Ca(2+)- and Sr(2+)-sensitive ATPase activity of slow skeletal myofibrils in comparison with fast skeletal and cardiac myofibrils.

Ca(2+)- and Sr(2+)-sensitive ATPase of myofibrillar preparations from slow skeletal (chicken anterior latissimus dorsi), fast skeletal (chicken breast) and porcine cardiac ventricular muscles was investigated. Sr2+ at concentrations about 6 times higher than Ca2+ was required for activating the ATPase of slow skeletal or cardiac myofibrils, while Sr2+ about 30 times higher than Ca2+ was required for activating fast skeletal myofibrils. The cooperativity of activation of slow skeletal myofibrils was almost the same as that of cardiac myofibrils, but lower than that of fast skeletal myofibrils. Sr2+/Ca2+ sensitivity ratio of the desensitized (troponin-tropomyosin-depleted) fast skeletal myofibrils reconstituted with troponin-tropomyosin from slow skeletal, fast skeletal, and cardiac muscles were 13, 34, and 5, respectively. The troponin C-depleted fast skeletal myofibrils showed the same Sr2+/Ca2+ sensitivity ratio and the same cooperativity of activation as those of the intact fast skeletal myofibrils when reconstituted with fast skeletal troponin C, and as those of the intact slow skeletal myofibrils when reconstituted with slow skeletal troponin C.