Regulation of the (Na+ + K+)-ATPase in cultured chick skeletal muscle. Modulation of expression by the demand for ion transport.

The levels of (Na+ + K+)-ATPase expression during muscle development and in response to modulation of demand for ion transport were studied in chick skeletal muscle cells in culture. The number of (Na+ + K+)-ATPase molecules on the myogenic cell surface, quantified with 125I-labeled monoclonal antibodies, increased 20-fold during muscle differentiation, with a substantial increase in (Na+ + K+)-ATPase molecules/unit area of membrane. The demand for sodium ion transport by the (Na+ + K+)-ATPase was modulated by activating voltage-sensitive sodium channels with veratridine or exposing cultures to low [K+]o (0.5 mM). Exposure to veratridine (10 microM) resulted in a 60-100% increase in cell surface and a smaller increase in intracellular (Na+ + K+)-ATPase over a 24-36-h period. Neither high [K+]o (50 mM) nor Ca2+ ionophore A23187 (1 microM) produced any such change, suggesting that neither membrane depolarization nor elevated cytosolic calcium was mediating the effect of veratridine. Veratridine stimulated up-regulation was specific for the (Na+ + K+)-ATPase, blocked by tetrodotoxin, and completely reversible. The kinetics of the reversal (down-regulation) process were much faster (t1/2 = 3 h) than those of up-regulation (t1/2 = 18 h). Up-regulation of the (Na+ + K+)-ATPase by veratridine occurred by a combination of two mechanisms: the first an early phase involving a stimulated biosynthesis of the (Na+ + K+)-ATPase and a later phase in which the biosynthetic rate returned to approximately control levels while the degradation rate slowed (t1/2 control = 31 h, t1/2 veratridine = 64 h).