Muscle expression of a local Igf-1 isoform protects motor neurons in an ALS mouse model.

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by a selective degeneration of motor neurons, atrophy, and paralysis of skeletal muscle. Although a significant proportion of familial ALS results from a toxic gain of function associated with dominant SOD1 mutations, the etiology of the disease and its specific cellular origins have remained difficult to define. Here, we show that muscle-restricted expression of a localized insulin-like growth factor (Igf) -1 isoform maintained muscle integrity and enhanced satellite cell activity in SOD1(G93A) transgenic mice, inducing calcineurin-mediated regenerative pathways. Muscle-specific expression of local Igf-1 (mIgf-1) isoform also stabilized neuromuscular junctions, reduced inflammation in the spinal cord, and enhanced motor neuronal survival in SOD1(G93A) mice, delaying the onset and progression of the disease. These studies establish skeletal muscle as a primary target for the dominant action of inherited SOD1 mutation and suggest that muscle fibers provide appropriate factors, such as mIgf-1, for neuron survival.

Tumor necrosis factor-alpha inhibition of skeletal muscle regeneration is mediated by a caspase-dependent stem cell response.

Skeletal muscle is susceptible to injury following trauma, neurological dysfunction, and genetic diseases. Skeletal muscle homeostasis is maintained by a pronounced regenerative capacity, which includes the recruitment of stem cells. Chronic exposure to tumor necrosis factor-alpha (TNF) triggers a muscle wasting reminiscent of cachexia. To better understand the effects of TNF upon muscle homeostasis and stem cells, we exposed injured muscle to TNF at specific time points during regeneration. TNF exposure delayed the appearance of regenerating fibers, without exacerbating fiber death following the initial trauma. We observed modest cellular caspase activation during regeneration, which was markedly increased in response to TNF exposure concomitant with an inhibition in regeneration. Caspase activation did not lead to apoptosis and did not involve caspase-3. Inhibition of caspase activity improved muscle regeneration in either the absence or the presence of TNF, revealing a nonapoptotic role for this pathway in the myogenic program. Caspase activity was localized to the interstitial cells, which also express Sca-1, CD34, and PW1. Perturbation of PW1 activity blocked caspase activation and improved regeneration. The restricted localization of Sca-1+, CD34+, PW1+ cells to a subset of interstitial cells with caspase activity reveals a critical regulatory role for this population during myogenesis, which may directly contribute to resident muscle stem cells or indirectly regulate stem cells through cell-cell interactions.

V1a vasopressin receptor expression is modulated during myogenic differentiation.

Neurohypophyseal peptides potently stimulate myogenic differentiation by acting through different receptors of the same family. Here, we show that L6C5 myogenic cells express, at a high density, a single class of V1a Arg8-vasopressin (AVP) receptor. The expression of the vasopressin receptor of type 1a (V1aR) is significantly higher in proliferating myoblasts than in differentiated myotubes. The differentiation-related decrease of V1aR expression was evident both at the mRNA and at the protein level as shown by the reduction of [(3)H]-AVP binding. However, in L6C5 cells transfected with a synthetic construct containing the luciferase gene driven by the 2 kb upstream region of V1aR, we observed a stimulation of the activity of the promoter when the cells were cultured in differentiative medium. The down-regulation of the V1aR correlated with a decreased half-life of its mRNA (half-life 5.86+/-0.74 hr in 10% fetal bovine serum [FBS] versus 3.53+/-0.72 hr in 1% FBS). Cyclosporine A and dexamethasone, but not 5'-azacytidine, treatments of cells in differentiation medium restored the V1aR level to that measured in proliferating L6C5 cells, thus confirming the role of post-transcriptional mechanisms in the modulation of V1aR expression. Taken together, these data show that mRNA stability plays a role in modulating protein expression during the myogenic differentiation process.

Cdk9-55: a new player in muscle regeneration.

Adult skeletal muscle contains a specialized population of myogenic quiescent stem cells, termed satellite cells, which contribute to repair myofibers after injury. During muscle regeneration, satellite cells exit their normal quiescent state, proliferate, activating MyoD and Myf-5 expression, and finally differentiate and fuse to reconstitute the injured muscle architecture. We have previously reported that cdk9 is required for myogenesis in vitro by activating MyoD-dependent transcription. In myoblasts induced to differentiate, MyoD recruits cdk9 on the chromatin of muscle-specific regulatory regions. This event correlates with chromatin-modifying enzyme recruitment and phosphorylation of cdk9-specific target residues at the carboxyl-terminal domain of RNA polymerase II. Here we report that a second cdk9 isoform, termed cdk9-55, plays a fundamental role in muscle regeneration and differentiation in vivo. This alternative form is specifically induced in injured myofibers and its activity is strictly required for the completion of muscle regeneration process.

Local expression of IGF-1 accelerates muscle regeneration by rapidly modulating inflammatory cytokines and chemokines.

Muscle regeneration following injury is characterized by myonecrosis accompanied by local inflammation, activation of satellite cells, and repair of injured fibers. The resolution of the inflammatory response is necessary to proceed toward muscle repair, since persistence of inflammation often renders the damaged muscle incapable of sustaining efficient muscle regeneration. Here, we show that local expression of a muscle-restricted insulin-like growth factor (IGF)-1 (mIGF-1) transgene accelerates the regenerative process of injured skeletal muscle, modulating the inflammatory response, and limiting fibrosis. At the molecular level, mIGF-1 expression significantly down-regulated proinflammatory cytokines, such as tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta, and modulated the expression of CC chemokines involved in the recruitment of monocytes/macrophages. Analysis of the underlying molecular mechanisms revealed that mIGF-1 expression modulated key players of inflammatory response, such as macrophage migration inhibitory factor (MIF), high mobility group protein-1 (HMGB1), and transcription NF-kappaB. The rapid restoration of injured mIGF-1 transgenic muscle was also associated with connective tissue remodeling and a rapid recovery of functional properties. By modulating the inflammatory response and reducing fibrosis, supplemental mIGF-1 creates a qualitatively different environment for sustaining more efficient muscle regeneration and repair.

Local expression of mIgf-1 modulates ubiquitin, caspase and CDK5 expression in skeletal muscle of an ALS mouse model.

OBJECTIVE: The functional connection between muscle and nerve is often altered in several neuromuscular diseases, including amyotrophic lateral sclerosis (ALS). Knowledge about the molecular and cellular mechanisms involved in the restorative reactions is important to our understanding of the processes involved in neuromuscular maintenance. We previously reported that muscle-restricted expression of a localized Igf-1 isoform maintained muscle integrity, stabilized neuromuscular junctions, reduced inflammation in the spinal cord and enhanced motor neuronal survival in SOD(G93A) mice, delaying the onset and progression of the disease. In this study, we analysed potential molecular pathways that are modulated by mIgf-1 to counteract muscle wasting and to preserve motor neurons activity. METHODS: We performed molecular and morphologic analysis to address the specific proposed questions. RESULTS AND DISCUSSION: Ubiquitin expression and caspase activity resulted markedly increased in SOD(G93A) muscle but maintained at very low levels in the SOD(G93A) x MLC/mIgf-1 (SOD(G93A)/mIgf-1) transgenic muscle. In addition, CDK5 expression, a serine-threonine protein kinase that has been implicated in a number of physiologic processes in nerve and muscle cells, was reduced in SOD(G93A) muscle but increased in SOD(G93A)/mIgf-1 muscle. Notably, while the toxic p25 protein accumulated in SOD(G93A) muscle, no accumulation was evident in the SOD(G93A)/mIgf-1 muscle. The maintenance of muscle phenotype was also associated with maintenance of a normal peripheral nerve, and a greater number of myelinated axons. CONCLUSION: These observations offer novel insights into the role of mIgf-1 in the attenuation of muscle wasting in the mouse model of ALS disease.

Vasopressin-dependent myogenic cell differentiation is mediated by both Ca2+/calmodulin-dependent kinase and calcineurin pathways.

Arg8-vasopressin (AVP) promotes the differentiation of myogenic cell lines and mouse primary satellite cells by mechanisms involving the transcriptional activation of myogenic bHLH regulatory factors and myocyte enhancer factor 2 (MEF2). We here report that AVP treatment of L6 cells results in the activation of calcineurin-dependent differentiation, increased expression of MEF2 and GATA2, and nuclear translocation of the calcineurin target NFATc1. Interaction of these three factors occurs at MEF2 sites of muscle specific genes. The different kinetics of AVP-dependent expression of early (myogenin) and late (MCK) muscle-specific genes correlate with different acetylation levels of histones at their MEF2 sites. The cooperative role of calcineurin and Ca2+/calmodulin-dependent kinase (CaMK) in AVP-dependent differentiation is demonstrated by the effect of inhibitors of the two pathways. We show here, for the first time, that AVP, a "novel" myogenesis promoting factor, activates both the calcineurin and the CaMK pathways, whose combined activation leads to the formation of multifactor complexes and is required for the full expression of the differentiated phenotype. Although MEF2-NFATc1 complexes appear to regulate the expression of an early muscle-specific gene product (myogenin), the activation of late muscle-specific gene expression (MCK) involves the formation of complexes including GATA2.

IGF-I-induced differentiation of L6 myogenic cells requires the activity of cAMP-phosphodiesterase.

Inhibition of type 4 cAMP-specific phosphodiesterase (PDE4) activity in L6-C5 and L6-E9 abolished myogenic differentiation induced by low-serum medium and IGF-I. L6-C5 cells cultured in low-serum medium displayed a PDE4 activity higher than cells cultured in serum-free medium, a condition not sufficient to induce differentiation. In the presence of serum, PDE4D3, the major isoform natively expressed in L6-C5 cells, translocated to a Triton-insoluble fraction, which increased the PDE specific activity of the fraction, and exhibited a Mr shift typical of phosphorylation of this isoform. Furthermore, serum promoted the localization of PDE4D3 to a vesicular subcellular compartment. In L6-C5 cells, IGF-I is a stronger inducer of myogenic differentiation in the presence than in absence of serum. Its ability to trigger differentiation in the absence of serum was restored by overexpressing wild-type PDE4D3, but not a phosphorylation-insensitive mutant. This finding was confirmed in single cells overexpressing a GFP-PDE4D3 fusion protein by assessing nuclear accumulation of myogenin in both L6-C5 and L6-E9. Overexpression of other PDE isoforms was less efficient, confirming that PDE4D3 is the physiologically relevant phosphodiesterase isoform in the control of myogenesis. These results show that downregulation of cAMP signaling through cAMP-phosphodiesterase stimulation is a prerequisite for induction of myogenesis.

AVP induces myogenesis through the transcriptional activation of the myocyte enhancer factor 2.

The neurohypophyseal nonapeptide Arg8 vasopressin (AVP) promotes differentiation of cultured L6 and L5 myogenic cell lines and mouse primary satellite cells. Here, we investigated the molecular mechanism involved in the induction of the myogenic program by AVP. In L6 cells, AVP treatment rapidly induces Myf-5, myogenin, and myocyte enhancer factor 2 (MEF2) mRNAs, without affecting the expression of known myogenic growth factors such as IGF-I, IGF-II, or their receptors. In the presence of cycloheximide, AVP up-regulates the expression of MEF2, but not of myogenin, indicating that the synthesis of a protein intermediate is not necessary for MEF2 induction. Notably, AVP treatment activates a calcium/calmodulin kinase signaling pathway that induces cytosolic compartmentalization of the histone deacetylase 4, a mechanism related to the transcriptional activation of MEF2. The activity of chloramphenicol acetyltransferase reporter constructs carrying the Myo184 and Myo84 fragments of the myogenin promoter is also induced by AVP. Mutation of the MEF2 site completely abolishes the response to AVP, whereas deletion of the E1 site present in pMyo84 does not impair this response. Together, these results show that AVP induces myogenic differentiation through the transcriptional activation of MEF2, a mechanism that is critical for myogenesis.

PKCθ signaling is required for myoblast fusion by regulating the expression of caveolin-3 and ß1D integrin upstream focal adhesion kinase.

Fusion of mononucleated myoblasts to form multinucleated myofibers is an essential phase of skeletal myogenesis, which occurs during muscle development as well as during postnatal life for muscle growth, turnover, and regeneration. Many cell adhesion proteins, including integrins, have been shown to be important for myoblast fusion in vertebrates, and recently focal adhesion kinase (FAK), has been proposed as a key mediator of myoblast fusion. Here we focused on the possible role of PKC, the PKC isoform predominantly expressed in skeletal muscle, in myoblast fusion. We found that the expression of PKC is strongly up-regulated following freeze injury-induced muscle regeneration, as well as during in vitro differentiation of satellite cells (SCs; the muscle stem cells). Using both PKC knockout and muscle-specific PKC dominant-negative mutant mouse models, we observed delayed body and muscle fiber growth during the first weeks of postnatal life, when compared with wild-type (WT) mice. We also found that myofiber formation, during muscle regeneration after freeze injury, was markedly impaired in PKC mutant mice, as compared with WT. This phenotype was associated with reduced expression of the myogenic differentiation program executor, myogenin, but not with that of the SC marker Pax7. Indeed in vitro differentiation of primary muscle-derived SCs from PKC mutants resulted in the formation of thinner myotubes with reduced numbers of myonuclei and reduced fusion rate, when compared with WT cells. These effects were associated to reduced expression of the profusion genes caveolin-3 and ß1D integrin and to reduced activation/phosphorylation of their up-stream regulator FAK. Indeed the exogenous expression of a constitutively active mutant form of PKC in muscle cells induced FAK phosphorylation. Moreover pharmacologically mediated full inhibition of FAK activity led to similar fusion defects in both WT and PKC-null myoblasts. We thus propose that PKC signaling regulates myoblast fusion by regulating, at least in part, FAK activity, essential for profusion gene expression.

Skeletal muscle regeneration in mice is stimulated by local overexpression of V1a-vasopressin receptor.

Skeletal muscle has a remarkable capacity to regenerate after mechanical or pathological injury. We show that the V1a receptor (V1aR) for vasopressin, a potent myogenic-promoting factor that stimulates differentiation and hypertrophy in vitro, is expressed in mouse skeletal muscle and modulated during regeneration after experimental injury. We used gene delivery by electroporation to overexpress the myc-tagged vasopressin V1aR in specific muscles, thus sensitizing them to circulating vasopressin. The correct localization on the surface of the fibers of the recombinant product was demonstrated by confocal immunofluorescence directed against the myc tag. V1aR overexpression dramatically enhanced regeneration. When compared with mock-transfected controls, V1aR overexpressing muscles exhibited significantly accelerated activation of satellite cells and increased expression of differentiation markers. Downstream of V1aR activation, calcineurin was strongly up-regulated and stimulated the expression of IL-4, a potent mediator of myogenic cell fusion. The central role of calcineurin in mediating V1aR-dependent myogenesis was also demonstrated by using its specific inhibitor, cyclosporine A. This study identifies skeletal muscle as a physiological target of hormones of the vasopressin family and reveals a novel in vivo role for vasopressin-dependent pathways. These findings unveil several steps, along a complex signaling pathway, that may be exploited as potential targets for the therapy of diseases characterized by altered muscle homeostasis and regeneration.

Skeletal muscle is a primary target of SOD1G93A-mediated toxicity.

The antioxidant enzyme superoxide dismutase 1 (SOD1) is a critical player of the antioxidative defense whose activity is altered in several chronic diseases, including amyotrophic lateral sclerosis. However, how oxidative insult affects muscle homeostasis remains unclear. This study addresses the role of oxidative stress on muscle homeostasis and function by the generation of a transgenic mouse model expressing a mutant SOD1 gene (SOD1(G93A)) selectively in skeletal muscle. Transgenic mice developed progressive muscle atrophy, associated with a significant reduction in muscle strength, alterations in the contractile apparatus, and mitochondrial dysfunction. The analysis of molecular pathways associated with muscle atrophy revealed that accumulation of oxidative stress served as signaling molecules to initiate autophagy, one of the major intracellular degradation mechanisms. These data demonstrate that skeletal muscle is a primary target of SOD1(G93A) -mediated toxicity and disclose the molecular mechanism whereby oxidative stress triggers muscle atrophy.

Cellular heterogeneity during vertebrate skeletal muscle development.

Although skeletal muscles appear superficially alike at different anatomical locations, in reality there is considerably more diversity than previously anticipated. Heterogeneity is not only restricted to completely developed fibers, but is clearly apparent during development at the molecular, cellular and anatomical level. Multiple waves of muscle precursors with different features appear before birth and contribute to muscular diversification. Recent cell lineage and gene expression studies have expanded our knowledge on how skeletal muscle is formed and how its heterogeneity is generated. This review will present a comprehensive view of relevant findings in this field.

Static magnetic fields enhance skeletal muscle differentiation in vitro by improving myoblast alignment.

Static magnetic field (SMF) interacts with mammal skeletal muscle; however, SMF effects on skeletal muscle cells are poorly investigated. The myogenic cell line L6, an in vitro model of muscle development, was used to investigate the effect of a 80 +/- mT SMF generated by a custom-made magnet. SMF promoted myogenic cell differentiation and hypertrophy, i.e., increased accumulation of actin and myosin and formation of large multinucleated myotubes. The elevated number of nuclei per myotube was derived from increased cell fusion efficiency, with no changes in cell proliferation upon SMF exposure. No alterations in myogenin expression, a modulator of myogenesis, occurred upon SMF exposure. SMF induced cells to align in parallel bundles, an orientation conserved throughout differentiation. SMF stimulated formation of actin stress-fiber like structures. SMF rescued muscle differentiation in the presence of TNF, a muscle differentiation inhibitor. We believe this is the first report showing that SMF promotes myogenic differentiation and cell alignment, in the absence of any invasive manipulation. SMF-enhanced parallel orientation of myotubes is relevant to tissue engineering of a highly organized tissue such as skeletal muscle. SMF rescue of muscle differentiation in the presence of TNF may have important therapeutic implications.

Tumor necrosis factor-alpha gene transfer induces cachexia and inhibits muscle regeneration.

Chronic disease states are associated with elevated levels of inflammatory cytokines that have been demonstrated to lead to severe muscle wasting. A mechanistic understanding of muscle wasting is hampered by limited in vivo cytokine models which can be applied to emerging mouse mutants as they are generated. We developed a simple and novel approach to induce adult mouse skeletal muscle wasting based on direct gene transfer of an expression vector encoding the secreted form of the murine tumor necrosis factor-alpha (mTNFalpha). This procedure results in the production of elevated levels of circulating mTNFalpha followed by body weight loss, upregulation of Atrogin1, and muscle atrophy, including muscles distant from the site of gene transfer. We also found that mTNFalpha gene transfer resulted in a significant inhibition of regeneration following muscle injury. We conclude that in addition to being a potent inducer of cachexia, TNFalpha is a potent inhibitor of myogenesis in vivo.

The block of ryanodine receptors selectively inhibits fetal myoblast differentiation.

Differentiation and morphogenesis of skeletal muscle are complex and asynchronous events that involve various myogenic cell populations and extracellular signals. Embryonic and fetal skeletal myoblasts are responsible for the formation of primary and secondary fibers, respectively, although the mechanism that diversifies their fate is not fully understood. Calcium transients appear to be a signaling mechanism that is widely utilized in differentiation and embryogenesis. In mature skeletal muscle, calcium transients are generated mainly by ryanodine receptors (type 1 and type 3), which are involved in excitation-contraction coupling. However, it is not clear whether the activity of these receptors is important for contractile activity alone or whether it may also play a role in regulating the differentiation/developmental processes. To clarify this point, we first examined the expression of the receptors during development. The results show that the expression of both receptors appears as early as E13 during limb muscle development and parallels the expression of skeletal myosin. The expression and the activity of both receptors is maintained in vitro by all myogenic cell populations isolated from different stages of development, including somitic, embryonic and fetal myoblasts and satellite cells. Blocking ryanodine receptor activity by using ryanodine inhibits in vitro differentiation of fetal myoblasts (judged by the expression of sarcomeric myosin and formation of multinucleated myotubes) but not of somitic or embryonic and satellite muscle cells. This block is caused by the transcriptional inhibition of markers characteristic of terminal differentiation, rather than commitment, as the expression of muscle regulatory factors is not impaired by ryanodine treatment. Taken together, the data reported in this paper demonstrate that, although calcium transients represent a general mechanism for the control of differentiation and development, multiple calcium-dependent pathways may be relevant in different myogenic populations during development. Moreover, since fetal myoblasts are responsible for the formation of secondary fibers during development, and therefore for the building of the bulk of muscular mass, these results suggest that calcium release from ryanodine receptors plays a role in the histogenesis of mammalian skeletal muscle.

PKCalpha-mediated ERK, JNK and p38 activation regulates the myogenic program in human rhabdomyosarcoma cells.

We have previously suggested that PKCalpha has a role in 12-O-Tetradecanoylphorbol-13-acetate (TPA)-mediated growth arrest and myogenic differentiation in human embryonal rhabdomyosarcoma cells (RD). Here, by monitoring the signalling pathways triggered by TPA, we demonstrate that PKCalpha mediates these effects by inducing transient activation of c-Jun N-terminal protein kinases (JNKs) and sustained activation of both p38 kinase and extracellular signal-regulated kinases (ERKs) (all referred to as MAPKs). Activation of MAPKs following ectopic expression of constitutively active PKCalpha, but not its dominant-negative form, is also demonstrated. We investigated the selective contribution of MAPKs to growth arrest and myogenic differentiation by monitoring the activation of MAPK pathways, as well as by dissecting MAPK pathways using MEK1/2 inhibitor (UO126), p38 inhibitor (SB203580) and JNK and p38 agonist (anisomycin) treatments. Growth-arresting signals are triggered either by transient and sustained JNK activation (by TPA and anisomycin, respectively) or by preventing both ERK and JNK activation (UO126) and are maintained, rather than induced, by p38. We therefore suggest a key role for JNK in controlling ERK-mediated mitogenic activity. Notably, sarcomeric myosin expression is induced by both TPA and UO126 but is abrogated by the p38 inhibitor. This finding indicates a pivotal role for p38 in controlling the myogenic program. Anisomycin persistently activates p38 and JNKs but prevents myosin expression induced by TPA. In accordance with this negative role, reactivation of JNKs by anisomycin, in UO126-pre-treated cells, also prevents myosin expression. This indicates that, unlike the transient JNK activation that occurs in the TPA-mediated myogenic process, long-lasting JNK activation supports the growth-arrest state but antagonises p38-mediated myosin expression. Lastly, our results with the MEK inhibitor suggest a key role of the ERK pathway in regulating myogenic-related morphology in differentiated RD cells.

A bimodal modulation of the cAMP pathway is involved in the control of myogenic differentiation in l6 cells.

We have previously shown that myogenesis induction by Arg8-vasopressin (AVP) in L6 rat myoblasts involves a sustained stimulation of type 4 cAMP-phosphodiesterase. In this model, we observed that a transient cAMP generation occurs in the minutes following AVP addition. Evidence suggests that cAMP generation is due to the prostaglandins produced in response to AVP binding to V1a receptors and subsequent activation of phospholipase A2. The early cAMP increase was effective in activating cAMP-dependent protein kinase (PKA) and increasing phosphorylation of CREB transcription factor. Inhibition of PKA by compound H89 prior to AVP addition led to a significant reduction of expression of the differentiation marker creatine kinase, whereas H89 added 1-5 h after AVP had no significant effect. Furthermore, PKA inhibition 24 h after the beginning of AVP treatment potentiated differentiation. This shows that both an early activation and a later down-regulation of the cAMP pathway are required for AVP induction of myogenesis. Because phosphodiesterase PDE4D3 overexpressed in L6 cells lost its ability to potentiate AVP-induced differentiation when mutated and rendered insensitive to PKA phosphorylation and activation, we hypothesize that the early cAMP increase is required to trigger the down-regulation of cAMP pathway through stimulation of phosphodiesterase.

Stem cell-mediated muscle regeneration is enhanced by local isoform of insulin-like growth factor 1.

We investigated the mechanism whereby expression of a transgene encoding a locally acting isoform of insulin-like growth factor 1 (mIGF-1) enhances repair of skeletal muscle damage. Increased recruitment of proliferating bone marrow cells to injured MLC/mIgf-1 transgenic muscles was accompanied by elevated bone marrow stem cell production in response to distal trauma. Regenerating MLC/mIgf-1 transgenic muscles contained increased cell populations expressing stem cell markers, exhibited accelerated myogenic differentiation, expressed markers of regeneration and readily converted cocultured bone marrow to muscle. These data implicate mIGF-1 as a powerful enhancer of the regeneration response, mediating the recruitment of bone marrow cells to sites of tissue damage and augmenting local repair mechanisms.

Increase in cytosolic Ca2+ induced by elevation of extracellular Ca2+ in skeletal myogenic cells.

Cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) variation is a key event in myoblast differentiation, but the mechanism by which it occurs is still debated. Here we show that increases of extracellular Ca(2+) concentration ([Ca(2+)](o)) produced membrane hyperpolarization and a concentration-dependent increase of [Ca(2+)](i) due to Ca(2+) influx across the plasma membrane. Responses were not related to inositol phosphate turnover and Ca(2+)-sensing receptor. [Ca(2+)](o)-induced [Ca(2+)](i) increase was inhibited by Ca(2+) channel inhibitors and appeared to be modulated by several kinase activities. [Ca(2+)](i) increase was potentiated by depletion of intracellular Ca(2+) stores and depressed by inactivation of the Na(+)/Ca(2+) exchanger. The response to arginine vasopressin (AVP), which induces inositol 1,4,5-trisphosphate-dependent [Ca(2+)](i) increase in L6-C5 cells, was not modified by high [Ca(2+)](o). On the contrary, AVP potentiated the [Ca(2+)](i) increase in the presence of elevated [Ca(2+)](o). Other clones of the L6 line as well as the rhabdomyosarcoma RD cell line and the satellite cell-derived C2-C12 line expressed similar responses to high [Ca(2+)](o), and the amplitude of the responses was correlated with the myogenic potential of the cells.