Increased K+ inhibits spontaneous contractions reduces myosin accumulation in cultured chick myotubes.

Abstract

Increasing the K+ from 5.4 mM to 12 mM in the culture medium of developing chick myotubes causes an immediate cessation of spontaneous contractions and leads to an inhibition of myosin accumulation. The synthesis of myosin continues at the same rate in 12 mM K+ as in 5.4 mM K+ as measured by [3H]leucine incorporation into myosin corrected for differences in pool specific activity. Total protein synthesis and total protein accumulation are unaffected by growth in 12 mM K+. In addition, growth in 12 mM K+ did not alter the type of myosin heavy-chain isoform expression nor did it alter the pattern of myosin light-chain synthesis. However, the rate of myosin turnover increased threefold in cultures grown in 12 mM K+ compared to cultures grown in 5.4 mM K+, while total protein turnover was only marginally increased. We conclude that suppressed electrical or contractile activity of myotubes leads to an increased rate of myofibrillar protein turnover and that spontaneous mechanical and or electrical activity is required for continued myotube maturation in culture.

Myocyte differentiation generates nuclear invaginations traversed by myofibrils associating with sarcomeric protein mRNAs

“…This is probably because incompletely organized myofibrils are formed as detected by electron microscopic observations even after treatment with these reagents (Soeno et al, 1999). Both BDM and KCl block the contraction of cultured muscle cells (Bandman and Strohman, 1982;Soeno et al, 1999). Thus, these results also demonstrate that contraction is not responsible for the formation of the curved or branched grooves and invaginations.…”

“…In addition, despite the fact that the biochemically differentiated cells without cell fusion never contracted, they also had deformed nuclei. Moreover, although both BDM and KCl block the contraction of cultured muscle cells (Bandman and Strohman, 1982;Soeno et al, 1999), the treatment of C2C12 myotubes with BDM or KCl did not interfere with the formation of curved or branched nuclear grooves and invaginations. Consequently, the nuclear deformation is not ascribed to mechanical forces generated by contraction, at least in C2C12 cells.…”

“…To inhibit terminal differentiation, 5 mM sodium butyrate or 2% dimethyl sulfoxide (DMSO) was included in the differentiation medium (Endo and Nadal-Ginard, 1987). To impair myofibrillogenesis, 10 mM 2,3-butanedione monoxime (BDM) (Sigma) or 12 mM KCl were added to the differentiation medium (Bandman and Strohman, 1982;Soeno et al, 1999). Mouse C3H/10T1/2 (10T1/2) fibroblasts (Reznikoff et al, 1973) were cultured in the growth medium.…”

“…To confirm this, we added 10 mM BDM or 12 mM KCl to the differentiation medium. The addition of either BDM or KCl impairs the assembly of myofibrils without inhibiting myotube formation (Bandman and Strohman, 1982;Soeno et al, 1999) (T.O. et al, unpublished observations).…”

Failure to make normal α ryanodine receptor is an early event associated with the Crooked Neck Dwarf (cn) mutation in chicken

“…Consequently, if mechanical stresses generated by forming skeletal muscle are important for muscle-connective tissue interactions, the absence of these forces could lead to the loss of connective tissue components. In addition, conditions resulting in skeletal muscle inactivity also lead to the failure to properly organize sarcomeres (Bandman and Strohman, 1982;Cerny and Bandman, 1986). Data from several experimental systems suggest that mechanical stresses are important for the formation of a normal skeletal muscle morphology.…”