+)-ATPase in the myocytes. 6. In K(+)-free solution, re-introduction of K(+) (to 4.6 mM) hyperpolarized myocytes by 20.9+/-0.5 mV, an effect unchanged by 500 nM ouabain but abolished by 500 microM ouabain. 7. We conclude that under basal conditions, Na(+)/K(+)-ATPases containing alpha(2)- and/or alpha(3)-subunits are partially responsible for the observed K(+)-induced effects. The opening of myocyte K(+) channels (by levcromakalim or phenylephrine) creates a 'K(+) cloud' around the cells which fully activates Na(+)/K(+)-ATPase and thereby abolishes further responses to [K(+)](o) elevation.

FXYD7 is a brain-specific regulator of Na,K-ATPase alpha 1-beta isozymes

Recently, corticosteroid hormone-induced factor (CHIF) and the gamma-subunit, two members of the FXYD family of small proteins, have been identified as regulators of renal Na,K-ATPase. In this study, we have investigated the tissue distribution and the structural and functional properties of FXYD7, another family member which has not yet been characterized. Expressed exclusively in the brain, FXYD7 is a type I membrane protein bearing N-terminal, post-translationally added modifications on threonine residues, most probably O-glycosylations that are important for protein stabilization. Expressed in Xenopus oocytes, FXYD7 can interact with Na,K-ATPase alpha 1-beta 1, alpha 2-beta 1 and alpha 3-beta 1 but not with alpha-beta 2 isozymes, whereas, in brain, it is only associated with alpha 1-beta isozymes. FXYD7 decreases the apparent K(+) affinity of alpha 1-beta 1 and alpha 2-beta 1, but not of alpha 3-beta1 isozymes. These data suggest that FXYD7 is a novel, tissue- and isoform-specific Na,K-ATPase regulator which could play an important role in neuronal excitability.

K(+) cycling and its regulation in the cochlea and the vestibular labyrinth

Potassium (K(+)) plays a very important role in the cochlea. K(+) is the major cation in endolymph and the charge carrier for sensory transduction and the generation of the endocochlear potential. The importance of K(+) handling in the cochlea is marked by the discovery of several forms of hereditary deafness that are due to mutations of K(+) channels. Deafness results from mutations of KCNQ4, a K(+) channel in the sensory hair cells, as well as from mutations of the gap junction proteins GJB2, GJB3 and GJB6 that may facilitate cell-to-cell movements of K(+). Deafness results also from mutations of KCNQ1 or KCNE1, subunits of a K(+) channel that carries K(+) from strial marginal cells and vestibular dark cells into endolymph. Further, deafness results from mutations of KCNJ10, a K(+) channel that generates the endocochlear potential in conjunction with the high K(+) concentration in strial intermediate cells and the low K(+) concentration in the intrastrial fluid spaces. This review details recent advances in the understanding of K(+) transport and its regulation in the cochlea and the vestibular labyrinth.

The role of Na,K-ATPase in human sperm motility

A fourth Na,K-ATPase alpha isoform, which was found to be abundant in testes, was proved to be a catalytical subunit of the enzyme. Recently, it has been shown that the alpha 4 isoform along with alpha 1 is expressed in the midpiece of the flagellum of mature rat sperm and the inhibition of alpha 4 with ouabain led to sperm immotility. In this study, sperm from 135 males with normal semen profile and 50 males with oligoasthenospermia were treated with 10-5 and 10-2 M ouabain solutions to inhibit alpha 4, and alpha 4 plus alpha 1 isoforms, respectively. In males with normal semen profile, sperm motility has been demonstrated to decrease with time to almost the same level with both ouabain solutions. In oligoasthenospermic males motility was also found almost completely lost. These observations showed us that the alpha 4 isoform may be held responsible for human sperm motility. When sperm plasma membrane Na,K-ATPase activity was assayed for both normal and oligoasthenospermic males, a significant decrease in enzyme activity of males with oligoasthenospermia was observed (p < 0.05). In our recent study, sperm motility was found decreased by treatment with peroxynitrite (ONOO-). To investigate the effect of ONOO- on sperm Na,K-ATPase activity, sperm plasma membranes were treated with 100 microM ONOO- and plasma membrane Na,K-ATPase activity was observed to be significantly decreased (p < 0.05). Although total sulfhydryl (SH) content of sperm plasma membrane was also found significantly lower, no correlation was found between Na,K-ATPase activity and SH content.

Betam, a structural member of the X,K-ATPase beta subunit family, resides in the ER and does not associate with any known X,K-ATPase alpha subunit

betam, a muscle-specific protein, is structurally closely related to the X,K-ATPase beta subunits, but its intrinsic function is not known. In this study, we have expressed betam in Xenopus oocytes and have investigated its biosynthesis and processing as well as its putative role as a chaperone of X,K-ATPase alpha subunits, as a regulator of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA), or as a Ca(2+)-sensing protein. Our results show that betam is stably expressed in the endoplasmic reticulum (ER) in its core glycosylated, partially trimmed form. Both full-length betam, initiated at Met(1), and short betam species, initiated at Met(89), are detected in in vitro translations as well as in Xenopus oocytes. betam cannot associate with and stabilize Na,K-ATPase (NK), or gastric and nongastric H,K-ATPase (HK) alpha isoforms. betam neither assembles stably with SERCA nor is its trypsin sensitivity or electrophoretic mobility influenced by Ca(2+). A mutant, in which the distinctive Glu-rich regions in the betam N-terminus are deleted, remains stably expressed in the ER and can associate with, but not stabilize X,K-ATPase alpha subunits. On the other hand, a chimera in which the ectodomain of betam is replaced with that of beta1 NK associates efficiently with alpha NK isoforms and produces functional Na,K-pumps at the plasma membrane. In conclusion, our results indicate that betam exhibits a cellular location and functional role clearly distinct from the typical X,K-ATPase beta subunits.