Regulation of the sodium‐potassium pump in cultured rat skeletal myotubes by intracellular sodium ions

The properties of the Na‐K pump and some of the factors controlling its amount and function were studied in rat myotubes in culture. The number of Na‐K pump sites was quantified by measuring the amount of [³H]ouabain bound to whole‐cell preparations. Activity of the pump was determined by measurement of oua‐bain‐sensitive ⁸⁶Rb‐uptake and component of membrane potential. Chronic treatment of myotubes with tetrodotoxin (TTX), which lowers [Na]i, decreased the number of Na‐K pumps, the ouabain‐sensitive ⁸⁶Rb uptake, and the size of the electrogenic pump component of Em. In contrast, chronic treatment with either ouabain or veratridine, which increases [Na⁺]i, resulted in an elevated level of Na‐K pump sites. This effect was blocked by inhibitors of protein synthesis. Neither rates of degradation nor affinity of pump sites in cells treated with TTX, veratridine, or ouabain differred from those in control cells. The number and activity of Na‐K pump sites were unaffected by chronic elevation in [Ca]i or chronic depolarization. We conclude that alterations in the level in intracellular Na ions play the major role in regulation of Na‐K pump synthesis in cultured mammalian skeletal muscle.