Mechanical stimulation of skeletal muscle generates lipid-related second messengers by phospholipase activation

Repetitive mechanical stimulation of cultured avian skeletal muscle increases the synthesis of prostaglandins (PG) E2 and F2 alpha which regulate protein turnover rates and muscle cell growth. These stretch-induced PG increases are reduced in low extracellular calcium medium and by specific phospholipase inhibitors. Mechanical stimulation increases the breakdown rate of 3H-arachidonic acid labelled phospholipids, releasing free 3H-arachidonic acid, the rate-limiting precursor of PG synthesis. Mechanical stimulation also increases 3H-arachidonic acid labelled diacylglycerol formation and intracellular levels of inositol phosphates from myo-[2-3H]inositol labelled phospholipids. Phospholipase A2 (PLA2), phosphatidylinositol-specific phospholipase C (PLC), and phospholipase D (PLD) are all activated by stretch. The stretch-induced increases in PG production, 3H-arachidonic acid labelled phospholipid breakdown, and 3H-arachidonic acid labelled diacylglycerol formation occur independently of cellular electrical activity (tetrodotoxin insensitive) whereas the formation of inositol phosphates from myo-[2-3H]inositol labelled phospholipids is dependent on cellular electrical activity. These results indicate that mechanical stimulation increases the lipid-related second messengers arachidonic acid, diacylglycerol, and PG through activation of specific phospholipases such as PLA2 and PLD, but not by activation of phosphatidylinositol-specific PLC.

Computer-aided mechanogenesis of skeletal muscle organs from single cells in vitro

Complex mechanical forces generated in the growing embryo play an important role in organogenesis. Computerized mechanical application of similar forces to differentiating skeletal muscle myoblasts in vitro generate three-dimensional artificial muscle organs. These organs contain parallel networks of long unbranched myofibers organized into fascicle-like structures. Tendon development is initiated and the muscles are capable of performing directed, functional work. Kinetically engineered organs provide a new method for studying the growth and development of normal and diseased tissue.

Insulin and IGF-I induce pronounced hypertrophy of skeletal myofibers in tissue culture

Skeletal myofibers differentiated from primary avian myoblasts in tissue culture can be maintained in positive nitrogen balance in a defined serum-free medium for at least 6-7 days when embedded in a three-dimensional collagen gel matrix. Incubation of established myofiber cultures for 3-7 days with insulin (1 microM) or insulin-like growth factor I (IGF-I, 32 nM) stimulates both cell hyperplasia and myofiber hypertrophy. Mean myofiber diameter increases 71-98%. Insulin-like growth factor II stimulates cell hyperplasia but not myofiber hypertrophy. Cell growth results from a 42-62% increase in total protein synthesis and a 28-38% decrease in protein degradation. Myosin heavy-chain content increases 183-258% because of a 55% stimulation of myosin synthesis and 33-61% inhibition of degradation. Associated with myofiber hypertrophy is a 87-148% increase in the number of myofiber nuclei per unit myofiber length. The results indicate that insulin and IGF-I, but not IGF-II, can induce rapid myofiber hypertrophy in vitro, most likely by stimulating myoblast proliferation and/or fusion to established myofibers.

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.

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.

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.