Adenosine Triphosphatase Activities of Muscle Sarcolemma

Isolated sarcolemma hydrolyzed ATP in the presence of Mg2+ and Ca2+. MgATPase was stimulated by low concentrations of Ca2+ and inhibited by high concentrations of this cation. Membranes hydrolyzed p-nitrophenylphosphate in the presence of Mg2+; this activity was stimulated by Ca2+ and K+. La3+ stimulated MgATPase at low concentrations (up to 50 µm) and caused inhibition at higher concentrations (above 0.2 mm). Phosphoprotein was formed when the membranes were incubated with [γ-32P]ATP. Formation of phosphoprotein was strongly dependent on Ca2+ and Mg2+. The phosphoprotein was acid-stable and hydroxylamine-sensitive. In the presence of Mg2+ high concentrations of Ca2+ (above 0.3 mm) inhibited ATPase activity but not the level of phosphoprotein formation.