ADENOSINE TRIPHOSPHATASES DURING MATURATION OF CULTURED HUMAN SKELETAL-MUSCLE CELLS AND IN ADULT HUMAN MUSCLE

Na+/K+-ATPase, Mg2+-ATPase and sarcoplasmic reticulum (SR) Ca2+-ATPase are examined in cultured human skeletal muscle cells of different maturation grade and in human skeletal muscle. Na+/K+-ATPase is investigated by measuring ouabain binding and the activities of Na+/K+-ATPase and K+-dependent 3-O-methylfluorescein phosphatase (3-O-MFPase). SR Ca2+-ATPase is examined by ELISA, Ca2+-dependent phosphorylation and its activities on ATP and 3-O-methylfluorescein phosphate. Na+/K+-ATPase and SR Ca2+-ATPase are localized by immunocytochemistry. The activities of Na+/K+-ATPase and SR Ca2+-ATPase show a good correlation with the other assayed parameters of these ion pumps. All ATPase parameters investigated increase with the maturation grade of the cultured muscle cells. The number of ouabain-binding sites and the activities of Na+/K+-ATPase and K+-dependent 3-O-MFPase are significantly higher in cultured muscle cells than in muscle. The Mg2+-ATPase activity, the content of SR Ca2+-ATPase and the activities of SR Ca2+-ATPase and Ca2+-dependent 3-O-MFPase remain significantly lower in cultured cells than in muscle. The ouabain-binding constant and the molecular activities of Na+/K+-ATPase and SR Ca2+-ATPase are equal in muscle and cultured cells. During ageing of human muscle the activity as well as the concentration of SR Ca2+-ATPase decrease. Thus the changes of the activities of the ATPases are caused by variations of the number of their molecules. Na+/K+-ATPase is localized in the periphery of fast- and slow-twitch muscle fibers and at the sarcomeric I-band. SR Ca2+-ATPase is predominantly confined to the I-band, whereas fast-twitch fibers are much more immunoreactive than slow-twitch fibers. The presence of cross-striation for Na+/K+-ATPase and SR Ca2+-ATPase in highly matured cultured muscle cells indicate the development and subcellular organization of a transverse tubular system and SR, respectively, which resembles the in vivo situation.