Ca2(+)- and Sr2(+)-sensitivity of the ATPase of myofibrils and natural actomyosins prepared from various muscles.

Ca2(+)- and Sr2(+)-sensitivity of myofibrils and natural actomyosins prepared from various kinds of muscles was investigated by examining the ATPase activity. Sr2+ at concentrations about 30 times higher than Ca2+ was required for activating the ATPase of myofibrils or natural actomyosins from fast skeletal, smooth and scallop striated muscles, while Sr2+ 8-13 times higher than Ca2+ was required for activating the slow skeletal or cardiac system. These findings were consistent with the previous report on the Ca2(+)- and Sr2(+)-sensitive superprecipitation of various natural actomyosins.