Figure 3. Ca-ATPase activity as a function of the substrate and cofactor concentrations. SR vesicles (0.1 mg/mL) from masseter (•), medial pterygoid (), and fast muscles () were incubated at 37°C in a basic medium containing MOPS-Tris (pH 7.2), ATP, MgCl₂, KCl, CaCl₂, EGTA, and calcimycin modified as follows. (A) Ca-ATPase activity as a function of [Ca²⁺]. SR vesicles were incubated in the above basic medium except for the addition of various free calcium and EGTA concentrations to render concentrations as indicated in the abscissa. (B) Ca-ATPase activity as a function of [Mg²⁺]. SR vesicles were incubated in the above basic medium except for the addition of various free magnesium concentrations as indicated in the abscissa. (C) Ca-ATPase activity as a function of [KCl]. SR vesicles were incubated in the above basic medium except for the addition of various KCl concentrations as indicated in the abscissa. (D) Ca-ATPase activity as a function of [ATP]. SR vesicles were incubated in the above basic medium except for the addition of various ATP concentrations as indicated in the abscissa. (A-D) Reactants at constant concentration within each Fig. were as in Fig. 2. The reactions and the Ca-ATPase activity determinations were performed as indicated in MATERIALS & METHODS. Error bars indicate SD; n = 4.