Ontogeny of the (Na+,K+)-ATPase during chick skeletal myogenesis

The role of Na+/K+-ATPase during chick skeletal myogenesis

The formation of a vertebrate skeletal muscle fiber involves a series of sequential and interdependent events that occurs during embryogenesis. One of these events is myoblast fusion which has been widely studied, yet not completely understood. It was previously shown that during myoblast fusion there is an increase in the expression of Na⁺/K⁺-ATPase. This fact prompted us to search for a role of the enzyme during chick in vitro skeletal myogenesis. Chick myogenic cells were treated with the Na⁺/K⁺-ATPase inhibitor ouabain in four different concentrations (0.01-10 μM) and analyzed. Our results show that 0.01, 0.1 and 1 μM ouabain did not induce changes in cell viability, whereas 10 μM induced a 45% decrease. We also observed a reduction in the number and thickness of multinucleated myotubes and a decrease in the number of myoblasts after 10 μM ouabain treatment. We tested the involvement of MEK-ERK and p38 signaling pathways in the ouabain-induced effects during myogenesis, since both pathways have been associated with Na⁺/K⁺-ATPase. The MEK-ERK inhibitor U0126 alone did not alter cell viability and did not change ouabain effect. The p38 inhibitor SB202190 alone or together with 10 μM ouabain did not alter cell viability. Our results show that the 10 μM ouabain effects in myofiber formation do not involve the MEK-ERK or the p38 signaling pathways, and therefore are probably related to the pump activity function of the Na⁺/K⁺-ATPase.

Inhibition of myogenesis by ouabain: effect on protein synthesis

Ouabain, a specific inhibitor of the sodium- and potassium-activated adenosine triphosphatase, causes reversible inhibition of the fusion of myoblasts to form myotubes. We further examined this observation to investigate whether control of Na/K-ATPase activity may normally contribute to the regulation of myogenesis. In control cultures, fusion was preceded by a small decrease in intracellular sodium concentration, but intracellular sodium and potassium increased significantly during fusion. Levels of ouabain that produce prolonged inhibition of fusion (400 microM) virtually eliminated sodium and potassium gradients. However, lower ouabain levels (10-100 microM) also produced significant changes in intracellular potassium and/or sodium along with little apparent decrease in the eventual extent of fusion. The effect of ouabain on protein synthesis was also examined. Low levels of ouabain (<50 microM) that did not affect myogenesis also did not affect incorporation of radiolabeled amino acids, while higher concentrations produced a decline in protein synthesis that paralleled decreases in the rate of myoblast fusion. Levels of metabolic labeling were reduced 90% in cultures treated with 400 microM ouabain. Inhibition of protein synthesis would prevent membrane remodeling required for fusion and other events in myogenesis. Thus, our results do not support any specific role for the sodium- and potassium-activated adenosine triphosphatase in regulating myogenesis.

Adenosine triphosphatases during maturation of cultured human skeletal muscle cells and in adult human muscle

Na+/K(+)-ATPase, Mg(2+)-ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase are examined in cultured human skeletal muscle cells of different maturation grade and in human skeletal muscle. Na+/K(+)-ATPase is investigated by measuring ouabain binding and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase). SR Ca(2+)-ATPase is examined by ELISA, Ca(2+)-dependent phosphorylation and its activities on ATP and 3-O-methylfluorescein phosphate. Na+/K(+)-ATPase and SR Ca(2+)-ATPase are localized by immunocytochemistry. The activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase show a good correlation with the other assayed parameters of these ion pumps. All ATPase parameters investigated increase with the maturation grade of the cultured muscle cells. The number of ouabain-binding sites and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-MFPase are significantly higher in cultured muscle cells than in muscle. The Mg(2+)-ATPase activity, the content of SR Ca(2+)-ATPase and the activities of SR Ca(2+)-ATPase and Ca(2+)-dependent 3-O-MFPase remain significantly lower in cultured cells than in muscle. The ouabain-binding constant and the molecular activities of Na+/K(+)-ATPase and SR Ca(2+)-ATPase are equal in muscle and cultured cells. During ageing of human muscle the activity as well as the concentration of SR Ca(2+)-ATPase decrease. Thus the changes of the activities of the ATPases are caused by variations of the number of their molecules. Na+/K(+)-ATPase is localized in the periphery of fast- and slow-twitch muscle fibers and at the sarcomeric I-band. SR Ca(2+)-ATPase is predominantly confined to the I-band, whereas fast-twitch fibers are much more immunoreactive than slow-twitch fibers. The presence of cross-striation for Na+/K(+)-ATPase and SR Ca(2+)-ATPase in highly matured cultured muscle cells indicate the development and subcellular organization of a transverse tubular system and SR, respectively, which resembles the in vivo situation.

Modulation of Na+/K+ exchange potentiates lipopolysaccharide-induced gene expression in murine peritoneal macrophages

The role of Na+/K+ exchange in regulating lipopolysaccharide (LPS)-mediated induction of cytokine gene expression has been examined in murine peritoneal macrophages. Depletion of K+ from the culture medium resulted in a three- to five-fold potentiation of tumor necrosis factor-alpha (TNF alpha), KC (gro), and IP-10 mRNA expression in LPS-treated macrophages. The potentiating effect was apparently the result of inhibition of Na+/K+ exchange through the Na+/K(+)-adenosine triphosphatase (ATPase) because ouabain-mediated inhibition of Na+/K(+)-ATPase was also able to potentiate cytokine mRNA expression as much or more than did K+ depletion. The effects of K+ depletion or ouabain treatment were not caused by depolarization of the macrophage membrane because depolarization mediated by elevating extracellular K+ levels was inhibitory to cytokine mRNA expression. Depletion of Na+ by substitution with choline in the culture medium also markedly potentiated LPS-induced gene expression. The Na+/H+ antiporter was not, however, involved in potentiating cytokine expression because treatment of macrophages with amiloride either had no effect on or was inhibitory to the LPS-induced changes in mRNA levels. The potentiation of gene expression was selective and was at least partially the result of increased transcriptional activity of each gene. Whereas Na+ depletion and ouabain both inhibited 86Rb+ uptake by macrophages, treatment with LPS had no effect either on Rb+ uptake or on efflux. Thus altered Na+/K+ exchange is not a component of the primary signalling pathway(s) mediating response to LPS. Nevertheless, modulation of macrophage Na+/K+ exchange by agents encountered during an inflammatory response may be an important determinant of the magnitude and quality of specific gene expression.

Na(+)-K(+)-ATPase in adipocyte differentiation in culture

Differentiation of 3T3-L1 cells from a fibroblast to an adipocyte phenotype results in an approximately 50% decline in Na(+)-K(+)-ATPase activity and ouabain-sensitive 86Rb uptake. Kinetic analysis revealed a K 1/2 for Na+ of approximately 14 mM, a Km for ATP of approximately 0.4 mM, and maximal activation by sodium dodecyl sulfate at a 0.05 (wt/wt) detergent/protein ratio in both mature fibroblasts and adipocytes. Both fibroblasts and adipocytes exhibited Na(+)-K(+)-ATPase activity with an inhibition constant (Ki) for ouabain of approximately 10(-4) M. In addition, adipocytes exhibited a second component representing 30% of total activity with a Ki of approximately 5 x 10(-7) M. The emergence of biphasic ouabain inhibition kinetics in adipocytes raised the possibility of a change in alpha-subunit isoform composition with cytodifferentiation. This inference was evaluated by isoform-specific mRNA analysis (Northern blots) and by alpha-isoform-specific immunoassays (Western blots). Northern blots revealed a modest decrease in mRNA alpha 1, a striking increase in mRNA alpha 2, and a significant loss of mRNA beta content with differentiation of fibroblasts to adipocytes. By immunoassay, fibroblasts exhibited the alpha 1-isoform. Adipocytes exhibited an admixture of alpha 1- and alpha 2-isoforms, with alpha 2 being the more abundant isoform. There was no one-to-one correspondence either between the mRNA isoform and alpha-subunit abundances or between alpha-subunit abundances and enzymatic activity, suggesting that regulation occurs at multiple levels in this system. Findings indicate, however, that a shift in alpha-isoform composition accompanied by a change in ouabain inhibition kinetics occurs with cytodifferentiation.

Molecular genetics of Na,K-ATPase

Researchers in the past few years have successfully used molecular-genetic approaches to determine the primary structures of several P-type ATPases. The amino-acid sequences of distinct members of this class of ion-transport ATPases (Na,K-, H,K-, and Ca-ATPases) have been deduced by cDNA cloning and sequencing. The Na,K-ATPase belongs to a multiple gene family, the principal diversity apparently resulting from distinct catalytic alpha isoforms. Computer analyses of the hydrophobicity and potential secondary structure of the alpha subunits and primary sequence comparisons with homologs from various species as well as other P-type ATPases have identified common structural features. This has provided the molecular foundation for the design of models and hypotheses aimed at understanding the relationship between structure and function. Development of a hypothetical transmembrane organization for the alpha subunit and application of site-specific mutagenesis techniques have allowed significant progress to be made toward identifying amino acids involved in cardiac glycoside resistance and possibly binding. However, the complex structural and functional features of this protein indicate that extensive research is necessary before a clear understanding of the molecular basis of active cation transport is achieved. This is complicated further by the paucity of information regarding the structural and functional contributions of the beta subunit. Until such information is obtained, the proposed model and functional hypotheses should be considered judiciously. Considerable progress also has been made in characterizing the regulatory complexity involved in expression of multiple alpha-isoform and beta-subunit genes in various tissues and cells during development and in response to hormones and cations. The regulatory mechanisms appear to function at several molecular levels, involving transcriptional, posttranscriptional, translational, and posttranslational processes in a tissue- or cell-specific manner. However, much research is needed to precisely define the contributions of each of these mechanisms. Recent isolation of the genes for these subunits provides the framework for future advances in this area. Continued application of biochemical, biophysical, and molecular genetic techniques is required to provide a detailed understanding of the mechanisms involved in cation transport of this biologically and pharmacologically important enzyme.

Single K+ currents during differentiation of embryonic muscle cells in vitro

After 3-7 days in culture, chicken myotubes possess five types of K+ channel: two high-conductance channels of 195 and 105 pS which are sensitive to tetraethylammonium (TEA), an ATP-sensitive channel of 64 pS and two low-conductance channels of 40 and 15 pS which are insensitive to TEA and ATP. The same population of channels is to be found in EGTA-treated muscle cells with blocked fusion and, with the exception of the ATP-sensitive channel, also in 1-day-old myoblasts. There are differences between myoblasts and myotubes in the percentage of incidence of individual channel types. High-conductance K+ channels are most frequently to be observed in myotubes, but they are rare in myoblasts and EGTA-treated cells where low-conductance K+ channels predominate.

Regulation of the sodium-potassium pump in cultured rat skeletal myotubes by intracellular sodium ions

The properties of the Na-K pump and some of the factors controlling its amount and function were studied in rat myotubes in culture. The number of Na-K pump sites was quantified by measuring the amount of [3H]ouabain bound to whole-cell preparations. Activity of the pump was determined by measurement of ouabain-sensitive 86Rb-uptake and component of membrane potential. Chronic treatment of myotubes with tetrodotoxin (TTX), which lowers [Na]i, decreased the number of Na-K pumps, the ouabain-sensitive 86Rb uptake, and the size of the electrogenic pump component of Em. In contrast, chronic treatment with either ouabain or veratridine, which increases [Na+]i, resulted in an elevated level of Na-K pump sites. This effect was blocked by inhibitors of protein synthesis. Neither rates of degradation nor affinity of pump sites in cells treated with TTX, veratridine, or ouabain differred from those in control cells. The number and activity of Na-K pump sites were unaffected by chronic elevation in [Ca]i or chronic depolarization. We conclude that alterations in the level in intracellular Na ions play the major role in regulation of Na-K pump synthesis in cultured mammalian skeletal muscle.

Changes in myoblast membrane electrical properties during cell-cell adhesion and fusion in vitro

Characteristic of the process of myogenesis are the changes in the composition and organization of the cell membrane. While poorly understood, these changes have biochemical and biophysical relevance. Recently, changes in molecular order of the myoblast membrane which accompany differentiation in vitro have been observed (Santini, M.T., Indovina, P.L. and Hausman, R.E. (1987) Biochim. Biophys. Acta 896, 19-25). To further investigate these cell fusion processes we have examined additional physical parameters: conductivity and permittivity of the myoblast membrane during differentiation which reflect the molecular arrangement of the membrane. The determination of these parameters is possible because in the radio frequency range suspensions of cells in an electrolyte buffer show a characteristic conductivity dispersion due to the interfacial polarization. An analysis of our experimental data based on a 'single-shell' model showed that conductivity and permittivity of the membrane of pre- and post-fusion myoblasts varied significantly and abruptly. The conductivity of the cell interior (cytosol) remained constant. We discuss the significance of the observed changes in these membrane parameters for myogenesis.

Measurement of cytoplasmic free Ca2+ concentration in cultured muscle cells by aequorin and quin 2

Ca2+ has been proposed to regulate expression of the gene for the Ca2+ pump of the sarcoplasmic reticulum in developing chicken myoblasts (A. N. Martonosi, L. Dux, R. L. Terjung, and D. Roufa. Ann. NY Acad. Sci. 402: 485-514, 1982. In the present study, intracellular free Ca2+ ([Ca2+]i) was measured with the photoprotein, aequorin, incorporated by a reversible hyperpermeabilization technique, and with the fluorescent probe, quin 2. Because aequorin reacts irreversibly with Ca2+ and is inactivated by heat, at 37 degrees C the active aequorin content declined markedly. The resting glow (1/10(6) of the maximum light) disappeared after a few hours, whereas the light from the total active aequorin remaining was still detectable when cells were lysed 3 days later. A new approach, which compared the rate of disappearance of aequorin in treated and control cells, was developed on the basis that aequorin would be inactivated more quickly in cells with higher [Ca2+]i. A23187, ionomycin, and trifluoperazine, all of which accelerated gene expression (A. N. Martonosi et al.), increased the rate of decay of active aequorin and therefore increased [Ca2+]i. "Ca shock", and ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid, which also increased the gene expression (A. N. Martonosi et al.), caused small increases of [Ca2+]i not detectable by aequorin but detectable by quin 2. X537A and ouabain had no effect on the gene expression (A. N. Martonosi et al.) and did not raise [Ca2+]i. The results support the proposal that Ca2+ may regulate expression of this gene.

The role of the Na+/H+ exchange system in the regulation of the internal pH in cultured cardiac cells

The Na+/H+ exchange system is not the major mechanism that regulates the internal pH value (pHi) of chick cardiac cells in culture under normal physiological conditions in the absence of carbonate. In cardiac cells in which the internal pH has been lowered to 6.6-6.7, the Na+/H+ exchanger becomes the major mechanism to bring back pHi to normal values (pHi = 7.3). The blockade of the Na+/H+ exchange activity with an active amiloride derivative, ethylisopropylamiloride, prevents internal pH recovery. The internal pH dependence of the Na+/H+ exchanger activity has been carefully studied. The [H+]i-dependence is very cooperative. For an external pH of 7.4, the system is nearly completely inactive at pHi 7.8 and nearly completely active at pHi 6.9-7.0 with half-maximum activation at pHi = 7.35. The increased activity of the Na+/H+ exchange system which follows the acidification of the internal medium produces an activation of the (Na+,K+)-ATPase.

Estimation of stability of [3H]-ouabain binding site concentration in rat and human skeletal muscle post mortem

The post mortem stability of the [3H]-ouabain binding site concentration and 3-O-methylfluorescein phosphatase (MFPase) activity was evaluated in rat skeletal muscle. As compared with the values measured in fresh tissue, the [3H]-ouabain binding site concentration in rat soleus muscle only dropped by around 1% per hour post mortem and a significant decrease was only seen after 12 h (15%, p less than 0.02). The 3-O-MFPase activity in rat gastrocnemius muscle showed a similar decrease. After 4 days, both parameters had dropped by 65% (p less than 0.001). In contrast, when intact fresh rat soleus muscles were incubated in Krebs-Ringer bicarbonate buffer at 20 degrees C for 4 days no significant decrease was seen in the [3H]-ouabain binding site concentration. In 10 human subjects the concentration of [3H]-ouabain binding sites was measured in specimens of the vastus lateralis muscle obtained within half an hour and at 6 and 12 h post mortem. The relative decrease after 6 h was insignificant (8%, p less than 0.30), whereas it was significant after 12 h (29%, p less than 0.005). These results have shown that the [3H]-ouabain binding sites in human skeletal muscle are resistant to post mortem degradation during the first 6 h after death. This makes it possible to perform measurements post mortem of the [3H]-ouabain binding site concentration in human skeletal muscle.

A method for the determination of the total number of 3H-ouabain binding sites in biopsies of human skeletal muscle

A new method based on vanadate facilitated binding of 3H-ouabain has been applied for the quantitative determination of the number of 3H-ouabain binding sites (Na-K-pumps) in needle biopsies of human skeletal muscle. Samples of the vastus lateralis muscle weighing 2-8 mg showed specific and saturable binding of 3H-ouabain with an apparent KD of 1.9 X 10(-8) mol/l. In 20 healthy human subjects in the age range 25-80 years, the number of 3H-ouabain binding sites was 278 +/- 15 pmol/g wet weight with no relation to age or sex. In samples of the intercostal and rectus abdominis muscles, the number of 3H-ouabain binding sites varied from 225 to 280 pmol/g wet weight. These values are at least 2 times higher than those previously reported for human skeletal muscle. The number of 3H-ouabain and 3H-digoxin binding sites were identical, and ouabain (10(-3) mol/l) completely displaced specifically bound 3H-digoxin. When biopsies were frozen in liquid N2 immediately after withdrawal, storage at -20 degrees C for up to 11 weeks caused no significant change in the number of 3H-ouabain binding sites. The method allows quantitative determination of the number of 3H-ouabain binding sites in standard biopsies of human skeletal muscle to be performed by simple procedures within a few hours. This can be used for the study of conditions where the number of Na-K-pumps is known to undergo fluctuations.

Monoclonal antibodies associated with sodium channel block nerve impulse and stain nodes of Ranvier

Monoclonal antibodies were generated against native eel electroplax sodium channels in their natural membrane. These antibodies block nerve conduction in rat central (optic) and peripheral (sciatic) nerve. The antibody binding to eel electroplax membrane fragments and to rat brain synaptosomes can be modulated by neurotoxins. Thus it implies that active sites of the sodium channels are immunogenic in their natural membrane. Unlike the antibodies described in the past, our antibodies recognize antigenic determinants which are associated with the physiological activity of the channel and have been conserved through evolution.

The age-dependent changes in the number of 3H-ouabain binding sites in mammalian skeletal muscle

The influence of age on the binding of 3H-ouabain in skeletal muscle has been characterized in rats, mice and guinea pigs. Measurements performed using biopsies and intact fibers obtained from different types of rat muscles showed that from birth to the 4th week of life, the number of 3H-ouabain binding sites per unit weight increases up to 5-fold, followed by almost the same relative decrease to a plateau around 250 pmol/g wet wt at an age of 22 weeks. These changes were not associated with any major alterations in apparent KD (1.7-3.1 X 10(-7) M) dissociation rate or heterogeneity in binding characteristics. Measurements of 3-O-methylfluorescein phosphatase activity, an enzyme activity which is closely correlated to the Na-K-ATPase activity, confirmed the 3H-ouabain binding data. In mice, the number of 3H-ouabain binding sites showed similar, albeit less pronounced changes with age, a maximum being reached at the 4th week of life. In guinea pigs, the number of 3H-ouabain binding sites per unit weight decreased by 60% from birth to maturity. The results indicate that the early development and differentiation of individual skeletal muscles is associated with a marked increase in the number of Na-K-pumps (when expressed as pmol/muscle), until at maturity a plateau is reached. However, when expressed as pmol/g wet wt the increase is followed by a decrease to a plateau. This may in part account for the relatively low digitalis sensitivity seen in infants as compared to newborn and mature individuals.

Comparative changes of levels of nitrendipine Ca2+ channels, of tetrodotoxin-sensitive Na+ channels and of ouabain-sensitive (Na+ + K+)-ATPase following denervation of rat and chick skeletal muscle

Three major ion transport systems, the nitrendipine-sensitive Ca2+ channels, the tetrodotoxin-sensitive Na+ channel and the ouabain-sensitive (Na+ + K+)-ATPase, have been studied in skeletal muscle from rat and chick after chronic denervation. It is shown that the situation found for the Ca2+ channel differs dramatically from that found for the Na+ channel and the (Na+ + K+)-ATPase and that regulation of the nitrendipine-sensitive Ca2+ channel in denervated muscle also differs widely from that of the tetrodotoxin-sensitive Na+ channel and the ouabain-sensitive (Na+ + K+)-ATPase which show a quite similar evolution.

(Na+ + K+)-ATPase activity of crude homogenates of rat skeletal muscle as estimated from their K+-dependent 3-O-methylfluorescein phosphatase activity

A highly sensitive fluorimetric assay using 3-O-methylfluorescein phosphate as substrate was used in the determination of K+-dependent phosphatase activity in preparations of rat skeletal muscle. The gastrocnemius muscle was chosen because of mixed fibre composition. Crude, detergent treated homogenate was used so as to avoid loss of activity during purification. K+-dependent phosphatase activities in the range 0.19-0.37 mumol X (g wet weight)-1 X min-1 were obtained, the value decreasing with age and K+-deficiency. Complete inhibition of the K+-dependent phosphatase was obtained with 10(-3) M ouabain. Using a KSCN-extracted muscle enzyme the intimate relation between K+-dependent phosphatase activity and (Na+ + K+)-activated ATP hydrolysis could be demonstrated. A molecular activity of 620 min-1 was estimated from simultaneous determination of K+-dependent phosphatase activity and [3H]ouabain binding capacity using the partially purified enzyme preparation. The corresponding enzyme concentration in the crude homogenates was calculated and corresponded well with the number of [3H]ouabain binding sites measured in intact muscles or biopsies hereof.