Relationship of muscle growth in vitro to sodium pump activity and transmembrane potential

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Abstract
Serum stimulates embryonic avian skeletal muscle growth in vitro and the growth‐related processes of amino acid transport and protein synthesis. Serum also stimulates myotube Na pump activity (measured as ouabain‐sensitive rubidium‐86 uptake) for at least 2 h after serum addition. Serum‐stimulated growth depends on this Na pump activity since ouabain added at the same time as serum totally inhibits the growth responses. The relationship of myotube growth, Na pump activity, and transmembrane potential was studied to determine whether serum‐stimulated Na pump activation and growth are coupled by long‐term membrane hyperpolarization. When myotube amino acid transport and protein synthesis are prestimulated by serum, ouabain was found to have little inhibitory effect, indicating that the already stimulated growth‐related processes are not tightly coupled to continued Na pump activity. Serum‐stimulated protein synthesis is tightly coupled to Na pump activity, but only during the first 5–10 min after serum addition. When myotube transmembrane potentials were measured using the lipophilic cation tetraphenylphosphonium, serum at concentrations that stimulate myotube growth and Na pump activity was found to have little effect on the cell's transmembrane potential. Furthermore, partial depolarization of the myotubes with 12‐ to 55‐mM extracellular potassium does not prevent serum stimulation of myotube growth. Monensin was found to hyperpolarize the myotubes, but causes myotube atrophy. These results indicate that although Na pump activity is associated with initiation of serum‐stimulated myotube growth, continued Na pump activity is not essential, and there is little relationship between myotube growth and the myotube's transmembrane potential.