Dependence of Na+,K+-ATPase and electrogenic component of Em in cultured myotubes on cell fusion

Neural agrin changes the electrical properties of developing human skeletal muscle cells

Recent investigations suggest that the effects of neural agrin might not be limited to neuromuscular junction formation and maintenance and that other aspects of muscle development might be promoted by agrin. Here we tested the hypothesis that agrin induces a change in the excitability properties in primary cultures of non-innervated human myotubes. Electrical membrane properties of human myotubes were recorded using the whole-cell patch-clamp technique. Cell incubation with recombinant chick neural agrin (1 nM) led to a more negative membrane resting potential. Addition of strophanthidin, a blocker of the Na(+)/K(+) ATPase, depolarized agrin-treated myotubes stronger than control, indicating, in the presence of agrin, a higher contribution of the Na(+)/K(+) ATPase in establishing the resting membrane potential. Indeed, larger amounts of both the alpha1 and the alpha2 isoforms of the Na(+)/K(+) ATPase protein were expressed in agrin-treated cells. A slight but significant down-regulation of functional apamin-sensitive K(+) channels was observed after agrin treatment. These results indicate that neural agrin might act as a trophic factor promoting the maturation of membrane electrical properties during differentiation, confirming the role of agrin as a general promoter of muscle development.

Thyroid hormone up-regulates Na+/K+ pump alpha2 mRNA but not alpha2 protein isoform in cultured skeletal muscle

Thyroid hormone (T(3)) is known to up-regulate the physiological expression of the Na(+)/K(+) pump in cultured skeletal muscle. We recently reported that primary cultured rat skeletal muscle expresses only the alpha(1), beta(1) and beta(2) protein isoforms of Na(+)/K(+) pump. Interestingly, alpha(2) mRNA is detectable while the alpha(2) protein isoform is not. We therefore examined whether T(3) might up-regulate the expression of Na(+)/K(+) pump alpha(2) isoform at the protein and mRNA level. We also examined the regulation by this hormone of the other isoforms of the pump. Primary cultures were treated with T3 for 48 h from day 4 to day 6 of differentiation. Protein and mRNA isoforms of Na(+)/K(+) pump were identified by Western blotting and Northern blotting, respectively. T(3) induced a marked increase in the beta(1) protein and a slight increase in the alpha(1) protein. T(3) did not affect expression of the beta(2) protein. The alpha(2) protein was not detected in either untreated or T(3)-treated cells. In contrast, alpha(2) mRNA was highly up-regulated by T(3) treatment compared to the other isoforms. The lack of expression of the alpha(2) protein isoform following T(3) treatment suggests that posttranscriptional events related to this isoform may be dependent on other growth factors or hormones.

Na(+)/K(+) pump expression in the L8 rat myogenic cell line: effects of heterologous alpha subunit transfection

We have characterized the physiological and biochemical properties of the Na(+)/K(+) pump and its molecular expression in L8 rat muscle cells. Pump properties were measured by [(3)H]ouabain binding and (86)Rb uptake. Scatchard plot analysis of specific ouabain binding indicated the presence of a single family of binding sites with a B(max) of approximately 135 fmol/ mg P and a K(D) of 3.3 x 10(-8). (86)Rb uptake due to specific pump activity was found to be 20% of the total in L8 cells. The results indicated lower affinity of L8 cells for ouabain and lower activity of the pump than that reported for chick or rat skeletal muscle in primary culture. Both the alpha(1) and beta(1) protein and mRNA isoforms were expressed in myoblasts and in myotubes, while the alpha(2), alpha(3), and beta(2) isoforms were not detectable. We attempted to overcome low physiological expression of the Na(+)/K(+) pump by employing a vector expressing an avian high affinity alpha subunit. This allowed identification of the transfected subunit separate from that endogenously expressed in L8 cells. Successful transfection into L8 myoblasts and myotubes was recognized by anti-avian alpha subunit monoclonal antibodies. Fusion index, Na(+)/K(+) pump activity, and the level of the transmembrane resting potential were all significantly greater in transfected L8 (tL8) cells than in non-tL8. The total amount of alpha subunit (avian and rat) in tL8 cells was greater than that (only rat) in non-tL8 cells. This relatively high abundance of the Na(+)/K(+) pump in transfected cells may indicate that avian and rat alpha subunits hybridize to form functional pump complexes.

Insulin induces specific interaction between insulin receptor and protein kinase C delta in primary cultured skeletal muscle

Certain protein kinase C (PKC) isoforms, in particular PKCs beta II, delta, and zeta, are activated by insulin stimulation. In primary cultures of skeletal muscle, PKCs beta II and zeta, but not PKC delta, are activated via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. The purpose of this study was to investigate the possibility that PKC delta may be activated upstream of PI3K by direct interaction with insulin receptor (IR). Experiments were done on primary cultures of newborn rat skeletal muscle, age 5--6 days in vitro. The time course of insulin-induced activation of PKC delta closely paralleled that of IR. Insulin stimulation caused a selective coprecipitation of PKC delta with IR, and these IR immunoprecipitates from insulin-stimulated cells displayed a striking induction of PKC activity due specifically to PKC delta. To examine the involvement of PKC delta in the IR signaling cascade, we used recombinant adenovirus constructs of wild-type (W.T.) or dominant negative (D.N.) PKC delta. Overexpression of W.T.PKC delta induced PKC delta activity and coassociation of PKC delta and IR without addition of insulin. Overexpression of D.N.PKC delta abrogated insulin- induced coassociation of PKC delta and IR. Insulin-induced tyrosine phosphorylation of IR was greatly attenuated in cells overexpressing W.T.PKC delta, whereas in myotubes overexpressing D.N.PKC delta, tyrosine phosphorylation occurred without addition of insulin and was sustained longer than that in control myotubes. In control myotubes IR displayed a low level of serine phosphorylation, which was increased by insulin stimulation. In cells overexpressing W.T.PKC delta, serine phosphorylation was strikingly high under basal conditions and did not increase after insulin stimulation. In contrast, in cells overexpressing D.N.PKC delta, the level of serine phosphorylation was lower than that in nonoverexpressing cells and did not change notably after addition of insulin. Overexpression of W.T.PKC delta caused IR to localize mainly in the internal membrane fractions, and blockade of PKC delta abrogated insulin-induced IR internalization. We conclude that PKC delta is involved in regulation of IR activity and routing, and this regulation may be important in subsequent steps in the IR signaling cascade.

Role of hyperpolarization attained by linoleic acid in chick myoblast fusion

Our previous report has suggested that hyperpolarization generated by reciprocal activation of calcium-activated potassium (K(Ca)) channels and stretch-activated channels induces calcium influx that triggers myoblast fusion. Here we show that linoleic acid is involved in the process of generating hyperpolarization in cultured chick myoblasts and hence in promotion of the cell fusion. Linoleic acid dramatically hyperpolarized the membrane potential from -14 +/- 3 to -58 +/- 5 mV within 10 min. This effect was partially blocked by 1 mM tetraethylammonium (TEA) or 30 nM charybdotoxin, a selective K(Ca) channel inhibitor, and completely abolished by 10 mM TEA. Single-channel recordings revealed that linoleic acid activates TEA-resistant potassium channels as well as K(Ca) channels. Furthermore, linoleic acid induced calcium influx from extracellular solution, and this effect was partially blocked by 1 mM TEA and completely prevented at 10 mM, similar to the effect of TEA on linoleic acid-mediated hyperpolarization. Since the valinomycin-mediated hyperpolarization promoted calcium influx, hyperpolarization itself appears capable of inducing calcium influx. In addition, gadolinium prevented the valinomycin-mediated increase in intracellular calcium level under hypotonic conditions, revealing the involvement of stretch-activated channels in calcium influx. Furthermore, linoleic acid stimulated myoblast fusion, and this stimulatory effect could completely be prevented by 10 mM TEA. These results suggest that linoleic acid induces hyperpolarization of membrane potential by activation of potassium channels, which induces calcium influx through stretch-activated channels, and thereby triggers myoblast fusion.

Rat skeletal muscle in culture expresses the alpha1 but not the alpha2 protein subunit isoform of the Na+/K+ pump

Studies from this laboratory have shown that the physiological expression of the Na+/K+ pump in primary cultures of rat skeletal muscle increases with development. The molecular mechanisms underlying these changes are not known. Therefore, we have examined the expression of alpha and beta subunits of the Na+/K+ pump at both the protein and mRNA levels during myogenesis of primary skeletal muscle cell cultures obtained from newborn rats. Protein isoforms were identified by Western blotting techniques with specific monoclonal and polyclonal antibodies and subunit mRNA was studied with specific cDNA probes. Freshly isolated skeletal muscle from newborn rats expressed both alpha1 and alpha2 protein subunits. From day 1 after plating, primary cultures expressed only the alpha1 protein isoform. In contrast, both beta1 and beta2 isoforms were expressed in freshly isolated muscle and in primary cultures, with beta1 expression being stronger in both preparations. Studies on RNA expression showed that mRNA for alpha1, alpha2, beta1, and beta2 isoforms was identified both in freshly isolated muscle and after plating of cells in culture. These findings indicate that the lack of alpha2 protein expression in primary muscle cell cultures reflects a form of posttranscriptional regulation. There did not appear to be a quantitative difference in isoform expression as a function of age or of fusion in spite of developmental increases in Na+/K+ pump activity and its dependence on cell fusion. The lack of expression of the alpha2 subunit isoform suggests that the developmental changes in physiological expression of the Na+/K+ pump in primary cultures of skeletal muscle may be attributable either to the changes in activity of the alpha1 subunit or to differential activities of alphabeta complexes involving either of the beta subunits.

A possible role of inwardly rectifying K+ channels in chick myoblast differentiation

We examined the developmental change of inwardly rectifying K+ channels (IRK) and its possible role in myogenesis. Northern blot analysis revealed an increase in the level of IRK mRNA during myogenesis. Accordingly, IRK current was not detectable in replicating myoblasts but first appeared in aligned myoblasts that were competent for fusion and gradually increased thereafter. The time course change of IRK activity was closely related to the increase in resting membrane potential during myogenesis. Application of 0.5 mM Ba2+ to the bath depolarized the membrane and blocked IRK currents dramatically but not outwardly rectifying K+ currents. Myoblasts devoid of IRK had low resting K+ permeability, whereas myotubes that possess IRK had high resting K+ permeability. In some aligned myoblasts, anomalous hyperpolarization was elicited by increasing extracellular K+ concentration, which may be attributable to the increased conductance of IRK. Noteworthy was the fact that maximal fusion was obtained at this range of K+ concentration. These findings imply that IRK is responsible for the change in the K+ permeability during chick myogenesis, which may provide a larger driving force for Ca2+ influx that is a prerequisite for myoblast fusion.

Characterization and regulation of apamin-binding K+ channels in skeletal muscle

The Pattern of development and regulation of the apamin receptor (afterhyperpolarization channel) was studied in cultures of skeletal muscle prepared from 1-2-day-old rat pups. Expression was measured by the specific binding of (125)I-apamin. Apamin binding was virtually undetectable until the time of fusion (3-4 days in culture) of single myoblasts into myotubes. Mature myotubes (5-7 days in vitro) displayed a Bmax of 7.4 fmol/mg protein and a Kd of 376 pmol/L. When studied in mature muscle cells apamin binding was found to increase twofold in response to tetrodotoxin (TTX) and elevated Ko, which resulted in decreased Na(i). In contrast, treatments causing an increase in Na(i), such as monensin and veratridine, caused a decrease in apamin binding. The increase in apamin binding following TTX treatment was due mainly to synthesis of new channels, as the effect was blocked by cycloheximide. Alterations in cytosolic Ca2+ by calcium ionophore or Ca-channel blockers were without effect on apamin-sensitive channel expression. We conclude that afterhyperpolarization channel expression is regulated by the level of intracellular Na+ ions.

Sodium and potassium currents in freshly isolated and in proliferating human muscle satellite cells

1. Human muscle satellite cells (SC) were studied either immediately after dissociation of muscle biopsies or later, as they proliferated in culture. A purification procedure combined with clonal cultures ensured that electrophysiological recordings were done in myogenic cells. Hoechst staining for the DNA attested that cells were mononucleated. 2. The goals of this study were to examine (i) whether the electrophysiological properties of freshly isolated SC resembled those of SC that proliferated in culture for several weeks, (ii) whether freezing and thawing affected these properties, and (iii) whether SC constituted a homogeneous population. 3. We found that there were only subtle differences between the electrophysiological results obtained in freshly isolated SC and in proliferating SC with or without previous freezing and thawing. Most SC expressed two voltage-gated currents, a TTX-resistant Na+ current and a calcium-activated potassium current (IK, Ca). 4. The level of expression of the Na+ current and of IK, Ca was affected in a different way by cellular proliferation; the normalized Na+ conductance (pS pF-1) of proliferating cells resembled that of freshly isolated SC, whereas the IK, Ca conductance increased 10 times. The analysis of the amplitude distributions of the Na+ current and of IK, Ca in the various SC preparations suggested that there was only one class of SC.

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.

Verapamil regulation of Na-K pump levels in rat skeletal myotubes: role of spontaneous activity and Na channels

The effects of low and high doses of verapamil on expression of Na-K pump and Na channels in cultured rat skeletal muscle were studied and compared. Myotubes were treated for 3 days with either 20 or 100 microM verapamil, and measurements were made of transmembrane resting potential, spontaneous action potential frequency, and specific binding of [3H]ouabain and [3H]saxitoxin. Low verapamil upregulated both Na-K pumps and Na channels, whereas high verapamil down regulated the former and upregulated the latter. Changes in channel levels preceded those in pump levels. Spontaneous contractile activity could be observed during treatment with low but not high verapamil. Simultaneous treatment with verapamil and elevated external K+ reversed the effect on high verapamil-induced changes in pump levels, and potentiated the effects of both concentrations of verapamil on Na channel levels. Scatchard analysis showed that verapamil caused changes in Bmax without altering Kd. The verapamil-induced changes in both Na-K pumps and Na channels were blocked by inhibition of protein and RNA synthesis. We conclude that the differences in effects on Na-K pumps obtained with the two doses are due to the different effects on spontaneous activity and associated changes in intracellular Na concentration.

Nerve growth factor and fibroblast growth factor influence post-fusion expression of Na-channels in cultured rat skeletal muscle

We have examined effects of nerve growth factor (NGF) and fibroblast growth factor (FGF) on the density of tetrodotoxin (TTX)-sensitive Na-channels in cultured rat skeletal muscle. Measurements were made of specific binding of [3H]saxitoxin (STX) and the frequency and rate of rise of spontaneously occurring action potentials, the physiological expression of Na-channel density. Cells were transferred to various growth conditions at 6 days in vitro, and measurements were made beginning 24 hr later. Both growth factors (GF) caused dose-related increases in Na-channels compared with myotubes maintained in normal, serum-supplemented growth medium. Maximum effects occurred with a concentration of NGF of 50 ng/ml and FGF of 15 ng/ml. Scatchard analysis of specific STX binding showed an increase in Bmax with no significant change in Kd. Similar increases occurred on rate of rise and frequency spontaneous action potential. Treatment of cultures with cycloheximide or actinomycin D, inhibitors of protein and RNA synthesis, completely prevented the increase in STX-binding induced by GF treatment. The results indicate that NGF and FGF have important effects on regulation of excitable cell gene products after differentiation.

Synaptic aging as revealed by changes in membrane potential and decreased activity of Na+,K(+)-ATPase

Age-related changes in the membrane potential of nerve terminals were investigated by monitoring the accumulation of tritium-labeled triphenylmethylphosphonium ion, [3H]TPMP+, in mouse cortical synaptosomes. The resting membrane potential became less negative with advancing age, that is, it changed from -64.5 +/- 0.8 to -58.1 +/- 2.3 mV between 6 and 27 months of age. The intrasynaptosomal potassium concentration was found to decrease concomitantly by 13% in aged mice (56.6 +/- 0.9 mM) as compared to young-adult mice (64.9 +/- 0.5 mM). The ouabain-sensitive Na+,K(+)-ATPase activity of synaptic plasma membranes decreased in late senescence to 82% of the adult level. To examine the correlation with the decreased Na+,K(+)-ATPase activity, the membrane lipid composition was analyzed. Among the membrane phospholipids, only the content of phosphatidylcholine decreased in the course of senescence. The changes in the Na+,K(+)-ATPase activity were found to be positively correlated with the changes in the phospholipid content, and more specifically with the changes in the phosphatidyl-choline content. These results suggest that age-related alterations in the microenvironment constructed by phospholipids may decrease the activity of Na+,K+-ATPase, resulting in neuronal ion imbalance and decreased membrane potential. This might be responsible in part for altered functions of nerve terminals in aging brain.

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.

Characterization of the relation between sodium channels and electrical activity in cultured rat skeletal myotubes: regulatory aspects

The relation among sodium channel density, frequency of electrical activity and maximal rate of rise of the action potential was studied in developing and mature rat skeletal myotubes in culture. The number of tetrodotoxin (TTX)-sensitive Na-channels was determined by measurements of the amount of [3H]saxitoxin (STX) bound to the cultures, and electrical properties were recorded with intracellular microelectrodes. The EC50 for TTX-induced decreases in maximal STX-binding, frequency and rate of rise of action potentials was in the range 8-20 nM. The 3 variables increased in parallel with age in culture to reach peak values at age 7-8 days, and then decreased in parallel until 10-12 days in culture. The age-related increase in Na-channel density was decreased, but not abolished, by prevention of myoblast fusion. Treatment with the Ca2+ ionophore, A23187, down-regulated, and blockade of Ca-channels with verapamil up-regulated the number of Na-channels. Na-channel density was also increased by chronic treatment with TTX and elevated external [K+], which eliminated spontaneous electrical and contractile activity. Parallel effects were observed on frequency and rate of rise of action potentials. Up-regulation of Na-channels was prevented by simultaneous treatment of myotubes with inhibitors of protein synthesis. We conclude that electrical and mechanical activity of cultured myotubes regulate de novo synthesis of Na-channels through alterations in the level of cytosolic Ca2+.

Characterization of resting membrane potential and its electrogenic pump component in cultured chick myotubes

The role of the electrogenic Na+-K+ pump in the determination of the level of the resting membrane potential in cultured chick limb muscle was investigated. Transmembrane resting potential and ouabain-sensitive 86Rb-uptake were measured in myotubes at different ages in culture from 2 to 10 days in vitro. Inhibition of the Na+-K+ pump with ouabain prevented the developmental increase in membrane potential which normally follows fusion of myotubes (day 2-3). In mature myotubes, ouabain caused a dose-related decrease in both membrane potential and 86Rb-uptake, with values for EC50 and maximal effect being nearly the same on both variables. The decrease in membrane potential by ouabain, up to 20 mV maximum, occurred within 2-5 sec and was not accompanied by detectable changes in input resistance. Membrane potential was also reduced by a decrease in temperature of the recording medium and removal of extracellular K+, both of which reduce Na+-K+ pump activity. We also found that the relation between membrane potential and extracellular K+ concentration was completely attenuated by ouabain in the physiological range (2-10 mM). We conclude that the electrogenic Na+-K+ pump plays an important role in the determination of the resting membrane potential of chick myotubes and that regulation of its level is not entirely explained by the diffusion potential hypothesis.

Effects of carbamylcholine on membrane potential and Na-K pump activity of cultured rat skeletal myotubes

1. We measured changes in resting membrane potential (Em) and Na-K pump activity, assayed by ouabain-sensitive 86Rb uptake, in response to carbamylcholine (CCh) and its continued presence in single rat skeletal myotubes in culture. 2. CCh caused immediate depolarization from control Em (-80 to -85 mV) to near 0 followed by repolarization of varying degrees depending on the age of the culture and temperature of the recording medium; repolarization of Em was most apparent by culture age 8-9 days in vitro (DIV), Em reaching values as high as -60 mV by 5-10 min after peak depolarization at 37 degrees C. 3. Input resistance, which decreased during CCh depolarization, increased only slightly during the initial phase of repolarization and then remained essentially unchanged during the major component of membrane repolarization in the presence of CCh. 4. Ouabain, given before CCh, prevented repolarization of Em and, when given after repolarization had begun, reversed it and caused Em to return to about -7 mV. 5. Na-K pump activity was decreased in myotubes in which Em did not repolarize or did so only slightly, and was increased by over 40-50% in myotubes whose Em repolarized by 40-60 mV, even though CCh was still present in the medium. Inhibition of pump activity in non repolarizing myotubes was related to Na influx, inhibition being reversed to stimulation when CCh was administered to myotubes in Na-free medium. 6. Repeated (three or four times) or prolonged (up to 60-min) administration of CCh to myotubes in which repolarization was hardly expressed (age 6-7 DIV) caused increases both in the amount of repolarization and in 86Rb uptake, both being related to the number or duration of CCh exposures. 7. We conclude that repolarization of Em following CCh-induced depolarization of cultured rat skeletal myotubes depends to a large extent on an increase in activity of the electrogenic Na-K pump.

Nerve growth factor supports growth of rat skeletal myotubes in culture

Effects of nerve growth factor (NGF) were examined on the growth of rat skeletal myotubes in culture and the expression of Na-K pump activity in this preparation. We found NGF to cause an immediate increase in electrogenic Na-K pump activity as determined by electrogenic component of membrane potential (Em) and ouabain-sensitive 86Rb uptake. When given chronically, NGF was able to replace serum as an essential supplement for development of cultured myotubes. Thus, when maintained in a serum-free, basal nutrient medium (DMEM), myotubes progressively deteriorated as indicated by morphological appearance, Em and the number of [3H]ouabain binding sites compared with myotubes grown in normal, serum-supplemented growth medium (GM). In contrast, the presence of NGF in DMEM completely prevented the deterioration of these properties, their values actually exceeding those in GM. These findings demonstrate a trophic effect of NGF on bioelectric properties of neonatal mammalian muscle cells.

Role of Na-K ATPase in regulation of resting membrane potential of cultured rat skeletal myotubes

The role of Na-K ATPase in the determination of resting membrane potential (Em) as a function of extracellular K ion concentration was investigated in cultured rat myotubes. The Em of control myotubes at 37 degrees C varied as a function of (K+)0 with a slope of about 58-60 mV per ten-fold change in (K+)0. Inhibition of the Na-K pump with ouabain or by reduced temperature revealed that this relation consists of two components. One, between (K+)0 of 10 and 100 mM, remains unchanged by alterations in enzyme activity; The second, between (K+)0 of 1 and 10 mM, is related to the amount of Na-K pump activity, the slope decreasing as pump activity decreases. Indeed, with complete inhibition of the Na-K pump, Em does not change over the range of (K+)0 1 to 10 mM. Measurements of 86Rb efflux and input resistance of individual myotubes showed that membrane permeability does not change as (K+)0 increases from 1 to 10 mM but increases as (K+)0 increases further. Monensin, which increases Na ion permeability, increases Em at values of external K+ below 10 mM, and is without effect at higher values of K+ concentration. The effect of monensin is blocked by ouabain. Tetrodotoxin, which blocks voltage-dependent Na+ channels, decreases Em at low (2-10 mM) K+. We conclude that changes in Em as a function of extracellular K+ concentration in the physiological range are not adequately explained by the diffusion potential hypothesis of Em, and that other theories (electrogenic pump, surface-absorption) must be considered.

Influence of various growth factors and conditions on development of resting membrane potential and its electrogenic pump component of cultured rat skeletal myotubes

The effects of different growth factors and growth conditions were studied on the development of resting membrane potential and its electrogenic--ouabain-sensitive--pump component in cultured rat myotubes. Resting potential and its electrogenic pump component were dependent on the initial plating density of the myotubes, both values increasing with increasing density. Medium from cells plated at high density, when used to replace the medium of low density cells, increased both the resting potential and its electrogenic pump component of low density myotubes. Treatment of myotubes with cytosine-arabinoside delayed the appearance of [3H]ouabain binding sites and electrogenic pump component of resting potential, but by 8 days in culture there was no difference between treated and control cells. Similarly, cells plated initially in 5% horse serum developed resting potential and its electrogenic pump component more slowly than those in 15% horse serum, but by 8-10 days in vitro, the differences were no longer apparent. Chick embryo extract was found to have little, if any, influence on development of resting potential and its electrogenic pump component. We conclude that the different growth conditions and factors to the extent that they influence membrane potential, do so by altering the time of appearance of Na-K ATPase, the activity of which contributes a considerable component to resting potential.

Effects of chronic ouabain treatment on [3H]ouabain binding sites and electrogenic component of membrane potential in cultured rat myotubes

The effects of incubation of cultured rat skeletal myotubes in ouabain were studied on the number of [3H]ouabain binding sites and electrogenic component of membrane potential. Ouabain treatment for 2-6 days increased the number of binding sites, resting membrane potential (Em) and the ouabain-sensitive component of Em in the muscle cells. The findings strengthen the idea that Na,K-ATPase has an important role in regulation of Em in cultured skeletal muscle and suggest that Na-K pump inhibition during development may be a regulatory mechanism for cellular synthesis of Na,K-ATPase.