Calcium sensitivity of vertebrate skeletal muscle myosin

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Abstract
The Ca sensitivity of vertebrate skeletal muscle myosin was investigated. ATPase activity was assayed in a reconstituted system composed of either purified rabbit myosin plus actin or myosin plus actin, tropomyosin and troponin. The Ca sensitivity of actomyosin Mg-ATPase activity was directly affected by the ionic strength of the assay medium. Actomyosin assayed at approximate physiological ionic strength (120 mM KCl) demonstrated Ca sensitivity which varied between 6 and 52%, depending on the myosin preparation and the age of the myosin. Mg-ATPase activity was increased when Ca was present in the assay medium at physiological ionic strength. Conversely, actomyosin Mg-ATPase activity assayed at a lower ionic strength (15 mM KCl) was inibited by addition of Ca. Addition of tropomyosin and troponin to the assay increased the Ca sensitivity of the system at the physiological ionic strength still further (up to 99% Ca sensitivity) and conferred Ca sensitivity on the system at the lower ionic strength (> 90% Ca sensitivity). A correlation also existed between myosin''s Ca sensitivity and the phosphorylated state of L chain 2.