Differentiation of cell membranes in cultures of embryonic chick breast muscle

The RNA-binding protein HuR binds to acetylcholinesterase transcripts and regulates their expression in differentiating skeletal muscle cells

During myogenic differentiation, acetylcholinesterase (AChE) transcript levels are known to increase dramatically. Although this increase can be attributed in part to increased transcriptional activity, posttranscriptional mechanisms have also been implicated in the high levels of AChE mRNA in myotubes. In this study, we observed that transfection of a luciferase reporter construct containing the full-length AChE 3'-untranslated region (UTR) resulted in significantly higher (5-fold) luciferase activity in differentiated myotubes versus myoblasts. RNA-electrophoretic mobility shift assays (REMSAs) performed with a full-length AChE 3'-UTR probe and the AU-rich element revealed that the intensity of RNA-binding protein complexes increased as myogenic differentiation proceeded. Using several complementary approaches including supershift REMSA, mRNA-binding protein pull-down assays, and immunoprecipitation followed by reverse transcription-PCR, we found that the mRNA-stabilizing protein HuR interacts directly with AChE transcripts. Stable overexpression of HuR in C2C12 cells increased the expression of endogenous AChE transcripts as well as that of the luciferase reporter construct containing the AChE 3'-UTR. In vitro stability assays performed with protein extracts from these cells versus controls resulted in a slower rate of AChE mRNA decay. The down-regulation of HuR expression mediated through small interfering RNA further confirmed the role of HuR in the regulation of AChE mRNA levels. Taken together, these studies demonstrate that HuR interacts with the AChE 3'-UTR to regulate posttranscriptionally the expression of AChE mRNA during myogenic differentiation.

Changes in architecture of the Golgi complex and other subcellular organelles during myogenesis

Myogenesis involves changes in both gene expression and cellular architecture. Little is known of the organization, in muscle in vivo, of the subcellular organelles involved in protein synthesis despite the potential importance of targeted protein synthesis for formation and maintenance of functional domains such as the neuromuscular junction. A panel of antibodies to markers of the ER, the Golgi complex, and the centrosome were used to localize these organelles by immunofluorescence in myoblasts and myotubes of the mouse muscle cell line C2 in vitro, and in intact single muscle fibers from the rat flexor digitorum brevis. Antibodies to the ER stained structures throughout the cytoplasm of both C2 myoblasts and myotubes. In contrast, the spatial relationship between nucleus, centrosome, and Golgi complex was dramatically altered. These changes could also be observed in a low-calcium medium that allowed differentiation while preventing myoblast fusion. Muscle fibers in vivo resembled myotubes except that the ER occupied a smaller volume of cytoplasm and no staining was found for one of the Golgi complex markers, the enzyme alpha-mannosidase II. Electron microscopy, however, clearly showed the presence of stacks of Golgi cisternae in both junctional and extrajunctional regions of muscle fibers. The perinuclear distribution of the Golgi complex was also observed in live muscle fibers stained with a fluorescent lipid. Thus, the distribution of subcellular organelles of the secretory pathway was found to be similar in myotubes and muscle fibers, and all organelles were found in both junctional and extrajunctional areas of muscle.

The differentiation of avian skeletal muscle in culture: changes in responsiveness of adenylyl cyclase to prostaglandin E1 and adrenergic agonists

The responsiveness of adenylyl cyclase during avian myogenesis in vitro has been examined. Measurements of cyclic AMP generation in intact cells revealed that the precursor myoblast is highly responsive to prostaglandin E1 (11-fold maximum stimulation); whereas its response to isoproterenol is much smaller (2-fold). From the onset of terminal differentiation, responsiveness to the beta-adrenergic agonist increases progressively, reaching a 5.5-fold maximum response by 96 hr of culture. In contrast, there is little change in the cell population's responsiveness to prostaglandin E1. The rise in catecholamine responsiveness is consistent with previously reported increases in beta-receptors accompanying differentiation. DL-propranolol blocks the response of myoblasts and myotubes to 10(-6) M isoproterenol with the same half maximal inhibition value of 1 X 10(-8) mol. The results also suggest a change in the adrenergic character of the receptors and/or coupling to adenylyl cyclase as myoblasts differentiate. First the alpha-adrenergic antagonist phentolamine (10(-7)-10(-4) mol) inhibits the myoblast's response but enhances that of the myotube. Second, the potency ratios for the responses to isoproterenol, epinephrine, and norepinephrine shift from 1.1:1.0:1.0 in the myoblast to 3.3:2.1:1.0 in the myotube. The findings are discussed with reference to the role of prostaglandins in the positive control of muscle differentiation and the changes in the catecholamine-responsive adenylyl cyclase system as an aspect of the expression of the muscle phenotype.

The all-or-none role of innervation in expression of apamin receptor and of apamin-sensitive Ca2+-activated K+ channel in mammalian skeletal muscle

The long-lasting after-hyperpolarization(s) (AHP) that follows the action potential in rat myotubes differentiated in culture is due to Ca2+-activated K+ channels. These channels have the property to be specifically blocked by the bee venom toxin apamin at low concentrations. Apamin has been used in this work to analyze, by electrophysiological and biochemical techniques, the role of innervation in expression of these important channels. The main results are as follows: (i) Long-lasting AHP that follows the action potential in rat myotubes in culture disappears when myotubes are cocultured with nerve cells from the spinal cord under the conditions of in vitro innervation. (ii) Extensor digitorum longus muscles from adult rats have action potentials that are not followed by AHP but AHP are systematically recorded after muscle denervation and they are blocked by apamin. (iii) Specific 125I-labeled apamin binding is undetectable in innervated muscle fibers but it becomes detectable 2-4 days after muscle denervation to be maximal 10 days after denervation. (iv) Apamin receptors detected with 125I-labeled apamin are present at fetal stages with biochemical characteristics identical to those found in myotubes in culture. The receptor number decreases as maturation proceeds and 125I-labeled apamin receptors completely disappear after the first week of postnatal life, in parallel with the disappearance of multi-innervation. All these results taken together strongly suggest an all-or-none effect of innervation on the expression of apamin-sensitive Ca2+-activated K+ channels.

Acetylcholinesterase activity in the myotome of the early chick embryo

The myotome of early chick embryos was investigated histochemically by means of the acetylcholinesterase (AChE) reaction. Light-microscopically, at the cervical level, the myotome was first recognized and AChE activity demonstrated at stage 13 (2-day-old embryo). Subsequently, the myotome elongated ventro-laterally along the inner surface of the dermomyotome and reached the ventro-lateral end of the dermomyotome at stage 17 to 18 (3 day-old embryo). AChE activity in the myotome showed subsequent increase in intensity during the course of development. The myotome consisted mainly of AChE-positive cells displaying enzymatic activity along the nuclear membrane and within the cytoplasm. In contrast, almost all cells of the dermomyotome and the interstitial cells were AChE-negative. Electron-microscopically, the myotome cells of the 2 day-old embryo and the cells in the dorso-medial portion of the myotome of the 3 day-old embryo were morphologically undifferentiated; AChE activity was detected in the nuclear envelope and in single short profiles of the endoplasmic reticulum (ER). On the other hand, in the 3 day-old embryo the cells in the ventro-lateral portion of the myotome showed AChE activity in the nuclear envelope, numerous profiles of the ER and some Golgi complexes. These AChE-positive cells were regarded as developing myogenic cells based on their morphological characteristics. The present findings indicate (i) that the appearance of AChE activity in the cytoplasm is the first sign of the differentiation of myogenic cells, and (ii) that in these myogenic cells the increase in AChE activity is based on the development of the ER.

Appearance and distribution "in situ" of nicotinic acetylcholine receptors in cervical myotomes of young chick embryos. Radioautographic studies by light and electron microscopy

Localization of the acetylcholine (nicotinic) receptor sites was investigated in the developing cervical myotomes of the early chick embryo by radioautography at the light and electron microscope level, using 125I-alpha-bungarotoxin. The presence of cholinergic receptor sites was detected in situ as early as 60 hours of incubation (stage 17); their relative density increased in the myotome during the differentiation of the somite. Specific labeling of these receptor sites was detected in the myotomal tissue but not in the notochord, spinal cord or periaxial mesenchyme. The distribution of the receptor sites was uniform in the myotome at 3 days in ovo. An anterior-posterior asymmetry of the density appeared at 4 days in ovo and developed up to the 6th day. The highest density of these toxin-binding receptor sites was observed near the spinal motor nerve bundle as revealed by silver staining. These observations, made in situ, are discussed with respect to the possible neurotrophic or physical effects of the early motor innervation.

Muscle protein analysis by two-dimensional gel electrophoresis

Two-dimensional electrophoresis was first applied to the analysis of muscle proteins in 1976 when the occurrence of multiple forms of actin was discovered. Since that time the technique has become widely accepted as a new approach to studies of myogenesis, muscle differentiation, and muscle pathology. In addition, two-dimensional electrophoresis now is being used to investigate contractile proteins present in nonmuscle cells. This review will discuss, in general, the technique of two-dimensional electrophoresis in polyacrylamide gels which combines isoelectric focusing and sodium dodecyl sulfate electrophoresis. The application of the technique specifically to muscle protein analysis will be discussed through a review of existing literature on two-dimensional electrophoresis of cultured muscle cells and tissue homogenates. Attention will be given to contractile protein heterogeneities such as alpha, beta, and gamma actin and the embryonic forms of myosin light chains, all discovered through the use of two-dimensional electrophoresis. New information concerning gene expression during muscle differentiation revealed by differences in two-dimensional electrophoresis protein patterns and the use of two-dimensional electrophoresis for studying human muscle pathology through analysis of tissue biopsies will also be discussed.

Interaction of the cytoskeletal framework with acetylcholine receptor on th surface of embryonic muscle cells in culture

To monitor the interaction of cell surface acetylcholine (AcCho) receptors with the cytoskeleton, cultured muscle cells were labeled with radioactive or fluorescent alpha-bungarotoxin and extracted with Triton X-100, using conditions that preserve internal structure. A significant population of the AcCho receptors is retained on the skeletal framework remaining after detergent extraction. The skeleton organization responsible for restricting AcCho receptors to a patched region may also result in their retention after detergent extraction.

Carbohydrate requirement for expression and stability of acetylcholine receptor on the surface of embryonic muscle cells in culture

We have investigated the significance of protein glycosylation for metabolism of acetylcholine receptors (AcChoR) in primary cultures of embryonic chicken muscle cells. Tunicamycin, a specific inhibitor of the glycosylation of asparagine residues on glycoproteins, decreased AcChoR accumulation and accelerated its degradation. In contrast, there was no evidence that tunicamycin treatment affected AcChoR biosynthesis, intracellular transport, or incorporation into surface membranes. Leupeptin, an inhibitor of intracellular proteases, markedly increased accumulation of AcChoR on the external surface of muscle cells treated with tunicamycin. Our findings indicate that impairment of protein glycosylation prevents accumulation of AcChoR by increasing its susceptibility to degradation by cellular proteases.

Control of mouse myoblast commitment to terminal differentiation by mitogens

Regulation of the transition of mouse myoblasts from proliferation to terminal differentiation was studied with clonal density cultures of a permanent clonal myoblast cell line. In medium lacking mitogenic activity, mouse myoblasts withdraw from the cell cycle, elaborate muscle-specific gene products, and fuse to form multinucleated myotubes. Addition of a purified mitogen, fibroblast growth factor, to mitogen-depleted medium stimulates continued proliferation and prevents terminal differentiation. When mitogens are removed for increasing durations and then refed, mouse myoblasts irreversibly commit to terminal differentiation: after 2-4 h in the absence of mitogens, myoblasts withdraw from the cell cycle, elaborate muscle-specific gene products, fuse in the presence of mitogens that have been fed back. Population kinetics of commitment determined with 3H-thymidine labeling and autoradiography suggests the following cell-cycle model for mouse myoblast commitment: 1)if mitogens are present in the extracellular environment of myoblasts in G1 of the cell cycle, the cells enter S and continue through another cell cycle; 2) if mitogens have been absent for 2 or more hours, cells in G1 do not enter S; the cells commit to differentiate, permanently withdraw from the cell cycle (will not enter S if mitogens are refed), and they subsequently elaborate acetylcholine receptors and fuse (even if mitogens are refed); 3) cells in other phases of the cell cycle continue to transit the cell cycle in the absence of mitogens until reaching the next G1. the commitment kinetics and experiments with mitotically synchronized cells suggest that the commitment "decision" is made during G1. Present results do not, however, exclude commitment of some cells in other phases of the cell cycle.

A transient increase in amino acid transport modulated by insulin in differentiating muscle cells

During synchronous differentiation of embryonic chick muscle cells in cultures, the Na-dependent uptake of an amino acid analog, alpha-amino isobutyric acid (AIB) undergoes in abrupt, transient increase. The increase in AIB uptake is concomitant with the rapid fusion of mononucleated myoblasts, and precedes the accumulation of muscle-specific proteins. Subsequently, Na-dependent AIB transport diminishes markedly during postfusional differentiation of myotubes. The rate of AIB uptake is increased by insulin both before and after myoblast fusion. This stimulation by insulin is restricted to the Na-dependent component of total AIB uptake but is apparently not the result of insulin-mediated increase in the trans-membrane Na gradient.

Intrathymic pathogenesis of myasthenia gravis: transient expression of acetylcholine receptors on thymus-derived myogenic cells

Differentiation of myogenic stem cells from undifferentiated thymic stem cells is thought to play a critical role in the pathogenesis of myasthenia gravis. The expression of membrane acetylcholine receptor (AChR) on the membranes of developing muscle clones in cultures of murine thymus reticulum was followed and found to be transient. AChR are first expressed shortly after fusion of myotubes. In subsequent stages of myogenic development, the density of homogenously distributed AChR is strongly increased, and, in addition, concentrated "hot spot" AChR areas appear. During further maturation, membrane AChR are lost. Highly mature myotubes (3 months in culture) lack substantial amounts of homogenous AChR, as well as hot spots.

Acetylcholinesterase differentiation during myogenesis in early chick embryonic cells caused by an inducer RNA

In the stage 4 chick blastoderm, an area located 0.6 mm posterior to Hensen's node, the post-nodal piece (PNP), consists of an undifferentiated population of cells, since the explants when cultivated in vitro in a variety of media do not develop into any histologically identifiable structures. However, addition of a specific low molecular weight RNA isolated from the 16-day-old chick embryonic heart promotes the appearance of a distinct mode of morphological and biochemical changes that is similar to that of embryonic cardiogenic process. The RNA-induced changes in the PNP also include a marked increase in acetylcholinesterase activity. The increase in enzymatic activity can be measured biochemically, as well as visualized histochemically.

Interrelated lipid alterations and their influence on the proliferation and fusion of cultured myogenic cells

We have cultured myogenic cells derived from primary explants and a cell line (L6) in a lipid-depleted medium (LDM) and produced large alterations of the fatty acyl and polar headgroup composition and of the cellular sterol levels. These alterations were produced by altering the composition of the media as follows: removing biotin and providing exogenous fatty acid; removing choline and providing exogenous ethanolamine or choline analogues; and by adding 25-OH cholesterol, an inhibitor of 3-hydroxy-3-methylglutarate (HMG)-CoA reductase. Relatively small, secondary alterations of other lipid classes accompany the large primary alteration. In general, they are not obviously compensatory for the primary alteration by retaining some physical property. We have explored the influence of these lipid alterations on myoblast proliferation and fusion into myotubes. In general, considerable variability appears tolerated, but there also appear to be limits. Long-term cultures grown in media containing a single fatty acid do not proliferate indefinitely, and the fatty acid does not become the sole fatty acyl component of the phospholipids. This phenomenon is also observed for cultures enriched in phosphatidylethanolamine (PE) or phosphatidyldimethylethanolamine (PDME). The influence of the lipid alterations on fusion is particularly interesting. The inclusion of 25-OH cholesterol inhibits fusion. Enrichment of the fatty acyl chains with elaidate or the polar headgroups with PE also inhibits fusion, but in contrast to that by 25-OH cholesterol, a significant fraction of the myoblasts are aligned and interacting with each other. Oleate enrichment enhances the rate of fusion.

Development of electrophysiological and biochemical membrane properties during differentiation of embryonic skeletal muscle in culture

Newly fused chick myotubes undergo simultaneous and rapid changes in cell membrane properties during synchronous differentiation in culture. These changes are coordinately regulated and include increases in acetylcholine receptor, acetylcholinesterase, and resting potential, as well as the appearance of action potentials in discrete membrane areas upon stimulation. Subsequently, the acetylcholine receptor reaches maximal levels, whereas the development of electrical properties is marked by a further increase in resting potential, changes in the characteristics of the elicited action potential, and the recruitment of additional membrane areas for action potential generation. Maturation of electrical excitability, marked by the acquisition of the ability to fire repetitively and to conduct action potentials along the membrane, occurs well after resting potential has reached a maximum. During post-maturational development, myotubes exhibit spontaneous electrical and contractile activity, and levels of acetylcholine receptor accessible to externally applied 125I-labeled alpha-bungarotoxin decrease markedly. It is suggested that electrophysiological membrane maturation is autonomously regulated with no requirement for neuronal intervention and involves the coordinated biosynthesis of discrete membrane components and their subsequent organization in the myotube membrane.

A new myasthenic syndrome with end-plate acetylcholinesterase deficiency, small nerve terminals, and reduced acetylcholine release

A new myasthenic syndrome is described in a patient whose symptoms began soon after birth and included generalized weakness increased by exertion, easy fatigability, hyporeflexia, and refractoriness to anticholinesterase drugs. Electromyography showed a decremental response at all frequencies of stimulation and a repetitive response to single nerve stimulation. Miniature end-plate potentials (mepps) were of normal amplitude but of decreased frequency. The mepp duration and half-decay time were prolonged, and prostigmine was without any addtitional effect. The quantum content of the end-plate potential was decreased due to a reduced store of quanta immediately available for release, but the probability of release was normal. Quantitative electron microscopy demonstrated a 3-fold to 4-fold decrease of nerve terminal size and reduced postsynaptic membrane density. The postsynaptic folds showed focal degeneration, and many were distended by labyrinthine membranous networks that communicated with the synaptic space. Degenerating nuclei were found in the junctional sarcoplasm. The ultrastructural localization of the acetylcholine receptor protein was normal. Acetylcholinesterase (AChE) was absent from the motor end-plates by histochemical and electron cytochemical criteria. Biochemical studies indicated total absence of the end-plate-specific 16 S species of AChE and marked decrease in total muscle AChE. A congenital defect in the molecular assembly of AChE or in its attachment to the postsynaptic membrane might represent the basic abnormality and condition the morphological and physiological alterations.

Protein synthesis and actin heterogeneity in calf muscle cells in culture

Pulse-labeled cytoplasmic proteins from cultured fetal calf-muscle cells at various stages of development were analyzed by one- and two-dimensional gel electrophoresis. The high resolution of the two-dimensional technique allows the determination of those protein species that begin to be synthesized after cell fusion. In addition, actin has been found to exist in three forms possessing similar biochemical properties and identical molecular weights but having slightly different isoelectric points. Two of the forms are found in prefusion dividing myoblasts and also in cultured kidney cells. The third form is the only one found in fetal muscle tissue and is predominant in cultures of fused muscle cells. Thus, it would seem that actin can exist in several isozymic forms of which one is specific to fused muscle tissue.

Induction of acetylcholine receptors in muscle cultures

Acetylcholine receptors in muscle cells differentiated in vitro were monitored by using 125 I-alpha-bungarotoxin. The number of cholinergic receptors was increased 4-8 fold in 2 days due to inhibition of spontaneous contraction of the muscle fibers. The inhibition of this activity, whether mediated through tetrodotoxin, lidocaine or D-600, did not affect the biochemical differentiation of muscle, as represented by creatine-phosphokinase and acetylcholinesterase activity. "Induction" of receptors by tetrodotoxin was inhibited by cycloheximide, actinomycin-D, or 5-bromotubericidine. Dystrophic muscle responded in vitro to inhibition of contraction similar to normal tissue.

Calcium-related changes of enzyme activities in energy metabolism of cultured embryonic chick myoblasts and myotubes

Changes in activity of enzymes involved in energy metabolism have been determined in unfused, fused as well as in fusion-inhibited chick embryo muscle cells in vitro. Functionally related enzymes which supposedly are coded by "gene clusters" show a similar degree and rate of enzyme activity increase. Hexokinase and glucose-6-phosphate dehydrogenase reveal only slight activity changes during muscle cell development under the conditions studied. The elevation of phosphofructokinase can be distinguished from that of the other glycolytic enzymes by its higher rate of increase and from that of phosphorylase by its time-course of activity change. The Ca2+ dependence of the phosphorylase activity increase runs parallel to myoblast fusion rate. Experiments in which calcium was removed from cultures which had reached the final morphological state of mature myotubes 24 h after onset of fusion show that increases of enzyme activities are irreversible and that these increases proceed at unchanged rates. Experimental evidence suggest that although fusion and enzyme syntheses may be uncoupled, both are similarly triggered by being dependent on Ca2+ concentration.

Synthesis of myosin heavy and light chains in muscle cultures

The weight ratio of myosin/actin, the myosin heavy chain content as the percentage of total protein (wt/wt), and the kinds of myosin light chains were determined in (a) standard muscle cultures, (b) pure myotube cultures, and (c) fibroblast cultures. Cells for these cultures were obtained from the breast of 11-day chick embryos. Standard cultures contain, in addition to myotubes, large numbers of replicating mononucleated cells. By killing these replicating cells with cytosine arabinoside, pure myotube cultures were obtained. The myosin/actin ratio (wt/wt) for pure myotube, standard muscle, and fibroblast cultures average 3.1, 1.9, and 1.1 respectively. By day 7, myosin in myotube cultures represents a minimum of 7% of the total protein, but about 3% in standard cultures and less than 1.5% in fibroblasts cultures. Myosin from standard cultures contains light chain LC1, LC2, and LC3, with a relative stoichiometry of the molarity of 1.0:1.9:0.5 and mol wt of 25,000, 18,000 and 16,000 daltons, identical to those in adult fast muscle. Myosin from pure myotubes exhibits light chains LC1 and LC2, with a molar ratio of 1.5:1.6. Myosin from fibroblast cultures possesses two light chains with a stoichiometry of 1.8:1.8 and mol wt of 20,000 and 16,000 daltons. Clearly, the faster migrating light chain, LC3, found in standard cultures is synthesized not by the myotubes but ty the mononucleated cells. In myotubes, both the assembly of the sarcomeres and the interaction between thick and thin filaments required for spontaneous contraction occur in the absence of light chain LC3. One set of structural genes for the myosin light and heavy chains appears to be active in mononucleated cells, whereas another set appears to be active in multinucleated myotubes.

Differences among myosins synthesized in non-myogenic cells, presumptive myoblasts, and myoblasts

Myosins synthesized in non-myogenic cells and replicating presumptive myoblasts differ from those synthesized in postmitotic mononucleated myoblasts and myotubes. Myoblasts and myotubes synthesize the definitive light chains, MLC1 and MLC2. These light chains display different molecular weights in sodium dodecyl sulfate-polyacrylamide gels from the fibroblast light chains FLC1 and FLC2 synthesized in non-myogenic cells and presumptive myoblasts. There are immunological differences between the myosin heavy chains synthesized in myoblasts and myotubes and those synthesized in non-myogenic cells and presumptive myoblasts. Fluorescein-labeled antibodies against skeletal light meromyosin are bound only along the lateral edges of emerging and definitive A-bands. This antibody to light meromyosin is not bound to the outside of, or the microfilaments subtending, the plasma membrane in non-myogenic cells or in myoblasts or in myotubes. These findings suggest that: (1) non-myogenic cells and replicating presumptive myoblasts synthesize similar myosin heavy and light chains; (2) replicating presumptive myoblasts synthesize a different set of myosins from those synthesized by their postmitotic daughters, the myoblasts; (3) the myosins associated with the plasma membranes of non-myogenic and myogenic cells are products of structural genes distinct from those coding for the myosins for skeletal myofibrils.

Thick and thin filaments in postmitotic, mononucleated myoblasts

Addition of cytochalasin B to primary muscle cultures allows the physical separation of postmitotic myogenic cells from replicating presumptive myoblasts and replicating fibroblasts. Mononucleated, postmitotic myoblasts proceed without fusion to synthesize myosin and actin and to assemble these proteins into thick and thin filaments. Although sarcomeres oriented in tandem are not evident and A, H, and I bands are atypical in these mononucleated myoblasts, the irregularly scattered clusters of myofilaments are assembled into remarkably normal interdigitating arrays. These scattered clusters of stacked thick and thin filaments permit the cell to contact spontaneously in the presence of cytochalasin B.

Acetylcholine receptor turnover in membranes of developing muscle fibers

[125I mono-iodo-alpha-bungarotoxin is used as a specific marker in a description of acetylcholine receptor metabolism. It is concluded that acetylcholine receptors in the surface membranes of chick and rat myotubes developing in cell cultures have a half-life of 22-24 h. Alpha-bungarotoxin (bound to a receptor which is removed from the membrane) is degraded to monoiodotyrosine which appears in the medium. Several observations are consistent with a model in which receptors or alpha-bungarotoxin-receptor complexes are internalized and then degraded: (a) the rate of appearance of iodotyrosine does not reach its maximal rate until 90 min after alpha-bungarotoxin is bound to the surface receptors; (b) 2,4-dinitrophenol, reduced temperature, and cell disruption all inhibit the degradation process. The degradation of surface receptors is not coupled to the process by which receptors are incorporated into the membrane. Evidence suggest that receptors are incorporated into the surface membrane from a presynthesized set of receptors containing about 10% as many alpha-bungarotoxin binding sites as does the surface. Additionally, a third set of acetylcholine receptors is described containing about 30% as amny binding sites as does the surface. These "hidden" recptors are not precursors yet are not readily accessible for binding of extracellular alpha-bungarotoxin. These findings are discussed in relation to both plasma membrane biosynthesis and control of chemosensitivity in developing and denervated skeletal muscle.

Effects of cytochaslasin B and colcemide on myogenic cultures

Muscle cultures treated with cytochalasin B yield mono- and oligonucleated cells of two kinds: (i) arborized, replicating precursor myogenic cells and fibroblasts; and (ii) round, post-mitotic, terminally differentiating myoblasts and myotubes. The arborized cells do not bind fluorescein-labeled antibody against myosin, do not contract rhythmically, and do not display hexagonally stacked thick and thin filaments. The round, mono-nucleated myoblasts and round, oligonucleated myotubes bind the fluorescein-labeled antibody against myosin, contract rhythmically, and display clusters of hexagonally-stacked thick and thin filaments. When cytochalasin B is removed and replaced by colcemide, the arborized cells, but not the post-mitotic muscle cells, acquire a radial symmetry and are induced to assemble massive, meandering cables that may occupy over 25% of the cell volume. These tortuous calbes are positively birefringent and consist exclusively of enormous numbers of 100-A, intermediate-sized filaments.