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

Abstract

Increasing the K+ from 5.4 to 12 mM in the culture medium of developing chick myotubes caused an immediate cessation of spontaneous contractions and led to an inhibition of myosin accumulation. The synthesis of myosin continued 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 were unaffected by growth in 12 mM K+. Growth in 12 mM K+ did not alter the type of myosin H chain isoform expression or the pattern of myosin L chain synthesis. The rate of myosin turnover increased 3-fold in cultures grown in 12 mM K+ compared to cultures grown in 5.4 mM K+, while total protein turnover was only marginally increased. Suppressed electrical or contractile activity of myotubes leads to an increased rate of myofibrillar protein turnover. Spontaneous mechanical and or electrical activity is required for continued myotube maturation in culture.