Stretch-induced growth of skeletal myotubes correlates with activation of the sodium pump.

Skeletal myotubes responded to passive stretch by increased amino acid uptake (as measured with [3H]alpha-aminoisobutyric acid), increased incorporation of amino acids into total cellular protein and myosin heavy chains, and increased accumulation of total cellular protein and myosin heavy chains. These alterations were preceded by an increase in the uptake of ouabain-sensitive rubidium-86 (86Rb+), a potassium tracer used to measure membrane sodium pump activity (Na+K+ATPase). This stretch-induced stimulation of 86Rb+ uptake resulted from a 60-70% increase in the Vmax of the Na pump with little change in the Km. [3H] ouabain binding studies showed no stretch-induced change in the number of membrane Na pumps, indicating that stretch activates the Na pumps that are already present on the cell surface. Since the stretch-induced increases in amino acid transport and amino acid incorporation into proteins were inhibited by ouabain, Na pump activation may be involved in stretch-induced cell growth of skeletal muscle cells by hypertrophy.