Cell shape and growth regulation in skeletal muscle: exogenous versus endogenous factors

Molecular and Biomechanical Adaptations to Mechanical Stretch in Cultured Myotubes

Myotubes are mature muscle cells that form the basic structural element of skeletal muscle. When stretching skeletal muscles, myotubes are subjected to passive tension as well. This lead to alterations in myotube cytophysiology, which could be related with muscular biomechanics. During the past decades, much progresses have been made in exploring biomechanical properties of myotubes in vitro. In this review, we integrated the studies focusing on cultured myotubes being mechanically stretched, and classified these studies into several categories: amino acid and glucose uptake, protein turnover, myotube hypertrophy and atrophy, maturation, alignment, secretion of cytokines, cytoskeleton adaption, myotube damage, ion channel activation, and oxidative stress in myotubes. These biomechanical adaptions do not occur independently, but interconnect with each other as part of the systematic mechanoresponse of myotubes. The purpose of this review is to broaden our comprehensions of stretch-induced muscular alterations in cellular and molecular scales, and to point out future challenges and directions in investigating myotube biomechanical manifestations.

A novel bioreactor for the generation of highly aligned 3D skeletal muscle-like constructs through orientation of fibrin via application of static strain

The generation of functional biomimetic skeletal muscle constructs is still one of the fundamental challenges in skeletal muscle tissue engineering. With the notion that structure strongly dictates functional capabilities, a myriad of cell types, scaffold materials and stimulation strategies have been combined. To further optimize muscle engineered constructs, we have developed a novel bioreactor system (MagneTissue) for rapid engineering of skeletal muscle-like constructs with the aim to resemble native muscle in terms of structure, gene expression profile and maturity. Myoblasts embedded in fibrin, a natural hydrogel that serves as extracellular matrix, are subjected to mechanical stimulation via magnetic force transmission. We identify static mechanical strain as a trigger for cellular alignment concomitant with the orientation of the scaffold into highly organized fibrin fibrils. This ultimately yields myotubes with a more mature phenotype in terms of sarcomeric patterning, diameter and length. On the molecular level, a faster progression of the myogenic gene expression program is evident as myogenic determination markers MyoD and Myogenin as well as the Ca(2+) dependent contractile structural marker TnnT1 are significantly upregulated when strain is applied. The major advantage of the MagneTissue bioreactor system is that the generated tension is not exclusively relying on the strain generated by the cells themselves in response to scaffold anchoring but its ability to subject the constructs to individually adjustable strain protocols. In future work, this will allow applying mechanical stimulation with different strain regimes in the maturation process of tissue engineered constructs and elucidating the role of mechanotransduction in myogenesis.

STATEMENT OF SIGNIFICANCE

Mechanical stimulation of tissue engineered skeletal muscle constructs is a promising approach to increase tissue functionality. We have developed a novel bioreactor-based 3D culture system, giving the user the possibility to apply different strain regimes like static, cyclic or ramp strain to myogenic precursor cells embedded in a fibrin scaffold. Application of static mechanical strain leads to alignment of fibrin fibrils along the axis of strain and concomitantly to highly aligned myotube formation. Additionally, the pattern of myogenic gene expression follows the temporal progression observed in vivo with a more thorough induction of the myogenic program when static strain is applied. Ultimately, the strain protocol used in this study results in a higher degree of muscle maturity demonstrated by enhanced sarcomeric patterning and increased myotube diameter and length. The introduced bioreactor system enables new possibilities in muscle tissue engineering as longer cultivation periods and different strain applications will yield tissue engineered muscle-like constructs with improved characteristics in regard to functionality and biomimicry.

Bioreactors for guiding muscle tissue growth and development

Muscle tissue bioreactors are devices which are employed to guide and monitor the development of engineered muscle tissue. These devices have a modern history that can be traced back more than a century, because the key elements of muscle tissue bioreactors have been studied for a very long time. These include barrier isolation and culture of cells, tissues and organs after isolation from a host organism; the provision of various stimuli intended to promote growth and maintain the muscle, such as electrical and mechanical stimulation; and the provision of a perfusate such as culture media or blood derived substances. An accurate appraisal of our current progress in the development of muscle bioreactors can only be made in the context of the history of this endeavor. Modern efforts tend to focus more upon the use of computer control and the application of mechanical strain as a stimulus, as well as substrate surface modifications to induce cellular organization at the early stages of culture of isolated muscle cells.

Myogenic differentiation by human processed lipoaspirate cells

The use of undifferentiated cells for cell-based tissue engineering and regeneration strategies represents a promising approach for skeletal muscle repair. For such strategies to succeed, a readily available source of myogenic precursor cells must be identified. We have previously shown that cells isolated from raw human lipoaspirates, called processed lipoaspirate cells, display multilineage mesodermal potential in vitro. Because human liposuctioned fat is available in large quantities and can be harvested with low morbidity, it may be an ideal source of stem cells for tissue-engineering applications. In this study, processed lipoaspirate cells were isolated from raw lipoaspirates harvested from eight patients who underwent cosmetic surgery. Processed lipoaspirate cells were placed in promyogenic conditions for up to 6 weeks, and the expression of the myogenic markers MyoD1 and myosin heavy chain was confirmed by using structure, histology, and reverse transcriptase-polymerase chain reaction. Histologic results were quantitated as an indicator or myogenic differentiation levels. We found that induced human processed lipoaspirate cells form multinucleated cells after 3 weeks of induction, indicative of the formation of myotubes. In addition, MyoD1 and skeletal muscle myosin heavy chain are expressed at distinct time points during differentiation with MyoD1 expression preceding expression of myosin. Finally, approximately 15 percent of human processed lipoaspirate cells can be induced toward myogenic differentiation 6 weeks after induction. In summary, our findings suggest that human processed lipoaspirate cells differentiate into myogenic cells. Furthermore, these cells may be a useful source for skeletal muscle engineering and repair.

ANG II is required for optimal overload-induced skeletal muscle hypertrophy

ANG II mediates the hypertrophic response of overloaded cardiac muscle, likely via the ANG II type 1 (AT(1)) receptor. To examine the potential role of ANG II in overload-induced skeletal muscle hypertrophy, plantaris and/or soleus muscle overload was produced in female Sprague-Dawley rats (225-250 g) by the bilateral surgical ablation of either the synergistic gastrocnemius muscle (experiment 1) or both the gastrocnemius and plantaris muscles (experiment 2). In experiment 1 (n = 10/group), inhibiting endogenous ANG II production by oral administration of an angiotensin-converting enzyme (ACE) inhibitor during a 28-day overloading protocol attenuated plantaris and soleus muscle hypertrophy by 57 and 96%, respectively (as measured by total muscle protein content). ACE inhibition had no effect on nonoverloaded (sham-operated) muscles. With the use of new animals (experiment 2; n = 8/group), locally perfusing overloaded soleus muscles with exogenous ANG II (via osmotic pump) rescued the lost hypertrophic response in ACE-inhibited animals by 71%. Furthermore, orally administering an AT(1) receptor antagonist instead of an ACE inhibitor produced a 48% attenuation of overload-induced hypertrophy that could not be rescued by ANG II perfusion. Thus ANG II may be necessary for optimal overload-induced skeletal muscle hypertrophy, acting at least in part via an AT(1) receptor-dependent pathway.

Mitogenic activity of fetal bovine serum, fish fry extract, insulin-like growth factor-I, and fibroblast growth factor on brown bullhead catfish cells--BB line

Bioassays were performed to assess the effects of different levels of growth medium supplementation with fetal bovine serum (FBS), fish fry extract (FE), combinations of FBS and FE, and addition of insulin-like growth factor I (IGF-I) and fibroblast growth factor (FGF) on the proliferation of brown bullhead catfish cells (BB line). Treatments (n = 4) were: 2.5, 5, 10, and 15.0% FBS or FE and 5/2.5, 5/5, 10/2.5, and 10/5 of a FBS/FE combination as supplement to the growth medium, or the addition of 0.1, 1, 2.5, 10, 25, and 75 ng/ml of either IGF-I or FGF to the growth media. Initial cell density was 1.1 x 10(6) cells per well on uncoated 24-well plates. Incubation temperature was 29.5 +/- 0.7 degrees C. Six hours after plating, initial culture medium was removed, plates rinsed with Dulbecco's phosphate buffered saline, treatment media added, and cells allowed to proliferate for 24 hours. Another bioassay was performed with rat myoblast omega cells (RMo) using the same levels of growth medium supplemented with FBS, FE and FBS/FE. Base growth medium was Dulbecco's MEM. The initial cell density was 7.2 x 10(6) cells per well, and the bioassay was carried out at 36.0 +/- 0.5 degrees C, on a 95% air, 5% CO2 incubator. Increasing levels of FBS had a positive effect (P < 0.05) on the proliferation of both BB and RMo cells. Increasing levels of FE had a negative effect (P < 0.05) on the proliferation of BB cells and totally inhibited the proliferation of RMo cells at any level of supplementation. Higher levels of FE on the FBS/FE combinations presented a negative effect on the proliferation of both BB and RMo cells (P < 0.05). Insulin-like growth factor I had a positive quadratic effect (P < 0.05) on the proliferation of BB cells. Apparently, mammalian growth factors slightly stimulated mitogenic activity in fish cells, while FE contained factors which inhibited the mitogenic activity of RMo and BB cell lines.

Effect of serum and mechanical stretch on skeletal alpha-actin gene regulation in cultured primary muscle cells

The purpose of this study was to determine whether mechanical stretch or serum availability alters pretranslational regulation of skeletal alpha-actin (SkA) in cultured striated muscle cells. Chicken primary skeletal myoblasts and cardiac myocytes were plated on collagenized Silastic membranes adherent to nylon supports and stretched 8-20% of initial length 96 h postplating. Serum dependence of SkA gene regulation was determined by maintaining differentiated muscle cells in growth/differentiation (G/D; skeletal myotubes, 10% horse serum-2% chick embryo extract; cardiac myocytes, 10% horse serum) or growth-limiting (G-L; 0.5% horse serum) medium. Skeletal myotubes had higher SkA mRNA and SkA promoter activity in G/D than in G-L medium. Cardiac myocyte SkA mRNA was higher in G-L than in G/D medium. Serum response factor (SRF) protein binding to serum response element 1 (SRE1) of SkA promoter increased in skeletal cultures in G/D compared with G-L medium. Western blot analysis demonstrated that increased SRF-SRE1 binding was due, in part, to increased SRF protein. Stretching skeletal myotubes in G-L medium reduced SkA mRNA and repressed SkA promoter activity. The first 100 bp of SkA promoter were sufficient for stretch-induced repression of SkA promoter activity, and an intact transcriptional enhancer factor 1 (TEF-1) binding site was necessary for this response. Serum and stretch appear to repress SkA promoter activity in skeletal myotubes through different DNA binding elements, the SRE1 and TEF-1 sites, respectively. Stretching increased SkA mRNA in cardiac myocytes in G-L medium but did not alter SkA mRNA level in cardiac cells in G/D medium. These results demonstrate that stretch and serum interact differently to alter SkA expression in cultured cardiac and skeletal muscle cells.

Ectopic skeletal muscles derived from myoblasts implanted under the skin

We investigated the potential of cultured myoblasts to generate skeletal muscle in an ectopic site. Myoblasts from a clonal cell line or from expanded primary cultures were injected under the skin of the lumbar region of adult syngenic Balb/c mice. One to 7 weeks after injection, distinct muscles, of greater mass in mice injected with clonal myoblasts (6–78 mg, n=37) than in mice injected with primary myoblasts (1–7 mg, n=26), had formed between the subcutaneous panniculus carnosus muscle and the trunk muscles of host animals. These ectopic muscles exhibited spontaneous and/or electrically-evoked contractions after the second week and, when stimulated directly in vitro, isometric contractile properties similar to those of normal muscles. Histological, electron microscopical and tissue culture examination of these muscles revealed their largely mature morphology and phenotype. The fibres, most of which were branched, were contiguous, aligned and capillarised, exhibited normal sarcormeric protein banding patterns, and expressed muscle-specific proteins, including desmin, dystrophin, and isoforms of developmental and adult myosin heavy chain. Enveloping each fibre was a basal lamina, beneath which lay quiescent satellite cells, which could be stimulated to produce new muscle in culture. Presence of endplates (revealed by alpha-bungarotoxin and neurofilament staining), and the eventual loss of expression of neural cell adhesion molecule and extrasynaptic acetylcholine receptors, indicated that some fibres were innervated. That these muscle fibres were of implanted-cell origin was supported by the finding of Y-chromosome and a lack of dystrophin in ectopic muscles formed after subcutaneous injection of, respectively, male myoblasts into female mice and dystrophin-deficient (mdx) myoblasts into normal C57Bl/10 muscle. Our results demonstrate that an organised, functional muscle can be generated de novo from a disorganised mass of myoblasts implanted in an extramuscular subcutaneous site, whereby the host contributes significantly in providing support tissues and innervation. Our observations are also consistent with the idea that myogenic cells behave like tissue-specific stem cells, generating new muscle precursor (satellite) cells as well as mature muscle. Subcutaneous implantation of myoblasts may have a range of useful applications, from the study of myogenesis to the delivery of gene products.

Strain reorganizes focal adhesions and cytoskeleton in cultured airway smooth muscle cells

Abnormal mechanical stress on pulmonary structures is associated with increased airway resistance and impaired gas exchange as a result of increased airway smooth muscle (ASM) deposition. Using an in vitro system with cultured ASM cells, we have demonstrated that cyclic deformational strain increases ASM cellular myosin and myosin light chain kinase. To determine if these contractile protein increases were accompanied by ultrastructural changes in cells indicating phenotypic modulation, cells subjected to strain were compared to cells grown under static conditions by transmission electron microscopy (TEM) and fluorescent staining. The strained ASM cells oriented perpendicular to the strain direction were more elongated and contained more actin stress fibers than identical cells grown under physically static conditions. The stress fiber bundles were thicker and reorganized parallel to the long axis of the cell. Marked increases in the numbers and lengths of focal adhesions between the cell membrane and the substratum were found by both TEM and immunostaining for talin. Mechanical strain thus increases organization of cytoskeletal elements in cultured ASM cells. Similar effects in vivo may serve to promote the expression of the contractile phenotype of cultured ASM cells independent of other in vivo factors and alter cell contractility. Increased organization of cytoskeletal elements might also increase the efficiency of signal transduction from the extracellular matrix into the cell interior.

Mechanical stimulation of skeletal muscle increases prostaglandin F2 alpha production, cyclooxygenase activity, and cell growth by a pertussis toxin sensitive mechanism

Repetitive mechanical stimulation of differentiated skeletal muscle in tissue culture increased the long-term production of prostaglandin F2 alpha, an anabolic stimulator of myofiber growth. Within 4 h of initiating mechanical stimulation, the enzymatic activity of cyclooxygenase (prostaglandin GH synthase [PGHS]), a regulatory enzyme in prostaglandin synthesis, was increased 82% (P < .005), and this increase was maintained for at least 24 h. Kinetic analysis of stretch-activated cyclooxygenase activity indicated a two to threefold decrease in the enzyme's Km, with little change in its Vmax. Immunocytochemical analysis of the cell cultures indicated the presence of high levels of the mitogen-inducible isoform of cyclooxygenase (PGHS-2) in the skeletal myofibers compared to the interstitial fibroblasts. While the stretch-induced increase in cyclooxygenase enzymatic activity was not inhibited by tetrodotoxin and therefore was independent of cellular electrical activity, the G protein inhibitor pertussis toxin prevented stretch-induced cyclooxygenase activation. Pertussis toxin also inhibited stretch-induced increases in PGF2 alpha production, phospholipase D activation, and cell growth. It is concluded that stretch of skeletal muscle increases muscle cell growth through a G protein-dependent process involving the activation of cyclooxygenase, an immediate early gene product.

Hormonal induction of c-fos and HSP68 mRNAs on an isolated coronary perfused adult rat heart

Transient growth signals which can be related to protein synthesis and cellular growth are of particular interest in the heart because of the incidence of cardiac hypertrophy in man. The isolated coronary perfused adult rat heart or the so-called Langendorff preparation, is an useful model in exploring not only protein synthesis but also c-fos/c-myc protooncogene and Heat Shock Protein (HSP) gene expression. Phenylephrine infusion in this preparation induces c-fos expression whether the heart is beating or reversibly or irreversibly arrested by solutions enriched in KCl. Norepinephrine has the same effect. Quantitative analysis with slot blots shows that in both cases the adrenergic effect has a dual origin since it is inhibited both by propranolol, a beta-adrenergic antagonist, and terazosine, a soluble alpha 1-adrenergic antagonist. We conclude that the isolated heart is a useful tool to explore the early changes in gene expression which occur in this tissue in response to various physiological stimuli.

Mechanically induced alterations in cultured skeletal muscle growth

Model systems are available for mechanically stimulating cultured skeletal muscle cells by passive tensile forces which simulate those found in vivo. When applied to embryonic muscle cells in vitro these forces induce tissue organogenesis, metabolic adaptations, and muscle cell growth. The mechanical stimulation of muscle cell growth correlates with stretch-induced increases in the efflux of prostaglandins PGE2 and PGF2 alpha in a time and frequency dependent manner. These prostaglandins act as mechanical "second messengers" regulating skeletal muscle protein turnover rates. Since they also effect bone remodelling in response to tissue loading and unloading, secreted prostaglandins may serve as paracrine growth factors, coordinating the growth rates of muscle and bone in response to external mechanical forces. Cell culture model systems will supplement other models in understanding mechanical transduction processes at the molecular level.

Cyclic biaxial strain of pulmonary artery endothelial cells causes an increase in cell layer-associated fibronectin

Bovine pulmonary artery endothelial (PAE) cells were cultured on an artificial compliant substrate (Mitrathane) and were strained biaxially at a frequency of 1/s for 2, 4, 6, 7, or 24 h. Total protein synthesis, determined by estimating the incorporation of radiolabeled precursors into nondialyzable protein, was increased in cultures that had been biaxially strained for 6, 7, or 24 h, with differences more apparent in the cell layer fraction than in the medium fraction. Medium and cell layer-associated fibronectin were quantitated by enzyme-linked immunosorbent assay and by densitometric analysis of the autoradiograms of electrophoresed protein. Fibronectin levels in the medium of biaxially strained cells were initially depressed in comparison to nonstrained controls but, with time, began to approach control values. Cell layer-associated fibronectin of biaxially strained cultures was significantly elevated at 24 h, whereas DNA synthesis was not altered. Immunohistochemical localization of fibronectin and factor VIII-von Willebrand antigen revealed a more intense staining pattern in strained cultures. Distribution of stress fibers containing fibrous actin was visualized by staining with rhodamine-phalloidin and was altered in strained cultures. These observations indicate that cells respond to cyclic biaxial strain by selectively enhancing structural components associated with cell adhesion.

Stretch-induced prostaglandins and protein turnover in cultured skeletal muscle

Intermittent repetitive mechanical stimulation of differentiated avian skeletal muscle cells in vitro for 48 h stimulates skeletal muscle growth [Am. J. Physiol. 256 (Cell Physiol. 25): C674-C682, 1989]. During the first 2-3 h of stimulation, temporary muscle damage occurs based on increases in creatine kinase efflux, total protein degradation rates, and several proteinase activites. With continued mechanical stimulation for several days in serum-containing medium, the proteinase activities return to control levels, and total protein degradation rates decrease to levels less than static controls. Decreased protein degradation thus contributes to stretch-induced cell growth. The efflux of prostaglandins (PG) E2 and F2 alpha but not 6-keto-PGF1 alpha increase with mechanical stimulation. During the first 5 h of stimulation, PGE2 and PGF2 alpha efflux rates increase 101 and 41%, respectively. PGE2 efflux returns to control levels by 24 h of mechanical stimulation, whereas PGF2 alpha efflux is continuously elevated (41-116%) for at least 48 h. The long-term stretch-induced elevation of PGF2 alpha efflux correlates with a 52-98% long-term increase in total protein synthesis rates. The prostaglandin synthesis inhibitor indomethacin partially blocks early stretch-induced cell damage and long-term stretch-induced cell growth. The results indicate that both of these processes are partially dependent on stretch-induced increases in prostaglandin synthesis.

Fibronectin controls capillary endothelial cell growth by modulating cell shape

An in vitro system has been developed to study the mechanism by which fibronectin (FN) regulates capillary endothelial cell growth in the presence of soluble angiogenic mitogens. Endothelial cells were cultured in chemically defined medium containing a constant, saturating amount of basic fibroblast growth factor. Formation of cell-FN contacts was then varied in a controlled fashion by three different techniques: (i) nonadhesive, bacteriological dishes were precoated with increasing densities of FN; (ii) soluble RGD peptides were used to progressively inhibit binding of cell-surface integrin receptors to adsorbed FN; and (iii) FN-coated surfaces were covered with increasingly thick layers of polyhydroxyethylmethacrylate (a nonadhesive polymer) to physically restrict cell access to FN binding sites. Endothelial cells became more extended and proliferated more rapidly as FN coating concentrations were raised from approximately 250 to approximately 10,000 FN molecules per micron 2. Computerized morphometric analysis confirmed that cell shape (projected cell areas) was determined by the density of FN contacts and that DNA synthetic levels were tightly coupled to the extent of cell spreading, regardless of the method used to perturb cell adhesion. In contrast, neither soluble FN nor cell-surface binding of FN-coated microbeads (diameter, 4.5 microns) had any effect on growth when cells were grown in suspension and cell spreading was prohibited. These results suggest that FN controls capillary endothelial cell proliferation based on its ability to support tension-dependent alterations of cell shape--i.e., both by binding to cell-surface integrins and by resisting mechanical loads that are applied to these receptors.

Increased cyclic AMP content accelerates protein synthesis in rat heart

Elevation of cyclic AMP (cAMP) content in perfused rat hearts by exposure to glucagon, forskolin, and 1-methyl-3-isobutylxanthine (IBMX) increased rates of protein synthesis during the second hour of perfusion with buffer that contained glucose in the absence of added insulin. When tetrodotoxin was added to arrest contractile activity, glucagon, forskolin, and IBMX still elevated cAMP content and rates of protein synthesis. Perfusion of beating rat hearts at elevated aortic pressure (120 mm Hg vs. 60 mm Hg) also accelerated rates of protein synthesis and raised cAMP content and cAMP-dependent protein kinase activity during the second hour of perfusion. Insulin accelerated rates of protein synthesis in beating hearts during the first and second hour of perfusion but did not increase cAMP content. Elevation of aortic pressure in insulin-treated hearts raised cAMP content but had no further effect on rates of protein synthesis. Perfusion of arrested hearts for as little as 2 minutes at 120 mm Hg resulted in a rapid and sustained increase in cAMP content, cAMP-dependent protein kinase activity, and rate of protein synthesis after 60-120 minutes of additional perfusion at 60 mm Hg. Exposure of arrested hearts to 0.2 mM methacholine, a muscarinic-cholinergic agonist, for 5 minutes before elevation of perfusion pressure blocked the pressure-induced increases in cAMP content, cAMP-dependent protein kinase activity, and rates of protein synthesis. When hearts were removed from pertussis toxin-treated animals, methacholine did not block the effects of forskolin on these same three parameters. These studies indicated that elevation of tissue cAMP by hormone binding, direct activation of adenylate cyclase, or inhibition of phosphodiesterase resulted in acceleration of protein synthesis. Furthermore, the effects of increased aortic pressure to accelerate synthesis appeared to involve a cAMP-dependent mechanism that was independent of changes in contractile activity but could be blocked with a muscarinic-cholinergic agonist. Acceleration of protein synthesis by insulin was not associated with an elevation of cAMP.

Longitudinal growth of skeletal myotubes in vitro in a new horizontal mechanical cell stimulator

A new computerized mechanical cell stimulator device for tissue cultured cells is described which maintains the cells in a horizontal position during mechanical stretching of up to 400% in substratum length. Mechanical stimulation of myogenic cells in this device initiates several aspects of in vivo skeletal muscle organogenesis not seen in normal static tissue culture environments. Embryonic skeletal muscle cells from avian m. pectoralis are grown in the device attached to the collagen-coated elastic substratum. Dynamic stretching of the substratum in one direction for 3 d at a rate (0.35 mm/h) that stimulates in vivo bone elongation during development causes the myoblasts to fuse into parallel arrays of myotubes which are 2 to 4 times longer than myotubes grown under static culture conditions. This longitudinal myotube growth is accompanied by increased rates of cell proliferation and myoblast fusion. Prestretching the collagen-coated substratum before cell plating also results in increased cell proliferation, myotube orientation, and longitudinal myotube growth. The effects of substratum stretching on myogenesis in this model system thus occur by alterations in the cell's extracellular matrix and not by acting directly on the cells.

Skeletal muscle growth is stimulated by intermittent stretch-relaxation in tissue culture

Avian pectoralis muscle cells differentiated in vitro are mechanically stimulated by repetitive stretch-relaxation of the cell's substratum using a computerized mechanical cell stimulator device. Initiation of mechanical stimulation increases the efflux of creatine kinase from the cells during the first 8-10 h of activity, but the efflux rate returns to control levels after this time period. Decreased total cell protein content accompanies the temporary elevation of creatine kinase efflux. With continued mechanical stimulation for 48-72 h, total cell protein loss recovers and significantly increases in medium supplemented with serum and embryo extract. Myotube diameters increase and cell hyperplasia occurs in the stimulated cultures. In basal medium without supplements, mechanical activity prevents myotube atrophy but does not lead to cell growth. Mechanically induced growth is accompanied by significant increases in protein synthesis rates. The increases in protein synthesis and accumulation induced by mechanical stimulation are not inhibited by tetrodotoxin but are significantly reduced in basal medium without supplements. Mechanically stimulated cell growth is thus dependent on medium growth factors but independent of electrical activity.

Role of proto-oncogenes in myocardial hypertrophy

A question of major clinical significance in cardiology is the nature of the signals that initiate and maintain the various types of myocardial hypertrophy, either in response to hemodynamic loading or in the absence of altered load. This review suggests that the proto-oncogene model, a concept derived from the study of cancer, can be very useful in identifying these signals. The proto-oncogene model conceives of cell growth regulation in terms of a limited number of classes of critical regulatory proteins: growth factors, growth factor receptors, intracellular transducing proteins and ribonucleic acid (RNA) transcription factors. Growth of all cells has dissociable components: hypertrophy (growth in size), deoxyribonucleic acid synthesis, mitosis and cytokinesis. Hypertrophy may be the end result of activation of RNA transcription. The various types of hypertrophy could reflect transcription of specific myocyte genes in response to different growth factors. At least 1 member of each functional class of proto-oncogenes has been detected in the myocardium or myocytes, or both. The alpha 1-adrenergic receptor has been shown to be a growth factor receptor and to regulate RNA transcription. Continued work on proto-oncogenes in myocytes may open the way to manipulate the growth of these cells.

Maintenance of highly contractile tissue-cultured avian skeletal myotubes in collagen gel

Highly contractile skeletal myotubes differentiated in tissue culture are normally difficult to maintain on collagen-coated tissue culture dishes for extended periods because of their propensity to detach as a sheet of cells from their substratum. This detachment results in the release of mechanical tension in the growing cell "sheet" and, consequently, loss of cellular protein. We developed a simple method of culturing high density contractile primary avian myotubes embedded in a collagen gel matrix (collagel) attached to either a stainless steel mesh or nylon support structure. With this system the cells are maintained in a highly contractile state for extended periods in vitro under tension. Structural integrity of the myotubes can be maintained for up to 10 d in basal medium without serum or embryo extract. Total cellular protein and myosin heavy chain accumulation in the cells can be maintained for weeks at levels which are two to three times those found in time-matched controls that are under little tension. Morphologically, the myotubes are well differentiated with structural characteristics of neonatal myofibers. This new collagel culture system should prove useful in the analysis of in vitro gene expression during myotube to myofiber differentiation and its regulation by various environmental factors such as medium growth factors, innervation, and mechanical activity.

Faster ribosome synthesis induced by elevated aortic pressure in rat heart

An increase in aortic pressure from 60 to 120 mmHg accelerated ribosomal protein synthesis in rat hearts during 1 or 2 h of labeling with 0.4 mM [3H]phenylalanine. When hearts were perfused with buffer that contained 20 mM glucose and normal plasma concentrations of 19 other amino acids without added insulin, ribosomal protein synthesis relative to the rate of total protein synthesis increased from approximately 0.22 to 0.36 and 0.30 as aortic pressure was raised from 60 to 120 mmHg during 1 or 2 h of labeling, respectively. With the addition of insulin, the relative rate of ribosomal protein synthesis averaged 0.33 at an aortic pressure of 60 mmHg and increased to 0.42 when aortic pressure was raised to 120 mmHg. These results indicate that elevation of aortic pressure has a preferential effect on synthesis of new ribosomes. This response appears to be an early and physiologically significant event in cardiac hypertrophy.

Modulation of protein synthesis and secretion by substratum in primary cultures of rat hepatocytes

Hepatocytes isolated by perfusion of adult rat liver and cultured on substrata consisting of one or more of the major components of the liver biomatrix (fibronectin, laminin, type IV collagen) have been examined for the synthesis of defined proteins. Under these conditions, tyrosine amino transferase, a marker of hepatocyte function, is maintained at similar levels in response to dexamethasone over 5 days in culture on each substratum, and total cellular protein synthesis remains constant. By contrast, there is a rapid decrease in synthesis and secretion of albumin and a 3-7-fold increase in synthesis and secretion of alpha-fetoprotein which are most marked on a laminin substratum, but least evident on type IV collagen, and an increased synthesis of fibronectin and type IV collagen. The newly synthesized matrix proteins are present in the cell layer as well as in cell secretions. The enhanced synthesis of fibronectin is less in cells seeded onto a fibronectin substratum than on laminin or type IV collagen substrata, and its synthesis by hepatocytes seeded onto a mixed substratum of laminin and fibronectin is down-regulated by fibronectin in a dose-related manner. Similarly, type IV collagen synthesis is less when the cells are seeded on the homologous matrix protein substratum than on heterologous substrata. These results indicate that hepatocytes cultured in serum-free medium on substrata composed of components of the liver biomatrix maintain certain functions of the differentiated state (tyrosine amino transferase), lose others (albumin secretion) and switch to increased synthesis of matrix components as well as fetal markers such as alpha-fetoprotein. The magnitude of these effects depends on the substratum on which the hepatocytes are cultured.

Contractile activity is required for the expression of neonatal myosin heavy chain in embryonic chick pectoral muscle cultures

The expression of neonatal myosin heavy chain (MHC) was examined in developing embryonic chicken muscle cultures using a monoclonal antibody (2E9) that has been shown to be specific for that isoform (Bandman, E., 1985, Science (Wash. DC), 227: 780-782). After 1 wk in vitro some myotubes could be stained with the antibody, and the number of cells that reacted with 2E9 increased with time in culture. All myotubes always stained with a second monoclonal antibody that reacted with all MHC isoforms (AG19) or with a third monoclonal antibody that reacted with the embryonic but not the neonatal MHC (EB165). Quantitation by ELISA of an extract from 2-wk cultures demonstrated that the neonatal MHC represented between 10 and 15% of the total myosin. The appearance of the neonatal isoform was inhibited by switching young cultures to medium with a higher [K+] which has been shown to block spontaneous contractions of myotubes in culture. Furthermore, if mature cultures that reacted with the neonatal antibody were placed into high [K+] medium, neonatal MHC disappeared from virtually all myotubes within 3 d. The effect of high [K+] medium was reversible. When cultures maintained in high [K+] medium for 2 wk were placed in standard medium, which permitted the resumption of contractile activity, within 24 h cells began to react with the neonatal specific antibody, and by 72 h many myotubes were strongly positive. Since similar results were also obtained by inhibiting spontaneous contractions with tetrodotoxin, we suggest that the development of contractile activity is not only associated with the maturation of myotubes in culture, but may also be the signal that induces the expression of the neonatal MHC.

Effect of intraventricular pressure on protein synthesis in arrested rat hearts

A modification of the Langendorff technique for perfusion of rat hearts was developed to allow a constant intraventricular pressure to be imposed on the left ventricle of arrested-catheterized hearts. This model was used to assess effects of increased aortic (60 and 120 mmHg) and intraventricular (0 and 25 mmHg) pressures on the rate of protein synthesis between 70 and 130 min of perfusion and on contents of ATP and creatine phosphate (creatine-P) in ventricles. Rates of protein synthesis in atria also were measured. Increased intraventricular or aortic pressure elevated the rate of protein synthesis (41%) compared with hearts supplied an aortic pressure of 60 mmHg and an intraventricular pressure of 0 mmHg. Higher intraventricular pressure also decreased the ribosomal subunit content. No change in the rate of protein synthesis was observed when intraventricular pressure was raised in hearts supplied an aortic pressure of 120 mmHg. Rates of atrial protein synthesis and contents of ATP and creatine-P were unchanged by elevations of either intraventricular or aortic pressure. These experiments indicate that stretch of the ventricular wall due to higher intraventricular or aortic pressure accelerated protein synthesis by maintaining the in vivo balance between rates of peptide chain initiation and elongation.

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.